Effect Of Rare-earth Yttrium And Heat Treatment On Microstructure And Properties Of ZL114A Alloy

Hypoeutectic Al-Si alloy is widely used in aerospace,military and automotive industries because of its advantages such as good casting performance,excellent corrosion resistance and higher strength.However,eutectic silicon is rod-like or needle-like under as-cast conditions which will cause stress concentration,decrease mechanical properties of the alloy.Therefore,the hypereutectic Al-Si alloy need to be modified to change the morphology and size of the primary ?-Al phase and the Si phase,and improve the mechanical properties.Rare earth is a green,efficient,and long-acting modifier.It can purify the melt,and has a good modification effect on the hypereutectic Al-Si cast alloy.In this paper,ZL114A alloy was used as the research object.The effects of different rare earth Y addition,different metamorphic times,different remelt weights and heat treatment on the microstructure and properties of ZL114A were studied by means of metallographic microscope,scanning electron microscope,hardness and room temperature tensile test,polarization curve test and thermal analysis experiment.The main research results are as follows:(1)The results indicated that the secondary dendrite arm spacing of a-Al in ZL114A alloy was decreased.When the amount of Y was 0.3%-0.4wt%,the distance of secondary dendritic arm was reduced from 42?m to 19?m-22?m.The eutectic silicon was well modified because of the addition of Y.The optimum amount of Y was 0.3wt%,the tensile strength,the elongation and the hardness of the alloy were 181 MPa,3.9%,and 83HV,which were improved by 15%,77%,20%respectively compared to the unmodified alloy.(2)Rare earth Y can reduce the eutectic growth temperature of the ZL114A alloy,?TEG was increased and the metamorphic effect was enhanced by the increase of?TEG.When the amount of Y was 0.4wt%,?TEG reached the maximum value,the eutectic reaction temperature decreased by 7.4 ?.(3)The corrosion property of the alloy was improved by the addition of Y.The corrosion current density of the alloy after adding 0.2wt%Y in 3.5%NaCl solution decreased from 5.43×10-7A·cm-2 to 3.26×10-7A·cm-2.(4)Rare earth Y have good remelting property and long-term efficiency.When the proportion of heavy melt is less than 20wt%,the alloy can maintain good mechanical properties and the tensile strength is slightly increased.Rare earth Y has a good ability of fade resistance,there will be a good effect after 30 minutes.The tensile strength and elongation of the alloy can still reach 178MPa and 3.6%after 5 hours of deterioration,the reduction of tensile strength is less than 2%.(5)After heat treatment the eutectic silicon of ZL114A alloy which contains 0.3wt%Y changed from fibrous form to spherical.The best heat treatment process is:solid solution 540 ?/10h,aging 170 ?/10h.The tensile strength,elongation and hardness of the alloy reached 339MPa,6.2%and 129HV respectively.