Research On Grinding Mechanism Of Hardened Cold-work Die Steel Based On Single Grain Cutting

Hardened cold-work die steel is widely used in the die manufacturing industry due to its high hardness and good wear resistance. Grinding is an important process to improve the surface quality and geometrical precision. But AISI D2 is a typical hard-to-cut material. Burn and white layer usually occur in the ground surface or sub-surface during AISI D2 grinding. And micro-crack and residual tensile stress in the ground surface greatly influence the product performance and life. Therefore, it is necessary to deeply study the grinding mechanism of AISI D2 steel. Combining with the measurement and modeling of grinding surface topography, the cutting mechanism of single grain is related with the grinding forces, grinding temperature and ground surface integrity during AISI D2 grinding. It reveals the relationship between grinding conditions and surface integrity, which provides the base of surface quality control and grinding process parameters optimization. The main contents in this study include:(1) Modeling of grain distribution on the grinding wheel surface based on the measurement of grinding surface topography. Wyko NT9300 white light interferometer was employed to measure the surface topography of alumina grinding wheel with different grain size. The power spectral density (PSD) analysis and Fourier filtering was used to remove the high frequency component of the measurement results and white noise. The characterization of wheel topography, such as grain density, grain protrusion height, grain shape and sharpness, were characterized through the employment of "Birmingham set" three-dimensional (3D) surface characterization parameters. The statistical normal distribution of grain protrusion height of alumina grinding wheel topography model was built.(2) The modeling and verification of finite element models of single grain cutting processes. The mechanistic model of single grain cutting was built based on the physical model of single grain cutting. The finite element model of single grain cutting was built including the grain geometrical model based on the measurement of grinding wheel topography, material model based on Split Hopkinson Pressure Bar (SHPB), friction model of alumina grain against AISI D2 steel. The material model of AISI D2 steel was verified by the orthogonal turning experimrnts. The FEM model of ball plate friction has been built to compare with the experiments, which validated the FEM model of single grain cutting process. The influence of grain size and cutting parameters on material deformation, plowing-cutting transition critical depth of cut, cutting forces (ratio), chip thickness and shear angle were studied by the aid of single grain cutting simulation.(3) Orthogonal array and single factor experiments of hardened cold-work die steel AISI D2 grinding. The orthogonal experiments of AISI D2 grinding were performed to build the empirical model of grinding forces with grain size of 60# and 80#. The analysis method of variance was used to analyze the influence degree of grain size and grinding parameters on grinding forces. The single factor grinding experiments were performed and the influence of grain size and grinding parameters on grinding forces (ratio) were analyzed by the aid of conclusions from single grain cutting mechanism.(4) Prediction and verification of grinding forces based on simulated single grain cutting forces and grain distribution on the grinding wheel surface. It supposed that the material removal volume in the given grinding time was equal to the total volume removed by all the single grains in the contact zone. Based on the normal distribution model and grinding parameters, the depth of cut of each single grain was calculated. The related single grain cutting forces were integrated to predict grinding forces. The experimental grinding forces showed good agreement with the predicted grinding forces.(5) Prediction of grinding temperature and study on the ground surface quality. The influence of grinding process on the workpiece materials was similar to the influence of temper process. The micro-hardness of surface/sub-surface in the different temper temperatures was obtained. Comparing with the micro-hardness of surface/sub-surface on the ground surface, the average grinding temperature was predicted. The surface integrity of AISI D2 grinding was analyzed by white light interferometer, metallurgical microscope, micro-hardness tester and X-ray residual stress tester. The generation mechanism of ground surface integrity was presented based on the single grain cutting mechanism. It revealed the influence rule of grain size and grinding parameters to the surface integrity.