[Tim Hancock]

Check the back side to see if you are getting penetration. You will still see the joining line, but it should be melted there and will not be a crack anymore. If an actual crack is still there, you did not penetrate enough, at least when you are welding thin stock. Also try to break your sample, if it breaks right where the original crack is, you did not penetrate well enough. I had to practise alot before I could trust my tig welds on >050 aluminum aircraft fuel tanks. Initially I thought it was fairly easy and my sample welds "looked" awesome, but I soon found that I was not getting full penetration and the welds would easily fail at the joint. On thin stock you should actually see a slight bulge on the backside to be sure you actually penetrated.

All this info should translate to mig welds as far as penetration goes.
Cleanliness is critical and it looks like your sample wall cleaned well. Just make sure you use a dedicated alum only stainless steel wire brush for a final clean right before you weld. It is also desirable to clean your tack weld before finish welding as there will be impurities in the tacks you just laid down.