Bi-Metal Basics

Bi-Metal Casting allows manufacturers to join two metals in almost any desired shape-without visible seams.

The casting of one metal onto another-otherwise known as bi-metal casting-is not a new technique. The dental industry, for example, has successfully used it for more than 30 years to create dentures and other prosthetic pieces. But in the jewelry industry, bi-metal casting has been used only sparingly.

The reasons for this, I have found, are not based on economic considerations, but on a lack of knowledge about metallurgy and other technical factors. Once that “knowledge deficit” has been filled, manufacturers find bi-metal casting has its advantages: It allows them to join two metals in almost any desired shape, without the creation of a visible seam that may tarnish or oxidized. Also, bi-metal casting can create a stronger bond than brazing, sintering, or similar methods.

Any jewelry manufacturer with access to casting has the capability to learn and use this technique. In this article, I will address the pros and cons, providing practical advice on the most common problems-and how to overcome them.

Metallurgical Requirements

First, some fundamental metallurgical principles must be explained. Bi-metal casting involves a surface reaction between solid and liquid metal: As the liquid meets the solid under proper conditions, a permanent metallurgical bond is created. This involves three requirements:

1. The two metals must make direct contact. To accomplish this, both the solid metal and its liquid counterpart must be free of non-metallic layers, oxidation in particular.
2. The alloys used must be capable of creating an inner metallic compound. The thermal relationship should be such that a good wetting of the metal structure takes place, and an alloying occurs at the joining area.
3. To prevent a spotty connection, the metal should not freeze immediately at contact but stay in a liquid state for a short time (normally about two to three seconds, depending on the metal combination). If additional force is applied by centrifugal or pressure casting, that time will shorten accordingly.

Either gaseous or liquid media can be used to fulfill the requirements for a clean surface. The solid surface, for example, can be coated with a flux that operates within a certain temperature range. (I’ve found paste flux based on borax [boric acid with fluoride additives] especially useful.) The surface can also be galvanically coated; I’ve found gold or silver plating works well, with a minimum coating thickness of 10 to 20 microns.

For additional help in reducing oxidation, pre-heat the flask under a cover gas. Often this is not an option, as many casters do not have burnout kilns that offer atmosphere control. However, vacuum or pressure casting machines do offer the possibility of flushing the already-hot flask with a cover gas. This too will reduce oxidation at the contact surface.

One of the most important tasks in bi-metal casting is to create and set a timely, localized, accurate temperature field at the connecting surface of the invested metal part. (This will take care of requirements two and three.) The thermal parameters, such as the pre-heat temperature of the invested metal section and the casting temperature of the casting alloy, depend on the geometry of the piece to be made. The determining factor is the relationship between the mass of the casting metal of the piece (not the casting tree) versus the contact surface of invested metal.

During the casting process, the temperature of the contact surface of the invested metal has to increase to a point slightly above the solidus temperature of the casting alloy. When there is a wet metal connection at the joining surface, a good interface is possible. If the chosen metals create problems in reaching this required liquid state, those problems can be overcome by coating the connecting surface with a lower-melting solder. This will create a wet surface and make a bond possible.

Selecting the Right Alloys

When selecting alloys for bi-metal casting, manufacturers should always obtain technical information from the metal supplier. Several points must be considered:

1. The alloys must be able to blend together. (It is always a good decision to do a test casting for compatibility.)
2. Jewelry alloys with low copper content are preferred. If a red/pink gold is required, then the contact surface must be plated to prevent oxidation.
3. The solidus temperature of the invested section has to be at least 50°C1122°F to 100°C1212°F above the liquidus point of the casting metal.
4. Casting alloys should have no complex characteristics, such as age hardening.
5. The alloys should have a similar expansion coefficient.
6. Cold-formed parts made of sheet, wire, or tube should be used for investing.
7. Alloys that absorb and release a lot of gas should not be used.

Absorbent alloys often cause problems. This type of alloy releases a great deal of gas during cooling; if the atmospheric pressure is less than the gas pressure, a bubble forms. During a normal casting, this gas will be carried through the investment and dissipate. Not so at the interface surface of a bi-metal casting: The danger of gas build-up can lead to gas porosity, which in turn can lead to a weakening of the connecting area and, ultimately, breakage.

Alloys containing silicon work best with bi-metal casting, since they develop much less oxidation than other alloys. The oxidation that does occur is very thin and transparent, and will be flushed away when the metals contact. In addition, that thin layer can protect the alloy from heavier oxidation.

One final note: Platinum is an ideal choice for the invested metal. Most platinum alloys do not oxidize, and the flask temperature can be widely varied because of platinum’s high melting point.

Casting Preparations

A successful bi-metal casting begins with proper planning and communication at several levels, from the designer to the model maker to the caster. They all need a thorough understanding of the process and its dynamics.

The designer must be able to design a piece that is castable. For bi-metal casting, this means the connecting surfaces should be as simple as possible: Simple geometric forms are better than complicated ones. The caster, however, should determine how to sprue the piece.

To ready a wax tree for investing, the wax part and the metal part for the bi-metal casting must be joined. This can be done in several ways. As in stone-in-place casting, the metal sections can be simply pressed into the wax. Another method is to create a holding device inside a rubber mold, insert the metal into the mold, and inject the wax around it. The third possibility is to model the wax to the metal section by hand. With all three methods, the metal must protrude 0.2 to 0.3 mm over the wax, since casting metal can melt portions of the invested metal. This protrusion will guarantee that the investment holds the metal in place after burnout.

