Guo Qun Zhao

Numerical Investigation of Viscoelastic Flow and Swell Behaviors of Polymer Melts in the Hollow Profile Extrusion Process

The viscoelastic flow and swell behaviors of polymer melts in the profile extrusion process can significantly influence the performance and dimension of the final products. In the study, the viscoelastic flow pattern of a commercial low density polyethylene (LDPE) extruded through out of the hollow profiled extrusion die is investigated by means of finite element simulation. The mathematical model of three-dimensional viscoelastic flow and swell of polymer melts is established with a differential Phan-Thien and Tanner (PTT) constitutive model. A penalty method is employed to solve the non-linear problem with a decoupled algorithm. The computation stability is improved by using the discrete elastic-viscous split stress (DEVSS) algorithm with the inconsistent streamline-upwind (SU) scheme. A streamface-streamline method is introduced to adjust the swelling free surface of the extrudate. The essential viscoelastic flow characteristics of LDPE flowing through out of the hollow profile extrusion die is investigated based on the proposed numerical scheme. Both the redistribution of flow velocity and the release of stress are found to be the reasons for the swell phenomenon.

Modeling Material Removal in Fracture Erosion for Brittle Materials by Abrasive Waterjet

The abrasive waterjet machining is a powerful tool in processing various materials, especially, for brittle materials, such as ceramic, glass and so on. However, the material removal of a brittle material when impacted by abrasive waterjet is not understood in detail. In this paper, the material removal model in fracture erosion of brittle materials by abrasive waterjet has been developed.

Numerical Simulation and Die Optimal Design of a Complex Section Thin-Walled Aluminum Profile Extrusion

The most important way to improve the quality of aluminum extrudates is to assure the material flow through die exit with the same velocity. Usually, aluminum profile extrusion is a very complex process of metal flow, which is mainly dependent on the level of the die design. Yet current design of extrusion die is primarily based on the trial-and-error method, which is to a large extent is determined by the experience of the die designers. In this research, a numerical model was developed to simulate the extrusion process for a complex section thin-walled aluminum profile with HyperXtrude. According to the non-uniformity of metal flow with the original die, several modifications, such as drainage channels, baffle plates and local bearing length, were performed to optimize the die structure. Finally, an optimal die design with more uniform flow velocity was achieved. The research results showed that the numerical model developed in this paper was effective for aluminum profile extrusion die optimization.

Effects of Welding Chamber Step on Extrusion Process of a Complex Hollow Profile

The extrusion die is of great importance in the quality control of profile production. Yet in practice, the design of extrusion die is mainly dependent on the experience and intuition of die designers, which is difficult to guarantee product quality and productivity. In this paper, a numerical model was developed based on HyperXtrude with an attempt to investigate the effects of the shape of the welding chamber on metal flow and weld quality. The porthole dies with different steps of welding chamber were designed and applied to extrude an identical profile. Numerical results showed that with an increasing step of welding chamber, more uniform velocity and temperature distributions in the cross-section of the extrudate were observed. In addition, the weld quality was improved owing to increasing welding pressure, when adopting the multi-step welding chamber.

Coupled Simulation of Plastic Part and Mold Temperatures in the Filling Process of Rapid Heat Cycle Injection Molding

A fully transient method for the simulation of the coupled plastic part and mold temperature in the filling stage of Rapid Heat Cycle Molding process was presented. Unlike the constant temperature boundary conditions in conventional injection molding simulation methods, the mold temperature was assumed to be variable in the filling process. The mold temperature in the heating process was first simulated. In the filling process, the flow equations were solved only on the cavity domain, while the energy equations were solved in a coupled manner for the cavity and mold domain at the matrix level. The temperature evolution in a two-dimensional filling process with heated mold was simulated. Numerical results showed that the proposed coupled simulation method provides reliable temperature estimations in mold filling process.

