The effect of welding speed on microstructures of cold metal transfer deposited AZ31 magnesium alloy clad

Highlights: • CMT process could be used for stable cladding of AZ31 magnesium alloy. • AZ31 magnesium alloy is sensitive to pores and liquation cracks during welding. • Local melting in HAZ was inevitable in CMT cladding of AZ31. • In HAZ eutectic α-Mg contained more Mg but less Al than α-Mg due to local melting. - Abstract: Magnesium alloys are sensitive to heat input and susceptible to grain coarsening, pores and hot cracking during welding. Cold metal transfer (CMT) with low heat input exhibits a great potential for magnesium alloy welding. In this paper, the effect of welding speed on microstructures of CMT deposited AZ31 magnesium alloy clad (when wire feed speed was 4 m/min) was investigated. A high-speed camera was used to observe the arc characteristics. And optical microscope, scanning electron microscope and energy dispersive spectrometer were employed to characterize the microstructural evolution. The results demonstrated that, the fusion zone (FZ) consisted of fine dendrite α-Mg and dispersed β-Mg₁₇Al₁₂. Pores were found in FZ at a higher welding speed of 14 mm/s. Grain coarsening, precipitation of β-Mg₁₇Al₁₂ and local melting were witnessed in the heat-affected zone (HAZ). With the decrease of welding speed, β-phase accumulated and formed the shape of band at grain boundaries, and the HAZ became more susceptible to the liquation cracks. To get a qualified cladding of AZ31 magnesium alloy, pulsed-CMT, with an optimal welding speed of 12 mm/s, was preferred.