Study On Microstructure And Properties Of Al-10.7Zn-2.4Mg-0.9Cu Alloy Prepared By Rapid Solidification And Hot Extrusion

With the continuous warming of the global climate in recent years,environmental crisis and energy consumption have become major problem for human beings.The lightweight of the car-body is beneficial to environmental protection,and the application of aluminum alloys can greatly reduce the weight of the car-body structural parts,to achieve the purpose of reducing energy consumption and waste gases emissions.Al-Zn-Mg-Cu alloy is a kind of heat-treatable wrought aluminum alloy with advantages of low density,high strength,good fracture toughness and strong corrosion resistance.In recent years,it has been widely used as a lightweight material in the manufacturing of important structural components in the transportation fields,such as automobiles,ships,high-speed trains and space shuttles.In order to improve the mechanical properties of Al-Zn-Mg-Cu alloy,researchers have conducted extensive research on the optimization design of alloy composition,development of new preparation process,and the relationship between material microstructure and properties at home and abroad.Aluminum alloy prepared by rapid solidification technology has the characteristics of fine grain,less segregation and high solid solubility,which is beneficial to improve the performance of Al-Zn-Mg-Cu alloy.Based on the above reasons,this paper uses Al-10.7Zn-2.4Mg-0.9Cu alloy as the experimental material,and the processing technology of melt-spinning+cold-pressing+hot-pressing+hot extrusion and subsequent heat treatment are used to prepare high-strength aluminum alloy rods with fine and uniform grains.The main conclusions of this study are as follows:The results of ribbon preparation by melt spinning show that rapid solidification has a significant effect on grain refinement.The thickness of the Al-10.7Zn-2.4Mg-0.9Cu alloy ribbon obtained by the melt-spinning is 92.7?153.7?m,and the ribbon is mainly composed of equiaxed fine grains with an average grain size of 3.8?m,which is only 5%of the grain size of metal-mold casting ingot.Coarse?phase?Al₂Cu?exists on the grain boundary of the casting samples,and the main intragranular particles are?phase?Mg Zn₂?.Because rapid solidification expands the solid solubility of alloy elements in the Al matrix,most of alloy elements are dissolved in the Al matrix,and no coarse second phase was found in the ribbon.The lattice distortion occurs in aluminum matrix,and the lattice constant increases slightly.The EBSD pole figure analysis results show that there is no obvious preferred orientation in the ribbon.TEM microstructure analysis results indicate that there are a few?'metastable phases and high dislocation density in the ribbon.Extrusion densification experiments show that after hot extrusion of cold-pressed billet,as the extrusion ratio increases,the alloy grain size decreases and the homogeneity increases,the metallurgical bonding quality of the ribbon improves continuously,and the mechanical properties of Al-10.7Zn-2.4Mg-0.9Cu increase accordingly.With the increase of extrusion temperature,the aluminum grains continue and the second phase coarse,and the mechanical properties first increase and then decrease.The cold-pressed billets are further hot pressed,and then extruded at an extrusion ratio of 39.1 and 420?.When the hot-pressing temperature is200?,the mechanical properties of extruded rod are the best,and there is no obvious ribbon bonding interface in the fracture surface.The alloy elements precipitate from the ribbon matrix,forming a large number of finely dispersed?phases.The extruded alloy has obvious preferred orientation along the extrusion direction,mainly forming<111>and<001>textures.By comparing the microstructure and properties of the alloy after the hot deformation of three kinds of billets,namely cold-pressing billet,200?hot-pressing billet and metal casting ingot,it is found that the grain size of the metal casting ingot is still large after extrusion,and the plastic deformation could not completely eliminate the coarse non-equilibrium eutectic phase formed in the traditional metallurgical process,and the mechanical properties are the worst.Although the grains are the finest after the cold-pressed billet is extruded,its mechanical properties are still not ideal since the metallurgical bonding quality of the ribbon are not quite good.Due to the improvement of the metallurgical bond quality of the ribbon,fine grain strengthening,texture strengthening and second phase strengthening,the mechanical properties of the extruded rod of 200?hot-pressed billet are the best,with its tensile strength and elongation of 577.9 MPa and 16.8%,respectively.The heat treatment experiment shows that after the solution treatment of 480?/0.5 h,the peak aging process is 140?/3 h.The T6 alloy has finely dispersed precipitations inside the grains and a continuous precipitation chain distributed at the grain boundary.There is no obvious precipitates free zone?PFZ?near the grain boundary.And its tensile strength and elongation are 693.4 MPa and 7.9%,respectively,the fracture mechanism is ductile-brittle mixed fracture.The optimal two-step aging process of the alloy is 120?/2 h+160?/2 h.At this time,the size of the precipitated phase in the alloy grows slightly,and the precipitated phase in the grain boundary is intermittently distributed,and a wide PFZ exists near the grain boundary.The tensile strength and elongation of the two-step aging alloy are 594.5 MPa and15.5%,respectively,and its fracture mode is ductile fracture.The alloy can obtain better comprehensive properties after the RRA treatment of 140?/3 h+180?/20 min+140?/3 h,and the microstructure exhibits the characteristics of one-step aging and two-step aging,and a narrow PFZ exists along the grain boundary.The sample shows a tensile strength of 662.8MPa and an elongation of 13.8%,and the fracture mode is ductile fracture.