Papers by Author: Xin Zhao

Abstract: In this paper, the comparisons of the polishing performance between prehardened P20 and S45C steels under the different hardness were carried out by using surface grinding, manual polishing and PVA grinding wheel polishing tests. The results show that the surface roughness of the two kind steels decrease with the increase of hardness and the roughness of S45C is lower than that of P20 during the surface grinding test. After polishing process, the surface roughness of the two kind steels decrease with the increase of hardness as well. However, the surface roughness of S45C is higher than that of P20 after both manual polishing and PVA grinding wheel polishing tests.

Abstract: A single-point load will be applied on the ring in the cold ring rolling process when the ring vibrates violently. This arouses hourglass in finite element simulation of the process using reduced integration element. A one-point quadrature element with hourglass control developed by Hu is used, and the user-defined control parameters are not required. Hourglass energy can be reduced to 3 percent of the internal energy. The element is applied to simulation of a pure radial cold ring rolling with rectangular section. Compared with the experimental results, the simulation results are proved to be accurate. Key-Words: one-point quadrature; Hourglass control; finite element; cold ring rolling

Abstract: The warm deformation behavior of as-quenched and as-annealed ASTM 1045 steel was studied by isothermal compression testing on a Gleeble3500 machine. The temperature range was 550-700°C and the strain rate range 0.001-0.1s^-1. Transmission electron microscopy (TEM) was used to study the microstructures associated with the observed deformation phenomenons. The results show that the flow stress of quenched specimens is higher than that of annealed ones at 550°C when strain rates are greater than 0.001s^-1. However, at 600-700°C and strain rate of 0.001s^-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the dynamic recrystallization easily occurs in quenched specimens during warm compression, which results in the above phenomenons. Keywords: warm deformation, flow stress, steel, quenching, annealing

Abstract: Specimens of medium carbon steel were quenched and warm-compressed on a Gleeble 3500 Machine. The microstructure of the specimens was studied by using an optical microscope and a transmission electron microscopy. And the properties were investigated by using tensile tests and hardness tests. Results show that the starting microstructure is lath martensite with a small amount of flake martensite. After 50% compression at 550-650°C, ultrafine grains can be observed in the specimens. The microstructure of the specimens compressed at 600°C is equiaxed ultrafine ferrite grains + nano-carbides and a good combination of strength and ductility is obtained. The tensile strength and total elongation are 861MPa and 19.1%, respectively. The hardness is 233.81Hv.

Abstract: To improve the efficiency of process planning, an CAPP system is developed according to the complexity and particularity of process design for large forgings,. The feature functions of the system are analyzed. The key techniques are discussed. The application results show that the process plans of about 70% large forgings are designed by the system and the efficiency of process planning is improved 4-8 times.

Abstract: The two methods of artificial damping and artificial stiffness are developed in dynamic explicit FEA code for ring rolling. Advantages and disadvantages of artificial damping method and artificial stiffness method are analyzed. A rectangular section cold ring rolling is simulated. The hourglass energy and the ratio of the hourglass energy to the internal energy are compared between the two hourglass control methods. The hourglass control coefficient criterion of the two methods in the cold ring rolling simulation is proposed.

Abstract: Mg-6Zn-0.6Zr magnesium alloy (ZK60) was extruded by equal channel angular processing (ECAP) equipment via 2, 4, 6 passes at 400°C. The corresponding microstructure and hardness were examined. The change of the crystal plane in this processing was analyzed by X-ray diffraction. The results show that, ECAP favors recrystallization and refines grains with the increase of pass times. The grains are significantly refined to 10um after 6 passes and the hardness is improved to 79.8HV after 4 passes.