Liquation of Brazing Filler Metals in Brazing (What it is, and how to deal with it

When a brazing filler metal (BFM) is melted during a brazing process, it is not uncommon for "liquation" to occur.  Liquation, in brazing, is defined as the tendency of the lower-melting constituents of a BFM to separate out and flow away (by capillary action) from the higher-melting constituents of the BFM during heating, sometimes leaving behind a non-melted "skull" of alloy at the point where the BFM was applied.  Liquation is usually apparent in BFMs having a wide melting range, i.e., having a large difference between the solidus (See explanatory Note 1 at end of this article) and liquidus (See Note 1) temperatures of the BFM in question, and occurs when the BFM is heated slowly through that melting range (such as when furnace brazing).  Liquation is not typically encountered when rapid brazing techniques are used, such as when flame-brazing or induction brazing.