Introduction to Welding
The ability to put things together to build a useful tool has been important since the dawn of humanity. Early civilizations used vines or rope to tie stones to sticks to make tools such as axes. Later, glues or cements were used to hold parts together. Forge welding (FOW) was used to join smaller pieces of metal that could be heated in a forge and hammered together. At the dawn of the Iron Age, rivets were used to fabricate large metal structures like bridges, boilers, trains, and ships. But with the advent of modern welding, cutting, and brazing, civilization began advancing more rapidly. In fact, modern civilization could not exist without welding. Today, everything we touch was manufactured using some welding process or was made on equipment that was welded. The skills of welding, cutting, and brazing are an essential part of metal fabrication.
Metal fabrication is the building, shaping, and assembling of a product, equipment, or machine from raw metal stock. Metal fabrication can be done using rivets, bolts, welding, and so forth.
A welded metal fabrication is primarily assembled using one or more of the following processes: welding, thermal cutting, or brazing.
A weldment is an assembly in which its component parts are all joined by welding.
In some cases, a welded fabricated part may require some postweld finishing such as grinding, drilling, machining, or painting to complete the fabrication.
Fabrication Steps
The process of metal fabrication can be divided into several, often distinct steps. Following are the primary steps for fabrication:
Layout—the process of drawing lines on the raw metal stock according to the parts drawings and specifications,
Cut out—the process of removing all of the unwanted material around the laid-out part or sometimes just cutting material to the desired length. Some of the most common methods of cutting out the parts are flame cutting, plasma cutting, sawing, and punching.
Assembling—the process of placing all the parts together in the correct location and orientation with each other. The parts may be held in place with small welds called tack welds or by some type of clamp.
Welding—the process of permanently attaching the parts together to form the finished part.
Finishing—can be accomplished by any number of different processes such as grinding, polishing, drilling, machining, painting, etc.
Not all metal fabrication includes all of the steps, and the difficulty of each step varies with the complexity of the fabrication. In addition, sometimes the order in which each step is done may change. For example it may be necessary to wait until part of the assembly has been welded before laying out the location of an additional part; or a part may be trimmed to fit once other parts have been welded in place.
WELDING AND CUTTING PROCESSES
Gas Metal Arc Welding
Gas metal arc welding (GMAW) uses a solid electrode wire that is continuously fed from a spool, through the welding cable assembly, and out through the gun. A shielding gas flows through a separate tube in the cable assembly, out of the welding gun nozzle, and around the electrode wire. The welding power flows through a cable in the cable assembly and is transferred to the electrode wire at the welding gun. The GMA weld is produced as the arc melts the end of the continuously fed filler electrode wire and the surface of the base metal. The molten electrode metal transfers across the arc and becomes part of the weld. The gas shield flows out of the welding gun nozzle to protect the molten weld from atmospheric contamination. GMA welding is extremely fast and economical because it can produce long welds rapidly that require very little postweld cleanup. This process can be used to weld metal ranging in thickness from thin gauge sheet metal to heavy plate metal by making only a few changes in the welding setup.
Flux Cored Arc Welding
Flux cored arc welding (FCAW) uses a flux core electrode wire that is continuously fed from a spool, through the welding cable assembly, and out through the gun. The welding power also flows through the cable assembly. Some welding electrode wire types must be used with a shielding gas, as in GMA welding, but others have enough shielding, which is produced as the flux core vaporizes. The welding current melts both the filler wire and the base metal. When some of the flux vaporizes, it forms a gaseous cloud that protects the surface of the weld. Some of the flux that melts travels across the arc with the molten filler metal where it enters the molten weld pool. Inside the molten weld metal, the flux gathers up impurities and floats them to the surface where it forms a slag covering on the weld as it cools. Although slag must be cleaned from the FCA welds after completion, the advantages of this process, including high quality, versatility, and welding speed offset this requirement. Gas metal arc welding and flux cored arc welding are very different welding processes, but they use very similar welding equipment. Both GMA and FCA welding are classified as semiautomatic processes because the filler metal is automatically fed into the welding arc, and the welder manually moves the welding gun along the joint being welded. GMA and FCA welding are the first choice for many welding fabricators because these processes are cost effective, produce high-quality welds, and are flexible and versatile. In addition to welding supply stores, many others stores such as hardware stores, building supply stores, automotive supply stores, and others carry GMA/FCA welding equipment and filler metals.
