Influence of laser beam variables on AZ91D weld fusion zone microstructure

Abstract

The influence of the laser beam variables on rapidly solidified magnesium alloy AZ91D weld microstructures was investigated using a continuous 1.2 kW Nd-YAG laser. To generate two- or three-dimensional heat flow thermal cycles, 2 mm thickness specimens were bead on plate welded using a focused 0.7 mm beam, 600 and 900 W power, and travel speeds from 10 to 110 mm s^-1. The fusion zones were examined via microscopy, electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD) and tested for microhardness. Fusion zone dimensions were first measured and correlated with laser beam variables, and then used to estimate weld solidification times from well known formulations. Measurements showed that fusion zone hardness increased considerably for short solidification times (of the order of 15 ms). Hardness was lower near the fusion line, but greater where the last liquid solidified. Using EDS and XRD it was demonstrated that short solidification times led to greater fractions of redistributed aluminium in the last liquid left at the weld surface when heat flow was three-dimensional. Hardness was correlated not only with β-Mg₁₇Al₁₂ fractions, but also with finer microstructures, consistent with the Mg-Al phase equilibria and concepts of solidification.