Comparison of the master sinter curves of water- and gas-atomized AISI 4340 low-alloy steel in binder jetting additive manufacturing

Abstract

Master sinter curves (MSC) are empirically derived models used to predict bulk densification behavior of a powder-compact exposed to heat in a controlled nitrogen-hydrogen atmosphere. In this study, solid-state sintering MSC models were developed for two types of AISI 4340 powders: water-atomized and gas-atomized, which were deployed in binder jetting additive manufacturing. The models were constructed based on shrinkage measurements obtained through push-rod dilatometry. The results showed that the bulk densification behavior of the water-atomized parts was generally similar to the gas-atomized, indicating its relevance in BJAM when it comes to sintering. Some of the differences in bulk densification were attributed to the powder organization in the green state. The water-atomized powder was found to have a larger activation energy value (341 kJ/mol) than that of the gas-atomized (312 kJ/mol) and exhibited a slightly slower densification rate. The models were validated through independent sintering trials, resulting in model prediction accuracies of 91.2% and 99.4% for the water- and gas-atomized powders respectively. The starting green densities in the validation trials were 49% and 50% respectively, with final density targets of 70–73%. Examination of validation samples from both powders revealed similar particle necking behavior. Further optimization of the powder spreading for WA powders may maximize the powder packing and further improve the densification rate.