Evaluation of the microstructure and microhardness of laser-fabricated titanium aluminate coatings

Abstract

Titanium aluminide intermetallics are very brittle at room temperature, hence they are challenging to fabricate even by conventional manufacturing techniques such as casting and forging. The production of TiAl from elemental powders using industrial processes is also challenging; in particular regarding the aspects of microstructural tailoring for improved high-temperature performance and oxidation resistance. To circumvent the difficulties, pre-alloyed TiAl powders are used to make industrial components. Electron beam melting was used successfully to produce miniature turbine blades from pre-alloyed TiAl powders. The innovative aspect of this work lies in process development for the fabrication of TiAl materials from elemental powders of titanium and aluminum. The laser metal deposition technique was used to produce TiAl coatings on titanium alloy substrates using a 3 kW laser system. The effects of laser power on the resulting TiAl microstructure were investigated. Scanning electron microscopy (SEM) analysis showed the inclusion of colonies of unmelted TiAl particles and surface pores and cracks. SEM images also revealed that an increase in laser power leads to a microstructural transformation from lamellar to dendritic. The overall hardness of the coating is also a function of laser power. The EDS mapping and hardness measurements confirmed that all the coatings were TiAl phase.