Study On Fatigue Fracture Mechanism Of Additive Manufactured 5087 Aluminum Alloys

With the rapid development of rail transit industry,the operation speed level of high-speed EMU has been continuously improved.The vehicle body and related components have been in a complex and changeable load environment for a long time,which may cause local fatigue or fracture due to cumulative damage and seriously threaten safety.However,the strong passenger demand for high-speed EMUs requires that the vehicles maintain a certain passenger capacity,which also puts forward higher requirements for the lightweight and durability of the car body structure.Aluminum alloy has become an important way to achieve structural lightweight because of its advantages,such as high specific strength,good sealing,and corrosion resistance.At the same time,aluminum alloys processed by additive manufacturing(AM)are widely used in the integrated molding and lightweight design of high-end equipment by the material’s lightweight advantages,as well as the characteristics of related manufacturing processes in material utilization and complex structure manufacturing.AM technology also shows excellent application potential in the production of complex structures of high-speed EMUs.Based on this,5087 aluminum alloys processed by standard wire + arc additive manufacturing(WAAM)and hybrid in situ rolled wire + arc additive manufacturing(HRAM)were selected as the research object in this thesis,and the integrated research on the process,microstructure,mechanical response,and fracture performance of AM-processed aluminum alloys.The main research contents and conclusions are as follows:The microstructure characteristics of WAAM-and HRAM-processed 5087 aluminum alloys were obtained using microscopic testing techniques.The results show that the HRAM sample has fewer internal defects,finer grains,and uniform phase precipitation compared to the WAAM sample.According to the tensile stress-strain curves,the strength of the HRAM sample is slightly increased,but the elongation was significantly increased,which is mainly attributed to the microstructural changes of the materials caused by the in situ rolling process.Further,the in-situ tensile test was carried out using in situ electron backscattering diffraction(EBSD)technology to analyze the microstructure and dislocation density evolution of the sample surface during the tensile process.In situ fatigue tests were carried out by means of scanning electron microscopy(SEM)to study the fatigue short crack initiation and propagation behavior of WAAM and HRAM samples.The results show that in the short crack stage,the crack growth rate decreases several times due to the influence of the microstructure,and the crack growth rate of the HRAM sample decreases more times.At the same time,the short crack growth path of the HRAM sample is relatively more tortuous,which is more likely to lead to crack tip closure and will further reduce the crack growth rate.Therefore,the in situ rolling process can improve the short fatigue crack propagation resistance of HRAM specimens by enhancing the material microstructure.The fatigue crack growth(FCG)rate of WAAM-and HRAM-processed alloys was studied,and the FCG behavior of HRAM samples under two notch orientations and three stress ratios was emphatically analyzed.The results show that the notch orientation has little influence on the crack growth rate when the stress ratio is small,while the effect of the notch on the crack growth rate shows a weakening and convergence trend as the stress ratio increases.Then,the microstructure on both sides of the crack propafation path was characterized using the EBSD technique,and it was found that the crack tip of the CT-X1 sample propagated through or along the equiaxed crystal,while the crack tip of the CT-Y1 sample propagated vertically through the columnar crystal.Combining the fatigue crack growth curves and the grain morphology through the crack tip,it can be seen that the CT-Y1 specimen has better resistance to crack expansion,which indicates that the microstructure difference is the main reason affecting the fatigue crack expansion.The evolution characteristics of the crack tip plastic zone of WAAM-and HRAM-processed 5087 aluminum alloys were compared by FCG test based on digital image correlation(DIC)technology.DIC is used to track the change of displacement field and strain field on the specimen surface under different load levels and crack lengths.The results show that the grain refinement and defect closure caused by in situ rolling can effectively improve the mechanical properties and resistance to long crack growth of HRAM specimens.In the processing of crack propagation,the plastic zone size at the crack tip of HRAM sample with same crack length is relatively small.In addition,the traditional single-parameter or two-parameter fracture mechanics theory can be used to determine the area of the plastic zone.However,when studying the stress field far from the crack tip,it has significant advantages to use the high-order terms of the Williams expansion series to approximate the stress field.