Aluminum IN NUCLEAR (CASE STUDY

Aluminum IN NUCLEAR 

introduction

Aluminum is the world’s most abundant metal and is the third most common element, comprising 8% of the earth’s crust. The versatility of aluminum makes it the most widely used metal after steel. Although aluminum compounds have been used for thousands of years, aluminum metal was first produced around 170 years ago. In the 100 years since the first industrial quantities of aluminum were produced, worldwide demand for aluminum has grown to around 29 million tons per year. About 22 million tons is new aluminum and 7 million tons is recycled aluminum scrap. The use of recycled aluminum is economically and environmentally compelling. It takes 14,000 kWh to produce 1 tone of new aluminum. Conversely it takes only 5% of this to remelt and recycle one tone of aluminum. There is no difference in quality between virgin and recycled aluminum alloys is soft, ductile, and corrosion resistant and has a high electrical conductivity. It is widely used for foil and conductor cables, but alloying with other elements is necessary to provide the higher strengths needed for other applications. Aluminum is one of the lightest engineering metals, having strength to weight ratio superior to steel. By utilizing various combinations of its advantageous properties such as strength, lightness, corrosion resistance, recyclability and formability, aluminum is being employed in an ever-increasing number of applications. This array of products ranges from structural materials through to thin packaging foils.

History

it is often said that Aluminium has had a relatively brief history, and under the name Aluminium it is certainly true. But using aluminium for its properties in compounds (some sources reckon) started at around 5300 BC. It is thought that potters in ancient Persia made their strongest cooking vessels from a clay that consisted largely of aluminium silicates. Aluminium compounds are thought to have been used more by the Egyptians and Babylonians around 4000 years ago as fabric dyes and cosmetics.

                                        

                                                            Fig.1 (Red clay containing bauxite)

Despite these uses in the very far past the element aluminum itself wasn't discovered or named until the early 1800's when Sir Humphrey Davy established its existence, but even he was unable to actually make any. Just over 10 years later a French scientist discovered hard, red clay containing over 50% aluminum oxide in southern France. It was named bauxite, aluminum’s most common ore. As aluminum is so combined in nature, and never occurs naturally, even up to this time no pure aluminium had been produced..

Friedrich Wohler.

In 1825 a small lump of aluminium metal was finally produced by Hans Christian Oersted of Denmark, whose work was continued and developed by Friedrich Wohler in Berlin who managed to isolate aluminium as a powder in 1827, in a process involving potassium and anhydrous aluminium chloride. Wohler also established the density of aluminium, and discovered its key lightness in 1845. Nine years later, another French scientist, Henri Sainte-Claire Deville, became involved with aluminium, and improved upon Wohler's method to create the first commercial production process.

Hall / Heroult.    

At this time aluminium was more expensive than gold and platinum¹, and it was regarded as "the new precious metal". A bar of aluminium was even exhibited at the Paris Exhibition in 1855. But over the next ten years its value fell by over 90% although it was still far too expensive to be adopted by industry as the metal of choice. Gradually methods were improved upon and in 1885 a process was developed with an annual output of 15 tones. The real revolution occurred a year later with the remarkable, separate but simultaneous development of an aluminium isolation method on either side of the Atlantic. Paul Louis Toussaint Héroult of France and Charles Martin Hall of the USA both came up with a process of dissolving aluminium oxide in molten cryolite (to lower the melting point and therefore the energy required) and passing through a large electrical current. When this was done pure aluminium collected at the bottom. The process was named after both scientists and we still use the hall heroult method today.

                                                  

                                                                Fig.2( Hall / Heroult)

However in 1889 Karl Josef Bayer developed a new, more efficient process (the Bayer process) for the extraction of aluminum oxide from bauxite. Again the price of aluminum fell. The quantities of aluminum produced began to grow. In 1900 8 thousand tones were produced, in 1946 the output was 681 thousand tonnes, and in 1999 24 million tones of aluminum were made. Also in 1999 some 7 million tonnes of recycled aluminum were produced, giving 31 million tones in total. This is more than the production of all the non-ferrous metals combined, even though they were discovered hundreds of years earlier.

Extraction of Aluminum

The first step in extracting of  aluminium is to remove it from the Earth in mining. This is relatively simple given the abundance of the material. However aluminium is never found isolated in the Earth (due to its reactivity) but instead it is found bound to other elements in compounds. This means that aluminium alone in can never be dug up, but compounds of it, often containing oxygen and silicon, ore.

Bayer process

The bauxite then has to be purified using the Bayer process, whose development changed the course of aluminium's history. The process occurs in two main steps. Firstly the aluminium ore is mixed with the sodium hydroxide in which the oxides of aluminium and silicon will dissolve, but other impurities will not. These impurities can then be removed by filtration. Carbon dioxide gas is then bubbled through the remaining solution, which forms weak carbonic acid neutralizing the solution and causing the aluminium oxide to precipitate, but leaving the silicon impurities in solution. After filtration, and boiling to remove water, purified aluminium oxide can be obtained.

                              

                                  

                                                                      Fig.3 ( Bayer process on industrial scale)  

The hal heroult process

Once purified aluminium oxide has been manufactured aluminium can be removed from it by the Hall-Heroult Method. In this the aluminium oxide is mixed with cryolite (made of sodium fluoride and aluminium fluoride) and then heated to about 980 °C to melt the solids. This is much lower than the temperature required to melt pure aluminium oxide so much energy is saved. The molten mixture is then electrolysed with a very large current and the aluminium ions are reduced to form aluminium metal (at the cathode) and oxygen gas is formed at the anode, where it reacts with the carbon the anode is made from to give carbon dioxide gas.

As the process is so long and requires so much energy (in electricity) the aluminium metal obtained is quite expensive, but still it is competitively priced in relation to other metals unlike earlier in its history.

      

             

                                                           Fig.4   (The Hall Heroult Process)

Summary: Aluminium is extracted from the ground in compounds, it is the purified to alumina (aluminium oxide) in the bayer process, and the metal is finally obtained after electrolysis in a cryolite solution. The full process needs a lot of energy and is quite expensive.

Commercially pure aluminium

Commercially pure aluminium is the product of the electrolytic cell process. It contains a low level of impurities, usually much less than 1%. Commercially pure aluminium is light in weight (2,700 kg.m-3 compared with iron at 7,870 kg.m-3) and melts at 660 °C. A lump of aluminium that has been heated to just below the melting point and allowed to cool slowly (annealed) is light in weight, is not very strong, is soft and ductile, is corrosion resistant and has high thermal and electrical conductivities (see Figure 1201.01.01 for data). If the lump is mechanically deformed at room temperature, then it becomes noticeably harder and less ductile - the material has been “work hardened”; the mechanism of work hardening will be explained later in this section. The mechanical and physical properties of commercially pure aluminium may be also be changed by deliberate additions of other elements, for example, copper (Cu), magnesium (Mg), silicon (Si) - the products are alloys and the aim of industrially useful alloys is to enhance their properties and hence make them more suitable for fabrication into useful products. Again, the mechanisms involved will be the subject of much discussion later in this chapter. Such alloy additions are small in amount (typically up to a few percent); consequently they have only a very small effect on the density, which remains low at typically 2800 kg.m-3. An exception is additions lithium (Li), density 540kg.m-3, of up to a few percent and specially developed for aerospace applications, where the aluminium-lithium alloy density is lower at 2200-2700 kg.m-3. Also, alloys of aluminium with small additions of lithium are stiffer than other aluminium alloys, which is a feature of benefit to some applications.

             

Atomic structure of aluminium

Let us return to the specific case of aluminium. The atomic number of an aluminium atom is 13 by definition this is because it has 13 protons in its nucleus, together with 14 neutrons. The atomic structure is shown pictorially in Figure 1201.02.04. The outer electron shell – the valence shell - contains three electrons; these contribute to the “free electron gas” of aluminium crystals and give such crystals excellent electrical conductivity. As a general rule, the electrical conductivity is reduced by the addition to aluminium of impurities and by the deliberate addition of other elements to form aluminium alloys.

                       

Crystal structures

As already indicated above, aluminium atoms assemble into an array to form a crystal lattice.The crystal lattice has a face-centred cubic structure (often abbreviated to fcc). Aluminium, in common with most other metals and their alloys, pack atoms together in a highly compacted way - mostly, this is achieved by one of three different lattices, face-centred cubic, close-packed hexagonal or body-centred cubic - see Figure 1201.02.05.

                     

Applications

Aluminium is used excessively in the modern world, and the uses of the metal are extremely diverse due to its many unusual combinations of properties. No other metallic element can be used in so many ways across such a variety of domains, like in the home, in transport, on land, sea and in air, and in industry and commerce. Aluminium's uses are not always as obvious as they may seem, with sizeable proportions of manufactured aluminium and aluminium oxide going into other separate processes, like the manufacture of glass, rather than towards the common consumer products that we most readily associate Aluminium with.

                                 

                                                Fig.5 (Aluminium cooking equipments)

One of the most common end uses of aluminium is packaging, including drinks cans, foil wrappings, bottle tops and foil containers. Each of these relies on aluminium to provide a way of containing the food cleanly, and to protect it from changes in the local environment outside the packaging. Aluminium is still used in a very big way in the food packaging industry despite recent health worries linking aluminium to Alzheimer's disease. Aluminium's natural resistance to corrosion aids it in its role in packaging (and many other areas), as unlike in iron, aluminium oxide forms a protective and not destructive layer. Aluminium is also completely impermeable, (even when rolled into extremely thin foil), and also doesn't let the aroma or taste out of food packaging, the metal is non-toxic and aromaless itself too, making it perfect for packaging.

                                                             

                                                              Fig.6 (drinking cans) 

Aluminium's unbeatable strength to weight ratio¹ gives it many uses in the transport industry. Transport is all about moving things around and to do so a force is always required. As force = mass x acceleration (Newton's Second Law of Motion), less force is needed to move a lighter object to a certain acceleration than is needed to get a heavier object to the same acceleration. As aluminium is so lightweight this means that less energy needs to be used to move a vehicle made with aluminium than one made from a heavier metal, say steel. Although aluminium isn't the strongest of metals its alloys use other elements to pin dislocations in its structure to increase its strength. With trains, boats and cars aluminium is useful for this lightweight property (which gives fuel efficiency) but not essential, in planes however maintaining a relatively low weight is vital (in order to level the ground), and aluminium allows planes to have to this. In modern planes aluminium makes up 80% of their (unlade) weight, and a normal Boeing 747 contains about 75 000 kg of the metal. Its corrosion resistance is an advantage in transport (as well as packaging) as it makes painting planes unnecessary saving some hundreds of kilograms of further weight.

                                 

                                                           Fig.7(aerospace industrry)

Aluminium alloys

Introduction

Even earlier than 1886 aluminium had been employed as an alloying constituent in bronze and as a de-oxidant in steel making. The first official alloy designation that denotes commercially pure aluminium dates back to 1888. Since then the introduction of new wrought and casting alloys, each developed for specific qualities, has continued steadily until the present day.The range of alloy choice is important. The number of widely used commercial alloys is of course much smaller. Designers should try to avoid a fixation on the familiar specification, it may not be a logical choice for a new product or application.

