M. Shahriarifar 1, 2*, M. Doré 3, X. Zhang 1, M.K. Khan 1
1 Faculty of Engineering Computing and Environment, Coventry University, UK
2 National Structural Integrity Research Centre, TWI Ltd, Cambridge, UK
3 TWI Ltd, Cambridge, UK
Powder bed based additive manufacturing techniques have been gaining an impressive and growing attention owing to their ability to fabricate near-net shape parts with complex geometries. However, the presence of process-induced defects such as gas pores and lack of fusion has detrimental impact on the mechanical behavior of the AM components. These defects can act as stress raisers reducing the strength of the material especially under fatigue loading. The aim of this work is to elucidate the influence of defects on the high cycle fatigue strength of austenitic stainless steel 316L produced by laser powder bed fusion (L-PBF) process. As-received virgin and recycled powders were used to produce two batches of specimens resulting in different levels of defects in each batch; samples built by recycled powders were found having higher defect population. Optical and electron microscopy were employed for characterization of the microstructure. Tensile and force controlled fatigue tests were carried out on these two groups of specimens. X-ray micro-computed tomography was utilized for characterization of defect size distribution in fatigue test specimens. Fractography on fatigue specimens revealed the size and location of the crack-initiation defect. Different fatigue life prediction models are used to investigate the fatigue cracking phenomenon.
Preliminary results indicate that samples produced from virgin powder (Batch A) are almost fully dense with a few small spherical gas pores (less than 100 μm diameter), whereas larger pores and lack of fusion defects of irregular shape were observed in specimens fabricated from recycled powders (Batch B). Re-use of powders is currently practiced in additive manufacturing; its effect on mechanical properties is a topic of current research. Samples from both batches have similar yield and tensile strengths, which are higher than the wrought material with the strength of batch A slightly higher than that of batch B. However, the ductility of the specimens produced from recycled powders was found to be significantly lower than specimens from virgin powders.