The influence of ultrasonic assistance and laser melting on the microstructure and properties of AZ91D magnesium alloy

The study investigates the effects and mechanisms of laser melting and ultrasound-assisted laser melting treatments on the microstructure and properties of AZ91D magnesium alloy. The findings indicate that the grain size of the samples subjected to laser melting and ultrasound-assisted laser melting is decreased to 40.89 mu m (number fraction) and 54.14 mu m (area fraction), and 23.21 mu m (number fraction) and 29.56 mu m (area fraction), respectively. The decrease in grain size can be ascribed to the rapid melting and solidification of the sample surface caused by the high-energy laser beam. This leads to a notable rise in nucleation rate and the development of smaller grains. The effectiveness of laser melting is further enhanced with the introduction of ultrasound. The average microhardness of the samples subjected to laser melting and ultrasound-assisted laser melting shows a significant improvement. Furthermore, the microhardness distribution of the sample subjected to ultrasoundassisted laser melting exhibits greater uniformity. The wear resistance of the samples subjected to laser melting and ultrasound-assisted laser melting shows a significant improvement, as indicated by the friction coefficients of 0.688 and 0.382, respectively. These values represent only 50.7 % and 28.15 % of the friction coefficient of the untreated samples. The improvement of wear resistance can be attributed to the enhancement and homogenization of the microhardness of the samples. The application of laser melting treatment leads to an enhancement in the corrosion resistance of the sample primarily as a result of microstructure densification. However, the introduction of ultrasound does not lead to further improvement in corrosion resistance, possibly as a result of significant grain refinement, which in turn leads to an increase in high-energy grain boundaries.