Objective Titanium alloys have low density, high specific strength, and excellent corrosion resistance, thus they are commonly used in the engineering field to improve the mechanical properties of structural parts. However, their usage for various industrial applications is severely constrained owing to low hardness and poor wear properties. Several critical studies have been conducted regarding the surface modification of titanium alloys to solve their defects, including thermal spray, ion implantation, vapor deposition, and laser cladding, which have effectively improved their mechanical properties. In this work, a laser cladding composite coating was prepared on the Ti811 surface using the coaxial powder feeding method to improve the micro-hardness and wear resistance of the Ti811 alloy. The coating microstructure was characterized to study its phase compositions and elemental distribution. Moreover, the micro-hardness, friction, and wear properties of the coating were tested, and the CeO₂ dispersive strengthening mechanism in the coating was revealed. The research results here can provide a valuable experimental and analysis basis for studying the laser cladding of composite coatings on Ti811 alloys.

Methods The TruLaser Cell 7040 laser-processing center with a 4 kW laser at a 1030 nm wavelength was used for the laser cladding process. The coaxial powder feeding laser cladding technology was developed to synthesize the TiC/Ti₂Ni composite coating. The laser process parameters are as follows: laser power of 900 W, scanning speed of 400 mm/min, powder feeding speed of 5.0 g/min, protective gas flow of 11 L/min, powder feeding gas flow of 7 L/min, spot diameter of 3 mm, and working distance of 16 mm. The chemical compositions(mass fractions)of the materials used for laser cladding are Ni60(74%),TC4(20%),NiCr-Cr₃C₂(5%),and CeO₂(1%). A Ti811 (Ti-8Al-1Mo-1V) cuboid specimen of 40 mm× 30 mm × 8 mm was used as the substrate material. The chemical compositions (mass fractions) of the substrate are Al(8.1%), Mo(1.05%), V(0.99%), Fe(0.05%), C(0.03%), N(0.01%), O(0.06%), and Ti (Bal.). Before laser cladding, the working surfaces of the specimens were sandblasted to remove the oxide skin and cleaned in absolute ethanol using an ultrasonic cleaner for 20 min. The specimens were then cut off along the section of the coating via wire cut electrical discharge machining into a size of 15 mm×7 mm×8 mm as the metallographic samples. A mixture of HF, HNO₃, and H₂O was used as the etchant. The etching time was 10--25 s. The precipitation order of the phases was predicted using thermodynamic calculation. X-ray diffraction, scanning electron microscope (SEM), field emission electron probe (EPMA), Vickers hardness, friction, and wear testers were used to analyze and study the phase compositions, microstructures, elemental distributions, micro-hardness, and wear resistance of the coating.

Results and Discussions The composition analysis showed that the final phase mainly consisted of ceramic-reinforced phase TiC and TiB₂, intermetallic compound Ti₂Ni, and α-Ti solid solution. The thermodynamic calculation revealed a precipitation order of TiB₂→TiC→Ti₂Ni→α-Ti (Fig. 4). The SEM analysis and the EPMA results showed that TiC had a petal shape, TiB₂ had a long rod shape, Ti₂Ni had a strip shape, and nano-CeO₂ was dispersed in the coating (Figs. 5 and 7). The micro-hardness improvement was mainly attributed to the effect of fine grain strengthening, dispersion strengthening, and solution strengthening. The highest micro-hardness of the coating was 902 HV, 2.37 times that of the Ti811 substrate (Fig. 8). The wear volume of the coating was reduced by 27.9% compared with that of the substrate. The friction coefficient was stable between 0.38 and 0.42. The wear mechanism was mainly adhesive and slight abrasive wear with an excellent wear resistance (Figs. 9 and 10).

Conclusions In this study, an in-situ TiC/Ti₂Ni composite coating was prepared on Ti811 alloy using the coaxial powder feeding laser cladding technology. The final phase of the coating mainly consists of ceramic-reinforced TiC and TiB₂, intermetallic compound Ti₂Ni, and α-Ti solid solution. The precipitation order of this phase was TiB₂→TiC→Ti₂Ni→α-Ti. The improvement in the coating micro-hardness was mainly attributed to the dispersion strengthening effects of TiC, TiB₂, Ti₂Ni, and nano-CeO₂, and the solid solution strengthening effect of α-Ti. The highest coating micro-hardness was 902 HV, which is approximately 2.37 times that of the Ti811 alloy. The wear volume of the coating was approximately 27.9% less than that of the substrate. The friction coefficient of the coating was stable between 0.38 and 0.42. The wear mechanism of the coating was dominated by adhesive wear and slight abrasive wear with excellent wear resistance.