High-strength low-alloy steel fabricated by in situ interlayer hot forging arc-based directed energy deposition assisted with direct cooling

Abstract

Controlling thermal cycles during arc-based Directed Energy Deposition (DED), typically known as Wire Arc Additive Manufacturing (WAAM), is crucial to reduce heat buildup and prevent issues such as distortions, formation of brittle microstructures, grain growth, anisotropy, and consequent reduction in mechanical properties. In-situ interlayer hot forging coupled with WAAM (HF-WAAM) provides grain refinement and pore closure. The effect of HF-WAAM can be combined with the control of peak temperature and cooling rates, benefiting the material's microstructure and mechanical properties. In this context, the aim of this work was to evaluate the effect of direct cooling on the mechanical and microstructural properties of a high-strength low-alloy (HSLA) steel manufactured by WAAM and HF-WAAM. A pneumatically actuated system with a cooling system was specifically designed, where two pumps with a flow rate of 1.8 kg/min each were used to pump G13 antifreeze fluid at approximately −25 °C. In the actuator design, a double counterflow cooling system was used, as it promotes greater thermal homogenization and higher heat transfer rate, thus allowing greater thermal energy removal. Analyses of the mechanical and microstructural properties of the parts were carried out through uniaxial tensile testing, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Thermal cycles and cooling system control were conducted using a thermal imaging camera and thermocouples installed at the inlet and outlet of the actuator's cooling ducts. The results showed that samples manufactured with HF-WAAM had a greater number of less hard structures in their microstructure than those manufactured by conventional WAAM. The fabricated samples exhibited high tensile and yield strength values, with calculated anisotropy below 2 %. All samples showed ductile fracture characteristics after the tensile test, confirmed by fractography.