Micromachining imposed subsurface plastic deformation in single-crystal aluminum

Mechanical removal of metal induces deformation and changes to microstructural characteristics of the newly created surfaces. The mode and extent of deformation can be difficult to predict since it depends on the local crystallographic orientation, which varies significantly for polycrystalline metals. In this work, we analyzed the deformation mode and extent beneath machined surfaces of different crystallographic orientations. This was accomplished by orthogonal micromachining of single-crystal aluminum along six different crystallographic orientations orthogonal to the sample [111] zone-axis, followed by electron backscatter diffraction (EBSD) analysis to evaluate the resulting subsurface microstructure and crystal lattice rotation. The results indicate that differences in the initial material crystallographic orientation produce significant variations in the depth of deformation (compared to the uncut chip thickness), the degree of grain refinement and the extent of lattice rotations. We grouped the orientation as "hard" or "soft" based on the measured cutting force. The soft orientations exhibit deformation modes consisting of shear bands and lattice rotations; whereas hard orientations exhibit deformation modes consistent with strain hardening: localized dynamic recrystallization, highly entangled dislocations and minimal crystal lattice rotations. The depth of subsurface deformation for some orientations was extensive, reaching depths far greater than the uncut chip thicknesses. Overall, we conclude that the cutting force required to machine a given orientation does provide some insight on the local deformation mode, and orientations can be easier or harder to machine based on local susceptibility to shear and lattice rotation.