PSI - Issue 75

John Hock Lye Pang et al. / Procedia Structural Integrity 75 (2025) 29–34 Author name / Structural Integrity Procedia (2025)

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and enhances fatigue resistance. However, it can also relieve beneficial residual stress, which may lower yield strength, and potentially negate some of the fatigue resistance gains [3, 4]. Surface machining is typically applied after LPBF to eliminate surface irregularities and reduce surface stress concentrations [5]. Additionally, shot peening utilizes the mechanism of surface plastic deformation to further improve fatigue performance by introducing compressive residual stress and refining surface grains [6, 7].

Nomenclature AM

additive manufacturing

HIP hot isostatic pressure LPBF laser powder bed fusion

While various post-processing techniques have been individually studied and applied in fatigue performance improvement, the effectiveness of combining these methods to enhance the fatigue resistance of metals processed by LPBF has not yet been thoroughly validated. This study assesses two sequential post-processing methods: (i) HIP followed by surface machining and (ii) HIP followed by surface machining and shot peening, in improving the fatigue performance of as-built LPBF-processed 316L stainless steel. In this study, experimental fatigue testing, fractographic analysis, and surface profile analysis are utilized to quantify fatigue life and identify crack initiation mechanisms, as well as investigate the effect of surface plastic deformation, through the two proposed post-processing methods.

2. Experimental Methods 2.1. Test Specimen Preparation

Figure 1 (a) illustrates the proposed two sequential post-processing methods for LPBF-processed 316L stainless steel: (i) HIP followed by surface machining, and (ii) HIP combined with surface machining and an additional shot peening step. The commercial LPBF system M290 from EOS GmbH was used to fabricate ASTM E466 hour-glass flat fatigue test specimens for subsequent post-treatments. The 316L stainless steel powders , sized from 15 to 65 μm, were used as raw materials; the material properties are detailed in Ref. [8]. According to the guidelines provided by EOS GmbH, the laser energy density was set to 57.72 J/mm 3 for manufacturing 316L stainless steel fatigue test specimens with less than 0.02% internal porosity. The proposed two sequential post-treatments both involve HIP and surface machining. The schematic in Figure 1 (c) depicts the HIP process, in which argon maintained an isostatic pressure of 137 MPa on as-built 316L stainless steels while keeping the internal temperature at 700 ℃ for four hours to close internal pores. After the HIP process, surface machining was employed to remove 0.2 mm of material from the four lateral surfaces, eliminating surface irregularities. Based on Sequence 1, Sequence 2 incorporates an additional shot peening step, using 0.58 mm diameter shots with a mass flow rate of 3 kg/min. Figure 2 (a) shows the dimensions of post-treated 316L fatigue test specimens. Figure 2 (b) exhibits the as-built specimens processed by LPBF, which were fabricated vertically on the baseplate, aligning the fatigue loading direction with the build direction known to have the weakest mechanical properties. Figure 2 (c) and (d) separately illustrate the specimens processed through the two proposed post-treatment sequences. The dimensional deviations were measured and found to be within 3%, ensuring that the fatigue testing results are comparable.