When I weld up any material that is cracked, I will use a liquid dye penetrant, and some dye developer to find the very end of the crack, use a small drill, drill out the ends of the crack, then wheld it up. Ive found especially with aluminum, and Cast iron that as it heats up the crack will chase out on you, and just get longer. the drilled hole takes out the stress risor, and eliminates where the chase can start.

In college we would have contests on who can crack a test piece of aluminum in half from start to finish by adding a lot of filler really fast, and the piece will split right down the center and you dont even need a crack to have it start. So aluminum welding can be tricks.

Kasey