Nowadays, research on additive manufacturing of Ti6Al4V alloy is growing exponentially but there are just a few studies about additive manufacturing of metal matrix composite components. In this work, highly reinforced Ti6Al4V matrix composites with SiC particles have been additively manufactured by direct laser deposition (DLD). Ti6Al4V powder and SiC particles have been deposited layer by layer to form an additive thin wall structure. The geometry, microstructure, and microhardness of the samples are strongly influenced by the laser scanning speed used during de fabrication process. In addition, the effect of the SiC increment in reinforcement concentrations and the influence of SiC particle sizes in the microstructure have been evaluated, and the reaction mechanisms have been established. The percentage of reinforcement measured is lower than expected due to the reinforcement-matrix reactivity that results in partially dissolved SiC particles and the formation of a TiC and Si₅Ti₃ ring around them. The size and number of particles and reaction products depend on the initial size and percentage of reinforcement and the DLD scanning speed. The higher the size and percentage of SiC particles and reaction products in the matrix, the higher the hardening effect of the composite matrix. Graphic Abstract

The difficulty of getting a correct distribution of the reinforcement in the metal matrix and the complexity for achieving a good-metallurgy matrix-reinforcement bonding has limited the development of additive manufacturing of metal matrix composites. This research proposes the use of a reactive atmosphere during the fabrication process to obtain titanium matrix composites reinforced with TiN. The relation between the carrying gas and the process parameters used with the presence of porous and defects, the microstructure, and microhardness has been obtained. Nitrogen was used as the carrying gas of the titanium powder. Under laser irradiation, the particles melt and react with nitrogen, resulting in the formation of a titanium matrix composite highly reinforced with TiN. The composite obtained had a microhardness increase between 50 and 100% in comparison with titanium samples fabricated in the same conditions in an argon atmosphere. Three reaction mechanisms have been proposed to take place in the microstructure, depending on the amount of nitrogen in the titanium particles, and its diffusion in them during the manufacture. Graphic

Ti6Al4V is the most used alloy for implants because of its excellent biocompatibility; however, its low wear resistance limits its use in the biomedical industry. The additive manufacturing (AM) of Ti6Al4V is a well-established technique that is being used in many fields. However, the AM of Ti6Al4V composites is currently under investigation, and its manufacture using laser powder bed fusion (L-PBF) would result in a great benefit for many industries. The one-step novel concept proposed uses a gas-controlled L-PBF system that enables the AM of layers with different compositions. Six millimeter-edged cubes of Ti6Al4V were manufactured in an Ar atmosphere and coated with in situ Ti6Al4V/TiN layers by using an Ar–N₂ mixture given the direct reaction between titanium and nitrogen. Unreinforced Ti6Al4V presented a martensitic microstructure, and TiN reinforcement dendrites and a minor Ti₂N phase were gradually introduced into an α + β basketweave titanium matrix. The composites’ microhardness, nanohardness, and elastic modulus were 2, 3, and 1.5 times higher, respectively, than those of the Ti6Al4V. Porosity levels (caused by a lack of fusion, trapping gases, and interdendritic porosity), ranged from 7 to 12% (most measured 20–40 µm) and increased with the reinforcement content (15 to 25%). A scaled-up, proof-of-concept design of a hip implant stem was 3D printed using this nitriding method. Since the neck of the stem (top part) is more susceptible to the fracture and fretting corrosion process, the resulting graded material part consisted of unreinforced Ti6Al4V at the bottom and Ti6Al4V/TiN at the top. This change was controlled by gradually adding nitrogen to the atmosphere; moreover, it was found that the more nitrogen in the chamber, the more TiN reinforcement formed in the part. A microhardness of ~450 HV_0.1 was measured at the bottom and gradually increased to ~900 HV_0.1, with the increment corresponding to the in situ TiN reinforcement amount.