When the steel from the mig cools down it does so much faster than the cast and contracts, you may hear some tinkling noises coming from the casting as it cools, the noise is cracks forming, the best way to weld cast with mig is to braze it, you need to heat the cast up to over 700degrees and then let it cool down very slowly in an over to save the cracking. BUT mig is by far the best way to weld cast and it should be done properly.