Coating of ti - HA composites on Tİ-6AL-4V biomedical alloy surfaces with tig welding method and investigating the welding current effect

Within the scope of the thesis, composite coatings with 5% Hydroxyapatite (HA) were applied to the Ti-6Al-4V (Ti64) alloy using the Tungsten Inert Gas (TIG) welding method, and evaluated for biomedical applications at welding currents of 90A, 100A, and 110A. In the macro cross-section images, a narrow and symmetrical weld zone was observed at 90A, while at 100A, a homogeneous melt and strong bonding with the base metal were achieved. In 110A, however, due to high heat input, a wide weld zone, irregular phase distribution, and micro-cracks have formed. EDX and XRD analyses have shown that in 90A, the chemical and crystal structure of HA is preserved, and the Ca/P ratio is consistent with theoretical HA. Limited HA integration was observed in 100A, while phase separation and amorphization were noted in 110A. In three-point bending tests, 110A exhibited the highest bending force (2.26 kN) and demonstrated balanced ductile behavior. 90A (1.43 kN) exhibited limited coating integrity, while 100A (2.12 kN) showed brittle fracture. In the examination of fracture surfaces, 110A exhibited strong matrix adhesion and ductile-mixed fracture, while 100A showed brittle fracture and cracks. In the wettability tests of the coating surfaces, 100A provided an ideal surface for osteoblast adhesion by offering high hydrophilicity with the lowest contact angle. 90A and 110A were interpreted to show lower wettability due to uneven HA distribution and micro-cracks, respectively. In electrochemical in vitro corrosion tests, both OCP and EIS tests showed that 110A exhibited the highest corrosion resistance (wide Nyquist curve, positive OCP), while 100A provided balanced electrochemical stability. In the PDP curves, it showed the lowest corrosion current density (6.93E-03 μA·cm-2) and high polarization resistance (12800 Ω) at 100A. In SBF immersion tests, 100A exhibited high osteoinductive activity with homogeneous and dense apatite deposits, while limited Ca-P formations were observed at 90A and sparse Ca-P formations at 110A. In conclusion, the 100A welding current has been determined to be the most suitable parameter for biomedical implants due to its homogeneous microstructure, chemical stability, high mechanical strength, biological activity, and adequate corrosion resistance. Although 110A provides an advantage in corrosion resistance, it is thought that 90A has low biological performance due to HA degradation and cracks, and offers insufficient integration with low energy. As a result of the study, it has been revealed that biocompatible coatings can be successfully produced on various surfaces using the TIG welding method.