Development of method for processing brittle-hard materials using Laser Metal Deposition (LMD)

Abstract: New materials capable of operating at ultra-high temperatures in gas turbine engines are highly desired. Materials such as silicides of refractory metals present great potential to substitute single crystal nickel-based superalloys. Among this class of materials, the Mo-Si-B alloy is a prominent candidate to significantly push the gas-turbine operating temperatures higher, hence increasing their efficiency. Additive Manufacturing (AM) is a technique capable of producing fully functional, geometrically complex parts with economic benefits. Laser Metal Deposition (LMD), is one of such AM techniques that can be utilized to build parts layer-by-layer. LMD uses a high-power laser to melt metal powder being delivered in situ, providing the opportunity to generate functionally graded or custom-tailored structures as well as to repair them. This work provides an overview of the advances and challenges of processing Mo-Si-B alloys using LMD. An evaluation of the deposition of single tracks has been made in the first place, assessing the influence of laser power, powder mass flow, pre-heating temperature, scanning speed and focal point on the resulting tracks. Afterward, a method for obtaining cubic samples of Mo-Si-B on Nickel-based substrates has been conceived, which in turn, enabled the evaluation of defects in the structures. The observations performed enabled the classification of defects in three categories, delamination from the substrate, cracking during the process and porosity. To tackle these challenges, different approaches based on graded structures have been tested. Firstly, it has been shown how a variation in dilution influences the delamination of the samples and can significantly improve it in certain situations. The cracking occurring on the upper part oft he samples has been attributed to the accumulation of residual stresses; it has been observed how an increase in pre-heating temperature and decrease in scanning speed alleviates this defect. Additionally, functionally graded material (FGM) structures have been designed to improve the adhesion of the samples to the substrate and to reduce the residual stresses. A strategy consisting on introducing an interlayer of a refractory metal between the substrate and the Mo-Si-B alloy has been tested, showing some improvement of the process. A composite Mo-Si-B alloy with Ni-based interlayers has been created; the residual stresses were significantly reduced using this approach. Finally, a relation between porosity and oxygen content has been observed, with oxidation mechanisms seemingly increasing the porosity.