Research On Extrusion Flow Behavior And Interface Behavior Of The Continuous Fiber-reinforced Aluminum-based Composite Profile

Aluminum alloy profiles are widely used in construction,rail transit,aerospace,and other industries due to their low density,high specific strength,excellent corrosion resistance and good recyclability.The hot extrusion process is a main method to produce aluminum alloy profiles where the high-temperature aluminum bars are extruded through die orifices to form profiles with specific cross-section shapes However,the ability to improve the mechanical strength of aluminum alloy profiles through heat treatment was limited,thus a new hot extrusion process named as the continuous extrusion of the fiber-reinforced aluminum-based composite profiles was proposed.By using the modified porthole extrusion die,the composite profile can be obtained by embedding the reinforcing fiber and making them bond together.The novel process has the great potential for improving the mechanical properties and functional diversity of aluminum alloy profiles,therefore,it has attracted much attention of relevant manufacturers and researchers.During the extrusion process of the continuous fiber-reinforced aluminum-based composite profiles,the aluminum alloy billet is divided into multiple strands of metal flow by porthole bridges.Thus,under high-temperature and high-pressure,multiple welding lines form on the profile's surface along with solid-state bonding and microstructure recombination.In addition,the aluminum alloy is in contact with the reinforcing fibers(such as wire rope)during the extrusion process,thus the Fe/AI diffusion layer is formed owing to the diffusion of different elements on the contact interface.Therefore,the mechanical properties of the fiber-reinforced aluminum-based composite profiles are directly determined by the welding quality of the Al/Al interface and the diffusion behavior of the Fe/Al interfaceBased on the numerical simulation and experimental examination of the extrusion process of continuous fiber-reinforced aluminum-based composite profiles in this work,the plastic deformation behavior of matrix material and the influence of special die structure on the reinforced fiber were studied,and the stress concentration of fiber were analyzed.Besides,the welding behavior of Al/Al solid-phase interface,the diffusion behavior of steel/Al solid-phase interface and the evolution of interface microstructure in the whole extrusion process of composite profiles were revealed.The main work and conclusions of this study are as follows:(1)The extrusion die of continuous fiber-reinforced aluminum-based composite profiles was designed.Based on the numerical simulation of finite element software(Deform),the flow behavior of matrix material,the distribution of stress,strain,welding pressure,temperature and velocity in the composite profiles and the stress characteristics of reinforced fiber were studied.Then,the extrusion experiment was carried out successfully and the composite profiles were obtained.(2)It was found that at the initial unsteady extrusion stage,the Al/Al welding interface consisted of a large number of continuous micro-voids.Meanwhile,the grains on both sides of the interface were not firmly bonded together,but only the physical contact.Subsequently,under high-temperature and high-pressure,the size of micro-voids and the distance between A1 atoms decreased continuously,the interface grain boundaries were formed and the bridging behavior was found.Finally,the intimate contact was achieved at the welding interface and the sound atomic bonding was formed.(3)Along the Al/Al welding path,the continuous voids and gaps on the bonding interface gradually evolved into discontinuous micro-voids,and the interface grain boundaries was found,where the size of micro-voids at the junction of grain boundaries was lager than that at grain boundaries.Interestingly,because of the decrease of micro-voids and the accumulation of driving force at the Al/Al welding interface,the interface grain boundaries broke away from the dragging effect of the micro-voids and migrated,thus the micro-voids remained inside the grains,which indicated that the new grains through the welding interface were formed.Finally,under high-temperature and high-pressure,the micro-voids further shrunk and then closed,and finally a firm bonding zone was formed.(4)At the steel/Al interface,the existence of micro-voids and the gathering of minor elements at Fe/Al diffusion interface had noticeable effects on the diffusion behavior of Fe and Al.One was that the increase of the number of the micro-voids on the aluminium matrix side of diffusion zone would decrease the Al and Fe flux and thereby restricted the growth of diffusion layer.Another was that the enrichment of Si and Zn elements at the interface took up a number of vacancies and consequently retarded the diffusion of Fe and Al elements,leading to the slow growth of diffusion layer.(5)The embedded fiber had a certain effect on the mechanical properties of aluminum profiles.It was found that the hardness of Fe-Al solid solution in the diffusion zone was dramatically higher than that in the aluminum matrix and wire rope,while the elastic modulus was slightly lower than that of the wire rope zone which was far away from the Fe/Al interface.Besides,the fracture mechanism of aluminum matrix was dominated by intergranular fracture.Compared with the mechanical properties of the conventional profiles,the tensile strength of the composite profiles was slightly improved,but its elongation was decreased because of the insertion of fiber.On the fracture surface of composite bar,the cracks originating from the Fe/Al interface and the debonding gaps between the aluminum matrix and wire rope could be observed,which could explain the reason why the tensile strength of composite bar had not been greatly improved.