Papers by Author: Zhi Min Zhang

Abstract: The deformation behavior and microstructure evolution of LPSO phase of Mg-11Gd-4Y-2Zn-0.5Zr magnesium alloy were investigated. This alloy was deformed by hot compression using Gleeeble 3500 thermal simulation machine at different temperatures. The microstructure was analyzed by optical microscopy (OM), scanning electron microscopy (SEM) with spectroscopy (EDS) and XRD. The results showed that the kink bands of LPSO structure of Mg-11Gd-4Y-2Zn-0.5Zr alloy, after thermal compression processing, aggravated as temperature increasing. The fine lamellar LPSO phase could be observed in the alloy. At 450 °Cand 500 °C, some fine lamellar LPSO phase in the grain had been broken into short rod or small block forms, meanwhile, a new rod-like LPSO structure appeared along the grain boundary. Moreover, the decomposition of LPSO structure was more obvious with the increasing of temperature. During hot compression deformation, the segregation of Y, Zr had eliminated partially and the diffraction peaks of W-phase had disappeared. Moreover, there was an increase in hardness as the dispersion distribution of LPSO phase increased.

Abstract: Uniaxial hot compression was conducted on Gleeble-3500 thermo simulation machine. Based on stress-strain curves, the constitutive relationship and the dynamic recrystallization kinetics model of Mg-8Gd-3Y-1Nd-0.5Zr were established. Simultaneously, dynamic recrystallization mechanism of this alloy under different deformation condition was investigated by SEM, EBSD and OM. The critical strain equation and the dynamic recrystallization kinetics model were obtained. The results showed that the dynamic recrystallization volume fraction increased with the increasing of the strain.The twin dynamic recrystallization (TDRX) was the mainly DRX mechanism at 350°C;the dynamiac recrysallization mechanism was dominated by continuous dynamic recrystallization (CDRX) at 400°C and 450°C. At higher temperature (500°C), the dynamic recrystallization was dominated by discontinuous dynamic recrystallization (DDRX) with a small amount of CDRX.

Abstract: A hot rolling method to produce Mg-Gd-Y-Nd-Zr alloy bars through oval-vertical elliptical-round pass system is put forward. The mechanical model of Mg-Gd-Y-Nd-Zr alloy has been established. Using Deform-3D software, the rolling process of Mg-Gd-Y-Nd-Zr alloy bar is simulated at rolling temperature 450°C, rolling speed 0.4m/s. And equivalent stress distribution and load variation has been obtained. It provides a theoretical basis for the hot rolling bars of magnesium alloy. The experiment verifies that the hot rolling process is feasible. The organizational structure is analyzed with metallographic microscope, and the results show that dynamic recrystallization is occurred, the grain size is obviously refined, and the mechanical property of the material is improved in hot rolling process.

Abstract: Mechanical properties of Al-12Zn-2.4Mg-1.2Cu alloy extruded sheet were investigated by tensile tests. Microstructures were investigated by optical microscopy (OM) and scanning electron microscope (SEM). The result shows that no matter in the L direction or in the T direction, the tensile strength and yield strength decrease with the increasing of the extrusion temperature in different states. The tensile strength and yield strength in the L direction are higher than in the T direction at different extrusion temperatures and different treatment states. When temperature is 340°C, the highest tensile strength is 780 MPa and the highest yield strength is 753 MPa in the two-stage solution and two-stage aging state. The reason for the higher mechanical properties are in the L direction in different states is mainly depend on the distribution direction of the grains.

Abstract: By multi-axis active load deformation method, the equivalence diameter tee joint was formed on multi-axis numerical control hydraulic press machine. The loading route was determined by numerical simulation. The experiment results showed that two loading method can complete form the parts. The different directions metal flow was observed under multi-axis loading conditions. To simplify analysis, the flow field was divided into several regions. In every region, the metal flow direction was only one. In multi-axis loading, the way of deformation follows priority deformation principle: The metal flows only choose one direction even if the work piece were under the complicated coupling field condition. The mathematical model of the deformation force and metal flow rate was established. The theoretical calculation had been provided.

