Aluminum Workshop: Using 4043, 4643 when welding 6061-T6

Q: Most 6061-T6 is welded using 4043 filler wire. If you heat-treat the weld, will the 4043 get stronger? What happens if I use 4643 filler instead of 4043?

A: Is the weld heat-treatable? Well, yes and no. Remember, the first step to producing T6 temper is heat treating at around 1,050 degrees F to put all the alloying elements in solution and then quenching the material to keep everything in solution. The second step is a low-temperature aging.

To answer your question, most welds in 6061-T6, whether made with 4043 or 4643, are heat-treatable to a degree. This happens because enough 6061-T6 is melted in the weld so that magnesium from it is added to the filler and makes it essentially an Al-Si-Mg alloy, which is heat-treatable.

However, if all you do after welding is perform an additional aging treatment, the weld won’t increase in strength. In order to approach the unwelded properties of 6061-T6, you need to perform a complete heat treatment—that is, solution heat treatment, quenching, and aging.

Remember, the 4043 is becoming heat-treatable because of weld dilution from the 6061. In welds in thin material, say, 3⁄8 in. or less, there usually is enough weld dilution to make the weld heat-treatable. But what happens in thicker materials? Multiple passes are needed for each weld layer, especially near the top of the weld. In a case like this, the weld dilution is normally low in the passes toward the center of the weld (that is, away from the weld fusion line), and the weld won’t contain enough magnesium to make it heat-treatable.

The 4643 filler is essentially 4043 but with enough magnesium added to make it heat-treatable. It is designed for 6061 weldments, especially those thicker than about 1⁄2 in. that will get a complete heat treatment after welding. It will make welds in thinner 6061 heat-treatable as well. However, 4643 has no benefit over 4043 if the weld will not be heat-treated or if it will receive only an aging treatment.