Papers by Author: Min Jie Wang

Abstract: Some typical kinds of PCD and PCBN compacts are selected to be machined by WEDM, and a series of processing tests are taken. After machining, the surface roughness of cutting section, the processing quality of cobalt-rich interface layer and the edge of superhard material layer are measured by surface profiler and 3D microscope. The results show that processing quality is affected by superhard particle size and concentration greatly, and better processing quality can be obtained after several cutting of WEDM. The minimum sharpening allowance of PCD cutting tools can be controlled within 4~15μm after WEDM, and within 10μm for PCBN BNX20, while BZN6000 needs larger follow-up workload of sharpening.

Abstract: The adiabatic shear, which may produce serrated chip, usually occurs for a large number of materials in high speed machining. Adiabatic shear band is an important damage model for metals under high-velocity deformation process. The damage evolution of micro-voids in adiabatic shear bands resulted in material fracture finally. Now the thermal softening effect, the strain rate harding effect and the strain harding effect have been discussed extensively in literature, but there is very little research on its damage effect. Based on the experiments of predecessors, this paper presents a new damage evolution equation that is dependent on strain, strain rate and is suitable for the description of voids damage evolution in adiabatic shear band. The corresponding rate-dependent constitutive relation taking account of damage evolution and temperature are proposed. The predicted results are in good agreement with the experiment datum.

Abstract: Orthogonal cutting experiments of Fe-36Ni invar alloy are performed to investigate the influence of cutting conditons on adiabtic shear, which occurs in the process of chip formation of many materials. It is found that the cutting speed, cutting depth and rake angle all have influence on adiabatic shear and there is a critical cutting speed at which the adiabatic shear appears. By metallurgical observation, the critical cutting speed under different cutting depth and rake angles are given. A model based on linear pertubation analysis is used to predict the adiabatic shear critical ctting conditions of Fe-36Ni invar alloy. The comparison of prediction results and that of expriments shows that this prediction model is available.

Abstract: A model is presented for the prediction of critical cutting conditions of adiabatic shear in orthogonal high speed cutting. Considering the influence of compression stress, a critical criterion of adiabatic shear in orthogonal high speed cutting is given by linear perturbation on the compress-shear deformation continuum mechanics basic equations of the primary shear zone. Combining with the relationship of cutting conditions and deformation ones, also the materials constitutive realtionship, a pridiction model, which expressed by cutting conditions and materials properties is established. As an example, the adiabatic shear critical conditions of AISI 1045 steel are predicted, and cutting experiments are performed. The prediction results are consistent with that of experiments.

Abstract: The chip morphology and the formation and development of the adiabatic shear band within the serrated chips formed in high speed cutting of 30CrNi3MoV steel with two tempering hardness were observed and analyzed using optical microscope and SEM. The investigation shows that as the adiabatic shear phenomenon occurs and develops, the chip morphology changes as follows: ribbon chip→serrated chip with deformed band→serrated chip with transformed band→fractured chip. The cutting speed and tempering hardness is the two main factors affecting adiabatic shear, in the case of lower cutting speed the formation and development of adiabatic shear band are more sensitive to tempered hardness increase. The deformed shear bands are constituted by large deformed microstructure, while the formation of the transformed shear bands has experienced the large plastic deformation and grain refinement.

Abstract: The microstructure observation and microhardness measurement were performed on the adiabatic shear bands in primary shear zone in the serrated chips formed during high speed machining of two tempering hardness of hardened high strength steel under different cutting speeds by optical microscope, SEM, TEM and microhardness tester. The investigation results show that two types of adiabatic shear bands are formed as cutting speed increases. One is deformed band with heavy elongated microstructures generated under lower cutting speed, another is transformed band with fine grains under higher cutting speed. The increase of the cutting speed little influences on the microhardness in the transformed bands, and the microhardness in deformed band results from strain hardening, whereas transformation hardening leads to very high microhardness in transformed band.

Abstract: The rheological characteristics of four polymer melts (PS, PMMA, PP and HDPE) are investigated by a capillary rheometer with the die diameters from 1.5mm to 0.5mm.The effects of temperature on shear viscosity and the effects of both temperature and shear rate on non-Newton exponent of melts are discussed when the die diameter is 0.5mm. The results show that the shear viscosity of four polymers decreases with the rise of shear rate. The shear viscosity of PS and PMMA increases with the decrease of die diameter, the shear viscosity of PP and HDPE decreases with die diameter and minute differences of shear viscosity with different die diameters are observed with the growth of shear rate. When the die diameter is 0.5mm, Arrhenius equation is found to be suitable to describe the relations between shear viscosity and temperature for four polymer melts. The non-Newton exponents of four polymer melts increase with the temperature and decrease with the rise of shear rate.

Abstract: Compared with hot embossing, microfluidic chips injection molding is higher efficiency process and more suitable for mass production, but the quality control for injection molding is much more complex. Experiments indicate that the incomplete replication of the micro-channel and the sink mark for microfluidic chips are the chief defects to the molding. Simulation and theoretical analysis show that the stagnant flow of the melt in micro-channel and the shrinkage difference of the chips in different directions are the main reasons for molding defect. A set of new methods that how to control process parameter, design mold, and select polymer material is proposed to reduce or avoid the defects.

Abstract: A method was presented for calculating the temperature distribution of primary shear zone in orthogonal high speed cutting based on the non-uniform volume moving heat source. The temperature distribution of primary shear zone in orthogonal high speed cutting was calculated by the dynamic plastic constitutive relationship and the distribution of strain and strain rate of primary shear zone. The results show that the temperature distribution of primary shear zone is uneven, from the original plane to the cutoff plane, the cutting temperature increases continuously. In the middle of primary shear zone, the change of cutting temperature is larger, at the position near to original plant and cutoff plane, the change of cutting temperature is smaller. The cutting temperature increases with the increase of cutting speed and cutting depth, but decreases with the increase of rake angle. The comparison with existing method shows that the method presented in this paper is not only available, but also simple, convenient and more accord with the fact of orthogonal high speed cutting.

Abstract: The metallurgical observations of microstructure characteristics of the adiabatic shear bands(ASB) within the primary shear zones of the serrated chips produced during high speed machining high strength steel have been performed by using optical microscope, SEM and TEM. The observations showed that the microstructure between the matrix and the center of the ASB gradually was changed, the fine equiaxed grains appeared with size of about 0.4～0.6μm in the center of the adiabatic shear band. The serrated chip formation was likely due to material softening that occurred in the primary shear zones. The microstructural development of dynamic recovery and rotational dynamic recrystallization is the dominant metallurgical process leading to material softening in primary shear zone during high speed machining. A model of microstructural development in primary shear zone during serrated chip formation in high speed machining was suggested by analyzing material softening mechanism.