| As the lightest structural metallic material,magnesium(Mg)alloys exhibit considerable specific strength and stiffness,superior heat dissipation and electromagnetic shielding performance,having broad applications prospects in the fields of aerospace,transportation and electro-communication.Under the strategic target of "peaking carbon dioxide emissions by 2030 and achieving carbon neutralization by 2060",Mg alloys,which are easily recycled,environmentally friendly,and known as the green engineering material in 21st century,will obtain an unprecedented development opportunity.Hence,it is extremely urgent to develop high-performance Mg alloy components and its advanced forming technology.Hollow Mg alloy profiles fabricated based on porthole die extrusion process can minimize the weight of structural components.Great efforts have been devoted on the investigation of extrusion process,microstructure and properties of Mg alloy profiles.However,a few research was available that focused on the porthole die extrusion of Mg alloy profiles,and there are still some key issues needed to be solved.The materials flow and microstructure evolution of Mg alloys were extremely complicated,which was further intensified by the solid welding behaviors of charge and longitudinal weld seams,making it difficult to design and control the extrusion die and process.Materials at different positions experienced significantly different histories of deformation and temperature,and the existence of longitudinal welds divided the profiles into welding and matrix zones with completely different microstructure,leading to the microstructure and properties inhomogeneities of Mg alloy profiles.The mechanical properties of Mg alloy profiles were deteriorated due to the occurrence of abnormal grain growth during the extrusion and subsequent hot treatment processes.Focusing on the aforementioned issues,the solid-state bonding and abnormal grain growth behaviors in the porthole die extrusion of ZK60 Mg alloy profiles were investigated using extrusion experiments,numerical simulation and theoretical analysis.The effects of charge and longitudinal welds on the microstructure inhomogeneity,mechanical properties and corrosion resistance were enclosed.The microstructure evolutions of Mg alloys along various extrusion deformation path were studied.The mechanisms of abnormal grain growth on the longitudinal welds and surface of Mg alloy profiles were illuminated,and a method based on room temperature pre-tensile deformation was proposed to suppress the abnormal grain growth on the welding zone during solution process.The implementation of this subject can provide theoretical guidance for the solid-state bonding and the control of microstructure and properties during porthole die extrusion process of Mg alloy profiles.The main research works are as follows:(1)The formation and evolution of charge weld during porthole die extrusion of Mg alloy profiles were investigated,the effects of key extrusion die structures and processing parameters on the charge weld length were analyzed,and the relationship between charge welds and mechanical properties of Mg alloy profiles were established,contributing to the determination of the optimal discharged position in view of the existence of charge welds.It was found that the material flow velocity and the adhesive friction of old billet with die cavity were key factors affecting the evolution of charge weld and the length of charge weld.Hence,lower temperature,higher extrusion ratio,higher extrusion speed,and the reasonable baffle plate were beneficial to reduce the charge weld length.Some amounts of impurities and oxides distributed on the charge weld and the different microstructure features between new and old billets could be observed,which deteriorated the mechanical properties of Mg alloy profiles.With the extension of charge weld,the density and size of impurities and oxides gradually decreased and the microstructure of new and old billets gradually converged,contributing to the improvement of mechanical properties.(2)The mechanisms of microstructure inhomogeneity on the porthole die extruded Mg alloy profiles were investigated,the effects of longitudinal welds and extrusion speed on the microstructure homogeneity were revealed,and the relationship between extrusion speed,microstructure,mechanical properties and corrosion resistance were established.It was found that the different degree of dynamic recrystallization and grain growth at different position of Mg alloy profiles induced by the inhomogeneous distribution of temperature,effective strain and velocity during the porthole die extrusion should be responsible for the formation of inhomogeneous microstructure.The Mg alloy profiles were divided into welding zone with uniform equiaxed grains and matrix zone showing bimodal characteristics by longitudinal welds,which further intensified the inhomogeneous distribution of microstructure.Increasing extrusion speed could increase the grain size of the Mg alloy profiles and dynamic recrystallization degree of the matrix zone,while reduce the density of precipitations,leading to the improvement of microstructure inhomogeneity.Moreover,the strength and elongation of Mg alloy profiles were deteriorated by high speed extrusion,while corrosion resistance of welding zone was enhanced,improving the consistency of the corrosion behaviors of welding and matrix zones.(3)The effects of longitudinal welds,tensile temperature and loading direction on the deformation behaviors and mechanical properties of Mg alloy profiles were investigated.It was found that the tensile deformation of welding zone at low temperature was mainly controlled by the dislocation slips with the association of some twins,while twining played significant roles in the accommodation of c-axis strain of the coarse grains on the matrix zone.During the low temperature tension along extrusion direction(ED),the deformation of Mg alloy profiles was based on basal slip and small amount of twining,exhibiting a slip-dominated deformation at the macro level.However,the deformation along transverse direction(TD)was controlled by basal slip,prismatic slip and twinning,showing a twining predominant deformation.With the increase of deformation temperature,the dislocation cross slips of pyramidal slip were activated,and the twining vanished,resulting in the converging of deformation mode on welding and matrix zones.Besides,the effects of loading direction on deformation mechanisms were greatly weakened under high temperature deformation.The welding zone showed superplastic behavior during tensile deformation at higher than 300℃ due to the grain boundary sliding.The fine grain structure resulted in the superior room temperature tensile properties along extrusion direction of welding zone.With increasing the tensile temperature,the strength of Mg alloy profiles suddenly dropped,and the tensile properties at various position of Mg alloy profiles tended to be equivalent.