Materials Science Forum Vols. 546-549

Abstract: By the severe plastic deformation, the equal channel angular pressing method (ECAP) has been used for producing metal materials with the ultra-fine grain size and specific mechanical properties, in particular high yield strength. Analytical approaches have also been studied by few researchers. However, none of the previous analyses have taken into account the strain hardening of the material by considering the microstructure evolution. In this paper, the deformation behavior and the strain harden of aluminum during equal channel angular pressing was calculated on the basis of a dislocation evolution model. Then simulated stress, strain and strain distribution and strain hardening were analyzed. The strain is seen to rapidly increase when the material passes the shear area. This makes a maximum value of stress. And for the congregation of dislocation density, the maximal value of strain increase with the process continuing.

Abstract: The horizontal direct chill (HDC) casting process is a well-established production route for an aluminum alloy ingot; however, the ingot may suffer from inhomogeneous microstructures and serious macrosegregation due to the unbalanced cooling condition and gravitational effect. In order to control the defect, a low frequency electromagnetic field has been applied in the HDC casting process and its influences on microstructures and macrosegregation have been studied. The results show that the low frequency electromagnetic field can improve macrostructures, reduce inhomogeneous microstructures and macrosegregation in HDC product; and two main parameters of the electromagnetic field—density and frequency influence the microstructures and solution distribution along the diameter of the ingot significantly. In the range of ampere-turns and frequency employed in the experiments, the optimum ampere-turns and frequency have been found to be 10000At, 30Hz.

Abstract: On the basis of analyzing the relationship between filling mould ability and surface tension of Al-Si alloy, a new method was put forward that filling mould ability can be fast evaluated by surface tension. To fast test surface tension of Al-Si alloy in front of furnace, a new apparatus had been developed and a new mould had been designed to appraise the ability of filling acute angle of Al-Si alloy by authors. By means of the self-developed new apparatus and new mould, the relationship between surface tension and filling mould ability had also been proved and gotten by many experiments on Al-Si alloys. Depending on the relationship the filling mould ability of Al-Si alloy can be evaluated by surface tension in few seconds before cast.

Abstract: Aluminum alloy sheet is becoming one of the main materials to take the place of steel components to reduce the vehicle mass due to the advantage of low special density, high strength and corrosion resistance. To predict the formability during the aluminium alloy sheet forming accurately, , a ductile fracture criterion developed by the authors, was for numerical simulation, in this paper. Fracture behavior in square-cup forming of the aluminium alloy LF21M was predicted by means of the criterion. Comparison of the predicted results with experimental values shows that the critical punch stroke and the fracture initiation position in square-cup forming of the aluminium alloy have been predicted successfully by the criterion.

Abstract: Low frequency electromagnetic casting is a new developed technology that appears in the recent years. In this paper, a comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during LFEC (low frequency electromagnetic casting) process. The model is based on a combination of the commercial finite element package ANSYS and the commercial finite volume package FLUENT, with the former for calculation of the electromagnetic field and the latter for calculation of the magnetic driven fluid flow, heat transfer and solidification. Moreover, the model has been verified against the temperature measurements obtained from one 7XXX aluminum alloy billet of 200mm in diameter, during the LFEC casting processes, respectively. There was a good agreement between the calculated results and the measured results. Further, the effects of electromagnetic frequency on fluid flow, temperature field and solidification during LFEC process have investigated numerically by using the mathematic model. The choosing criterion of the electromagnetic frequency during LFEC process has been used in order to obtain the best structure of the billets by analyzing the effects of fluid flow and temperature field on the solidification process in the presence of electromagnetic field.

Abstract: The key to enhance the precision of sheet metal stamping parts is accurate prediction and control of the springback. And one of the dominant factors of springback is the elastic modulus. The change of elastic modulus with plastic deformation was experimentally studied for the annealed LY12 and the annealed LF21 aluminum alloy respectively, and the changing role of elastic modulus with plastic deformation was obtained. By inverse pole figure from the X-ray diffraction analysis, the effects of the material original textures and their changing with plastic deformation on the elastic modulus were studied. And by positron annihilation lifetime spectrum , the influence of lattice distortion caused by crystal defect during plastic deformation on the elastic modulus was investigated. The micromechanism of the elastic modulus changing with plastic deformation for aluminum alloy is revealed, which lays a foundation for improving the stamping parts’ precision.

Abstract: A systematic homogenization treatment on Al-Li-Cu-Mg-Mn-Zr alloy, within the temperature range from 500
C to 540
C for 24h or 36h was conducted. Microstructure and alloying elements distribution before and after homogenization treatment were studied by optical microscope, SEM, EDS and XRD. The results showed that in the as-cast condition Cu segregated at grain boundary and formed θ(Al2Cu) phase. With the homogenization temperature increased and time prolonged, the amount of the remained particles at grain boundaries decreased, and the alloying elements distribution became more homogeneous. It has been found that the homogenization temperature has more obvious effects on the microstructure and mechanical properties than time. However, the alloy homogenized at temperatures above 540
C was overheated. Furnace cooling generates large amounts of precipitates in the matrix. The best homogenization parameter of the Al-Li-Cu-Mg-Mn-Zr alloy was determined as 530
C for 24 h followed by air cooling.

Abstract: The effect of pulse electric current with different density on solidification structures of hypoeutectic Al-7%Si, Al-10%Si and hypereutectic Al-22%Si alloys was studied. The shape and distribution of α-Al and Si were analysed.The experimental results showed that α- Al of Al-7% Si alloy is shortened when applied a pulse electric current during the solidification, and the microstructure turns from the obvious columnar crystal into rosette. After treatment with pulse electric current, the primary Si of Al-22%Si alloy was refined obviously. When the discharge voltage was 3kV, Si tended to gather at the edge of the sample and the primary α- Al appeared in this hypereutectic alloy. When the discharge voltage was 6kV, the primary Si was refined much more and distributes uniformly. Pulse electric current treatment had no evident effect on the approximate eutectic Al-10%Si alloy. The magnetic force was analysed under pulse electric current, and the reason of the phenomenon was discussed

Abstract: According to the characteristics of transient solidification and continuous rheology in roll casting process, and by modifying clamp system and loading way of molten metal, physical simulation of continuous roll casting process was realized in conjunction with Gleeble-1500 thermal-mechanical simulation testing machine in the present study. And relative rheologica1 constitutive models fitting the process were set up. A method to conduct multi-nonlinear thermo-mechanical coupling analysis suitable for aluminum strip roll casting process was explored by the secondary development of ANSYS finite element analysis software. The variation rules of temperature and deformation field were obtained. It has been found that the simulating models are compared with experimental results.

Abstract: In the investigation, the continuous extrusion forming (CONFORM) process of aluminum alloy rectangular hollow conductor has been studied by three-dimensional finite-element method based on Software DEFORM-3D. The rigid-viscoplastic constitutive equation was employed in the model. Distributions of velocity field, strain field, stress field and temperature field were obtained in the forming process. The results will give effective guidelines to optimize the processing parameters and to select, the die structure and die materials.