The types of aluminum used in gun frames if NOT considered weldable, tig, mig, micro or otherwise. Does not matter with theses alloies it they were cast or machined from a billet. Notice how airplane bodies and wings are riveted and not welded? . Close to the same alloy. The weld itself is not the problem, the problem is the heat effected zone beside the weld will be very brittle and very prone to cracking and there is nothing that can be done about it, YET. I have heard of a experimental process where they are putting titanium particles in the weld zone to combat this with some success. At this point you would spend more money than a frame is worth to have a sub par frame.

A cracked grip frame could probably be welded and hold up as it is not normally a high stress area.