Impact of power modulation on weld appearance and mechanical properties during laser welding of AZ31B magnesium alloy

Abstract

A new laser welding process with power modulation was proposed to promote the weld formation and improve the quality of welds over magnesium alloys. The impact of the parameters of power modulation on laser welding was investigated experimentally at the aspects of weld formation, keyhole entrance, molten pool flow, and microstructure and mechanical properties of welds. The experiments were performed for the butt joints on AZ31B magnesium alloy sheets with a thickness of 3 mm, and the results shown that the laser welding with a sinusoidal power modulation increased the melting volume and improve the efficiency of energy coupling in comparison with that with a constant power. Power modulation in laser welding could minimize even eliminate the underfill over the top surface of weld. The higher the frequency of power modulation was, the longer the columnar grains formed near the fusion line of weld. With an increase of the amplitude of power modulation, both of the cell grains and columnar grains were decreased in width; while the average size of equiaxed grains was increased. Power modulation in laser welding improved the tensile strength and the elongation of magnesium alloy at welds; when the power was modulated with the frequency of 200 Hz and the amplitude of 300 W, the maximum tensile strength was 237 MPa, and the ultimate elongation was 7.26% and the weld was broken by a mixed ductile–brittle fracture.