Dental Casting Alloys

Base metal alloys 

are often marketed for both all-metal and metal-ceramic prostheses because of their oxide formation at room temperature.

oxide formation

Base metal alloys are often marketed for both all-metal and metal-ceramic prostheses because of their ? at room temperature

Oxide formation importance:

• For metal-ceramic prostheses, that oxide layer helps porcelain stick to the metal framework.

• For all-metal prostheses, it shows that the metal is chemically stable and resistant to corrosion.

• High noble

• Noble

• Predominantly base metal

Alloy classification by noble metal content

Goppirr

• Gold

• Osmium

• Palladium

• Platinum

• Iridium

• Rhodium

• Ruthenium

What are the 7 noble metals? Only ? which have the lowest melting temperatures of the seven noble metals, are currently of major importance in dental casting alloys.

PPCS

• PLATINUM

• PALLADIUM

• COPPER

• SILVER

High Noble and Noble Alloys

↑ hardness & elasticity of gold

↑ melting temperature of the alloy

Platinum purpose

MAKES THE ALLOY HEAT-TREATABLE

It means that when enough copper is mixed with gold in a dental alloy, the metal can be hardened by heating and cooling — this process is called heat treatment.

In simple terms:

• Pure gold is soft.

• When you add enough copper, the gold alloy can be heated and then cooled to make it stronger and harder, which is useful for dental restorations that need to withstand biting forces.

Copper purpose

• With gold and palladium: forms solid solutions for better strength.

• With gold-copper alloys: removes the reddish tint caused by copper.

• With palladium-based alloys: gives the alloy a white color.

Silver purposes

• mixes well with gold, silver, and copper

• gives good mechanical strength, tarnish/corrosion resistance, and biocompatibility.

Palladium purpose

Gold-based alloys

Generally yellow in color

• soft and designed for inlays

• supported by teeth and not subjected to significant mastication forces.

Type 1 gold alloys use

• inlays - superior mechanical properties

• but they have less ductility than type 1 alloys

Type 2 gold alloys use

• constructing crowns and onlays

• for high-stress areas

Type 3 gold alloys use

• used in high-stress areas

• bridges and partial denture frameworks

Type 4 gold alloys use

copper, silver, and palladium.

Changes of alloy color caused by the reduction in gold content are compensated for by an increase in ?

silver and copper content

Higher ? reduces the corrosion resistance of these alloys.

• Softening heat treatment is referred to as a solution heat treatment.

• The casting is placed in a furnace for 10 minutes at 700°C and then quenched in water.

• Tensile strength, proportional limit, and hardness are reduced by such a treatment, and the ductility is increased.

• Indicated for structures that are ground or reshaped plastically to a different form, either in or out of the mouth.

Heat Treatment of Gold-Copper Alloys

solution heat treatment

Softening heat treatment is referred to as a ?

• white and predominantly silver in composition

• contain at least 25% palladium to provide nobility and increase tarnish resistance

What is the composition and purpose of palladium in silver-palladium alloys?

• poor castability

• require precise control of casting and mold temperature to produce acceptable castings

Why do silver-palladium alloys need precise control of casting and mold temperature?

greater potential for:

1. tarnish

2. corrosion

What is the major limitation of silver-palladium alloys?

They may also contain copper and a small amount of gold

What other elements can be present in silver-palladium alloys?

alloy lose its reddish color and acquire a gold color

How does increasing palladium content affect the color of the alloy?

• increase ductility and

• improve castability of the alloy for dental applications

Why are silver, copper, or gold added to silver-palladium alloys?

Silver, copper, and/or gold

can be added to increase ductility and improve castability of the alloy for dental applications.

Ni-Cr

Co-Cr

 two main groups of base metal dental alloys:

• improves castability

• promotes the formation of a stable metal oxide for porcelain bonding

What is the function of beryllium in Ni-Cr alloys?

• with beryllium

• without beryllium

How are Ni-Cr alloys classified based on their composition?

partial denture frameworks are easy to finish and polish!!!!!!!

The Ni-Cr alloys used as… 

• small castings

  • crowns & FPDs

What are Ni-Cr alloys commonly used for?

increase strength = improve castability

• Beryllium refines grain size (increasing strength)

• lowers the fusion temperature of the alloys, improving castability

What are the effects of adding beryllium to dental alloys?

Manganese and silicon also improve the castability of the alloys

Which elements help improve the castability of dental alloys besides beryllium?

aluminum in ni-cr = nickel and al compound = tensile & yield strength increase

Aluminum in Ni-Cr alloys forms a nickel and aluminum compound = increases both tensile and yield strength

What is the role of aluminum in Ni-Cr alloys?

Nitrogen

also improves the overall qualities of the casting

Carbon forms carbides with components in the alloy that increase hardness and yield strength but decrease ductility

How does carbon affect the mechanical properties of dental alloys?

CARBON!!!

forms carbides with components in the alloy that increase hardness and yield strength but decrease ductility

Titanium and titanium alloys

First reported application was in 1977.

Titanium and titanium alloys

Considered the most biocompatible metal.

• all-metal & metal-ceramic:

  • prosthesis

  • implans

  • rpd frameworks

What are the dental uses of titanium and titanium alloys?

Ti-6Al-4V

The most widely used titanium alloy in dentistry

is ???, a beta alloy.

Titanium alloys

are highly resistant to sag deformation.

For all-metal restorations:

has a high melting point (1668°C) and a high rate of oxidation above 900°C.

Because the metal’s internal structure (crystals) must be reset or “relaxed” by softening heat treatment first. Then, when it’s reheated and cooled properly, the hardening process works better — giving the right mix of strength and ductility.

CQ

Why must Type 3 and Type 4 gold-copper alloys be softened by a heat treatment before they are hardened by another heat treatment?

Fast cooling → harder but less ductile
Slow cooling → softer but more ductile

CQ

What effects can the cooling rate of a heat treated Type 3 gold-copper alloy have on its hardness and ductility?

When gold-copper alloys are heated to 700°C then quenched, their structure becomes disordered, making them soft and ductile (easy to bend and shape).

When reheated at 200–450°C, the atoms realign into an ordered structure, making the alloy hard and strong but less ductile.

CQ

What happens within gold-copper alloys when subjected to a prescribed heating-cooling cycle?

Palladium is used to replace gold in dental alloys because it is noble, resists tarnish and corrosion, and helps maintain the alloy’s stability.

It also gives the alloy good strength, a gold-like color, and similar melting and casting behavior to gold-based alloys.

CQ

What characteristics of palladium make it a natural choice of element to replace gold for dental alloys?

Chromium’s role:
Chromium forms a thin protective chromium oxide layer on the alloy’s surface, which prevents corrosion and strengthens the alloy through solution hardening.

Beryllium’s role:
Beryllium refines the grain size (making the metal stronger) and lowers the melting point, which improves castability and helps form a stable oxide layer for better porcelain bonding.

CQ

What is the role of chromium in predominantly base metal alloys? How does beryllium improve the properties of nickel-based metal alloys?

• All-metal alloys focus on strength and corrosion resistance

• metal-ceramic alloys must also bond with porcelain and withstand porcelain firing heat.

CQ

How do alloys for all-metal prostheses differ from those required for metal-ceramic prostheses?