Imaging And Characterization Of Fatigue Cracking In Fusion Welded Al Alloys Used In High-Speed Train By Synchrotron Radiation X-Ray Microtomography

To investigate the initiation and propagation of fatigue cracking related with micro-pores inside hybrid laser arc welded 7020-T651 alloys, both high resolution in situ synchrotron X-ray computed microtomography and traditional finite element based numerical modeling have been undertaken.The effective diameter, sphericity and distance from barycentre of pores to free surface of samples were measured, statistically summarized and fitted. According to the 3D characterization, micropores inside hybrid welded Al alloys are mainly metallurgical pores, which are symmetrically distributed about the seam centerline, having a sphericity larger than 0.65. Moreover, pores inside upper welds appear to be larger in effective diameter, but denser in heat affected zone and lower welds. Besides, pores with diameter less than 20μm are existed extensively, with a frequency of 65% and 85% in the upper and lower weld, respectively. Furthermore, it is found that the connections of pore-pore and pore-hot-crack together with the hot cracks result in the smaller sphericity of gas pores in the lower welds. Finally it can be confirmed that the higher welding speed gives rise to the smaller pore volume fraction, but has little influence on the distribution of pore position and sphericity.Multi cracks are initiated from pores at the surface or subsurface but only very few of them can propagate into a regular shape, with typical low cycle fatigue characteristics. Crack fronts are locally complex and nonlinear due to the influence of microstructures. However they keep regular on the whole during the stable growth stage, which accounts at least 30% of the total fatigue lifetime of each sample. For the fusion weld, the studied corner fatigue crack can be described, despite its small dimension, in the domain of linear elastic fracture mechanics. Based on the fatigue crack geometry and morphology, a finite element model has been established to evaluate the crack growth rate parameters, the closure effect and fracture toughness via determination of the crack front every 500 loading cycles. A quantitative agreement between experiment and prediction of crack shape under executed loading cycles is observed when a cubic spline is adopted to simulate the spatial evolution of the crack front. By using the critical stress intensity factor along the crack line, the 3D fracture toughness of a given hybrid laser weld can also be determined numerically for three-dimensional part-through curved fatigue cracks.The above investigation about 3D morphology of pores and pore-induced crack and prediction of related material constants can provide theory instruction and critical data for improved manufacturing and fatigue evaluation of welded structures.