Research on AlSi7Mg Alloy Semi-Solid Billet Fabricated by RAP
JIANG Jufu1(), ZHANG Yihao1, LIU Yingze1, WANG Ying2, XIAO Guanfei1, ZHANG Ying1
1.School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China 2.School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China
Cite this article:
JIANG Jufu, ZHANG Yihao, LIU Yingze, WANG Ying, XIAO Guanfei, ZHANG Ying. Research on AlSi7Mg Alloy Semi-Solid Billet Fabricated by RAP. Acta Metall Sin, 2021, 57(6): 703-716.
Semi-solid metal processing is a metal-forming technology that combines the advantages of casting and forging, realizing near-net forming high-performance parts with complex structures. Research on semi-solid processing of AlSi7Mg alloys mainly focuses on rheology, and the preparation of high solid fraction AlSi7Mg semi-solid billets by the solid phase method has been largely neglected. In fact, semi-solid technology is more significant than casting at higher solid fractions. The present study investigates semi-solid billets of AlSi7Mg aluminum alloy with a high solid fraction, prepared by the recrystallization and partial re-melting (RAP) method. The effects of upsetting temperature, compression ratio, semi-solid isothermal treatment temperature, and holding time on the billet microstructure were investigated by DSC test, upsetting experiment, semi-solid isothermal treatment experiment, OM observations, and Image Pro Plus image processing software. The microstructure of the semi-solid billet during isothermal compression was slightly affected by temperature but was beneficially refined by increasing the compression ratio. The optimal hot upsetting parameters were 240oC and 40% deformation. During the semi-solid isothermal treatment, increasing the holding temperature gradually increased the size of the solid phase grains in the microstructure. As the holding time increased, the solid phase particles in the semi-solid structure initially grew slowly, and thereafter rapidly grew to a stable size. The changes in roundness of the solid particles were more complicated. The average grain size of the billet prepared by the RAP method was 64~117 μm, and the shape factor was 0.76~0.89. The linear relationship between cubic coarsening of the average semi-solid grain size and isothermal time was nonobvious at isothermal temperatures below 599oC but was evident at temperatures of 599oC. Below 599oC, the grain coarsening is affected by Ostwald ripening, coalescence, recrystallization, and melting; while at 599oC, the grain coarsening was dominated by Ostwald ripening.
Table 1 Chemical composition of AlSi7Mg aluminum alloy
Fig.1 DSC curve of AlSi7Mg aluminum alloy (a) and its liquid phase ratio-temperature curve (b)
Fig.2 Upset samples under different deformations of AlSi7Mg aluminum alloy
Fig.3 The influence of different deformation temp-eratures and deformation amounts on AlSi7Mg aluminum alloy semi-solid billet
Fig.4 OM images of microstructures with different deformation temperatures at 40% deformation of AlSi7Mg aluminum alloy semi-solid billet (isothermal temperature T = 599oC, isothermal time t = 11 min)
Fig.5 OM images of microstructures with different deformation temperatures at 30% deformation of AlSi7Mg aluminum alloy semi-solid billet (T = 599oC, t = 11 min)
Fig.6 OM images of microstructures with different deformation temperatures at 20% deformation of AlSi7Mg aluminum alloy semi-solid billet (T = 599oC, t = 11 min)
Fig.7 OM images of microstructures of AlSi7Mg aluminum alloy semi-solid billet at different isothermal time (T = 581oC)
Fig.8 OM images of microstructures of AlSi7Mg aluminum alloy semi-solid billet at different isothermal time (T = 584oC)
Fig.9 OM images of microstructures of AlSi7Mg aluminum alloy at different isothermal time (T = 588oC)
Fig.10 OM images of microstructures of AlSi7Mg aluminum alloy at different isothermal time (T = 593oC)
Fig.11 OM images of microstructures of AlSi7Mg aluminum alloy at different isothermal time (T = 599oC)
Fig.12 Scatter diagram and linear fitting diagram of D3 with isothermal time (K—coarsening rate of solid grain, R2—coefficient of determination )
Fig.13 D (a) and f (b) with isothermal temperatures in different isothermal time groups
Fig.14 D and f with different isothermal temperatures at isothermal time of 11 min
Fig.15 Comparison of microstructures of AlSi7Mg aluminum alloy semi-solid billets prepared by different methods (RAP—recrystallization and partial remeltling, MHD—magnetohydrodynamic, MITP—melt isothermal treatment process)
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