Investigation of titanium - hydroxyapatite composite surface modification and hydroxyapatite effect on Ti-6Al-4V biomedical alloys by tig welding method

In this study, Hydroxyapatite (HA) modified %0HA-%100Ti, 5%HA–95%Ti, and 10%HA–90%Ti composite coatings were applied onto the surface of Ti-6Al-4V (Ti64) alloy, which is of great importance for biomedical applications, using the TIG (Tungsten Inert Gas) welding method. The resulting coatings were systematically evaluated in terms of microstructure, mechanical properties, corrosion resistance, surface wettability, and bioactivity. The cross-sectional images obtained with the applied TIG welding parameters revealed a clear distinction between the base metal (Ti64), heat-affected zone (HAZ), and fusion zone. Optical microscopy and SEM analyses showed that increasing HA content led to a thicker HAZ and more pronounced acicular structures and microstructural irregularities in the fusion zone. In particular, 10%HA-containing coatings exhibited pores and microcracks on the surface, which were attributed to the ceramic nature of HA and differences in thermal conductivity. EDX analysis confirmed the presence of Ca and P elements in the fusion zones of HA-containing samples, indicating that HA was successfully incorporated into the metal matrix. XRD analysis revealed only the α-Ti phase in the pure Ti coating, while HA-specific crystalline phases were detected in the composite coatings. Mechanical testing indicated that increasing HA content caused a notable decrease in flexural strength and ductility; however, the 5%HA composition maintained mechanical integrity while providing biological advantages. Surface contact angle measurements showed that HA addition made the surfaces more hydrophilic, with the lowest contact angle (~58°) observed in the 10%HA coating. This suggests improved interaction with biological fluids, cell adhesion, and protein adsorption. Electrochemical test results indicated that the 10%HA coatings exhibited the highest corrosion resistance and the most stable oxide film behavior. In immersion tests performed in simulated body fluid (SBF), significant Ca-P deposition was observed on the 10%HA surfaces, supporting their high bioactivity potential. Overall, the results demonstrate that HA-reinforced Ti composite coatings produced via TIG welding are highly promising candidates for surface modification of biomedical implants, offering both structural and biological functionality.