Study On Microstructure Development And Mechanical Properties Of AZ31 Mg Alloys Profiles Extruded By Porthole Die Extrusion

With the increasingly prominent environmental problems in China,the demand for energy saving and emission reduction in industry is increasing,such as automobile,rail transit,aerospace and shipbuilding.Thus,the lightweight of traffic carrier has became a hot research topic.Mg alloy is becoming a matter of concern for more and more people as the lightest structural metal.Mg alloy has many advantages,such as high strength-to-weight ratio,high specific stiffness,easy cutting,easy recovery.However,Mg alloy usually exhibits poorer plastic formability at room temperature,due to its hexagonal close-packed(HCP)crystal structure.Thus,hot forming processes have been main methods to produce Mg alloy components,such as hot rolling,hot forging and hot extrusion.Among these processes,hot extrusion of Mg alloy is focused,because it is the efficient and convenient one to produce profiles.Moreover,compared with other hot forming processes,hot extruded profiles have better microstructure and higher mechanical properties.Porthole die extrusion is the main way to produce hollow magnesium alloy profile.During porthole die extrusion,the material undergo some processes,such as splitting,welding,breaking,and so on.The processes of material flow and microstructure evolution are complex,and profile with several longitudinal seams is formed after porthole die extrusion.At present,the investigation of Mg alloy extrusion mainly focus on plat extrusion.The study of Mg alloy porthole die extrusion has not received adequate attention,and the vary rule of microstructure and mechanical properties of Mg alloy material during porthole die extrusion is not clear.In response to the above problems,this paper focused on porthole die extrusion of Mg alloy and related numercial simulation.This study deeply investigated the microstructure and mechanical properties of Mg alloy profiles extruded by porthole die.The main contents and conclusions are shown as follow:(1)After porthole die extrusion of Mg alloy at 380?,the extruded profile and dies were immediately quenched into iced water to save initial deformed microstructure.The material inside dies were taken out for microstructure observation.The material flow line were confirmed by the numerical simulation results of Deform3D.And,the sampling spots in flow lines were picked for microscopic characterization.Based on the above results,the microstructure evolution of AZ31 Mg alloy during porthole die extrusion were investigated,such as grain morphology,grain size,texture,and so on.(2)The porthole die extrusion experiments at different temperatures(350?,380? and 410?)of Mg alloy were tried.Then,the microstructure in cross-sections of extruded profiles were observed,and the quality of L-seams were tested by the tensile tests perpendicular to L-seams.The microstructure distribution of extruded profiles was explained by the Deform3D results.The Q values and J values of L-seams were calculated to estimate the welding quality by the results of Deform3D.Combined with the microstructure observation and the results of the tensile tests,it is found that the quality of L-seams is related to the welding quality and the microstructure in the welding zones.(3)The porthole die extrusion with different billets(as-cast,as-homogenized and as-extruded billets)were finished.Then,the microstructure and the mechanical properties of the extruded profiles were tested and analyzed.In this study,the second phase particles apparently promote dynamic recrystallization(DRX)of Mg alloy in porthole die extursion,and it plays an important role in grain refinement and texture weakening.Thus,the profile extruded with as-cast billet has the best mechanical properties because of its the finest grain and the weakest deformation texture.