Parameter study for the optimization of heat treatments for VDM Alloy 780 processed by laser powder bed fusion

Abstract

This thesis investigates the influence of different heat treatment strategies on the microstructural development and mechanical properties of LPBF-fabricated VDM Alloy 780. Solution heat treatments, isothermal and double aging, and cooling conditions were systematically studied. Characterization was carried out using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDS) and Vickers hardness tests (HV5). Results show that γ grain growth was modest for shorter solution heat treatments, with possible secondary recrystallization after extended durations. γ’ precipitates coarsened in line with an Ostwald ripening model, initially forming small spheres and transforming into cuboids and octocubes. Slower cooling rates and higher aging temperatures promoted larger γ’ particles, whereas lower aging temperatures favored higher phase area fractions. Double aging with air cooling between steps produced a bidisperse γ’ size distribution. Hardness correlated primarily with γ’ phase area fraction, with finer, denser precipitates leading to higher hardness at constant fractions. Suspected high-temperature phases formed primarily at grain boundaries during extended high-temperature aging and slow furnace cooling, depleting adjacent regions of γ’. MC-type carbides with two compositions—(Nb,Ti)C (most common) and (Nb,Mo,Ti)C—were found both intergranularly and at grain boundaries, exhibiting globular and plate-like morphologies, and sometimes forming chains or agglomerates. Oxide inclusions connected to spatter and powder recycling were observed.