Study on Warm and Hot Tensile Deformation Behavior and Flow Stress of AZ31B Magnesium Alloy Sheet

The true stress-strain curves of AZ31B magnesium alloy sheet were obtained by using the uniaxial tensile tests at the temperature ranging from 50°Cto 300°C and the initial strain rate ranging from 0.001/s to 0.1/s. The influence of temperature and strain rate on the flow stress was analyzed. The results show that the flow stress decreases and the elongation in fracture increases with increasing temperature and decreasing strain rate，and the plastic performance is improved obviously. Through the analysis of the true stress-strain curves，a mathematical model of the flow stress was established based on an exponential form whose power is a quadratic function. All the coefficients in the model were fitted as functions of temperature and strain rate. The comparison of the calculated results with experimental data shows that the model established in this study can accurately reflect work hardening and strain softening effect of AZ31B magnesium alloys during the hot deformation. It can be used for the prediction of flow stress for AZ31B magnesium alloys under hot work conditions and numerical simulation of forming processes.

Multiple Objective Preform Die Shape Optimization Design for Controlling Forging’s Shape and Deforming Force during Forging Process

In order to decrease the cost of the material and energy during the forging process, multiple preform die shape optimization design was carried out in this paper. Based on the FEM, a sensitivity analysis method was used to perform the optimization procedure. The shape of the forging and deforming force of the final forging was used to express the cost of material and energy respectively. Using the weighted sum method, the total objective function was gotton. The coordinates of the control point of the B-spline used to represent the preform die shape was determined as the optimization design variable. The sensitivity equations of the total objective function with respect to the design variables was developed. The multiple objective perform design optimization software was developed by FORTRAN language. And then, the preform die shape of an H-shaped forging process is optimized. The total objective function, sub-objective function, the shape of the preform die and the final forging during the optimization were given. After the optimiztion, a near net shape forging was obtained. At the same time, the deforming force decreased. The optimization results are very satisfactory.

Computer Simulation of Evolution of Ordering Phases and the Boundaries of Ni-Cr-Al Alloy

Based on the microscope phase-field model，the evolution of atom morphology, the long range order(lro) parameter and concentration can be gotten, and atomic clustering and ordering during the precipitation process of Ni-Cr-Al alloy could be obtained. The Ni-14at.%Cr-15.5at.%Al alloy is studied and the temperature of precipitation are 973K. It was showed that the ordering of both Al and Cr atoms take place simultaneously during the precipitation process of Ni-Al-Cr alloy, Cr atoms transfer to the boundaries of L12 phases, the domain of rich Cr atoms are formed. At the boundaries of L12 phases, Cr atoms may substitute the Al sublattice, and the D022 phases are formed.

An Automatic Mesh Generator for Hexahedral Element Mesh Using an Improved Grid-Based Method

An automatic mesh generator AUTOMESH-3D is developed by the authors in this paper. The generator can adaptively produce hexahedral element meshes with an improved inside-out grid-based method. This method involves seven steps: STL model construction, geometric feature identification, cubic grid structure generation, jagged core mesh generation, surface-gap filling, boundary matching and quality improvement. Adaptive refinement, local refinement and boundary matching techniques are employed to generate eligible elements according to the model’s features and users’ requirements. Several examples of hexahedral element mesh generation with AUTOMESH-3D are given, and the effectiveness and robustness of the mesh generator are demonstrated.

Numerical Simulation and Experimental Study of AZ31 Magnesium Alloy Deformation Behavior in ECAP

This article investigated deformation behavior and microstructure of AZ31 magnesium alloy in equal channel angular pressing (ECAP) process at 250 C. The proper process parameters were obtained through investigating the deformation behavior and properties of AZ31 magnesium alloy by using finite element (FE) simulation under different deformation conditions. The ECAP experiments were also carried out by using the optimized design of the channel die. The micro-structures of experimentally pressed work-pieces are in good agreement with finite element analysis results. The research results provided reliable basis for the further development and use of magnesium alloy precision forming.