Plastic deformation and grain refinement in surface layer induced by thermo-mechanical loads for hard milling process

Abstract

Microstructure alteration of machined surface induced by the coupled thermal and mechanical loads in the hard milling process has great influence on surface integrity. Hence, it affects the performances of the workpiece. The hard milling experiments of AISI H13 steel with different cutting speeds and feed rates were carried out and the microstructure evolution of machined surface was investigated by means of some advanced characterization methods. The experimental result indicates that plastically deformed layer was evident with three distinguishable zones, that is, unaffected zone, plastic deformation zone, and white layer zone. X-ray diffraction analysis revealed that no retained austenite is detected in the white layer. Numerous nano-sized equiaxed grains were induced by dynamic recrystallization (DRX), as a result of severe shear plastic deformation on the machined surface. As the cutting speed increases, the grain refinement becomes more obvious. However, the original cementite was greatly refined and well distributed compared with bulk material. A significant improvement in nano-hardness was witnessed in both for machined surface and subsurface. The grain refinement as well as well-dispersed cementite played a vital role in the improvement of nano-hardness. The varied depth of plastic deformation was observed with respect to change of cutting forces. It is the main source of mechanical load inducing plastic deformation with the assistance of material thermal softening. The present work can provide more visibility on the surface integrity along with the quality control of the workpiece.