Da Yong Li

Influence of Dynamic Recrystallization on Tensile Properties of AZ31B Magnesium Alloy Sheet

Dynamic recrystallization (DRX) occurring in tensile deformation of AZ31B magnesium alloy sheet has been investigated. In particular, in order to predict the effect of DRX on tensile properties of AZ31B magnesium alloy sheet, a modified constitutive model is presented and used in the finite element method (FEM) model of the tension. Results from this analysis are compared with the experimental data for tensile tests. The results indicate that the modified DRX model can validly express the effect of DRX on tensile properties of AZ31B magnesium alloy sheet. The softening stage of flow stress simulated by this model can approach that of experiment considerably. In addition, the evolution of DRX grain is examined with increasing temperature. Furthermore, a comparison is made between calculated and tested DRX grain size.

Dynamic recrystallisation and dynamic precipitation in AA6061 aluminium alloy during hot deformation

Abstract Deformation behaviour of AA6061 alloy was investigated using uniaxial compression tests at temperatures from 400 to 500°C and strain rates from 0·01 to 1 s−1. Stress increases to a peak value, then decreases monotonically until reaching a steady state. The dependence of stress on temperature and strain rate was fitted to a sinh-Arrhenius equation and characterised by the Zener–Hollomon parameter with apparent activation energy of 208·3 kJ mol−1. Grain orientation spread analysis by electron backscattered diffraction indicated dynamic recovery and geometrical dynamic recrystallisation during hot compression. Deformation at a faster strain rate at a given temperature led to finer subgrains, resulting in higher strength. Dynamic precipitation took place concurrently and was strongly dependent on temperature. Precipitation of Q phase was found in the sample deformed at 400°C but none at 500°C. A larger volume fraction of precipitates was observed when samples were compressed at 400°C than at 500°C.

Explicit Simulation of Roll Forming Process with EAS Solid-shell Elements

Solid‐shell elements can be seen as a class of typical double‐surface shell elements with no rational degrees of freedom, which are more suitable for analyzing double‐sided contact problems than conventional shell elements. In this study, an EAS‐based solid‐shell element is implemented into the explicit finite element formulation to simulate roll forming process. A twelve‐parameter enhanced assumed strain (EAS) method is adopted to solve for the locking pathologies. Accuracy of the explicit solid‐shell finite element model is excised through two NUMISHEET benchmark tests. Afterwards, a U‐channel forming is simulated with the present explicit model. Numerical results of longitudinal strains and final geometries are compared with experiment as well as calculated by the commercial software ABAQUS. The solid‐shell element is found more applicable in dealing with roll‐forming process than ABAQUS inherent elements. Potential of the explicit solid‐shell model in analyzing cold roll forming process is confirmed.

The simulation of sheet metal forming processes via integrating solid-shell element with explicit finite element method

Solid-shell elements can be seen as a class of typical double-surfaced shell elements with no rational degrees of freedom, which are more suitable for analyzing double-sided contact problems than conventional shell elements. In this study, a solid-shell finite element model is implemented into the explicit finite element software ABAQUS/Explicit as a user-defined element, through which the sheet metal forming processes are simulated. The main feature of this finite element model is that the solid-shell element formulation is embedded into an explicit finite element procedure, compared to the previous studies on the solid-shell elements under the implicit finite element framework. To obtain a straightforward element, a complete integration scheme is adopted. No loss of generality, a twelve-parameter enhance assumed strain method is employed to improve the element’s behavior. Two benchmarks from the NUMISHEET conference and a U-channel roll-forming process are simulated with this explicit solid-shell finite element model. The calculated results are comparable with experimental and numerical results presented in the literatures.

Numerical and Experimental Study on Seam Welding Behavior in Extrusion of Micro-Channel Tube

Micro-channel tube with submillimeter-diameter channels is a kind of newly developed heat transfer tube based on theory of micro-scale heat transfer. As the micro-channel tube has multiple welding points and work under high pressure condition, welding strength is one of the key problems for the extrusion process. This paper presented a new method in evaluation of the seam welding strength formed in the extrusion process. Firstly, FE simulation is carried out for the status of the billet in the welding chamber during the extrusion process. Then, thermo mechanical experiment is done for the relationship between the welding strength and three key factors. Combining the relationship with the numerical results, welding strength of the micro-channel tube can be evaluated. Pressure bearing test shows that the evolution method is reliable. The study is helpful for the optimization the extrusion process and improvement of the seam weld quality.

