Defect induced fatigue failure behavior and life assessment of laser powder bed fused Al-Si alloy under different building directions

The influence of the building direction (0°, 45°, and 90°) on the high cycle fatigue properties of a laser powder bed fusion (LPBF) Al-Si alloy was investigated by uniaxial loading fatigue tests under a stress ratio of 0 in combination with multi-dimensional imaging and electron backscattered diffraction (EBSD) analysis. Surface/subsurface failures were induced by single irregular lack of fusion defect that originated from unmelted powder or inhomogeneous microstructure. The fatigue properties decreased with increasing building angle, which was attributed to the size difference of the original defect dominating crack nucleation and the distribution difference of melting pool boundaries deflecting crack growth. According to the stress intensity factor evaluation, the threshold value of long crack growth and the transition size of crack growth exhibited a decreasing tendency as the building angle increased. Considering the influence of building direction on defect size distribution, a fatigue index parameter-controlled crack nucleation life prediction model was established, and the prediction result was satisfactory for safety design.