Welding aluminum alloy 356

[ewee]

I purchase a part cast from alloy 356 that is received with the T5 temper. The part is approximately 30" in diameter, 10" deep, and 2" thick (hollow in the center). On the inside surface tapped holes are machined for nozzles. The tapped holes intersect the as-cast water channels on the outside of the casting. The water channels are enclosed by rolling 6061-T6 flat bar straps and welding the flat bar on each side to seal the channel so that it is water tight. No pre-heat is used prior to welding and the flat bar is joined to the casting by MIG using 4043 filler metal. The casting is allowed to cool in air. The final processing step involves polymer impregnating the casting to fill any remaining porosity that is not visible.

The problem we are having is with the outer row of tapped holes near the edge of casting that develop a crack in the casting on the sealing surface of the tapped holes. The material thickness that remains from the edge of the tapped hole and the water channel on the outside of the casting is about 3/16". We are considering a design change that will leave more material thickness, however I am wondering if there are processing changes that may help us eliminate the problem without a redesign. For instance, would it be better to receive the casting in the as-cast condition, weld it, and then temper to T5? Thanks for your consideration and looking forward to your reply.

 

[swall]

I have seen T5 castings with high levels of residual stress and that could be the source of your cracking problem. If you machine the casting in the as cast condition, the lower strength condition may allow the material to yield slightly rather than crack to relieve the machining stresses. Then apply the T5 ageing cycle after all machining and welding to get the higher strength. I would try some and see what happens.