Aluminum Workshop: Preheating aluminum

Q: What preheat temperature should I use when welding aluminum? I’ve heard different numbers, and I know they can’t all be right.

A: Let me begin with a bold statement: If you have the right equipment, you never need to preheat when welding aluminum.

I have gas tungsten arc-welded (GTAW) 7-inch-thick aluminum without preheat. I admit, I used a 500-amp power supply and a ¼-in.-dia. tungsten; if I’d had a 200-amp power source, I would have been out of luck. I would have needed to preheat.

A little preheat is OK. Preheating to 250 degrees F maximum won’t hurt anything and will drive off water vapor or condensation, which minimizes porosity. But higher preheat temperatures can degrade aluminum’s mechanical properties significantly.

For heat-treatable alloys such as 6061 and 6063, the last heat treatment step is aging at 325 to 400 degrees. Heat treatment higher than this will overage the alloy and reduce its welded tensile strength from the 25 kilopounds per square inch (KSI) normally required to as low as 15 KSI.

If you hold non-heat-treatable alloys at temperatures above 150 degrees for long periods of time, you can sensitize them to stress corrosion cracking.

Preheat can help you weld two pieces of metal with different thicknesses. For example, if you want to join a 3/32-in. piece of 6061-T6 sheet to a 3/8-in. A356 casting, the thinner sheet will melt before the thicker casting. Preheating the casting to 200 degrees makes it easier to weld them together.

Within limits, preheating can be helpful, although it’s usually not required. Just don’t think that if 200-degree preheat is good, 600 degrees must be three times as good. It doesn’t work that way.