Hi Thrutraffic,
First of all I would have to have one of those units, Which I don't.
Who knows what type of ali it is, I don't do you? I doubt the welder did.
Second I have lots of ali projects here take a look.
Next if you use a grinder that has been used on steelwork you have just contaminated the ali surface. The sections are thin and grinding 1mm off would have weakened it.
Third take a good look at the weld, is there black soot? That is not good.
Forth and most important did he cure the disease or just treat the symptoms. If this unit is getting loaded in some unusual way than he should not fix it unless the original fabricator has agreed to its mod. Then gusset plates or struts could be added to solve the issue.
Work place accidents are big money for lawyers and insurance companies not to mention the poor Joe that gets hurt.

Please post your ali welds and may be someone here can help you on your journey.