Laser surface processing of Ti6Al4V in gaseous nitrogen: corrosion performance in physiological solution

Laser surface processing was carried out in gaseous nitrogen atmosphere at ambient temperature. The laser scan speed was varied (50-150 cm/min) at constant power of 1500 watts and resulting changes such as microstructural evolution, hardness, and electrochemical response of modified surface in Ringer's physiological solution at varying pH were studied. Increase in laser scanning speed was found to reduce the thickness of alloyed zone from 258 to 87 microm. The microstructure of laser-modified surface contains dendrites grown perpendicular to the laser traverse direction, beneath which basket weave structure of acicular alpha (martensite) was prevalent. Hardness at the top surface of laser-processed at 50 cm/min was approximately 1137 kg/mm2 that reduced with increase in the laser scan speed (577 kg/mm2 at 150 cm/min). Laser surface processing shifted the corrosion potential of Ti6Al4V towards noble side as compared to untreated alloy; the maximum shift by approximately 494 mV was recorded in pH approximately 9 solution. Passivation after laser surface modification was improved as currents were at least 1/3 of the untreated Ti6Al4V in passive region. While the pitting potential of untreated material was found to increase from 1.84 V for 4.0 pH to >2.5 V for 9.0 pH, the pitting potential after laser treatment was observed to drop from maximum of 74% for 4.0 pH (at 100 cm/min) to maximum of 42% for 9.0 pH (at 150 cm/min).