Wire-feed welding does have a few distinct advantages over GTA welding. One is initial cost. A MIG machine is much less expensive to buy and easier to operate than a TIG machine. Many MIG welders will run on 110V household circuits; you can literally plug them in and run a bead. You don't even need shielding gas if you use flux-cored wire. As with stick welding, the flux inside the wire will shield the weld. Better-looking welds are obtained, however, by using solid wire and an 85 percent argon/15 percent carbon dioxide mix. Undoubtedly, the biggest advantage of GMA welding is speed. In the hands of a professional, MIG welding can be as much as three times faster than TIG welding. In places like collision repair shops where speed is of the essence, you'll trip over MIG machines all day long before you find a TIG machine.
Lincoln Welding School instructor Karl Hoes says MIG welding is the easiest to learn, but may be the most difficult to get right. In theory, it's simple: Since the machine feeds the wire automatically, the operator need only set the proper heat range and wire speed appropriate for the type of materials being welded, and pull the trigger. As long as the operator moves the torch at the same rate as the wire is being fed, it's pretty easy to make a good-looking weld. Problems can arise because of insufficient penetration, though. You may have only laid a good-looking bead on top of the joint being fused. Even with the correct heat setting, MIG welds can often experience insufficient penetration at the start of a bead, a situation called a "cold start." In this case, the weld wire solidifies before the base materials are heated enough to fuse the joint. It takes a lot of practice to get the technique right. In some ways, the benefits MIG welding have over TIG welding (ease of use and speed) can be a disadvantage to a novice welder.
 Here's the progression you...  Here's the progression you can expect in a few days of practice. With our having never TIG-welded before, the first day of class produced the embarrassing-looking welds on the pieces on the left side of the picture. Compare them to the pieces on the right side made on the last day of class-not quite as bad-looking, but still, we need more practice. |  |  Mike Fratena, who races in...  Mike Fratena, who races in NMCA's Street Race class, welds brackets on his header tubes on a Sunday morning at last year's event in Joliet, Illinois. This illustrates several advantages of MIG welding: It's quick and portable, and many of the machines can run on household current. |
SMAW
Though shielded metal arc welding, or stick welding as it's commonly called, is not normally used in automotive applications, it's worth mentioning here because it's a term you'll hear a lot, and any TIG machine you can currently buy can also be set up to do stick welding. It's an arc-welding process that uses a flux-coated welding rod. The flux is vaporized in the arc and forms a cloud of shielding gas around the weld area. Stick welding is usually done with DC positive current, where the welding rod is positive and the workpiece is negative. Stick welding is the oldest form of electrical arc welding and is usually used on heavy materials in industrial settings like shipyards or construction sites, and for pipefitting.
Materials
Carbon SteelThe most common metal in automotive applications is mild steel, which is good because it's the easiest to weld. Though a full-blown discussion of metallurgy wouldn't be appropriate here, it is crucial to note that different metals must be welded in different ways, and each alloy or blend requires the use of a compatible welding rod or filler wire.
Steel is usually classified according to its carbon content: low-, medium-, or high-carbon. Generally, the greater the carbon content, the harder the steel. However, this is not necessarily a good thing: High-carbon steel can be very brittle and susceptible to cracking under certain forces. Also, steel with higher levels of carbon reacts differently to the heat of welding. Too much heat or improper cooling of the weld can make an already brittle piece even more brittle-a potentially dangerous condition if it is a structural component such as a rollcage or suspension piece. The operator must be aware of this and adjust his technique to suit the composition of the base metal.