Two-pass laser welding of AZ31B magnesium alloy

• Decomposition of magnesium hydroxide was considered as a main mechanism causing pore formation in the weld bead.
• Two-pass laser welding could effectively mitigate pore formation in the weld.
• Calculated electron temperature could be correlated to defect formation in the weld bead.
• Tensile test results revealed that two-pass welded samples have a higher value in comparison with single-pass welded samples.

A two-pass laser welding process was applied to AZ31B magnesium sheet in a zero-gap, lap-shear configuration. The first pass decomposed the magnesium hydroxide into magnesium oxide and molecular water while the second pass, which was designed for keyhole welding of the magnesium, provided a path for the vaporized water to escape and thereby producing a pore free weld. Two groups of samples including one pass laser welding (OPLW) and two pass laser welding (TPLW) were studied. In the two pass laser welding procedure, the first pass was performed by a defocused laser beam on the top of the two overlapped sheets in order to preheat the faying surface prior to laser welding, while the second pass was applied to melt and eventually weld the samples. The need for preheating using a defocused laser beam was related to the decomposition of magnesium hydroxide that was the cause of pore formation in one pass laser welding process. The chemical compositions of the welds and metal sheet surfaces were evaluated using an energy dispersive spectroscopy (EDS). The presence of the oxide layer on the faying surface of two overlapped sheets resulted in an unstable process. Tensile and microhardness tests were used to measure the mechanical properties of the laser welded samples. A spectrometer was used in real-time to correlate pore formation with calculated electron temperature using the Boltzmann plot method. The experimental results revealed that a two pass laser welding process could effectively mitigate gas pore formation at the faying interface of two overlapped sheets.