Effects of Cooling Rate and Silicon Content on Microstructure and Mechanical Properties of Laser Deposited Ti-6Al-4V Alloy

Abstract

The growth in the use of titanium alloy components in marine industries, aerospace, medical apparatus and power machinery has increased because of its good properties. Some detriments such as low hardness, poor thermal stability and poor tribological properties has however limits its wide range application to the industries. This has led to a search for techniques to improve the surface of Ti-6Al-4V alloy to enhance longevity in service. Laser deposition is one of the feasible techniques to modify the surface of titanium alloy which involves addition of reinforcements to improve properties without changing the bulk properties while optimizing the parameters.Hence, laser surface modification by incorporating chemical barrier coatings can be very beneficial and this lead to investigation aimed at enhancing the surface properties of Ti-6Al-4V alloy by incorporating Al-Si-Ti coatings. For this purpose, a 3-kW continuous wave ytterbium laser system attached to a KUKA robot which controls the movement during the alloying process was utilized to deposit coatings with stoichiometry Al-12Si-3Ti and Al-17Si-5Ti. The alloyed surfaces were investigated in terms of its mechanical properties as function of the laser processing conditions. Hardness measurements were done using a Vickers micro-hardness tester. The microstructures of the coated and uncoated samples were characterized by optical and scanning electron microscopy. The optimum performances were obtained for an alloy composition of Al-17Si-5Ti, at laser power of 900 W and coating speed of 1.2 m/min. Its performance enhancement compared to the unprotected substrate comprised a significant increase in hardness from 296 to 689 HV which translates to 57.04% in hardness values above that of the substrate.The enhanced increase in hardness of the coating can be traced to the presence of silicon and the various hard titanium aluminides intermetallic (TiAl, TiAl3, and Ti3Al) phasese.