Papers by Author: Ke Ke Zhang

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Authors: Ke Ke Zhang, Yun Yue, Z.L. Zhang, Ning Ma, S. Liu, Hong Xin Shi, Hua Yu, Yao Li Wang
Abstract: Taking 1.4%C and 1.6%C ultrahigh carbon steels (UHCSs) with ultra-fine structures developed through thermo-mechanical processing as research objects, the influences of superplastic deformation conditions and electric field on compression superplasticity were conducted without electric field and with electric field. The experimental results indicate that 1.4%C and 1.6%C UHCSs have respectively superplasticity or electro-superplasticity under the conditions of deformation temperature of 800°C, the initial strain rate of 0.75-3.75×10-4 s-1 and deformation temperature of 780 °C, the initial strain rate of 0.5-5.0×10-4 s-1 and the suitable external electric field. Its strain rate sensitivity are respectively 0.32 and 0.46.When the electric field intensity of 3 kV/cm in the manner of the specimen connected to positive terminal, the steady flow stress of 1.6%C UHCS decreases more than 10.5% as compared with electric field intensity of 0 kV/cm.
Authors: Jin Yong Zhang, Gang Bai, Ke Ke Zhang, Yu Hang He, Shun Yang
Abstract: In the paper, the Sn2.5Ag0.7Cu solder alloy is selected out by comprehensive comparison. Minute a mount of rare earth (RE) was added into Sn2.5Ag0.7Cu solder aiming at examining the effect of different a mount of RE on microstructure,the physical properties and mechanical properties of Sn2.5Ag0.7Cu solder.the aim is to define the optimization range of the RE addition.
Authors: Hua Yu, Ke Gao, Ran Feng Qiu, Ke Ke Zhang, Hong Xin Shi
Abstract: Steel 35CrMo is widely used to produce the principal axis of machine, but the principal axis is often abraded or scratched during its service. The CoCr coating produced by electro-spark deposition on the machine principal axis of 35CrMo can solve the problem and resume its application. In this study, the interface behavior of joint between the CoCr coating and the 35CrNo steel substrate was studied. The microstructure of CoCr coating was also observed and analyzed by using a scanning electron microscope with an energy dispersive X-ray spectroscopy analysis. A narrow layer of elements diffusion containing Co0.72Fe0.28, CoCr and Co7Fe3 was detected in the bonding interface between the CoCr coating layer and the 35CrMo steel substrate. The CoCr coating with higher microhardness was obtained on the substrate. The results reveal that a joint between the CoCr alloy and the steel 35CrMo substrate with stable quality can be obtained by electro-spark deposition.
Authors: Hua Yu, Hong Xin Shi, Yao Li Wang, Ke Ke Zhang, Wen Yan Wang, Li Juan Han, Qing Hua Pang
Abstract: Strengthened coating on the surface of 35CrMo steel alloy was deposited by electrospark process using NiCr electrode. The bond strength and microhardness of the coating was checked and the microstructure of the strengthened coating was analyzed using the optical micro-scope, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectrum (EDS). The results show that the bond strength between the coating and the base metal is 495.1MPa, and the hardness of the coating surface is about 2 times than that of the base metal, and the property of the workpieces surface has become better. The coating is a metallurgical reacting bonding layer between the base metal and the NiCr electrode, and its main ingredients are FeNi and NiCrFe.
Authors: Ke Ke Zhang, Yao Li Wang, Yan Li Fan, Jie Yiang, Yan Fu Yan, Xin Zhang
Abstract: Creep property of solder alloys is one of the important factors to effect the reliability of surface mount technology (SMT) soldered joints. The creep behavior and its rupture life of Sn2.5Ag0.7CuXRE lead-free soldered joints were separately investigated and predicted under constant temperature by a single shear lap creep specimen with a 1mm2 cross sectional area and finite element method (FEM) in this paper. Results show that the creep property of Sn2.5Ag0.7Cu0.1RE is superior to that of the commercial employed lead-free solder Sn3.8Ag0.7Cu and the creep rupture life of its soldered joints is 8.4 times more than that of Sn2.5Ag0.7Cu solder. The creep rupture life of Sn2.5Ag0.7CuXRE lead-free soldered joints indirectly predicted by FEM is better in accord with that of actual testing results, which are important to design the reliability of lead-free soldered joints for SMT.
