The Experts below are selected from a list of 132 Experts worldwide ranked by ideXlab platform
Guoqun Zhao - One of the best experts on this subject based on the ideXlab platform.
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analysis of longitudinal weld seam defects and investigation of solid state bonding criteria in porthole die extrusion process of aluminum alloy profiles
Journal of Materials Processing Technology, 2016Co-Authors: Junquan Yu, Guoqun Zhao, Liang ChenAbstract:Abstract In porthole die extrusion process of aluminum alloy profiles, the formation of longitudinal weld seams (L-seams) is inevitable. Analysis of L-seam defects and investigation of solid-state bonding criteria are important issues for practical production. In this study, a set of modular porthole extrusion dies with different depths of welding chambers were designed and manufactured. The profiles extruded with different depths of welding chambers were obtained by performing extrusion experiments.The welding quality of extruded profiles was characterized by means of microstructure observation, tensile test and fracture analysis. The true stress-Strain curves of homogenized AA6063 aluminum alloy with various Strain Rates and deformation temperatures were obtained by means of isothermal hot compression test. Three-dimensional transient numerical simulation models of the porthole die extrusion processes were established. Flow behaviors of aluminum alloy in porthole die extrusion process were investigated, and the formation processes of L-seams and their defects were revealed. The solid-state bonding processes of metal particles were traced and their welding paths were determined. Finally, based on the plastic deformation and diffusion mechanisms for closure behaviors of micro voids on contact interfaces, a new dimensionless solid-state bonding criterion related to stress triaxiality, Effective Strain Rate, temperature and contact time was proposed. The bonding quality of L-seams of extruded profiles without any macro defect was predicted successfully by using this new criterion. In addition, it was found that the formation of a macro hole in the profile extruded with a shallow welding chamber is attributed to metal flow behavior, and does not relate to solid-state bonding process.
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Modeling of aluminum alloy profile extrusion process using finite volume method
Journal of Materials Processing Technology, 2008Co-Authors: Guoqun Zhao, Rui Wang, Xianghong WuAbstract:Abstract The most popular numerical method used for simulating aluminum alloy extrusion process is finite element method (FEM). But aluminum alloy profile extrusion process is a metal forming process with severe and large plastic deformation. If finite element method is used to simulate aluminum profile extrusion process, mesh always distorts quickly and frequent remeshing is needed. In addition, the checks for node-separation-from or node-contact-to die surfaces are also frequently needed. The continuous remeshing and node-separation-contact checks usually decrease the accuracy of FEM-based simulation results and increase the CPU time. In this paper, finite volume method (FVM) based on Euler mesh is used to simulate three-dimensional steady or transient aluminum alloy profile hot extrusion process. Semi-implicit method for pressure-linked equations (SIMPLE) algorithm is used to calculate the velocity field and the pressure field of the extrusion process. The dynamic viscosity coefficient of the material mostly dependent on temperature and the Effective Strain-Rate is updated in every SIMPLE inner iteration. At every exterior time step, the method of volume of fluid (VOF) is applied to catch the transient free surface of the material. “Moving grids system” is also used to deal with the moving problem of the boundaries of the calculated domain caused by the moving of the extrusion ram. To ensure the stability and efficiency of the simulation, automatic adjustment of the time increment is realized. Two typical extrusion processes are simulated by the FVM model of this paper. Some results are compared with those simulated by FEM. The material flow characteristics from transient state of extrusion process to steady state of extrusion process are also discussed.
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Preform die shape design for uniformity of deformation in forging based on preform sensitivity analysis
Journal of Materials Processing Technology, 2002Co-Authors: Xinhai Zhao, Guoqun Zhao, Guangchun Wang, Tonghai WangAbstract:Abstract A finite element based sensitivity analysis method for preform die shape design in metal forging by controlling the deformation uniformity is developed in this paper. The optimization problem is to minimize the Effective Strain variation within the final forging through optimizing the preform die shape, so that a more uniform deformation within the final forging can be obtained. The preform die shapes are presented by cubic B-spline curves. The control points of the B-spline curves are used as the design variables. The objective function expressed by the Effective Strain variation is constructed. For two-dimensional forging problems, the sensitivity equations of the objective function, elemental volume, elemental Effective Strain-Rate and the elemental Strain-Rate with respect to the design variables are developed. The optimization procedures of the method are given. The preform die shapes of two H-shaped forging processes in axisymmetric and plane-Strain deformation are designed using the method.
Giuseppe Mirone - One of the best experts on this subject based on the ideXlab platform.
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Variability of the Effective Strain Rate in SHTB tests and related effects on the dynamic stress amplification
EPJ Web of Conferences, 2018Co-Authors: Giuseppe Mirone, Raffaele BarbagalloAbstract:Although the standard procedure for SHTB testing is based on the hypothesis that the Strain Rate is nominally constant during each test, it is known by many experimental evidences that the Effective Strain Rate histories cannot be really constant, and this may cause the function expressing the dynamic amplification of the stress to substantially differ from that inferred under the hypothesis of Strain Rate constancy. This aspect is stressed out in this work by experiments and by numerical analyses, relating the variability of the Strain Rate to the finite rise time necessary in Hopkinson bars for reaching the plateau of the loading wave, and to the abrupt large increase in the Effective Strain Rate naturally induced by the necking onset. Also the interplay between the achievement of the Strain Rate plateau and the achievement of the necking initiation Strain is analysed, for assessing if and eventually how the same Strain Rate plateau achieved before or after the necking onset can affect the resulting dynamic amplification of the stress17 mm from the left and right page margins and justified..
