Study On Joining Processes Of SiC_p/8009Al Composite And Aluminum Alloys And Stability Of Al₁₂（Fe,V）₃Si

SiC_p/8009Al composite is an ideal material for lightweight engine pistons due to its excellent heat resistance,high specific strength and specific stiffness and good abrasion and corrosion resistance.However,SiC_p/8009Al composite also occupies a poor forming performance and needs to be joined to aluminum alloys for application.A356 aluminum alloy has good castability and can achieve ideal combination of high strength,good plasticity and impact attenuation through appropriate heat treatment.In this work,compound casting was employed to achieve metallurgical joining of SiC_p/8009Al composite and A356 aluminum alloy.SEM,EDS,XRD,EBSD and tensile tests were used to investigate the optimal compound casting processing parameters and the formation mechanism of the compound casting transition zone.AlSi20 and rapidly solidified 8009Al alloy powders were used to prepare AlSi20/8009Al alloy.By studying the stability of Al₁₂（Fe,V）₃Si dispersion and its evolution to Al₉Fe₂Si₂and the nano-sized V-rich phases under the condition of external Si addition,the formation mechanism of Al₉Fe₂Si₂phase in the transition zone was revealed.The main results are listed as following:（1）As increasing the preheating temperature of the SiC_p/8009Al composite from 395 to 450 ^oC,the compound casting interfacial bonding strength of SiC_p/8009Al composite and A356 aluminum alloy increased first and then decreased.The optimal compound casting process parameters were obtained:the pouring temperature of the A356 aluminum alloy melt was 730 ^oC,and the preheating temperature of the SiC_p/8009Al composite was 420 ^oC.（2）The thickness of the compound casting transition zone increased as increasing the preheating temperature of the SiC_p/8009Al composite.The transition zone containedα-Al,SiC,Si and Al₉Fe₂Si₂phases.SiC particles were uniformly distributed throughout the transition zone.In the transition zone,from the SiC_p/8009Al composite side to the A356 aluminum alloy side,the average grain size ofα-Al increased,the size and distribution density of Al₉Fe₂Si₂phase increased and decreased respectively,and both the size and distribution density of eutectic Si increased.The formation of the compound casting transition zone was based on the mutual diffusion of SiC particles,Fe element and Si element andα-Al and Al₉Fe₂Si₂phase nucleated on the SiC_p/8009Al composite.（3）The external Si addition promoted the instability and phase evolution of Al₁₂（Fe,V）₃Si dispersion in the originally stable temperature range.Increasing annealing temperature（480～540 ^oC）or annealing time（540 ^oC,15～60 min）decreased the contents of Si phase and Al₁₂（Fe,V）₃Si dispersion while promoted the formation of acicular Al₉Fe₂Si₂phase in the AlSi20/8009Al alloys.Along with the formation of Al₉Fe₂Si₂phase,a nano-sized V-rich phase was formed.（4）During heating to near-melting point temperatures and cooling processes of AlSi20/8009 aluminum alloys,the phase evolution behavior of Al₁₂（Fe,V）₃Si dispersion under the external Si addition was as following.When the AlSi20/8009Al alloys were held at 580～600 ^oC,Al₁₂（Fe,V）₃Si dispersion and Si phase evolved into acicular Al₉Fe₂Si₂and the nano-sized V-rich phases.Increasing holding temperature to 620～640 ^oC,Al₉Fe₂Si₂and the nano-sized V-rich phases reversibly evolved into Si and coarse hexagonal Al₁₂（Fe,V）₃Si phases.During the AlSi20/8009Al alloy（after holding at 640 ^oC）furnace cooling to 575 ^oC,its microstructure maintained stable.While during the AlSi20/8009Al alloy（after holding at 640 ^oC）furnace cooling to570 ^oC or lower,Si and coarse hexagonal Al₁₂（Fe,V）₃Si phases evolved into strip-like Al₉Fe₂Si₂and the nano-sized V-rich phases.（5）During the formation of compound casting transition zone of the SiC_p/8009Al composite and A356 aluminum alloy,the coarsening of Al₁₂（Fe,V）₃Si dispersion occurred under the external Si addition provided by the A356 aluminum alloy melt,then Si and the coarse hexagonal Al₁₂（Fe,V）₃Si phases evolved into Al₉Fe₂Si₂and the nano-sized V-rich phases.（6）The surface of SiC_p/8009Al composite was covered with a layer of 6061aluminum alloy utilizing friction stir welding to avoid the melting of SiC_p/8009Al composite,the Si element of A356 aluminum alloy melt diffusing into the SiC_p/8009Al composite and the formation of Al₉Fe₂Si₂phase.Under conditions of tool rotational speeds of 1200 r/min and 1400 r/min,welding speed of 60 mm/min and tool pin offset of 1 mm to 6061 aluminum alloy side,the SiC_p/8009Al composite and 6061 aluminum alloy formed defect-free alternate lamellae structure and Al₁₂（Fe,V）₃Si dispersion was slightly coarsened in the stir zone.After postweld T6treatment,the highest tensile strength of joints was 272MPa.