Investigation On ?-phase Segregation And Room Temperature Properties Of High Strength Casting Al-Cu-Mn Alloy

ZL205A alloy is a kind of high strength casting aluminum alloy developed by Beijing Aeronautical Materials Research Institute in the 1970 s.It has the characteristics of high specific strength,excellent corrosion resistance and machinability,and used in aerospace,weapon,metallurgical fields,etc.But with wide crystallization temperature range,shrinkage,segregation and other casting defects were easy produced in casting process,it also has the tendency of intergranular corrosion and stress corrosion,thus limiting widely applied in high-performance mechanical parts and civilian industries.The purpose of this paper is to broaden the industrial application of high strength cast aluminum alloy in complex thin wall casting and precision forming areas,the microstructures and room temperature properties of ZL205 A alloy,and influence mechanism were investigated from three aspects of casting process parameters,microalloying and heat treatment respectively.We hope to lay the foundation for further controlling the materials solidification behavior and improving the microstructure precise control theory.Pouring temperature and temperature gradient are two key parameters to control materials solidification behavior.With different pouring temperature(680?~740?),the microstructures and properties of as-cast and heat treatment castings with different thicknesses(5 mm ~ 25 mm)were studied.The results indicated that with increasing pouring temperature,the ? phase in ZL205 A cast alloy tends to exist in granular and elongated strip shape and the overall residual amounts decreases and the distribution also becomes relatively uniform,the dissevering effect on matrix is reducing.The temperature gradient caused by wall thickness has a certain effect on the ? phase morphology and the radial component gradient.The concentration of Cu atoms in the residual ? phase decreases with wall thickness increased,and the temperature gradient can promote the degree of Cu atoms solid soluting into matrix,thereby enhancing solution strengthening effect.After T6 heat treatment,the amount of residual white structure which distributed along with grain boundaries is very small,and micro alloying elements are basically dissolved in the matrix during the stage of solution treatment and aging process.Thus component segregation is greatly reduced,and structure density is also obviously higher than that of as-cast state.The influencing mechanism between different microalloying elements and matrix is not same,while the resulting intermetallic compounds have a significant difference with the matrix structure,this will have an important effect on the precipitation process of the secondary phase,and seriously affect the comprehensive mechanical properties of alloy after solution treatment alloy.In this paper,with single addition of Er element and multiple additions of Er and Zr elements respectively,the segregation behavior of ? phase and the effect mechanism of castability,mechanical properties and fracture performance were compared and analyzed.The results show that single addition of Er element and multiple additions of Er and Zr elements both can increase the fluidity of liquid alloy and reduce the casting defects such as shrinkage,porosity,sand inclusion,etc.This effectively improved the casting performance of ZL205 A alloy.Single addition of Er element will have a great influence on the diffusion and solid solution of Cu atoms,resulting in the ? eutectic phase occurring near grain boundary,and with increase of addition amount(0.1% ~ 0.5%),the residual content of eutectic structure increased.Er has high chemical activity,which is easy to adsorb on the interface of atomic group and crystal embryo during the solidification process,reducing interface energy and nucleation activation energy of the ?,phase,and slightly shifting the TTT curve to the left,thus accelerating the desorption progress of GP zones and ?,phase during solution and aging process,and also promoting to precipitated a small amount of ? equilibrium phase.Trace Er and Zr elements can refine the grains,at a certain degree aggravated the ? phase branching and segregating in some cases,resulting in a decrease in the width of grain boundary structure,or ever fusing into granular form while extending into bulk form at the intersection of grain boundaries.T5 heat treatment can promote organization and composition even,while leading solute atoms fully soluted into the matrix.It can hinder the movement of dislocation and the migration of sub-boundary.