Papers by Author: Cheng Qiu

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Authors: Fang Chai, Da Tong Zhang, Wen Zhang, Cheng Qiu
Abstract: Normal and submerged friction stir processing (SFSP) were conducted to AZ91 magnesium alloy plates with 6mm in thickness, and influence of processing speed (ν) on microstructures and mechanical properties of the experimental materials was investigated. The results revealed that fine and equiaxed grains were observed in the stirred zone (SZ). As the processing speed increased from 60mm/min to 150mm/min, the average grain size in the SZ of normal FSP material decreased. However, the grain size of the SFSP specimens first increased with the processing rate increasing from 60mm/min to 120mm/min, and then decreased when the processing rate increased to 150mm/min. Microstructure of the SFSP specimen was much finer compared with the normal FSP one, and the grain size of α-Mg was about 1.2µm when the processing speed was 60mm/min during SFSP. Because of much finer microstructure of SFSP, the microhardness, tensile strength and elongation were all improved. SEM fracture observation showed that fine dimples and tearing edges could be observed on SFSP specimen which showing good ductility. In addition, high temperature tensile tests showed that SFSP AZ91 alloys exhibited excellent superplasticity at high strain rate, with an elongation of 1202% at 623 K with a strain rate of 3x10-3s-1. The present study demonstrated that SFSP possesses great potential in preparing fine-grained materials.
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Authors: Jun Lin, Da Tong Zhang, Wen Zhang, Cheng Qiu
Abstract: Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. In this paper, the microstructure and mechanical properties of ZK60 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The first-pass was subjected with 800r/min-100mm/min and the second-pass was subjected with 600r/min-100mm/min. The results show that the coarse second phase was dissolved into magnesium matrix and the α-Mg grains were remarkably refined in stir zone after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 6.8μm and 6.0μm respectively. The microhardnesses of the specimens were 70HV and 65HV, the ultimate tensile strengths were 276MPa and 272MPa, and the elongations were 31.6% and 42.5%, respectively. Through increasing the processing pass, the microstructure became finer and the second phase is dissolved more thoroughly. The combined effect of grain refinement and second phase dissolved was responsible for the change of mechanical properties.
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Authors: Xi Cai Luo, Da Tong Zhang, Wen Zhang, Cheng Qiu
Abstract: Friction stir processing (FSP) has been considered as a promising technique for grain refinement. In this study, two-pass FSP was conducted under water to enhance the cooling rate during processing, and a fine-grained AZ61 magnesium alloy was prepared through this method. Compared with one-pass FSP, the microstructure of two-pass FSP alloy is more homogeneous and finer, with an average grain size of 1.8 μm. Both the tensile strength and ductility of the two-pass FSP sample are improved due to grain refinement. A tensile elongation of 31.8% was achieved in the two-pass FSP sample. The microstructure evolution mechanism during the processing was discussed based on the microstructure examination.
272
Authors: Cheng Qiu, Zhen Xing Zheng, Wei Xia, Zhao Yao Zhou
Abstract: There are multiple nonlinearities during the course of powder rolling. Considering Material and Geometrical Nonlinearity during powder rolling, a constitutive model complying with an elliptical yield criterion aiming to the powder rolling is constructed based on the Updated Lagrange (U.L.) formulation by which the basic theory of numerical simulation is deduced. The numerical algorithm is discussed and it is implemented into user-subroutines of Marc. With the code, numerical simulation of powder rolling is performed. It is shown that the result of simulation was consistent with that of experiment and the whole result is dependable. The constitutive model and code developed in this work are correct and have good convergence during calculation. Its robustness is to be validated in simulations with complicated geometrical shape and boundary conditions.
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Authors: Ze Qin Liang, Da Tong Zhang, Cheng Qiu, Wen Zhang, Yuan Yuan Li
Abstract: The high mechanical properties in 7xxx aluminum alloy are obtained by controlling the precipitation hardening microstructure. In this work, the relationship between the microstructures and mechanical properties of 7A04 (Al-Zn-Mg-Cu) aluminum alloy during isothermal aging at 140 oC after different quenching rates has been studied in order to find its useful hardening conditions. The as-extruded samples were solution heat treated at 480 oC and cooled in air, 70 oC water, 40 oC water and 0 oC ice water. Tensile test were performed and the ultimate tensile strength and percentage of elongation were obtained. The difference in the amount of precipitates is known by DSC and the morphology of the precipitates is characterized by TEM. The results indicate that the artificial age hardening response is strongly dependent on the quenching rate. Lower quenching rate results in lower tensile strength but higher percentage of elongation in the peak age condition, and this corresponds to the difference in the size and number density of the precipitates. It is concluded that the highest ultimate tensile strength is obtained in the fastest quenching rate in 0 oC ice water (up to 870Ks-1) while stable and high percentage of elongation is achieved in the intermediate quenching rate (about 226 Ks-1).
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