Synergistic improvement of mechanical property and thermal expansion of Wire-arc DED Invar alloy enabled by a novel deposition strategy

Invar alloy has been widely used in composite molds due to its low coefficient of thermal expansion (CTE). The cold metal transfer (CMT) based wire arc directed energy deposition (Wire-arc DED) makes the rapid fabrication of large-scale composite molds a reality. The present study proposes a novel deposition strategy that combines voltage and current online monitoring and interlayer temperature control. The results show that the optimization of the process parameters obtained by online monitoring of the electrical signals and extending the interlayer dwell time (IDT) contributes to improved mechanical properties and elimination of tensile anisotropy. The elongation (EL) at fracture of vertical and horizontal specimens is increased by 29.9 % (EL: 30 %) and 28.1 % (EL: 34.6 %), respectively. This is attributed to the improvement of heterogeneous microstructure and the significant refinement of the long-chain-like micron-scale (Nb, Ti) C. Based on the analysis of grain size, (Nb, Ti) C distribution and defect distribution, the main mechanisms for the synergistic strengthening-ductility effect are discussed in detail. Controlling the interlayer temperature enhances the residual compressive stress within the component, resulting in excellent low CTE (0.33 × 10⁻⁶/°C, 25–100 °C). This provides a convenient way for the improvement of mechanical properties and thermal expansion of Wire-arc DED Invar alloy.