That's twice you mentioned the grinding dust... Now my question to you is this... Are some welders in the same perimeter of where the Aluminum welding and grinding is, welding and grinding steel? If so, that could be a source of contamination especially if some of the steel particulates from the grinding is falling on to the Aluminum surface and being smeared all over the weld zones where it's no longer visible... The area's where steel is being fabricated and welded should be separate from each other...
Still, I have to agree with Lawrence stating that your problem is most likely process related and he provides an excellent and descriptive set of points to consider... Too many of the points he has described are correct and makes sense as well as not to be overlooked in determining the root cause(s) of your problem.
Respectfully,
Henry