Small crack initiation and propagation behavior of selective laser melting TC4 alloy under cyclic loading

Based on the in-situ fatigue test technique of scanning electron microscope （SEM），the small crack behavior of selective laser melting （SLM） TC4 alloy is studied. The focused ion beam （FIB） technique is used to manufacture the simulative initial defects in the process of additive manufacturing，and the initiation and propagation behavior of small cracks induced by defects are observed under cyclic loading. The results show that under cyclic loading，small cracks are easy to initiate at the surface defect，and the crack fatigue initiation life is short. The fatigue life of SLM TC4 alloys is mainly consumed in the small crack propagation stage. Affected by the microstructure，the small cracks are more likely to propagate along the α/β interface. Due to the random orientation of α laths，the path of small cracks is deflected several times in the initial propagation stage，exhibiting a mixed mode of propagation along α lamellae and through α lamellae within grains， resulting in significant scatter in crack growth rates. At the stable crack propagation stage，the plastic slips of the crack tip are obvious，the influence of microstructure is reduced，and the propagation path tends to straighten. Comparative analysis with long crack growth data indicates that SLM TC4 alloy exhibits a significant “small crack effect”.