Aluminum and its alloys offer an extremely wide range of capability and applicability, with a unique combination of advantages that make it the material of choice for numerous products and markets. It is the purpose of this presentation to:

 (a) Provide an over view of the various types of aluminum alloys that are available to engineers, designers ,and others considering aluminum for new products or applications,

(b) To describe the properties and characteristics that make aluminum alloys so useful.

The breadth of individual alloys and of applications is so broad that it will be necessary to hit only the highlights and provide representative examples. For more detail on the alloys, their properties ,and the  applications.

                                     

          Fig.8 ( Extruded tubes)                                                                          fig.9 ( Formed cans)

Summary: Alloys of aluminum are more useful that aluminum itself, as they give real strength to the material, by pinning dislocations whilst maintaining aluminum’s other properties. Using different materials in alloys gives slightly different effects, so alloys can be handpicked for their specific job.

Understanding the Aluminum Alloy Designation System

With the growth of aluminum within the welding fabrication industry, and its acceptance as an excellent alternative to steel for many applications, there are increasing requirements for those involved with developing aluminum projects to become more familiar with this group of materials.  To fully understand aluminum, it is advisable to start by becoming acquainted with the aluminum identification / designation system, the many aluminum alloys available and their characteristics.

                             

The Aluminum Alloy Temper and Designation System

In North America, The Aluminum Association Inc. is responsible for the allocation and registration of aluminum alloys.  Currently there are over 400 wrought aluminum and wrought aluminum alloys and over 200 aluminum alloys in the form of castings and ingots registered with the Aluminum Association.  The alloy chemical composition limits for all of these registered alloys are contained in the Aluminum Association’s Teal Book entitled “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys” and in their Pink Book entitled “Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot.  These publications can be extremely useful to the welding engineer when developing welding procedures, and when the consideration of chemistry and its association with crack sensitivity is of importance.

Types of aluminium alloys

Aluminum alloys can be categorized into a number of groups based on the particular material’s characteristics such as its ability to respond to thermal and mechanical treatment and the primary alloying element added to the aluminum alloy.  When we consider the numbering / identification system used for aluminum alloys, the above characteristics are identified.  The wrought and cast aluminums have different systems of identification; the wrought having a 4-digit system, and the castings having a 3-digit and 1-decimal place system.

Wrought Alloy Designation System

Ø  1xxx - Pure Al (99. 00% or greater)

Ø  2xxx - Al-Cu Alloys

Ø  3xxx - Al-Mn Alloys

Ø  4xxx - Al-Si Alloys

Ø  5xxx - Al-Mg Alloys

Ø  6xxx - Al-Mg-Si Alloys

Ø  7xxx - Al-Zn Alloys

Ø  8xxx - Al+Other Elements

Ø  9xxx - Unused series

      The first digit (Xxxx) indicates the principal alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series, i.e., 1000 series, 2000 series, 3000 series, up to 8000 series .

the second single digit (xXxx), if different from 0, indicates a modification of the specific alloy, and the third and fourth digits (xxXX) are arbitrary numbers given to identify a specific alloy in the series.  Example: In alloy 5183, the number 5 indicates that it is of the magnesium alloy series, the 1 indicates that it is the 1st modification to the original alloy 5083, and the 83 identifies it in the 5xxx series.

The only exception to this alloy numbering system is with the 1xxx series aluminum alloys (pure aluminums) in which case, the last 2 digits provide the minimum aluminum percentage above 99%, i.e., Alloy 1350 (99.50% minimum aluminum).

Casting alloys designation system

Ø  lxx.x - Pure Al (99.00% or greater)

Ø  2xx.x - Al-Cu Alloys

Ø  3xx.x - Al-Si + Cu and/or Mg

Ø  4xx.x - Al-Si

Ø  5xx.x - Al-Mg

Ø  7xx.x - Al-Zn

Ø  8xx.x - Al-Sn

Ø  9xx.x - Al+Other Elements

Ø  6xx.x - Unused Series

     The cast alloy designation system is based on a 3 digit-plus decimal designation xxx.x (i.e. 356.0).  The first digit (Xxx.x) indicates the principal alloying element, which has been added to the aluminum alloy.

The second and third digits (xXX.x) are arbitrary numbers given to identify a specific alloy in the series. The number following the decimal point indicates whether the alloy is a casting (.0) or an ingot (.1 or .2).  A capital letter prefix indicates a modification to a specific alloy.

Example: Alloy - A356.0 the capital A (Axxx.x) indicates a modification of alloy 356.0. The number 3 (A3xx.x) indicates that it is of the silicon plus copper and/or magnesium series.  The 56 (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) indicates that it is a final shape casting and not an ingot.

Temper designation system

Ø  F - As-fabricated

Ø  0 - Annealed

Ø  H - Strain-hardened (wrought products only)

Ø  W- Solution heat-treated

Ø  T - Thermally treated to produce tempers other than F,O,H (usually

Ø  solution heat-treated,quenched and precipitation hardened)

Ø  Numeric additions indicate specific variations

Ø  e.g.,T6 = solution heat treated and artificially aged.

 If we consider the different series of aluminum alloys, we will see that there are considerable differences in their characteristics and consequent application.  The first point to recognize, after understanding the identification system, is that there are two distinctly different types of aluminum within the series mentioned above.  These are the Heat Treatable Aluminum alloys (those which can gain strength through the addition of heat) and the Non-Heat Treatable Aluminum alloys.  This distinction is particularly important when considering the affects of arc welding on these two types of materials.

The 1xxx, 3xxx, and 5xxx series wrought aluminum alloys are non-heat treatable and are strain hardenable only. The 2xxx, 6xxx, and 7xxx series wrought aluminum alloys are heat treatable and the 4xxx series consist of both heat treatable and non-heat treatable alloys. The 2xx.x, 3xx.x, 4xx.x and 7xx.x series cast alloys are heat treatable.   Strain hardening is not generally applied to castings.

The heat treatable alloys acquire their optimum mechanical properties through a process of thermal treatment, the most common thermal treatments being Solution Heat Treatment and Artificial Aging.  Solution Heat Treatment is the process of heating the alloy to an elevated temperature (around 990 Deg. F) in order to put the alloying elements or compounds into solution.  This is followed by quenching, usually in water, to produce a supersaturated solution at room temperature.  Solution heat treatment is usually followed by aging.  Aging is the precipitation of a portion of the elements or compounds from a supersaturated solution in order to yield desirable properties.  The aging process is divided into two types: aging at room temperature, which is termed natural aging, and aging at elevated temperatures termed artificial aging.  Artificial aging temperatures are typically about 320 Deg. F.  Many heat treatable aluminum alloys are used for welding fabrication in their solution heat treated and artificially aged condition.

The principal characteristics and applications

Wrought alloys

Ø  1xxx - Pure Al

Ø  Strain hardenable

Ø  High formability, corrosion resistance and electrical conductivity

Ø  Electrical, chemical applications

Ø  Representative designations: 1100,1350

Ø  Typical ultimate tensile strength range:10-27 ksi

        The 1xxx series represents the commercially pure aluminum, ranging from the baseline 1100 (99.00% min. Al) to relatively purer 1050/1350 (99.50% min. Al) and 1175 (99.75 % min. Al).Some,like 1350 which is used especially for electrical applications, have relatively tight controls on those impurities that might lower electrical conductivity. The 1xxx series are strain-hardenable ,but would not be used where strength is a prime consideration. Rather the emphasis would be on those applications where extremely high corrosion resistance, formability and/or electrical conductivity are required,e.g., foil and strip for packaging, chemical equipment, tank car or truck bodies, spun hollowware, and elaborate sheet metal work.

            

                                                      

fig.10(Food packaging trays

                  of  pure aluminum (1100))                                                fig.11( Decorated foil pouches for  

                                                                                                                                      food(110)

            

                                 

                                             

Ø  2xxx - Al-Cu Alloys

Ø   Heat treatable

Ø   High strength,at room & elevated temperatures

Ø   Aircraft, transportation applications

Ø   Representative alloys:2014,2017, 2024,2219,2195

Ø  Typical ultimate tensile strength range:27-62 ksi

      

        The 2xxx series are heat-treatable ,and possess in individual alloys good combinations of high strength (especially at elevated temperatures),toughness, and, in specific cases, weld ability; they are not resistant to atmospheric corrosion, and so are usually painted or clad in such exposures. The higher strength 2xxx alloys are primarily used for aircraft (2024) and truck body (2014) applications;

these are usually used in bolted or riveted construction. Specific members of the series (e.g.,2219 and2048) are readily welded, and so are used for aerospace applications where that is the preferred joining method.

Alloy 2195 is a new Li-bearing alloy for space applications providing very high modulus of elasticity along with high strength and weldability.There are also high-toughness versions of several of the alloys (e.g.,2124, 2324,2419), which have tighter control on the impurities that may diminish resistance to unstable fracture,all developed specifically for the aircraft industry. Alloys 2011,2017, and 2117 are widely used for fasteners and screw-machine stock.

Fig.12:  Aircraft internal structure includes extrusions and plate of  2xxx and 7xxx alloys like 2024,2124 and 2618. External sheet skin may be alclad 2024 or 2618;the higher purity cladding provides corrosion protection to the Al-Cu alloys that will darken with age otherwise.

            

                                                                      

Fig.13:The fuel tanks and booster rockets of the Space Shuttle are 2xxx alloys, originally 2219and 2419,now Al-Li“Weldalite”alloy2195.

fig

      

                         

                                                                       

                                                                                                                          (Booster rocket)

Ø  3xxx - Al-Mn Alloys

Ø  High formability, corrosion resistance, and joinability:medium

Ø   strength

Ø   Heat transfer, packaging, roofing-siding applications

Ø   Representative alloys:3003,3004,3005

Ø   Typical ultimate tensile strength range:16-41 ksi

       The 3xxx series are strain-hardenable, have excellent corrosion resistance, and are readily welded, brazed and soldered. Alloy 3003 is widely used in cooking utensils and chemical equipment because of its superiority in handling many foods and chemicals, and in builders’ hardware. Alloy 3105 is a principal for roofing and siding. Variations of the 3xxx series are used in sheet and tubular form for heat exchangers in vehicles and power plants. Alloy 3004 and its modification 3104 are among the most widely used aluminum alloys because they are drawn and ironed into the bodies of beverage cans.

                               

                                                

                                         Fig.14: Alloy 3003 tubing in commercial power plant heat exchanger

Ø  4xxx- Aluminum-Silicon Alloys

Ø  Heat treatable

Ø  Good flow characteristics, medium strength

Ø  Typical ultimate tensile strength range: 175 to 380 MPa (25–55 ksi)

Ø  Easily joined, especially by brazing and soldering

Of the two most widely used 4xxx alloys, 4032 is a medium high-strength,alloy used principally for forgings in applications such as aircraft pistons. Alloy 4043 on the other hand is one of the most widely used filler alloys for gas-metal arc (GMA) and gas-tungsten arc (GTA) welding 6xxx alloys for structural and automotive applications. The same characteristic leads to both applications: good flow characteristic provided by the high silicon content, which in the case of forgings ensures the filling of complex dies and in the case of welding ensures complete filling of crevices and grooves in the members to be joined. For the same reason, other variations of the 4xxx alloys are used for the cladding on brazing sheet, the component that flows to complete the bond.

