Study On The Microstructure And Properties Of Nano Al₂O₃/A356 Composites

A356 aluminum alloy has excellent casting performance and fine corrosion resistance in natural environment.It is the material used most widely for vehicle and motorcycle hub,but its strength and plasticity is not prominent,which limits its application in higher mechanical property requirement.In order to improve the strength and plasticity of A356 aluminum alloy,nano Al₂O₃/A356 composites were prepared by stir casting method and semi-solid squeeze method,in which the A356 aluminum alloy was used as matrix and nano Al₂O₃particle as reinforcement.Then the effects of chemical nickel-plating with nano Al₂O₃particle,semi-solid squeeze and T6 heat treatment on the microstructure and tensile properties of the composites were studied.The main results are as follows:?1?After chemical nickel-plating,metal Ni was deposited on the surface of nano Al₂O₃ particles and nano Ni-Al₂O₃ particles,compound of Ni and nano Al₂O₃particles,was obtained.?2?The study of the microstructure and properties of stir casting 1.5wt.%nano Al₂O₃/A356 composite and 1.5wt.%nano Ni-Al₂O₃/A356 composite show that chemical nickel-plating with nano Al₂O₃ particle can effectively refine the grain of the matrix,improve the dispersion of particles and increase the tensile properties of the composite.The particles in the 1.5wt.%nano Al₂O₃/A356 composite are seriously agglomerated and the grain size of the matrix is 78?m,which is 77?m smaller than A356 alloy.The particles in the 1.5wt.%nano Ni-Al₂O₃/A356 composite are slightly agglomerated and the grain size of the matrix is 45?m,which is 110?m smaller than A356 alloy.Compared with A356 alloy,tensile strength of 1.5wt.%nano Ni-Al₂O₃/A356 composites is increased 10.4%,elongation is descended 7.8%.?3?The microstructure and tensile properties of the stir casting and semi-solid squeeze 1.5wt.%nano Ni-Al₂O₃/A356 composites were compared,the results showed that semi-solid squeeze can obviously reduce grain size of the matrix,improve dispersion of nano Ni-Al₂O₃ particles and increase the tensile properties of the composites.The nano particles in semi-solid squeeze?squeeze temperature is 585?and squeezed 3 passes?1.5wt.%nano Ni-Al₂O₃/A356 composites are dispersed in the grain boundary and the grain size of the matrix is 23?m.The tensile strength and elongation of the semi-solid squeeze 1.5wt.%nano Ni-Al₂O₃/A356 composite are298MPa and 9.0%,respectively,which were 22.6%and 5.9%higher than those of stir casting 1.5wt.%nano Ni-Al₂O₃/A356 composites.?4?The optimum squeeze temperature for the preparation of semi-solid squeeze nano Ni-Al₂O₃/A356 composites is 585?,and the best squeeze channel is 4 pass.The grain size of the semi-solid squeeze?squeeze temperature is 585?and squeezed 4passes?1.5wt.%nano Ni-Al₂O₃/A356 composite is 18?m,the yield strength,tensile strength and elongation are 212Mpa,293MPa and 10.6%,respectively.?5?The effect of T6 heat treatment on the microstructure and properties of stir casting nano Ni-Al₂O₃/A356 composites and semi-solid squeeze nano Ni-Al₂O₃/A356composites was studied and the results show that after T6 heat treatment,the microstructure of the stir casting 1.5wt.%nano Ni-Al₂O₃/A356 composite was homogenized.The grain is transformed from dendrite and columnar crystal to ellipsoidal crystal,and the tensile strength is increased by 52.8%,the elongation is reduced by 15.8%.Compared with the non heat treated state,tensile strength of the semi-solid squeeze 1.5wt.%nano Ni-Al₂O₃/A356 composites is increased by 53.4%,the elongation is reduced by 8.6%after T6 heat treatment.