PSI - Issue 68

7

Amy Milne et al. / Procedia Structural Integrity 68 (2025) 666–673 Milne et. al. / Structural Integrity Procedia 00 (2025) 000–000

672

From Figure 6 the X and Y sample data fall along the same trend. X (vertical) orientated samples are expected to be most affected by the stress concentrating effect of lack of fusion defects, which are located along the build layers, since they are loaded perpendicular to their major axis as outlined by Williams et al. (2019). However, the creep strain rate in vertical samples are expected to be an order of magnitude slower for a given stress than horizontal samples, since grains grow in the direction of heat extraction (in this case the build direction) generating elongated grains in the Y direction, thus reducing grain boundary diffusion, Williams et al. (2020a). Therefore, lack of fusion defects and grain structure have opposing effects on the creep crack growth rate of X orientation samples. Y orientation samples are expected to be less affected by lack of fusion defects than X samples, however every few layers defects may exist that are loaded normal to their major axis. Y samples will however have a higher creep strain rate than X samples. Z orientation samples are also expected to have higher creep strain rate than X samples (similar to Y samples), however the crack grows through each build layer sequentially and the lack of fusion defects are expected to have their major axis parallel to the loading direction, hence reducing their stress concentrating effect. The stress concentrating effect of lack of fusion porosity is therefore deemed more important than grain structure in controlling CCG in LPBF components.

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Figure 6: Creep crack growth test duration against stress intensity factor.

5. Conclusions The CCG behaviour of 316L SS C(T) samples manufactured by LPBF at 650 °C has been investigated. The samples were manufactured in three orthogonal orientations to determine the influence of crack growth and loading direction on CCG resistance of the LPBF samples. The CCG behaviour is influenced by both creep strain rate and lack of fusion porosity. The creep strain rate is influenced by grain elongation in the build direction, hence the creep strain rate in Y and Z samples is expected to be higher than X samples. The impact of lack of fusion porosity depends on their stress concentrating effects, which is maximised when loaded normal to their major axis (which lies along a build layer). For X samples, all lack of fusion pores are loaded perpendicular to their major axis, whereas for Y samples only fraction of the lack of fusion pores are loaded normal to their major axis. For Z samples, lack of fusion pores are generally not expected to be loaded along their major axis, hence, despite the higher creep rates expected in Z samples, Z samples generally have the highest CCG resistance. It is therefore clear that the control of porosity is the most important factor in controlling the CCG behaviour of LPBF components.

References ASTM 2023. Standard Test Method for Measurement of Creep Crack Growth Times in Metals. United States: ASTM International.