Increasing Ti–6Al–4V brazed joint strength equal to the base metal by Ti and Zr amorphous filler alloys

Abstract

Microstructural features developed along with mechanical properties in furnace brazing of Ti–6Al–4V alloy using STEMET 1228 (Ti–26.8Zr–13Ni–13.9Cu, wt.%) and STEMET 1406 (Zr–9.7Ti–12.4Ni–11.2Cu, wt.%) amorphous filler alloys. Brazing temperatures employed were 900–950 °C for the titanium-based filler and 900–990 °C for the zirconium-based filler alloys, respectively. The brazing time durations were 600, 1200 and 1800 s. The brazed joints were evaluated by ultrasonic test, and their microstructures and phase constitutions analyzed by metallography, scanning electron microscopy and X-ray diffraction analysis. Since microstructural evolution across the furnace brazed joints primarily depends on their alloying elements such as Cu, Ni and Zr along the joint. Accordingly, existence of Zr2Cu, Ti2Cu and (Ti,Zr)2Ni intermetallic compounds was identified in the brazed joints. The chemical composition of segregation region in the center of brazed joints was identical to virgin filler alloy content which greatly deteriorated the shear strength of the joints. Adequate brazing time (1800 s) and/or temperature (950 °C for Ti-based and 990 °C for Zr-based) resulted in an acicular Widmanstätten microstructure throughout the entire joint section due to eutectoid reaction. This microstructure increased the shear strength of the brazed joints up to the Ti–6Al–4V tensile strength level. Consequently, Ti–6Al–4V can be furnace brazed by Ti and Zr base foils produced excellent joint strengths.