| The ZL114 A alloy is extensively used in the automobile, rail transportation, aviation and other fields, due not only to its excellent properties in castability, weldability, corrosion resistance, but also to its low density and excellent mechanical properties. The dimension and morphology of eutectic silicon in ZL114 A alloy are crucial for the mechanical properties of the alloy, in particular fracture elongation. Modification can significantly improve the size and morphology of eutectic silicon, thus improving the mechanical properties of the alloy. Furthermore, modification of eutectic silicon is closely related to solidification process. The dimension and morphologies of eutectic silicon at different cooling rates and modified by Al-10 Sr alloys with different states were investigated, the sensitivity of modification of eutectic silicon with Al-10 Sr alloy to the cooling rate was studied, and the differences of modification with Al-10 Sr alloys with different states were disscssed.By remelting and rapid-solidification and subsequent rolling process to Al-10 Sr master alloy, rapid-solidified and rolled Al-10 Sr master alloys were prepared. Microstructure showed that the dimensions of primary Al4 Sr phase in conventional as-cast, rapid-solidified and rolled Al-10 Sr alloys decrease gradually, the average width were 26.4μm, 15.8μm and 3.3μm respectively. DSC thermal analysis showed that their melting enthalpy were 441.8J/g、375.9J/g and 315.6J/g respectively。ZL114A alloys modified by Al-10 Sr master alloys with different states were investigated, the Sr content was 0.02 wt.% and the melt was hold for 30 minutes at 750 ℃. Experimental results showed that the modification effects of conventional as-cast, rapid-solidified and rolled Al-10 Sr alloys become worse, which is caused by a combination of insufficient addition amount of strontium and the different burning loss of strontium during isothermal holding. In addition, increasing the cooling rate during solidification process can narrow the difference in modification effect to a certain extent.ZL114A alloys modified by Al-10 Sr master alloys with different states were studied, the Sr content was 0.02 wt.% and the melt was hold for different time at 750 ℃. The results showed that the best modifications of eutectic silicon for conventional as-cast and rapid-solidified Al-10 Sr are obtained at 30 min and 15 min respectively, while the modifications for rolled Al-10 Sr is terrible at different holding time at 750 ℃. In order to obtain the optimum of modification process, we planed to lower the melt temperature. ZL114 A alloys modified by rolled Al-10 Sr master alloys with different holding time were investigated, the Sr content was 0.02 wt.% and the melt temperature was 720 ℃. The results indicated that modification of eutectic silicon for rolled Al-10 Sr reach to the best performance at 15 min. Thus, the bad modifications of eutectic silicon for rolled Al-10 Sr alloy at 750 ℃ are due to the long existence of free strontium atoms in the melt as a result of the short incubation time, the severe burning loss of strontium and the insufficient addition amount of strontium, all these lead to a low effective Sr content in the melt. Based on the above analysis, the best incubation time of modification of eutectic silicon for Al-10 Sr alloy relates not only to the dimension of primary Al4 Sr phase, but also to the melt temperature. With the decrease of the dimension of primary Al4 Sr and the increase of melt temperature, a shorter incubation time is obtained, but the burning loss of strontium aggravates at the same time. It should be noted that the short effective time for modification of eutectic silicon in the experiment is because of the insufficient addition amount of strontium and the burning loss of strontium during isothermal holding.ZL114A alloys modified by Al-10 Sr master alloys with different states were investigated, the Sr content was 0.04 wt.% and the melt was hold for 15 minutes at 720 ℃. Experimental results showed that the modification effects of conventional as-cast, rapid-solidified and rolled Al-10 Sr alloys become better, the dimensions of eutectic silicon are getting smaller and the the branches of silicon increase significantly. These differences in modification effect are due to structural heredity effect of Al-10 Sr master alloys and solid-liquid interface reaction kinetics of decomposoition of Al4 Sr phase.Research on unmodified ZL114 A alloys solidified in sand mould, cast iron mould and copper mould showed that, the increase of cooling rate can not only refine the primary α-Al, but also significantly improve the size and morphology of eutectic silicon, which transforms the eutectic silicon from coarse plate-like to fine needle-like and markedly increase the branches of eutectic silicon. With the improvement of the cooling rate during solidification, the average dimension and aspect ratio of eutectic silicon in T6 temper alloy decrease gradually, thus the mechanical properties, especially fracture elongation can be significantly improved. Besides, the fracture mode of the material transforms from brittle fracture to ductile fracture.Research on as-cast Al-10 Sr modified ZL114 A alloys solidified in sand mould, cast iron mould and copper mould showed that, the modification of eutectic silicon with Al-10 Sr alloy is sensitive to the cooling rate. At low cooling rate, modification of eutectic silicon is not sufficient, the dimension and spacing of eutectic silicon are large. With the increase of cooling rate, the dimension of eutectic silicon is significantly decreased and the branches of eutectic silicon show a sharp increase. For the T6 temper alloy solidified in copper mold, the ultimate tensile strength reach 331.7 ± 4.3MPa, elongation reach 9.4 ± 2.7% and the quality index reach 477.6MPa, which increased by 10.8%, 327.3% and 36.2% respectively, compared with T6 temper alloy solidified in sand mold. |
PDF Full Text Request