Research On Microstructure And Mechanical Property In Porthole Die Extrusion Of 7075 Aluminum Alloy

In recent years,more and more countries have taken energy conservation and emission reduction as an important development direction,due to the increasingly prominent environmental pollution.As one of the typical high strength AI-Zn-Mg-Cu series alloys,7075 aluminum alloy has the advantages of low density,high fracture toughness and good workability.It is considered to be the promising candidates to substitute the traditional steel for lightweight design in the fields of automobile,high speed train and aerospace,to conserve energy and reduce emissions.Thus,the industrial application of 7075 aluminum alloy has attracted more and more attentions.Porthole die extrusion has great advantage in producing hollow profiles,and thus is increasingly used in industry.However,longitudinal weld seams along the entire length of the profile inevitably forms due to the metal separation and welding during the porthole die extrusion.If the solid welding process is not well controlled,the longitudinal weld seam tends to become the weakest part and degenerate the mechanical properties of the profile.Moreover,the material at different positions experiences varied strain,strain rate and temperature during the porthole die extrusion,and thus leads to the inhomogeneous microstructure distribution of the profile.Based on above issues,the microstructure,mechanical properties were comprehensively studied by the experiment and numerical simulation of 7075 aluminum alloy to reveal the microstructure evolution during the porthole die extrusion and improve the mechanical properties of the profile.The implementation of this subject can build theoretical basis for the porthole die extrusion of 7075 aluminum alloy and furtherly improve the quality of the extruded profile,which is of great theoretical significances and engineering application values.The main work and conclusions of this study are as follows:(1)The strain compensated Arrhenius and modified Johnson-Cook constitutive equation were established based on the true stress-strain curve obtained from isothermal hot compression tests.Both equations show high accuracy in predicting flow stress of 7075 aluminum alloy.Through the analyses of the established kinetic equation of 7075 aluminum alloy,it is found that both high deformation temperature and low strain rate are favorable for the occurrence of dynamic recrystallization(DRX).(2)The porthole die extrusion experiment was conducted to obtained the profile of 7075 aluminum alloy,and it is found that significant microstructure inhomogeneous exists on the cross section,which can be divided into welding zone,transition zone and matrix zone.Complete DRX occurred in the welding zone,and the recrystallization fraction decreases along the direction of matrix zone.According to the experimental and simulated results,the welding quality is the worst at the front side of the profile and enhanced along the inverse direction of the extrusion direction.Moreover,the increase of the extrusion temperature is helpful for the improvement of the welding quality.(3)Through the study of microstructure inside the chamber of porthole die,it was found that DRX fraction varies at different position due to the strain disparity.Moreover,{101}<111>and {101}<001>textures are the main texture in welding zone,while the mainly textures in matrix zone are Copper and R-Copper texture.Moreover,Al23CuFe4 phases cannot dissolve into Al-matrix and break into chainlike particles due to the mechanical deformation during the porthole die extrusion.However,MgZn2 phases firstly dissolve into Al-matrix and then homogeneous precipitate at the die exit.(4)Solution treatments with different time have been conducted for the porthole die extruded profile,and it is found that the solution treatment at 470? for 15 min is sufficient for the dissolution of MgZn2.The profile with best mechanical properties can be obtained with solution treatment of 15 min,and the mechanical properties degenerate with the prolongation of the solution treatment time.In addition,the texture intensity and volume fraction increase and then decrease with the prolongation of the solution treatment time,while the types of texture component remain unchanged.