dental casting alloys - minia.edu.eg 25-3-2020dent… · dental casting alloys are those used for...

Dr/Asmaa Abdel-Hakeem

Def:

Dental casting alloys are those used for the

construction of indirect metallic

restorations e.g. inlays, onlays, crowns,

bridges, endodontic post and core, and

removable partial denture framework


Requirements:

A - Functional Requirements

1. High yield stress

2. Stiffness

3. Resilience

4. High Fatigue resistance

5. Compatibility with porcelain

6. Tarnish and corrosion resistance

7. High sag resistance

8. High Fitness


B- Working characteristics:

1- Easy casting,

2-Easy soldering

3-Easy burnishing.


Classification according nobility:

1. High noble (HN) ……………………40 % gold(Au) 60 % Noble metal

2. Noble (N) ………………………..More than 25% noble metals

3. Predominantly base metal (PB) ……………………More than 75 % Base metal or less than 25 % Noble metal

Classification according to Mechanical Properties (A.D.A. Classification NO:5 ).

Type I (Soft)

Stresses ≤140MPa

Type II (Medium)

Stresses 140 – 200 MPa

Type III (hard)

Stresses 200– 340MPa

Type V(Extra hard)

Stresses ≥ 340MPa


Classification according to uses of dental

casting alloys:

Alloys for all metal restorations.

Alloys for metal ceramic restorations.

Alloys for partial denture frame works


Composition of gold alloys:

Gold is alloyed with several metals in order to

modify the properties.

A) Noble metals

B) Base metals


A) Noble metals and their effect on properties:

1) Gold: a high noble metal that resists tarnish and corrosion. It has low strength, hardness and high ductility. Its specific gravity is 19.3.

2) Platinum: a noble metal that resists tarnish and corrosion. It increases strength and hardness of gold. Its melting temperature is 1755°C, and specific gravity 21.37.

3) Palladium: cheaper than platinum, and is used as a replacement for platinum. Its specific gravity is half that of platinum 11.4. It is a malleable and ductile metal. Palladium quantities as low as 5% have a pronounced effect on whitening the color of gold alloy.

4) Silver: malleable, ductile, stronger and harder than gold. it can whiten the color of gold. Pure silver occludes oxygen in the molten state which is evolved during solidification. This phenomenon is reduced by addition of 5 to 10 % copper.

5) Irridium, Ruthenium, and Rhodium: As little as 0.005% is effective in refining the grain size of gold alloy


B) Base metals and their effect on properties:

1) Copper: malleable and ductile metal with a characteristic red color. It imparts strength and hardness to gold alloys. It is an important factor in heat treatment.

2) Zinc: acts as a scavenger (deoxidizing agent) during melting and casting of gold alloys,in low concentration as 0.5-2% improve castbility and fluidity.

3) Indium & Tin: soft metals with a low melting points Indium Gray white Oxidizing agent. Play important role for porcelain bonding used as replacement of zinc. They produce oxides in porcelain alloys, which help bonding with porcelain Tin. Play important role for porcelain bonding Lustrous white and make Hardening effect with Pl&Pd

4) Iron: Increase hardness, strength and modulus of elasticity by formation of Pt3Fe


ClassificClassifications of gold alloys:

ation of gold alloy:

Acc. to gold content Acc. to mechanical

prop

Acc. To alloy

description

1. Carat (1/24 parts)

2. Fineness (1/1000 parts)

2. Type II (Medium)

3. Type III (hard)

4. Type V (extra

hard)

1. Type I (soft)

Color description

Economy golds 42% > Au > 58%

White gold Pd > 5%

Yellow alloy ↑ Au & Cu

Gold alloys: Composition and mechanical properties:-

Typ

e Au % A g% Cu % Pt % Pd % Zn %

Y.S. (At 1%

offset)

V.H.N. Elong. %

I 87 9 4 - - - 100-120 50-90 35

II 76 13 8 - 2.5 0.5 150-180 95-120 30

III 70 15 10 1 3 1 S

H

200

310

S

H

120

170

S

H

35

20

IV 65 12 15 2 3 2 S

H

275

510

S

H

150

250

S

H

30

8 Dr/Asmaa Abdel-Hakeem

Low gold content (Economy gold) with

palladium more than 5% and silver addition.

The gold content reduced to 42-58 % and was

replaced by silver and palladium for Low gold

alloys 1% palladium was needed for every 3%

silver to resist the tarnish tendency of silver.

.


It allows for atomic diffusion in the solid state by heating the solid below its solidus temperature.

It is performed to control the physical properties of dental gold alloy.

There are two processes:

A-Softening heat treatment (solution heat treatment).

Heating the alloy to 700 oC in electric furnace for 10 min then rapid cooling by water quenching, to obtain disordered solid solution.

The tensile strength, proportional limit and hardness are decreased but ductility is increased.

Softening heat treatment is indicated for structures subjected for cold working.


•The alloy should be subjected to softening heat

treatment first.

• Then alloy heated to 450 oC for 2 minutes in hot

oven. Slowly cooled within 30 minutes to 250 oC

then quenching in water at room temperature.

•Hardening heat treatment increase strength (tensile,

yield strength and P.L), increase hardness and

decrease ductility.

B- Hardening heat treatment: (Age hardening)


N.B.

Only Type III and Type IV respond to heat treatment because their gold copper ratio allows the precipitation of the ordered structure (AuCu) in the parent alloy.

This is not the case in Type I and Type II, therefore they don’t respond to heat treatment.

Indication of hardening heat treatment:

1-Metallic partial dentures.

2-Bridges and other similar structures.


It used in construction of crown and bridges as

alternative to gold alloys at less expensive cost.

Types:

Silver palladium.

Palladium-silver. Palladium 60%, silver 30% with

other metals as tin, indium, gallium as balances.

Problem with their use:-

1- Low density.

2- Low ductility.

3- Porosity due to gas absorption.


dental casting alloys - minia.edu.eg 25-3-2020dent… · dental casting alloys are those used for...

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