An investigation into specimen property to part performance relationships for laser beam powder bed fusion additive manufacturing

Abstract

The influence of part size and geometry on the melt pool size, microstructural features, and resulting mechanical properties of additive manufactured 17-4 precipitation hardening (PH) stainless steel (SS), fabricated using a laser beam powder bed fusion (LB-PBF) process is investigated. Three different types of geometries including a dog-bone part (i.e. representing the specimen or the witness coupon) and two square rods with different cross-sectional sizes (i.e. representing the parts) were designed and fabricated using the same set of process parameters. Mechanical properties were determined under quasi-static tensile and uniaxial strain-controlled fully-reversed fatigue loading conditions. Experimental results confirmed a minimal effect of part geometry on the tensile behavior of the LB-PBF 17-4 PH SS as the yield strength ranged from 1145 to 1198 MPa and ultimate tensile strength varied from 1171 to 1231 MPa. On the other hand, part geometry had a significant impact on the fatigue behavior in the high cycle regime. Specimens fabricated from the large square blocks contained the lowest amount of porosity, and consequently, exhibited the highest fatigue resistance. On the contrary, the highest amount of porosity was observed in the specimens fabricated from the dog-bone parts resulting in an inferior fatigue resistance as compared to other geometries. Microstructural analysis was performed to estimate the effect of part geometry on the thermal history experienced during manufacturing by measuring the melt pool dimensions. Some effects of geometry on the melt pool dimensions were observed as the melt pool depth was found to be shorter for dog-bone specimens and longer for large block specimens. The less elongated melt pools in dog-bone specimens can be attributed to higher cooling rate experienced by the specimens resulting in more entrapped gas pores.