Double-Sided Arc Welding of AZ31B Magnesium Alloy Sheet
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Magnesium alloys are of interest to the automotive industry because of their high specific strength and potential to reduce vehicle weight and fuel consumption. In order to incorporate more magnesium components into automotive structures, efficient welding and joining techniques must be developed. Specifically, a method of making butt-joint welds must be found in order to use sheet magnesium alloys in the form of tailor-welded blanks for structural applications. The existing welding processes each have disadvantages when applied to magnesium alloy sheet. The double-sided arc welding (DSAW) process has been shown to produce high quality welds in aluminum alloy sheet, for tailor-welded blank applications. The DSAW process has not yet been applied to AZ31B magnesium alloy, which has thermo-physical and oxide forming properties similar to those of aluminum alloys. Therefore, this research explores the weldability of AZ31B magnesium alloy, using the DSAW process. Experimental, butt-joint configuration welds were made in 2 mm thick AZ31B-H42 magnesium alloy sheet. Acceptable welds have been produced using welding speeds ranging from 12 mm/s to 100 mm/s and welding powers from 1.6 kW to 8.7 kW. The influence of these parameters on the appearance, geometry, mechanical properties and microstructure of the resulting welds was investigated. Optimal appearance, geometric profile and mechanical properties were obtained at the lowest welding speeds and powers. Under these conditions, mechanical properties of the weld metal were equivalent to those of the fully annealed (0-temper) base metal. However, progressive deterioration in appearance, geometry and mechanical properties occurred at higher welding speeds. The deterioration in mechanical properties was associated with 2 microstructural defects that were observed at higher welding speeds: 1) the formation of larger amounts of Mg17Al12 -phase particles, at the grain boundaries, and 2) the formation of solidification shrinkage micro-porosity at these same inter-granular locations. This research demonstrates that the DSAW process is capable of producing acceptable quality, butt-joint welds in AZ31B magnesium alloy sheet at welding speeds up to 100 mm/s. However, in order to achieve the highest quality welds, low welding power, and, low welding speed, should be used. The highest quality welds were produced at welding speeds of 12 mm/s.