Microstructure analysis of TIG weld joints of Ni-based superalloys produced by Laser Powder Bed Fusion

The industrial use of laser powder-bed fusion (L-PBF) in turbomachinery is gaining momentum rendering the inspection and qualification of certain post-processing steps necessary. This includes fusion techniques that allow to print multiple parts separately to take advantage of e.g., various print orientations and join them subsequently. The main motivation of this study is to validate the tungsten inert gas (TIG) welding process of L-PBF manufactured parts using industrial specifications relevant for gas turbines to pave the way for the industrial production of modular build setups. For this, two commonly used nickel-based super alloys for high-temperature applications, Inconel 718 and Inconel 625 are chosen. Since their defect-free printability has been established widely, we focus on the suitability to be joined using TIG welding. The process is evaluated performing microstructural examination and mechanical tests in as built as well as heat-treated samples. The welds are assessed by applying a general weld qualification approach used at Siemens Energy - Gas and Power. It was found that both materials can be joined via TIG welding using standard weld parameters causing minimal defects. A solution annealing heat treatment before welding is not necessary for a positive outcome, but still recommended for Inconel 718.