      Fig.15: Refrigerator coolant circulation system in brazed unit of high-Si brazing alloy sheet.

      Fig.16: Alloy 4043 is one of the most widely used weld wires.        

                                                     

       

Ø  5xxx - Al-Mg Alloys

Ø   Strain hardenable

Ø   Excellent corrosion resistance, toughness, weldability; moderate strength

Ø   Building & construction ,automotive, cryogenic, marine applications

Ø   Representative alloys:5052,5083,5754

Ø   Typical ultimate tensile strength range:18-51 ksi

Al-Mg alloys of the 5xxx series are strain hardenable,and have moderately high strength, excellent corrosion resistance even in salt water, and very high toughness even at cryogenic temperatures to near absolute zero.They are readily welded by a variety of techniques, even at thicknesses up to 20 cm. As a result, 5xxx alloys find wide application in building and construction,highways structures including bridges, storage tanks and pressure vessels,cryogenic tankage and systems for temperatures as low as -270°C (near absolute zero),and marine applications. Alloys 5052, 5086,and 5083 are the work horses from the structural standpoint, with increasingly higher strength associated with the increasingly higher Mg content.Specialty alloys in the group include 5182,the beverage can end alloy, and thus among the largest in tonnage;5754 for automotive body panel and frame applications;and 5252,5457,and 5657 for bright trim applications, including automotive trim.

                                       Fig.17 (The internal hull stiffener structure of a high-speed yacht )                                 

                               

Fig.18: Rugged coal cars are provided by welded 5454 alloy plate construction.

Fig.19: High speed single-hull ships like the Presario employ 5083- H113/H321 machined plate for hulls, hull stiffeners, decking and superstructure

                                                                          

                                                                                                  

                                                        ’       

                       

Ø  6xxx - Al-Mg-Si Alloys

Ø  Heat treatable

Ø  High corrosion resistance, excellent  extrudibility; moderate strength

Ø  Building & construction, highway, automotive, marine applications

Ø  Representative alloys: 6061,6063, 6111

Ø  Typical ultimate tensile strength range:18-58 ksi

The 6xxx alloys are heat treatable,and have moderately high strength coupled with excellent corrosion resistance.They are readily welded. A unique feature is their extrudability, making them the first choice for architectural and structural members where unusual or particularly strength- or stiffness-criticality is important. Alloy 6063 is perhaps the most widely used because of its extrudability;it was a key in the recent all-aluminum bridge structure erected in only a few days in Foresmo,Norway, and is the choice for the Audi automotive space frame members. Higher strength 6061 alloy finds broad use in welded structural members such as btruck and marine frames, railroad cars,and pipelines. Among specialty alloys in the series:6066-T6, with high strength for forgings;6111 for automotive body panels with high dent resistance; and 6101and 6201 for high strength electrical mbus and electrical conductor wire, respectively.

Fig.20:Geodesic domes,such as this one made originally to house the“spruce Goose”in Long Beach,CA, the largest geodesic dome ever constructed,at 1000 ft across, 400 ft. high.

Fig.21: The new Mag-Lev trains in development in Europe and Japan employ bodies with 6061 and 6063 structural members.

                                                                                 

             

                                                                                                                    

Note: The 5000 and 6000 series aluminum alloys are extensively used in research reactors. To validate the choice of the suitable Al-alloys for the core components and the experimental devices for the conception of the nuclear reactor Reactor (RJH), a characterization program of some highly irradiated components was performed.

Ø  7xxx - Al-Zn Alloys

Ø  Heat treatable

Ø  Very high strength; special high toughness versions

Ø  Aerospace, automotive applications

Ø  Representative alloys: 7005,7075, 7475, 7150

Ø  Typical ultimate tensile strength range:32-88 ksi

The 7xxx alloys are heat treatable and among the Al-Zn-Mg-Cu versions provide the highest strengths of all aluminum alloys. There are several alloys in the series that are produced especially for their high toughness, notably 7150 and 7475, both with controlled impurity level to maximize the combination of strength and fracture toughness. The widest application of the 7xxx alloys has historically been in the aircraft industry, where fracture-critical design concepts have provided the impetus for the high-toughness alloy development. These alloys are not considered weld able by routine commercial processes, and are regularly used in riveted construction. The atmospheric corrosion resistance of the 7xxx alloys is not as high as that of the 5xxx and 6xxx alloys, so in such service they are usually coated or, for sheet and plate, used in an alclad version. The use of special tempers such as the T73- type are required in place of T6-type tempers whenever stress corrosion cracking may be a problem.

Ø  8xxx - Alloys with Al+Other Elements (not covered by other series)

Ø   Heat treatable

Ø   High conductivity, strength, hardness

Ø   Electrical, aerospace, bearing applications

Ø   Representative alloys: 8017,8176, 8081,8280,8090

Ø   Typical ultimate tensile strength range:17-35 ksi

The 8xxx series is used for those alloys with lesser used alloying elements such as Fe,Ni and Li.Each is used for the particular characteristics it provides the alloys: Fe and Ni provide strength with little loss in electrical conductivity and so are used in a series of alloys represented by 8017 for conductors.Li in alloy 8090 provides exceptionally high strength and modulus, and so this alloy is used for aerospace applications where increases in stiffness combined with high strength reduces component weight.

Cast alloys

In comparison with wrought alloys,casting alloys contain larger proportions of alloying elements such as silicon and copper.This results in a largely heterogeneous cast structure,i.e. one having a substantial volume of second phases.This second phase material warrants careful study, since any coarse,sharp and brittle constituent can create harmful internal notches and nucleate cracks when the component is later put under load.The fatigue properties are very sensitive to large heterogeneities. As will be shown later, good metallurgical and foundry practice can largely prevent such defects.

 The elongation and strength,especially in fatigue,of most cast products are relatively lower than those of wrought products.This is because current casting practice is as yet unable to reliably prevent casting defects. In recent years however, innovations in casting processes have brought about considerable improvements,which should be taken into account in any new edition of the relevant standards.

Ø  2xx.x - Al-Cu Alloys

Ø   Heat treatable/sand and permanent mold castings

Ø   High strength at room and elevated temperatures; some high

Ø   toughness alloys

Ø   Aircraft, automotive applications/engines

Ø   Representative alloys: 201.0,203.0

Ø   Approximate ultimate tensile strength range:19-65 ksi.

The strongest of the common casting alloys is heat-treated 201.0/AlCu4Ti. Its castability is somewhat limited by a tendency to microporosity and hot tearing,so that it is best suited to investment casting.Its high toughness makes it particularly suitable for highly stressed components in machine tool construction, in electrical engineering (pressurized switchgear casings), and in aircraft construction.

Besides the standard aluminum casting alloys,there are special alloys for particular components, for instance, for engine piston heads,integral engine blocks,or bearings. For these applications the chosen alloy needs good wear resistance and a low friction coefficient, as well as adequate strength at elevated service temperatures. A good example is the alloy 203.0/AlCu5NiCo,which to date is the aluminum casting alloy with the highest strength at around 200°C.

 Fig.22: Landing flap mountings and other aircraft components are made in alloys of the 201.0 or in A356.0 types.

                                 \

                                                                                  

Ø  3xx.x - Al-Si+Cu or Mg Alloys

Ø   Heat treatable/sand,permanent mold,and die castings

Ø   Excellent fluidity/high strength/some high-toughness alloys

Ø   Automotive and applications/pistons/pumps/electricAl

Ø   Representative alloys:356.0, A356.0,359.0, A360.0

Ø   Approximate ultimate tensile strength range:19-40 ksi

The 3xx.x series of castings are one of the most widely used because of the flexibility provided by the high silicon contents and its contribution to fluidity plus their response to heat treatment which provides a variety of high-strength options.Further the 3xx.x series may be cast by a variety of techniques ranging from relatively simple sand or die casting to very intricate permanent mold,lost foam/lost wax type castings,and the newer thixocasting and squeeze casting technologies.

Among the workhorse alloys are 319.0 and 356.0/A356.0 for sand and permanent mold casting,360.0, 380.0/A380.0 and 390.0 for die casting, and 357.0/A357.0 for many type of casting including especially the squeeze/forge cast technologies. Alloy 332.0 is also one of the most frequently used aluminum casting alloys because it can be made almost exclusively from recycled scrap.

Fig-23: Gearbox casing for a passenger car in alloy pressure die cast 380.0.

Fig.24: A356.0 cast wheels are now widely used in the US industry

                                              .

                                                                                                                                    

Ø  4xx.x - Al-Si Alloys

Ø  Non-heat treatable/sand,permanent mold,and die castings

Ø  Excellent fluidity/good for intricate castings

Ø  Typewriter frames/dental equipment/marine/architectural

Ø  Representative alloys:413.0,443.0

Ø  Approximate ultimate tensile strength range:17-25 ksi

Alloy B413.0/AlSi12 is notable for its very good cast ability and excellent weldability, which are due to its eutectic composition and low melting point of570°C.It combines moderate strength with high elongation before rupture and good corrosion resistance. The alloy is particularly suitable for intricate, thin walled, leak-proof, fatigue resistant castings.

Ø  5xx.x - Al-Mg Alloys

Ø  Non-heat treatable/sand,permanent mold,and die

Ø  Tougher to cast/provides good finishing characteristics

Ø  Excellent corrosion resistance/machinability/surface appearance

Ø  Cooking utensils/food handling/aircraft/highway fittings

Ø  Representative alloys:512.0,514.0,518.0, 535.0

Ø  Approximate ultimate tensile strength range:17-25 ksi

The common feature which the third group of alloys have is good resistance to corrosion. Alloys 512.0 and 514.0 have medium strength and good elongation, and are suitable for components exposed to sea water or to other similar corrosive environments.These alloys are often used for door and window fittings, which can be decoratively anodized to give a metallic finish or in a wide range of colors.Their castability is inferior to that of the Al-Si alloys because of its magnesium content and consequently long freezing range. For this reason it tends to be replaced by 355.0/AlSi5Mg,which has long been used for similarapplications. For die castings where decorative anodizing is particularly important,the alloy 520.0 is the most suitable.

Ø  7xx.x - Al-Zn Alloys

Ø   Heat treatable/sand and permanent mold cast (harder to cast)

Ø   Excellent machinability/appearance

Ø   Furniture/garden tools/office machines/farm/mining equipment

Ø   Representative alloys: 705.0,712.0

Ø   Approximate ultimate tensile strength range:30-55 ksi

Because of the increased difficulty in casting 7xx.x alloys,they tend to be used only where the excellent finishing characteristics and machinability are important.

