Whats making my weld look like this, metal impurities?

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Is there something inside this cooking out while I'm welding, and its forming pockets? This is an ATV engine sidecase that has been exposed to God only knows what during its lifespan. You'll notice that in the middle, where my filler is at, the pockets are not there. Only around the edges where the filler and original material meet. I've also been told I might have a leak somewheres inbetween my argon tank and my tig torch which is letting the gas become contaminated. The welder is a Miller Dialarc HF, the material is mostly some sort of casted aluminum.  What do you guys think?
Reply:No one has any ideas?
Reply:looks like to me that u didnt allow the cast aluminum to melt all the way i have done alot of cast and it always pops till i get it all to melt together adding filler to it at the same time.
Reply:So I probably rushed it, and didn't wait long enough for it to flow? Hmm...I'll keep that in mind.
Reply:Castings like these are always going to be a bit of pot luck. Unfortunately there is no way of knowing what alloying elements have been added during the manufacturing process. Often in these castings a lot of zinc is added to get it to flow in tight areas easily and zinc is the main problem for us in casting repairs. I use either 4043 5% silicone or 4047 10% silicone rods for casting repairs, 4047 if the casting isn't welding nicely. Before welding I groove out the crack using a die grinder with a tungsten cutter and then clean alongside where I am welding both sides of the groove with the die grinder as well.Clean out the groove thoroughly with acetone and then wipe dry and preheat till hot to touch (just a bit more than you are happy to hold with your hand)Try 4043 1st, if no good then try 4047, if still no good try a buffering run of pure aluminium then if it's ok when you have a buffer to the casting change to 4043 to finish.Sometimes even pure aluminium won't even do it and you're stuck with whatever you can get as far a porosity goes, sometimes it won't weld at all.Another tip: Sometimes you will lay a weld that seems ok but as it cools you can hear it cracking quite loudly, in these cases they are usually magnesium castings which will need magnesium filler wire to weld.Remember that getting the weld zone clean is one of the most important issues for casting (and any aluminium) welding.Regards Andrew.We are Tig welders, gravity doesn't worry us.Miller Dynasty 350. 350 amps of awesomeness.OTC AVP 300 hybrid wave AC/DC inverter tig. (now retired)Kemppi MLS 230 amp for home, a sweet little AC/DC Tig
Reply:Actually 4043 is what i was using there. The parts were cleaned till the surface was shiny with with a sanding flap on an air die grinder. I did not go over it with a solevant, but I was pretty careful to try and not touch it with bare hands in that area. I'll try some different rod next time and wipe down the surface with some acetone and see what I can get. Thanks for the suggestions.
Reply:You MAY be able to just weld it slower, giving the impurities more time to burn out...and the pockets of gasses more time to come out.  You may also find it necessary to leave your weld on and not grind it down.  By layering it, you will be able to close up any porosity, since you'll be welding filler metal on filler metal instead of on the dirty base metal.Proud to be a UNION worker.  Better pay, better benefits, better work environment.  UA Unions = working for the working class!!
Reply:hey goat,  you know aluminum and carbon dont mix right when it comes to welding.  engine oil is what?   Carbon.  And castings are extremely pourous.  Therefore you must really bake that carbon out.  Ive heard of people heating with rosebud and using kitty litter to clean it up.  Whatever you do you just have to totally burn that crap out first.  You'll never get it all out, but you do your best. Thats what your seeing though, its from the oil.   Just gotta get it cleaned out better.  Sorry for the bad newsCHRIS