Microstructure Evolution And Property Control Of Hypereutectic Al-Si Alloy Prepared By A Composite Process Of Serpentine Pouring Channel And Chemical Modifier

Hypereutectic Al-Si alloys are widely used in automobiles,tractors,motorcycles,ships and other equipment due to their low weight,low thermal expansion and high wear resistance.However,the coarse primary Si particles(PSPs)and the needle-like eutectic Si severely degrade the mechanical and machinability properties of these alloys.Simultaneous refinement of primary Si and modification of eutectic Si is extremely difficult owing to the fact that reactions occur between popular primary Si refiners and eutectic Si modifiers.In this paper,the semi-solid slurries of hypereutectic Al-Si alloys with Si contents of 17 wt.%,22 wt.%and 28 wt.%,respectively,were prepared by a composite process of copper serpentine pouring channel(SPC)and strontium(Sr)or antimony(Sb)modifier and subsequently molded by rheological high pressure die casting(Rheo-HPDC)in an attempt to achieve simultaneous refinement of primary Si and modification of eutectic Si.The effects of copper SPC-treatment,modifier type and addition,preparation temperature of slurry(Tp)and silicon content on the microstructure evolution,mechanical properties and wear resistance of semi-solid slurry and Rheo-HPDC casting were systematically investigated.The main results in this paper are as follows.The results indicate that the composite process of the copper SPC and Sr-or Sb-modifier can simultaneously achieve the sufficient refinement of primary Si and effective modification of eutectic Si.It has broken through the technical difficulties faced for decades in producing hypereutectic Al-Si alloys by sufficiently refining the primary Si while effectively modifying the eutectic Si and provided a good basis for further improvement of mechanical and other properties of hypereutectic Al-Si alloy casting.The sufficient refinement of primary Si is mainly achieved and dominanted by the copper SPC processing.The primary Si nuclei are nucleated by chilling effect generated by the serpentine pouring channel inner-wall and then multiplied continuously by the subsequent melt scouring.The addition of Sr or Sb only slightly increases the average size of the PSPs,and their effects in this respect are almost negligible.The average equivalent diameter of the PSPs in the A390 aluminum alloy slurry was refined from 60.2 ?m to 25.2 ?m at the Tp for 660?.The refinement of the PSPs,however,was slightly weakened with the increase of-the Si content.The average equivalent diameters of the PSPs in the Al-22SiM and Al-22Sip alloy slurries were refined from 75.1 ?m to 28.0 ?m and 29.7 ?m,respectively,at the Tp for 775?.In the Al-28Si alloy slurry,the average equivalent diameter of the PSPs was refined from 166.8 ?m to 63.2 ?m at the Tp for 830?.Strontium inhibits the nucleation of the PSPs in hypereutectic Al-Si alloys by poisoning the heterogeneous nucleation sites,but has a negligible effect on the refinement of the PSPs by the SPC processing.Strontium transforms the morphology of eutectic Si in the hypereutectic Al-Si alloys from needle-like into fibrous as it does in hypoeutectic Al-Si alloys.The eutectic Si in semi-solid slurries of A390 aluminum alloy and Al-22Sip alloy can be effectively modified to fibrous when the additions of Sr were 0.02 wt.%and 0.09 wt.%,respectively.The lower impurity level of the Al-Si alloys contributes more to the effective modification of eutectic Si.In the Al-22Sim alloy slurry,Sr addition was increased to 0.12 wt.%and still failed to modify the eutectic Si.In contrast,in the higher purity Al-28Si alloy slurry,the Sr addition of 0.03 wt.%can effectively modify the eutectic Si.The primary and eutectic Si is mainly affected by three factors,that is,strontium poisoning of heterogeneous nucleation sites,decreasing the solid-liquid interface energy and increasing the activation energy for diffusion of silicon atoms across the solid-liquid interface.Increasing antimony addition results in an increase and then a decrease in the nucleation temperature of primary Si.When antimony addition exceeds a critical value,the antimony-rich intermetallics nucleate earlier than the primary Si,which plays a limited role in poisoning heterogeneous nucleation sites.The Sb-rich intermetallic compounds,such as AlSb and Mg3Sb2,cannot act as heterogeneous nucleation sites for the PSPs.Antimony alters the growth rate of primary Si by changing the actual growth undercooling of primary Si and blocking the active position for primary Si growth.Antimony only refines rather than modifies the eutectic Si,and promotes the nucleation of eutectic Si by changing the liquid phase structure to aggregate the Si atoms.The eutectic Si in the A390 alloy slurry can be refined when the Sb addition reached 0.10 wt.%.The semi-solid slurry of hypereutectic Al-Si alloy molded by the Rheo-HPDC has the finer PSPs and effectively modified eutectic Si,and its mechanical properties are significantly improved.Neither strontium and antimony altered the fracture mechanism of hypereutectic Al-Si alloy in the quasi-cleavage manner.Under the condition of composite Sr-modifer treatment,the as-cast Rheo-HPDC A390 alloy with optimal mechanical properties at the Tp of 700? and Sr addition of 0.02 wt.%,in which the average equivalent diameter of the PSPs was refined to 15.8 ?m,increased the ultimate tensile strength(UTS)by 23%to 258±6 MPa and the elongation(El)by 65%to 0.66±0.05%.The as-cast Rheo-HPDC Al-22Sip alloy with optimal mechanical properties at the TP 795? and Sr addition of 0.12 wt.%,in which the average equivalent diameter of the PSPs was refined to 18.2 ?m,increased the ultimate tensile strength(UTS)by 38%to 255±3 MPa and the elongation(El)by 64%to 0.46±0.01%.In the Rheo-HPDC,strontium aggravates the tendency of macro-segregation and hot cracking and inhibits the formation of ?-Al halos encompassing the PSPs.The surface of the PSPs exposed in the liquid phase serves as a substrate for eutectic Si nucleation.Strontium absorbed in the growth steps of primary Si is entrapped within the PSPs resulting in an increasing tendency for the PSPs to crack.The wear mechanisms of hypereutectic Al-Si alloy include abrasive,adhesive and oxidation wear.With the increase in the strontium addition,the abrasive wear gradually dominates the wear process.In the Al-22Sip alloy,the lowest friction coefficient of about 0.29±0.02 was achieved at the Sr addition of 0.06 wt.%.With Sb as a modifier,the as-cast Rheo-HPDC A390 alloy with optimal mechanical properties at the TP of 680? and Sb addition of 0.30 wt.%,in which the average equivalent diameter of the PSPs was refined to 16.9 ?m,increased the ultimate tensile strength(UTS)by 24%to 257±6 MPa and the elongation(El)by 44%to 0.52±0.04%.Sb-rich intermetallics within the PSPs act directly as a source of cracking for cleavage fracture of the PSPs.