Halo or Distortion after Weld

I am having an issue where we weld on the back side of the aluminum, no penetration at all, fully grind and prep the surface with a satin finish. But when it comes back from anodizing, on some pieces there is a halo or distortion on the face; but not on every piece. Can you explain and help?

When you're welding as you well know, you're heating the part you're welding to. If the aluminum gets too hot you can cause the alloying elements in the part to migrate and this can cause the distortion you're describing.

So the reason it shows up on some parts and not others is because the aluminum is being heated more on those parts where it does show up.

You could probably remove the distortions with an Acid Etch. Of course, since you don't know what parts it's going to show up on before their anodized, that could present an issue. I suppose you could talk to whoever is doing your anodizing and see if they have an acid etch to test a part with.

My company sells an Acid etch they could use, but they very well might already have one or their current provider may be able to provide them with it if they don't.