Troglodyte
New Member
- Messages
- 8
- Location
- UK
Looking for advice on welding 7000 series alloy.
I know you shouldn't, but I have to modify some cycle crank arms shortening them from 175 mm to 125mm throw. Can't be sure what the material is but they are forged so likely 6000 or 7000 series, and I've measured the density at 2.8 g/ mm3 so I'm thinking 7075. The section is around 12mm deep x 25 wide at the ends where the original pedal thread is located but reduces to around 6mm deep (U section) where the new pedal thread goes... a bit too thin. My solution is to build up with weld within the U section, back to original thickness. I've done some previously with MIG and that worked OK - the first I did failed after 2 years and 3000 Km, the second is still going strong. But it was fiddly to do with lots of birds nesting (no spool gun) and definitely not pretty as welded, tho' it cleaned up OK and seemed to have good fusion to the parent metal.
Now I have a shiny new TIG to play with (R tech 170 AC/DC) and want to use that, when I've got my technique up to scratch. My thoughts are to use 5183 wire, clamp the crank to a heavy heatsink and build up with several small beads to minimise HAZ. Post weld heat treatment would be a possibility, I do have a muffle furnace with good temp control, though I would need to identify the material more precisely before trying that.
But I'm no welder, just an enthusiastic amateur and wonder what the experts think?
I know you shouldn't, but I have to modify some cycle crank arms shortening them from 175 mm to 125mm throw. Can't be sure what the material is but they are forged so likely 6000 or 7000 series, and I've measured the density at 2.8 g/ mm3 so I'm thinking 7075. The section is around 12mm deep x 25 wide at the ends where the original pedal thread is located but reduces to around 6mm deep (U section) where the new pedal thread goes... a bit too thin. My solution is to build up with weld within the U section, back to original thickness. I've done some previously with MIG and that worked OK - the first I did failed after 2 years and 3000 Km, the second is still going strong. But it was fiddly to do with lots of birds nesting (no spool gun) and definitely not pretty as welded, tho' it cleaned up OK and seemed to have good fusion to the parent metal.
Now I have a shiny new TIG to play with (R tech 170 AC/DC) and want to use that, when I've got my technique up to scratch. My thoughts are to use 5183 wire, clamp the crank to a heavy heatsink and build up with several small beads to minimise HAZ. Post weld heat treatment would be a possibility, I do have a muffle furnace with good temp control, though I would need to identify the material more precisely before trying that.
But I'm no welder, just an enthusiastic amateur and wonder what the experts think?