Effects of microstructural heterogeneity and structural defects on the mechanical behaviour of wire + arc additively manufactured Inconel 718 components

Abstract

This study investigates the extent to which the build orientation and heat treatment schedule affect the microstructure and mechanical properties for thin-walled and additively manufactured IN718 components produced with the cold metal transfer process (CMT-WAAM). Uniaxial tensile tests using digital image correlation (DIC), microhardness analysis and fractography were used to characterise the mechanical behaviour, both in the as-deposited condition and after heat treatments used in the aerospace and oil and gas industries. Wrought material was also tested to benchmark the measured properties. The solution treatment of 1040 °C for 1 h in the oilfield specification reduced the area fraction of Laves phases significantly (∼80%) and promoted higher homogenisation of ageing constituents. However, grain growth near interlayer boundaries resulted in localised low hardness (∼50 HV0.2 below the average) after age hardening. In the as-deposited condition, the yield strength was ∼10% lower along the build direction and changed to being 8–13% higher along the same direction in heat-treated samples, while the elastic modulus relative to deposition orientation was unaffected. Furthermore, solidification defects, such as porosity and hot cracking, caused strain localisation during tensile testing and substantial scatter in macroscopic strain. The ductility was improved with oil and gas heat treatment, but it was substantially lower along the build direction due to the longer axis of defects being perpendicular to loading direction. This study highlights the importance of optimising process parameters to minimise defects and tailoring heat treatments to achieve a higher ductility in IN718 processed by WAAM.