Welding two pieces of metal together forms a permanent bond in which the metals are heated to a melting point, mixed together and then cooled, creating a single object. This type of bond is stronger than other bonds, such as soldering, which can be reversed.
There are four basic types of welding: MIG, TIG, flux-cored, and stick. The process and welding techniques to be used depend on the application, the types of metal to be welded, and the skill and experience of the welder.
New welders just learning how to weld should start with metal inert gas (MIG) welding, also known as gas metal arc welding (GMAW). MIG is a type of electric arc welding, which relies on creating an electrical circuit that runs through the objects to be welded together and a welding wire, which acts as an electrode.
When the welding wire touches the metal object, the circuit is completed. The wire is then pulled back a short distance, causing an electric arc that heats up to thousands of degrees Fahrenheit, melting the wire and partially melting the two pieces of metal. This creates a melt pool where all three metals merge to become one as it cools and solidifies, leaving a bead along the welded seam.
As the welding action continues down the seam, the wire melts off and more wire is fed continuously from the tip of the welding gun. At the same time, a gas is diffused from the gun, spreading around the arc to shield the welded area from contaminants in the air, such as oxygen and nitrogen.
Tungsten inert gas (TIG) welding requires more skill and takes longer than MIG welding, but it offers more precision. TIG, also known as gas tungsten arc welding (GTAW), can be used to weld aluminum and alloys, such as 4130 chrom-moly.
TIG welding is similar to MIG, but instead of a consumable wire, the electrode used is a tungsten metal rod inside the welding gun. The welder holds the gun in one hand while feeding a filler rod in the other hand. As the arc creates a melt puddle from the two work pieces, the heat of the melt puddle melts the filler rod, so that all three mix together in the puddle. Like MIG welding, TIG welding guns disperse a shielding gas to protect the new weld.
A TIG welding machine also includes a foot pedal to adjust the current running through the electrical circuit created between the metal objects and the tungsten electrode. This can be used to slowly increase or decrease the heat applied to the metal, which can help prevent brittleness caused by thermal shock.
A third type of welding, called flux-cored arc welding (FCAW), uses a continuously fed electrode tube in place of the wire used for MIG welding. The tube is metal on the outside, with a flux agent in the core. As the flux melts during welding action, it creates a liquid slag and gas that shields the weld from contaminants. This provides better protection for the weld than the shield gas used in MIG welding, especially where strong breezes might disperse the gas, but it also results in more spatter which must be cleaned up afterwards.
Otherwise known as shielded metal arc welding (SMAW), stick welding is a two-handed method, like TIG. Stick welding uses a metal filler rod, or stick, coated with flux. As the stick material melts in the heat, the flux coating also melts, creating both a gas and a liquid slag, which act to shield the weld from oxidation. As with FCAW, shield gas does not have to be dispensed during the welding process but the leftover slag leaves a mess requiring cleanup using a sander or solvents.
Once the method of welding has been decided, the next step is to select the proper welding equipment. Welders can choose either a MIG welder or a MIG/Stick/TIG multi-process welder which can be used for all four basic types of arc welding.
Along with the proper machine, the next important piece of equipment required is a welding helmet to protect the face and especially the eyes, from the extreme heat and bright light created by the electric arc. The latest technology has made possible auto-darkening helmets with vision screens that instantly adjust to the light level so that the welder has constant visual input.
Welders also need sturdy leather gloves and shoes, as well as caps, long-sleeved cotton shirts, bibs, overalls, and/or aprons to protect their skin from sparks.
Materials needed for welding include consumables, such as MIG wire and flux-cored wire in various diameters and materials, welding tips, electrode sticks or tubes, flux, and TIG or gas brazing rods.
Other useful equipment includes magnets and clamps to hold metal objects in place during welding, adjustable welding tables and workstands, and fiberglass welding blankets to prevent the spread of sparks.
Weld-on tabs in a variety of sizes and shapes are used to create flanges, holes, handles, and other mechanical parts when welded to a pipe or other metal object.
Each welder develops his or her preferred welding techniques. One of the most commonly used is nicknamed “stacking dimes,” which ends up looking like a string of round coins overlapping along the length of the weld.
As the welding arc liquefies a small pool of metal, the welder pushes the melt pool ahead with the electrode, using a fluid motion similar to writing a series of the letter “e” in cursive. Alternative motions may be described as, “figure 8,” or “half-moon.” The key to each type of movement is to ensure that the electrode pushes the melt pool back and forth evenly between the two workpieces, so that both are fully welded.
Some welders prefer to pull, rather than push the melt pool along, depending on their handedness (right or left) and the position of the workpiece. When MIG welding, the choice is a personal preference.
With TIG, the welder pushes the puddle along the seam, making sure to move back and forth, all the while dipping the end of the rod in and out of the puddle with the other hand.
However, stick and flux-cored welding requires a pulling motion, to avoid welding over the melted flux slag, which creates porosity and “wormholes” in the finished weld.
Welding different types of metal varies, depending on the different physical characteristics of each. For example, stainless steel doesn’t transfer heat as well as other metals, so it’s easy to build up too much heat in the weld area, causing the steel to warp and reducing its corrosion resistance. To control the heat, reduce the current on the welding machine and speed up the movement of the torch.
Compared with all types of steel, aluminum has a much higher thermal conductivity, as well as a lower melting point. It is also highly reactive to air, creating a hard oxidation layer on the surface. Because this oxidation layer melts at a much higher temperature than the aluminum underneath, it must be removed with a wire brush or chemical solvent before the welding begins.
Aluminum can be welded using both TIG and MIG methods, but aluminum filler wire is quite soft and can easily get tangled in the wire feeder of a MIG gun. To prevent this, use a Teflon or plastic liner in the wire feeder and guide tubes to support the wire from the feeder to the gun.
Aluminum welding requires a higher amperage welding machine and faster welding speed than steel to avoid “burning through” the base metal, melting a hole with too much heat. Aluminum welds also must be well protected from oxidation with an inert shield gas, such as argon.