EFFECTS OF POST-PRINTING HEAT TREATMENT AND ADDITION OF CERAMIC-BASED NANOPARTICLES ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF WIRE ARC ADDITIVE MANUFACTURED PH 13-8Mo MARTENSITIC STAINLESS STEEL

Abstract

Precipitation hardening martensitic stainless steels offer an outstanding combination of strength and corrosion resistance under harsh environmental conditions while maintaining their moderate toughness. The reasonable weldability of these alloys makes them suitable candidates for wire arc additive manufacturing (WAAM) with a significant deposition rate, high material usage efficiency, and lower capital and fabrication costs as compared to traditional subtractive manufacturing methods. Aiming to accelerate the widespread adoption of this state-of-the-art manufacturing technology, in the first phase of this research, the fabrication feasibility of the PH 13-8Mo martensitic stainless steel through the WAAM was investigated. The obtained results proved the successful fabrication of a sound and defect-free PH 13-8Mo part with a great combination of hardness, ductility, and tensile strength. However, the complex thermal history experienced by different locations of the component caused a gradual increase of microhardness and ultimate tensile strength values along the building direction owing to the non-uniform distribution of residual δ-ferrite and retained austenite. Moreover, the intrinsic directional heat sink towards the substrate resulted in the formation of a columnar structure of primary δ-ferrite grains, leading to a strong cubic texture and anisotropy in ductility. To address this issue, in the second phase of the performed research, different post-printing heat treatment cycles, including solution and aging treatments at different temperatures, were applied on the additively manufactured PH 13-8Mo samples to modify the microstructural features, promote the equiaxed grain structure, eliminate the anisotropic behaviour, and improve the mechanical properties. It was revealed that the solution treatment at 1050 °C for 1 h resulted in the dissolution of undesired δ-ferrite and removal of the columnar structure and anisotropic ductility, while the following aging at 500 °C for 4 h led to the maximum hardness and tensile strength due to the formation of optimum sized β-NiAl precipitates in a fully martensitic matrix. Despite the benefits of post-printing heat treatment, it increases the lead time and cost of the fabrication process, which is contrary to the value proposition of WAAM technology. Thus, in the third phase of this research, the potential grain refinement and strengthening through the introduction of TiC and TiB2 reinforcing ceramic nanopowders into the molten pool during WAAM processing was investigated. Although both inoculant agents refined the columnar primary δ-ferrite grains and provoked the precipitation of in-situ hard TiC/M3B2 phases, TiB2 nanopowders were found to be a more effective grain refiner as compared to TiC reinforcing particles. Overall, the fabrication feasibility of PH 13-8Mo alloy via WAAM was validated, while further microstructural and mechanical properties improvement was accomplished through the implementation of appropriate post-printing heat treatment cycle or by the addition of TiC and TiB2 nanopowder inoculants during the deposition process.