casting alloys dental material

Casting AlloysCasting Alloys

Requirements of casting alloys

I. Biologically

1. Casting alloys should not cause toxicity, allergy or even irritation in service or during the fabrication process (casting and finishing).

Beryllium-containing alloys could cause berylliosis if inhaled during finishing

The metal nickel is known to be allergenic

2. The alloy should resist the degradation in oral fluids


II. Interfacially and chemically

1. Casting alloys should have low surface energy to reduce the plaque attachment

2. If the alloy is going to be covered with porcelain should be able to form surface oxide layer

3. The alloy should be resistant to both tarnish or corrosion (Nobility and passivity)

4. Alloy surface should not be affected by the oral environment (show no pitting)


III. Mechanically

1. High strength (P.L, Y.S. & U.S.) to resist the permanent deformation or even fracture during service

2. Ductility is also required in certain situation where burnishing and marginal closure are needed

3. Alloys with higher hardness are difficult to be finished (Require sandblasting or electro-polishing) and could cause wear to the opposing natural teeth.

Requirements of casting alloysV. Practicability

1. Inexpensive and able to be soldered and repaired

2. Melting range It is preferred to use alloys that fuses below

1000oC

Alloys with higher melting range require;1. Either phosphate or silica-bonded investment2.Special melting equipments (oxy acetylene gas

torch or electric induction machine)

3. The Density Dense alloys are more easier in casting under

relatively lower casting force Lighter alloys requires more casting force and

are more liable to casting defects (incompleteness and porosity)

4. The reactivity at the molten state Many casting alloys (e.g. Titanium) are highly

reactive at the molten state either to the surrounding atmosphere …….or Investment materials


This fact could leads to 1. alloy oxidation, 2. Complication of the finishing procedure ….or

even3. Enhancement of alloy corrosion

To avoid these adverse effects, Proper selection of the following items is so important;

1. The alloy (Should be low-reactive …..such as gold alloys)

2. The investment (Should contain reducing agent) …………and

3. The melting method (Proper usage of flame and using gas containing no or little amount of carbon)


5. The casting shrinkage All Metals expand on heating ( inter-atomic

distances) and shrink on cooling ( inter-atomic distance and density)

Alloys with little solidification and cooling shrinkage are able to produce more accurate casting

Alloys with higher shrinkage rates require special support (Both the die and investment materials should show higher expansion rates)


A. HIGH NOBLE CASTING ALLOYS1. Gold alloys2. Low gold containing alloys

B. NOBLE CASTING ALLOYS2. Pd-Ag alloys

C. BASE METAL CASTING ALLOYS1. Co-Cr alloys2. Ni-Cr alloys3. Titanium alloys

Types of Casting alloys

Pure gold is yellow, soft (ductile & malleable) metal that welded (cohere) together and easily deformed under pressure.

Pure gold is used as direct filling material to restore small tooth cavities

Alloying the metal gold with Copper, Silver, Platinum, Palladium, Zinc and sometimes minute amount of Indium improves its mechanical properties to fit the requirements of different applications

Gold Alloys

1. Gold alloy’s melting range (its MP = 1063oC) the golden yellow color the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure)2. Copper alloy’s melting range (its MP = 1083oC) the reddish color the strength and hardness (forms solid

solution with gold heat treatment) the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure)

Role of elements in the gold alloys

3. Silver alloy’s melting range (its MP = 960oC) the whitish color that neutralize the red color

of copper the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure)4. Platinum alloy’s melting range (its MP = 1773oC) the whitish color the strength and hardness the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure)


5. Palladium alloy’s melting range (its MP = 1553oC) the whitish color the strength and hardness the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure) the weight of the alloy6. Zinc alloy’s melting range (its MP = 788oC) the oxidation of the alloy (act as scavenger) the castability of the alloy ( flow during

casting)


