Grain refining of magnesium welds by arc oscillation

Grain refining is known to improve the solidification-cracking resistance and mechanical properties of welds. Effective grain refining of Mg welds was achieved by transverse arc oscillation. The effects of the oscillation amplitude, oscillation frequency, torch travel speed and alloy composition were investigated. The grain refining mechanism was identified as dendrite fragmentation. Transverse arc oscillation caused the weld pool to zigzag (left figure). Cooling curves recorded during weld pool solidification showed that transverse arc oscillation caused reheating (i.e., temperature rise) during solidification (right figure), which could in turn cause dendrite fragmentation by remelting the dendrite arms. The curves also showed transverse arc oscillation reduced the cooling rate and hence the temperature gradient G significantly. Thus, transverse arc oscillation also increased constitutional supercooling to help dendrite fragments survive and grow into fine equiaxed grains.342