Welding

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space. Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join iron and steel by heating and hammering. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding, electron beam welding, electromagnetic pulse welding and friction stir welding in the latter half of the century. Today, the science continues to advance. Robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality.

Power supplies

To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common welding power supplies are constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.

The type of current used also plays an important role in arc welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.

Gas welding

The most common gas welding process is oxyfuel welding,also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work.

The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C.The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.

Oxyacetylene Gas Welding

Advantages of Oxy-Acetylene Welding

• It's easy to learn.
• The equipment is cheaper than most other types of welding rigs (e.g. TIG welding)
• The equipment is more portable than most other types of welding rigs (e.g. TIG welding)
• OA equipment can also be used to "flame-cut" large pieces of material.

Disadvantages of Oxy-Acetylene Welding

• OA weld lines are much rougher in appearance than other kinds of welds, and require more finishing if neatness is required.
• OA welds have large heat affected zones (areas around the weld line that have had their mechanical properties adversely affected by the welding process)

Materials Suitable for OA Welding in the PRL

• Most steels
• Brass

Lap joint

In woodworking or metal fitting, a lap joint is a technique for joining two pieces of material by overlapping them. A lap may be a full lap or half lap.

In a full lap, no material is removed from either of the members to be joined, resulting in a joint which is the combined thickness of the two members. In a half lap joint, material is removed from each of the members so that the resulting joint is the thickness of the thickest member. Most commonly in half lap joints, the members are of the same thickness and half the thickness of each is removed.

Applications

• Frame assembly in cabinet making
• Temporary framing
• Some applications in timber frame construction

T Joint

A joint formed when the edge of one piece is welded to the surface of another piece at a 90° angle.

Butt joint

One of the most common types of weld joints in manufacturing is the butt joint. This joint is formed when the two pieces to be welded are simply placed face to face and the welding head run over it. In the case of GTAW and PAW this joint can only be used on very thin pieces without extensive preparation and the use of filler wire. Both the laser and the electron beam, on the other hand, can butt weld very thick pieces, up to 30 cm for the electron beam. This is accomplished by using the keyhole method, in which the beam is used to bore a path through the piece for itself, allowing it to distribute energy evenly across the joint, regardless of its depth. This makes the electron beam, and to a lesser extent the laser, the ideal welding system for many jobs.

Advantages

The butt joint has many advantages over other types of joint. The first of these is that it results in a uniform surface, which allows them to be used in places where fit or appearance is extremely important. A second advantage is strength. Due to the fact that the area of the weld is nearly the same as that of the pieces being welded, the tensile strength can be comparable to that of the base metal. The third advantage of the butt joint is simplicity to set up and weld. Unlike some of the other joints, which require complicated geometry, such as flanges, to work, the butt joint only requires a smooth interface.

Disadvantages

There are also disadvantages to the butt joint. The first of these is that they are especially sensitive to weld defects. Because all of the forces on the joint are absorbed by the weld, defects such as porosity, inclusions, cracks, etc, can cause easily cause the joint to fail catastrophically. A second disadvantage of the butt weld is that it is usually not self-aligning. Whereas some other joint types will hold together before welding, the butt joint will not. In many cases this greatly increases the complexity of the fixturing necessary to hold the pieces to be welded in place before and during the welding processes. A third disadvantage of the butt weld is that it is nearly impossible to butt weld very thin materials, due to the fact that aligning the faces properly is very difficult.