Microstructure And Properties Of ZL114A Containing Sc Element Under Different Cooling Rates And Heat Treatment Conditions

At present,Al-Si casting alloys are widely used in aerospace and automotive industries because of their excellent castability,corrosion resistance,high strength to weight ratio and low coefficient of thermal expansion.The mechanical properties of Al-Si casting alloy mainly depend on chemical composition and microstructure.The mechanical properties and fracture behavior of Al-Si casting alloy are mainly determined by the grain size of?-Al and the size and morphology of eutectic silicon.Sc,as a rare earth element,has been found to have the influence of both refining?-Al grains and modifying Al-Si eutectics in Al-Si based foundry alloys.However,few literatures have systematically reported the effects of Sc on the microstructure characteristics,precipitation behavior,tensile properties and fracture behavior of ZL114A Alloy under different cooling rates,especially on the solidification behavior of?-Al and modification of eutectic Si.In this paper,the effects of adding Sc on the microstructure characteristics,tensile properties,precipitation behavior and fracture behavior of ZL114A Alloy at different cooling rates,as-cast and T6 were systematically studied.The following results are obtained:(1)Obvious grain refinement effect was observed in ZL114A alloy containing Sc.The addition of Sc reduces the sensitivity of mechanical properties of ZL114A Alloy to cooling rate.Especially under the condition of low cooling rate,the elongation of ZL114A Alloy is greater than 4.88%due to the grain refinement of Sc.At high cooling rate(metal mold),the secondary dendrite of the alloy is obvious,and the relationship between SDAS and yield strength follows hall Petch equation,which has become the main factor affecting the mechanical properties of the alloy.Under low cooling rate(sand mold),the addition of Sc will lead to the degeneration of secondary and tertiary dendrites of?-Al,and the grain size and yield strength are confirmed to Hall Petch equation,while SDAS and yield strength are no longer applicable to Hall Petch equation.At this time,the grain size becomes the main factor affecting the mechanical properties.(2)Eutectic Si takes precedence over the formation of Mg₂Si and Al Sc₂Si₂ phases.At the end of the eutectic stage,Mg₂Si,Al Sc₂Si₂ and Si undergo eutectic growth.The addition of Sc has a strong modification effect on Al-Si eutectics,changing the eutectic Si from plate shape into fibrous morphology.When the cooling rate is high,the eutectic Si phase solidifies rapidly and fails to fully diffuse,contributing to the fine and fibrous eutectic Si.When the cooling rate is low,eutectic Si has enough time to diffuse to the grain boundary and appears as coarse fibers.(3)After T6 heat treatment,the UTS and YS of ZL114A alloy increased by 71.5%and154.1%respectively,but the elongation decreased slightly.The UTS and YS of ZL114A-0.5Sc alloy increased by 69.2%and 143.0%respectively,and the elongation increased by 117.3%in metal mold.In sand mold,the UTS and YS of ZL114A alloy increased by 93.2%and 179.6%respectively,and elongation increased by 38.2%.The UTS and YS of ZL114A-0.5Sc alloy increased by 98.9%and 173.3%respectively,but the elongation decreased slightly.