Influence of Brazing Time on Liquid Migration and Cell Structure of Porous Aluminum

Abstract

The aim of this study was to clarify the structural evolution of porous aluminum during brazing with an Al–Si-based alloy brazing sheet. High-quality metallurgical bonds between porous aluminum and solid aluminum sheets are required for effective thermal management applications. The authors have proposed a new brazing method using an Al–Si-based alloy brazing sheet that can control the liquid metal for the bonding in smaller amounts. Although the metallurgical bond was realized without filling the pores, the new brazing method influenced the cell structure because of liquid migration. The liquid migration and cell structure evolution in sintered dense aluminum and porous aluminum were experimentally investigated as a function of brazing time. The experimental results indicated that the liquid migration distance in both the dense and porous aluminum obeyed a parabolic law. The velocity of liquid migration in the porous aluminum was higher than that in the sintered dense aluminum, attributed to the high cell wall surface area in the porous aluminum. During liquid migration, the liquid filled former powder boundaries before and after grain coarsening occurred. Therefore, grain coarsening was attributed to liquid film migration, which directly affected evolution of the cell structure. The grain diameter was linearly related to the porosity, where extrapolation of this relationship showed that the grain diameter was equal to the diameter of the raw aluminum powder when the porosity was equal to the initial porosity.

Local porosity (P) at X = 0.5 mm for various times as a function of the grain diameter near the interface (G).