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Research on Thixo-forging Process of Semi-solid6061Alloy

Author: LiXiaoFeng
Tutor: WangPing
School: Northeastern University
Course: Materials Processing Engineering
Keywords: 6061alloy semi-solid thixo-forging reheating FEM simulation heat treatment
CLC: TG249.9
Type: Master's thesis
Year: 2011
Downloads: 104
Quote: 0
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Abstract


By the trend of automotive lightweight, casting aluminum wheels have been popularized in the auto industry with its characteristics of relatively low price, good performance, beautiful appearance and so on. But forging aluminum wheels with complex production process and high cost are used in special areas such as sports car, as casting ones can no longer meet the performance requirements. Semi-solid forging is a near net shape technology, and the forming parts with this technology have close performance but lower cost to forging ones and more effective than casting ones in lightweight, so it has an important meaning to apply this technology in the production of automobile wheels.Semi-solid forming technology, FEM,6061alloy have been introduced in this paper, and the advantages and prospects of the application of thixo-forging with semi-solid6061alloy in the production of automobile wheels have been described. In the experimental parts, the structure characteristics of the semi-solid6061alloy billet was studied prepared by electromagnetic semi-continuous casting technology with raw material of pure aluminum etc., and reheating experiments were also carried out to study the microstructure evolution and to define the optimum reheating process; using DEFORM-3D FEM simulation platform, the characteristics and variation of metal flow, temperature field, strain field, stress field and die load were studied to get optimum forming process; Finally, the influence of solution temperature, solution time, aging temperature and aging time on microstructure and performance of forming parts were studied to define the optimal heat treatment process, and a verification experiment conducted. The results are as follows:(1) Semi-solid6061alloy ingot with good surface quality, uniform and roselike grains can be obtained by electromagnetic semi-continuous casting; the equal-area-circle grain diameters of rim, middle and center part are64.0μm,84.1μm and80.5μm, and the minimum roundness reached1.5; the solidus and liquidus of obtained alloy are580.34℃and658.70℃, and with53.08%liquid fraction at650℃(2) The quenching structure changed significantly and became uniform equiaxed grains by reheating at630℃for5～10min; structure with similar uniform equiaxed grains and proper solid fraction obtained by reheating at650℃for5min;(3) The temperature of individual regions were below the solidus in the final stage of filling and healing at the pressing speed of25mm/s, so there were likely folding and insufficient filling defects; the optimum forming process is press at the speed of50mm/s under the mold temperature of400℃, and the vertical load of top die is382kN at final forging moment;(4) Strength of the specimen after heat-treat significantly improved with the increase of solution temperature and time, and it also improved with the increase of aging temperature; strength appeared a downward trend after aging for more than10h with the emergence of the phenomenon of over-aging; aging temperature and time affected elongation obviously; elongation decreased and then increased significantly with the increase of solution temperature, and increased with the increase of solution time, but decreased significantly with the increase of aging temperature and time;(5) The main strengthening phase of semi-solid thixo-forging parts after heat-treat is Mg2Si dispersed in the base body and formed significant precipitation belt along grain boundaries with the increase of solution temperature and time; the solution process accelerated and the solution amount of segregated phase increased with the increase of solution temperature; there is a small amount of β(Fe2Si2Al9) phase along grain boundaries;(6) The optimal heat treatment process obtained by orthogonal experiment is as follows: solution process at555℃for65min followed by water quenching at room temperature, then aging at165℃for8h immediately with air cooling finally; the hardness of the specimen obtained by the optimal heat treatment process is142.3HV, and tensile strength reached392MPa with a elongation of11.5%.

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