Aluminum is soft, and it is...
Aluminum is soft, and it is possible to accidentally grind rust particles into the metal if you try to clean it with a wire brush that is also used on steel. Andy Weyenberg, Miller Electric's welding technician for the racing industry, recommends keeping two sets of brushes marked for either steel or aluminum.
Pedaling Power TIG welders offer the advantage of being able to continuously control the amount of amperage flowing from the torch to give better control of the weld. When welding materials with high heat conductivity, you need to establish enough heat to create a weld puddle quickly. Then, you will need to gradually reduce the amperage as the heat builds up in the metal for the longer welds. For better control, the maximum amperage is normally set on the console of the main unit. Welders are also classified by their maximum amp output. Weyenberg recommends welding at one amp per thousandths of thickness of the material to be welded. Since everything on a race car is less than 11/44 inch thick (or at least it should be), a 250 amp welder should do just fine.
Controlling the amperage is relatively easy with a foot control. Operating exactly like a gas pedal, the foot control (there are other designs, like a sliding fingertip control, but few are as easy to use) provides a level of control not possible with typical MIG welders. Once an arc is struck between the torch and the material to be welded, the welder looks for the affected area to appear bright, shiny silver (from behind a welder's helmet) before applying the filler metal. It is easy with limited experience to tell how much is enough. If the area to be welded isn't hot enough, the filler metal will not melt correctly because the metal won't be melted. It will also appear medium to light gray instead of silver. If the area is too hot, the weld will still be good; the silver area will just be larger than necessary.
One thing that makes TIG welding so versatile is the variety of filler metals available for use. For example, there are currently 7 filler metal options for welding steel, 13 for titanium, and 12 for aluminum. Of course, that amount of variety can be intimidating, but once you know your needs, it's easy to settle on a few options and sizes. When it comes to aluminum, an excellent all-purpose rod is ER4043. The "ER" stands for electrode rod, and the "4043" designates a specific chemical composition.
Here's a nice example of TIG...
Here's a nice example of TIG welding on a race car chassis. Notice how the overlapping "droplets" of filler material are nice and tight and the center of the droplet is never exposed. Many people describe the look of a good TIG weld as a roll of quarters on its side.
It's easy to check with a tech rep for the correct filler metals for your welding needs, but generally you need to use a filler metal that matches both the tensile strength and malleability of the metals you are welding. Although it may seem safe to go with a super-strong material as a filler metal, it actually isn't the best idea, especially for critical welds like rollcage tubing. The mild steel tubing is designed to bend or flex on impact and not break or tear. If you use a stronger filler metal than the mild steel, it is likely to shear off in impact, significantly weakening the cage.
Safety In any welding process, using good safety procedures is a priority. Electric energy is used to heat the metal to a molten state. Remember that electricity will always take the path of least resistance. If you are standing in a puddle, you become that path. That may sound unlikely, but race cars are notorious for leaking coolant, brake fluid, and other wet stuff. Likewise, sweat increases the risk of electric shock. Although TIG welding doesn't produce slag or sparks, it's still a good idea to wear a good pair of welder's gloves to ward off the risk of getting a good shock.
TIG welding doesn't produce nearly the same volume of harmful fumes as many other methods, but they are still there. A respirator isn't usually necessary, but make sure you work in a well-ventilated area and keep your head out of the fumes. Because there is little smoke produced, ozone can be produced from the ultraviolet light emitting from the arc when it travels through the oxygen molecules in the air. Unfortunately, you can't exactly keep a fan blowing on yourself because this type of welding is a gas-shielded process and you don't want to blow the gas away from the weld too quickly.