PSI - Issue 13

Catrin M. Davies et al. / Procedia Structural Integrity 13 (2018) 1384–1389 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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fracture resistance of SLM components. In this work the influence of sample orientation and heat treatment are experimentally examined on Charpy impact and standard fracture toughness samples.

2. Sample Geometries and Build Details

All samples were manufactured from 316L stainless steel powder using a Renishaw AM250 SLM machine equipped with a 200W pulsing laser. The process parameter values used were recommended by the equipment manufacturer (see Davies et. al. 2018). The samples were built in an inert argon environment to prevent sample oxidation. The chessboard scanning strategy was employed where each layer was rotated by 67°, thus repeating the same pattern every 180 layers, which helps reduce the anisotropy level of the SLM parts. Both Charpy impact and fracture mechanics samples have been manufactured, as detailed below. 2.1. Charpy Samples Charpy samples were manufactured and tested according to the ASTM testing standard: E23 − 16b (ASTM, 2016). As shown in Fig. 1, samples were manufactured such that the fracture plane would be (a) parallel (horizontal sample) and (b) orthogonal (vertical sample) to the build direction. Charpy impact tests have been performed on the vertical (V) and horizontal (H) samples in their as-built (AB) condition and after heat treatment at either 700 °C or 900 °C for a duration of 60 minutes followed by air cooling. Preliminary Charpy results in the as-built and 700 °C heat treated condition have been presented by Davies et. al. (2018) however additional test results are presented here. Hardness tests were performed on a selection of the Charpy samples prior to testing, to determine the influence of the heat treatment on hardness.

(b) Vertical

Build direction

(a) Horizontal

Fig. 1: Schematic illustration of the Charpy samples indicating the sample orientation relative to the build direction.

2.2. Fracture Samples

Standard single edge notched bend, SEN(B), samples were manufactured according to ASTM E1820-18 (ASTM, 2018) and tested under three point bend (3PB) loading conditions. The samples were of overall dimensions as shown in Fig. 2(a). The samples were of width, W = 18 mm and thickness, B = 9 mm. The loading pins were configured such that the sample’s span , S , (distance between supporting pins) to width ratio was S/W = 4. The samples were manufactured in two orientations denoted horizontal and vertical, such that their crack planes were parallel and perpendicular to the build direction, respectively. Samples were tested in both orientations in the as built condition and after heat treatment to 700 °C for 60 minutes followed by air cooling. The samples were manufactured with a chevron notch of depth 8.1 mm and were subsequently pre-cracked by high cycle fatigue loading at a frequency of 10 Hz to a crack length to specimen width ratio a/W ≈ 0.5. Knife edges were attached to the mouth of the samples and the crack length was monitored by a clip gauge using the Instron software package. The test set-up is shown in Fig. 3. The samples were subsequently loaded monotonically until crack extension was visibly observed and the crack mouth opening displacement was monitored using a clip gauge.