| This paper presented a comparative study on themicrostructures and properties of7075Al alloy cast by Direct Chilling (DC)and Low Frequency Electromagnetic Casting(LFEC). The microstructure differences were compared firstly, and then the changes in microstructure and properties of ingots after homogenization and aging were studied in this paper to review the effects of LFEC on the processing behaviors of7075aluminum alloy.The results showed that the quality of LFEC ingots was improved. The microstructure of the ingots was greatly refined, its grains became fine and uniform, and the area fraction of the constituent phases on the grain boundary was also reduced. The over-heating temperature of7075aluminum alloy ingot was478℃.With increase in time during homogenization, non-equilibrium eutectic phase in the alloy gradually dissolved, the areal fraction of the residual phases decreased while the electrical conductivity of alloy gradually increased. And the dissolution of residual phases in LFEC ingot was faster than that in DC ingot. The eutectics transformed to S(Al2CuMg) phase with higher melting point during single-step homogenization treatment at465℃. The dissolved temperature of S phase was490℃. S phase could not be dissolved into matrix entirely after homogenization at465℃/24h。After three-step homogenization (200℃/2h+460℃/6h+480℃/12h), S phase dissolved into matrix entirely, over-heat didn’t occur, and the electrical conductivity of7075aluminum alloy was higher than that of single-step homogenization and as-cast.The7075aluminum alloy had a strong ability of aging strengthening. When aging at120℃,the alloy reached its peak strength in24hours in single-step aging. The strength, Vickers-hardness and electrical conductivity of extruded bar of the alloy DC cast was666.37MPa,189.87, and32.3%IACS respectively; the strength, Vickers-hardness and electrical conductivity of extruded bar of the LFEC alloy was673.50Mpa,194.62, and32.4%IACS respectively. LFEC extruded bar had a much stronger ability of aging strengthening. Compared with T6, the comprehensive properties of the alloy after retrogression and re-ageing were better. The temperature and time of retrogression had strenger effect on mechanical properties of the alloy. As the aging time of retrogression increased, its strength and hardness decreased while the electrical conductivity increased greatly. And LFEC extruded bar had better comprehensive properties at the same RRA condition. In this experiment, the best RRA process of7075aluminum alloy was120℃/24h+180℃/30min+120℃/24h, the strength, Vickers-hardness and electrical conductivity of DC extruded bar was669.14MPa,194.41, and35.1%IACS respectively; the strength, Vickers-hardness and electrical conductivity of LFEC extruded bar was676.64Mpa,197.88, and35.7%IACS respectively. |
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