Study On Grain Refinement And Heat Treatment Process Of Cast Al-2.5Li-5Mg-1Cu Alloy

In this paper,the grain refinement and heat treatment process of cast Al-2.5Li-5Mg-1Cu(hereinafter referred to as mass fraction)alloy is studied.The alloy is smelted under the protection of surface inert gas and formed by gravity casting.In this paper,the effects of different refinement additions and wall thickness of castings on the refining effect of the alloy were studied.Five different alloy refinement schemes were designed by changing the type of refiner and its addition amount.The test results show that among the five schemes,the alloys added by 0.2% Zr and 0.2% Al-5TiB mixed refining have the best refining effect,and the average grain size is 31.6 ?m.The average particle size of the alloy which was not refined was 136.6 ?m,which was 76.9% smaller than that of the alloy.Secondly,the alloy was added with 0.2% Zr.The average particle size after refining was 56.4 ?m;at the same mass fraction.The refining effect of the alloy with 0.2% Al-5Ti-B is reduced,and the average particle size is 92.4 ?m.Meanwhile,as the addition amount of Al-5Ti-B increases,the refining effect of the alloy gradually decreases.The addition of the refiner also changes the dendrite morphology of the alloy while increasing the nucleation rate.The addition of Zr element changes the dendritic morphology of the alloy from dendrites to cell crystals,and the addition of Al-5Ti-B is also the same.The dendritic morphology of the alloy is changed from dendrites to cellular dendrites.The wall thickness of the casting also has a certain influence on the grain size and dendritic morphology of the alloy.Generally,the smaller the wall thickness of the casting,the larger the cooling rate,the greater the degree of subcooling during solidification of the alloy,and the smaller the grain size.However,for the alloy with single Zr element refinement,as the wall thickness decreases,the cooling rate during solidification increases gradually,and the dendritic morphology of the alloy changes from cell crystal to dendrite,and the size of crystal grains increases.The solution temperature of the alloy heat treatment was set to 500 ?.The orthogonal test of heat treatment process was carried out with three factors: solid solution time,aging temperature and aging time.The test results showed that the alloy had the best performance after heat treatment at 500 ? for 6 h and 175 ? for 17 h.The quenching method after solution treatment is water quenching,and the cooling method after aging is air cooling.After the heat treatment of the alloy,the mechanical properties are improved to varying degrees.The tensile strength after heat treatment of 0.2% Zr refined alloy was 358.5MPa,which was 35.4% higher than that of the as-cast condition.The tensile strength of the alloy after adding 0.2% Al-5Ti-B was 313.7MPa.The properties of the alloy in the as-cast condition increased by 27.8%.After the heat treatment of the alloy after mixing and refining,the tensile strength was 354.6 MPa,which was 17.0% higher than that of the as-cast property.The fracture mechanism of the as-cast alloy of 0.2% Al-5Ti-B is dimple fracture,the toughness of the alloy is good,and the cleavage fracture characteristics appear after fracture after heat treatment,and the elongation rate is reduced.The alloys after mixing and refining and adding 0.2% Zr refinement have an increase in elongation after heat treatment.The reason may be related to the T1 phase precipitated at the grain boundary.The T1 phase changes the movement mechanism of dislocations,which makes it change from the cutting mechanism to the bypass mechanism.The mechanism improves the strength of the alloy while also improving the plastic toughness of the alloy.