Back to Basics: How to AC TIG Weld Aluminum
TIG expert Wyatt "Mr. TIG" Swaim and artist Kevin Caron tackle AC TIG welding aluminum .... First, Wyatt says to make sure to hit that "AC," or "alternating current," button on your welder to get started. You can hear the AC running constantly, giving it a totally different sound.
On machines built in the last 15 years or so, a cleaning action also is built in. That means that, aside from removing oil, you don't need to prepare aluminum metal at all before you begin TIG welding. Even though you can't see it, aluminum has an oxide layer that melts at about 3600 degrees Farenheit. The aluminum itself starts melting at about 1200 degrees, creating a "little physics problem." The machines' cleaning action is controlled by a knob that usually says something like "AC Balance" that allows you to adjust for just how dirty your metal is. It usually has words like "More Cleaning" or "Less Cleaning" to help. Once you visually determine how dirty your metal is, you can set your balance. If you find that your tungsten is balling up too much, your setting is too far into the positive range. Most of the time, Wyatt runs at about 70% negative. By running a bead on a plate and adding filler material, you can look adjacent to the weld, and make sure that you are getting cleaning action for a relatively small section outside the heat-affected zone. An inch wide cleaning zone is too much. Kevin then asks whether you need to clean the aluminum in real-world situations, but Wyatt says that using a wirebrush or otherwise cleaning the surface simply polishes the oxides. The welder actually bombards and vaporizes the contaminant and cleans the metal, except for oils, which you do need to remove.
They suit up and Kevin takes the torch using a 1/16 diameter, 2% thoriated tungsten, 100 percent argon, and about 100 - 110 amps. The AC balance is set at 70% negative. He begins welding without filler. The close-up shows the cleaning action on each side of the puddle - he's getting more than adequate cleaning. As Kevin reaches the end of the weld, he tapers off slowly. Wyatt points out that heat tends to build up quickly with aluminum. Some people ask if aluminum is harder to weld. Swain says it isn't, it's just different. Once they get used to it, he says, some people like aluminum better than other materials. Kevin says welding aluminum seems quicker than welding steel. For the next weld, Kevin uses filler material. Wyatt emphasizes how important it is to establish the puddle, which will be very clear and precise, almost mirror-like, before adding your filler. Not doing so, he says, is the number 1 mistake new TIG welders make. For this application, they're using 1/16 diameter, 4043 filler, which is pretty common for engineering.
The close-up shows Kevin dabbing the filler rod as he moves the tungsen down the metal and Wyatt gives a play-by-play. Kevin backs off slowly at the end. Wyatt points out that you may start out at 100 or 110 amps, but by the time you get to the end of the weld, your heat has caught up to you, and you have to back off or your weld gets wider and wider and, Kevin adds, flatter. That's a big difference between steel and DC welding and aluminum and AC welding. You also have to watch out for hot, short cracking at the end. Adding another little dab of filler at the end of your weld can avoid this problem. Wyatt points out that this is one big difference between the big industrial machines and the light-duty machines: the more expensive machines are more precise. With the higher-end machines, it's much easier to avoid cratering than it is with the less expensive welders. Once you get used to the light-duty machines, you can dab a little filler to overcome the cratering, but with the industrial welders, you can taper off to as little as 5 amps to avoid it altogether, which is critical when you're doing X-ray quality welds.
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