In Situ Measurement of the Thermal Contact Resistance of an Al Lap Joint During Braze Processing

A technique was developed, using a laser flash thermal diffusivity apparatus, to measure the thermal contact resistance, R _c, of an Al lap joint during braze processing. The method required a determination of the temperature dependence of the thermal diffusivity of the individual Al braze sheets, as well as a two-layer lap joint made from these sheets, under identical conditions. A two-layer modeling method was then used to analyze the above data to determine R _c for the lap joint as a function of processing temperature. This in situ analysis of R _c for a developing joint during brazing was able to distinguish both solid-state and liquid-phase reactions occurring at the faying surfaces of the joint. Consequently, it represents a powerful, new experimental tool that can be used to investigate the mechanisms of braze joint formation and the thermal properties of a joint as a function of processing condition. In the particular case of this study, the technique demonstrates that the use of a Ni-based fluxless brazing process to join aluminum (Al) sheets, reduced the R _c of the lap joint by 100-folds from 1.3 × 10⁻⁴ m² K/W before brazing to 1.35 × 10⁻⁶ m² K/W after brazing.