Microstructure and strength of brazed joints of Ti3Al base alloy with AgCuZn filler metal

Abstract

Brazing Ti3Al alloys with filler metal Ag–Cu34–Zn16 was carried out at 1073–1173 K for 60–1200 s. The relationship between brazing parameters and shear strength of the joints was investigated, and optimum brazing parameters obtained as follows: brazing temperature 1073–1083 K, brazing time 250–300 s. The maximum shear strength obtained for joints was 160–170 MPa. The reaction products and the interface structure of the joints were investigated by SEM, EPMA, and XRD. Three kinds of reaction product were observed to have formed during the brazing of Ti3Al alloys with Ag–Cu34–Zn16 filler metal: namely Ti(Cu, Al)2+TiCu intermetallic compounds formed close to the Ti3Al alloy, and Ag solid solution formed in the middle of the joint. The interfacial structure of brazed Ti3Al alloy joints with Ag–Cu34–Zn16 filler metal was Ti3Al/Ti(Cu, Al)2+TiCu/Ag solid solution/Ti(Cu, Al)2+TiCu /Ti3Al, and this structure does not change with brazing time once it forms. The formation of intermetallic compounds Ti(Cu, Al)2+TiCu results in embrittlement of the joint and poor joint properties. The thickness of Ti(Cu, Al)2+TiCu intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K0 of the Ti(Cu, Al)2+TiCu reaction layer in the brazed joints of Ti3 Al alloys with Ag–Cu34–Zn16 filler metal were 170 kJ mol–1 and 2·31 × 10−5 m2 s−1 respectively. The growth formula was y2=2·13 × 10−5exp(−20447.44/T)t. Careful control of the growth of the reaction layer Ti(Cu, Al)2+TiCu can influence the final joint strength.