X-ray imaging of defect population and the effect on high cycle fatigue life of laser additive manufactured Ti6Al4V alloys

Abstract

Laser powder bed fusion (L-PBF) Ti6Al4V samples were made to estimate their high cycle fatigue performance. The annealed samples were measured in the HCF regime and crack initiation was identified using scanning electron microscope. Defect population was detected using X-ray micro computed tomography. Additionally, their formation and effects on the fatigue life were characterized and discussed. The results demonstrated that optimized L-PBF process remained instability and induced metallurgical defects with random distribution, particularly in the horizontal orientation. The scatter and anisotropy of fatigue life were dependent upon the characteristics of surface keyholes. The pore defect presented a slight effect on crack propagation, while the persistent slip band at the surface can evolve into a crack initiation when the defect size was less than 40 μm. The lack of fusion defect significantly reduced the tensile properties and was sufficient to act as fatigue crack initiation when it appeared. The defect population in reconstructed three-dimensional volume was available for estimating fatigue life, and provided an attenuated exponential relationship using an X-parameter fatigue life prediction model considering the size and shape of surface keyholes. The results provide an understanding of the technological significance of process control and stability, and the effect of defects on the fatigue life.