Research On The Composite Process Of Superplastic Forming/Friction Stir Welding Of 2195 Al-Li Alloy Hollow Structure

Due to the urgent requirements for lightweight and high-strength materials in the aerospace industry,Aluminum-lithium alloy has attracted wide attention for its unique advantages such as low density,high elastic modulus,and high specific stiffness.Among them,the 2195 aluminum-lithium alloy in the 2xxx aluminum alloy is particularly prominent.The use of this alloy in aerospace vehicles can reduce the weight of parts by14%-30%and increase the rigidity by 15%-20%.Superplastic forming is a recognized advanced forming technology for manufacturing thin-walled complex overall structures.It has the advantages of no hardening springback after forming and high forming accuracy.Combining superplastic forming with various connection technologies,such as SPF/DB,SPF/LBW,etc.which can produce overall lightweight structures.However,due to the poor diffusion bonding performance of aluminum-lithium alloys,the application of traditional SPF/DB technology has natural limitations.To this end,this article proposes a superplastic forming/friction stir welding（SPF/FSW）composite forming process,using friction stir welding instead of diffusion bonding,which can apply the SPF/FSW multilayer structure integrated forming concept to aluminum-lithium alloy and also control the shape of the support unit through the distribution design of the local friction stir welding zone of the core plate and the panel,and can realize the integrated forming of thin-walled components of various forms of support structures.Compared with diffusion bonding,friction stir welding has stronger quality controllability and monitorability.Applying it to the local connection of aluminum-lithium alloy thin-walled structures and controlling the welding form according to needs is to promote aluminum-lithium alloy thin-walled hollow structures is the key to precise forming.Based on the superplastic forming/friction stir welding combined forming process,this article has carried out the forming research of the large-scale annular hollow structure of 2195 aluminum-lithium alloy.The high temperature tensile test of 2195 aluminum-lithium alloy under different temperature and different strain rate conditions was carried out,in order to find its best deformation parameters.The optimal deformation parameters are determined to be temperature T=475℃and strain rateε&=0.005s^-1.Under these conditions,the maximum elongation of 2195 aluminum-lithium alloy can reach 203.8%,which has good superplastic deformation properties.The friction stir welding of 2195 aluminum-lithium alloy under different parameters has been studied.The results show that the best parameters are the forward speed of the stirring head v=100 mm/min,and the rotation speed of the stirring headω=300 r/min.After welding,the tensile strength of the plate at low temperature reaches 461MPa,which is 77.0%of the base metal.The elongation is slightly higher than that of the base metal.When the temperature T=475℃and the strain rateε&=0.005s^-1,the highest elongation can be reached.72%,calculated to meet the forming requirements.Finite element numerical analysis was carried out on the load-bearing capacity,superplastic deformation of the unit and the overall forming process of the 2195aluminum-lithium alloy unit.According to the simulation results,it was determined that the unit aspect ratio was 3:1 and the overall arrangement was a 15-divided ring.With the hollow structure,the pressure-time change curve during the forming process is obtained,which provides a theoretical basis for the forming test.The forming test of the 2195 aluminum-lithium alloy annular hollow structure was carried out,and finally a structure with good surface quality,high welding rate and accurate size was obtained,which met the expected requirements.Set different process parameters,and conduct heat treatment research on 2195aluminum-lithium alloy according to process parameters,and the precipitation behavior of the precipitates was analyzed by XRD,SEM,EBSD,TEM and other means,and the cause of the sawtooth rheological phenomenon after solid solution was explained.The changes of the precipitated phase and the second phase during the aging process were analyzed.