Research On Squeeze Casting Processing Of SiCp/A356Composites Prepared By Stirring Technology

SiC particle reinforced aluminum matrix composite castings produced by traditional sandand gravity casting processes generally need sequent processing, but the existence of SiCparticles brings about great difficulties in machining processing due to short tooling servicelife. Therefore, development of near net shape forming process of the composites is desirable.Squeeze casting process combines the advantage of casting and forging methods, and canmake the castings with fewer machining allowance. In this paper, SiCp/A356compositesfabricated by direct and indirect squeeze casting methods were studied.According to the principle of manufacturing composite by mechanical stirring, anequipment for casting composite stirring had been designed and produced. The factorsaffecting quality of composite melts has been analyzed and discussed. Then semi-solid andliquid stirring method and its process parameters have been determined to prepare particlereinforced aluminum composite melt. The microstructure assessment shows that SiC particlespresent as agglomerate and associate with porosity after the semisolid stirring, and after theliquid stirring uniform particle distribution has been achieved.The SiCp/A356composite castings with average particle size of10μm and150μm wereproduced by direct squeeze casting process under different squeeze pressures, respectively. Itis found that defects of particle-porosity clusters can be clearly observed in the gravitycastings, and the defects and porosities dramstically reduce as the applied pressures increase.The tensile properties, hardness and elonglation of the as-cast and T6-treated castingsincreased due to reduce of the particle-porosity clusters and grain refine with the squeezepressure increase. By SEM analysis, tensile fractures of the composites in as-cast conditionshow a brttile fracture mode, while in the T6-treated condition the suqueez castA356-10μmSiCp and A356-150μmSiCp composites tend to ductile and brittle-ductile mixedfracture modes, respectively. Compared to the A356-10μmSiCp composites, theA356-150μmSiCp composites have higher tensile strength and hardness and lower elongation.The composite tensile propersites still improved after the squeeze pressure over50Mpa,which indicates that compared with the monothalic alloy squeeze casting higher squeezepressure is needed to fabricate high-performance composite castings. The indirect squeeze casting dies with square spiral shape were designed, and theA356-50μmSiCp spiral composite castings were fabricated with different injection speeds.The results show that the composite fluidity is very poor and lack of filling was found underthe low injection speed of0.08m/s, but in the high injection speed of0.16m/s the fuidity hasbeen greatly improved which helped to form the sound spiral castings. The influence ofdifferent injection speed on SiC particles distribution, microstructure and mechanicalproperties of the A356-50μmSiCp castings was analyzed. Under the high injection speed,particle distribution is very complex and the particle fraction is the main factor in determiningthe mechanical properties. On the contrary, as for the composite castings with the lowinjection speed, the particle volume fractions increased with increasing filling distance andthe mechanical properties are determined by the density and shrinkage defects rather than theparticle fractions.