Process variation effects linked to feature size on fatigue performance in additively manufactured 316L stainless steel

Abstract

Metal-based additive manufacturing provides a substantial degree of design flexibility for metal parts, garnering interest from various industries, including Vestas Aircoil, a project partner. The focus is on utilising this technology to design innovative heat exchangers aimed at reducing vibration-induced fatigue and enhancing heat transfer. Laser-based powder bed fusion (LPBF) technology, in particular, has gained prominence for manufacturing high-strength, intricate parts. However, inherent challenges such as poor surface quality, defects, microstructure inhomogeneity, and material anisotropy limit its application. The influence of part geometry size and scale on crystallographic and porosity microstructural features, crucial in early-stage fatigue crack initiation, necessitates a comprehensive investigation. This study aims to examine the size scale effect on microstructure and fatigue behaviour, contributing to the development of a lifing technique applicable to generic heat exchanger structures.
Despite the design freedom offered by LPBF, it presents unique challenges. A thorough analysis was conducted to explore the impact of process parameters, including build direction, energy density, build location, and geometry size scale, on average grain size, boundary distribution, preferred crystallographic orientation, and recrystallization within grains. The study revealed a robust crystallographic orientation in rectangular bend bar specimens with the longest dimension oriented in the LPBF build direction (Z builds), accompanied by larger grains and lower recrystallization compared to their counterparts, indicating high material anisotropy across all batches of specimens. Subsequent analysis of area size scale effects on microstructure using two batches of micro tensile specimens consistently showed a large grain size in heat-restricted zones, along with a high percentage of recrystallized grains in critical gauge areas. These findings underscore the interplay of LPBF laser parameters, orientation, and specimen geometry in shaping microstructures and properties. Generally, higher strength and ductility result from orientations with larger layer areas and lower energy density, while restricted heat flow can impact the recrystallization process, potentially weakening material strength.
In the context of high cycle fatigue (HCF) behaviour, this study focused on LPBF-processed 316L stainless steel (SS) rectangular bend bars. Employing a three-point bending setup, specimens underwent cyclic loading at two stress levels with a frequency of 10 Hz and a stress ratio (R) of 0.1. The study particularly emphasized early-stage crack initiation and coalescence mechanisms, monitored through a silicone replication technique. Scanning electron microscopy (SEM) and surface roughness profilometry were employed to examine fracture surfaces, sub-surface defects, porosities, and material surfaces after etching.
Specimens built in horizontal (X and Y), and vertical (Z) directions were investigated to assess the influence of material defects on crack initiation and subsequent effects on fatigue lifetime. Additionally, the study explored the impact of various stress states and the influence of yield strength on fatigue behaviour. Generally, Z build specimens exhibited the highest number and percentage of porosity compared to X and Y build specimens. Lower yield strength was associated with smaller fatigue life due to persistent slip band-induced early-stage crack initiation.
Fatigue behaviour evaluation for LPBF-processed material proved to be complicated. Porosity and defects played a pivotal role in initiation, with crack coalescence and initiation events dominating overall fatigue lifetime, leading to multiple coalescences. The study also observed stress shielding and anti-shielding behaviours of cracks across all specimens. Coalescence between two cracks significantly influenced the stress state of other surface cracks, generally shielding parallel cracks and anti-shielding aligned cracks. These results underscore the significance of investigating surface defect distribution and coalescence mechanisms in controlling fatigue behaviour.
A finite element (FE) model was developed using a constitutive model of 316L SS to simulate tension-tension fatigue tests of micro tensile specimens. The model, validated against elastic-plastic responses obtained from micro tensile fatigue tests, revealed an overestimation of fatigue lifetime. Fractography of micro tensile specimens identified surface roughness as the predominant factor controlling fatigue, with process-induced defects and porosity often initiating or interacting with cracks, accelerating propagation behaviour. The final failure stress calculated from the fracture surface was nearly twice the ultimate tensile strength (UTS) for all the specimens.
In conclusion, this study provides valuable insights into the intricate relationship between LPBF process parameters, specimen geometry, and resulting microstructures and properties. The findings contribute to the development of lifing techniques for generic heat exchanger structures and enhance the understanding of the high cycle fatigue behaviour of LPBF-processed 316L stainless steel.

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Identifiers

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

ORCID for Andrew Hamilton: orcid.org/0000-0003-4627-849X

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