Ultrasonic-assisted brazing of sapphire with high strength Al-4.5Cu-1.5Mg alloy

The method for joining alumina using high strength aluminum alloys as a filler metal has been desired for years because of the very limited strength of the joints using pure aluminum in the current methods. Here we show a novel ultrasonic-assisted brazing method for making sapphire (monocrystalline α-alumina) joints using a high strength aluminum alloy of Al-4.5Cu-1.5Mg as a filler metal. The bonding between the sapphire alloy and the Al-4.5Cu-1.5Mg was realized through an interfacial oxidation of metallic elements, resulting in formation of MgAl₂O₄ at the interface. Ultrasonic wave was used to promote the interfacial oxidation during brazing and an annealing heat treatment was used during cooling. Nominal shear strength of the joints made with Al-4.5Cu-1.5Mg was ∼125 MPa, 2.7 times higher than that made with pure Al. Finite element method (FEM) was used to study the difference on fracture behaviors of the joints made with the Al-4.5Cu-1.5Mg alloy and pure aluminum. The actual bonding shear strength at the sapphire/Al-4.5Cu-1.5Mg interface was estimated as high as 230 MPa. Thus, a new technical route for joining ceramics with aluminum alloys is proposed, which is hopefully an alternative of conventional active brazing.