Papers by Author: Zhi Min Zhang

Abstract: The multi-pass hot compression deformation behavior of the cast alloy with the composition of Mg-13Gd-4Y-2Zn-0.6Zr, was investigated, and the four-pass compression tests were conducted at the temperatures ranging from 350°C to 500°C and strain rate 0.01 s^-1. The experimental results showed that the alloys incurred different degrees of softening by multipass deformation. The microstructure evolution for the deformed alloy was investigated, the influence of the microstructure on the hardness properties of the alloy discussed. The tests reveal that dynamic recrystallization is not the main softening mechanism for this alloy; rather, kink deformation refines the grains to achieve the observed softening effect. The hardness test curve showed that the hardness increased gradually with an increasing number of deformation passes. The improvement of the main mechanical properties related to the strengthening by the grain refinement. In multipass deformation, the misorientation of the kink belt gradually increased,and refined the grains. On the other hand, the grain size of the eutectic phase at the grain boundary decreased with increase of deformation passes. In addition, the mechanical properties were improved by the distribution dispersion of tiny cuboidal particles and acicular-like phases in the matrix.

Abstract: Isothermal compression experiment of Al-W alloy was carried out using Gleeble1500 thermodynamic simulator within a temperature range of 420°C-570°C and a strain rate range of 0.001s^-1-1s^-1。And the constitutive relationship model for this alloy was successfully developed using BP neural network. In the proposed model, the input variables are strain, strain rate and deformation temperature while the flow stress is the output variable. The results show that absolute maximum error between predicted and experimental values of flow stress is less than 10.0Mpa, the correlation coefficient is 0.993. It was found that the established constitutive relationship model could provide a good representation of the test data and describe the whole deforming process better compared with the traditional method.

Abstract: Hot compression tests of 7075 aluminum alloy were carried out on the Gleeble-1500 thermal simulation machine at strain rates of 0.01 s-1 ,0.1s-1, 1s-1 and at temperatures of 320°C,400°C,480°C. The compression direction are aligned at 0°and 45°to the axis direction of the bar. Microstructure was studied using Optical Microscopy. The results show that the true stress of both specimen increases with the decrease of temperature and the increase of strain rates. The true stresses rose rapidly to peak value and then held constant or decreased with the increase of the true strains at different deformation conditions. The yield strength and stable flow stress of the specimens in 0° to the axis direction is higher than that of 45° specimen, which could be attributed to the elongated grain structures of the bar and the texture of the extruded bar.

Abstract: In this paper, the effects of hot extrusion and T5、T6 heat treatment on the microstructures and mechanical properties of ZK60 magnesium alloys are investigated by optical microscope, electronic scanning microscope and mechanical testers. The result shows that both the tensile strength and the elongation of the ZK60 alloy extruded at 380°Care much higher than that of the as-cast alloys, as there are much granular second phases precipitated during the extrusion. The tensile strength of the extruded and T5 treated alloy increases while the elongation decreases faster than that of the extruded alloy. The strengthening effect of the T6 treatment is inferior to that of the T5 treatment. The tensile fracture of the as-cast alloy is brittle fractured while that of the extruded and T5 treated alloy is ductile fractured with lots of deep and even dimples.

Abstract: In order to study the effect of deformation extent on microstructure and mechanical properties of as-cast AZ91D magnesium alloy, experiments of direct extrusion were performed at temperature of 420 and different extrusion ratios. The microstructure and mechanical properties of billets and extrudates were measured. Experimental results show that the grain size of as-cast AZ91D magnesium alloy can be dramatically refined by extrusion. Direct extrusion can obviously improve the mechanical properties of as-cast AZ91D magnesium Alloy, comparing with the pre-extruded billet, the tensile strength, yield strength and elongation of extrudate can be improved by at least 83%, 154% and 150% respectively. As the extrusion ratio increases, the tensile strength and yield strength of extrudate will increase at first and then fall.

