Anisotropic creep behavior of stainless steel produced by selective laser melting

In this study, cubic blocks of 316L austenitic stainless steel (SS316L) were fabricated from gas-atomized powder with certain process parameters for selective laser melting (SLM). The microstructural characteristics and tensile properties were investigated for the blocks made in two different directions of the material by using the SLM method. Series of small punch (SP) creep tests were conducted at 650 °C under various applied load levels in the vertically oriented specimen and horizontally oriented specimen. The results suggested that the creep life and the creep resistance of the vertical specimen were significantly higher than those of the horizontal specimen. The creep voids and micro-cracks nucleated and grew along the grain boundaries and the intra-granular cellular networks, where the accumulated dislocations has caused creep cavity nucleation and generated physical damages at both the boundaries. An image analysis method was used to measure the area fraction of the internal creep damage. Furthermore, the results for the horizontal and the vertical specimens were compared after the SP creep tests. The difference in creep damage mechanism between the two directions was explained by the melt pool boundary (MPB) characteristics. Two types of MPB such as “layer-layer” and “track-track” MPBs, were generated by overlapping of multiple melt pools between the layers and between the tracks during the SLM process. The bonding force of the “layer–layer” MPBs seemed to be stronger than that of the “track–track” MPBs. The optimal direction with a high creep resistance was determined for vertical specimens. The creep test results obtained by SLM showed slightly weaker creep resistance compared with the reported data of the conventionally manufactured 316L stainless steel.