Study On The Thin Wall Effect And Microstructure Formation Mechanism Of ZL205A Alloy Non-equilibrium Solidification

ZL205A has been widely used in key fields such as aerospace vehicles due to its good mechanical properties.However,due to the wide crystallization temperature range of the alloy and the complex phase structure formed under different cooling conditions,the alloy is severely limited in its wider application in high-performance parts and national economy.In this study,by designing a stepped structure with different wall thicknesses,it can be used in near equilibrium solidification condition I(slow cooling in clay sand molds),near equilibrium solidification condition II(solidification in coated sand molds)and high temperature gradient cooling conditions III(fast cooling in thick-walled metal molds).ZL205A alloy samples with different thicknesses were prepared under different cooling conditions,and the solidification structure and mechanical properties of the alloys in different solidification conditions,asymmetrical heat dissipation of the wall and T5 treatment in time were systematically studied,and the phase transformation law and the mechanical properties of the alloy were explored.The results show that the structure and properties of the ZL205A alloy cast in different molds are significantly different,and the grains are more refined at the thin wall.As the wall thickness increases,the alloy grains become larger under near equi-librium solidification conditions,and the grain boundaries also become wider as the wall thickness increases.The tensile strength of the alloy in near-equilibrium solidifi-cation condition?is greater than that in condition?,and the elongation is also quite different,and the elongation of the material in sand type?reaches a maximum of 10.9%at the wall thickness of 10 mm.Under the same state The sand mold I castings only reached 4.6%.After 14 hours of solid solution in the as-cast alloy,a large amount ofqphase dissolves in thea-Al matrix,and as the wall thickness increases,the solubility of Cu atoms continues to increase;Correlation effect of organization and performance.The content of Cu element in thea-Al matrix increases with the extension of the solid solution time.After T5 treatment of ZL205A alloy,the strength and hardness of the alloy are significantly improved.After T5 treatment,the content of precipitates at the grain boundaries increases significantly.Through software analysis,it is found that when the ZL205A alloy tends to near equilibrium solidification,the phase composition of the alloy is Al,Al2Cu andAl₂₀Cu₂Mn₃,The equilibrium phase composition at room temperature is 92.13%Al,6.18%Al₂Cu and 1.69%Al₂₀Cu₂Mn₃;When it tends to non-equilibrium solidifica-tion,its room temperature phase composition is 93.5%Al,6.4%Al₂Cu and 0.14%Al₂₀Cu₂Mn_3.The high degree of supercooling at low temperature is conducive to the nucleation of the precipitated phase,but the diffusion of elements is small and the growth is slow,while the nucleation at high temperature is more difficult but the dif-fusion of elements is easy.This paper studies the wall thickness effect of ZL205A alloy under different tem-perature gradients.Generally speaking,whether it is a low temperature gradient or a high temperature gradient,the wall thickness effect of thin-walled castings is very ob-vious.As the wall thickness increases,the microscopic grains of the castings are grad-ually getting larger,the grain boundaries are getting wider and wider,and the content of grain boundary precipitates is also increasing.