Why does welding repair cause second fracture in cold-drawn aluminium tubing?

[hafiz88a]

Homework Statement

Lightweight bicycle had been fabricated from aluminium alloy tubing, with joints made by adhesive bonding. When the frame fractured near one of the joints, the owner requested that a repair be made using conventional gas tungsten arc welding. Shortly thereafter, the frame broke again. This time the fracture was adjacent to the repair weld and the the characteristics of the break were different. The first fracture was somewhat brittle in nature, the second fracture occurred in the tube material, not the weld itself.

Homework Equations

If the material had been cold drawn aluminium tubing (i.e. strain hardened), explain what might occurred during repair. What is the possible cause of the second fracture?

The Attempt at a Solution

Cold drawn products could have internal cracks which the heat from welding causes thermal cracks to occur.

 

[SteamKing]

The fact that the second break occurred in the tube material and not the weld means the weld was not at fault, i.e. the weld was done properly.
The underlying problem is that the aluminum tubing is not strong enough to support the loads being imposed on it.
The owner is going to have to get a stronger bike.