PHASE CONSTITUENTS AND MICROHARDNESS OF LASER ALLOYED Ti-6Al-4V ALLOY

摘要

Ti-6Al-4V alloy possesses excellent mechanical and chemical properties which make it a favourite material for the automobile, aerospace and aeronautical industries, but the alloy has low hardenability and poor wear resistance. This is because of its low resistance to plastic shearing, low work hardening and the low protection offered by its surface oxide. This study was designed to modify the microstructure and enhance the microhardness of the two-phase Ti-6Al-4V alloy. The alloy was laser coated with a premixed ratio of Mo+Zr+Stellite 6 using 4.4 kW continuous wave (CW) Rofin Sinar Nd:YAG laser processing system fitted with an off-axis nozzle for powder feeding. Optical and scanning electron microscopes were used to study the microstructural evolution in the laser coatings, while phase constituents were identified and studied by X-ray diffractometer. A through-thickness hardness indentation was measured using Vickers hardness tester. New intermetallic compounds and alloy phases were precipitated which confirms metallurgical reaction between the substrate and the powder mix. The β-phase of the two-phase titanium alloy was retained. This is attributed to the presence of Mo, which is a beta phase stabilizer, in the powder mix. There was considerable increase in the Vickers hardness from 357.6 HV_(0.1) in the native alloy to 1145.2 HV_(0.1) in the composite coating.