Fatigue fracture of 316L steel manufactured by selective laser melting method

The kinetics of short cracks in 316L steel samples made by selective laser melting was studied. The structural sensitivity of such cracks, which form on technological defects at the early stage of fatigue is noted. The cracks develop predominantly along the fusion boundaries and slow down their growth at the boundaries of the melt pools. An increase in the crack opening of arrested cracks lead to the formation of a plastic zone at their tips, localization of deformation, a decrease in opening and continued growth with an increase in the number of cycles. The alternation of the processes of propagation and deceleration of short cracks during loading is reflected in the kinetic diagram of fatigue failure which has rate growth thresholds with spacing between them being close to the scanning step during steel manufacturing. The diagram is described by the Paris equation with the same exponent at the growth stage of both short and long cracks. The plotted fatigue curve was compared with fatigue curves for the same material produced by traditional and additive methods. It was shown that the plotted fatigue curve, as well as the fatigue curves for similar materials taken from literary sources, lie much lower than the fatigue curve for the same steel obtained by the traditional method. However, the use of optimal manufacturing modes, as well as subsequent heat treatment, leads to the fatigue characteristics of «additive» steel being similar to those of steels produced by the traditional method. The macro- and microrelief of the fracture surfaces of the samples was studied, the stages of stable and accelerated crack growth were identified, the crack lengths at fracture surfaces corresponding to these stages were evaluated and the predominate fracture mechanisms at each stage were described. It is shown that the observed knee point in the fatigue curve is accompanied by an increase in the damage of the lateral surface of the samples with an increase in the stress amplitude and transition to a more ductile fracture relief, which is explained by the switch from the plane-strain state to a plane-stressed state of the sample material realizing at the tip of the macrocrack.