Abstract: In the paper, the high-temperature compression test of BTi-62421S alloy was carried out in the temperature range from 850°C to 1050°C, strain rate range from 0.01 s ^-1 to 30 s ^-1, and the deformation of 70%. The results show that the flow stress of BTi-62421S alloy significantly increased with the decreasing of the temperature and the increasing of the strain rate. By calculating the activation energy of deformation and analyzing the stress-strain curve, it can be seen that the softening mechanism of hot deformation of BTi-62421S alloy is different near the （α+β）/β phase transforming temperature. The softening mechanism of hot deformation is mainly dynamic recrystallization in （α+β） area and it is mainly dynamic recovery above β phase transforming temperature. According to hyperbolic sine model, the alloy’s constitutive equation is established. Key word: BTi-62421S alloy; high temperature figuration; constitutive equation

Abstract: In order to study the effect of plastic deformation on microstructure and mechanical properties of as-cast AZ91 magnesium alloy, experiments of hot direct extrusion were performed at different extrusion temperatures and different extrusion ratios. The microstructure and mechanical properties of extruded billets and extrudate were measured. Experimental results show that the grain size of as-cast AZ91 magnesium alloy can be dramatically refined by extrusion. Hot extrusion can obviously improve the mechanical properties of as-cast AZ91 magnesium Alloy, comparing with the pre-extruded billet, the tensile strength, yield strength and elongation of extrudate can be improved by at least 69%, 117% and 150% respectively. As the extrusion temperature increases, the tensile strength and yield strength of extrudate will increase. As the extrusion ratio increases, the tensile strength and yield strength of extrudate will increase at first and then fall. At the time of extrusion temperature of 420°C and extrusion ratio of 45, the highest tensile strength of 381Mpa and yield strength of 303MPa can be achieved for the extrudate.

Abstract: In this paper, rigid plastic finite element method simulation of AZ31 wrought magnesium alloy radiator isothermal extrusion is studied according to the stress - strain data of magnesium alloy obtained from isothermal compression experiments and the mathematical model of warm forming by the regression. And the paper focuses on the AZ31 magnesium alloy during isothermal extrusion, the deformation force and metal flowing law. The simulated stress field, strain field, velocity field, load changes and so on from simulation can predict the defect in deformation, and offer the reliable reference basis for such parts processing in isothermal extrusion.

Abstract: The precipitation mechanism of secondary-precipitated phase β in AZ80 magnesium alloy during the hot compression deformation under different temperature was investigated. The results show that there are β-Mg₁₇Al₁₂ phases with different morphology and precipitation mechanism in the microstructure of AZ80 alloy deformed both in two-phase region、critical region and single-phase region. β-Mg₁₇Al12 phases were directly deformed and broken to be strip form during the compression under 200、250、300 and 350°C. A lot of fine granular second phase particles precipitated with the grain refinement simultaneously in AZ80 alloy deformed in two-phase region with tremendous deformation. There are also fine granular second phase particles precipitated in the alloy deformed in critical region. There are massive β-Mg₁₇Al₁₂ phases precipitated in the alloy deformed under higher temperature such as 350 and 400°C during the water cooling after the compression, the high power observation of which is fine and tightly lamellar microstructure. Compared with coarse reticular β-Mg₁₇Al₁₂ phases in the as-cast microstructure of AZ80 alloy, the morphology of β-Mg₁₇Al12 phases precipitated during the hot compression has obvious improvement.

Abstract: Microstructure observation by optical microscope and numerical simulation by MSC software were performed to study the reason of ring crack at the bottom of the extruded magnesium alloy tube. On this basis, the effective protective measures were proposed by numerical simulation analysis. The result shows that: there is a significant difference between the direction of backward extrusion metal flow and the direction of the original metal flow direction. Therefore, the plastic deformation is difficult, and then the crack initiates owing to severe stress concentration. Through simulation analysis, it is found that both increasing the bottom thickness and changing the bottom position could effectively prevent the crack.