Comparative Study on Microstructure and Strength of TiAl/Steel Joints Brazed with Ag-Cu-Ti and Ag-Cu-Zn Filler Metals

Abstract

Vacuum brazing of a TiAl-based alloy to a medium-carbon steel has been carried out at 1173 K for 2-40 min using Ag-Cu-Ti and Ag-Cu-Zn filler metals. The formation phases and interface structure or microstructure of the joints were investigated by SEM, EPMA and XRD, and the strength of the joints was determined by the shear testing method. The experimental results indicate that (1) Ti(Cu,Al)2, Ag(s.s.), Ag-Cu eutectic and TiC phases have occurred in the TiAl/steel joints, and their amount changes with the brazing time; (2) the interface structures of the joints brazed with the Ag-Cu-Ti filler metal is TiAl/Ti(Cu,Al)2+ Ag(s.s.)/Ag(s.s.)+Ti(Cu,Al)2/TiC/steel, while the microstructure of the joints brazed with the Ag-Cu-Zn filler metal is composed of a continuous TiC layer and a mixed zone of Ti(Cu,Al)2, Ag(s.s.) and Ag-Cu eutectic; (3) the shear strength of the joints brazed with the Ag-Cu-Ti filler metal monotonously decreases with increasing brazing time, while the shear strength of the joints brazed with the Ag-Cu-Zn filler metal first rises and then decreases, accordingly an optimum brazing time exists; (4) the maximum shear strengths of the TiAl/steel joints brazed with the Ag-Cu-Ti and Ag-Cu-Zn filler metals are 170 and 190 MPa, respectively.