Effect of binder jetting microstructure variability on low cycle fatigue behavior of 316L

Abstract

Despite the growing interest for Binder Jet Additive Manufacturing (BJAM), variability in the mechanical properties hinders its industry wide adoption. In order to reduce the variability, the underlying failure mechanisms should be understood. In the present work, a stainless steel 316 L alloy powder is used to generate BJAM parts with different microstructures via changing the powder particle size distribution and post-processing heat treatment. The effects of the microstructural features such as porosity, grain size, secondary phases and annealing twins on fatigue failure mechanisms are analyzed. The results show that the primitive porosity lines generated during BJAM process are the major cause of fatigue crack initiation and mechanical variability. These lines led to the formation of elongated pores during subsequent sintering, causing a reduction in the fatigue life and an increase in the data scattering. It is shown that a minimum level of green density prior to sintering is necessary to reduce the occurrence of this networked porosity. Moreover, the use of HIP treatment removed the process induced porosity and reduced scattering of the fatigue life but did not impact the average fatigue life significantly. The benefit of porosity reduction and pore roundness after HIP treatment appears to be greatly offset by grain coarsening and formation of annealing twins during HIP.