Microstructure and corrosion behavior of porous coatings on titanium alloy by vacuum-brazed method

The microstructural evolution and electrochemical characteristics of brazed porous-coated Ti-6Al-4V alloy were analyzed and compared with respect to the conventionally 1300°C sintering method. The titanium filler metal of low-melting-point (934°C) Ti-15Cu-15Ni was used to braze commercially pure (CP) titanium beads onto the substrate of Ti-6Al-4V alloy at 970°C for 2 and 8 h. Optical microscopy, scanning and transmission electron microscopy, and X-ray diffractometry (XRD) were used to characterize the microstructure and phase of the brazed metal; also, the potentiostat was used for corrosion study. Experimental results indicate that the bead/substrate contact interface of the 970°C brazed specimens show larger contact area and higher radius curvature in comparison with 1300°C sintering method. The microstructure of brazed specimens shows the Widmanstätten structure in the brazed zone and equiaxed α plus intergranular β in the Ti-6Al-4V substrate. The intermetallic Ti₂Ni phase existing in the prior filler metal diminishes, while the Ti₂Cu phase can be identified for the substrate at 970 for 2 h, but the latter phase decrease with time. In Hank's solution at 37°C, the corrosion rates of the 1300°C sintering and the 970°C brazed samples are similar at corrosion potential (E_corr) in potentiodynamic test, and the value of E_corr for the brazed sample is noble to the sintering samples. The current densities of the brazed specimens do not exceed 100 μA/cm² at 3.5 V (SCE). These results suggest that the vacuum-brazed method exhibits the potentiality to manufacture the porous-coated specimens for biomedical application.