Effects Of Sc On The Microstructure And Properties Of The Al-Si-Cu-Ni-Mg Piston Alloy

Due to the excellent comprehensive mechanical properties,cast Al-Si system alloys have become the preferred metallic materials for the fabrication of lightweight pistons.The performance requirements of Al-Si alloys for piston increase with the increasingly stringent requirements on the performance of pistons in industry.Therefore,development of new high-strength and lightweight alloys for piston has become research hotspots of Al-Si alloys.In this paper,the Al-12Si-4Cu-2.7Ni-1Mg alloy was micro-alloyed to achieve the adjustment and optimization of the microstructure morphology by adding different amounts of Sc element.The aim is to improve the comprehensive mechanical properties of the Al-12Si-4Cu-2.7Ni-1Mg alloy.The microstructure evolution and mechanism of Al-Si-Cu-Ni-Mg piston alloy micro-alloyed by Sc element were studied by means of metallographic microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,Brinell hardness,high temperature tensile testing and creep testing.The effect of Sc content on the morphology and size of theα-Al phase,the second phases and the existence of Sc element in the alloy,was analyzed.The influence law and mechanism of Sc content on the comprehensive mechanical properties of the Al-Si-Cu-Ni-Mg alloy were discussed.The main conclusions are as follows:（1）The content of Sc addition played an obvious role on the microstructure of Al-12Si-4Cu-2.7Ni-1Mg piston alloy.With the increase of Sc element content,the average size ofα-Al matrix and primary Si phase in as-cast alloy decreased significantly.The morphology of eutectic Si was modified from lamellar to fibrous.The morphology ofδ-Al₃Cu Ni precipitated from the piston alloy during solidification evolved as follows:cluster fishbone→rod→cluster fishbone.While for the heat treated samples,the newly formed fibrous eutectic Si is more prone to spheroidization through T6 treatment（500℃solution for6h+215℃aging for 3h）,and Sc element promotes the aging precipitation and formation of Al₂Cu precipitates.（2）The solidification rate has an effect on the presence form of Sc.In alloys cast using iron molds,the existence of Sc was mainly in the form of Sc Si Ni phase which usually presents two regions,closely bound to the primary Si and exists around the primary Si,or isolated in the matrix.Both the two forms of Sc Si Ni phase maintain a non-coherent interface with the matrix.With the solidification rate increasing,Sc was mainly consolidated in theδ-Al₃Cu Ni phase in an alloy cast using copper molds.（3）Compared with the matrix alloy,the hardness and tensile strength of 0.2wt.%Sc-contained alloy in T6 state decreased while the elongation increased obviously.Further increasing the Sc element content to 0.4wt.%,0.6wt.%and 0.8wt.%,the hardness of the piston alloy increased gradually while the tensile strength varied slightly.When Sc content is0.2wt.%,the alloy exhibited the best creep resistance.The steady creep rate decreased about45%compared with the that of Al-12Si-4Cu-2.7Ni-1Mg alloy without Sc elelemt.When Sc element was added to 0.4wt.%and 0.6wt.%,the creep resistance of the alloy decreased.The results of mechanical properties testing showed that Al-12Si-4Cu-2.7Ni-1Mg piston alloy had the best comprehensive mechanical properties when the Sc element content was 0.2wt.%.