Ø  8xx.x - Al-Sn Alloys

Ø   Heat treatable/sand and permanent mold castings

Ø  (harder to cast)

Ø   Excellent machinability

Ø   Bearings and bushings of all types

Ø   Representative alloys:850.0,851.0

Ø   Approximate ultimate tensile strength range:15-30 ksi

Like the 7xx.x alloys, 8xx.x alloys are relatively hard to cast and are used only where their unique machining and bushing characteristics are essential.In concluding this section on casting,it is worth noting that conventional die casting tends to yield parts with relatively low elongation values, which are therefore unsuitable for safety-critical components.In recent years,higher pressure types of casting (e.g., squeeze casting and thixocasting) have been developed to a commercial level. As a result, elongation values of well over 10% are now attainable,together with higher strengths.This considerably widens the range of application of aluminum alloy castings.

Summary -An Aluminum Alloy for Every Application.

It seems apparent from the foregoing overview that aluminum alloys possess a number of very attractive characteristics which, together with their very light weight, make then extremely attractive for many applications. Further, their versatility with respect to options of how to shape them and strengthen them provide an amazing variety of choices when you are looking for an ideal material for a special application.

Ease of extrudibility is one characteristic that justifies some special emphasis in wrapping up this summary because the extrusion process, most uniquely suitable to aluminum alloys, allows the designer to create special shapes that place the metal where it can carry the required load most efficiently. There is no need, as with steel or most titanium alloys, to be limited to “standard” shapes. The economics and metallurgy of the extrusion process permit you to economically create unique shapes, even multi-hollow shapes, for unique applications, perhaps combining what would otherwise be several separate parts that would have to be joined, into one specially shaped piece. Extrusion is one of aluminum’s “aces in

The hole”, as the expression goes.

Fig25: fuel tanks of the Space Shuttle are 2xxx alloys, originally 2219 and 2419; now sometimes aluminum-lithium “Weldalite” alloy 2195

Fig26: Internal railroad car structural members are sometimes 2xxx alloys (also sometimes 6xxx alloys).

   

                                                                                              

Basic Physical Metallurgy

Ø  Work hardening

Ø  Dispersion hardening

Ø  Solid solution hardening

Ø  Precipitation hardening

There are four basic ways in which aluminium can be strengthened: work hardening,dispersionhardeningsolidsolution hardening and precipitation hardening. These hardening processes areeffective because theyproduce conditions that impede the movement of dislocations. Dislocations are faults that enable metal crystals to slip at stresses very much below those that would be required to move two perfect crystal planes

one another.

Work Hardening

Whenever aluminium products are fabricated by rolling, extruding, drawing, bending, etc., work is done on the metal. When work is done below the metal's recrystallisationtemperature (cold work), it not only forms the metal, but also increases it strength due tothe fact that dislocations trying to glide on different slip planes interact causing a "trafficjam" that prevents them from moving. Fabricating processes carried out above the metal'srecrystallization temperature (hot work) do not normally increase strength over theannealed strength condition.

With non heat-treatable wrought alloys, cold work is the only way of increasing strength.With heat treatable alloy, cold work applied after heat treating can increase strength stillfurther. Work hardening of non heat treatable aluminium magnesium and pure aluminiumalloy is shown in Figure 1501.04.01.

                             

Dispersion Hardening

Fine particles of an insoluble material are uniformly distributed throughout the cristal lattice in such a way as to impede the movement of dislocations (eg 3000 series). Withaluminium, dispersion-hardening in two ways:

− by the addition of alloying elements that combine chemically with the metal or eachother to form fine particles that precipitate from the matrix− by mixing particles of a suitable substance (for example A1203) with powdered aluminium and then compacting the mixture into a solid mass.

Solid Solution Hardening

Most alloys are solid solutions of one or more metals dissolved in another metal: either thealloying of atoms take over the lattice positions of some of the base-metal atoms(substitutional solid solutions) or they occupy spaces in the lattice between the base-metal(interstitial solid solutions). In both cases, the base-metal lattice is distorted, retarding themovement of dislocations and hence strengthening the metal. The 5000 series with

magnesium as the solute is a good example.

Most aluminium alloys reflect some solid solution hardening as a result of one or more elements being dissolved in the aluminium base, each element's contribution to the strength of the alloy is roughly additive. Usually these alloys are further strengthened by heat treatment or by work hardening.

Precipitation Hardening

Precipitation hardening is a two stage heat treatment. It can be applied only to thosegroups of alloys which are heat treatable (i.e. 2000, 6000 and 7000 wrought series).Firstly, a supersaturated condition is produced by solution heat treatment. Secondly the"ageing" process that occurs after quenching may be accelerated by heating the alloy untila second and coherent phase is precipitated. This coherent phase strengthens the alloys byobstructing the movements of dislocations.

Solution treatment involves heating the alloy to a temperature just below the lowestmelting point of the alloy system, holding at this temperature until the base metal dissolvesa significant amount of the alloying elements (Figure 1501.04.02). The alloy is thenrapidly cooled to retain as much of the alloying elements in solution as possible and soproduce a supersaturated solid solution. This supersaturated condition is usually unstableand therefore heat-treatable alloys are used in this condition, i.e. T4.

                  

After solution heat-treatment most heat-treatable alloys exhibit some age-hardening at room temperature. The rate and extent of natural age-hardening at room temperature varies from alloy to alloy. For example, 2024 reaches a stable condition in four days and is therefore widely used in naturally aged tempers. By contrast, 7075 and most other aluminium-zinc-magnesium-copper alloys continue to age-harden indefinitely at room temperature and are seldom used in naturally aged temper.

Heating above room temperature accelerates the precipitation reaction, in practice, therefore, precipitation-hardened alloys are usually ″artificially aged' (precipitation heat treated) to develop maximum properties as quickly as possible. The temperature range within which control of the precipitation reactions is feasible is 120-180°C. The actual temperature depends on such variables as the alloy, the properties desired and productionschedule.

An aluminium alloy that responds to precipitation hardening must contain amounts of soluble alloying elements that exceed the solid solubility limit at room temperature. Figure 1501.04.02. shows one corner of the phase diagram of such an alloy. In addition, the alloy must be able to dissolve the excess of soluble alloying elements and then to precipitate them (or the compounds they form) as distinctive constituents within the crystal lattice. The constituents precipitated must have a structure different from the solid solution. Careful control of this precipitation reaction is essential, otherwise the hardening constituents become too coarse and contribute little to the strengthening. The effect of time and temperature on the precipitation process is shown in Figure 1501.04.03

                

Mechanical Properties

Ø  Tensile strength

Ø   Strength/weight ratio

Ø   Elastic properties

Ø   Compression

Ø   Shear

Ø   Hardness

Ø   Ductility

Ø   Creep

Ø   Properties at elevated temperatures

Ø   Properties at low temperatures

Ø   Impact strength

Ø   Fracture characteristics

Ø   Fatigue

Tensile Strength

Behaviour under tension is generally considered the first yardstick of an engineering material, and Figure 1501.05.01 shows typical tensile stress/strain curves for four different aluminium alloys and compares them with a range of engineering metals. The alloys are: 99.5% pure aluminium (1050A) in the fully annealed state, suitable for deep pressing; a 4.5% magnesium-aluminium alloy (5083) after strain-hardening, by rolling, to the ″half-hard″ temper, used in marine and welded structures; a magnesiummanganese- silicon alloy 6082 after solution treatment and ageing to the fully heat treated ″T6″-condition, used in commercial structures and a zinc-magnesium-copperaluminium alloy 7075 in the fully heat treated condition used in aircraft construction.

                  

Strength/Weight Ratio

As can be seen from Figure 1501.05.01 the high tensile steels have the highest strengths of all the metals. These are followed by Titanium and the aircraft aluminium alloys and some way below these the commercial structural alloys 5083-H12 and 6082-T6. If wen now considers the strength available for a given mass by dividing the tensile strength by the density we get quite a different picture (Figure 1501.05.02). We now find the 7075 at the top with the commercial structural alloys moving to the mid range above the common mild steel.

          

Elastic Properties

From Figure 1501.05.03 it can be seen that for the initial part of the stress-strain curve the strain per unit increase of stress is much higher for aluminium than for steel, measurement shows that it is three times higher. The slope of this part of the curve determines the Modulus of Elasticity (Young’s Modulus) e.g. stress divided by strain. It follows therefore that the Modulus of Elasticity for aluminium is one-third that of steel, being between 65500 and 72400 MPa for most aluminium alloys.

From the information already given it is clear that when a steel structural member is replaced by one of identical form in an aluminium alloy the weight will be one third butthe elastic deflection will be about three times as large. From this we can deduce that an aluminium member of identical dimension to one in steel will absorb three times asmuch energy, but only up to the point where the stress in the aluminium remains below

the limit of proportionality.

It is worth noting that stiffness is defined as the product of the Modulus of Elasticity and the Moment of Inertia of a section (E x I) and it is this which determines the deflection when subjected to a bending load. This allows the application of another attribute of aluminium, its ability to be made into a variety of complex structural shapes by extrusion. The extrusion process provides the designer with the opportunity to shape the metal to achieve maximum efficiency in the design of a section usually by making itdeeper. However, making a section deeper often sacrifices some of the potential weight saving with the result that it only weighs about half that of the steel member instead of a third.

Figure 1501.05.04 shows two different approaches of saving weight when using aluminium instead of steel for the main beams of a road trailer. All sections have the same bending stiffness, the aluminium 'I' beam has been designed with a maximum overall extrusion dimension and minimum extrusion thickness, while the aluminium box beam has been designed to the same width as the steel beam but with additional special features to improve the build. The aluminium I beam exhibits an improved section modulus and consequently a lower induced stress in bending in addition to a 57% weight saving, but because of its slender shape has inherent poor torsional stability.

The aluminium box beam exhibits an even greater improvement in section modulus combined with a considerable improvement in torsional stability but only a 33% weight saving. By changing the design any combination of characteristics inside the practical manufacturing limits can be obtained.

Young#s Modulus can vary by as much as 40% with the addition of up to 15% Manganese but for commercial alloys it only varies one or two percent and this variation is ignored in standard structural calculations.

          

The Torsional Modulus or Modulus of Rigidity of aluminium e.g. shear stress divided by angular strain is again about a third of that for steel being 26000 MPa for aluminium compared to 82700 Mpa for steel. The same rules should therefore be applied by the designer when looking at aluminium designs in torsion as in bending.

Poisons Ratio e.g. lateral strain divided by longitudinal strain is ν = 0.33.

=

Compression

The behaviour of aluminium alloys under compressive loading does not receive the attention given to tensile properties, perhaps because the strength of structural members is so often limited by buckling, and the actual compressive strength of the metal is not approached (Figure 1501.05.06).

For most engineering purposes it is customary to use the same design stress for compressive work as for tensile. In the testing machine, an aluminium alloy will show an apparently higher strength in compression than in tension, but this can in part be attributed to the changing cross-sectional areas of the specimens, increasing in one case and decreasing in the other, while the stress is based on the original area. Cylindrical

specimens of the softer aluminium alloys can be compressed to thick discs before cracking, and even then may still sustain the load. The harder alloys show a more definite failure point and pronounced cracking.

