High speed laser surface modification of Ti–6Al–4V

Titanium and its alloys have been commonly used for biomedical implant applications for many years; however, associated high coefficient of friction, wear characteristics and low hardness have limited their long term performance. This article investigates the effects of the high speed laser surface modification of Ti–6Al–4V on the microstructure, surface roughness, meltpool depth, phase transformation, residual strain, microhardness, and chemical composition. Laser treatment was carried out using a 1.5 kW CO2 laser in an argon gas environment. Irradiance and residence time were varied between 15.7 to 26.7 kW/mm2 and 1.08 to 2.16 ms respectively. Laser treatment resulted in a 20 to 50 μm thick surface modified layer without cracks. An increase in residence time and irradiance resulted in higher depth of processing. Surface roughness was found to decrease with increase in both irradiance and residence time. Metallography showed that a martensite structure formed on the laser treated region producing acicular α-Ti nested within the aged β matrix. The laser treatment reduced volume percentage of β-Ti as compared to the non-treated surface. Lattice stains in the range of 0.81% to 0.91% were observed after laser surface modification. A significant increase in microhardness was recorded for all laser treated samples. Microhardness increased up to 760 HV0.05 which represented a 67% increase compared to the bulk material. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that laser surface modification produced a more homogenous chemical composition of the alloying elements compared to the untreated bulk metal.