An Investigation of the Processing-Microstructure-Property Trinomial in the Production of High-Wear-Resistant Coatings by Multi-Material L-DED

340 p.
This PhD thesis aims to investigate aspects related to the process-microstructure-property trinomial in multi-material Additive Manufacturing (AM) when applied to the production of high-wear-resistant coatings. In the course of this research work, several topics have been covered in order to approach the Laser-Directed Energy Deposition (L-DED) of metal-ceramic multi-material structures from a holistic perspective. Therefore, from aspects related to the process set-up and the behaviour of multi-material powder mixtures in L-DED nozzles to the influence of the processing conditions on the microstructure and subsequent material properties have been investigated. In addition, the performance of these coatings has been evaluated from a mechanical and tribological perspective, to demonstrate the capabilities and potential benefits of metal-ceramic coatings for wear-resistant applications.Based on the current state-of-the-art and motivated by industrial and societal requirements, WC-Co Metal Matrix Composite (MMC) coatings are established as a suitable solution for the coating of components subject to high frictional loads and operating at high temperatures. In addition, L DED is proposed as the most suitable process for depositing such coatings. On this basis, the most relevant research questions are identified, namely, the challenges associated with multi-material L-DED from a system design perspective and the need for a methodology to set-up the L-DED process and target specificmicrostructures and properties, when multi-material powder feedstocks are involved.The first part of this PhD thesis focuses on investigating the fluid-dynamic behaviour of metal-ceramic multi-material powder mixtures in continuous coaxial L-DED nozzles. Indeed, due to the different inertial properties of WC and Co-alloys, powder segregation is likely to occur, which will subsequently affect the composition of the deposited coatings. For this purpose, a Computational Fluid Dynamic (CFD) model is