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the dynamic effect of necking in hopkinson bar tension tests
Mechanics of Materials, 2013Co-Authors: Giuseppe MironeAbstract:Abstract The determination of the stress–Strain curves from static and dynamic tension tests is affected by the necking which locally modifies the stress distributions and the stress state, so that uniformity and uniaxiality of the stress state cease to apply and the load-area reduction measurements do not allow anymore to calculate the equivalent plastic Strain and the equivalent stress at any material point within the resisting cross-section. In case of dynamic tension tests, the necking also influences the Effective Strain Rate, causing it to substantially differ from the nominal applied Strain Rate. The effects of the necking on the Strain Rate and on the related material response are investigated here, and it is also checked whether or not a material-independent function previously developed for correcting the post-necking true curves in quasi static tests, can also be used for correcting the stress–Strain curves from Hopkinson bar testing and transforming them into equivalent stress vs. equivalent Strain curves at a given Strain Rate. Finite elements analyses simulating experimental tests are compared to experimental data from the literature so that, from the validated numerical results, stress and Strain distributions in the interiors of the specimens can be investigated in detail.
O Richmond - One of the best experts on this subject based on the ideXlab platform.
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singular plastic flow fields near surfaces of maximum friction stress
International Journal of Non-linear Mechanics, 2001Co-Authors: S Alexandrov, O RichmondAbstract:Assuming a rigid/perfectly plastic material model with arbitrary isotropic smooth yield criterion, it is shown that the velocity fields adjacent to surfaces of maximum friction, except for some special planar flows, must be describable by non-differentiable functions where the maximum shear Strain Rate and the Effective Strain Rate approach infinity. This is consistent with experimental results that show very large gradients of velocity near such surfaces and with computational results that indicate difficulty in describing such behavior with finite elements using simple interpolation functions. Moreover, this result leads naturally to the definition of a Strain Rate intensity factor which has similar meaning to the stress intensity factor in linear elastic fracture mechanics. As an example of the application of the singular velocity field, simple compression of a plastic layer between parallel, rough plates is considered. An upper bound solution is found by assuming the singular field in the layer. As expected, it lies between the upper bound obtained by assuming simple compression in the layer and a slip line solution.
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Strain Rate potential for metals and its application to minimum plastic work path calculations
International Journal of Plasticity, 1993Co-Authors: Frederic Barlat, K Chung, O RichmondAbstract:Abstract In this work, a definition of the Strain Rate potential for plastically deforming metals is proposed. This potential is defined in six-dimensional deviatoric Strain Rate space, and its gradient provides deviatoric stresses in the flowing material. For isotropic FCC metals, it is shown that the plastic behavior predicted with this proposed phenomenological description is identical to the behavior predicted with the Taylor/Bishop and Hill polycrystal plasticity model. For orthotropic FCC metals, six material coefficients characterize the anisotropy. This potential provides a definition of the Effective Strain Rate. Together with a work-hardening curve, this equation completely describes the plastic behavior of isotropically hardening metals. This definition is useful for the calculation of work along minimum plastic work paths, as is illustRated for an isotropic FCC metal and a strongly textured aluminum alloy, subjected both to pure shear and simple shear deformation modes.
Hui-ju Gao - One of the best experts on this subject based on the ideXlab platform.
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modelling of recrystallization behavior and austenite grain size evolution during the hot rolling of gcr15 rod
Applied Mathematical Modelling, 2010Co-Authors: Chong-xiang Yue, Liwen Zhang, Jinhua Ruan, Hui-ju GaoAbstract:Abstract In this paper, four 3-D finite element models are developed to simulate the whole rod rolling process of GCr15 steel. The distribution and evolution of different field-variables, such as Effective Strain, Effective Strain Rate and temperature, are obtained. Based on the simulated results and the microstructure evolution models of the steel, the paper designs a FORTRAN program to predict the evolution of recrystallization behavior and austenite grain size in rolled piece during the rolling. The surface temperatures of rolled piece calculated by FEM agree well with measured values. Comparison between calculated values and measured ones of grain size shows the validity of the program.
G D Wang - One of the best experts on this subject based on the ideXlab platform.
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an analytical solution for tube sinking by Strain Rate vector inner product integration
Journal of Materials Processing Technology, 2009Co-Authors: H J Du, Gang Wang, G D WangAbstract:Abstract A new integration method of Strain Rate vector inner-product is proposed in present paper. First, cylindrical coordinate Effective Strain Rate is established and expressed in terms of two-dimensional Strain Rate vector for tube sinking. Then, plastic power expressed by the vector inner-product is integRated term by term. Second, sum of the integRated results yields the total power. By minimization of the power, the stress Effective factor, the maximum possible reduction Ro/Rof and the optimal die angle are obtained. It is proved that the expression of power by linear integration is similar with that by traditional immediate integration. In order to avoid wrinkling in one process, the reduction should be designed according to the constant wall thickness region defined by the ratio of the outer diameter to the original thickness of the tube. Also, the solution is compared with that of Avitzur. The basic law is analyzed.
