Influence of heat treatments on the mechanical properties and microstructure of VDM Alloy 780 components manufactured by laser metal deposition

Abstract

VDM Alloy 780 is a novel nickel-based superalloy using γ′-precipitation hardening, thus enabling application temperatures of up to 750 °C. However, when using laser metal deposition (LMD) as a manufacturing process, the resulting parts show anisotropic mechanical properties and directional grain growth along the buildup direction. These effects as well as the grain coarsening caused by low cooling rates lead to reduced mechanical properties compared to wrought material. In order to achieve comparable mechanical properties, LMD-manufactured components must be subjected to postheat treatments. In this work, the influence of postheat treatments on the mechanical properties and microstructure of powder-based LMD VDM Alloy 780 components consisting of solution annealing and double aging is investigated. Additionally, hot isostatic pressing (HIP) is applied to determine the influence on the porosity and the mechanical properties. The specimens are examined metallographically using optical microscopy and scanning electron microscopy. The grain orientation along the microstructure is determined by electron backscatter diffraction analysis of the heat treatment stages. To evaluate the mechanical properties, tensile tests in perpendicular and parallel directions as well as macrohardness measurements are carried out. The results show that the ultimate tensile strength and the macrohardness were increased by postheat treatments. The porosity in the component can be reduced by the HIP process, thus improving the mechanical properties further.