It is great that you caught this when you did. As for how they break, it happens when the engine heat and expansion exceeds that of the tensioner casting. The head grows at one rate and the tensioner casting at another (since its exposed to cooler air and under much less heat due to combusion transfer). I suspect that the only way to weld one of these is to do it while it is properly torqued to the head, otherwise you risk the two pieces being slightly skewed after welding. If even slightly mismatched, they will be under stress once torqued to the flat mating surface on the head.
I've been welding for most of my life. Personally, the only way I see this happening would be from possible contamination or Cooling irregularities in the casting of the peace due to the thickness. OR excessive pressure put on that ear when tightening it down. I Could be wrong however..