(4)The abnormal grain growth on the longitudinal welds of Mg alloy profiles during porthole die extrusion was investigated,and the effects of extrusion temperature and speed on microstructure and mechanical properties of extruded profiles were well studied.It was found that the abnormal grain growth on the longitudinal welds of Mg alloy profiles during porthole die extrusion could be attributed to the growth priority of some grains with the advantages of grain size and high energy grain boundaries,and was driven by the grain and sub-grain boundary energy.Besides,the restriction on normal grain growth by the inhomogeneous distribution of precipitates further promoted the occurrence of abnormal grain growth.The formation and evolution of the fine grains without abnormal grain growth depended on the dynamic recrystallization and grain growth.The occurrence of dynamic recrystallization promoted the grain refinement and the formation of<10-10>-<11-20>double fiber texture during splitting stage,while the grain growth caused the increase of grain size and the transformation of grain orientation to<10-10>fiber texture during solid bonding and extruding stages.The formation of abnormal coarse grains on the welding zone deteriorated the mechanical properties of extruded profiles.Higher extrusion temperature or speed increased the sizes of both abnormal coarse grains and the other grains,resulting in the decrease of strength and elongation.(5)The formation mechanism of abnormal grain growth on the welding zone of Mg alloy profiles during solution process was well investigated,and the effects of solution and aging treatments on the microstructure,mechanical properties and corrosion resistance were studied.It was found that abnormal coarse grains were formed on the welding zone during solution process.High solution temperature could greatly decrease the incubation time,and increase both the growth rate and final size of abnormal coarse grains.The main driving force of abnormal grain growth was the stored energy gradient.<11-20>//ED grains with low quantity and stored energy were selected to achieve abnormal grain growth by consuming the<1010>//ED grains with high stored energy to reduce the total free energy of the system.Besides,high proportion of high energy grain boundary helped the<11-20>//ED grains easily to release from pinning effects of the second phase particles.When static recrystallization and grain growth of matrix zone were activated,lots of stored energy was consumed,resulting in the ending of abnormal grain growth.With the increase of solution time,the strength and elongation of Mg alloy profiles gradually decreased,while the corrosion resistance was greatly enhanced.Aging treatment making an obvious precipitation strengthening effects on Mg alloy profiles,while the corrosion resistance of aged sample was deteriorated again due to the precipitation.The formation of abnormal coarse grains strongly deteriorated the mechanical properties of Mg alloy profiles,while the corrosion resistance was greatly improved after solution treatment.(6)In order to suppress the abnormal grain growth behaviors on the welding zone of Mg alloy profiles during solution process,a technology based on room temperature pre-tensile deformation was proposed.The effects of pre-tension deformation on abnormal grain growth and the properties of Mg alloy profiles were investigated.It was found that the size and stored energy of grains with different orientation was reassigned by pre-tensile deformation,inhibiting the orientation-dependance abnormal grain growth.Moreover,high density dislocations and some amounts of twins as well as stacking faults were induced by pre-tension,which could greatly promote the occurrence of static recrystallization and refine the welding zone.The abnormal grain growth on the welding zone of Mg alloy profiles was effectively suppressed based on the above two aspects.With increasing pre-tensile strain,the grain size on the welding zone gradually decreased after solution treatment,while that on the matrix zone firstly increased and then decreased.The density of precipitates raised,and the type of precipitates was changed after pre-tension.With increasing pre-tensile strain,the strength of the profiles after solution and aging treatment firstly decreased and then increased,and the fracture mechanism transformed from mixed type to ductile fracture.(7)The mechanism of abnormal grain growth on the surface of Mg alloy profiles during extrusion were investigated,and the surface microstructure evolution and its effects on hardness and wear properties were studied.It was found that surface materials experienced higher degree of deformation and temperature during porthole die extrusion process.The abnormal grain growth easily occurred in the subsequent air cooling process,which distributed inhomogeneously along the length direction of Mg alloy profiles.The materials closed to the profile front experienced low effective strain and temperature during porthole die extrusion process,and thus formed large amounts of precipitates,restricting the occurrence of recrystallization and grain growth.Hence,their surface was composed of<11-20>//ED abnormal coarse grains,the<10-10>//ED coarse deformed and fine recrystallized grains.With the proceeding of extrusion process,the deformation temperature and strain gradually increased,and the density of precipitates gradually decreased,which greatly promoted the recrystallization and the abnormal grain growth of<11-20>//ED grains.The abnormal growth of surface grains had an adverse effect on the hardness and wear properties of the Mg alloy profiles. |
