Study On Refinement And Modification Of Ce/La Additions In A17SiMg Alloy And Its Application In Semisolid Die Casitng Process

The refinement of primary ?-Al and the modification of eutectic Si in hypoeutectic Al-Si alloys are the main methods to improve the casting performance and mechanical properties of alloys.Rare earth(RE)elements have been proven to refine the primary ?-Al grains and modify the coarse flake morphology of eutectic Si,moreover,low cost advantage of RE elements also promotes its research and application in casting process.However,the refining effect of RE is unstable and is easily influenced by content of RE addition and casting process parameters like melt holding time and cooling rates during solidification.The refinement mechanism of RE also has not been accurately concluded.Moreover,it is investigated the modification effect of RE is noticeably enhanced at higher cooling rate.The modification mechanism of RE on eutectic Si still lacks experimental research.Besides,the research on the effect of RE on the microstructure and mechanical properties of hypoeutectic Al-Si alloys is mainly under gravity die casting process.For the die casting process with higher cooling rate,whether RE has the refining effect on ?-Al grains and modification effect on eutectic Si,and its influence on the mechanical properties of the hypoeutectic Al-Si alloys need further study.Regarding the questions above,this paper studies the refinement and modification behavior of Ce and La in Al-7.0Si-0.3Mg(wt.%)(hereinafter denoted as A17SiMg)alloy,and applies it to the semi-solid die casting process to improve the mechanical properties of the alloy.In terms of refining,the effect of the holding time of the melt and the cooling rate during solidification on the Ce/La refining effect were studied,and the ?-Al grain nucleation and growth process were analyzed,the refining mechanism of Ce/La was proposed.In terms of modification,the effect of Ce/La contents and cooling rate on the size,morphology and twinning of eutectic Si were studied.The distribution of Ce/La atoms on eutectic Si at high cooling rate was obtained through multi-scale microscopic observations.The mechanism of Ce/La modified eutectic Si at different cooling rates is proposed.In the semi-solid die casting process,the influence of 0.1 wt.%La addition on the microstructure and mechanical properties of A17SiMg alloy was also investigated in the as-cast and heat-treated condition.The conclusions are described as follows:(1)The refining efficiency reaches 33%when the Ce/La content is 0.4 wt.%,and the size of ?-Al grains do not change with the holding time of the melt.In addition,at different cooling rates(0.2,1.5 and 7.5 K/s),Ce/La has an obvious refining effect,and its refining efficiency is up to 20%-40%.The Ce and La solutes increase the constitutional supercooling at the front of the solid-liquid interface,thereby increasing the driving force of the nucleation of ?-Al grains.(2)At low cooling rate(0.2 K/s),the growth temperature of eutectic Si decreases from 573? to 567? when the Ce/La contents increase from 0 wt.%to 1.6 wt.%.The size and morphology of eutectic Si are only refined when the cooling rate is relatively low.The Ce/La elements are mainly in the form of AlxSix(La,Ce)compounds.(3)At high cooling rate(150 K/s),when the La content is 0.4 wt.%,high volume fraction of La solute is located in eutectic Si,some La atoms enrich along the<112>si growth direction and at the intersection of twins.This investigation highlights the high cooling rate promotes the enrichment of La atoms within eutectic Si,which induces the formation of multiple twins and thus modifies the eutectic Si to a fibrous morphology.(4)In the gravity die casting,with increasing cooling rates,the grain size in A17SiMg alloys was greatly decreased,which results to the noticeable enhancement in UTS.For alloys containing 0.4 wt.%La,the grain size is decreased by Ce/La refiner,the modification effect of eutectic Si is significantly enhanced.Compared to the A17SiMg alloys,the UTS of alloys with 0.4 wt.%La is increased from 156 MPa to 185 MPa at higher cooling rate(7.5 K/s),the elongation is increased from 2.1%to 4.6%.(5)In the semi-solid die casting,at the cooling rate of 30 K/s,when the La content is 0.1 wt.%,the eutectic Si is modified to fiber-like morphology.After the heat treatment,the length and aspect ratio of eutectic Si were 2.2 ?m and 1.9.The modification of eutectic Si significantly improved the elongation of the alloy.The elongation of alloys containing 0.1 wt.%La has an increase of 20%and 15%under as-cast and heat-treated condition,respectively.