Aluminum-Copper Solid Solution Hardening

Solid solution hardening means you dissolve copper in the aluminum lattice. The copper atoms stretch the lattice and block dislocations, making the material stronger. When welding, this is a problem: The heat destroys this state, the structure becomes soft. When welding such alloys, you have to keep the heat low and, if in doubt, re-harden afterwards. Otherwise, you'll have a soft spot at the weld that will break under load.