Microstructural evolution in ultra-precision grinding of Al/SiCp metal matrix composites

Silicon carbide (SiC) particle reinforced aluminum (Al) metal matrix composites (Al/SiCp MMCs) have been utilized in many engineering applications because of their superior properties. However, poor surface integrity is normally induced in conventional machining of Al/SiCp MMCs due to the heterogeneous constituents, which substantially undermines its performance and service life. This is because although a high surface quality with excellent surface finish is achieved, subsurface damage (SSD) of machined workpieces is usually neglected in conventional machining of Al/SiCp MMCs. To preserve the surface integrity of Al/SiCp MMCs, this study performs ultra-precison grinding (UPG) to explore speed effect on the microstructural evolution and material removal mechanisms of Al/SiCp MMCs. Surface morphology observation reveals that grinding scratches, grinding chips, surface pits and side flow are the major characteristics due to broken SiC particles and drastic plastic deformation caused by the grinding processes. Although an increased grinding speed does not induce significant difference to the morphology of ground surfaces, it leads to distinct microstructural alterations and substantially reduced depth of subsurface damage. SSD characterization reveals that the ground subsurface is characterized by a topmost hybrid layer and an underlying plastic deformation layer. Continuous dynamic recrystallization is determined as the mechanism of Al grain refinement based on the features of dislocation arrays in the refined Al grains. Strain-rate effect plays a dominant role during Al alloy matrix deformation in UPG because of the reduced plastic deformation in comparison to the workpieces subjected to conventional grinding. The reduced SSD depth in UPG of Al/SiCp indicates a high surface integrity, revealing a result of damage skin effect. This study indicates that the damage skin effect is widely applicable in grinding of Al/SiCp MMCs at an increased grinding speed through mitigating the plastic deformation of the ductile matrix.