Cracked right in the middle of the aluminum weld

[froader]

If you made a simple fillet weld with the to pieces mated inside edge to inside edge, the center of the weld could have been very thin, if there was incomplete penetration and not enough filler was used.

With some thicker material this would be a good idea.

Your weld lacks penetration. You have managed to only create a bead on the surface. probably using the corner of each piece of metal as filler. Just to note your bung weld is too hot and not enough filler. You need to slowly end the weld and add filler at the very end so you don't leave craters, which are basically a crack.

Make sure to scotch brite the metal inside the joint as well. A wipe with acetone is also a good idea. When I built AL bike frames, we had a scotch brite type of abrasive wheel to clean up the weld area and then dipped it into an acetone container. The cleaner you get the weld and keep your gloves/work area the cleaner the weld will be - that part is up to you.
Alternatively you could make this into a lap joing by putting a 90 on the side piece so the top would sit right on top.