Because of its special nature, platinum requires a different technique. Invested platinum is usually already polished; consequently, the wax portion should be slightly larger than the metal to compensate for the polishing that has yet to be done to the gold.

Occasionally a metal section will block or interfere with sprueing. In that case, hollow stock, such as tube sections, can help. The casting alloy will flow through the tube, which will heat the metal evenly. This flow-through (or flow-over, as the case may be) has a positive effect or removing all non-metallic particles and thus leaving a clean surface. It is possible in special cases to roughen the surface of the metal insert or install mechanical connectors {e.g., small fins, or small holes drilled in the invested metal) to make the casting alloy adhere.

It is important to be clean and exact when connecting the metal to the wax, and the entire unit to the wax tree. {The soldering point between sprue and stem should have a hollow core center to prevent turbulence during casting.) When the investment is poured, there can be no wax between metal and investment, since this may cause the metal to loosen during burnout. It is also important that no gap exists between the wax and the metal, to prevent investment from interfering with the connection.

To create the proper thermal conditions, place same or similar pieces on the tree. Also, do not wax the parts too close together, otherwise you can transfer the heat of liquid metal from one casting part to another and cause uncontrolled solidification. Units should be waxed to the tree at a 45° angle with the contact area between the wax and the metal at a 90° angle to the flow of the metal. This will help with metalostatic and centrifugal forces during casting.

Investing and Burnout

To invest the wax tree for bi-metal casting, use either gypsum- or phosphate-based investment powders. Gypsum-based investments require an 11 to 13 hour burnout cycle, while phosphate investments can be burned out in five to six hours. These long burnout cycles, however, can damage the metal inside the flask, increasing grain size and causing deep oxidation.

In the last few years, so-called “high speed” investments have been developed for the dental industry. These investments make it possible to cast in one to two hours after set-up: The investment can be immediately placed in a kiln pre-heated to 900°C/1,652°F {regular investments always begin burnout at room temperature or, at maximum, 150°C/302°F) and the flask will cool to casting temperature in about 30 minutes. However, while they help avoid metal damage, these investments can be used only with metals that have a solidus point over 900°C/1,652°F. Also, because burnout cycles with dental investment begin with higher temperatures and do not last as long as normal cycles, the airflow in the kiln cannot remove all the wax vapors. Consequently, if the kiln door is opened during the burnout phase, the additional air can ignite the vapors.

To reach the proper temperature requirements for bi-metal casting, it is very important to choose the proper flask temperature. This can be done by experimentation. If the proper temperature has been determined for a particular piece, that information can then be used to estimate temperatures for different situations. For this, the guidelines in Figure 2 should be applied.

Casting

Bi-metal casting requires the exact controlled casting, manufacturers should make sure they use a machine that can measure casting temperatures, exert external force {such as centrifugal pressure), and provide atmospheric control through a vacuum or a cover gas. Techniques without such controls, such as torch casting, are not recommended.

For each alloy combination, the exact flask and casting temperatures should be logged and used repeatedly. If the casting temperatures are not exactly adhered to, the casting could fail and the entire tree be lost. If the casting temperature is too low, the connecting surfaces might not bond. If the temperature is too high, the metal can create too big an interface area or, worse, dissolve where it was to be joined. Achieving the right temperature requires experimentation; begin with the melting range of the metals and calculate from there.

The time in which the flask rests in the casting machine before casting should also remain constant. If these times vary greatly, the thermal conditions at the connecting surface will vary too. As the flask temperature has the greatest influence on the quality of bi-metal casting, always make sure this area is controlled.

Cooling, De-vesting, and Finishing

The cooling of a flask is a critical process. Three different materials {two metals and the investment) are cooling at the same time, and this can lead to stress in the jewelry as well as cracking where the two metals interface.

After casting, the flask must cool to room temperature. Quenching should be avoided, since it can crack the metals at the interface {thermal shock cracking). Also, when removing the investment from the flask, avoid hitting the button with a hammer. For easy de-vesting of the hard gypsum-bonded investment, line the flask with a ceramic liner before investing. This makes .it possible to push the hard investment out of the flask after casting.

The pieces should next be sand blasted and cleaned, and their interface surfaces checked with a loupe or a microscope for hairline cracks.

If the pieces contain no cracks, anneal them with a torch or in a kiln. Annealing will remove built-up stresses in each piece, which may have been created during casting.

The casting can now be finished at the bench. Any excess metal can be filed away, and the pieces finished and polished as usual.

Considering the advantages that bi-metal casting offers, it is surprising that
more manufacturers don’t use it. However, as the manufacturing industry continues to search for new ways to manufacture jewelry, and as designers continue to combine different metals or alloys, bi-metal casting may become more common. Time-and experimentation-will tell.

References
1. “Verbundgiessen-Wege zur optimalen Werkstoffanpassung,” A. Lane, A. Issleib, VDI Bericht 133
2. “Verbundgiessen fur VerschleiBteile,” A. Ibleib, A. Friedel, I. Lubojanski, GieBeret Praxis Nr. 7/8 1995
3. Fertigungstechnik, VDI, Verlag
4. Werkstoffkunde der Dental Edelmetall, Legierungen, E. Wagner
5. “Casting Gold to Platinum,” by Jurgen Maerz, Platinum Guild International; paper presented at 1998 Santa Fe Symposium on Jewelry Manufacturing Technology.
6. “Process Control; Power, Value, and Advancement for the Jewelry Industry by Timothy L. Donohue and Helmut F. Frye, TechForm LLC; paper presented at 1998 Santa Fe Symposium on Jewelry Manufacturing Technology.

Klaus Wiesner
www.c-hafner.de
AJM, September 1998