Shielded Metal Arc Welding
Shielded metal arc welding (SMAW) uses a 14-in.-(350-mm) long consumable stick electrode that both conducts the welding current from the electrode holder to the work, and as the arc melts the end of the electrode away, it becomes part of the weld metal. The welding arc vaporizes the solid flux that covers the electrode so that it forms an expanding gaseous cloud to protect the molten weld metal. In addition to protecting molten weld metal, fluxes also perform a number of beneficial functions for the weld, depending on the type of electrode being used. SMA welding equipment can be very basic compared to that used in other welding processes. It can consist of a welding transformer and two welding cables with a work clamp and electrode holder. There are more types and sizes of SMA welding electrodes than there are filler metal types and sizes for any other welding process. This wide selection of filler metal allows welders to select the best electrode type and size to fit their specific welding job requirements. So, a wide variety of metal types and metal thicknesses can be joined with one machine.
Gas Tungsten Arc Welding
Gas tungsten arc welding (GTAW) uses a nonconsumable electrode made of tungsten. In GTA welding the arc between the electrode and the base metal melts the base metal and the end of the filler metal as it is manually dipped into the molten weld pool. A shielding gas flowing from the gun nozzle protects the molten weld metal from atmospheric contamination. A foot or thumb remote-control switch may be added to the basic GTA welding setup to allow the welder better control. This remotecontrol switch is often used to start and stop the welding current as well as make adjustments in the power level. GTA welding is the cleanest of all of the manual welding processes. But because there is no flux used to clean the weld in GTA welding, all surface contamination such as oxides, oil, and dirt must be cleaned from the part being welded and the filler metal so it does not contaminate the weld. Even though GTA welding is slower and requires a higher skill level as compared to other manual welding processes, it is still in demand because it can be used to make extremely high-quality welds in applications in which weld integrity is critical. In addition, there are metal alloys that can be joined only with the GTA welding process.
Oxyacetylene Welding, Brazing, and Cutting
Oxyacetylene welding (OAW) and torch or oxyfuel brazing (TB) can be done with the same equipment, and oxyfuel gas cutting (OFC) uses very similar equipment. In OA welding and TB, a high-temperature flame is produced at the torch tip by burning oxygen and a fuel gas. The most common fuel gas is acetylene; however, other combinations of oxygen and fuel gases (oxyfuel gas [OF]) can be used for welding such as hydrogen, MappÒ, or propane. In OF welding the base metal is melted, and a filler metal may be added to reinforce the weld. No flux is required to make an OF weld. In TB, the metal is heated to a sufficient temperature but below its melting point so that a brazing alloy can be melted and bond to the hot base metal. A flux may be used to help the brazing alloy bond to the base metal. Both OF welding and TB are used primarily on smaller, thinner-gauge metals.
THERMAL CUTTING PROCESSES
There are a number of thermal cutting processes such as oxyfuel gas cutting and plasma arc cutting (PAC). They are the most commonly used in most welding shops. Air carbon arc cutting (AAC) is also frequently used, and many larger fabrication shops have started using laser beam cutting (LBC).
Oxyfuel Gas Cutting
Oxyfuel gas cutting (OFC) uses the high-temperature flame to heat the surface of a piece of steel to a point where a forceful stream of oxygen flowing out a center hole in the tip causes the hot steel to burn away, leaving a gap or cut. Because OF cutting relies on the rapid oxidation of the base metal at elevated temperatures to make a cut, the types of metals and alloys that it can be used on are limited. OF cutting can be used on steel from a fraction of an inch thick to several feet, depending on the capacity of the torch and tip being used.
Plasma Arc Cutting
Plasma arc cutting (PAC) uses a stiff, highly ionized, extremely hot column of gas to almost instantly vaporize the metal being cut. Most ionized plasma is formed as high-pressure air is forced through a very small opening between a tungsten electrode and the torch tip. As the air is ionized, it heats up, expands, and exits the torch tip at supersonic speeds. PAC does not rely on rapid oxidation of the metal being cut like OFC, so almost any metal or alloy can be cut. PA cutting equipment consists of a transformer power supply, plasma torch and cable, work clamp and cable, and an air supply. Some PA cutting equipment has self-contained air compressors. Because the PA cutting process can be performed at some very high travel speeds, it is often used on automated cutting machines. The high travel speeds and very low heat input help to reduce or eliminate part distortion, a common problem with some OF cutting.