A proof stress, at which there is a small measurable departure from the elastic range, is therefore usually quoted, and will be roughly equal to the corresponding tensile proof stress; in cast or forged metal it is usually slightly higher. Sheet and extruded products, however, are often straightened by stretching, an effect of which is to lower them ,compressive proof stress and raise the tensile proof stress by small amounts.

Shear

In the wrought alloys the ratio of ultimate sheer stress to ultimate tensile stress varies with composition and method of fabrication from about 0.5 to 0.75. When test resultsare not available, a ratio of 0.55 is safe for most purposes (Figure 1501.05.06).

Rivets in low and medium strength alloys, with shear strengths up to 200 MPa can be driven cold. Small rivets in stronger alloys can be driven in the soft state immediately following solution treatment and, on natural age-hardening, shear strengths up to 260 MPa will be developed.

Hardness

Resistance to surface indentation is an approximate guide to the condition of an alloy, and is used as an inspection measure. Brinell (steel ball), Vickers (diamond) and Shore Scleroscope (diamond Hammer) testing machines are applied to aluminium alloys; typical Brinell values range from 20 for annealed commercially pure-metal to 175 for the strongest alloy (Figure 1501.05.06). Hardness readings should never be regarded as

a quantitative index to tensile strength, as is often done with steels, for in aluminium the relation between these two properties is far from constant. The surface hardness of aluminium can be increased considerably by the process of hard anodising (500VPN) and is therefore often employed to improve the wear resistance of components.

We have said the elongation of a tensile test piece at fracture is a useful but not a key to the ductility of an alloy.

Simple bend tests are widely used as a further indication of workability. A strip of metal with smooth rounded edges is bent through 90° or 180° by hand or mallet over a steel former of prescribed radius. By using successively tighter formers, a minimum bend radius, at which there is no cracking, can be found, and is usually quoted as a multiple of sheet thickness ″t″, for example, 1½.

T obtain a measure of ductility a sample of sheet that is intended for deep drawing or pressing is often subjected to the Erichsen cupping test in which a hemispherical punch is forced by a hand-operated screw against one side of the sheet, stretching the metal into a dome or cup (Figure 1501.05.07). The depth of penetration at fracture gives an indication of the amenability of the metal to deep drawing processes involving stretching, though not necessarily to other pressing operations.

Much of the value of this test lies in its ability to show up to two phenomena that will prevent successful drawing: a coarse grain structure produces roughness of the cup and perhaps an early failure through local thinning; and directionality or variation of properties in relation to the direction of rolling affects the shape of the fracture, which should be circular.

          

Ductility

We have said the elongation of a tensile test piece at fracture is a useful but not a conclusive key to the ductility of an alloy.

Simple bend tests are widely used as a further indication of workability. A strip of metal with smooth rounded edges is bent through 90° or 180° by hand or mallet over a steel former of prescribed radius. By using successively tighter formers, a minimum bend radius, at which there is no cracking, can be found, and is usually quoted as a multiple of sheet thickness ″t″, for example, 1½ t.

To obtain a measure of ductility a sample of sheet that is intended for deep drawing or pressing is often subjected to the Erichsen cupping test in which a hemispherical punch is forced by a hand-operated screw against one side of the sheet, stretching the metal into a dome or cup (Figure 1501.05.07). The depth of penetration at fracture gives an indication of the amenability of the metal to deep drawing processes involving stretching, though not necessarily to other pressing operations.

Much of the value of this test lies in its ability to show up to two phenomena that will prevent successful drawing: a coarse grain structure produces roughness of the cup surface and perhaps an early failure through local thinning; and directionality or variation of properties in relation to the direction of rolling affects the shape of the fracture, which should be circular.

                                                                                           

Creep

In the preceding discussions of tensile, compressive and shear properties it is implied that the stress is increased continuously and that the accompanying strains are independent of time under any given stress. If, however, a stress less than the ultimate strength is constantly maintained for a long period of time, the strain increases continuously (Figure 1501.05.08). If the stress is high enough or held long enough, the specimen eventually fails in the mode which would have occurred under continuously increasing loading. In this respect, the behaviour of aluminium is like that of other metals, and the term used for this form of failure is Creep Rupture.

The creep strength of metals reduces as the operating temperature increases, again aluminium's behaviour is the same as other metals. It follows, therefore, that Creep strength cannot be expressed by a single number but must be related to operating temperatures, time and amount of deformation. Figure 1501.05.08 illustrates these relationships for an Al-Cu alloy.

These data are important to the designer of a structure which is subject to stress and temperature, such as hot tarmac carrying vehicles (required life 1000's hrs), some forms of pressure vessels used in process plant (required life 100,000 hrs). It may also be necessary for predicting the life of a structure in hazard situations such as a safety critical structure surrounded by a fire (30 mins), or even a very short rupture life as maybe required in a rocket shell (2 mins). In all of these cases the time to failure at a given stress level and temperature is the design criterion, and the data are usually applied with a suitable safety allowance on time.

Properties at Elevated Temperatures

The strength of aluminium alloys decreases with the increase in temperature excluding the effects of age-hardening within narrow temperature ranges for various holding periods. The time of exposure is important in the case of cold worked or heat-treated alloys (Figure 1501.05.09) but has little or no effect on the properties of anneale dalloys. The heating time at test temperature is often quoted as 10,000 hrs, but with the time-temperature dependence of strength it may be necessary for other exposure times to  considered.

Shear, compression, bearing and fatigue strengths vary with temperature in much the same way as tensile strength; ratios of these strengths to tensile strength may be taken as constant.

      

The reduction in strength caused by exposure to elevated temperatures can only be regained by heat treatment or cold work or a combination of these processes which is usually impractical in the case of fabricated items. The tensile strength of an AlCu4MgSi alloy, tested at room temperature after exposure at elevated temperature, is shown in Figure 1501.05.10. After either short term exposure at high temperature or long term exposure at medium temperature the material approaches a super soft annealed condition and the lower limit strength becomes constant.

      

The modulus of elasticity of aluminium alloys also decreases as the operating temperature increases but unlike strengths which stabilise at a lower annealed value, the modulus of elasticity returns to its room temperature value after exposure (Figure 1501.05.11).

            

Properties at Low Temperatures

Aluminium and its alloys have no ductile to brittle transition at low temperatures, indeed, their strengths increase with decreasingtemperature. The strengths of stable temper aluminium alloys are not influenced by the time of exposure at low temperatures neither are the strengths at room temperature after exposure at low temperature. However, freshly solution treated heat treatable alloys can be held in this condition for long periods by storing them at a low temperature because of the retardation of the ageing process. This is used to good effect when placing aircraft rivets of the AlCuMgSi type which may be solution treated prior to use by heating to 4950c for a period of time between 5 and 60 minutes, depending upon the size and quantity of rivets being processed, after which they are quenched in cold water. The rivets remain soft after quenching for up to two hours at ambient, but at minus 50C this is extended to forty-five hours and at minus 150C to one hundred and fifty hours.

The increase in strength of aluminium alloys at low temperatures is negligible down to minus 500C but begins to increase significantly below minus 1000C (Figure 1501.05.12). The elongations of most aluminium alloys also increase with the reduction in temperature down to minus 196°C whereupon some alloys notably with higher magnesium content (4.5% and above) begin to reduce again but not below the ambient figure.

Shear, compression and bearing strengths - all improve at low temperatures, also the moduli of elasticity under tensile, compressive and shear loading are 12% higher at minus 1960C than at room temperature.

       

Impact Strength

As already indicated the low elastic modulus of aluminium alloys is an asset when a structure is subjected to shock-loading conditions: an aluminium alloy member will absorb three times as much energy before permanent damage occurs than a steel member of equal moment of inertia and strength.

Energy absorption figures from tests or notched specimens in Izod or Charpy pendulum machines are, as with other metals, not directly applicable to design work. Again, the results from different alloys of aluminium are so varied and so unrelated to performance under structural conditions, that this type of test is little used.

Fracture Characteristics

By this we mean a materials tendency to exhibit rapid propagation of a crack without appreciable plastic deformation. Information on this form of failure is vital for the design of structures working at stress levels and containing high elastic energies where sudden failure would be catastrophic. Elongation and reduction of area from tensile tests and the ratio of yield to tensile strength, both give indications of fracturecharacteristics, but for the engineer these indications are seldom sufficient to be used alone as a basis for design.

Charpy and Izod notched bar impact tests have been widely employed to determine the transition temperatures for ferritic steels, i.e. temperatures at which the alloys begin to exhibit brittle fracture characteristics, but these tests are generally unsuitable for aluminium and its alloys because the latter do not exhibit a transition temperature. Also notched bar impact test values for aluminium alloys are almost constant from ambient down to temperatures of minus 2680C; in addition most wrought alloys are so tough the test bars do not fracture. Therefore no useful data are obtained.

To overcome this problem an adaption of the Navy tear test, originally developed by Noah Kahn to investigate the sudden failures of welded steel ships, is often employed to assess a fracture rating factor for aluminium and its alloys. In this test the energies required to initiate and propagate a crack in a specially prepared test piece (Figure 1501.05.13) are obtained by calculating the appropriate areas under the load extension curve. The energy required to propagate a crack in the tear test divided by the net cross

sectional area of the specimen is referred to as the "unit propagation energy". It provides a measure of tear resistance and, indirectly, a measure of fracture toughness.

The unit propagation energy obtained from the test can be related directly to the strainenergy release rate for alloys that conform to the fracture mechanics theory, thereby providing a realistic measure of the resistance to rapid crack propagation.

Test procedures have also been developed which relate the fracture strength of a material to a flaw or crack size or specific design detail, thereby providing a measure of "fracture toughness". Fracture toughness can be described as the resistance of a material to unstable crack propagation at elastic stresses, or to low ductility fracture of any kind. Testing for fracture toughness requires the initiation of a crack of known length in a specially prepared test piece either by fatigue loading but usually by cutting a very thin slot followed by loading in the same manner as the Navy tear test (Figure 1501.05.13),. A relationship between the stress intensity factor K, uniform gross tensile stress σa, and the length of the crack 2a is given by K = σa 2π a . The stress intensity factor K (at the onset of unstable crack propagation) decreases with the increase in metal thickness and approaches a constant minimum value which is identified as KIc the "critical elastic stress intensity factor".or the plain strain fracture toughness. KIc is analogous to yield stress since it is the minimum stress intensity at which failure can start at a given temperature and at full thickness for plain strain conditions. The fracture toughness route is not suitable for highly ductile alloys since they do not exhibit rapid crack propagation under elastic conditions. The test is therefore usually confined to the high strength heat treatable alloys.

Fatigue

In common with other metals aluminium will fracture when subjected to variable or repeated loads at stress levels considerably lower than it would be the case with static loads. This type of failure which consists of the formation of cracks under the action of the fluctuating loads is known as fatigue. The fluctuating load practice could be caused by live loads, vibration or repeated temperature changes. The direction in which the fatigue crack propagates is always perpendicular to the line of action of the stresses causing the crack. As the crack progresses the stress on the residual cross section increases so that there is a corresponding increase in the rate of crack propagation.

