Investigation On Semi-solid Stir Casting Technology Of SiCp/A357 Composites

Characterized with high specific strength and specific stiffness,good wear resistance and low thermal expansion,particle reinforced A1 matrix composites are widely applied in the transportation,aerospace and electronic industry.The demand for large-sized composites component is increasing with the development of application field of composites.It's hard and costly to machine the composites due to the high hardness and wear resistance.However stir casting is a near-net shape and low cost process which is most protential to prepare large-sized composites.But it faces some difficulties such as particle aggregation,low density and fluidity.Therefore,intense researching for SiCp/Al composites stir casting process and realizing the casting of large-sized composites become a research hotspot.In this paper,a paddle-grid compound stirrer used for stirring larged-sized composites melt was designed,and then the stir casting technology of SiCp/A357 composites was investigated.The SiCp distribution,density and property of composites were analyzed.The effect of SiCp addition on the apparent viscosity of SiCp/A357 composites was investigated.At last,the distribution rule of SiCp in the flow of composites and the mold filling behavior of composites were investigated by means of numerical simulation and experiment.The main research results are shown as follows:(1)The stirrer structure used for large-sized composites melt was investigated and optimized.It was found that the velocity of single stirrer was lower and its function range was limited.The stir speed and function range of compound stirrer were higher than single stirrer.Paddle-compound stirrer could circuit and shear the composites melt in the crucible.The paddle on the upper layer could drive the melt at the surface moving to the lower layer,and the grid stirrer could shear the melt on a large scale,which attributes to the stir casting of large scale composites.The optimized structure of stirrer was 150mm of diameter,40mm of layer spacing and 40mm of distance to the bottom.(2)The large-sized composites ingot with uniform SiCp distribution and high density was successfully fabricated by pre-treatment of SiCp and optimization of stirring process.The rinse and high temperature oxidation could remove the submicron order SiCp and avoid harmful interfacial reaction.The powder adding and stirring process were investigated respectively.It was found that the powder adding efficiency at 40%matrix solid fraction was higher and the stirring at 10%solid fraction could eliminate the necklace-like segregation at the grain boundary of matrix.The optimized parameters of stirring process were 610? of stir temperature,800rpm of stir speed and 20min of stir time.The optimized parameters of T6 heat-treating process were 2h of solid solution at 545 ? and 6h of aging at 175?.The solution and aging time were shotter than A357 alloy.The distribution of SiCp had significant effect on the mechanical property of composites.Micro holes usually appeard near the SiCp cluster.The crack initiation and growth were easier at the micro holes and brittle fracture occurred in composites.(3)The influence mechanism of SiCp to the apparent viscosity of SiCp/A357 composites was investigated.It was found that the apparent viscosity of composites was higher than A3 5 7 alloy at the same matrix solid fraction due to the addition of 10vol.%,15vol.%,20vol.%SiCp.The apparent viscosity increased with SiCp volume fraction.But the apparent viscosity of composites was lower than A357 alloy at the same total solid fraction due to the reduction of primary a-Al.The addition of SiCp could hinder the aggregation of primary a-Al and modify its morphology.(4)The two-phase flow and casting filling process of SiCp/A357 composites were investigated by means of numerical simulation and experiment.The results showed that the SiCp volume fraction increased and then decreased along flow length direction.The SiCp volume fraction at flow front was the lowest.The SiCp distributed nonuniform at the direction of gravity.The fluidity of SiCp/A357 composites was lower than A357 alloy at the same pouring temperature.The flow length at semi-solid casting was shorter than liquid casting,but the homogeneity of SiCp was higher than liquid casting.Rising the mould temperature and pouring temperature could improve the flow length,which attributed to the casting process.