Abstract: In arc welding of aluminum, a mixture gas, with argon, helium and N₂,can improve the welding process. In comparison to the pure argon MIG welding, the arc is more concentrated and allows a higher application of energy, which increases the microstructure of weld bead. Welding thermal cycle curve of Ar-He-N₂ gas shielding metal arc welding and argon metal arc welding are measured, which the peak temperature is higher, the cooling rate is faster than the former. A staining method is used for the microstructure analysis. In comparison to argon metal arc welding, the grain size is larger and porosity is lower in build up welding. In comparison to argon metal arc welding, the partially melted zone is narrower in butt welding. The results of experiment show that the characteristic of welding temperature field is the basic reason for promotion of welded joint mechanical properties.