Fatigue strength of PBF‐LB/M and wrought 316L stainless steel: effect of post‐treatment and cyclic mean stress

Moritz Braun,Sören Ehlers,Igor Kryukov,Eduard Mayer,Aliakbar Taghipour,Rachael Elizabeth Wu,Christian Wolf,Shahram Sheikhi,Stefan Böhm

doi:10.1111/ffe.13552

Abstract

AbstractAdditive manufacturing (AM) enables the cost‐effective production of complex components, many of which are traditionally manufactured using costly subtractive processes. During laser‐based powder bed fusion of metals (PBF‐LB/M), internal pores and rough surfaces are typically inevitable, reducing fatigue and corrosion resistance compared to traditional processes. Additionally, large defects often occur near to or at surfaces. Thus, this study investigates the effect of hybrid additive and subtractive manufacturing on the fatigue strength of AISI 316L. To this goal, different post‐treatment routes are compared with wrought material. Additionally, computed tomography is used to determine the necessary machining depth of the surface layer. In this study, heat treatment and machining are both found to significantly increase fatigue strength (17% and 87%). Finally, the mean stress sensitivity M of as‐built PBF‐LB/M and wrought material is found to be highly affected by the assessed number of cycles to failure and residual stresses in PBF‐LB/M material.

Highlights

Additive manufacturing (AM) is a rapidly developing technology that combines computer-aided design with material processing and shaping to produce structures with complex geometry
A different mean stress sensitivity was determined for N = and N = 2 Â cycles for both materials. While this effect is related to the change in slope exponent of the wrought material, it is probably related to relaxation of residual stresses for the PBF-LB/M material
The fatigue strength of additively manufactured components produced by PBF-LB/M depends on factors such as the microstructure, surface roughness, residual stresses, and above all, porosity and other defects

Summary

Introduction

Additive manufacturing (AM) is a rapidly developing technology that combines computer-aided design with material processing and shaping to produce structures with complex geometry. AM is suitable for low-volume components, tailor-made, and valuable products.[1] Laser-based powder bed fusion of metals (PBF-LB/M) known as direct metal laser melting (DMLM) or selective laser melting (SLM), a form of AM, has recently experienced rapid industrialization and popularity and is the most widely used process for manufacturing using metal powders.[2] This process employs a building platform, onto which thin layers of powder are applied. A laser beam selectively fuses the particles (“scanning”) before the platform is lowered to add another thin layer of powder; this process is repeated until the whole model is complete.[3] This form of manufacturing is “additive,” as each layer builds on the last, enabling lightweight structures with complex geometry—which would require costly methods such as casting, welding, and machining in Fatigue Fract Eng Mater Struct. This form of manufacturing is “additive,” as each layer builds on the last, enabling lightweight structures with complex geometry—which would require costly methods such as casting, welding, and machining in Fatigue Fract Eng Mater Struct. 2021;44:3077–3093

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