Characterization of high deposition rate wire arc directed energy deposition of 316L stainless steel for pressure boundary components

Abstract

Wire-arc directed energy deposition (WA-DED) or wire-arc additive manufacturing (WAAM) presents a novel fabrication method for construction of pressure retaining components within nuclear power plants; reducing lead times for construction of replacement parts and potentially preventing millions of dollars in losses from power plant down time. However, ASME code changes are required before WA-DED 316L can be utilized for this application. This work aims to support these code changes. A WA-DED process parameter study was undertaken to better understand how travel speed, interpass temperature, and filler wire metal selection impact the microstructure and mechanical properties of 316L. It was found that interpass temperature and travel speed displayed no significant impact on the mechanical properties of WA-DED 316L within the parameter ranges evaluated. However, the selection of 316LSi filler wire metal resulted in an increase in yield strength, tensile strength, and ductility over 316L filler metal, likely due to solid solution strengthening and a reduction in stacking fault energy. Across conditions, heat treated samples exceeded the ASME minimums of a yield strength of 172 MPa, a tensile strength of 482 MPa, and an elongation of 43.6% for 316L at room temperature. A heat transfer model was developed to allow for prediction of the thermal history of the WA-DED process. The heat transfer model was used in conjunction with solidification models to make predictions of dendrite spacing and growth morphology showing good agreement with experimental measurements. Further work was undertaken to understand the role of hatch spacing on lack of fusion porosity in WA-DED builds. By increasing the ratio of hatch spacing to weld bead width, lack of fusion defects were observed in-situ using an infrared thermal camera. It was found that lack of fusion begins to form at a critical hatch spacing to bead width ratio of over 0.68. Lastly, a large-scale WA-DED 316L body was produced to gain understanding of print strategy development and part design for WA-DED. This large-scale body was separated into three distinct sections for production and surfaces were overbuilt by approximately 6.4 mm to allow for post process machining. A 323 mm (12.7 in) tall 316L representative valve body was successfully produced through WA-DED following the print strategy developed.