Table 7 provides a summary of the above-mentioned mechanical properties.

Titanium alloys have various aerospace, marine, biomedical, structural and industrial applications. They owe excellent strength to weight ratio, malleability, formability, deterioration resistance, and biological compatibility characteristics. However, these alloys yield reduced hardness and wear attributes. A suitable way to increase the hardness and tribological characteristics of Ti-alloys is to mix the Ti-matrix with tough precipitates to achieve Ti-based composites (TMCs). These can be classified into two sub-categories based on the type of reinforcement: (i) continuous reinforced TMCs (ii) discontinuous (particles) reinforced TMCs [102,103,104]. Table 6 summarizes the continuous and discontinuous TMCs formation techniques regarding illustration, advantages, and disadvantages.

In the last few years, the discontinuously strengthened Ti-composites have experienced rapid expansion. They have an elevated strength, toughness, wear resistance, and thermal reliability as compared to pure Ti-alloys. Therefore, they are potentially considered for applications, including aerospace and automotive. Different particulates have been selected for reinforcement so far such as TiB₂, TiN, B₄C, ZrC [42,104], nano-SiC [114], TiB, TiC [115,116], Al₂O₃ [117], and Si₃N₄, which were found unstable due to the formation of titanium silicide and carbon nanotubes [118,119,120]. For the continuous fibers, TMCs using boron reinforcing fibers (coated with silicon carbide) were produced; however, these fibers are expensive as compared to the other fibers, which leads to discontinuation of TiB fibers [104,121,122]. Besides this, various researches have been carried out using SiC [123,124,125,126,127], carbon [104,128,129], SCS-6 and Sigma [130,131] reinforced fibers.

Figure 9a displays the mechanical and physical properties of various discontinuous TMCs. It can be seen that the TiB and graphene present the least and highest melting point, respectively, as compared to the other discontinuous reinforcements (DRFs). It means that a low amount of laser energy will be needed to melt down the TiB. In contrast, an opposite behavior can be observed for graphene. Moreover, La₂O₃ presents the highest density, while graphene possesses the least density value. To the best of our knowledge, the elastic modulus of TiB and La₂O₃ containing MMCs are not reported in the literature, which identifies the potential area for future research. The B₄C, SiC, TiB₂, TiC, and TiN have almost the same elastic moduli; but, the carbon nanotubes (CNTs) and graphene present the highest elastic moduli as compared to the rest of DRFs. Similarly, the La₂O₃ has the highest thermal expansion coefficient as compared to the rest of DRFs. Figure 9b shows a comparison of various continuous reinforcements (CRFs) regarding their diameter and ultimate tensile strength (UTS). The two major types of CRFs, including SiC and Al₂O₃, have been presented. The majority of the CRFs such as SM 1140+, trimarc, SCS-ultra, and SM-6 belong to the SiC category, while sapphire belongs to the Al₂O₃ type. The output explains that SCS-ultra, with a diameter of 140 µm, shows the maximum UTS. In contrast, for the rest of the CRFs, a compromise should be reached between the particulate size and UTS. By keeping in view the trend presented in the Figure 9, one can conclude that the proper selection of DRFs and CRFs has to be made based on the specific requirements, and a compromise should be made between the thermal and physical properties [104].

Ceramic reinforced nickel matrix composites, also known as NMCs, usually possess high fatigue and corrosion resistance with better hardness and wear resistance properties as compared to the simple nickel matrix. They are considered to be prospective materials in aerospace, biological, and petrochemical manufacturing due to above-mentioned properties. TiC strengthened NMCs via LMD were produced by Hong et al. [132]. They found that the existence of TiC was beneficial to refine grain size, from 34.1 µm to 27.2 µm. Moreover, the results displayed that the high energy can lead to an effective Marangoni convection confined by the melt pool, which induces refined and normalized spreading of TiC reinforcements. Thus, resulting in improved wear resistance and malleability. Furthermore, an increment in the thermal energy input per unit length resulted in uneven columnar dendrites formation, declining the wear and tensile properties of NMCs. Li et al. [133] produced Ni + TiC NMCs by LMD. A total of three TiC compositions, including 20, 40, and 60 (vol.%), were selected. The influence of TiC vol.% on phase transformation, microstructure evolution, hardness, and wear resistance, was analyzed. The analyses exhibited that the composites consisted of TiC + Ni phases, demonstrating that TiC was produced through in-situ reaction. Moreover, TiC particulate size increased from 3 to 10 μm, when the TiC (vol.%) was enhanced from 20 to 60%. In addition, the hardness was improved from 365.6 HV_0.3 to 1897.6 HV_0.3, while the value of wear resistance changed from 20 to 6 × 10⁻³ g.

