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when i weld cast aluminum i blacken the part with acetylene soot . light the torch with just the acetylene on do not turn the oxygen on wave the torch on the part should turn black then turn the oxygen on preheat the part the black will vanish when it hot enough then weld away learned from a old buck with a funny hat and moustache
Water can get trapped in the aluminum oxide and cause hydrogen when you weld on it.
Heres a couple more things ive found happening to me.
1. Check to make sure your tungsten isnt bending. If u weld at too high of amps for the size ur tungsten can bend. Then the heat from it will melt ur filler before it gets gas coverage and put contamination into the weld. Usually when this happens u can see the black speckles come from the filler.
2. Try a new torch head. I had the problem u are describing one time at work. I traced it down to the torch. Once i replaced that it went away. I think there was a small leak inside the head that was putting coolant in with the gas. It wasnt enough to drip but was messing up my welds.
3. I know this is pry a dumb one but ive seen it alot. Make sure u have a clean tungsten. Ive seen people touch it then keep on welding. It will melt off and contaminate the weld. Once i touch anything the slightest bit i stop and resharpen. If u do this trust me u will get sick of sharpening and get real good at not touching your tungsten.
this may or may not be of any use. I made the clipboard project the other day,
This was a super sized one. I wanted the edges rounded smooth with a bead look.
So I went around the whole thing 99% no filler, I was seeing pinholes develop behind the puddle as the arc was moved forward. I could let off the heat some and move back onto the pinholes and rewet it just a little then move forward easy and the pinholes would fill in.
Like maybe not enough heat to start with or moving to fast for no filler. Porosity is a pain to stop or get away from but if it bad technique, I believe it can be controlled by dealing with it as it happens.