Ultimately a stage is reached when the remaining area is insufficient to support the applied load and final rupture occurs. Fatigue cracks may be very difficult to detect since unlike tensile failures there is no visible surface contraction at the point of failure.

When assessing fatigue three basic factors need to be known:

Ø  Number of stress cycles.

Ø  A definition of the stress cycle.

Ø  Surface finish or contour shape of the component.

Fabrication processing of aluminium  alloys

Fabrication processes are used to shape, machine and join metals. The metal which is operated is in the form of an ingot obtained by reducing or refining the metal ore. The fabrication processes are basically casting, forging, metal machining, metal joining and finishing. Fabrication of parts with other methods is time consuming.

Casting of aluminium alloys

Three steps are involved in a casting process:

1) heating metal till it becomes molten

 2) pouring molten metal into a mould

3) allowing the metal to cool and solidify in the shape of the mould.

Casting is used in the automobile industry to produce engine blocks or cylinder heads. Metal casting is vital to our economy and security. Different metals are cast by many different processes for different applications. Cast metal products and processes offer advantages unavailable from products made by other metal forming and fabricating techniques.

Types of casting

1) Sand Casting

2) Casting in metal moulds

3) Centrifugal casting

4) Shell moulding

5) Investment casting

6) Continuous casting.

 

                                     

Casting Defects

The cast aluminium alloys are subject to:

Shrinkage – Al/alloys shrink by 4–6% during solidification (depending on alloy type)

Gas porosity- Molten aluminium picks up hydrogen which is expelled during solidification giving rise to porosity. Molten aluminium picks up hydrogen from the atmosphere or from refractories. The solubility of hydrogen in solid aluminium is low and it is high with molten aluminium. To obtain good castings melts are degassed. Hydrogen has a high solubility in molten aluminium which increases with melt temperature. Hydrogen comes from water vapour in the atmosphere/from burner fuels, refractories, moist fluxes, oily/dirty scrap/foundry tools. To reduce hydrogen pickup, refractories, crucibles, tools and oily scrap should be thoroughly preheated.

Oxide inclusions- Molten aluminium exposed to air oxidizes forming a oxide which may be entrained into the casting. The other casting defects are cold shuts, hot tears, etc.

Other

Molten metal is brought in ladles from the furnace and it is later poured in the sand mould or metal mould to obtain a casting. The selection of pouring temperature depends upon the metal composition and the type of casting to be made. Improper pouring temperatures can cause casting defects. Molten metal poured at temperatures lower than the optimum causes cold shuts. If temperature of molten metal is high gas content will increase especially for molten aluminium.

Forging

Forging is a manufacturing process in which metal is pressed, pounded, or squeezed under great pressure into high-strength parts. This is usually done by heating the metal, but some forgings are produced without heat.

Generally, forged components are shaped by either a hammer or a press. Forging by hammer is carried out in a succession of die impressions using repeated blows. In a press, the component is usually hit only once in each die impression.

The Forging process

The three basic types of aluminum alloy forgings are:

Ø  open-die forgings,

Ø  closed-die forgings,

Ø  rolled rings.

Open-die forging

In open-die forging, the work component is not completely confined as it is being shaped by the dies. This process is commonly associated with large parts such as shafts, sleeves, and disks. The part’s weight can range from 5 to 500,000 lbs.

Most open-die forgings are produced on flat sides. Round swaging dies and V dies are also used in pairs or with a flat die.

As the forging workpiece is hammered or pressed, it is repeatedly manipulated between the dies until it reaches final forged dimensions. Because the process is inexact and requires a skilled forging operator, substantial workpiece stock allowances are retained to accommodate forging irregularities. The forged part is rough machined and then finish machined to final dimensions.

In open-die forging, metals are worked above their recrystallization temperatures. Since the process requires repeated changes in workpiece positioning, the workpiece cools below its hot-working or recrystallization temperature. It then must be reheated before forging can continue.

Closed-die forging

Impression-die forging accounts for the majority of forging production. In impression-die forging, two dies are brought together and the workpiece undergoes plastic deformation until its enlarged sides touch the die side walls.

Some material flows outside the die impression, forming flash. The flash cools rapidly and presents increased resistance to deformation, effectively becoming part of the tool. This builds pressure inside the bulk of the workpiece, aiding material flow into unfilled impressions.

Ring rolling

Ring rolling has evolved from an art into a strictly controlled engineering process. In the ring-rolling process, a preform is heated to forging temperature and placed over the internal roll of the rolling machine. Pressure is applied to the wall by the main roll as the ring rotates. The cross-sectional area is reduced as the inner and outer diameters are expanded.

Rings can be rolled into numerous sizes, ranging from roller-bearing sleeves to rings of 25 feet in diameter with face heights of more than 80 inches.

Applictions

In automotive applications, forged components are commonly found at points of shock and stress. Forged automobile components include connecting rods, crankshafts, wheel spindles, axle beams, pistons, gears, and steering arms.

Forgings are also used in helicopters, piston-engine planes, commercial jets, and supersonic military aircraft. Many aircraft are "designed around" forgings and contain more than 450 structural forgings, including hundreds of forged engine parts.

"Forged" is the mark of quality in hand tools and hardware. Pliers, hammers, sledgers, wrenches, garden implements, and surgical tools are almost always produced through forging.

                           

                           

                                                           Fig.27 (forged rods)

                                                          

Extrusion

Extruded products constitute more than 50 % of the market for aluminium products in Europe of which the building industry consumes the majority. Aluminium extrusions are used in commercial and domestic buildings for window and door frame systems, prefabricated houses/building structures, roofing and exterior cladding, curtain walling, shop fronts, etc. Furthermore, extrusions are also used in transport for airframes, road and rail vehicles and in marine applications.

The term extrusion is usually applied to both the process, and the product obtained, when a hot cylindrical billet of aluminium is pushed through a shaped die (forward or direct extrusion, see Figure 1). The resulting section can be used in long lengths or cut into short parts for use in structures, vehicles or components. Also, extrusions are used for the starting stock for drawn rod, cold extruded and forged products. While the majority of the many hundreds of extrusion presses used throughout the world are covered by the simple description given above it should be noted that some presses accommodate rectangular shaped billets for the purpose of producing extrusions with wide section sizes. Other presses are designed to push the die into the billet. This latter modification is usually termed "indirect" extrusion.

                         

The extrusion process

The fundamental features of the process are as follows: A heated billet cut from DC cast log (or for small diameters from larger extruded bar) is located in a heated container, usually around 450°C - 500°C. At these temperatures the flow stress of the aluminium alloys is very low and by applying pressure by means of a ram to one end of the billet the metal flows through the steel die, located at the other end of the container to produce a section, the cross sectional shape of which is defined by the shape of the die.

Aluminium Alloys and Extrusion

All aluminium alloys can be extruded but some are less suitable than others, requiring higher pressures, allowing only low extrusion speeds and/or having less than acceptable surface finish and section complexity. The term ‘extrudability’ is used to embrace all of these issues with pure aluminium at one end of the scale and the strong aluminium/zinc/magnesium/copper alloys at the other end. The biggest share of the extrusion market is taken by the 6000, AlMgSi series. This group of alloys have an attractive combination of properties, relevant to both use and production and they have been subject to a great deal of R & D in many countries. The result is a set of materials ranging in strength from 150 MPa to 350 MPa, all with good toughness and formability. They can be extruded with ease and their overall ‘extrudability’ is good but those containing the lower limits of magnesium and silicon e.g. 6060 and 6063 extrude at very high speeds - up to 100 m/min with good surface finish, anodising capability and maximum complexity of section shape combined with minimum section thickness.

Extrusion Machines

Press load capacities range from a few hundred tonnes to as high as 20,000 tonnes although the majority range between 1,000 and 3,000 tonnes. Billet sizes cover the range from 50 mm diameter to 500 mm with length usually about 2-4 times the diameter and while most presses have cylindrical containers a few have rectangular ones for the production of wide shallow sections.

                     

Product forms

Aluminium is available in both wrought and cast forms.The wrought forms comprise hot and cold rolled sheet, plate, rod, wire and foil. The ductility and workability of aluminium mean that extrusion is a simple method of producing complex shapes, particularly for long, structural members such as I and H beams, angles, channels,T-sections, pipes and tubes. Forging, both hot and cold, is used extensively as a fast, economical method of producing simple shapes. Precision forging is particularly suitable for aluminium

alloys, giving advantages of good surface finish, close tolerances, optimum grain flow and the elimination of machining. The four most commonly used methods of casting are sand casting, lost wax casting, permanent steel mould casting and die-casting. The requirement for high fluidity in a casting alloy means that many are based on aluminium–silicon alloys although heat-treatable (age-hardening) alloys are often used for sand, lost wax and permanent mould castings. Lost wax and die-casting give products with smooth surfaces to close tolerances and are processes used extensively for aerospace products.

Welding: a few definitions

Welding can be described as the joining of two components by a coalescence of the surfaces in contact with each other.This coalescence can be achieved by melting the two parts together – fusion welding – or by bringing the two parts together under pressure, perhaps with the application of heat, to form a metallic bond across the interface. This is known as solid phase joining and is one of the oldest of the joining techniques, blacksmith’s hammer welding having been used for iron implement manufacture for some 3500 years. The more modern solid phase techniques are typified by friction welding.

Brazing is also an ancient process, is one that involves a braze metal which melts at a temperature above 450 °C but below the melting temperature of the components to be joined so that there is no melting of the parent metals.

Soldering is an almost identical process, the fundamental difference being that the melting point of the solder is less than 450°C.Welding that involves the melting and fusion of the parent metals.

Introduction

Ideally a weldment – by this is meant the complete joint comprising the weld metal, heat affected zones (HAZ) and the adjacent parent metal – should have the same properties as the parent metal.There are, however, a number of problems associated with the welding of aluminium and its alloys that make it difficult to achieve this ideal.The features and defects that may contribute to the loss of properties comprise the following:

• Gas porosity.

• Oxide inclusions and oxide filming.

• Solidification (hot) cracking or hot tearing.

• Reduced strength in the weld and HAZ.

• Lack of fusion.

• Reduced corrosion resistance.

• Reduced electrical resistance.

Types of welding

·         Tig welding

·         Mig welding

·         Other welding process

Tig welding process

introduction                                                                                                        

Tungsten arc inert gas shielded welding, EN process number 144 abbreviatedto TIG, TAGS or GTAW (USA), is an arc welding process that uses a non-consumable tungsten electrode and an inert gas shield to protect the electrode, arc column and weld pool, as illustrated in Fig. 6.1. The welding arc acts as a heat source only and the welding engineer has the choice of whether or not to add a filler wire. The weld pool is easily controlled such that unbacked root passes can be made, the arc is stable at very low welding currents enabling thin components to be welded and the process produces very good quality weld metal, although highly skilled welders are required for the best results. It has a lower travel speed and lower filler metal deposition rate than MIG welding, making it less cost effective in some situations.

