Effect Of Rare Earth Er On Microstructure And Properties Of A356 Aluminum Alloy

A356 Aluminum Alloy has good comprehensive properties,its good fluidity,air tightness,shrinkage and thermal cracking tendency.It has sub eutectic composition and a wide solid-liquid coexistence zone,has been widely used in the aviation,automotive and other fields.In the condition of as-cast,the primary ?-Al dendrites in the A356 aluminum alloy are coarse,and the eutectic Si is easy to form the thick needle and flake.The coarse eutectic Si causes the matrix to be seriously divided and the strength and plasticity of the alloy are greatly reduced.Therefore,the modification method is usually adopted to improve the mechanical properties of the alloy.In this paper,the rare earth Er was chosen as the grain refiner.The effect of rare earth Er on microstructures and properties of as cast A356 aluminum alloy were studied by means of using differential scanning calorimetry(DSC),X-ray diffraction(XRD)and scanning electron microscopy(SEM).The optimum amount of rare earth was preliminary drawn on the basis of the analysis of the best rare earth addition.The influence of traditional T6 process of the alloy was studied by means of different heat treatment of the modified alloy.It optimized the process parameter,the effect of T6I6 two-stage on the alloy was further researched.Based on the modification of rare earth Er,the effect of compound modification on microstructures and properties of as cast alloy was studied by adding trace element Zr.The research results show that:Rare earth Er element not only has a refinement effect on the primary phase ?-Al of the A356 alloy,but also has a modification effect on the eutectic silicon.It is found that ?-Al and Al3 Er compounds are produced by the eutectic reaction between rare earth and aluminum by means of DSC.The first precipitated phase of the eutectic reaction is the ?-Al that can be used as the nucleation cores of the primary phase of the alloy,which can play a role in refining the grain and reducing the secondary dendrite.Constitutional supercooling was formed by the rare earth Er element in the front of the precipitated silicon phase,and the original growth direction of silicon phase was restrained.The part of rare earth atoms adsorbed in the twin grooves of the silicon phase embeds the silicon lattice to form foreign atomic defects,which causes the lattice distortion.This distortion makes silicon producetwin in more direction and the eutectic silicon change from coarse lath to short rod and fine granular,which has obvious metamorphism.When the content of rare earth Er is 0.4 wt.%,the microstructures and the mechanical properties of as cast alloy are obviously improved.A356+0.4% Er alloy has excellent combination property under T6 heat treatment optimized process of 530?×4h + 175?×6h.At the same time,we study the effect of T6I6two-stage process on microstructures and properties of alloy and optimize the T6I6 two-stage process parameters.Compared with T6 process,the tensile strength,elongation and hardness of the alloy increase by 11.9%,4.4%,18.2% respectively when the pre-aging process parameter is 110?×2h and the re-aging process parameter is 200?×4h.The grain size is obviously refined with the addition of Zr in A356+0.4%Er alloy,especially when the content of Zr is 0.2 wt.%,the refinment is the best.The tensile strength can reach 225 MPa,the elongation is 6.4 wt.% and the hardness is 75.3HV,compared with single Er modification increase respectively by 32.7%,4.9%,23.8%.