Papers by Author: Jie Zhou

Abstract: In order to increase heating efficiency and decrease heating time, a new hot-forging method by means of direct resistance heating was investigated in this paper. Based on the approach, the hot upsetting experiments with cylindrical billet of 42CrMo4 were performed. Moreover, the influence of the multi-layer aluminium foils inserted between the billet and the die as a forming condition on heating and forming was researched. The results of the experiments show that prior to forming the billet could be heated quickly to forming temperature in about 10 seconds. During the upsetting process the billet cooling rate was effectively decreased and the forming time was extended in relation to the resistance heating. The insertion of multi-layer aluminium foils not only improved the efficiency of the heating, but also avoided the plastic deformation of the die and the occurrence of cracks on the billet’s surface. Keywords: Resistance heating, Hot forging, upsetting, Aluminium foils Introduction

Abstract: Based on the platform ABAQUS, a 3-D FEM model of spirogyrate ring rolling process has been developed to investigate the ring’s deformation behavior and the expansion law of the diameter. According to the FEM analysis, this kind of profile ring rolling process can be divided into two phases, i.e. the profile forming phase and the diameter increasing phase. During the former phase, the deformation of spirogyrate ring focuses on the profile formation and the diameter increases slowly. In the latter phase, the ring diameter expands rapidly. FEA illustrates that the material flowing to the outer diameter areas is greater than that towards the inner diameter areas in the profile forming phase. The maximum PEEQ is in the pit of the outer profile, whereas the minimum PEEQ locates near the end face. The variation of outer and inner diameter of the spirogyrate ring shows monotonic increasing trend during the whole ring rolling process, in which the expanding rate of inner areas is greater than that of the outer areas.

Abstract: In order to reveal the forming laws，eliminate the forming defects appearing in the typical aluminum alloy web-rib parts，two types of preforms have been used in the final forging with numerical simulation software.Based on the simulation，the comparisons of metal flow，effective strain distribution are obtained between the two types of preforms applied in the final forging process，the forming laws of a symmetrical rib-web component have been obtained ,and the forming mechanism of defects often happened in web- rib parts also has been analyzed,the relationship between the fillet radius and metal flow velocity/ final load was obtain. The results show that the fiber breaking,vortex could be avoided，a high forming quality of rib-web part could be gained by the method of using the thin web-high rib preform in finish-forging.

Abstract: Based on the platform ABAQUS, 3-D FEA model for ring rolling has been constructed to investigate the effect of drive roll rotation speed on the metal flow in the ring rolling process. It can be seen clearly that the axial metal flow in the outside diameter of the rolled ring increases with the increasing of drive roll rotation speed and with the decreasing of the feeding magnitude per revolution , which causes the increasing of fishtail coefficient FT and maximum spread coefficient and in turn makes the quality deterioration of end-plane in the rolled ring. The rolling force and rolling torque necessary to execute the rolling process are reduced when the drive roll rotation speed is elevated, which results in the lower requirement for the characteristic of force and energy in the rolling machine. Besides that, average equivalent plastic strain PEEQ also increases, which indicates the enhancement of plastic deformation and is beneficial to the improvement of the mechanical property of the formed ring. But one point that we should pay attention is that uneven deformation and possibility of the inner defects in the rolled ring might be increased with the increasing of the drive roll rotation speed.

Abstract: In the present study, an axis-symmetric electro-thermo-mechanical model has been developed to analyze a deformation process by continuous resistance heating. To obtain the transient temperature field prior to forming, a novel temperature-dependent model of the contact resistance was developed in the thermal-electrical analysis. The influences of the contact resistance, the current intensity and the die geometry on the temperature distribution were investigated. In the subsequent electro-thermo-mechanical analysis of the forming process by continuous resistance heating, the variations of the billet temperature distribution, forming force were obtained. The simulation results correspond well with experimental measured values. Furthermore, the influence of a current increasing during forming on the billet temperature and forming force was predicted in order to optimize the forming technology by continuous resistance heating.

Abstract: The formability of different gauge tailor welded blanks is difference to single material blanks. As an effective and practical material property test method, tensile test were applied to study it. Base material and tailor welded blanks test specimens had been designed for tensile testing. The results of tensile test indicated that the strength difference and weld line location in the TWBs plays an important role on the formability aspect.

Abstract: This work was focused on the compressive deformation behavior of 42CrMo steel at temperatures from 1123K to 1348K and strain rates from 0.01s-1 to 10s-1 on a Gleeble-1500 thermo-simulation machine. The true stress-strain curves tested exhibit peak stresses at small strains, after them the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. And the stress level decreases with increasing deformation temperature and decreasing strain rate. The values of strain hardening exponent n, and the strain rate sensitivity exponent m were calculated the method of multiple linear regression, the results show that the two material parameters are not constants, but changes with temperature and strain rate. Then the two variable material parameters were introduced into Fields-Backofen equation amended. Thus the constitutive mechanical discription of 42CrMo steel which can accurately describe the relationships among flow stress, temperature, strain rate, strain offers the basic model for plastic forming process simulation.

Abstract: Research on microstructure and behavior of natural biomaterials contributes to understanding the biomimetic design. Yakbone bioceramic’s cross section with innumerable voids was observed via SEM approach, then a digital image processing technique was adopted to analyze the images. It is supposed that the pores can be classified into n series by different size level. A non-linear function between the pore size and series number got fitted which indicates a regression trend to zero as n rising to a large number. A non-linear convergent analysis of the function results in the porosity of porous microstructure of yakbone bioceramic. Furthermore, a plane helices characterization model was assumed to descript the multilevel voids distribution, then a series of helices parameters were designed for bionic yakbone porous microstructure with porosity tolerance of 2.5%. This parameter description of yakbone, a porous bioceramic with high strength and light weight, contributes to further research on the relationships between mechanical property and microstructure.

Abstract: A Kumar-type constitutive relationship of TA15 alloy was investigated by non-linear regression analysis, and the equations were established based on test data from hot process simulator. Applying this constitutive equation in commercial FEM software of SFTC/Deform, the hammer forging process of TA15 alloy beam was simulated, meanwhile the final shape of forging piece, the hammering times, the height reduction variation and the maximal effective stress variation along with forging time were predicted. The hammer forging test was carried out and the test results showed that the simulation based on Kumar-type constitutive relationship meet the practical need to a great extent.

Abstract: The strength ratio between two blanks of tailor welded blanks (TWBs) plays an important role in the formability aspect. To study the strength ratio effects on the formability of TWB is significant while more and more TWBs were applied in auto industry. The formability of TWBs square cup was analyzed by numeric simulation technology in the terms of different strength combination TWBs. The results indicate that the formality goes worse dramatically when the strength ratio bigger than 2.0 and the crack located on the higher strength blank side when the crack appeared. The results are applicable.