TIG tends to be limited to the thinner gauges of aluminium, up to perhaps 6 mm in thickness. It has a shallower penetration into the parent metal than MIG and difficulty is sometimes encountered penetrating into corners and into the root of fillet welds.

Process principles

The basic equipment for TIG welding comprises a power source, a welding torch, a supply of an inert shield gas, a supply of filler wire and perhaps a water cooling system. A typical assembly of equipment is illustrated in Fig. 6.2. For welding most materials the TIG process conventionally uses direct current with the electrode connected to the negative pole of the power source, DCEN. As discussed in Chapter 3 welding on this polarity does not give efficient oxide removal. A further feature of the gas shielded arc welding processes is that the bulk of the heat is generated at the positive pole. TIG welding with the electrode connected to the positive pole, DCEP,results in overheating and melting of the electrode. Manual TIG welding of aluminium is therefore normally performed using alternating current,AC, where oxide film removal takes place on the electrode positive half cycle and electrode cooling and weld bead penetration on the electrode negative half cycle of the AC sine wave. The arc is extinguished and reignited every half cycle as the arc current passes through zero, on a 50Hz power supply requiring this to occur 100 times per second, twice on each power cycle. To achieve instant arc reignition a high-frequency (HF), high-voltage (9–15 000 V) current is applied to the arc, bridging the arc gap with a continuous discharge.

                                 

Mig welding

Introduction             

The metal arc inert gas shielded process,EN process number 131,also known as MIG,MAGS or GMAW, was first used in the USA in the mid 1940s. Since those early days the process has found extensive use in a wide range of industries from automotive manufacture to cross-country pipelines. It is an arc welding process that uses a continuously fed wire both as electrode and as filler metal, the arc and the weld pool being protected by an inert gas shield. It offers the advantages of high welding speeds, smaller heat affected zones than TIG welding, excellent oxide film removal during welding and an all positional welding capability. For these reasons MIG welding is the most widely used manual arc welding process for the joining of aluminium.

Process principles

MIG welding process, illustrated in Figs. 7.1 and 7.2, as a rule uses direct current with the electrode connected to the positive pole of the power source, DC positive, or reverse polarity in the USA.. Recent power source developments have been successful in enabling the MIG process to be also used with AC. Most of the heat developed in the arc is generated at the positive pole, in the case of MIG welding the electrode, resulting in high wire burn-off rates and an efficient transfer of this heat into the weld pool by means of the filler wire.When welding at low welding currents the tip of the continuously fed wire may not melt sufficiently fast to maintain the arc but may dip into the weld pool and short circuit.This short circuit causes the wire to melt somewhat like an electrical fuse and the molten metal isdrawn into the weld pool by surface tension effects. The arc re-establishes itself and the cycle is repeated. This is known as the dip transfer mode of metal transfer. Excessive spatter will be produced if the welding parameters are not correctly adjusted and the low heat input may give rise to lack -fusion defects.At higher currents the filler metal is melted from the wire tip and transferred across the arc as a spray of molten droplets, spray transfer. This condition gives far lower spatter levels and deeper penetration into the parent metal than dip transfer.When MIG welding aluminium the low melting point of the aluminium results in spray transfer down to relatively low welding currents, giving a spatter-free joint.

             

                                  

                                                                            

                                                                   

          

Other welding process

While MIG and TIG welding may be regarded as the most frequently usedprocesses for the joining of aluminium and its alloys there are a large number of other processes that are equally useful and are regularly employed although perhaps in rather more specialised applications than the conventional fusion welding processes. Some of these processes are given below:

·         Plasma-arc welding

·         Laser welding

·         Electron beam welding

·         Friction welding

                                                    

               Summary  

                                                                                                                                                                

          

Nuclear materaials

Introduction

Nuclear plants are designed for decades of operation. One of the challenges in their maintenance is how to control the degradation of materials in reactor plant structures and components caused by radiation, high temperature, high pressure cyclic stresses, and a relatively corrosive environment. The safe, reliable and economic operation of such plant is critically dependent on good materials performance and, in particular, on understanding and mitigating specific environmental degradation processes (e.g. mechanical, corrosion and radiation effects).

This article will briefly address the degradation problems, in particular, due to corrosion and how to control them. It will cover the following aspects:

Ø  Materials used in nuclear reactor and their corrosion mechanism after interaction with water

Ø  The effect of radiation on corrosion

Ø  How to control corrosion problem in research reactors .

Ø  Corrosion problem in nuclear power plant.

Reactor Materials

Properties and requirements

The requirements of material properties in nuclear reactor can be divided into two main categories, i.e. general properties or basic consideration and special properties or particular consideration. The general properties are similar to the conventional engineering properties of material, which are required in most engineering design.

The special properties required for nuclear reactor materials arise from nuclear radiation, or irradiation, sources and circumstances of the reactor system.

                                  Table 1.2. General Properties of Nuclear Reactor Materials

Mechanical strength	Heat transfer properties
Ductility	Thermal stability
Structural integrity	Compatibility
Fabricability, machinability	Availability
Corrosion resistance	Cost

In general, nuclear applications require materials of adequate mechanical strength, ductility and toughness. This would make it possible to obtain a good structural integrity or mechanical stability, such as integrity of fuel element and control rod in a nuclear reactor. In addition the materials should also feasible to be put through machining processes (cutting, milling, rolling) and fabrication using standard process of forming, joining, welding and so on.

Corrosion, which can attack all metallic members in contact with corrosive fluid (liquid or gas coolant), should always be taken into consideration for material selection. The degree of corrosion resistance depends mainly on the service conditions. Unpredictable corrosion breakaway, or rapid change in corrosion rate, is undesirable and should be avoided. In particular, the fabricated joints and machined areas of the pressure vessel, fuel cladding, piping system, etc., in a light water reactor are relatively vulnerable to aqueous corrosion attack.

Heat transfer properties, particularly heat conduction and convection are also of major important in nuclear reactor design and materials selection. Thermal stability is an important property for materials that usually operate at elevated temperature. In most practical cases, the mechanical strength, structural integrity, and corrosion resistance of the structure and piping materials decrease with increasing temperature.

Material compatibility is a major criterion that requires all elements and all components in a given system to be compatible. In other words, the materials selected for each element or component of the system must function properly and consistently.

Material selection in engineering or nuclear reactor design also involves compromise of many factors including those mentioned above as well as cost and availability of the materials concerned. The economical consideration dominated by availability and cost is probably the final test in the procedure of engineering design and material selection.

                                         Table 1.3. Special Properties of Nuclear Reactor Materials

Neutronic properties	Chemical interaction
Induced radioactivity	Particle interdiffusion
Irradiation Stability	Ease of fuel reprocessing

                                                  

Neutrons play the most important role in a nuclear fission reactor. The neutronic properties consist mainly of neutron absorption, both fission and capture, and neutron scattering, or collision. A measure of the probability of neutron absorbing and scattering properties is called the absorption cross section and the scattering cross section, respectively. Various component materials, such as fuel, structure, moderator, reflector, blanket, coolant, shielding, and control, of a nuclear fission reactor will have different specific requirement of the neutronic properties. For neutron economy, the structural material should have a small absorption cross section.

Absorption of thermal or fast neutrons in a nuclear reactor material can initiate nuclear transmutation and isotope (stable or unstable) production. The radiation (a ray, b ray, g ray, etc) emitted from the nuclear transmutation and isotope productions are referred to as the induced radioactivity of nuclear reaction. The induced radioactivity should have a short half-life with weak radiation energy.

In regard to the property changes produced in nuclear reactor materials by nuclear irradiation, the most important particles are fission fragments and neutrons. Although fission fragment have very high energies, their range is small, and consequently most physical changes are confined to the nuclear fuels. As a result, almost all radiation effect, or damage, in the reactor materials are chiefly produced by neutrons, especially by the bombardment of fast neutron. The primary radiation effects on the fuel material are irradiation growth, thermal-cycling growth, irradiation swelling, and irradiation creep. Similarly, the irradiation effects on the structural materials are thermal-cycling cracks and fatigue, irradiation swelling, and irradiation creep. Of these, irradiation swelling, irradiation creep, and thermal-cycling cracks and fatigue can reduce and limit the irradiation stability of the fuel and structural materials.

Chemical interactions and particle interdiffusion between fuel and cladding, or the fuel-cladding gap, of the fuel element irradiated at high temperature (approximately hotter than 500 ^oC at the outer cladding surface and 1500 ^oC in the oxide fuel) are often observed in post irradiation specimens. In the presence of high oxygen potentials, absorbed gas impurities, and fission product gases, low-density oxide fuel is more susceptible to the fuel-cladding chemical interactions and particle interdiffusion than high-density pellet fuel pins under the same or equivalent operating conditions. This can have a great effect on the fuel-cladding gap conductance, or gap heat transfer coefficient. In general, chemical interaction and particle interdiffusion can weaken the structural integrity and irradiation stability of fuel elements irradiated at high temperatures during service life.

A nuclear fuel used in a research or power reactor has a limited service life and requires chemical reprocessing. The main purpose of fuel reprocessing is to recover the valuable fissionable materials, uranium and plutonium, contained in depleted solid fuel elements. There are two reason that depleted fuel elements require chemical reprocessing:

Ø  The reactor reactivity of the fuel becomes too low because of consumption of fissionable material and accumulation of neutron-absorbing fission products.

Ø  The fuel element has gradually been damaged by corrosion, thermal, radiation, and mechanical effect (irradiation swelling and irradiation creep).

Processing depleted fuels by solvent-extraction techniques is in large-scale use for nuclear fission reactor because of the easy of recovering valuable uranium and plutonium. Therefore, the ease of fuel reprocessing is a special requirement of nuclear (fuel and cladding) material properties.

Main materials of primary components of nuclear reactors

The primary components of a nuclear reactor can be classified in seven main categories as: nuclear fuel, structure, moderator-blanket-reflector, control element, coolant, shields and safety systems. Figure 2.1 shows a simplified schematic diagram of the primary components of a reactor. The main materials used in each category are listed in Table 1.4.

Among the metals used for structural materials, beryllium, magnesium, aluminum and zirconium are suitable for use in thermal (research and power) reactors due to their low thermal neutron absorption cross section and austenitic stainless steels (or mild carbon steels) and nickel alloys are suitable for use in fast (research, power and breeder)  reactors due to their low fast neutron absorption cross section but high thermal neutron absorption cross section. In thermal reactors however, stainless steel and carbon steels may also be used in pressure vessel and leak-tight piping materials where the neutron absorption cross-section is not important. Stainless steels have excellent mechanical properties, corrosion and oxidation resistance at elevated temperatures, which render them as candidate for use at high temperatures in many reactors (vessel and piping system) and for radioactive waste and radioisotope containers and other nuclear applications. Another advantage with the utilization of stainless steel is its reasonable cost and availability.