In the LMD process, the rapid heating and solidification lead to the warpage, delamination, and cracks. Li et al. [134] carried out a study on the usage of invar. The experimental results showed that TiC reinforced invar, with the 64 wt.% Fe and 36 wt.% Ni composition, has a low thermal expansion coefficient, improved rigidity, and YS. Xiong et al. and Picas et al. [135,136,137] manufactured WC + Co using LENS. An improved microstructure, wear resistance, and mechanical properties were found with the addition of WC in Co. Choi and Maumder [138] reported a study on the manufacturing of Fe + Cr + C + W MMCs via the DMD, thereby, producing an innovative wear-resistant material. The results explained that the conformation and volume proportion of carbides could easily be handled by regulating the pre-heating temperature, input power density, and scanning speed. The matrix participation was carried out by M₆C and M₂₃C₆ type carbides; the rhombus-shaped M₆C carbides showed better tribological properties. Zhong et al. [139] synthesized the Ni-Al intermetallic layers and TiC (particulates) MMCs by laser cladding. The powder particles were added coaxially. The printed layers were cracks free and metallurgically bond to the substrate. The microstructure of the layers was mainly composed of β-Ni-Al phase and a few γ-phases. Moreover, un-melted dispersive fine precipitates of TiC particles and refined β-Ni-Al phase matrix were found in the composites. The hardness test shows that the microhardness for Ni-Al intermetallic layers was equal to 355 HV_0.1, and 538 HV_0.1 for Ni-Al+ TiC matrix composites.

MMCs present superior properties including hardness, YS, and UTS in comparison to the base alloys. Li et al. [141] manufactured MMCs through the LMD by feeding the WC powder particulates along with titanium wire into the melt pool generated by the laser beam. Major process parameters, including wire feeding rate, powder flow, and the laser power were used in the analyses. The microhardness of the MMCs was 500 HV_0.2, which is much higher than the titanium alloys (320 HV_0.2). It indicates that the presence of the WC + TiC phases within the printed layer improved the hardness and abrasive resistance. Bi et al. [142] carried out the deposition of inconel 625 + TiC nano-powders using the LMD. The mechanical properties were investigated. Three different compositions of TiC + inconel 625, including 0.25/99.75, 0.50/99.50 and 1.00/99.00 (wt.%), were synthesized. The hardness, UTS, YS, and elongation are shown in Figure 10a. It can be seen that the hardness, UTS, and YS increased proportionally with the increment in the quantity of TiC particulates (wt.%). However, elongation presented random behavior with the increment in TiC particulates (wt.%). The maximum elongation was exhibited by 0.50/99.50 composition. Gopagoni et al. [143] processed Ni (80 wt.%) + Ti (10 wt.%) + C (10 wt.%) MMCs via LENS technique. The manufactured MMCs showed the eutectic TiC and FCC-TiC structures. The tribological and mechanical analyses were conducted. The stationary friction coefficient was found equal to 0.50, inferior to pure Ni. Moreover, the hardness increased substantially up to 370 VHN, proving them a potential candidate for surface engineering operations. Crack-free functionally graded MMCs composed of TiC particulates + Ti6Al4V were manufactured by Wang et al. [56] using LMD. A volume fraction of TiC in between 0–30 (% wt.) was used to analyze the effect of TiC (vol.%) on the microstructure and mechanical characteristics of MMCs. They found that the hardness gradually increased with the increment in TiC (vol.%), which can be attributed to the presence of eutectic + TiC phases. When TiC increased up to 5 (vol.%), the tensile strength enhanced by 12.3% as compared to the Ti6Al4V alloy. Nevertheless, the tensile strength and elongation of the produced MMCs decline as the volume fraction of TiC surpass by 5%. It can be explained that the number of stiff un-melted TiC particulates, amount and dimension of dendritic TiC phases raised with the increment in TiC. These results are presented in Figure 10b.

Hong et al. [144] used the LMD to manufacture inconel 718 + TiC MMCs. The influence of the laser energy over the unit length (80–160 kJ/m) on the microstructures and hardness, was analyzed. The TiC experienced a tremendous transformation as the laser energy density increased from 80 to 120 kJ/m. The comparatively coarse polyhedral TiC particulates resulted when the laser energy was up to 100 kJ/m. As the laser energy increased beyond 100 kJ/m, completely liquified smooth TiC particulates were produced. Furthermore, when the laser energy increased beyond 160 kJ/m, the TiC particulates were significantly refined. However, a direct relationship was observed between laser energy input and microhardness for the produced MMCs, as given in Figure 11a. Sateesh et al. [145] manufactured MMCs using pre-heated nickel phosphide coated with SiC reinforced particles via the LMD process, under inert nitrogen atmosphere. An inclination in the hardness, UTS, YS, and elongation was observed with the increment in SiC (wt.%). These properties decline dramatically beyond 3 (wt.%) addition of SiC. For a given laser scanning speed and power, the increment in SiC (wt.%) resulted in excessive un-melted SiC particles, leading to the declination of hardness, UTS, YS, and elongation, as shown in Figure 11b.

