Study On Microstructure And Mechanical Properties Of Conventional Squeeze Casting And Rheo-squeeze Casting 7075 Aluminum Alloy

In recent years,in order to further achieve energy saving and emission reduction,the development of new energy vehicles in China is very rapid,and the demand for lightweight automotive structural parts is more and more urgent.The method of replacing steel with aluminium and forging with casting is an important way to realize lightweight of key complex structural parts such as automobile chassis and power system.As one of the ultra-high strength aluminum alloys,the tensile strength of 7×××series alloy is generally higher than 500 MPa,which makes it the preferred lightweight material for new energy vehicles.However,due to its high alloying degree and wide solidification temperature range,defects such as coarse structure,hot cracking and segregation are prone to occur during casting.It is difficult to produce complex structural parts directly by casting and extrusion,which greatly limits the application scope of this series of alloys in the field of automobiles.Therefore,the development of liquid forming technology of 7×××high strength aluminum alloy has very important application value.In this paper,7075 aluminium alloy is taken as the research object,and optical microscopy,scanning electron microscopy,X-ray diffractometer,differential scanning calorimeter,transmission electron microscopy,hardness and tensile test were used.The effects of process parameters on the microstructure and mechanical properties of 7075aluminum alloy in conventional squeeze casting and rheological squeeze casting were systematically studied.The effects of pressure,pouring temperature and micro-alloying element Sc on the solidification behavior and strength and toughness of 7075 aluminum alloy in conventional squeeze casting and rheo-squeeze casting were clarified.The optimized forming process parameters were obtained.On this basis,the heat treatment process of squeeze casting 7075 aluminium alloy is studied,which provides an optimized heat treatment scheme for its practical engineering application.The results show that:During the conventional squeeze casting process,with the increase of extrusion pressure,the coarse dendrites of?-Al grains are gradually transformed into smaller rosette-like structures,and the distribution of the second phase of grain boundary is more and more uniform.This is mainly because the pressure increases the liquidus temperature of the alloy,thus increasing the undercooling,increasing the heterogeneous nucleation rate of the melt and accelerating the heat transfer rate.With the decrease of pouring temperature,the grain size of?-Al decreases first and then increases slightly.When the melt temperature is low and the thickness of free solidification layer is moderate,the grain refinement effect is the best.The optimized extrusion pressure is 120 MPa and the pouring temperature is 680?.In addition,the heterogeneous nucleation core of primary Al₃Sc formed in the solidification of the alloy plays a better role in grain refinement.Under the same process,the yield strength,tensile strength and elongation of 7075-Sc alloy reach 258.5 MPa,344.5 MPa and 5.3%,which are 24.5%,17.3%and 17.8%higher than that of 7075 aluminum alloy.In the process of semi-solid slurry preparation,with the decrease of pouring temperature,primary?-Al gradually changed from coarse dendrite to small rosette-like grains,and the solid fraction increased continuously.The main reason is that the temperature of slurry decreases,which is conducive to the nucleation and growth of primary?-Al.With the increase of the rotational speed of the drum,the grain size of primary?-Al decreases at first and then remains basically unchanged,but the solid fraction increases greatly.The increase of drum speed improves the local chilling effect.The optimized pulping process parameters are pouring temperature700?and drum speed 60 r/min.The metallographic structure of the alloy formed by rheo-squeeze casting is composed of rosette-like and near-equiaxed grains.The addition of Sc element can first form primary Al₃Sc particles before the formation of primary?-Al,and further refine the slurry structure with the stirring action of the drum.At the same time,the microstructures of 7075-Sc alloy obtained by rheo-squeeze casting are mostly equiaxed grains with uniform size.The yield strength and tensile strength of 7075-Sc alloy are 276.7 MPa and 356.7 MPa respectively,which are 55.1%and 33.3%higher than 7075 aluminium alloy.And the elongation is 4.5%.The heat treatment process of squeeze casting 7075 and 7075-Sc alloys was studied.The results showed that:The second phase with high melting point at grain boundary can not be fully soluble under the single-stage aging at 470?.A two-stage solution process?470?×10h+500?×2h?was developed to improve the solution degree of the second phase.The peak aging was achieved at 120?for 48h.In the underaged state of the alloy at 24 h,GP zone is mainly formed in the grains.For Sc-containing alloys,Al₃Sc at grain boundaries plays an obvious role in pinning grain boundaries and significantly reduces grain growth during solid solution.At the same time,Al₃Sc phase plays a better role in dispersion strengthening and precipitation strengthening.After aging,the yield strength,tensile strength and elongation of 7075-Sc alloy by rheo-squeeze casting reach 529.3 MPa,612.2 MPa and 10.5%respectively.Its properties are basically consistent with those of 7075-T651 wrought aluminium alloy.