Abstract Scope |
Introduction: Metal matrix composites (MMCs) reinforced with ceramic particles are promising materials for many industrial applications owing to their favorable combination of mechanical and physical properties. Titanium alloys are attractive for several industries because of their low density, high strength, and corrosion resistance. Based on their useful set of properties, Ti alloys can be exploited as the matrix for MMCs containing various ceramic reinforcements. Additions of ceramic reinforcement serve to improve wear resistance and increase the elastic modulus of the resulting MMCs. Laser-Directed Energy Deposition (L-DED) additive manufacturing (AM) techniques offer a versatile option to fabricate fully dense and intricate components of MMCs. Laser Engineered Net Shaping (LENS) is a L-DED process that provides a fabrication route to overcome some of the challenges in processing of MMCs by conventional routes.
Procedure: Titanium/Titanium Carbide (Ti/TiC) composites with 20, 40 and 60 vol.% of TiC powders were deposited on Ti-6Al-4V substrates using a L-DED method. The evolution of microstructures and local chemical compositions in the deposits were analyzed using X-ray diffraction (XRD), electron probe micro-analyzer (EPMA) and scanning electron microscopy (SEM). Assessment of mechanical integrity of the deposits involved microhardness, tensile testing and fractography.
Results: SEM micrographs of the starting TiC powder showed cracks along some grain boundaries within the polycrystalline particles. Bulk relative densities of ~ 99% were achieved in the deposits. XRD patterns showed strong diffraction peaks in the deposits corresponding to the HCP α-Ti and an fcc phase that matched closely with the TiC0.55 compound. According to the Ti-C phase diagram, a range of non-stoichiometric TiCx compounds exist with a corresponding fraction of vacancies occupying the carbon sites. Additional peaks that match the stoichiometric TiC compound were seen on the shoulders on the TiC0.55 peaks, especially in the 60% TiC deposit.
SEM images of the deposits showed that the fraction of remaining TiC in the deposits increased with increasing volume fraction in the powder feed mixture. No cracks were observed in the 20% and 40% TiC deposit. However, cracks propagated across the substrate/deposit interface for the 60% TiC deposits. Remnant TiC particles (embedded in the α-Ti solidified matrix) that did not dissolve entirely during the deposition process were evident. Some of the original TiC in each deposit dissolved into the melt on heating and formed fine TiC0.55 particles (without cracks) during subsequent solidification. The fraction of fine TiC0.55 particles increased with increasing % TiC in the original powder mixture. The hardness in the deposits increased sharply with increasing fraction of TiC in the powder mixtures (and thus TiC0.55).
The existence of a heat affected zone (HAZ) in the Ti-6Al-4V substrate extending up to ~300 μm from the deposit and characterized by an α’ martensite structure resulting from rapid cooling was evident. The hardness values increased in the HAZ due to the α’ martensite in that region.
Conclusions: The most important observation in the present work is that the quality of the TiC used in the starting feedstock, in terms of pre-existing cracks from the process used for production, played a dominant role in the mechanical integrity of the deposits. This observation suggests that the mechanical performance of the Ti/TiC composite deposits could be improved with the reduction of undissolved TiC content in the deposit. An optimal combination of mechanical properties and constituent volume fraction can likely be achieved with the L-DED process to fabricate defect-free deposits. For example, use of a lower volume fraction of smaller TiC particles together with tailored processing parameters to promote formation of TiC0.55 compound may eliminate issues associated with pre-existing cracks and stress concentrations with large, pointed remnant TiC particles. |