Analysis of Brazing Imperfections by Metallographic Investigation / Optical Microscopy – Part 2 – Aluminium Brazing
In everyday practice we can encounter problems connected with brazing imperfections – often manifested as leaking. In order to identify the failure root causes, usually a detailed examination needs to be perform. Optical metallography is one of the most important tools for such an analysis.
The purpose of this article is to present the basic principles for performing analysis of brazing joints, and to show what information can be obtained from the investigation.
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The major fault in any brazed joint is porosity – or using other words: presence of voids. Their sizes can vary substantially from very small ones to much bigger ones. The fact is that for the CAB process there is no numerical criterion for determining the level of acceptable porosity. Case by case criteria are used for accepting the quality of joints, though the obvious standard is the so called leak test – to check if the part is tight.
When porosity is too large it can develop into a leak. Thus, identification of its origin is of great importance. As a first step it must be identified if the free surface of the porosity was wetted by the filler alloy. That can be determined by presence or absence of residual filler alloy clad layer on the joint surface. It is manifested by the presence of a white ribbon at the surface of porosity (see Fig. 5).
