Welding encompasses a handful of major processes, with several of them being especially prominent in industry. These are arc welding, gas welding, brazing, and resistance welding. All are used to melt metal; often a filler material is used. This article briefly describes these welding processes and some instances of where they are used.
Arc and gas welding
Arc and gas welding are what typically come to mind when we hear the word “welding.” They use high heat from electric arcs or oxy-acetylene torches to melt metal. In joining pieces together, a filler material is added to the joint, creating a weld bead. A properly executed weld can be as strong as the parent metal. Welding can be used to join almost any metal, though some materials require more advanced processes. It can join similar and, to a degree, dissimilar metals.
The big three arc welding techniques are shielded metal arc welding (SMAW or stick), gas metal arc welding (GMAW or mig), and gas tungsten arc welding (GTAW or tig). In all cases, molten metal requires shielding from the air to assure proper fusion. For stick welding, this shielding comes from a flux coating applied to the filler rod itself. Mig welding uses the same approach in flux-core processes or uses an inert shielding gas while feeding uncoated welding wire from a spool into the molten weld puddle. For both stick and mig welding, the stick or wire provides the electrical path that produces an arc at the workpiece. Tig welding combines elements of the other two processes: it uses discrete filler rods and shielding gas. Its primary difference is the electrode: made from tungsten, it does not melt.
Stick welding is a simple, yet somewhat violent process. The flux coating on the rod produces much smoke and sometimes spatter. Slag covers the weld and it must be chipped or brushed off. Still, it is a sound method of fusing metal in the real world. Electrodes are available that can burn through rust and scale, making for a robust process. It is used extensively in outdoor applications where the loss of shielding gas from wind is not a concern.
Mig welding gets around this problem with the use of flux-core wire, which supplies a shielding flux through the center of the welding wire. Similar to stick welding as regards smoke, spatter, and slag, its advantage is that the weld wire is supplied on spools allowing a weldor to put down long beads without having to replace a welding rod after so many inches. But the real application of mig is indoors, where shielding gas is used. This gas allows clear visibility of the joint during welding, eliminates smoke by eliminating the flux coating, and produces a finished slag-free joint.
Tig welding goes one step further and is almost exclusively done indoors in clean environments. The tungsten electrode can be controlled through either foot or hand controls, allowing the tig weldor exquisite discretion over the amount of heat going into the metal. It can be used for joining thin metals and very delicate parts.
Other arc processes include submerged arc welding, which uses a granulated flux that is lain down ahead of a traveling electrode to produce large and lengthy welds, and plasma welding, which uses a stream of ionized gas to produce high-quality, precise welds.
Gas welding uses torches and filler rods similar to those used for tig welding. Acetylene combined with oxygen provides a very hot flame that quickly and easily melts metal within a small area. Gas welding has been somewhat overtaken by arc welding in terms of popularity, but the same torches used for gas welding are still employed extensively for cutting ferrous metals and for preheating metals prior to arc welding.
Although much of the above discussion pertains to manual welding, automated welding is commonplace. It removes the variable of skill from the production of quality welds, not to mention the many hazards associated with welding itself.
Brazing
A torch is the principal method for manual brazing, using propane or oxy-acetylene mixtures. Brazing is also done in furnaces and through induction processes.
Brazing takes place above 800°F but below the melting temperature of the parent metals. It is this that distinguishes brazing from gas or arc welding, where the filler metal and parent metals usually have similar melting points. It also distinguishes brazing as a welding process and not a bonding process, such as soldering.
As with welding, the brazed joint can only develop its full strength in the absence of air. Thus, brazing rods are sometimes coated with flux, much like welding rods. Brazing furnaces are usually filled with inert gas atmospheres. Typical filler materials include copper alloys, silver alloys, or nickel alloys.
Brazing is used where distortion of the parent metals is unacceptable. It is often used to make repairs without affecting the metallurgical properties of the broken parts. Bronze is commonly used as a filler for brazing cast and wrought iron, steel, copper, brass, etc. Brazing can be used for joining dissimilar metals. Complex fabrications can be made by using fillers that have increasingly lower melting points. Preformed filler shapes such as brazing rings are available for such fabrications.
Sometimes a distinction is made between the terms brazing and braze welding. In the former, capillary action is used to draw the filler material into the interstice between mating parts, such as a tube and socket or a close-fitting butt joint. Clearances in these assemblies usually run 0.025-0.05 mm but special filler rods are used with wider gaps. Braze welding often produces a thicker bead much like arc welding but it does not rely on capillary action.
A primary distinction between brazing and arc welding is that with brazing, the filler material is drawn into the joint all at once. Once the base metal is sufficiently hot to melt the filler metal, capillary action takes over, producing a clean joint that requires no finishing. This makes it suitable for production in furnaces. For this reason, brazing is often likened to soldering. The phrase “silver soldering” actually describes a brazing process.
Resistance welding
Resistance welding is the term that applies to spot welding, seam welding, projection welding, capacitor-discharge welding, etc., where resistance to the flow of electricity produces heat and pressure is applied to complete the weld. Spot welding is used extensively in the manufacture of automobiles while seam welding is a common method of making tubing and similar products.
Other welding methods
Electron beam (EB) and friction welding are specialty processes. EB and laser welding are used in the high volume or high-value production of intricate parts for industries such as aerospace and automotive. Friction welding is a solid-state process—meaning no external heat source—used to join tubes, drives shafts, etc. Heat is produced by spinning the items to be joined against each other. As the metals plasticize they are pushed together with high pressure. Welding is also used for joining plastics, an endeavor which has its own specialized equipment such as sonic welders.
Summary
This article presented a brief discussion of several different welding processes for the purpose of contrasting welding vs. brazing. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products. More information about educational opportunities, certifications, standards, and other resources relating to welding may be found on the American Welding Society website.
