PSI - Issue 53

S. Senol et al. / Procedia Structural Integrity 53 (2024) 12–28

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Author name / Structural Integrity Procedia 00 (2019) 000–000

bi-directional scan strategy with 45° tilted scan vectors and 90° rotation between each layer. The Archimedes’ method was used to measure relative part densities and 2.82 g/cm 3 was used as the theoretical density, based on the powder density measured by gas pycnometry. The average part density was noted as 99.3 ± 0.1 %. In Appendix A, the OM and SEM images displaying the as-built microstructure of crack-free, dense (Ti+B 4 C)/Al-Cu-Mg are shared. The 3PBF sample dimensions (Beretta et al., 2020) utilized for this study are shown in Fig. 1(a).

Fig. 1. (a) 3PBF sample dimensions (in mm) (Beretta et al., 2020) with two representative supporting pins for fatigue testing. h s , h b , w , L , L t and R refer to sample height at the minimum cross section, maximum height, sample width, distance between the supporting pins, total sample length, and radius of curvature, respectively; (b) the 3PBF sample top surface to be modified (white arrow), XZ, and YZ cross sections are indicated; (c) 3PBF test set-up, red arrow displaying the applied force and white arrow is indicating the minimum cross sectional area with AB or modified surface conditions. 2.3. Surface treatment In order to reveal the effect of the dL-PBF treatment on the bending fatigue behaviour of (Ti+B 4 C)/Al-Cu-Mg and effectiveness of dL-PBF as surface treatment technique, four surface conditions were compared in this work, namely; (a) as-built (AB), (b) dual-laser powder bed fusion (dL-PBF) processed (R), (c) wire electric-discharge machined (EDM) as a non-conventionally machined surface reference, (d) milled (M) as conventionally machined surface reference. These surface treatments were applied to the top surface (Fig. 1(b)) of 3PBF samples. In the case of dL-PBF processed samples (R), following the completion of the 3PBF geometry using L-PBF (CW laser), the inert atmosphere in the build chamber was preserved, while the build job was allowed to cool down to room temperature, and the in-process dL-PBF was applied. A schematic describing the details of the in-process dL-PBF treatment applied in this study is displayed in Fig. 2. The dL-PBF process consists of two steps: 1) powder removal via LISW produced by pulsed wave (PW) laser ablation, 2) re-melting via continuous wave (CW) laser (Metelkova, Ordnung, et al., 2021). Evidently, as the whole part is completed at the end of the L-PBF processing, the region of interest (RoI) (curved region of 3PBF samples shown in Fig. 1(b), indicated with white arrow) was covered with powder (Fig 2(a)). Firstly, the powder covering the RoI, was removed by applying a selective powder removal step employing the PW laser (Fig. 2(b-d)), making the region accessible, and secondly, a re-melting step was applied using the CW laser (Fig. 2(e)). Both the first and the second step involve defocusing (DF), which refers to changing the vertical distance between the focal plane of the laser and the laser-matter interaction zone as defined in (Metelkova, Vanmunster, et al., 2021). Defocusing aims at efficiently removing the powder covering the RoI (step 1) and subsequent re-melting at the RoI (step 2), which was the minimum cross sectional area of 3PBF samples as mentioned above. Fig. 2(a) displays the positions of focal plane (orange), the AM part (blue), the powder bed (light grey), and the build platform (dark grey) at the end of L-PBF part production. For the first step of dL-PBF, the build platform was moved 3 mm upwards from its position at the end of the job (DF3) (Fig. 2(b)), while the PW laser was used to remove powder selectively, starting to scan region 1, followed by region 2 (Fig. 2(c)). Alternating scan strategy is utilized for PW laser scans; scanning the defined area bi-directionally 2 times, with a 180° rotation between scans (first from A to B, then B to A in Fig 2(c)). Both scan region 1 and 2 are scanned twice. Then, the build platform was raised 1 mm more (DF4) for the PW laser to scan once the full length of the 3PBF sample (Fig. 2(d)), aiming at the maximum powder removal efficiency at the center of the curved region. Finally, the build platform is lowered (DF3) and the CW laser is used to re-melt the full length of the sample’s top surface (Fig. 2(e)). The abovementioned dL PBF strategy is optimized for this geometry and material, combined with PW and CW laser processing parameters