MTS250S Weld Cast Iron and Aluim.

[Kempy]

The reason the black soot is left is because the Helium is virtually leaving the weld area right after it is applied. That's the down side to helium, no one talks about. It's lighter than air, and as soon as it exits the gun, starts to float up and off. Argon is heavier than air, and stays where it is put when it is done down hand. The black is oxidation actually.

I modified a nozzle to get the flow on the weld better, it did help the black. I made no changes to the settings just the nozzle would have to do more test to get the weld better. Maybe I will try welding upside down and let the helium work the way it wants.
The argon does make a nice weld and cleaner. It only took a few swipes with a wire brush and it was clean.