Microstructure Of Direct-chill Casting 7A56 Aluminum Alloy And Its Evolution During Homogenization Treatment

The perpetual subject of the 7xxx series aluminum alloys is to improve the strength and comprehensive properties as much as possible.To achieve this target,increasing the content of main elements has been proved as a doable and effective method in recent years.However,it will inevitably result in more non-equilibrium phases during the cooling process.As a result,it is more difficult to eliminate these coarse phases during the heat treatment process.Based on the direct-chill casting 7A56 alloy ingot with large size produced under the industrially conditions,the microstructure and its evolution during homogenization has been systematically studied.We hope that it will provide some innovatively theoretical and technical ideas for developing new homogenization tempers for high alloying content 7xxx series aluminum alloys.This study mainly focuses on the microstructure of as-cast 7A56 aluminum alloy and its evolution during homogenization treatment.First,the microstructure of as-cast 7A56 alloy ingot,including the second phases,is carefully studied.The macro homogeneity of the large ingot was discussed because it might interfere the analysis of property evolution during homogenization.Then the microstructure of the homogenized 7A56 alloy,especially the phase transformation and the dissolution of eutectic phase with low melting point,has been studied.Through the comparison of the result before and after homogenization,the evolution of microstructure is rigorously studied.It is discovered that the alloying content is lowest at the center part of the ingot while other places share a good chemical homogeneity;the center and middle part is tested with a higher hardness,a lower conductivity as well as a larger grain size than those of the edge part.These properties in the center and middle part are in good homogeneity.There is a large amount of non-equilibrium phases in as-cast 7A56 alloy,including T phase(rich in Al,Zn,Mg and Cu elements),Al7Cu2Fe phase and Al2Cu phase.T phase,whose lattice structure is as same as MgZn2,is the main second phase with a low melting point that was mainly focused during the homogenization.The average lattice constant of the ingot is 4.051 78A;the average hardness of the middle part is about 102HV and the electrical conductivity is 22.8Ms/m.After one-stage homogenization treatment,it is discovered that temperature plays a pivotal role for the transformation of phase T.After homogenized at 380?,most part of phase T still remain with the original eutectic structure with a continuous border,only a few of which transform into S phase(Al2CuMg).After homogenized at 430?,the eutectic structure of phase T is partially destroyed,the border of which turns to discontinuous in many areas;it is estimated that 50%-60%of phase T has been dissolved into the Al-matrix.After homogenized at 470'C,the eutectic structure of phase T has been completely devastated;more than 90%of phase T has been eliminated.Transformation from phase T to S is not observed under 430? or 470?.The dissolution of phase T can significantly influence the properties of Al-matrix that the lattice constant is larger;the hardness increases while the electrical conductivity decreases.The more phase T has been dissolved,the larger the vibrations is.After two-stage homogenization treatment,it is found that the residual phase T after the first stage treatment(430?/12h)will rapidly be dissolved during the second stage at a higher temperature(470?).Also,no phase transformation from phase T to S was observed.The effect on Al-matrix due to the dissolution of phase T is similar to that of single-stage treatment.The temperature of first-stage can influence the shape of precipitated Al3Zr particles,the increase of which leads to an increase of square Al3Zr particles.After 400?/12h+470?/24h,the Al3Zr particles is spheroid.After 430?/12h+470?/24h,beside the spheroid Al3Zr particles,a few square particles occurred.After 450?/12h+470?/24h,the volume of square Al3Zr particles obviously increases.Both spheroid and square particles have similar electron diffraction characterization.