Question: I’ve heard people use the term “liquidous” when they refer to a brazing filler metal melting and flowing. The words they used were: “When the filler metal goes liquidous it can join two surfaces.” What is the actual meaning of that word, and is that person using the term correctly?

Answer: No, the person you heard is using the word incorrectly, and also spelling it incorrectly, assuming you’ve also seen them spell the word as shown in your question. I, too, have heard people speak like that, and it is my opinion that they use that word incorrectly either out of ignorance or perhaps because they are just trying to sound “technical.”

The proper spelling for the word is “liquidus.” The term liquidus is NOT a synonym for the word liquid, but it is a specific temperature above which the filler metal (or any metal) is fully liquid (as determined by laboratory testing). It is a metallurgical term, which, along with its companion term “solidus,” helps metallurgists understand the melting and flowing characteristics of metal alloys.

The solidus temperature of an alloy is the temperature below which an alloy (or pure metal) is completely solid. During heating, the alloy will start to melt when you reach its solidus temperature. More and more of that alloy will melt as you continue to heat it higher and higher until it becomes completely liquid (under ideal conditions) when you cross the liquidus temperature for that alloy.

Therefore, the person who spoke to you should have said something like this:  “When the filler metal is heated to a temperature above its liquidus, it will become completely liquid, and that liquid filler metal can join two surfaces.”

Please remember once again that the words “solidus” and “liquidus” refer only to specific temperatures through which a metal will pass during its heating and do not represent a condition (or “state of being”) for that material.