| Lightweight structure has always been the goal in the fields of aerospace,high-speed rail transportation,automobile and so on.The application of lightweight components will not only help increase the carrying capacity of vehicles but also reduce energy consumption and environmental pollution,which is one of the important ways to achieve sustainable development.The third-generation Al-Li alloys have been widely applied in the aerospace industry and military fields due to their superior performance such as low density,high elasticity modulus,high specific strength,good weldability and corrosion resistance.The hollow Al-Li alloy profiles,which are produced by the porthole die extrusion process,are typical integrating components.The integrating and lightweight Al-Li alloy components are conducive to improve the carrying capacity of aircraft and reduce the costs of production.The bad service environment for the aerospace industry makes strict demands on the performance of hollow Al-Li alloy profiles.One or more longitudinal welds are unavoidable during the porthole die extrusion,and the welding defects and heterogeneous microstructure will form along the welding interface,which will decrease the performance of profile.Therefore,it is of great significance to study the control method of longitudinal welding quality for obtaining high performance Al-Li alloy hollow profiles.In recent years,researchers have carried out some studies on the porthole die extrusion process of aluminum profiles,including the research about material flow behavior during extrusion,the effects of extrusion parameters and die structure on the microstructure and mechanical properties of extrusion welds as well as the weld quality prediction of extrusion profiles,and have made big progress.However,the research about the porthole die extrusion process of Al-Li alloy could hardly be found,and some key issues still need to be studied and solved urgently.For example,the solid-state welding mechanism of Al-Li alloy,the effects of extrusion parameters on the microstructure evolution and welding quality of the weld seam,the microstructure evolution of longitudinal weld during heat treatment and its effects on the mechanical properties,and the formation mechanism,influence factors and suppression methods for the abnormal grain growth of longitudinal weld and so on.To solve the problems aforesaid,this paper mainly studies the solid-state welding behavior of Al-Li alloy and microstructure and properties of longitudinal weld in porthole die extrusion profile,and the main research contents and major conclusions are as follows:(1)To simulate solid-state welding process,hot compression bonding tests about as-cast 2196 Al-Li alloy were carried out at different temperatures,strain rates and strains,and the hot deformation behaviors and compression bonding behaviors of alloys were studied.The effects of the above deformation parameters on the bonding quality of as-cast 2196 Al-Li alloy were analyzed,it was found the bonding quality improves with the increase of strains,and the sound bonded samples were obtained when deformed at 470?525? with strain rates of 0.6?1 s-1.It was found the interfaces will realize sound bonding by the grain boundary bulging,the discontinuous DRX induced by the interfacial defects and strain concentration as well as the grain boundary migration through the interface.To analyze and summarize the effects of deformation parameters on the bonding quality,a relational expression between the deformation parameters and strength of bonded interface was proposed,which provides a quantitative method for quality prediction of solid-state welding of Al-Li alloy.(2)A porthole extrusion die with a removable bridge was designed and manufactured,and the as-sprayed 2195 Al-Li alloy profiles with and without longitudinal weld were obtained under the almost same extrusion condition.The evolution of grain morphology,grain orientation and the microvoids that distribute along the interface during the porthole die extrusion were characterized.It was found that the metal that flow through the bridge surface were subjected to drastic friction and shear deformation,and these metal flows down into the welding chamber and formed the longitudinal weld.There were many micro-bulges and discontinuous micro-voids distributed along the welding interface,which promote the micro-plastic deformation and discontinuous DRX of the welding area.The DRX degree is higher in the welding area,which led to the formation of fine equiaxed grains and local softening area.While the matrix area was subjected to smaller deformation and obvious dynamic recovery during porthole die extrusion,thus formed extrusion fibers with many low angle grain boundaries.(3)The effects of extrusion temperatures and speeds on the evolution of micro-defects,grain structure and orientation,secondary particles and mechanical properties of the longitudinal weld area of as-sprayed 2195 Al-Li alloy profiles were studied.It was found that the high extrusion speed is in favor of the welding of the interface,and increasing the extrusion temperature helps improve the bonding quality and hardness of the profile.The grain size and DRX degree of the welding area increased with the rise of extrusion temperature,but increased firstly and then decreased with the increasing of extrusion speed.The microstructure evolution of longitudinal weld with different defects during the solution and aging treatment were analyzed.It was found that the fine equiaxed grains of the welding area transformed into coarse grains by abnormal grain growth,and the welding defects got even worse during solution treatment due to the oxidizing Al-Li alloy,which prohibited the migration of grain boundary through the welding interface,lead to the brittle fracture of the profile along the welding interface.During aging treatment,some strengthening phases T1??' and ?' were precipitated,and the softening phenomenon of welding area was disappeared gradually and the tensile strength was improved obviously.With the increase of aging time,the precipitate-free zone of grain boundary became wider,led to the profile fractured along the grain boundaries of coarse grains in the welding area and the delamination fracture of profile.(4)The abnormal grain growth(AGG)behaviors of the welding area during the extrusion and solution treatment were analyzed in detail,and the AGG mechanism was clarified.It was found that the fine equiaxed grains of the welding area formed Copper texture,and some grains transformed into Cube and Goss orientated grains due to the DRX with the increase of extrusion temperatures and speeds.Some strain-free Cube grains that located at the different textures of welding area have high grain boundary energy,driving force and priority for grain growth,thereby these grains transformed into Cube coarse grain by "orientated growth".During the solution treatment,the dislocation annihilation,subgrain coalescence and dislocation migration were observed in the fine grains of the welding area by the in-situ heating TEM,which led to the formation of AGG and millimeter-sized grains with random orientation.While the matrix area has flat grain boundaries with a low driving force for grain growth,the dislocations within the matrix area tend to migrate along the grain boundaries directions,which are hard to meet and annihilate.Therefore,the matrix area kept Brass texture with fine grain during extrusion and heat treatment.(5)The strengthened phase precipitation behaviors of the AGG area around the longitudinal weld and the fine grain area in the matrix were observed by HRTEM,the effects of AGG on the precipitation behavior of profile were studied.It was found that the coarse AGG area without substructures had a faster diffusion rate of the solute atom,and it reached the peak aging state in a short time,and coarsening and low-density precipitated phases were easy to form during this process,which led to the poor mechanical properties of the welding area.While the abundant substructures in the matrix area promoted the precipitation of the fine and uniform T1 phases.Besides,the aging hardening process of AGG area and fine grain area during the pre-stretching(with a stretching quantity of 1%?11%)and aging process were researched,it was found that the dislocations formed in the AGG area during pre-stretching were hard to be reserved,which led to the rapid coarsening of precipitated phases during aging treatment.Some dislocations were introduced and stored in the matrix area,which promoted the uniform precipitation of T1 phases.The precipitation sequence of phases in the welding area was identified as SSS??'/?'+GP zone+T1??+?'+T1+?'/?'-T2+?+S'/S,and that in the matria area was SSS??'/?'+GP zone+T1??+?'+T1+?'/?'?T1+T2+?+S'/S+?'/?'.(6)An innovative heat treatment method,the intermediate annealing treatment?T6 treatment was proposed to suppress the AGG behavior of longitudinal weld during solution treatment.It was found that inserting an intermediate annealing treatment before solution treatment will promote the coalescence and homogeneous growth of the subgrains and decrease the driving force for grain growth,which led to the reduction of the AGG area width during the subsequent solution treatment.Compared with traditional T6 profiles,the intermediate annealing treatment?T6 treatment improved the elongation and tensile strength of profile,and decreased the area of intergranular fracture surface.This method alleviated the AGG phenomenon and the heterogeneous microstructure of profiles with longitudinal weld. |
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