Extrusion Simulation and Texture Study on Mg-Y Alloy

In order to study the influence of extrusion process on texture development of alloys, numerical simulation methods were used to simulate the round and shape extrusion process and deformation texture. Extrusion of Mg-Y magnesium alloy was carried out at the temperature of 673K with different ram speeds to verify the simulation results. Instead of using the Lagrangian FE method, the Arbitrary Lagrangian-Eulerian (ALE) method was employed in this study so that a more accurate description of the steady-state extrusion process can be achieved. By obtaining strain histories of specified material tracer particles, the coupling of deformation and crystal plasticity theory was applied to simulate the texture evolution in hot extrusion. The results showed that the texture simulation corresponded well with the experimental ones. The study proposes a method to analyze the steady-state extrusion process and texture evolution, and can be used as a useful tool in optimizing the extrusion process.

Simulation of the Extrusion Texture of Magnesium Alloy AZ31 Using Crystal Plasticity Finite Element Method

By using Eulerian adaptive modeling approach, both the round extrusion and sheet extrusion of magnesium alloy AZ31 were simulated. Furthermore, the history strains of material point flowing through the Eulerian domain was extracted and used as the foundation for defining the boundary conditions in the crystal plasticity finite element (CPFE) modeling for the extrusion texture. By virtue of this modeling method, the realistic grain boundaries can be approximated by using a 3D polycrystal generator and the intra-granular interactions can be well described. Both of the simulated round extrusion and sheet extrusion textures of alloy AZ31 show reasonable agreement with experimental results.

Thermo-mechanical coupled simulation of warm stamping of AZ31 magnesium alloy sheet

Uniaxial tensile test of a cross rolled magnesium alloy sheet was conducted under different temperatures and strain rates. The mechanical propriety of AZ31 magnesium alloy sheet was analyzed according to the true strain-stress curves. Then the non-thermal drawing process, during which the temperature of die, blankholder and blank is 200°C while the punch is kept at room temperature, was simulated by the thermo-mechanical coupled finite element method. The deformation behavior and the temperature change in the drawing process was investigated. Due to the heat conduction, there was non-uniform distribution of temperature along flange area, force transfer area and deformation area. Therefore the resistance of the force transfer area is enhanced and the warm formability of magnesium alloy sheet can be further improved. The thermo-mechanical coupled simulation provides a good guide for the development of non-isothermal drawing techniques.

Interference detection for direct tool path generation from measured data points

One of the main issues of the reverse engineering (RE) is the duplication of an existing physical part whose geometric information is partially or completely unavailable in measured form. In some industrial applications, physical parts are duplicated using three-axis CNC machines and ball-end mills. Many researches studied the problem of direct tool path generation from measured data point. However, up to the present, it appears that there is no reported study on interference detection in paths generated directly from measured data points. Interference detection is a curial problem in direct tool path generation from measured data points. This paper discusses the problem of local and global interference detection for three-axis machining in RE and proposes algorithms for local and global interference detection. With these algorithms, the measured data points captured from a physical part are analyzed and classified according to the shapes of the part. The method has been tested with several industrial parts, and it is shown to be robust and efficient especially for the part with free-form surfaces.

Effects of Hardening Models on CUO Forming and Springback Simulation of High Strength Line Pipes

The UOE process is an effective approach for manufacturing the line pipes used in oil and gas transportation. During the UOE process, a steel plate is crimped along its edges, pressed into a circular pipe with an open-seam by the successively U-O forming stages. Subsequently, the open-seam is closed and welded. Finally, the welded pipe is expanded to obtain a perfectly round shape. In particular, during the O-forming stage the plate is suffered from distinct strain reversal which leads to the Bauschinger effect, i.e., a reduced yield stress at the start of reverse loading following forward strain. In the finite element simulation of plate forming, the material hardening model plays an important role in the springback prediction. In this study, the mechanical properties of API X90 grade steel are obtained by a tension-compression test. Three popular hardening models (isotropic hardening, kinematic hardening and combined hardening) are employed to simulate the CUO forming process. A deep analysis on the deformation and springback behaviors of the plate in each forming stage is implemented. The formed configurations from C-forming to U-forming are almost identical with three hardening models due to the similar forward hardening behaviors. Since the isotropic hardening model cannot represent the Bauschinger effect, it evaluates the higher reverse stress and springback in the O-forming stage which leads to a failure prediction of a zero open-seam pipe. On the contrary, the kinematic hardening model overestimates the Bauschinger effect so that predicts the larger open-seam value. Specifically, the simulation results using the combined hardening model show good agreement in geometric configurations with the practical measurements.