Strain-Control Fatigue Anisotropy of Laser-Powder Bed Fusion 316L Following Hot-Isostatic Pressing
Open Access
- Author:
- Wietecha-Reiman, Ian
- Area of Honors:
- Materials Science and Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Todd Palmer, Thesis Supervisor
Robert Allen Kimel, Thesis Honors Advisor - Keywords:
- fatigue
316L stainless steel
hot-isostatic pressing
strain-control
lack of fusion
gas porosity
surface roughness
cyclic stress-strain hysteresis - Abstract:
- Additively manufactured components have increasing use in limited batches and legacy components, necessitating characterization. Strain-control fatigue behavior of hot-isostatically pressed (HIP) laser-powder bed fusion (L-PBF) 316L steel is studied, and a literature review is performed to compare L-PBF 316L with wrought 316L. The impact of build angle and surface roughness is characterized with respect to process induced defects. In literature, several behaviors may be distinguished by their fatigue strength exponents (b), fatigue strength coefficients (σf), and fractography. Most specimens deviate from wrought material by having higher b and σf, which correlates with intergranular crack propagation and secondary cracking, indicative of an embrittlement mechanism. Cracks initiate at sub-surface defects. Several subsets of specimens behave similarly to wrought material, failing predominantly transgranularly which is typical of 316L fatigue. Specimens produced in this study show increased cycles to failure (Nf) compared to strain-controlled specimens in literature, though Nf are still lower than strain-controlled wrought comparisons. Contour porosity initiates cracks, but the circular cross-section does not explain fatigue anisotropy. Anisotropy is correlated to the maximum pit depth (Sp) and the contour microstructure. As the build angle decreases from 90° to 45°, Sp increases, decreasing Nf. Metallography has shown that the average grain size increases and the contour microstructure becomes more diffuse for 45° build specimens. When lack of fusion porosity is mitigated through HIP processing, it is suspected that the surface topography and contour microstructure contribute to the stress state produced by contour porosity. These findings precede a completed microstructural evaluation.