Effect of heat treatment on fatigue crack initiation of laser powder bed fusion stainless steel 316L

Abstract

Abstract. Laser powder bed fusion (L-PBF) is an emerging additive manufacturing technique for building structural components. L-PBF processing defects, such as lack of fusion pores, promote fatigue crack initiation and shorten the fatigue life. With well-controlled processing, critical pores can be avoided such that the microstructure-driven intergranular crack initiation mode becomes operative. In this work, the fatigue crack initiation behaviours of as-built and solution annealed L-PBF stainless steel 316L were studied. Crack initiation of the as-built samples is driven by de-bonding of the dendritic grain boundaries. High temperature annealing results in the formation of thermally-induced defects, possibly via the reheat cracking mechanism and the nucleation of pre-existing gas pores. As heat treating could have led to recrystallization and annihilation of the original grain boundary defects, the thermally-induced defects became the new sites for crack initiation. In addition, heat treatment incurred significant reduction in yield strength, such that the interaction of fatigue and ratcheting strain accumulation dominated the deformation behaviour of the material. The resulting fatigue strength in the finite life regime was reduced by about 13% but the fatigue endurance limit was not affected.

High cycle fatigue and ratcheting interaction of laser powder bed fusion stainless steel 316L: Fracture behaviour and stress-based modelling

“…Studies by Zhang et al [15] had shown that the lower tensile strength after solution annealing subjects L-PBF stainless steel 316L to ratcheting, where the activation and accumulation of plastic strain takes place under stress-controlled cyclic loading with superimposed tensile mean stress. Cyclic hardening or softening affects the generation of plastic strain, which exhausts the ductility of the material with the progression of cycles till failure.…”

Fatigue behaviour of laser powder bed fusion (L-PBF) Ti–6Al–4V, Al–Si–Mg and stainless steels: a brief overview

“…Fatigue crack initiation is mostly from grain boundary defects. Annealing changes grain size and removes significant defects on boundaries, thus shifts the fracture tendency from grain defects to pore defects remaining after annealing (Zhang et al 2018b). This procedure provides a better fatigue life.…”

“…Heat treatment has been reported to improve the fatigue life of L-PBF 316L steel in the HCF regime and lower the fatigue life for LCF (Leuders et al 2014;Zhang et al 2018b). The reduction in LCF is coarsening grain size through the Hall Petch (Callister and Rethwisch 2012) effect as fatigue life of this alloy mainly depends on monotonic strength than pores (Leuders et al 2014).…”

In-situ monitoring of selective laser melting process

“…Although cylindrical specimens were printed for correlation studies, it must be independent of the printed parts' geometry. This is similar to the fact that defects are inevitably generated in printed parts during the SLM process, regardless of their geometries [84].…”