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How to: Stud Welding Aluminum Capacitive Discharge Process

Aluminum can be a tricky material to weld. Why? Aluminum is very thermally and electrically conductive. That means that aluminum conducts heat much better than steel or stainless steel alloys.

Why is that a problem? This is a problem in welding because the heat input from the welding power source is quickly conducted away from the weld zone. When you conduct the heat away from the weld zone to some other area of the material, the weld zone does not stay hot enough to form a quality weld. It is like trying to fill up a kiddie swimming pool that has a hole in it. All the water runs out of the pool. Likewise, when welding aluminum, the heat runs away from the welding zone.

In stud welding, there are 2 main processes Capacitive Discharge (CD) and drawn arc stud welding. The aluminum work piece must be properly prepared for welding and is the same for both stud welding processes. Aluminum quickly forms a surface oxide. This oxide is tough and nonconductive and is detrimental to quality welding. I recommend taking a wire brush (MUST BE stainless steel as a carbon steel brush leaves deposits on the surface of the aluminum which would prevent successful welding). This cleaning process should be done within a couple of hours of actual welding otherwise the oxide layer begins to form again.

When using the CD welding process, the weld times are short, on the order of 10 milliseconds. Short weld times are beneficial when stud welding aluminum to prevent heat conduction away from the weld zone. Even with the standard fast CD weld times, aluminum needs to be as fast as possible.

There are two primary techniques for CD welding Gap and Contact. When using the contact method, to increase the speed the user needs to add spring pressure to the tool. For the gap CD process the user needs to add more lift or initial gap to increase the speed.

Capacitive discharge initial contact process diagram - aluminum

Capacitive discharge initial contact process diagram

Capacitive discharge initial gap process diagram - aluminum

Capacitive discharge initial gap process diagram

If studs are failing, it is useful to look at the bottom of the failed weld. If the weld face of the aluminum stud is fully or partially shiny, it indicates that the aluminum was melted and then cooled prior to being fused. If the surface is rough and a dull gray this indicates areas of fusion. If it stud appears shiny, it did not reach the weld zone quickly enough and the speed must be increased.

Next time: Aluminum drawn arc stud welding.