Research On Microstructure And Properties Of Stationary Shoulder Friction Stir Welded 2A97 Alluminium-Lithium Alloy

As the lightest aluminum alloy,aluminum-lithium alloys are widely used in aeronautical and aerospace fields owing to their substantial weight-saving and specific strength characterized by conventional aluminum alloy.Also,they are of special interest when materials with high corrosion and fatigue resistance are required.The wide demand for aluminum-lithium alloys puts forward new technical requirements for connection technology.Friction stir welding,as a solid-state process,can solve the problems of element loss,welding porosity,hot cracking,and difficulty in finding matching fluxes in brazing.However,there are still thermal gradients and thinning phenomenon in the thickness direction,resulting in the decreasing mechanical properties of the joint.The stationary shoulder friction stir welding employs a non-rotating external shoulder and the heat input is reduced while being relatively uniform in the thickness direction,thus the above problems could be effectively improved.In this paper,2A97 aluminum-lithium alloy with the thickness of 1.5 mm was welded by stationary shoulder friction stir welding.A special welding tool was designed to reveal the characteristics of the welding process parameters.Meanwhile,the influence of the process parameters on microstructure and mechanical properties was analyzed,and the relationship between microstructure evolution and mechanical properties was established aiming to provide theoretical guidance for obtaining process specifications of high-quality joints and meeting the application requirements.According to the characteristics of aluminum-lithium alloy sheets and the actual requirements of stationary shoulder friction stir welding,the structure and size of shoulder and pin are designed with reference to conventional friction stir welding,and H13 steel is used as the processing material.The results of welding process parameters showed with the increasing rotation speed of the tool or the decreasing welding speed,the heat input and the peak temperature of the characteristic point increase,leading to the fact that softening degree of plastic material increases and the spindle torque decreases.The temperature field distribution in various areas during the whole welding process are obtained based on Abaqus software.The fitting effect with the measured results is good.The range of the central high temperature area expands with the decreasing welding speed or the increasing rotation speed.And the trend of welding temperature rise gradually becomes gentler as the welding speed decreases.The stationary shoulder friction stir welding process is stable,and the weld is wellformed,with the absence of flash.The cavity defects occur inside the joint as the process parameters is unreasonable.Similar to conventional friction stir welding,the cross section of the joint could be divided into four areas: the nugget zone,the thermal mechanical affected zone,the heat affected zone,and base metal according to the microstructure.As the welding speed increases or the rotation speed decreases,the width of nugget zone gradually decreases.The difference in the grain size of the nugget zone with the increasing rotation speed of the tool is small.The grain size of the nugget zone and the thermal mechanical affected zone decreases first and then increases with the increase of the welding speed.And the grain size in the thermal mechanical affected zone gradually decreases with the increases of the rotation speed.The grain growth trend in the heat affected zone decreases with the increase of the welding speed or the decrease of the rotation speed.The average grain size transitions from the base material to the nugget zone is gradually refined.And the grain orientation tends to become random and irregular.At the same time,the transition from the low angle grain boundaries to the high angle grain boundaries occurs by continuous dynamic recrystallization and geometric dynamic recrystallization.The maximum value of the pole density gradually decreases,and the casting texture is replaced by recrystallization texture due to the complete recrystallization.Similar to the conventional friction stir welding,the hardness distribution of the stationary shoulder friction stir welded joints present a "W" shape.The hardness value of various area is mainly affected by the precipitation phase.And fine grain strengthening plays a secondary role in improving the hardness value.With the increase of the rotation speed,welding speed and shoulder pressure,the tensile strength and elongation of the joint show the increasing first and then decreasing trend.As the cavity defects existed,the cracks sprout from the cavity,showing brittle fracture mode;As the joints are lack of penetration,the fracture path is along the central nugget zone,and the fracture mechanism is ductile fracture.The defect-free joint fails at the thermal mechanical heated zone on the retreating side which are the lowest hardness value.The SEM images show a toughbrittle mixed fracture mode.