Investigation of Spatters in Cold Metal Transfer + Pulse-Based Wire and Arc Additive Manufacturing of High Nitrogen Austenitic Stainless Steel

During wire and arc additive manufacturing (WAAM) of high nitrogen austenitic stainless steel (HNASS), the explosion of droplets with high-content nitrogen will induce grievous spatters, causing incomplete fusion, cracks and inclusions. In this study, the formation, characteristics and influence of spatters on cold metal transfer + pulse (CMT+P)-based WAAM of HNASS were investigated. The droplets exploded violently due to the sharp decrease of nitrogen solubility in liquid steel, which produced abundant spatters at the pulse stage. The spatter particles were a mixture of manganese oxides (MnO and Mn3O4) and molten metals of the wire. The overall particle size followed normal distribution. The majority of particles were spherically shaped in diameter of 20–150 µm, and over 80% of spatters were below 200 µm in diameter. Inclusion of manganese oxide spatter particles appeared in the deposition metals of single-pass single layer. The spatters at the root of the deposited metal can be hardly removed, which produced interpass defects in overlapping deposition. For multi-pass multilayer deposition, interlayer tearing occurred to reduce the mechanical properties of HNASS. The interlayer tearing was caused jointly by the incomplete fusion caused by spatter particles of manganese oxide and the large thermal stress. Hence, the spatters generated in the droplet transition for WAAM of HNASS must be controlled.