Aluminium black soot mid way through weld

[Logic0101]

I'm welding an aluminium frame at the moment and it's been going great except for one fillet weld that I can't seem to get right.

What happens is it welds perfectly at the start and then mid way, the weld pool has a pin hole in it, I try to add filler but it barely covers it.

So I thought it must be contaminated. I ground the area with a grinding disc and then spray acetone, then stainless steel wire brush the area and start again, same thing happens!

What is causing this? The pin hole area also builds up with black soot and is just awful to weld at that location?

 

[Shox Dr]

You need to dig the hole out to remove all the contamination. Running the torch over it afterwards to boil it out and to the top

 

[Logic0101]

Ok I guess I'll have to cut right through, does the type of cutting disc or grinding disc matter for aluminium? Is there a chance this is causing contamination also?

 

[Cheesie]

Ok I guess I'll have to cut right through, does the type of cutting disc or grinding disc matter for aluminium? Is there a chance this is causing contamination also?

The cheap cutting discs we use will cause contamination, not sure if there is a type that won't, this happened in my last job when they went for cheaper discs than normal rust started showing on the st/st pipework on site, costly repair

 

[Hood]

On aluminium only use discs that are suitable for stainless or in other words don't contain iron.

 

[Mullet52]

Do you have breather holes in the frame?