Impact of Laser Marking on Microstructure and Fatigue Life of Medical Grade Titanium

Medical devices are required to feature a unique device identification marking to enable traceability. These identification tags are extensively applied directly onto metallic devices. Universally, the technology utilized to apply such markings is laser marking, where the device is heated locally in atmospheric air to form a dark oxide layer and generate contrast. Despite the wide application, the consequences of laser marking on materials performance are largely unknown. In this study, the effect of laser marking on the most common titanium alloys in the medical industry, i.e., commercially pure titanium and Ti6Al4V, is investigated. The morphology and composition of the generated oxide layer were investigated, along with a characterization of the heat affected zone. Additionally, the effect on fatigue strength is investigated in relation to the application of laser-marked titanium in a dental implant. Morphologically the laser-marked titanium surfaces exhibited signs of melting on the very surface and the generation of an abundance of cracks which formed upon cooling. Scanning electron microscopy shows that these cracks are perpendicular to the surface and reach deep into the HAZ. Evidence of substantial oxygen ingress was demonstrated: various titanium-based oxides were identified via X-ray diffraction, as well as oxygen stabilized α-phase. The fatigue analysis showed a severe reduction in endurance limit for all investigated parameters and materials of up to 80%. This was attributed the notch effect resulting from crack formation during laser marking. The fractured surfaces were investigated and showed a clear crack propagation from the entire laser-marked surface.