Waq,

Those are all very good and important questions.

But they are also very high level and complicated questions that really can't be properly answered here.

Please let me suggest a book that I recommend as a place to start.

"Welding Aluminum Theory and Practice" by the Aluminum Association.

I also suggest that while you study this, you hire an engineering consultant who is a welding specialist to help you with your current challenges.

http://www.browntechnical.org/products/welding-aluminum-theory-and-practice.html?pa=sc7125

Waqas-

some good info....
http://awo.aws.org/wp-content/uploads/2014/weldcrackingconference/Anderson-CrackinginAluminumAlloys.pdf

Thanks,
Jarhead1