casting alloys dental material

1. Casting AlloysCasting Alloys

2. 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

3. Requirements of casting
alloys
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)

4. Requirements of casting
alloys
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.

5. Requirements of casting
alloys
V. Practicability
1. Inexpensive and able to be soldered and repaired
2. Melting range
 It is preferred to use alloys that fuses below 1000o
C
 Alloys with higher melting range require;
1. Either phosphate or silica-bonded investment
2.Special melting equipments (oxy acetylene gas torch
or electric induction machine)

6. 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
Requirements of casting
alloys

7.  This fact could leads to
1. alloy oxidation,
2. Complication of the finishing procedure ….or even
3. 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)
Requirements of casting
alloys

8. 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)
Requirements of casting
alloys

10. A. HIGH NOBLE CASTING ALLOYS
1. Gold alloys
2. Low gold containing alloys
B. NOBLE CASTING ALLOYS
2. Pd-Ag alloys
C. BASE METAL CASTING ALLOYS
1. Co-Cr alloys
2. Ni-Cr alloys
3. Titanium alloys
Types of Casting alloys

12.  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

13. 1. Gold
 ↑ alloy’s melting range (its MP = 1063o
C)
 ↑ 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 = 1083o
C)
 ↑ 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

14. 3. Silver
 ↓ alloy’s melting range (its MP = 960o
C)
 ↑ 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 = 1773o
C)
 ↑ the whitish color
 ↑ the strength and hardness
 ↑ the resistance to tarnish and corrosion
 ↑ the ductility of the alloy ( FCC structure)
Role of elements in the gold
alloys

15. 5. Palladium
 ↑ alloy’s melting range (its MP = 1553o
C)
 ↑ the whitish color
 ↑ the strength and hardness
 ↑ the resistance to tarnish and corrosion
 ↑ the ductility of the alloy ( FCC structure)
 ↓ the weight of the alloy
6. Zinc
 ↓ alloy’s melting range (its MP = 788o
C)
 ↓ the oxidation of the alloy (act as scavenger)
 ↑ the castability of the alloy ( ↑ flow during casting)
Role of elements in the gold
alloys

16. 7. Indium
 ↓ grain size (↑ the mechanical properties)
 Responsible for forming the surface oxide layer in
case of metal-ceramic alloys
8. Tin and iron
 Responsible for forming the surface oxide layer in
case of metal-ceramic alloys
Role of elements in the gold
alloys

17. A. According to the gold contents
Karat 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. Types of gold alloys
18K gold
alloy
??F gold
alloy
=
Pure gold
contains 24
K gold
Pure gold
contain100
0 F gold
18K gold alloy = 750 F gold alloy
=

19. B. According to the hardness and
strength
Type I (Soft)… used for small inlays
Type II (Medium)… used for large inlays & onlays
Type III (Hard)… used for crown and bridge
Type 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)
Types of gold alloys

20. Types of gold alloys
Alloy Au
%
Cu
%
Ag
%
Pt
%
Pd
%
Zn
%
VHN M R
I. Soft 87 4 9 0 0 0 50-90 943-
960o
C
II. Medium 76 8 13 0 2.5 0.5 90-
120
924-
960o
C
III. Hard 70 10 15 1 3 1 120-
150
924-
960o
C
IV. Extra-
Hard
66 15 12 2 3 2 >150 871-
921o
C

21. 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

22.  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 ttt
2. Hardening heat ttt
Heat treatment of gold
alloys

23. A. Softening heat treatment
(Annealing)
Indication
1. Before hardening heat treatment
2. To increase the workability of the alloy
3. For structured to be cold worked (shaped or
ground)
Technique
Heating 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 & hardness
2. ↓ P.L. & E
3. ↑ Ductility

24. B. Hardening heat treatment
(Age hardening)
Indication
1. To increase the strength of cold worked alloys
2. To decrease the workability of the alloy
Technique
Step 1. Softening heat treatment (relief all stresses
and start at disordered structure)
Step 2.
 Heating the alloy at 700o
C  bench cooling
 Heating the alloy at 450o
C  bench cooling to
from 450o
C to 250o
C  quenching
 Maintain the alloy between 350-450o
C for 15 min
 quenching

25. B. Hardening heat treatment
(Age hardening)
Mechanism
The 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 solution
2. Precipitation of super-lattices
Outcomes
 ↑ Strength & hardness
 ↑ P.L. & E
 ↓ Ductility

26. Phase diagram of gold-copper alloy
system
1083 o
C 1063 o
C
410 o
C
390 o
C
0%Au------------------------------ 40------60---70---90--100% Au
100% Cu ----------------------------------------------------0% Cu
1083 o
C 1063 o
C
410 o
C
390 o
C
1083 o
C 1063 o
C
410 o
C
390 o
C
0%Au------------------------------ 40------60---70---90--100% Au
100% Cu ----------------------------------------------------0% Cu

28. 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

29. Low gold containing alloys
Characters
1. Its Gold content = 45-50%
2. The high % of silver & Palladium gives the
characteristic whitish color of the alloy
3. Its % of elongation = 2% <<< Gold alloys = 20%
4. Other properties are similar to those of type III
and IV gold alloys
5. Has good clinical performance and cast using the
same equipments required for regular gold alloys