Welding intercoolers

I posted this over in r.c.m, but forgot to crosspost. -----
I am trying to weld 6061 and 2024 alloys to my intercoolers, which are
of unknown alloy.
I have an OLD Miller Dialarc HF250. I've replaced my Argon to make sure it's clean. I seem to get a white powder on the surface of the aluminum. I'm using 1/8 inch thoriated tungsten because it stands up to the heat generated by my welder. Pure tungsten eventually spatters into the weld.
I have tried pre-heating my parts, but it's difficult to get things really hot. No matter what I try, it seems like I can't get a decent puddle in the intercooler aluminum. Good puddles in the 6061 and 2024 no problem. This intercooler kind of curdles up, and doesn't clean very well. I've almost destroyed the cores trying to get something going.
My last attempt will be to put the cores in an oven at 400 or so degrees for a couple of hours. I don't know, perhaps they've absorbed water, oil, or some other stuff that extended heating will get out.
Any other thoughts?
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snipped-for-privacy@awpi.com wrote:

To start with 2024 is not a weldable alloy. Too much copper.
You can use a aluminum solder like Alstate #31

I suspect you have an Al-Mag alloy. Not so easy to weld. The Magnesium burns off at lower temps.
Again a aluminum solder would be a better solution. You will still need to preheat the casting.
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