MODELING OF BASE-METAL DISSOLUTION BEHAVIOR DURING TRANSIENT LIQUID-PHASE BRAZING

Detailed finite difference modeling of dissolution during transient liquid-phase (TLP) brazing of nickel using Ni-11 wt pct P and Ni-14 wt pct Cr-10 wt pct P filler metals is carried out assuming infinite and finite heating rates between the filler melting temperature (T(m)) and the brazing temperature (T(b)). Two conditions are modeled during brazing, namely, where the liquid width due to base metal dissolution is unrestrained (variable) and where the liquid width is kept constant due to expulsion of excess liquid during the brazing process. The time required for completion of dissolution is proportional to the square of the filler metal thickness when an infinite heating rate between T(m) and T(b) is assumed. This occurs for both for constant and unrestrained liquid width situations. Also, the advance of the solid-liquid interface is approximately related to the reciprocal of the square root of the solute diffusivity in the liquid, except in the final stages of the dissolution process. When a multicomponent filler metal composition (Ni-14 wt pct Cr-10 wt pct P) is used, the time for completion of dissolution exceeds that when using a simple binary filler metal composition. The phosphorus concentration in the liquid approaches that of the liquidus line when the heating rate between T(m) and T(b) decreases and thin filler metals are used. This tendency can be estimated using a homogenizing index (z) which combines the effects of filler metal thickness, diffusivity in the liquid, brazing temperature and heating rate, and a distribution index (a). When thick filler metals are used, lower heating rates produce more dissolution at temperatures between T(m) and T(b) and less dissolution at the final brazing temperature. Conversely, heating slowly when using thin filler metals produces less base metal dissolution (at any temperature) and negligible dissolution at the final brazing temperature. At very low heating rates, solidification can occur while heating from T(m) to T(b).