welding on threaded aluminum

[dannyho]

Hi,
I have a cast aluminum piece that is part of a convertible top mechanism. On one of the ends there's a threaded hole and the piece has cracked across the threads.
In lieu of being able to find a replacement part, welding the thing back together is the next step.
My question is how I should handle the threads.
Should I avoid them all together and weld around it best I can, or should I just go at it and go back and drill and tap the threads again?

A thought I had was to put a steel bolt or some other material in the threads and then weld. Kind of similar to how steel wire won't stick to copper or aluminum when plugging holes. I have a feeling that won't work though.

Thanks
Dan

[astroracer]

Are you sure the piece is cast aluminum? It could be zinc (pot metal). To get the strongest weld don't plug the hole. You need to weld it up solid then drill and tap. IF you can weld it. You may want to look into that Alumaloy stuff.
http://www.alumaloy.net
May work well in this situation.
Mark

[ATOMonkey]

My suggestion, as I'm in the repair business, so take it or leave it...

Drill and tap the hole oversized. (If possible shoot for a .120 ish wall in your plug after final machining)
Then install a threaded aluminum plug into the hole, leave some finishing stock sticking out of the hole.
Grind out the crack down to your plug.
Weld the area that you just ground so the plug and parent material are fused together.
Grind/machine the plug flush with the parent metal.
Drill and tap the hole in the correct location.

This should fully restore the thread and the part.

DO NOT use lock-tite or alodine or anything like that as it will foul the weld like no one's business. When you drill, tap, and grind, that should prep your materials sufficiently for a good weld.

You could try to weld the crack and chase the threads, but I have never found that to work well, and it's just as much if not more work than just doing a welded plug.

[astroracer]

=ATOMonkey;892294]My suggestion, as I'm in the repair business, so take it or leave it...

Drill and tap the hole oversized. (If possible shoot for a .120 ish wall in your plug after final machining)
Then install a threaded aluminum plug into the hole, leave some finishing stock sticking out of the hole.
Grind out the crack down to your plug.
Weld the area that you just ground so the plug and parent material are fused together.
Grind/machine the plug flush with the parent metal.
Drill and tap the hole in the correct location.

This should fully restore the thread and the part.

Sorry, but I disagree. I would rather see a complete solid weld, which becomes part of the parent material, then a plug which is welded only to one side or edge. Welding the hole solid, drilling and then tapping would be a much more robust repair.
Mark

[dannyho]

Thanks, maybe I wasn't clear, but its not just a crack on one side of the threaded hole. It's cracked in half, two pieces. It looks like my plan is to weld it as solid as I can, get it smooth on the side that meets up with the rest of the linkages for the convertible top, and drill and tap a new hole.
Also, its definitely aluminum, not zinc pot metal.
If this were a static piece that didn't move I might consider that alumaloy, but I just don't trust it on something like this.
Thank you guys.

[astroracer]

The alumaloy was mentioned just in case it WAS pot metal. Weld away friend, you are on the right track.

[chevelletiger]

Thanks, maybe I wasn't clear, but its not just a crack on one side of the threaded hole. It's cracked in half, two pieces. It looks like my plan is to weld it as solid as I can, get it smooth on the side that meets up with the rest of the linkages for the convertible top, and drill and tap a new hole.
Also, its definitely aluminum, not zinc pot metal.
If this were a static piece that didn't move I might consider that alumaloy, but I just don't trust it on something like this.
Thank you guys.

A pic would be nice.if its cracked in half i would bevel the edges as much as you can then sew her up.also if your tig welding this which i hope you are, pre heat it first.when thread are really f@%&ed as with manifolds we get wr drill out the old one and make a plug thats drilled then bevel a 45 on each part and tig then machine flush.

[ATOMonkey]

Sorry, but I disagree. I would rather see a complete solid weld, which becomes part of the parent material, then a plug which is welded only to one side or edge. Welding the hole solid, drilling and then tapping would be a much more robust repair.
Mark

Depends on the size of the hole and whether or not you want the part to be straight when you're done. That much weld puts a lot of heat into the part and has a large Heat Affected Zone (HAZ).

Doing a lot of welding also gives you more opportunity for an inclusion that will cause the piece to crack again later.

I've never had a part fail by doing a welded threaded plug. Even now, with it broke in two pieces, I'd still prefer to weld as little as possible. Of course, any kind of drilling and tapping would have to be done on a drill press at a minimum at this stage.

[ATOMonkey]

I would also stay away from alumalloy for repairing anything other than a hole in a boat, as it's a step between brazing and solder and really won't hold any kind of load.

[Jim Nilsen]

How hard would it be to just make a whole new piece? How expensive is it to just get a new/used one?

I too would clamp it together and weld it solid by filling the whole hole, then drill and tap. Putting in a helicoil would also help with the strength.Since this is not a vital saftey issue you should be just fine.

[DRJDVM's '69]

Post a picture....

By the time you weld the whole thing up, drill and tap a new hole etc, wouldnt it be easier to just make it from scratch?