welding 7000 series alu

[Dingo2024]

Hi, I am in the process of starting my second build using an old Schwin mtb frame which is 7005 alu. The top tube needs to be extended to increase wheelbase and the plan is to use a billet of 7075 (I think) cnc'd headtube. Also need plates welding onto the frame to support batteries and motor mount. The question I have is;

the frame is/has been heat treated but the billet (and plate)is not.... is it best to anneal the frame (and any other heat treated stuff) to return it to a 'soft state' before welding and then reheat treat everything after fabrication

if this is the case what is the temps / timings to anneal 7000 series

any problems joining 7005 to 7075

any suggestions/pointers greatly appreciated

thanks

Ian

 

[rocwandrer]

While it can be done it is generally accepted that you don't weld 7075. The effort required and or the resulting joint properties are almost always bad enough to negate the structural benefits of 7075.

 

[speedmd]

Good Link for aluminum alloy tubing. http://www.eastoncycling.com/bike/wp-content/uploads/2010/04/fab_instructions-7005_6061.pdf

I would keep the tubes as is, and not soften them unless you are bending them in your build. I would think that solid machined 7075 is a bit overkill for a head tube. May find it suitable to go with a slightly thicker aircraft grade tube (6061t6) that joins well with the correct filler rod.