In this installment of General Metallurgy, Paul Finelt finishes his discussion of phase diagrams and begins on practical applications for casting.
The last time
In the last metallurgy installment, we discussed two very important “binary” phase diagrams, the silver-gold, and the copper-nickel systems.
These phase diagrams were rather simple. In this article, we will try to understand how the more complex alloy systems operate and affect our work.
We will also begin to discuss the basic problems of creating castings and sheet stock that are clean, free of porosity and easy to work. In tying the art (practice) of jewelry manufacturing with science, I will rely on the readers’ knowledge and understanding of shop practice without review.
I strongly suggest that the reader review and understand the articles on porosity, casting technique, etc. that have been published in previous issues of this magazine. Understanding these basics will increase the significance of this article.
The silver-copper phase diagram (or sterling silver)
This is the tough one. The silver-copper phase diagram shows that there is an alloy form known as the eutectic alloy (see Figure I). The AgCu (chemical symbols; Ag=silver, Cu=copper) alloy of 28.1% copper (balance, 71.9% silver) is the eutectic alloy composition.
Recall that the liquidus and solidus lines form the boundaries of an area where both solid and liquid exist. At the eutectic point (composition) the liquidus and solidus lines come together. When the eutectic alloy solidifies, it goes from liquid to solid very rapidly (at one temperature). In this case the temperature is 1435°F (779°C). All the other alloys in the system solidify over a range of temperatures.
You should take a close look at this range of temperatures and notice that some alloys freeze over a wide range of temperatures and some alloys (e.g., those closest to the eutectic composition) freeze over a narrow range.
The range of temperatures over which an alloy freezes is very critical to its ability to cast up soundly. More on this later.
At the extreme sides of the phase diagram, new phases exist that we did not see in the silver-gold and copper-nickel diagrams. These are solid phases.
They are areas of composition where silver or copper are in solid solution with the other component.
Let’s take a moment to recall when we last discussed that one metal could be in solid solution with another metal. Think about how the other phase diagrams looked. Below the solidus of the copper-nickel diagram was an area where copper and nickel remained in solid solution with each other.
These little areas at the sides of the Ag-Cu diagram are areas where the same thing happens. The other alloy systems we looked at formed complete solid solutions along the entire range of alloys. This does not happen in the silver and copper alloys.
The large area is an area of supersaturation. This means that two phases (metal structures) exist separately. Supersaturation should be a familiar word by now.
If you could put one of those alloys under a microscope, you would see that there are two very distinct phases (you would also see the grain structure and size that we discussed in the first article of the series).
These two distinct structures or phases are the result of the supersaturation that occurred. One of the elements (copper or silver) could not be dissolved in the other. When the alloy cooled, silver or copper could not be held in solution with the other and “came out” of solution.
It should also be apparent from this discussion and the phase diagram that solubility is a function of temperature. That is, the higher the temperature the more one metal can dissolve in another.
The same principle is true for any element (such as oxygen, nitrogen, boron, etc.), whether it is a solid, liquid, or gas.
At this point, we will bid farewell to our discussions of the phase diagram. However, we will tie in much of the information presented in the phase diagram with the “good stuff” we’ll discuss from now on.
How can a gas become dissolved in a liquid metal? Aren’t metals resistant to gases (too “hard”) even when molten (liquid)? Aren’t holes in castings really from metal sloshing around in the mold?
The answer to these and other fascinating questions will soon become clear.
Gases in metals
I’m sure that all of you are aware of how gases wreak havoc with making a good piece of jewelry. Many have been bitten by the bug of a casting or piece of plate that looks just great until we solder or drill into it. And, of course, Murphy’s Law states that “The problem only surfaces after almost all the labor is completed.”
The main point of discussion will be to create a better understanding of the ways gases get into metals, where they get into metals, at what point in the manufacturing process this occurs, and how we can minimize the effect of gases and keep them at a level we can live with.
