This document discusses dental casting alloys, focusing on base metal alloys used as alternatives to gold alloys. It defines base metal alloys, describes their composition and the roles of each element. Cobalt-chromium and nickel-chromium alloys are discussed in detail and compared to gold alloys. Their properties, casting techniques, and uses in removable and fixed prostheses are outlined. Titanium and titanium alloys are also summarized, including their composition, properties and challenges in casting.
2. Objectives
To classify base metal alloys used in dentistry
To understand the composition of base metal alloys
To compare between gold alloys type III and IV and
base metal alloys.
To predict the properties of base metal alloys when
used for removable and fixed prosthesis
Discuss titanium alloys for dental implants
3. Defined as alloys which do not contain
noble metals.
They are alternative for gold alloys
type III & IV.
What are base metal alloys?
4. Uses in Dentistry
1. Removable partial denture framework.
2. Full denture bases.
3. Crown and bridge.
4. Dental implants.
6. CompositionandRoleof EachElement
1- Cobalt Strength,
Hardness
Modulus of elasticity
2- Chromium 12-30 %:
Passive layer (CrO):
a- Thin
b- Uniform
c- Non porous
d- Adherent
30% sigma phase ppt of
chromium, embrittle the
alloy
7. 3- Nickel:
Strength,
Hardness
Modulus of elasticity
Ductility
Nickel is a well-known metal to produce allergic
reaction in some patients. In such cases it is advised
also to use a Nickel-free Co-Cr alloys.
4- Molybdenum:
Strength (grain refiner)
5- Silicon and Manganese:
Fluidity of the molten alloy thus improve
castability, Act as deoxidizers
CompositionandRoleof EachElement
8. 6- Carbon 0.2%:
Discontinuous Carbides phase appears at the grain
boundaries at high melting temperature during casting
Strength,
Hardness
Modulus of elasticity
7- Aluminum:
Strength
With Nickel Ni3Al nickel aluminide
(intermetallic compound)
CompositionandRoleof EachElement
9. 8- Beryllium: Fluidity of the molten alloy thus
improve castability
Lowers the Tm of alloy by 100°C
Beryllium vapor is carcinogenic and may lead to
fibrosis of the lungs. Therefore many alloys are
available now free of beryllium.
9- Titanium: Tarnish and corrosion resistance
CompositionandRoleof EachElement
10. Comparison with Cast gold alloys
Property Cobalt-
chromium
Nickel-
chromium
Gold type
III IV
Comment
Resistance
to tarnish
&
corrosion
Excellent Excellent Excellent All well Resistant
Color Lustrous silvery white Yellow or
white
Used alone/
porcelain fused
Fusion Tm 1400°C 1300°C 800 -
1050°C
Different melting/
investment use
11. Comparisonwith Cast goldalloys
Property Cobalt-
chromium
Nickel-
chromium
Gold type
III & IV
Comment
Density 7-8 gm/cm3 15-
18gm/cm3
Difficult to produce defect-
free casting in Co/Ni/Cr
alloys
casting
shrinkage
2.3% 1.6% Compensated by correct
investment material
Modulus of
Elasticity
E (MPa)
250X103 200X103 100X103 Co/Cr most rigid for
equivalent thickness;
advantage for connectors;
disadvantage for
clasps
Yield
strength
(MPa)
600 - 700 300-500 Adequate for the
applications
12. Comparisonwith Cast goldalloys
Property Cobalt-
chromium
Nickel-
chromium
Gold type III
& IV
Comment
Hardness
(VHN)
3 5 0 VHN 250 VHN Co/Ni/Cr more
difficult to polish
but retains
polish during
service
% Elongation 1 – 2% 8-20% large values
suggest that
burnishing is
possible
Casting Carbon & Nitrogen No technique
sensitivity
difficult to
produce defect-
free casting in
Co/Ni/Cr alloys
Investment Carbon-free phosphate or silicate
investment with vents
Gypsum &
phosphate
Correct choice
effects cast
quality
13. Comparison with Cast gold alloys
Property Cobalt-
chromium
Nickel-
chromium
Gold type III
& IV
Comment
Casting ring Rubber ring Metallic ring Rubber rings
allow more
expansion to
compensate for
the casting
shrinkage
Melting crucible Ceramic Carbon or
ceramic
High Fusion Temp
Melting Oxygen acetylene
Electrical melting
Flame gas air
torch
