Fatigue Performance Assessment Of The Fusion Welded Joints Of High Strength 2219 Aluminium Alloy

2219 aluminum alloy has unique advantages in achieving structural light weight and adapting to cryogenic environment,thus it is widely valued in the manufacturing of key components of high-speed launch equipment.Arc welding is currently the most widely applicable welding method in the forming of high-strength aluminum alloy structural parts.However,due to the particularity of the welding manufacturing process,the arc-welded joints obtained are not uneven in terms of microstructure,chemical composition and mechanical properties,and it is difficult to avoid welding defects such as porosity,which makes the strength design and analysis of welded structures more complicated.In order to effectively evaluate the in-service performance of arc-welded joints of 2219 aluminum alloy,guided by the heterogeneous characteristics of the joint,the failure behavior of arc welding joint under static loading and fatigue loading is analyzed,and the fatigue strength and residual life of arc welding joint are evaluated.The research contents and conclusions are as follows:The microstructural characteristics and nanoindentation hardness distribution of each region of the arc welded joint were obtained using electron backscatter diffraction technique and nanoindentation testing,and the evolution of grain size,grain boundary characteristics,grain orientation,local orientation difference and precipitation phase in each region were comprehensively analyzed.The results showed that the nanoindentation hardness in the fine equiaxed zone is the lowest across the weld zone with a softening degree of 36%;the softening in the fine equiaxed zone is mainly related to the dissolution ofθ（Al₂Cu）phase and the segregation of Cu elements.A series of monotonic tensile tests were conducted in the range of 293-77 K to explore the temperature dependence of the mechanical properties of the base material and the weld zone.The results showed that the base material exhibited a double-increasing effect of simultaneous increase in strength and elongation at low temperatures.The strength of the weld zone had low-temperature stability,and the elongation showed a trend of increasing and then decreasing with decreasing temperature.The geometric characteristics of internal defects was quantitatively characterized using synchrotron radiation X-ray micro computed tomography.Combined with an in-situ loading machine compatibly mounted on the advanced light source imaging platform,the damage evolution behavior of the weld zone in the thermal-force coupling loading mode was studied.It was found that the damage nucleation in the weld zone is in the form of microporosity,which grew,aggregated and connected under the applied stress,resulting in the failure of the specimen.Low cycle fatigue tests were conducted to compare the cyclic stress response characteristics and strain-life relationships between the base material and the joint.High cycle fatigue tests were conducted and two failure modes were found in arc-welded joints:cracks originated from the surface or near-surface weld porosity tended to propagate along the weld zone,and cracks originated from the weld toe notch tended to propagate along the thickness direction of the specimen.The Kitagawa-Takahashi（KT）diagram was established to determine the critical defect size and to quantitatively evaluated the effect of weld porosity size on the fatigue strength of arc welded joints.The remaining life of the crack propagating along the weld zone was evaluated using the Paris model.The extended KT diagram including safe-life zone and defect-determined finite life zone was constructed to evaluate the fatigue strength and residual life of2219 aluminum alloy arc welded joint.