Inoculation as grain refinement strategy for Wire Arc Manufacturing of Invar 36

Abstract

Additive manufacturing (AM) refers to a series of techniques in which parts are created by successive deposition of layers. Among the different technologies, Wire Arc Additive Manufacturing (WAAM) employs a welding system in combination with a motion mechanism to deposit layers of molten metal. This technology possesses high deposition rates and an unconstrained build envelope, limited only by the reach of the motion mechanism making it suitable for fabricating large-scale parts. The use of this technique to manufacture technologically interesting materials as Invar 36, a low thermal expansion alloy, could lead to reduced material waste and components with a geometrical complexity only attainable via AM. The high heat input associated with WAAM has been proven to induce columnar grain growth causing anisotropic behavior in the materials, and in the case of Invar 36, it also promotes cracking. In this work, the addition of nucleating agents, known as inoculation, has been implemented during the deposition process to induce grain refinement and mitigate the aforementioned effects. TiC and NbC were selected as possible inoculants based on the results from the implementation of the edge-to-edge matching model as selection criterion. The inoculants, in the form of powders, were mixed with an organic carrier to create suspensions at 50 wt.% and 75 wt.% which then were applied as a coating to each layer during the deposition process. Invar 36 cuboidal specimens with dimensions of 15x15x120 mm were fabricated using GTAW-based WAAM with a heat input of 550 J mm^-1. Microstructural characterization showed that specimens with added inoculants achieved significant grain size reduction, reduced crack formation, and an increase in hardness. Defect-free depositions were attained in the specimens with inoculant-loaded suspensions at 75 wt.%.