Defect And Fatigue Resistance Of Hybrid In Situ Rolled Wire+Arc Additively Manufactured Al-Mg4.5Mn Alloy

Metal additive manufacturing(AM),as an advanced machining method with low energy consumption,high material utilization rate and large design freedom,has been tried to be applied in the field of integrated molding and lightweight design of high-end equipment such as aviation and aerospace.It also shows great application potential in the production of complex structure of high-speed train,such as gearbox,gear parts,etc.Hybrid in situ rolled wire and arc additive manufacturing(HRAM)is a potential3 D printing technique that combines standard wire and arc additive manufacturing(WAAM)and in situ micro-rolling.However,compared with the traditional process,there are metallurgical defects in the AM alloy,whose size,morphology and distribution are complex,inevitable and difficult to eliminate,which is an important factor controlling the mechanical properties and service reliability of components.In order to effectively evaluate the service performance of HRAM processed materials and realize the application of additive manufacturing aluminum alloy components in the field of high-speed transportation,it is necessary to establish an accurate and effective fatigue performance evaluation system for additive parts with defects.The geometrical characteristics(density,morphology,distribution,size,etc.),tensile mechanical properties and microstructure characteristics of the defects in AlMg4.5Mn alloys prepared by WAAM and HRAM were compared by high-resolution synchrotron radiation X-ray micro-computed tomography(SR-μCT),Electron backscatter diffraction(EBSD)and uniaxial tensile tests.An extreme statistical method was used to predict the maximum characteristic size of defects in samples under the two processes.Compared with WAAM,HRAM can reduce the number and size of defects in the prepared aluminum alloy samples,and the morphology of defects does not change significantly.Also,HRAM improved the grain refinement degree and material ductility.The fatigue properties and related failure modes of HRAM aluminum alloy were studied by low cycle fatigue test,high cycle fatigue test and fracture analysis.The experiments indicated that the fatigue failure sources of the materials are all located at the surface or near surface defects,and the dispersion of high cycle fatigue life appears.The defects are the main reason for this result.The probabilistic fatigue life(P-S-N)curves of materials were drawn.Combined with microstructure characterization test and fatigue crack growth rate test,the high cycle fatigue life of materials with defects was predicted by Multistage Fatigue(MSF)model and modified NASGRO model.The predicted results were all within the range of P-S-N curves and were conservative.The defect evolution of Al-Mg4.5Mn alloy formed by HRAM during fatigue test was observed by SR-μCT.The imaging results show that the fatigue crack initiation occurs at the surface defect,and the phenomenon of local acceleration and stagnation occurs in the process of crack propagation.Based on extreme statistics,the fatigue limit of materials is predicted by modified Murakami model,and the predicted results are reliable.The Kitagawa-Takahashi diagram of HRAM Al-Mg4.5Mn alloy was drawn to determine the maximum critical defect size,and the fatigue strength of the material was evaluated based on the high cycle fatigue test results.