Welding aluminium differs from welding steel in important ways: a surface oxide layer with a very high melting point, high thermal conductivity and greater thermal expansion. With the right process and consumables, high-quality joints are entirely achievable.
The main challenges
- The oxide layer: aluminium oxide melts at a far higher temperature than the metal itself and must be removed before and during welding.
- High thermal conductivity: heat spreads quickly, so more power and the right travel speed are needed.
- Porosity: hydrogen from moisture and contamination causes pores in the weld; clean surfaces and filler wire are essential.
Common processes
TIG (gas tungsten arc welding): with alternating current (AC), which breaks up the oxide layer; high quality for thin, precise parts.
MIG: faster, for thicker parts and volume production.
Choosing a filler wire
The two most common filler wires are 4043 (aluminium–silicon; good fluidity and less crack sensitivity) and 5356 (aluminium–magnesium; higher strength and a better colour match after anodizing). The choice depends on the base metal and the service conditions.
Welding cast parts
Die-cast parts usually weld poorly because of trapped gas, but gravity and low-pressure castings in alloys such as A356 can be welded and repaired. 7xxx-series alloys such as 7075 have poor fusion weldability.