                Table 1.4.Main materials of primary components of nuclear reactor

                      

                               

         Fig.28  (Schematic diagram of the components of nuclear  reactor with radiation containment)

Reactor use of aluminium alloys

        Aluminum, with its low cost, low thermal neutron absorption, and freedom from corrosion at low temperature, is ideally suited for use in research or training reactors in the low kilowatt power  and low temperature operating ranges. Aluminum, usually in the relatively pure (greater than 99.0%) 2S (or 1100) form, has been extensively used as a reactor structural material and for fuel cladding and other purposes not involving exposure to very high temperatures.Aluminum with its low neutron capture cross section (0.24 barns) is the preferred cladding material for pressurized and boiling water reactors operating in the moderate temperature range.

Aluminum, in the form of an APM alloy, is generally used as a fuel-element cladding in organicmoderated reactors. Aluminum has also been employed in gas-cooled reactors operating at low or moderately high temperatures. Generally, at high temperatures, the relative low strength and poor corrosion properties of aluminum make it unsuitable as a structural material in power reactors due to hydrogen generation. The high temperature strength and corrosion properties of aluminum can be increased by alloying, but only at the expense of a higher neutron capture cross section.

In water, corrosion limits the use of aluminum to temperatures near 100C, unless special precautions are taken. In air, corrosion limits its use to temperatures slightly over 300C. Failure is caused by pitting of the otherwise protective Al(OH)3 film. The presence of chloride salts and of some other metals that form strong galvanic couples (for example, copper) can promote pitting.

Aluminum is attacked by both water and steam at temperatures above about 150C, but this temperature can be raised by alloying with small percentages of up to 1.0% Fe (iron) and 2.5% Ni (nickel). These alloys are known as aerial alloys. The mechanism of attack is attributed to the reaction Al + 3H2O Al(OH)3 +3H+ when the hydrogen ions diffuse through the hydroxide layer and, on recombination, disrupt the adhesion of the protective coating.

Aluminum-uranium alloys have been used as fuel elements in several research reactors. Enriched uranium is alloyed with 99.7% pure aluminum to form the alloy.

Research has shown that radiation produces changes in both annealed and hardened aluminum and its alloys. Yield strength and tensile strength increase with irradiation. Data indicates that yield strengths of annealed alloys are more effected by irradiation than tensile strengths. The yield strengths and the tensile strengths of hardened alloys undergo about the same percent increase as a result of irradiation. Irradiation tends to decrease the ductility of alloys. Stress strain curves for an irradiated and an unirradiated control specimen are shown in Figure 8.

Figure 8 illustrates the effect of neutron irradiation in increasing the yield strength and the tensile

strength and in decreasing ductility.         

            

                 

 Fuel elements considerations

The fuel material must be carefully chosen in order far the fuel element to meet the SSN reactor fuel element design objectives of maximum bumup, maximum operating lifetime, c o d o n rematence and ability to withatand credible accident conditions.

                                           table 1.5 some physical properties of nuclear fuel.

                                                  

Uranium aluminde-aluminium dispersion  fuel

Dispersion type fuels are two phase alloys consisting of a fissile isotope bearing material that is uniformly dispersed in a matrix of nonfissile material or diluent. These fuels are usually prepared by powder metallurgy, a process in which fine powdera of the fissile phase and nonfiasile phase are mixed, compacted, sintered, and rolled to form a continuous fuel material.

The dispersion technique offers the following advantages when the diluent predominates in volume:

1)       Damage to the fuel material due to fission fragments is localized to the each fuel particle and the region immediately surrounding it.

         2) The potential for reaction between the fuel and the coolant is essentially eliminated in the event of          cladding rupture. Only particles on the surface of the fuel material can be exposed to the coolant.

3) The path for heat flow from the fissile particles is through a highly conducting metallic nonfissile medium which lowers the required operating fuel temperature.

The uranium bearing intermetallic compounds formed by uranium and aluminum, UA12, U& and U&, can be dispersed in a continuous matrix of aluminum to form uranium aluminide - aluminum ( U a- Al) dispersion type fuel. This fuel has been used extensively in research reactors not intended for power generation. Thus they can operate at a relatively low temperature. Simple aluminium metal melts at 660°C a temperature which can easily be exceeded during transient or accident conditions, it should not be used as a fuel matrix or cladding material for PWR operating conditions.

               

Cladding material consideration

In nearly all reactors, the fuel is covered with a protective material or cladding which prevents the release of radioactive fission products from the duel surface to the coolant channel. The cladding also prevents corrosion of the fuel and acte to retain the original shape of the fuel material during the operating life time of the fuel element. The cladding must remain intact both throughout the operation of the reactor and following removal of the fuel element from the reactor core.

In high fuel pump power producing reactors, the cladding must resist swelling due to internal pressure buildup caused by fission gases. Typical design limits are 1% cladding strain in commercial reactors. in the case of weak or defective cladding, fission gas pressure can result in failure of the cladding.

                                 Table 1.6 properties of cladding materials.

Corroision resistant

Corrosion is a leading cause of loss of availability in both fossil and nuclear power plants. The financial impact of corrosion to the power industry is on the order of billions of dollars per year6. Corrosion occurs within the steam cycle of nuclear and fossil power plants and on the fire-side of fossil fired power plants.

 Corrosion of aluminium alloys in high temperaturewater (260°-315°C) depends partly on the conditions under which the tests are run. According to J. E. Draley and W. E. Ruther (USA), corrosion rates largely vary when there is a high rate of flow of water past metal surface. They pointed out that seemingly minor variations in conditions cause large changes in corrosion rate. Also, the silicon content of aluminium-nickel-iron alloys was found to have an important influence on their corrosion behaviour. Alloys with extremely low silicon content behaved better than those with a more typical siliconcontent. Steam cycle related corrosion has been the most troublesome causing failures to major components such as boilers, steam generators, turbines, condensers and the piping throughout the steam plant. Multiple forms of corrosion can impact a given component in the system.

                                    

                                                                Fig.29 Steam generator

Commonly encountered forms of corrosion         

Nuclear plants have experienced capacity losses due to corrosion damage to a number of components the most serious of which are stress corrosion cracking of Boiling Water Reactor (BWR) coolant piping and reactor internals and PWR steam generator tubes. Other corrosion problems include, erosion-corrosion of carbon steel steam-system piping, particularly turbine cross-around piping and extraction lines. In the reactor, nuclear fuel cladding failures in BWRs and PWRs can lead to increased O&M costs, investment losses, reduced cycle efficiency and increased inspection and reconstitution costs as well as to operational restrictions and outage duration increases. Crud induced localized corrosion (CILC) in BWRs, was a major problem in the mid-70's has been controlled by reducing copper transport. PWR cladding failures have generally been associated with debris or are due to hydriding. Irradiation-assisted stress corrosion cracking (IASCC) of core internals is becoming of increasing importance as plants age. In both fossil and nuclear plants, problems with service water systems have included fouling, sedimentation and various forms of corrosion (including microbial induced corrosion). These corrosion issues have had a significant impact on maintenance costs at some plants.

Irradiation effect on corrosion

Physical, mechanical, and thermal properties of nuclear reactor materials determine the corrosion rate and behavior. Generally, in aqueous systems (LWR, HWR, etc.) corrosion rate increases due to irradiation. In an aqueous media, irradiation effect is accelerated by chemical reactions and this brings about an increase in corrosive activity. There are three mechanisms to be concerned in the enhancement of corrosion activity8:

Ø  Hydrogen, oxygen, hydroxyl ions, and hydrogen peroxide are formed as a result of radyolytic decomposition of water and this results in an increased corrosion.

Ø  Neutrons disturb the thin protective layer on the surface of the steel, resulting in pitting of the metal surface,

Ø  Change in mechanical, physical, thermal properties of the materials brings about changes in the corrosion rate.

 Types of corrosion encountered

The factors promoting corrosion are complex and interrelated. They often operate synergistically, making prediction of corrosion difficult. In wet storage of clad spent fuel, there are a number of corrosion mechanisms involved. The most important mechanisms as related to spent nuclear fuel are as follows:

Ø  Uniform corrosion

Ø  Galvanic corrosion

Ø  Crevice corrosion

Ø  Pitting corrosion

Ø  Hydrogen blisters

 These form of corroision has a significant effect on the reactor materials which may cause a serious disaster during its opertion.the effect can be minimize by using corrosin resistant alloys e.g AL alloys.

Corrosion problem in research reactors

About 700 nuclear research and test reactors have been constructed around the world since the beginning of the nuclear age. More than 250 of these reactors are still in operation. The age distribution of operating research reactors peaks at between 35 and 40 years, with 61% of them more than 30 years old9. Many of those permanently shut down are still managing spent fuel in interim storage.The most common storage location for spent fuel from research reactors is in at-reactor water pools or auxiliary away-from reactor pools. Most of the fuel used in research reactors in both eastern and western countries is fabricated with a core consisting of uranium–aluminium alloys and protected by an aluminium alloy cladding. A small percentage of the fuel is clad with stainless steel, zirconium or other alloys. Fuel is regarded as spent nuclear fuel, regardless of burnup, when it is discharged from the reactor core for the final time. It is then normally placed in pools for cooling and interim storage until a final disposition is made. Some of these aluminium clad spent fuels have been in water storage for more than 40 years and remain in pristine condition, while others are severely degraded by pitting corrosion. Pitting corrosion of the fuel can lead to breach of the cladding material and release of radioactivity to the storage basin.

Corrosion protection

The corrosion of reactor materials is dependent on a number of interrelated factors. These factors may operate singly or synergistically, making predictions difficult. In the case of spent fuel, many of the metallurgical factors are already inherent in the spent fuel when a reactor operator receives the fuel from the fuel manufacturer.Factors such as alloy composition, heat treatment, microstructure, nature and thickness of the protective oxide coating, inclusions and impurities in the alloy, and cold work play a role in the corrosion processThese factors cannot be controlled during wet storage. However, the environmental and service related factors that can be controlled and used to optimize corrosion protection of the concerned materials, such aluminium clad fuel during interim wet storage.

The following protection mechanisms are actually for corrosion protection of the aluminium cladding, to prevent breach of this cladding and subsequent corrosion of the fuel core. However, most research reactor fuel is fabricated from uranium–aluminium alloys, and this type of fuel exhibits corrosion behaviour similar to that of aluminium. Also, the protection mechanisms would be applicable to aquaeus corrosion in general. Therefore implementation of these protection guidelines should also, for example, minimize corrosion of the fuel core.

The protection mechanism to be outlined focuses mainly on maintaining high quality water in the fuel storage pool. It is the single most important factor in controlling corrosion of aluminium clad spent fuel assemblies and other aluminium alloy components stored in the pool. Treatment and purification of the water in the pool and any make-up water with the aid of filters and ion exchange resins is essential to achieve optimum storage performance.

summary

Aluminum is ideally suited for use in low kilowatt power and low temperature reactors due to its low cost, low thermal neutron absorption, and freedom from corrosion at low temperatures.  Aluminum, with its low neutron capture cross section is the preferred cladding material for moderate temperature ranges.

Aluminum has been ruled out for power reactor application due to hydrogen generation and it does not have adequate mechanical and corrosion-resistant properties at the high operating temperatures.