On the Probabilistic Prediction for Extreme Geometrical Defects Induced by Laser-Based Powder Bed Fusion
Research Paper Abstract: Compared to traditional subtractive manufacturing processes, powder bed fusion (PBF) shows promise for making complex metal parts with design freedom, short development time, and environmental sustainability. However, there is a consensus within the additive manufacturing (AM) community that the random geometrical defects (e.g., porosity, lack-of-fusion) produced in PBF processes pose a great challenge to fabricating load-bearing parts, particularly under dynamic loading conditions. Therefore, it is imperative to quantify defect sizes and distributions to estimate the critical, life-limiting defect size that significantly reduces fatigue life. This paper presents a comprehensive analysis of the defects induced by selective laser melting to quantify their sizes and statistical distributions. Then four cumulative distribution functions (i.e., Weibull, Gamma, Gumbel, and Lognormal CDFs) are leveraged and compared to predict the maximum defect size based on the principle of statistics of extremes. Both the peak over threshold (POT) approach and the block maxima (BM) approach are used for these predictions. The results show that the BM approach-based predictions for all CDFs to be much larger than the measured maxima while the POT approach-based predictions have less deviation. The Weibull and Gamma CDFs were best correlated to the data, measured by Pearson’s R correlation coefficient, while the Gumbel and Lognormal CDFs were also well correlated About this data: Geometrical defects from a bulk sample of SLMed SS-316L were measured using the Feret caliper diameter method. The FC diameter defect size was quantified by inscribing each defect within the smallest circle possible and using the resulting diameter as an effective defect size. The grouping or clustering effect of defects was not accounted for in the size of the defects. Only individual defects were measured to simplify this study. A total of 769 defects were collected and measured. A Keyence VR 3100 optical microscope was used for these measurements. To observe these defects, 14 sample slices of the bulk printed sample were obtained. Only the defects within and touching the border of a 5 mm x 5 mm area of the sample slices (from both sides of the sliced sample) were measured. This is because these areas corresponded to gauge sections of the sample slices., which can be used for fatigue testing. Therefore the gauge sections are the regions from which cracks are expected to propagate due to the higher stresses that arise from the reduction in specimen cross section. This means all defects outside of these regions are not as critical and are not included in the distribution. The attached files denote the sample number (1-14) and the sample side (top/bottom face) to which the data correspond.