Defect-induced cracking and fine granular characteristics in very-high-cycle fatigue of laser powder bed fusion AlSi10Mg alloy

[Display omitted] •Defect-induced cracking behavior in VHCF of SLMed AlSi10Mg alloy is investigated.•The influence of MPBs on the VHCF crack propagation is studied.•Short crack propagation modes and grain refinement characteristics are analyzed.•The correlation between grain refinement and crack propagation path is revealed.•The inhomogeneous fine grain distribution mechanism is proposed. The evolution of defects during very-high-cycle fatigue (VHCF) is important for assessing the lifetime of AlSi10Mg alloy produced by laser powder bed fusion. In the study, VHCF experiments were carried out with an ultrasonic fatigue machine at a stress ratio of −1. Detailed characteristics of short cracks in VHCF were investigated by metallographic serial sections using scanning electron microscope and electron backscatter diffraction. Results show that the defects with nominal stress of 49–60 MPa have different degrees of cracking. As a result of the difference in defect shape, nominal stress and microstructure, fatigue cracks exhibit different cracking modes and propagation paths. Grain refinement behavior occurs at short cracks under VHCF, and the heterogeneous distribution of fine grains underneath the crack surfaces is strongly related to the as-printed microstructure and crack propagation path. Statistical analysis suggests that regardless of the crystal orientation, the misorientation between fine grains and matrix grains tends to be 40–50°. The average crack growth rate in VHCF is approximately in the range of 10−12 ∼ 10−10 m/Cycle.