Study On Preparation Of Nano SiC_P/A356 Aluminum-Matrix Composites And Control Technologies For Microstructure And Property

Nanoparticles reinforced aluminum matrix composites are featured by light weight,high specific strength,high specific stiffness,good toughness and other excellent properties,and have broad application prospects in aerospace,transport and other fields.However,there are great difficulties in preparation of Al matrix composites by liquid method,due to the high specific surface area of nano-Si C_Pand poor wettability between Si C_Pand aluminun melt,which would result in aggregative trend of nano-Si C_P.It is urgent to study new preparation methods and related basic technical problems.Therefore,this paper proposes a new process that combines high-energy ball milling,ultrasonic vibration and squeeze casting to prepare nano-Si C_P/A356 aluminum matrix composite.It is realized that nano-Si C_Pare added to Al melt smoothly and the distribution of nano-particles has been effectly improved.Meanwhile,the action mechanism of nanoparticles on the microstructure and properties of composite is also studied.The pre-dispersion of nano particles is realized by high-energy ball miiling process,and is conducive to the addition into aluminum liquid.Due to the large specific surface area of nanoparticles,the interface energy to be overcome by expanding nano-Si C_P/Al in liquid aluminum is much larger than that of micron particles,which brings great difficulties for the addition of nanoparticles.Millimetre-sized Si C_P/Al pre-composed granuals replace Si C_P/Al interface with Al/Al interface,which solved the adding problem caused by poor wettability.Before high energy ball milling,the surface of 40 nm Si C_Pwas oxidized to obtain a dense oxide layer of Si O₂about 4 nm,which inhibited the interfacial reaction between the nanoparticles and aluminum.By studying apparent viscosity and fluidity of composite melt,the optimal ultrasonic treatment process was formulated,and uniform dispersion of nano Si C_Pwas realized.Nano particles in liquid Al are affected by van der Waals force,and have a great tendency to aggregate.The relationship between particle content,melt temperature,apparent viscosity and fluidity of melt was studied and the corresponding mathematical model was obtained.It was found that the addition of nano-Si C_Pwould increase the apparent viscosity of Al liquid and reduce the fluidity.As a result the effect of ultrasonic treatment would be limited.To solve this problem,the time and temperature of ultrasonic treatment were optimized,and the distribution regulation of nano Si C_P,microstructure and mechanical properties of composite were effectively promoted.The optimal process of ultrasonic treatment was obtained at 700℃and 3 min,and the ultrasonic time was reduced by about 80%compared with existing researches.With the optimum parameters,the yield strength,ultimate tensile strength and elongation in as-cast state of 2 wt.%nano-Si C_P/A356 composites are 163 MPa,268 MPa and 7.4%,which are increased by83%,29%and 71%compared with untreated A356 alloy,respectively.The addition of modification element Sr can refine eutectic Si phases of 1 wt.%nano-Si C_P/A356 composites to a certain extent,but there has no obvious modifying effect on 2 wt.%nano-Si C_P/A356 composite.Because of oxidation tendency of Sr,the active ingredient of modification is consumed by O element which is introduced by particle pre-treatment.The heat treatment of nano-Si C_P/A356 has been studied,it is found that aging precipitation can be accelerated by addition of nano-Si C_P,by comparing the aging behavior of A356 alloy and 1 wt.%nano-Si C_P/A356 composite.The peak aging hardness of 1 wt.%nano-Si C_P/A356 composite can reach 114.3 HV,10.7 HV,and its tensile strength,yield strength and elongation are 335 MPa,226 MPa and 5.6%,which are increased by 20%,18%and 6%compared with A356-T6,respectively.The addition of nano-particles reduces fluidity of the melt,it is difficult to fill the composite melt during casting.Squeeze casting and other forming processes with external force can solve the problem of the low fluidity.The effects of applied pressure on particle distribution,microstructure morphology and mechanical properties of the composites have been studied.With applied pressure increasing from 0 to 100 MPa,the distribution uniformity of nano-Si C_Pis improved.Secondary dendrite spacing ofα-Al and geometric parameters of eutectic Si decrease significantly in nano-Si C_P/A356 composites with the improvement of mechanical properties.When applied pressure changes from 100MPa to400MPa,the increase of microstructure and properties decreases.