Publication
Titanium-based Metal Matrix Composites (MMCs) offer remarkable mechanical, thermal, and corrosion resistance properties and are of high interest to several industrial sectors, including aerospace. One of the main commercial drawbacks of these materials, when produced with conventional methods, is their complex and expensive manufacturing process, which limits the geometrical complexity. The ability of Additive Manufacturing technologies such as Laser Powder Bed Fusion (L-PBF) to produce near-net-shape components allows MMCs to reach their full potential. This work focuses on stiffness-driven Ti-TiC MMCs produced by L-PBF for space applications, and is supported by the European Space Agency through NPI funding. Stiffness improvement requires significant reinforcement content, which often leads to poor ductility. The first aim of this thesis is to produce Ti-TiC MMCs displaying a notable Youngâ s modulus increase, while keeping a significant ductility. The second objective is to investigate the MMCs damage via the related decrease in performances. Of particular interest, is the influence of the TiC feedstock. Ti-TiC MMCs were produced by L-PBF and subsequently heat-treated. Ti grade 2 was used as matrix, and TiC reinforcement was introduced with (i) aggregated TiC powder, (ii) comminuted TiC powder, or (iii) carbon black. The latter reacts in-situ with Ti to produce TiC, during the L-PBF process. TiC partial dissolution during the L-PBF process produced a microstructure with large undissolved TiC particles and sub-stoichiometric TiC dendrites spread out evenly in the Ti matrix. The reduction of the C/Ti ratio in TiC led to its volume fraction increase, resulting in an effective TiC content of 17â 21 vol%, compared to the nominal value of 12 vol%. The heat treatment converted the dendritic TiC into equiaxed TiC, and partially homogenised the TiC stoichiometry. The mechanical properties were measured on near-net-shape standard tensile specimens. The produced MMCs achieved Youngâ s moduli between 145-160 GPa, accounting for a 24â 37% increase compared to unreinforced Ti. The heat treatment more than doubled the elongation at failure of the composites, with a maximum value of 3.1%. The ductility increased the most in the 1st hour of heat treatment thanks to the microstructure globularisation. The in-situ MMCs displayed significantly higher ductility compared to the other MMCs, e.g., 2.8% vs. 1.9% after 24h heat treatment. These elongations at failure exceed by 138% and 46% the ones reported in the literature for Ti-based MMCs produced by L-PBF with similar Youngâ s moduli. The damage rate was strongly influenced by the type of TiC feedstock: (i) aggregated TiC led to the highest rate; (ii) the larger size distribution of the comminuted TiC led to the second highest damage rate, followed by (iii) the smaller size distribution; finally, the in-situ produced TiC led to non-significant damage rate. This exhibits the correlation between the particles i