Phase Diagrams 101
Consider the phase diagram (constitution diagram) for the silver-copper alloy system (Fig. 5). The symbol for silver is “Ag” and for copper is “Cu,” both being abbreviations of the Latin words for each of those two metals.
Since there are only two metals involved in this chart, it is called a binary phase diagram. Notice that the line across the center of the chart is labeled “solidus” and occurs at a certain temperature (as shown along the left-hand vertical axis). Within the regions between the solidus and liquidus lines on the chart there is a so-called “mixture” of both solids and liquids present when the BFM is heated above its melting point (i.e. above its solidus). I call these regions “slush zones” since it describes the nature of what’s going on in those regions. It is this slush zone that gives rise to the liquation phenomenon.
Let’s look at what happens when a wide-melt-range composition in the Ag-Cu system is cooled, as represented by the vertical line in Figure 5. At point A on the vertical line representing the 30% Ag, 70% Cu alloy composition, the alloy is completely liquid. As it is cooled and the temperature crosses the liquidus line, this alloy composition will begin to solidify. The highest melting constituent of that alloy will be the first to start to solidify, and as cooling continues down the vertical line, more and more of the alloy will solidify.
Lever Law for Phase Diagrams
Metallurgically speaking, we can now introduce a useful concept known as the “lever law.” Using this, at any given point vertically along the line (or any similar vertical line at any chemistry), we can determine the actual chemistry of the constituents that are solidifying and coming out of solution as well as the chemistry of the liquid that remains in the joint. The lever law can also be used to determine percentages of phases.
As cooling continues down along the line to point B, the actual chemistry of the phases that are coming out of solution and solidifying will be as shown at point B2 at the far right side of the horizontal line (lever-arm) drawn through point B (a vertical dotted line from point B2, intersecting the bottom axis, tells us what that chemistry is). At the left end of the horizontal lever-arm through point-B, i.e. where it intersects the slope of the liquidus line (at point B1), is shown the chemistry of the liquid portion of point B that remains as a liquid in the capillary space in the joint being brazed. The same is true for points C and D as well.
Notice that as the cooling continues down the vertical line, the chemistry of the liquid remaining in the joint is continually changing and follows the slope of the liquidus line. Thus, the last chemistry to exist as a liquid in the joint just prior to complete solidification is the eutectic chemistry, and it is the last component to freeze upon cooling the BFM from brazing temperature.
Now let’s reverse our thinking and start from a solid BFM in a brazed joint to see what happens as we heat that alloy up to the brazing temperature. We will use the same chart (Fig. 5) and the same lever-law principles but in reverse. Again, let’s use a BFM composition represented by the same vertical line and see what happens as we heat it up to brazing temperature.
As soon as the BFM temperature exceeds the solidus temperature, it will start to melt, and the chemistry of the first liquid to form should then be – according to the lever law – the eutectic chemistry. This first liquid formed (the low-melting constituent mentioned in the definition of liquation) will flow out from the BFM by capillary action (either into the joint if the joint is up to proper temperature or over the outside surfaces of the components being brazed, whichever is hotter). The rest of the melting process for the BFM can be studied along the lines (in reverse) of what has been described in the previous paragraphs.
But the key point here is that the first liquid to flow into the joint being brazed is the eutectic composition. From Fig. 3(a), we see that the flow of eutectic, or eutectic-like, BFM compositions is smooth and uniform. People who use eutectic BFMs generally find them to be highly desirable for most brazing applications, producing joints of high quality in which the BFM penetrates easily the entire joint, flows freely and is usually highly compatible with the base metals being joined.
Look again at Figs. 1 and 2. The liquation is readily apparent, but we know the joints can be trusted. It’s now understood that the eutectic composition of the BFM system being used is what flowed into the joint first, and the assembly is probably fully acceptable based on the required strength or hermeticity (leak-tightness). The overall joint may not look very pretty if some liquation is present, but the assembly should perform fully up to any specification requirements if there is visual evidence that all the joints look fully brazed with no open voids, etc.
When Should I Reject Brazed Assemblies Exhibiting Liquation?
It is not uncommon for some people to scrap assemblies that exhibit liquation for the simple fact that liquation is present since such assemblies (in their opinion) “can’t be any good.” Such rejections can be very wasteful and could needlessly scrap parts that might perform perfectly well in service.
The question that should really be asked is: “What will any liquation residues do to the parts in service from a performance point of view?” If there will be no negative impact on the performance (such as strength, leak-tightness, etc.), then such parts should be put into service.
Liquation would need to be reworked to remove the surface lumpiness (by grinding, etc.) prior to being placed in service if:
- Smooth flow of air across a brazed surface is affected (such as in airfoils)
- Turbulence is created in the flow of liquids across surfaces or through channels where such flow is supposed to be smooth
- Germs are entrapped or fluids contaminated in the medical or food industries
Aesthetics of brazed components is another important aspect of liquation, such as in the jewelry business, where brazing is a common joining method. No person would be happy to receive a brazed piece of jewelry exhibiting poor brazing with lots of liquation on it just because it was available “for a bargain price.” No, there are times when “perfect brazing” is required by the end user, and liquation will not be tolerated.
Remember, the acceptability or unacceptability of liquation should always be based on its impact on end-use service conditions (such as airflow, fluid flows, medical concerns or aesthetics required by end user). It should never be based on the following false assumptions:
- Liquation will cause the parts to be weak – False
- Liquation will cause the parts to leak – False
- Liquation means the parts were brazed incorrectly – False
- Liquation indicates that the BFM is of poor quality – False
Yes, liquation can happen when you slowly heat a wide melting-range BFM in a furnace, when the BFM is placed on the outside of the joint being brazed. This article has explained what liquation is, what causes it and how to minimize or eliminate it. Now it’s your turn. How will you use this new understanding of liquation in your own brazing environment?
Note 1. The solidus temperature is the temperature at which a solid material will begin to melt when it is being heated. We often call it the melting point for that material. The liquidus temperature is the temperature at which a liquid metal will start to solidify when it is being cooled down from the molten state. Thus, it is the lowest temperature at which that BFM will be completely liquid. In brazing, the assumption is often made (incorrectly) that the liquidus temperature is reached when the BFM, upon heating, has finally become completely liquid. Such an assumption for “liquidus” can lead many people into erroneous conclusions about the flowing characteristics of BFMs.
Note 2. A eutectic BFM is an alloy of two or more metals that, when heated to its melting point (solidus temperature), will completely melt and turn to liquid at that same temperature (called the eutectic temperature or eutectic point). Thus, there is no melt range associated with eutectic alloys, and its solidus and liquidus temperatures are the same (it is isothermal). When the difference between the solidus and liquidus temperatures of a BFM is only 25°F/12°C or smaller, that BFM is known as a eutectic-type BFM since it will behave in much the way as a eutectic BFM.
Note 3. Liquid BFM likes to flow toward the heat, i.e. the “hot spots” in any brazing environment. Thus, it will often find the outside surfaces of components much more attractive than the cooler spaces inside the gap waiting to be brazed where the temperatures are somewhat lower than the outside surfaces of the parts.