That's a great picture of something in welding called "cold lap".

Basically, there was no fusion with the base material on one side of the part. This can be due to a number of things, but in this case it was probably due to either too low of a current being used or a bad grounding set-up.

Bad part fit-up can also cause this, but in this case it looks like there was a pretty tight fit between the parts interfacially.