Analysis of the effect of internal defects on fatigue performance of additive manufactured metals

Abstract

Additive manufacturing (AM) technologies are growing rapidly to generate unique complex parts with short lead times. To achieve industrial standards for critical applications such as in biomedical and aerospace industries, AM parts should provide the required mechanical performance. This includes acceptable fatigue performance because of the typical cyclic loadings applied to these parts during operation. Resistance to fatigue failure is greatly influenced by the existence of defects since fatigue cracks often start at defects. In this work the effect of processing and post-processing conditions on defect content of L-PBF (Powder Bed Fusion) Ti–6Al–4V and 17–4 PH specimens was studied and effect of defects on fatigue performance of Ti–6Al–4V specimens was analyzed using data from this study and the literature. Despite the significant scatter, for similar AM machines with constant layer thickness and hatch spacing, an optimal region of energy density level created smaller defects. The number of fatigue critical defects in Ti–6Al–4V machined surface specimens decreased significantly after HIPing, however, the size of fatigue damage initiating defect slightly decreased. Gas porosities observed in heat treated 17–4 PH specimens shrank in size but did not completely fuse after HIPing. Fatigue performance was consistently better for Ti–6Al–4V with a lamellar microstructure compared to a martensitic microstructure with similar defect size. The effect of defects on fatigue performance was found to be dominant at longer lives, and less scatter was observed for specimens with larger defects. Extreme Value Statistics (EVS) showed promising results for estimating the size of fatigue critical defects.

Publication Title

Materials Science and Engineering A

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