Grain refining of magnesium welds by arc oscillation

Grain refining is known to improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction. The present study was conducted to grain refine Mg welds by arc oscillation, which has not been investigated so far. First, significant grain refining was demonstrated by transverse arc oscillation. The effects of oscillation amplitude, oscillation frequency and torch travel speed on grain refining were shown. The effect of the alloy composition on grain refining was also demonstrated. Second, by using an overlap welding procedure, the grain refining mechanism was identified as dendrite fragmentation. Third, cooling curves recorded during welding showed that transverse arc oscillation caused reheating during solidification, which has been shown in casting/solidification to cause dendrite fragmentation by melting off dendrite arms. The cooling curves also showed that transverse arc oscillation significantly reduced the temperature gradient G along the torch travel direction, which suggested constitutional supercooling was increased. Thus, transverse arc oscillation not only caused dendrite fragmentation but also increased constitutional supercooling to help dendrite fragments survive and grow into fine equiaxed grains. Dendrite fragmentation by remelting, instead of mechanical breakup, of dendrites was discussed in the context of welding. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.