An Investigation of the Processing-Microstructure-Property Trinomial in the Production of High-Wear-Resistant Coatings by Multi-Material L-DED
- Autores: Marta Ostolaza Gaztelupe
- Directores de la Tesis: Aitzol Lamikiz Mentxaka (dir. tes.), Jon Iñaki Arrizubieta Arrate (dir. tes.)
- Lectura: En la Universidad del País Vasco - Euskal Herriko Unibertsitatea ( España ) en 2023
- Idioma: inglés
- Programa de doctorado: Programa de Doctorado en Ingeniería Mecánica por la Universidad del País Vasco/Euskal Herriko Unibertsitatea
- Materias:
- Enlaces
- Tesis en acceso abierto en: ADDI
- Resumen
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 developed and validated through experimental methods. The proposed tool has demonstrated its ability to predict the actual composition of the deposited coatings, thus allowing the powder segregation to be anticipated and the parameters of the powder feeder to be tuned to achieve a specific composition. It is worth noting that, due to the different behaviour of each constituent of the multi-material powder mixture, the composition of the clad can also be optimised by acting on the stand-off distance of the nozzle, which is quite versatile from an industrial point of view.The second part of this thesis aims to explore the correlation between the thermal cycle of the L-DED process and the microstructure and hardness of MMC coatings. One of the main concerns in multi-material AM refers to the high-temperature processing nature of these processes. This is already a key issue in single-material AM and becomes even more important in multi-material AM. In the case of metal-ceramic mixtures, the exposure of these material systems to high temperatures promotes their interaction. As a result, hierarchical microstructures are often generated, which significantly modify their microstructure and mechanical properties. Therefore, in the present work, the influence of the process parameters on the thermal cycle of the L-DED process is first studied with the aim of precisely controlling the thermal aspects during processing. Then, the correlation between the thermal cycle of the process and the metal-ceramic interaction in MMC coatings is studied by looking at the compositional modification of the metal matrix. Finally, the hardness of the compositionally modified matrix and the overall hardness of the composite are characterised. It is concluded that, depending on the composition of the metal-ceramic mixture, different strengthening mechanisms are dominant, namely grain refinement and carbide precipitation. In addition, microstructural observations demonstrate the dispersion of complex carbides throughout the matrix when higher ceramic contents are employed. It is therefore concluded that if a controlled ceramic-metal interaction is desired, low ceramic content mixtures should be sought.Finally, and based on the previous results, the durability and performance of MMC coatings are investigated. The durability of monolithic MMC coatings is first studied by means of flexural tests, supported by microstructural observations and hardness tests. It should be highlighted that the results obtained are in good agreement with the hypothesis reached in the previous experiments. In addition, the failure mechanism is studied, which is concluded to be a rapid coalescence of voids, which is more acute the higher the ceramic content. In addition, the performance of the MMC coatings is assessed through room and high-temperature reciprocating tests. These tests are also supported by microstructural and hardness characterisations to confirm the correct design of the MMC coatings. It is concluded that WC-Co MMC coatings are particularly advantageous under high-temperature conditions and that particle clustering promotes heterogeneous wear of the coating. In addition, the Functionally Graded Material (FGM) strategy is proposed to increase the surface integrity of MMC coatings as surface cracking is common. Furthermore, it has been demonstrated that this crack mitigation strategy does not affect the tribological performance of the coatings and is therefore a useful tool to increase the WC content of MMC coatings without adverse effects.To sum up, this work presents an investigation into the surface enhancement of high-added-value components through multi-material wear-resistant coatings produced by L-DED. The proposed methodology attempts to approach the process-microstructure-property trinomial in multi-material AM in order to facilitate the design and processing of metal-ceramic coatings. The results obtained demonstrate the suitability of L-DED to produce high-performance coatings to protect high-added-value components, such as hot-forming tools, and industrial or oil and gas components.
