Development of Particle Reinforced Metal Matrix Composite Materials for Selective Laser Melting Applications

English

Maraging steels are often used for high temperature applications, like forging dies, casting dies, or parts which are used in harsh environments. They are based on Fe with a relatively high amount of Ni (i.e. 17 - 19 %) and were developed for the aero, space and tooling industry. Maraging steels feature a high strength and toughness, but lack of resistance against wear, which limits their applications in some areas. Due to the weak tribological behaviour, parts made out of maraging steel sometimes do not last very long, if they are involved in an abrasive movement. In this thesis work, the metal matrix of maraging steel MS1 was reinforced with different volume contents of either vanadium carbide or titanium carbide. The goal was to improve the hardness values and the tribological behaviour of the created metal matrix composite (MMC). As traditional manufacturing methods like casting are often not applicable, a powder processing route had to be chosen. For this, mechanical mixing of the MS1 and the composite material was chosen for the first step of the process route. After mechanical mixing, the powders were processed with a selective laser melting (SLM) machine to additively manufacture solid parts. Computer aided design (CAD) files were used to laser weld contours of the metal powder and layer by layer, parts were built three dimensional. The first investigation was about the influence of the initial particle size of the composite material. As vanadium carbide has a significantly higher melting temperature, the ability to melt these particles in a SLM machine had to be analyzed. Furthermore, the tendency of agglomerations due to Van der Waals forces had to be considered. After performing a Design of Experiments to find the optimal parameters for SLM processing, specimens with improved hardness were manufactured, but had some design flaws. To further improve the results and to better understand the effects of vanadium carbide on the matrix material, different volume contents were analyzed. However, after these first investigations it was found that solely mixing is insufficient for a homogeneous particle distribution, which is why mechanical alloying (MA) was performed as a post-powder processing technique. MA allowed to crack the agglomerations of the carbides and to embed the particles into the metal matrix material. This processing step was important for the stability of the SLM process and significantly improved the results. The metal matrix composite system of MS1 and TiC was the most promising and achieved the best results. A higher energy density "eta" improved the part density and lead to the complete melting of VC and TiC particles, which then solidified as primary carbides. While designing advanced materials with metal matrix composites, it is important to understand the phase formations and the mechanical properties of the new material. Several metallurgical, mechanical and tribological tests were performed after SLM processing the mechanically alloyed powders. To find the optimal mechanical alloying parameters, various milling times were applied. Afterwards, the morphology and the particle size distribution was investigated. It was found that the addition of carbides through mechanical alloying can significantly influence not just the mechanical properties, but also the phase formation. Some chemical transformations were observed in the VC and TiC particles, which influenced the solidification of the melt and the phase transformation of the gamma-Fe into alpha-Fe. Future scientific work is suggested in the optimization of SLM parameters for higher part densities and as a result, higher mechanical properties. Electron backscatter diffraction and transmission electron microscopy could furthermore, help to understand the microstructures of metal matrix composites.