7. Indium grain size ( the mechanical properties) Responsible for forming the surface oxide

layer in case of metal-ceramic alloys8. Tin and iron Responsible for forming the surface oxide

layer in case of metal-ceramic alloys


A. According to the gold contentsKarat system The gold alloys are divided into 24 parts Karat is the number of parts represent the pure

gold in the alloy e.g. 18 K gold alloy = 18 parts of gold, 6 parts of

other metals

Fine system The gold alloys are divided into 1000 parts Fineness is the number of parts represent the

pure gold in the alloy e.g. 750 F gold alloy = 750 parts of gold, 250

parts of other metals

Types of gold alloys


18 K gold alloy

?? F gold alloy

=

Pure gold contains 24

K gold

Pure gold contain1000

F gold

18 K gold alloy = 750 F gold alloy

=

B. According to the hardness and strength

Type I (Soft)… used for small inlaysType II (Medium)… used for large inlays & onlaysType III (Hard)… used for crown and bridgeType IV (Extra-hard)… used for denture frameworks

The content of both gold and copper are the most effective in this classification

(See the table of alloys’ composition)



AlloyAu %

Cu%

Ag%

Pt%Pd%

Zn%

VHNM R

I. Soft874900050-90943-960oC

II. Medium7681302.50.590-120

924- 960oC

III. Hard701015131120-150

924- 960oC

IV. Extra-Hard

661512232>150871- 921oC

What can you discover from the table?

The gold content Type I Type IV The copper content Type I Type IV The ductility& %elongation Type I Type IV The hardness & strength Type I Type IV The melting range Type I Type IV The golden yellow color Type I Type IV

N.B.Heat treatment could alter these announced

properties

The mechanical properties of gold alloys could be altered through the solid state reactions (Heat treatment)

The ability of heat treatment presents only in types III & IV due to the higher % of copper and silver

2 types of heat treatment could be carried out;1. Softening heat ttt2. Hardening heat ttt

Heat treatment of gold alloys

A. Softening heat treatment(Annealing)

Indication1. Before hardening heat treatment2. To increase the workability of the alloy3. For structured to be cold worked (shaped or

ground)TechniqueHeating the alloy at 700oC for 10 min quenching Mechanism & outcomes

All the solid transformed into a disordered solid solution at the high temp., with rapid cooling the structure remains disordered, accordingly

1. Strength & hardness2. P.L. & E3. Ductility

B. Hardening heat treatment (Age hardening)

Indication1. To increase the strength of cold worked alloys2. To decrease the workability of the alloy

TechniqueStep 1. Softening heat treatment (relief all

stresses and start at disordered structure)Step 2. Heating the alloy at 700oC bench cooling Heating the alloy at 450oC bench cooling to

from 450oC to 250oC quenching Maintain the alloy between 350-450oC for 15

min quenching

B. Hardening heat treatment (Age hardening)

MechanismThe solid at the start has a disordered structure,

slow cooling or even maintaining the temperature for sometime helps the diffusion

of atoms that leads to;1. Rearrangement of atoms ordered solid

solution2. Precipitation of super-lattices

Outcomes Strength & hardness P.L. & E Ductility

Phase diagram of gold-copper alloy system

1083 oC 1063 oC

410 oC

390 oC

0%Au------------------------------ 40------60---70---90--100% Au100% Cu ----------------------------------------------------0% Cu

1083 oC 1063 oC

410 oC

390 oC

1083 oC 1063 oC

410 oC

390 oC

0%Au------------------------------ 40------60---70---90--100% Au100% Cu ----------------------------------------------------0% Cu

Alloy

Au%

Cu%

Ag%

Pt%

Pd%

Other

Conventional

75

10

9

2

2

2

Low- Gold

44

12

36

0

6

2

Low gold containing alloys

Composition

Low gold containing alloys

Characters1. Its Gold content = 45-50%2. The high % of silver & Palladium gives the

characteristic whitish color of the alloy3. Its % of elongation = 2% <<< Gold alloys = 20%4. Other properties are similar to those of type III

and IV gold alloys5. Has good clinical performance and cast using the

same equipments required for regular gold alloys

Education

casting alloys dental material