Research On The Microstructure And Properties Of Nano TiN Reinforced Al-7Si-0.3Mg Aluminum Matrix Composites

Particle reinforced aluminum matrix composites(PRAMCs)have widespread applications in aerospace,automobile,shipping,electronics and etc.due to the properties of low density,high specific stiffness and strength,good wear resistance and favorable thermal and electrical conductivity.Therefore the study of particle reinforced aluminum matrix composites has been the point in fields of metal matrix composites.For the traditional preparation of PRAMCs,it is difficult to disperse nano reinforcement in the matrix due to the high surface free energy of nano reinforcement and the poor wettability between ceramic particles and molten matrix.As a result,nano particle usually trend to agglomerate which weakens the strengthening effect.In this paper,TiN/Al-7Si-0.3Mg aluminum matrix composites with 0.1wt% TiN addition were prepared with assistance of mechanical stirring and ultrasonic processing.With the aid of core-shell structure TiN/Al coated powders,nano TiN can wet well with aluminum matrix,so as to improve the microstructure of the alloy and improve the properties of the alloy to some extent.In the experiment,Al-7Si-0.3Mg(A356)and TiN/Al-7Si-0.3Mg prepared by adding TiN directly were served as contrast samples.A series of test and analysis method,such as OM,XRD,SEM,tensile test,electrochemical test and etc.were applied to characterize A356 alloy and its composites.Some important conclusions were obtained as follow:1)According to the Minimum Energy Principle,in the system composed of molten aluminum,nano particles and molten salt,nano TiN can coat on the micro Al powders spontaneously to reduce the Gibbs free energy of the system,forming the TiN/Al cladding structure;2)Nano TiN can provide effective heterogeneous nucleation points during the solidification of A356 alloy,improving the nucleation rate of ?-Al.It can also refine the grain of composites,transform ?-Al dendrite to equiaxial crystal,and further cause eutectic silicon became tiny and scattered;3)In the quenching process,dislocation point will be introduced due to addition of TiN,which can promote the nucleation of eutectic silicon during the aging precipitation.Meanwhile,for composites with addition of TiN/Al,TiN which at the edge of eutectic silicon can impede the precipitation and growth of eutectic silicon and restrain the formation of needle shaped eutectic silicon to some extent.Therefore,there are a large number of small rounded eutectic silicons in the composite with addition of TiN/Al.4)With addition of TiN/Al,the mechanical properties of composite increased significantly after heat treatment compared to A356 alloy,especially for the elongation,compared with matrix,it is improved to 191.9 pct after heat treatment.The hardness of A356 with TiN/Al also achieved 3.7 pct and 13.1 pct improvements before and after heat treatment compared with the matrix;5)For solution treatment with incomplete aging treatment,the best heat treatment process for A356 and its composites is 540? solution for 4h and 155? ageing for 6h;for solution treatment with complete aging treatment,the best heat treatment process for A356 and its composites is 540? solution for 4h and 175? ageing for 2h;6)When there is no current in the circuit,A356 with TiN/Al showed the strongest corrosion resistance because compared to the other two samples it has the most continuous oxide film on the surface.When current flows through the circuit,from the perspective of corrosion thermodynamics,A356 with TiN/Al showed the strongest corrosion resistance.Based on corrosion kinetics,A356 with TiN showed the slowest corrosion rate.But according to the comprehensive results,A356 with addition of TiN/Al coated powder showed the lightest corrosion degree.