Development of novel CsF–RbF–AlF3 flux for brazing aluminum to stainless steel with Zn–Al filler metal

• A series of Zn–xAl filler metals were developed to braze 6061 Al and 304 stainless steel.
• The oxide film on the base metal could be effectively removed by the CsF–RbF–AlF3 flux.
• A shear strength level of 131 MPa was obtained by Zn–15Al filler metal.
• The thickness of the intermetallic layer had effects on the mechanical properties of the joint.

Brazing 6061 Al alloy to 304 stainless steel by flame brazing has been carried out with an improved CsF–RbF–AlF3 flux which matched Zn–xAl filler metals. The results showed that, the spreading area on stainless steel of Zn–xAl filler metals has been improved with the addition of RbF to CsF–AlF3 flux. It is found that a Zn-rich phase appeared between the brazing seam and the intermetallic compound (IMC) layer in the joints brazed with Zn–2Al and Zn–5Al filler metals, and the thickness of the IMC layer was approximately 1.76–6.45 μm which increased with the increase of Al added to the filler metals. Moreover, a Fe4Al13 phase formed in the IMC layer, while a Fe2Al5 phase appeared as the second layer in Zn–25Al brazed joint. Neither the Zn-rich phase nor Fe2Al5 phase was found in the joint brazed with Zn–15Al filler metal, so that the joint was exhibited the maximum shear strength which was up to 131 MPa. All the lap joints were fractured at the interfacial layer of the brazing seam and stainless steel.