Study On Microstructure And Properties Of A356-B₄C Composites Prepared By Melt Stirring

Particulate-reinforced metal matrix composites produced by melt stirring method exhibits many advantages,including low producing cost,simple fabrication process and highly isotropic properties,and is quite promising in practical application.The particulate distribution during melt stirring has a significant effect on the casting mechanical properties.In this paper,two kinds of B4C particles with different particle size were selected as the reinforcing particles,and the A356-B4C composite plate was prepared by the method of semi-solid mixing and rolling.Firstly,B4C particles were activated and the effect of B4C particle activity on the microstructure of composites was obtained.A356 semi-solid slurry was mixed,adding B4C particles after activation treatment.The effects of stirring time and particle size on the dispersibility of particles in the melt were obtained;The mechanical properties and wear properties of A356B4C plate after rolling were tested.The main research results are as follows:(1)The activation process of B4C particles was obtained.The acid pickling solution was 4%hydrochloric acid,the acid pickling time was 4 hours,and the acid pickling process was always accompanied by mechanical agitation.Acid pickling treatment can remove the impurities on the surface of particles,The impact and friction between the particles in the stirring process can remove the sharp corners,improve the wettability of the particle surface.The oxidation product of B4C in air was B2O3,Molten B2O3 can form wavy shape with the dropping of temperature.B2O3 can improve wettability and prevente the direct contact of Aluminum matrix and B4C particles to form the brittle phase Al4C3.At 500?,B4C particles began to oxidize,but the effect was not obvious.At 550?,the oxidation of B4C particles was obvious,and the thickness of the oxide layer increased with time.In the early stage of oxidation,the reaction rate was mainly controlled by the interfacial chemical reaction.In the later stage of the oxidation,the reaction rate was mainly controlled by the diffusion rate.(2)The effect of B4C particles on B4C dispersion and solidification structure was obtained.Under the same conditions,activation treatment can significantly improve the agglomeration of B4C particles in A356 alloy and obtain A356-B4C composites with uniform dispersion.The addition of B4C can refine the grains of A3 5 6 alloy during solidification,and reduce the average diameter and roundness of the grains.(3)Melt Stirring device was self-designed to improve the dispersion of B4C particles.During the stirring process,B4C particles moving on the same fluid layer were separated successively by centrifugal force,all of which were uniformly dispersed into the A356 alloy matrix.Under the effect of the shear stress of the fluid layer of the alloy,the B4C particles in different fluid layers were continuously sheared and dispersed,so that the agglomerated B4C particles were dispersed gradually.(4)The effect of stirring time on the dispersion uniformity of B4C was obtained.Through the stirring process,the distribution of B4C particles tends to be uniform,and the agglomeration phenomenon was improved significantly.The concentration of B4C particles in each position of the melt was gradually stable.Under the same stirring process,the larger size of B4C particles,the better dispersion uniformity of B4C particles in composites.(5)Effect of rolling deformation on microstructure and properties of A356-B4C composites was obtained.The mechanical properties of A356-B4C composites were greatly improved during the rolling process.The main reason is that the rolling deformation can the fine grain size,eliminate the casting defects,improve the interface bonding between B4C particles and the matrix,improve the dispersion uniformity of B4C particles.The smaller size of B4C particles it is,the higher the tensile strength and elongation of A356-B4C composites it will be.When the particle size was 5 ?m,the tensile strength reached 156 MPa,the elongation reached 7.3%.When the particle size was 20 ?m,the tensile strength reached 123 MPa,the elongation was 5.3%.When B4C particles were small,the failure was mainly interfacial debonding.With the increase of the B4C particle size,the failure of A356-B4C composites was changed from interfacial debonding to interfacial debonding and B4C particle breakage.The interface between B4C particles and matrix was a combination of chemical reaction;With the analysis of XRD,the phase of the interface was mainly Al2O3 and Al3BC.The phase had a high hardness,which can effectively prevent the movement of the dislocations in the crystal,and improve the bonding strength between the B4C particles and the matrix,improve the mechanical properties of the composites.(6)The wear mechanism of A356-B4C composites was mainly adhesive wear and abrasive wear.With the increase of the load,the width of the wear scar became wider,and the wear surface was changed from the relatively smooth to rough.The friction coefficient decreased and the wear rate increased.