Abstract: Research on multipass plastic deformation of 7A04 ultra-high strength aluminium alloy by isothermal compression experiments on the 6300KN extrusion press. Experiment results show that elongation reaches its maximal value 9.25% after the first deformation. It is obvious that fibrous tissues appeared along the metal flow direction in the deformed 7A04 ultra-high aluminium alloy, with heterogeneous distribution of precipitated η (MgZn2) phase in the matrix, which results in lower strength (Rm=335MPa, Re=212.5MPa). As the times of deformation increases, precipitated phases grow gradually and the plasticity of alloys decreases dramatically, which reaches its minimal value 5.17% after the fourth deformation. With η(MgZn2) phase disperses gradually, the strength of the alloy increases gradually, and reaches its maximal value 386.7MPa after the fourth deformations. It is proved that 7A04 high-strength aluminium alloy has better synthetic mechanical properties after four times deformation.

Abstract: This paper presents the results of an investigation of the effects of homogenizing heat treatment and extrusion on plasticity of the as-cast AZ80 magnesium alloy. Both the homogenized and non-homogenized billets of AZ80 alloy were forward extruded at several different temperatures and different extrusion ratios. The effects of homogenization and extrusion on plasticity were evaluated by conducting tensile tests on these billets at room temperature and comparing their elongations. The experimental results showed that the elongation of the as-cast AZ80 alloy was increased by 67% after the homogenization treatment. After extrusion, the elongation of both the homogenized and non-homogenized AZ80 alloy increased significantly. The elongation of the homogenized billets decreased gradually with increasing temperature. For the non-homogenized billets, however, the elongation decreased sharply with temperature from 300 to 350 °C and then increased gradually with increasing temperature. There was not clear correlation between the elongations of both the homogenized and non-homogenized billets and the extrusion ratio.

Abstract: This paper presents a case study of optimizing the forming process for a fan-shaped shell component. Numerical simulation was used to study the backward extrusion process of a fan-shaped shell. The underfill defect produced at the opening of the extruded shell due to the billet shape was solved and the minimal base thickness required to avoid the presence of the underfill defect at the bottom corner of the component was defined through the numerical simulation. The extrusion drawing and forming process of the fan-shaped shell were designed on the basis of the results of the numerical simulation. Forming experiments had been performed on the fan-shaped shell at 380 °C and cracking was found on the outside wall in the center of the extruded shell. Choked groove on the inner wall of the die and reducing the lubrication had been used to avoid the presence of cracking. The fan-shaped shell of AZ31 magnesium alloy has been successfully formed by the three-stage forming process of hot upsetting, hot backward extrusion and cold sizing.

Abstract: The flow stress behavior and constitutive equation of the nanometric Al₂O₃ particulate reinforced Al alloy matrix composites were investigated in the temperature range from 590k-710k, and at the strain rates range from 0.01s^-1-1s^-1. Hot compression tests were carried out with thermal simulation machine Gleeble-1500. The results showed that the values of the true stresses rose rapidly and then held constant to some extent after attaining the peak values with the increasing strains at different deformation condition. The flow stress for the composites increased with increasing strain rate, which means that the experimental material is a sensitive material of positive strain rate, and decreased with decreasing temperature. Dynamic recovery and dynamic recrystallization occurred during hot compression of the composites. The constitutive equation represented by a Zener-Hollomon parameter in an exponent-type and the deformation activation energy are as follows respectively: σ=71.43ln{(Z/4.37×10¹¹)^1/5.94+[(Z/4.37×10¹¹)^2/5.94+1]^1/2}, Q=197KJ mol^-1.

Abstract: The influences of solution treatment, deformation strain, and nanometric Al₂O₃ particulate on dry wear properties for nanometric Al₂O₃ particulate reinforced Al alloy matrix Composites manufactured by casting were investigated. The result showed that both the wear resistance of AL matrix and the composites increased rapidly with increasing solution temperatures. The wear resistance increased slightly with increasing deformation strain for the composites. The wear resistance of the composites is higher than the AL matrix after the same deformation and heat treatment. Microstructure observation revealed that the grain sizes of the composites increased with increasing solution temperature and decreased with more severe deformation. Abrasive wear was the main wear mechanism both for the AL matrix and the composites.