Study On The Fabrications And Properties Of A356 Foam Composites Reinforced By In-situ Nano-ZrB2 Particles

In this study,the closed-cell nano-ZrB₂/A356 foam composites with the porosity,pore size and ZrB₂ particle fraction adjustable were prepared via melt in-situ reaction technology and the prepared composites were foamed with the assistant of CaCO₃ agent and the metal calcium as the thickener.The influences of the selection and addition amount of thickener and foaming agent,the foaming temperature,the stirring parameters of the melt,the mass fraction of the ZrB₂ particles,and other process parameters were analyzed for the pore structure of the foam composites.Then the best process parameters were determined.The microstructure of foam composites was investigated by XRD,SEM and TEM.The compression properties of nano-ZrB₂/A356 foam composites and the bending resistance of ZrB₂/A356 foam composite sandwich panels prepared as core materials were investigated by a universal testing machine.The effects of nano-ZrB₂particle content,porosity and loading rate on the compression properties of nano-ZrB₂/A356 foam composites,the deformation mechanism of ZrB₂/A356 foam composite sandwich panels with 3wt.%particles were also studied.The results show that the optimum process parameters are as follows:the in-situ reaction temperature forming nano-ZrB₂ particles is 850-900?,and the reaction time is 25 minutes.The optimum foaming temperature is 730?,2 wt.%metal Ca of the melt as the thickener 1².5 wt.%CaCO₃ powders were added as the foaming agent.The stirring speed is controlled at 800¹000r/min.The study of phases and microstructure of foam composites show that:The prepared foam composite contains Al,Si,ZrB₂ and CaAl₂ phases.The ZrB₂ particles are distributed on the nearby region of the Si phase in the grain boundary.Research on the cell walls,nano-ZrB₂ particles with the average size of 40 nm exist in the cell wall,and dispersed more uniformly along the edge of the cell wall than the particles with average size 80 nm in the interior region.Simultaneously,The edge of the cell wall is composed of discontinuous lengths of different eutectic Si bands;the nano-ZrB₂ particles not only refine the secondary dendrite arm spacing of the material,but also modify the Si phase morphology,and the Si phase can be spheroidized by T6 heat treatment,which can improve the material strength.The foam composite compression test results show that:The compression curve of the nano ZrB₂/A356 foam composite exhibits three characteristics of the metal foam material.The compressive strength of the foam composite with a particle content of 3 wt.%is 8.0 MPa,which is 94.4%higher than that of the A356 aluminum foam at 4.1 MPa,the energy absorption is up to6.5 MJ/m³,which is higher 42.7%than that of the A356 foam at 4.5 MJ/m³.With the increase of porosity,the platform stress of nano-ZrB₂/A356 foam composites becomes smaller and the platform length grows slightly.With the increase of loading rate,the compression curve of nano-ZrB₂/A356 foam composites shows smoother,and the platform stress increases,and the platform length does not change significantly.The bending strength of the 3 wt.%nano-ZrB₂/A356 foam composite is 44.7 MPa,which is 27.9%higher than the 34.9 MPa of the A356 foam board.The analysis results of the strengthening mechanism of nano-ZrB₂ particles on foam composites are as follows:Load transfer enhancement mechanism,Orowan strengthening mechanism,Fine grain strengthening mechanism and Dislocation strengthening mechanism synergy.The model of ZrB₂ particle-reinforced nano-ZrB₂/A356 foam composite was established.