Effect Of Modification And Heat Treatment On The Microstructure And Property Of Al-Si Cast Alloy

Al-Si cast alloy is a kind of important alloy material which is widely used in aerospace andautomotive fields because of its advantages such as light quality, high strength, good wear resistanceand corrosion resistance, etc., however, the coarse eutectic silicon which often appears in the alloymicrostructure has serious adverse effect on the microstructure and property of the alloy, so processesof modification and heat treatment of the microstructure are required. Rare earth element which isconsidered as a “vitamin” for metal material has a good modification effect on the Al-Si cast alloy.In this paper, different contents of rare earth element Nd in the form of master alloy were added tothe ZL101Al-Si cast alloy for modifying. The results indicated that α-Al of the alloy was turned to beuniform from disorder, the secondary dendrite arm spacing was decreased, the eutectic silicon waswell modified because of the addition of Nd, the optimum amount was0.3wt.%of the alloy, thehardness, tensile strength and elongation of the alloy were72HBS,190MPa,2.9%, improved by28.6%,23.4%,52.6%respectively, compared to the alloy unmodified. Nd contained phaseAl8Si8MgNd2was formed and had a negative effect on the microstructure and property of the alloywhen the content of Nd was0.4wt.%or above.The modification of Nd is sensitive to the cooling rate,fast cooling rate is acquired when good modification effect is needed; the results of immersioncorrosion and electrochemical corrosion tests showed that the corrosion property of the alloy wasimproved as the addition of Nd, the corrosion potential of alloy modified by0.1wt.%Nd and0.3wt.%Nd in the3.5%NaCl solution were-0.786V and-0.703V respectively, compared to-0.926Vwhich was the potential of the alloy unmodified, while the corrosion current density was decreasedfrom4.046×10-5A/cm2to5.048×10-6A/cm2、2.215×10-6A/cm2, respectively; The DSC result indicatedthat the eutectic process was restricted because of the addition of Nd, the eutectic temperature wasdecreased from583.7℃to580.6℃,576.5℃, respectively, the modification effect was obtained by theway of affecting the growth not the nucleation process of the alloy; the results of long effectivenessand remelting property of the alloy indicated that the modification of Nd on ZL101had a good abilityof fade resistance, the small flake-like eutectic silicon was still obtained after six hours modificationand four times remelting; the result of heat treatment showed that the effect sequence of heattreatment parameters on the property of the alloy was the aging temperature> aging time> solutiontime> solution temperature, the optimum parameter of heat treatment was540℃×6hsolution+170℃×10h aging, while the hardness, tensile strength and elongation of the alloy were 98HBS,298MPa,4.02%, respectively, the eutectic silicon was spheroidized, tensile fracture showedobvious dimple morphology after heat treatment.