Analysis of Brazing Imperfections by Metallographic Investigation / Optical Microscopy – Part 1 – Aluminium Brazing
Fig 3: Examples of structural brazing phenomena
a) Silicon diffusion
b) Erosion
c) Liquid Film Migration
Silicon diffusion is a phenomenon where silicon diffuses from static liquid filler into the base metal and aluminum diffuses into the liquid filler – resulting in partial melting of the base metal at brazing temperature. The extent of this phenomenon is enhanced by too high brazing temperature and/or too long time at brazing temperature.
The phenomenon of diffusion sometimes can develop into so-called erosion. In such cases the liquid part of the base metal had flown away – leaving a gutter behind. That can potentially happen at the end of a brazing cycle, when significant temperature gradients develop during cooling. It should be noticed that this phenomenon, which results in thinning out of for example the tube wall cross section, is very dangerous for part performance, because the thinner tube wall could be a potential place for fatigue and/or corrosion failure.
The Liquid Film Migration [LFM] physically is an entirely different phenomenon than the two ones discussed above. During brazing, there are always some interactions between the filler alloy and the base alloy. They take place before and after melting of the filler alloy. However, in some situations they have a very specific character. It is a phenomenon of rapid silicon diffusion into the matrix alloy. It starts at temperatures below the brazing window. During that process a moving liquid interface is created, which sweeps from the clad/core interface into the core of the material.
The degree of LFM depends on cold deformation induced to the base metal before brazing and also strongly depends on the alloy type of the part. Since this process diminishes the amount of liquid available to form joints – and it can also destroy a sacrificial layer in the base alloy (e.g. used for corrosion protection) – it should be avoided in the brazing practice. It is worthwhile pointing out that minimization of LFM can be achieved by proper choice of the base material and its temper, thus for a given part it must be done at designing stage.