Let’s make a significant statement right now about the origins of gases in metals.
A gas can “get into” a metal by one or more of the following:
- Mixing into a molten metal.
- Being absorbed into a molten metal-or even a solid metal.
- Being entrapped in a solid metal as it solidifies.
Air is a gas that is made up of approximately 78% nitrogen and 20% oxygen. It is the source of all gases absorbed into molten metals. If we can shield the molten or hot metal from air, then we can prevent a large and important source of gas from entering the metal. Notice I didn’t say ALL the sources of gas.
Depending upon the method used we could create another gas which would not eliminate the “gas” problem.
Back to air. The gases we would eliminate by shielding are nitrogen and oxygen. Why are we concerned about them at all?
Two bad actors
What happens when air dissolves or is mixed in with a liquid metal?
Basically, the main components of air break up under the heat of the melt and form “new alloys ” (on a microscopic scale) with the metals in the melt. The gas mates with those metals, which are relatively more active than the others in the melt. Chemically speaking, gases are very attractive to metals and vice versa.
Nitrogen is known to be a “bad actor” in many alloy systems. It forms nitrides with metals in alloys; these are generally very hard. Nitrides, which are nonmetallic inclusions, will reduce malleability and create cracking problems.
Oxygen, the better known “bad actor,” forms oxides. It does this at relatively low temperatures and so is more critical to control. A common illustration of an oxide is rust, specifically iron oxide. We are all well aware that rust occurs at normal temperatures.
Oxides of some high temperature metals can form at low temperatures and can be very well controlled; witness the current trend of “coloring” the reactive metals such as tantalum, niobium and tungsten.
But the oxides that should concern us are those of the alloying elements we use. As we have discussed, these are, primarily:
These have been listed in order of oxidation resistance with the least resistant element at the top. Only the oxidation of zinc from the melt is readily recognized.
This is because zinc oxide is rather light and comes off as the whiteish smoke in almost every precious metal melt. If you melt alloy alone, without any flux or gold, you will see this phenomenon exaggerated.
Next comes copper. As noted, this one is difficult to identify. But let is suffice to say that if the zinc supply is reduced because of heavy oxidation, the next element to be oxidized will be copper.
It has been experimentally determined numerous times that not all of the “most” oxidizable element has to be removed (oxidized) before the reaction continues to the next element.
Temperature is extremely important in the oxidation process. Higher temperatures cause the process to go faster and farther.
With all of this theory, what do we do?
We all know that with every melt (heat, batch, crucible) a bit of “flux” or slag (glass) forming compound is added to “protect the metal ” as it melts down and is cast. This flux is generally borax (dessicated, without water).
What actually happens is that a layer of molten glass is formed on the metal as it melts and is superheated to casting temperatures. This flux is a boride or boron-bearing glass. The glass protects the melt from the air. In addition, boron is a great lover of oxygen. That’s one of the reasons it forms “glassy” borides. There is some free boron present in borax and this reacts with the melt to remove some of the oxygen in the molten metal.
We noted that borax (or any flux for that matter) should be purchased dry (dessicated), without water. This is extremely important and can make “the difference” when casting certain “touchy” alloys, particularly the white gold alloys.
Borax should be stored in a very dry place, heated to over 212°F. These materials are very hydroscopic (“water lovers”), and this will prevent the flux from picking up moisture from the atmosphere. A good place to store flux is the hottest place in our plant-especially in the summer!
Now that we’ve touted the benefits of the common flux, let’s discuss its bad points. Boron is a very tiny atom. It can “get in between ” atoms of gold, silver etc. (as does oxygen) and will harden the metal if present in high enough quantities.
Flux is important in all melting procedures but should not be overdone. This caution is especially important when casting at higher temperatures.
Let’s stop here and take a breather. We’ve covered some tough and important ground in this game. Let’s not forget to keep our flux warm and our melts as cool as we can cast ’em.
(To be continued)
AJM, October 1985
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