Ultrasonic-assisted brazing of Al-Ti dissimilar alloy by a filler metal with a large semi-solid temperature range

The solidification temperature range of filler metal is normally narrow to avoid solidification cracks. In this paper, a new filler metal was designed that had large solidification temperature range between 517.7 and 179.2 °C. The large solidification temperature range reduced the residual thermal stress at the braze interface. The shear strength of the Al-Ti dissimilar braze joint increased from 45 MPa to 76 MPa when using the newly developed filler metal (namely ASn-II). The ASn-II filler metal was fabricated by adding 15 wt.% Sn to AlSiZnCuNi filler metal (namely A-I). During cooling, the Al, Si and CuAl₂ phases first solidify from the ASn-II filler metal between 517.7 and 487 °C. During further cooling, residual Sn-rich liquid exists and fills the gaps between the pre-solidified grains by capillary effects to form a network. The semi-solid state is maintained in the temperature range of 487-179.2 °C, during which no residual thermal stress occurs at the braze interface. The softness of the continuous Sn phases could also be beneficial for releasing the residual thermal stress by plastic deformation. Ultrasonic assistance made it possible to conduct the process in air and without using flux. The effect of Sn addition on Ti oxide film removal is also discussed.