الفهرس 
Development of Laser Surface Treatment of (?+?) Titanium Alloys
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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 first
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section, 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