Casting Alloys, Wrought Alloys, and Solders; wataha Flashcards
noble metals
Gold and platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium), which are highly resistant to oxidation and dissolution in inorganic
acids.
alloys
An alloy is a material composed of two more elements; at least one of which should be a metal. Thus, an alloy is made by fusing two or more metals, or a metal and a nonmetal.
solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. This often happens when the two elements (generally metals) involved are close together on the periodic table; conversely, a chemical compound generally results when two metals involved are not near each other on the periodic table.
Classify dental casting alloys
The ADA currently classifies dental casting alloys into
three groups.
- High-noble alloys must have a noble metal content of at least 60% by weight and a gold content of at least 40%.
- Noble alloys must have a noble metal content of at least 25%, but no stipulation exists for gold content.
- Base-metal alloys have a noble metal content of less than 25%.
Thus, the ADA classification includes alloys with reduced gold content and alloys with little or no noble metal content.
Describe noble metal alloys in detail.
Type I (high noble) gold alloys
They are weak, soft, and highly ductile, and useful only in areas of low occlusal stress designed for simple inlays such as used in class I, III, and V cavities.
These alloys have high ductility so they can
be burnished easily. Such a characteristic is
important since these alloys are designed to be
used in conjunction with a direct wax pattern.
Type II (high noble) gold alloys
Type II gold alloys contain lesser amount of gold
than type I, hence they are less yellow in color.
These are used for conventional inlay or onlay restorations that are subjected to moderate stresses, thick three-quarter crowns, pontics, and full crowns. These alloys have good strength and hardness. Ductility is almost the same as that of type I alloy; however, yield strength is higher. Hence, these alloys are more difficult to burnish than type I since burnishability is a function of ductility and yield strength.
Type III (noble) gold alloys
These are used for fabrication of inlays subjected to high stress and for crown and bridge castings. In contrast to types I and II gold alloys, type III alloys can be age hardened. The ability to burnish is not as much as the other two, though strength is higher and of greater importance.
Type IV (noble) gold alloys
These are used in areas of very high stress,
crowns, long span bridges, and partial denture
framework. It has the lowest gold content of
all four types (less than 75%) and the highest
percentage of silver, copper, platinum, and palladium. Hence, it has lower ductility and malleability, but has good yield strength and is most responsive to heat treatment.
Describe Ni-Cr alloys in detail.
It is the most common base metal alloy used
in metal–ceramic prostheses. The composition
includes nickel (61.5%–77.5%), chromium (12.8%–22%), molybdenum (4%–14%), aluminum (0%–4%), and iron (0%–5%). Ni-Cr-Be alloys also contain 0%–2% beryllium.
Chromium contributes to the corrosion resistance, molybdenum decreases the CTE, and beryllium improves castability by decreasing the melting ranges. Aluminum-containing alloys form a precipitate Ni3Al that helps in strengthening the alloy. Beryllium is primarily added to lower the melting range of the alloy so that it can be used with gypsum-bonded investments.
It forms a thicker oxide layer for bonding to porcelain. Ni-Cr alloys containing Be corrodes more easily than non–Be-containing alloys.
Beryllium is known to be toxic, more so to the laboratory technician during alloy melting.
Toxicity that is termed berylliosis may be exhibited from mild to moderate symptoms such as contact dermatitis to severe chemical pneumonitis. Permissible maximum concentration of Be in air is 5 mg/m3, while Occupational Safety and Health Act (OSHA) guidelines permit an exposure of a maximum of 2 mg/m3. T e laboratory should be equipped with a good exhaust
system.
Nickel is also a known allergen, more so in females (4.5%) than males (1.5%). It results in contact dermatitis and hypersensitivity. OSHA regulations allow 15 mg/m3 of Ni in air.
Ideal requirements of dental casting alloys
- Biocompatibility—The alloy must tolerate oral fluids and not release any harmful products into the oral environment that might cause a toxic or allergic reaction.
- Tarnish and Corrosion Resistance—Corrosion is the physical deterioration of a material in the oral environment, and tarnish is a thin film that is adherent to the metal surface. Both phenomena affect the durability and appearance of the prostheses.
- Thermal Properties—The melting range of the casting alloys must be low enough to form smooth surfaces with the mold wall of the casting investment. For metal-ceramic prostheses, the alloys must have closely matched thermal expansion coefficients to be compatible with the given porcelains, and they must tolerate high processing temperatures without deforming via a creep process.
- Strength Requirements—The alloy must have sufficient strength for the intended application. For example, alloys for bridgework require higher strength than alloys for single crowns.
- Fabrication—The molten alloy should flow freely into the investment mold, without any appreciable interaction with the investment material, and wet the mold to form a surface free of porosity. It should be possible to cut, grind, finish, and polish the alloy to obtain a prosthesis with a satisfactory surface finish.
- Porcelain Bonding—The alloy must be able to form a thin adherent oxide that enables chemical bonding to ceramic veneering materials.
- Economic Considerations—The cost of metals used for single-unit prostheses or as frameworks for FDPs or RPDs is a function of the metal density, fluctuations in metal prices, and the cost per unit mass.
liquidus vs. solidus
Simply put, liquidus is the lowest temperature at which an alloy is completely liquid; solidus is the highest temperature at which an alloy is completely solid.
Bonding of ceramic to alloys
Generally, ceramic bonds to metal primarily through chemical bond with the oxide layer on the metal surface and to some extent due to micromechanical retention achieved by sandblasting the metal surface. The metal oxide forms on the surface of the metal during the degassing stage of porcelain fabrication. The thickness, color, and strength of the oxide layer
vary with the type of the alloy and are crucial for the durability and strength of the bond. The thickness of the oxide layer should be minimal since these layers are brittle in nature.
lost wax technique
Process in which a wax pattern, prepared in the shape of missing tooth structure, is embedded in a casting investment and burned out to produce a mold cavity into which molten metal is cast.
wrought alloy
A metal that has been permanently deformed to
alter the shape of the structure and certain mechanical properties, such as strength, hardness, and ductility.
The fibrous structure of wrought alloys reverts to
the original grain structure if too much heat is applied to the wrought form.
welding
Process of fusing two or more metal parts through the
application of heat, pressure, or both, with and without a filler metal, to produce a localized union across an interface between the workpieces.
soldering
Process of building up a localized area of a metal prosthesis with a molten filler metal or joining two or more metal components by heating them to a temperature below their solidus temperature and filling the gap between them using a molten metal with a lower liquidus temperature. If the melting temperature of the solder is greater than 450 °C, the process is called brazing.
annealing
The process of controlled heating and cooling that is
designed to produce desired properties in a metal. Typically, the annealing process is intended to soften metals, increase their ductility, stabilize shape, and increase machinability.
Describe the grain structure of a wrought alloy, and compare it to a cast alloy of the same composition. How did the wrought alloy get its grain structure? What can make the wrought structure change? How are the wrought forms used in dentistry?
Fibrous grain structure of wrought alloys under the
light microscope. If a cast alloy is mechanically worked, it is referred to as a wrought alloy, and its grain structure is altered by breaking up the cast grains into a fibrous form seen here. Alloys with a fibrous grain structure are generally stronger and more brittle than their cast counterparts. Fibrous grain structures are common in wires used for orthodontics or in wires for clasps on removable partial dentures.
