Microstructure and surface integrity of machined AZ91 magnesium alloy

AZ91 magnesium alloy is one of the most popular materials among all cast magnesium alloys due to its excellent corrosion resistance and good cast-ability. Although AZ91 is categorized as soft materials and easy-to-cut, there are many challenges in the finish machining of AZ91. In this research, a comprehensive as well as novel study on the surface integrity of machined AZ91 magnesium alloy is presented. To this end, AZ91 samples are machined under different cutting conditions and afterward, both surface and subsurface features are analyzed. Assessing the finished surface is conducted by surface 3D topography, roughness measurement, and scanning electron microscope. In terms of subsurface characterization, the effect of machining conditions cutting speed and feed rate on the micro hardness variation, subsurface microstructure changes, plastic deformations, and subsurface defects are studied. Results indicated that when machining AZ91, surface defects mostly are formed around the secondary phases. Moreover, twining and shear bands are the main microstructural changes close to the machined surface. Additionally, finishing machining of AZ91 induced subsurface plastic deformation to the depth of 180 μm as well as micro cracks propagation, in secondary phases, to nearly 100 μm under the machined surface.