Fabrication And Characterization Of In-situ Nano Highly Dispersed ?-Al₂O_3p/A356 Aluminium Matrix Composites

A356 alloy is the most representative hypoeutectic aluminum alloy in ASTM B686 standard,due to its high strength,good casting performance,low defect sensitivity,excellent machinability and welding performance,and good corrosion resistance in atmospheric environment.It is the best comprehensive performance of aluminum alloy.Since the beginning of the 21st century,people have begun to pay attention to the lightweight of automobiles,due to the key issues such as climate change and energy crisis.Aluminum alloys have high specific strength,low density and high specific stiffness.However,some questions still exist for the aluminum alloys,for example low strength and elongation compared with traditional steel materials.The strength,hardness,elongation of aluminum alloys can be effectively improved by adding ceramic particles into metal materials and microalloying with a small amount of alloy elements.Therefore,in this research,it has proposed to combine in-situ reaction synthesis technology with microalloying technology to fabricate nano?-Al₂O₃ particles reinforced A356 aluminum matrix composites with high strength and high plastic toughness,explore the synergistic effect of in-situ?-Al₂O₃ particles and alloying element Co on the structure and properties of A356 aluminum alloy,and study the evolution rules and strengthening mechanisms of hot rolling and heat treatment on the structure and properties of the composites.It shows that,for the structure analysis of as-cast composites,the in-situ reinforced particles of?-Al₂O₃ can be uniformly distributed in the aluminum matrix.The size of the reinforced particles is 5¹00nm and the average size is 80nm.The morphology of the particles is mostly spherical,and the interface between the nanoparticles and the matrix is clean and well bonded.In addition,the SDAS?secondary dendrite arm spacing?and maximum length of eutectic silicon in composites decrease first and then increase with the enhancement of volume fraction of in-situ reinforced particles.When the particle content is up to 0.6vol.%,the SDAS is 13¹4?m in average,and the maximum length of Si phase is 19?m.The reduced Al₉Co₂ phase can refine the silicon phase by neutralizing the?-Al₅FeSi phase.When the particulate content is up to 0.8vol.%,the composites exhibit the finest grain size,and the average grain size is about 200?m.There are a lot of holes in the direct-cast composites.Most of the holes can be eliminated after vacuum treatment,and the melt can be purified at the same time.It shows that,for the microstructure analysis of the composites after hot rolling,hot rolling can eliminate the defects such as porosities in as-cast composites.The second phase is streamlined along the rolling direction,the particle distribution is more uniform,the particle size is significantly reduced,and the average particle size under the optimized condition is 50 nm.In addition,different rolling deformation and rolling temperature have significant effects on the microstructures of the composites.The mechanical properties of hot-rolled materials show that the optimum rolling deformation is 83%and the rolling temperature is 450°C.Under this process,the ultimate tensile strength of the composites reaches the maximum value.The ultimate tensile strength is 175 MPa,which decreases by 20.45%compared with that before deformation,and the elongation is 9%,and increases by 111%compared with that before deformation.After T4 heat treatment,the second phase in the matrix alloy and composite is partially dissolved,and the remaining second phase is more uniformly distributed in the matrix.The strength of the material can be effectively improved by the solid solution of Si,Co and other elements into the aluminum matrix.At the same time,the elongation of the material is greatly increased due to the dissolution of the second phase.The mechanical properties of as-cast materials after T4 heat treatment show that the strength and elongation of the composites are greatly improved compared with that of A356 matrix.When the particle content is up to 0.6vol.%,the ultimate tensile strength and elongation of the composites are 277 MPa and 6.5%,respectively,in comparison with A356 matrix,it increased by 43.6%and85.7%respectively.The mechanical properties of hot-rolled materials after T4 heat treatment show that the optimum rolling deformation is 83%and the rolling temperature is 450°C.The ultimate tensile strength and elongation of the composites reach the maximum,which are increased by 1.4%and 161.5%from 277 MPa and 6.5%to 282 MPa and 17%respectively.It manifests that,for the analysis of strengthening and toughening mechanism of composites,the strengthening mechanism of in-situ particle reinforced composites is the result of the combination of Orowan strengthening and CTE strengthening;the mechanism of improving the strength and toughness of composites by hot rolling is Orowan strengthening and deformation strengthening;and the strengthening mechanism of composites strengthened by heat treatment is solution strengthening and Orowan strengthening.