Effect of NOCOLOK® flux dry-off temperature on brazed joint strength of aluminum-based heat exchangers

During the brazing of aluminum-based heat exchangers, the flux dry-off temperature plays a crucial role to get sound joints with maximum strength. In the present study, the NOCOLOK® flux consists of two phases of K2AlF5.H2O and KAlF4 with a melting point around 580 °C was used for brazing AA3003 as base metal with a clad-coating of AA4343 as filler metal. The slurry was applied on the joints and they dried at 220, 300 and 380 °C in air. The tensile shear test revealed that when the slurry dried at 300 °C, the joint withstand maximum shear stress of 44 MPa without defective features. At 220 °C and 380 °C, joint shear stresses were 34 MPa, 30 MPa respectively. However, drying at 380 °C under protective nitrogen gas enhanced the shear strength of up to 39 MPa. Having applied a change in current dryer temperature in the factory from 360 °C to 300 °C reduced the percentage of heat exchangers leakage from 3.2% to 0.6%, approximately, on a weekly basis.