Effect Of New Al-5Ti-1B-7Sm Refinement Modification On The Microstructure And Properties Of Hypoeutectic Al-Si Alloy

The hypoeutectic Al-Si alloy is received more attention,due to its high strength and low density,but it appears coarse primary α-Al phase and lamellar eutectic silicon microstructure during the solidification,thus reducing the mechanical properties of the alloy.Refinement modification can significantly improve the morphology of eutectic silicon,so it has been widely applied in industrial production.In this paper,the ZL114 aluminum alloy was selected as the research object.First,based on the 1 wt.%Al-5Ti-1B and variable rare earth Sm composited refinement modification,Al-5Ti-1B-XSm refinement modification was optimized and prepared.The effect of refinement modification time and temperature on the microstructure and the properties were studied,and refinement modification mechanism of the ZL114 alloy modified with Al-5Ti-1B-Sm was investigated.First,Al-5Ti-1B addition is fixed at 1 wt.%,Al-5Ti-1B-XSm refinement modification was optimized by changing Sm addition.The results show that the secondary dendrite arm spacing of the α-Al phase and the average ratio of the eutectic silicon are decrased firstly and then increased with the increase of Sm addition.When 0.07 wt.% Sm was added,the secondary dendrite arm spacing of the α-Al phase and the average ratio of the eutectic silicon are smallest,and are11.24 μm and 2.53,respectively.Meanwhile,the tensile strength and microhardness are maximum,and are 225 MPa and 63.2,respectively.According to the conservation of atoms,the optimum refinement modification is Al-5Ti-1B-7Sm.The secondary phases of Al-5Ti-1B-7Sm are identified as Ti₂Al₂₀ Sm and TiB₂.The Ti B2 distributed dispersedly,and no segregation phenomenon occurred.The optimum processing parameters of ZL114 alloy modified with1 wt.%Al-5Ti-1B-7Sm refinement modification are 720 ℃ × 60 min.The secondary dendrite arm spacing of the α-Al phase and the average ratio of the eutectic silicon are smallest,and are 9.28 μm and 1.84,respectively.The tensile strength and micro-hardness are maximum,and are 265 MPa and 70.4,respectively.The fracture of the specimen is characterized by ductile dimple,and fracture surface are obviously refined.The analysis of refinement modification mechanism show that there are eight groups orientation relationships between α-Al and Ti2Al20 Sm,which can be used as heterogeneous nucleation particles of α-Al,and improving degree of refinement of α-Al.When the alloy is refined and modified with 1 wt.%Al-5Ti-1B-7Sm,the nucleation temperature of primary phase is increased to 609.7 ℃,on the contrary,solidification time of primary phase is decreased.That is to say,the nucleation rate of primary phase increased and its growth was suppressed.Meanwhile,the nucleation temperature of eutectic phase are 567.5 ℃ and its solidification time 2.6 s,which are lower than unmodified alloy.The reduction of nucleus and the shortening of solidification time are major reasons for fibrous morphology of eutectic silicon.In addition,the low growth rate of eutectic silicon and the characteristic of rough growth interface are other factors for fibrous morphology of eutectic silicon.