الفهرس 
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Abstract
Titanium alloys are widely used in aerospace, marine and chemical industries owing to their intrinsic properties such as high specific strength, excellent corrosion and oxidation resistance. However, because of their low hardness and poor tribological properties, the application of titanium alloys is severely constrained under severe wear and friction conditions. Laser deposition is a promising technique that is widely used to enhance the surface properties of many kinds of metals. The TC21 alloy (Ti-6Al-3Mo-1.9Nb-2.2Sn-2.2Zr-1.5Cr) is considered a new (α/β) titanium alloy that replaced the commercial Ti-6Al-4V alloy in aerospace applications due to its higher operating temperatures. Recently, direct energy deposition was usually applied to enhance the hardness, tribological properties, and corrosion resistance for many alloys. Consequently, this study was performed by utilizing direct energy deposition (DED) on TC21 (α/β) titanium alloy to improve their tribological properties by depositing a mixture powder of stellite-6 (Co-based alloy) and tungsten carbides particles (WC). Different WC percentages were applied to the surfaces of TC21 using a 4 kW continuous-wave fiber-coupled diode laser at a constant powder feeding rate. This study aimed to obtain a uniform distribution of hard surfaces containing undissolved WC particles that were dispersed in a Co-based alloy matrix to enhance the wear resistance of such alloys. Scanning electron microscopy, energy dispersive X-ray analysis (EDAX), and X-ray diffractometry (XRD) were used to characterize the deposited layers. New constituents and intermetallic compounds were found in the deposited layers. The microhardness was measured for all deposited layers and wear resistance was evaluated at room temperature using a dry sliding ball during a ring abrasion test. The results showed that the microstructure of the deposited layer consisted of a hypereutectic structure and undissolved tungsten carbide dispersed in the matrix of the Co-based alloy that depended on the WC weight fraction. The microhardness values increased with increasing WC weight fraction in the deposited powder by more than threefold as compared with the as-cast samples. A notable enhancement of wear resistance of the deposited layers was thus achieved by values reaching 116 times. After reaching the optimum powder composition (40٪ stellite-6 and 60٪ WC) in the firstsection, various powder feeding rates (40, 60, 80, and 100 g/min) were applied in order to obtain higher deposited layer thickness with lower dilution rate from the substrate. The results showed a heterogeneous distribution of WC particle in the deposited layers during the DED, especially at 40, 60 and 80 g/min.The dilution ratio decreased from 23٪ to 5.3٪ with the substrate. Microcracks appeared around the fusion line due to generated residual thermal stresses caused by the difference in the coefficient of the thermal expansion between WC particles and Stellite-6 MMC. Finally at the highest powder feeding rate of 100 g/min, insufficient metallurgical bonding was obtained due to the low laser energy delivered to the substrate to melt it.