Authors: Ke Ke Zhang, Yao Li Wang, Yan Li Fan, Guo Ji Zhao, Yan Fu Yan, Xin Zhang
Abstract: The effects of Ni on the properties of the Sn-2.5Ag-0.7Cu-0.1Re solder alloy and its creep properties of solder joints are researched. The results show that with adding 0.05wt% Ni in the Sn-2.5Ag-0.7Cu-0.1Re solder alloy, the elongation can be sharply improved without decreasing its tensile strength and it is 1.4 times higher than that of the commercial Sn-3.8Ag-0.7Cu solder alloy. Accordingly the creep rupture life of Sn-2.5Ag-0.7Cu-0.1Re-0.05Ni solder joints is the longest, which is 13.3 times longer than that of Sn-2.5Ag-0.7Cu-0.1Re and is also longer than that of the commercial Sn-3.8Ag-0.7Cu solder alloy. In the same environmental conditions, the creep rupture life of Sn-2.5Ag-0.7Cu-0.1Re-0.05Ni solder joints can sharply decrease with increasing the temperature and stress.
Authors: Hong Xin Shi, Zhi Wei Wu, Jing Sun, Hua Yu, Ran Feng Qiu, Ke Ke Zhang
Abstract: The 40Cr steel surface waiting for being welded was ultra-fined through laser quenching, and isothermal superplastic solid state welding was conducted between the 40Cr steel and Cr12MoV steel. The mechanical properties of the joint were evaluated by tensile testing and microhardness of the joint and thereby. The microstructure of 40Cr steel’s quenching layer and the joint interface zone was observed and analyzed using scan electron microscope. The experimental results show that ultra-fine microstructure is achieved in the 40Cr steel’s surface waiting for being welded. The superplastic solid state welding can be carried out under welding parameters including welding temperature of 800 , pressure welding time of 4 min and initial strain rate of 1.5×10-4 S-1. The tensile strength of joint achieves 81.94% that of 40Cr steel. The microhardness of transition region in the Cr12MoV side is higher than other region. On the contrary, the microhardness in the 40Cr side is lower than other region. The new M7C3 type carbide is checked in the Cr12MoV side. In welding process, the carbon atom has diffused from 40Cr steel side to Cr12MoV steel side. Therefore, isothermal superplastic solid state welding with higher quality can be realized between Cr12MoV steel and the 40Cr steel after microstructure ultra-fining through laser quenching.
Authors: J.B. Wen, F.X. Chen, Ke Ke Zhang, Yong Shun Yang
Abstract: By means of superplastic tensile test above the Ac1 temperature (γ→α transformation temperature), superplastic deformation activation energy of ultrafine-grained commercial die steel CrWMn is investigated on the basis of the Arrhenius theory equation, exp( / ) 1 ε& = Aσ m −Q RT , which indicates the resistance of the superplastic deformation. According to the Arrhenius equation, the activation energy is estimated from the log σt vs 1/T relationship at a constant of sensitivity index of strain rate. The results show that the strain rate sensitivity index is a constant and rather high at the conditions of superplastic deformation for the CrWMn steel, the activation energy for superplastic deformation of steel CrWMn above the critical temperature is 187KJ/mol, and the superplastic deformation activation energy is approached to the grain boundary diffusion activation energy of γ-Fe. This indicates that the grain boundary sliding (GBS) in superplastic deformation of CrWMn steel is controlled by grain boundary diffusion. The characters of superplastic deformation of the steel above the critical temperature, on the other hand, are also analyzed in this paper.
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