An in-situ substrate heating strategy for residual stress reduction and mechanical property enhancement in wire-arc additive manufactured Nickel Aluminium Bronze (NAB) alloy

Abstract

Wire-Arc Additive Manufacturing (WAAM) offers a rapid manufacturing route for the fabrication of Nickel Aluminium Bronze (NAB) alloy marine components compared to conventional casting processes. However, high cooling rates and cyclic thermal conditions inherent to the WAAM process result in heterogeneous microstructures, residual stress, and anisotropic mechanical properties. In this context, the present study proposes an in-situ substrate heating method during WAAM using a TIG torch. The study systematically investigates the influence of substrate heating on the bead geometrical features, microstructural evolution, residual stress, hardness, tensile properties, and fractography of NAB alloy fabricated using WAAM at welding currents of 90, 100, and 110 amperes (A). Compared to the nonheated deposition, the preheating improved the wettability and reduced the contact angle, particularly at 90 A from 88.9° to 32.2°, subsequently increasing the bead width from 5.6 mm to 8 mm. Microstructural observations reveal that substrate preheating causes distinctive changes in the morphology and size of kappa phases due to the reduced thermal gradient and solidification rate. The XRD studies revealed that preheating tends to reduce the compressive transverse residual stress buildup during WAAM. The hardness and tensile results demonstrate a significant reduction in the anisotropy, particularly at lower heat inputs. Among all, the ultimate tensile strength (UTS) of vertical samples of 90 A increased from 562.7 to 634.9 MPa due to preheating. The preheating yielded the highest UTS of 706.6 MPa and 63.21% at 110 A in the horizontal direction. Fractography investigation of tensile fracture surfaces using scanning electron microscopy (SEM) revealed the presence of micro-voids, a characteristic feature of ductile fracture and plastic deformation. The microstructure heterogeneities and kappa phases acted as the potential sites for micro-void formation and its coalescence, leading to dimples of several sizes and plastically deformed tear edges. Overall, the substrate preheating using a TIG torch provided an effective means to control bead geometry and microstructure and to reduce the thermal gradient and residual stress and enhance the mechanical anisotropy in the NAB alloy produced using WAAM.