Fatigue Fracture of 316L Steel Manufactured by Selective Laser Melting

We have studied the kinetics of small cracks in 316L steel specimens produced by selective laser melting and demonstrated structural sensitivity of such cracks forming at process-induced defects in the early stage of fatigue. They propagate predominantly along fusion boundaries, and their growth rate is lower at melt pool boundaries. An increase in the opening of arrested cracks leads to the formation of a plastic zone at their tips and deformation localization, followed by a decrease in crack opening and further growth as the number of cycles increases. Alternation of small crack arrest and growth reflects in the kinetic diagram of fatigue fracture, which has growth rate thresholds with the spacing between them approaching the scan step in the steel preparation process. The diagram can be described by Paris’s equation with identical exponents for the growth of short and long cracks. The fatigue curve we constructed was compared to fatigue curves of 316L steel produced by a conventional and an additive method. We demonstrate that, like fatigue curves reported for 316L steel in the literature, the curve we plotted lies far below the fatigue curve of the steel produced by a conventional process. At the same time, optimization of specimen preparation conditions and subsequent heat treatment make fatigue characteristics of the “additive” steel more similar to those of steel produced by a conventional process. We have studied macro- and microrelief of fracture surfaces of specimens, identified stable and accelerated crack propagation stages, evaluated crack lengths corresponding to these stages on fracture surfaces, and described predominant fracture mechanisms in each stage. The observed knee point in the fatigue curve of the steel was shown to be accompanied by an increase in damage to the lateral surface of the specimens with increasing stress amplitude and a transition to a more ductile fracture surface microrelief, which can be accounted for by a change from a plane strain state to a plane stress state of the material of the specimen at the macrocrack tip.