Zhang et al. [146] used the LMD to manufacture Ti + TiC MMCs with various pre-mixed ratios of TiC (10, 20 and 40 vol.%) The mechanical testing, including UTS, YS, elongation, hardness, and wear, were conducted. The results are presented in Figure 12. It can be observed that the UTS slightly deviates with the accumulation of TiC; meanwhile, YS declines quickly, which is caused by the existence of solid and stiff TiC particulates. The hardness and wear resistance of the produced MMCs raised with the increment in TiC volume fraction due to the strengthening effect of the particulates and the optimum bonding between TiC and Ti. Based on the excellent adhesion between TiC + Ti MMCs, such coatings can be deposited on the surfaces, where superior wear resistance is required. The LMD process inherits the high laser energy density; moreover, Ti showed an excellent affinity to the Oxygen + Nitrogen, which was absorbed quickly in the interstitials. The combination of the aforementioned facts leads to strength increment. Furthermore, a declination in the elasticity was observed in the MMCs.

Liu et al. [147] analyzed the creep performance of TA15-Ti + TiC particulates (10.8 vol.%) MMCs at 873 and 923 K, respectively, deposited by the LMD. The creep resistance of the MMCs improved notably due to the accumulation of TiC reinforcing particulates in comparison to the monolithic TA15-Ti alloy designed in between 723 to 773 K. The creep life for TA15-Ti + TiC MMCs at 873 and 923 K, is shown in Figure 13a. The rupture of the TiC + TA15 MMCs was originated due to the particles’ cracking, interfacial debonding and voiding, which becomes dominant with the temperature elevation. Jiang and Kovacevic [148] performed a study on the behavior of TiC + H13 tool steel MMCs, manufactured by LMD. The influence of TiC (vol.%) on erosion resistance was analyzed. Figure 13b shows the erosion rates of the coated layers at different impact angles. For all the coatings, it can be observed that the impact angle influenced the erosion rate considerably. For the 30° impact angle, the lowest erosion rate was observed, followed by a 90° impact angle. The maximum erosion was found at 45° and 60° angles. Moreover, TiC with 80 (vol.%) presented the least erosion resistance, while the coating with 40 (vol.%) showed the highest ones.

Lei et al. [149] used the LMD for Si_p + 6063Al MMCs for 5, 12, 20 and 30 (wt.%) Si contents. The thermophysical properties of MMCs were investigated. The results are shown in Figure 14. They found that with the increment in Si (wt.%), the thermal conductivity of the MMCs declined due to the decrease in an α-Al phase, which exhibits the high-level conductivity. The thermal expansion coefficient presented an opposite behavior as compared to the thermal conductivity.

MMCs have outstanding load-bearing and wear resistance properties, which make them interesting for implant applications [150]. TiN and SiC reinforced TMCs [151,152] have been proved for excellent biocompatibility. Hence, they are used for cladding on the metallic substrates to upgrade the biocompatibility of implants. In vitro testing was carried out to assess the biocompatibility of the deposited layers. The results showed an excellent cell to material interaction, and no toxicity was found. It showed that they could be used for load-bearing implants such as hip, knee and shoulder joints [151,152]. Moreover, orthopedic bone prototypes and meshed cranial prostheses implants were successfully developed for biomedical applications [57,153,154].

In MMCs, the ceramic reinforcements within the matrix play as the load-bearing element, which can confine the plastic distortion, thus, preventing the matrix from deterioration. It makes MMCs a potential candidate for wear-resistant applications [5,155]. The depositions of TiN reinforced MMCs on a Ni-Ti substrate by LMD, in a nitrogen atmosphere, increased the substrate’ wear resistance by a factor of two [156]. TiB + TiN MMCs were prepared on a Ti-substrate using pre-mixed boron nitride (BN) + Ti6Al4V powders by the LMD. With the increase in the BN content up to 15 (wt.%), the surface hardness increased from 543 to 877 HV, thereby, increasing the wear resistivity of the deposited layers [157]. The deposition of WC/W₂C + Ni MMCs on steel substrate was carried out via LMD. It was found that an inclination in the WC/W₂C content and a declination in carbide dimension, can enhance the wear resistance of MMCs up to 200 times more than the pure Ni matrix [158]. The dry friction, along with better contact load, is an increasing demand for aerospace and heavy industries. In simple words, only the high wear resistance is not sufficient. For this difficulty, WC/Co (ceramic reinforcement) + Cu-Sn (solid lubricant) were manufactured by the LMD process. The developed structures presented better wear resistance and low frictional characteristics [159].

MMCs are used to increase electrochemical corrosion and erosion resistance. For this purpose, Ni₂Si composites were deposited on a steel substrate by the LMD technique [160]. Immersion and anodic polarization tests were conducted, which proved the deposited layer intermediated better bio- and electrochemical corrosion resistance. The novel layers of TiC + Satellite 6, WC + Co, MoSi₂ + stellate 6, and MoSi₂ + steel matrix composites, were deposited on a metallic substrate to improve erosion wear rate [161].

LMD technology has recently been used to repair brake disks and turbo-engine parts [162]. In one of the recent studies, LMD was demonstrated as a potential candidate to repair steel dies using the Fe-Cr and Fe-Ni layers [163]. Moreover, various applications of the LMD have been determined in the fields of aeronautics and refractory [164]. Furthermore, LMD technique has been tested for the manufacturing of multiple parts, including turbine and compressor blades [165], nozzle guide vanes [166], jet engines [167], casting dies [168], Z-notches [169], bearing seats, valves, shafts, cylinders and rods [170], and seals [171].