Nicrobraz Technical Articles Library

Brazing Range - The Range Between Solidus and Liquidus

Ever wonder what the commonly used terms solidus, liquidus, melting point and brazing range really mean, the difference between them and why these differences are important? Here, we will explain these terms and their relevance to brazing filler metals.

Melting Point vs. Melting Range The term melting point, as used by metallurgists, usually applies to pure metals that melt at a single temperature. However, brazing people sometimes use melting point to refer to the liquidus temperature of a brazing filler metal. This is not technically correct.

Most brazing filler metals are alloys, or combinations of pure elements or materials. Alloys have very different melting characteristics compared to their pure forms. Most alloys do not have a single melting temperature or melting point; instead they have a melting range. The upper and lower limits of this range are called the liquidus and solidus temperatures. Therefore, most metallurgists will say a pure metal has a melting point, while an alloy (brazing filler metal) has a melting range.

Solidus

During heating, solidus is that temperature at which an alloy begins to melt. Between the solidus and liquidus temperatures, the alloy will be a mixture of solid and liquid phases. Just above the solidus temperature, the mixture will be mostly solid with some liquid phases (like the consistency of snow, but hotter!). Just below the liquidus temperature, the mixture will be mostly liquid with some solid phases (like sleet).

Liquidus

Liquidus is the temperature above which an alloy is completely molten. In other words, when an alloy exceeds the liquidus temperature, there are no solid phases present. The alloy is completely melted and flowing. Eutectic Point A few special alloy compositions have a specific melting point like that of a pure metal, in other words the solidus and liquidus are the same temperature. Alloys of this type are called "eutectic compositions" and the melting point is known as a eutectic point. An example of a eutectic brazing filler metal composition is Nicrobraz10 (BNi-6). As the table on page 5 shows, this nickel-phosphorus filler metal has a solidus and liquidus both at 1610° F (875° C). Other examples of eutectic compositions are Nicrobraz 50 (BNi-7) and Nicrobraz 150 (BNi-9). Brazing Range In most cases, for a brazing filler metal to flow properly into a joint via capillary action, the filler metal must be completely molten. Thus, the liquidus is the lowest temperature that should be used for brazing, and usually temperatures above the liquidus are needed for proper brazing. The exact brazing range, however, will depend upon the properties of the brazing filler metal and base metals being joined, the brazing process, and joint design. When brazing joints with wide gaps, it is advantageous to braze with a more sluggish brazing filler metal that will not flow all over the joint. This is why, as shown on page 5, the low end of the brazing range for certain brazing filler metals is below the liquidus. For instance, Nicrobraz130 (BNi-3) has a solidus at 1800° F (980° C) and a liquidus at1900° F (1040° C), but a brazing range of 1850-2150° F (1010-1175° C). The brazing range actually begins 50° F below the liquidus. At the low end of its brazing range, Nicrobraz 130 will become fluid enough to flow into a wide joint gap, but not so fluid that it flows out of control. Other brazing filler metals with similar characteristics include Nicrobraz LC(BNi-1a) and Nicrobraz 160. Now that you know the difference between solidus and liquidus, melting point and melting range, and how they relate to the brazing process, you will be able to look at the table on page 5 with greater understanding. Basically, a brazing filler metal will begin to melt at its solidus and continue to melt until it reaches the liquidus where it is completely molten. In most cases, this is the minimum temperature at which brazing is done. To obtain good brazing filler metal flow and capillary action, a temperature somewhere within the brazing range must be maintained for a given period of time (time at temperature).

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