Study on microstructure and strength of AgNi alloy/pure Al vacuum diffusion bonded joints

Diffusion bonding of AgNi alloy to pure Al was conducted at different temperatures of 500, 530, 560 and 600 °C. The former three temperatures characterize the bonding mechanism as diffusion bonding in a solid-state mode and the latter as the transient liquid phase (TLP) mode. The effects of these two modes on the microstructures and shear strengths of the joints were investigated. In the solid-state mode, diffusion layers were found on both sides of the faying interface, i.e., a layer of Ag2Al phase formed homogeneously on AgNi alloy side and a layer with Ag2Al phase precipitated from Al crystal boundaries and sub-boundaries on Al side. The raising of temperature promotes interface reactions between Ag and Al, hence increases the bonding strength. In the TLP mode, a layer comprising (Ag2Al + Al) phases was formed at the interface by the solidification of the eutectic liquid obtained via the contact-melting of Ag and Al elements at the bonding temperature. This layer is Al-rich and ductile, which can absorb stresses and avoid cracks. Such bonding mechanism ensures the better bonding quality. The shear strengths of the joints bonded at 530, 560 and 600 °C are 46.8, 57.3 and 76.6 MPa, respectively.