Better melting
Casting
machines
Centrifugal + pressure Air pressure
casting machine
Different
requirements
Cooling Bench cool Rapid Cool High Fusion Temp
14. Property Cobalt-
chromium
Nickel-
chromium
Gold type
III & IV
Comment
Finishing &
polishing
Sand blasting for finishing
Electrolytic polishing (act as anode)
Polishing by
rubber cup &
paste
Acid pickling in
warm HCl for Au
but Not in
Co/Ni/Cr alloys
because it attack
the passive layer
Recasting No No Yes Co/Ni/Cr alloys
tech. sensitive
Microstructure Complicated coarse grain with
cored structure
Fine equiaxed
grains
Effect properties
Heat treatment Not Possible Possible Type IV gold heat
hardened
Uses Co/Cr for partial denture
frameworks, Ni/Cr for crown &
bridge
Can be used
for both
Base metals alloys
economical than
gold
Comparisonwith Cast goldalloys
15. Base metal alloys for removable
appliances
•For partial denture connectors high modulus and high
proportional limit is required to resist deformation
•In addition low density is preferred in denture base connectors
for a lighter weight in thin sections
•For clasps high flexibility and ductility is required to be resilient
and adjustable.
•Co/Cr alloys are more suitable for connectors while gold alloys
for clasps
•Ideal to solder both alloys but corrosion at the joint is inevitable
•In practice both connectors and clasps are cast with same alloy
•Co/Cr alloys most commonly used for denture framework
alternating gold alloys in properties and cost
16. Base metal alloys for removable
appliances
•Increased use of Ni/Cr alloys
for partial denture
framework castings. This is
due to the relative ease of
finishing and polishing
compared with Co/Cr as a
consequence of the
lower hardness value
17. Base metal alloys for fixedprosthesis
oNi/Cr preferred for fixed prosthesis over Co/Cr provided patient is not
nickel allergic
oNi/Cr rarely used for all metal cast restoration while widely used for
PFM restorations
oHigh rigidity with lower hardness of Ni/Cr is of advantage for long
span bridges
oCast gold is superior in terms of casting shrinkage, marginal fit and
accuracy
oNi/Cr casts can be ill fitting due to casting tech
sensitivity
oMarginal adaptation is less effected by polishing
oHigh hardness in Ni/Cr limits Occlusal adjustments
compared to cast gold crown
18. Biocompatibility
oBeryllium is a known animal carcinogen and
poses a potential threat to lab dental
personnel's
oFor the patient, the most immediate
biocompatibility risk concerns nickel and the
risk of allergic contact dermatitis
oMore than 1% of Nickel is labelled
oWarning for 0.02% beryllium or other
hazardous element
19. TitaniumandTitaniumAlloys
oIt was named Titanium after the Titans of Greek
mythology (the God of enormous strength)
oCommercially pure titanium is never present as pure
element in nature
o Contains Oxygen, nitrogen, carbon and hydrogen, which
are always present between the titanium atoms
oCommercially pure titanium occurs in 4 grades, grade I II
III IV, according to oxygen content (0.18% to 0.40 %) & iron
content (0.20 to 0.50 wt %)
20. Titaniumand TitaniumAlloys
oCommercially pure titanium (cp Ti) or its alloys are used for
dental implants, surface coatings, and crowns, partial and
complete dentures, endodontic files and orthodontic arch wires.
oTi-6Al-4V alloy is the most widely used.
oForms a very stable oxide layer, repassivates in seconds
therefore called bio-inert
22. TitaniumCasting
oProblems include:
High melting point (~ 1700°C)
o Chemical reactivity
Reacts with gaseous elements easily,
esp. at high temp (>600°C)
Need a well-controlled vacuum in
processing
The technology required makes
casting Ti expensive.
23. Properties of Titanium
oMechanical properties of (commercially pure Titanium) are
similar to gold alloy type III and IV.
oWhile those of Titanium alloy are similar to Cobalt Chromium
and Nickel chromium alloys.
oShaped by milling the surface
oSpark erosion for the fitting surface using electric discharge
machine