Metal & Alloy

Question 1

Gold alloys are classified by ADA Sp. No. 5 as:

Answer 1

1- Type I (Soft) used for restorations which subjected to very slight stress
such as small inlays.
2- Type II (Medium) used for restorations subjected to moderate stress such as inlays subjected to moderate stress.
3- Type III (Hard) used for high stress situations such as onlays,
crowns and short-span fixed bridges.
4- Type IV (Extra-hard) used for extremely-high stress cases such
as removable partial dentures and long-span bridges.

Question 2

Ingredients of Casting Gold Alloys:

Answer 2

Gold: Contributes to the colour & corrosion and tarnish resistance,
and ductility.
Copper: Increases hardness, strength and reduces melting
temperature.
But it reduces corrosion and tarnish resistance, and gives reddish colour to the alloy.
Silver: Contributes to hardness and strength, but it lowers the
corrosion and tarnish resistance.
Platinum: Strengthens the alloy, improves corrosion and tarnish
resistance, raises the fusion point.
Palladium: Strengthens the alloy, raises the fusion point, but
whitens the alloy.
Iridium: Produces fine-grained alloys, i. e. gives stronger and more
ductile alloys.
Zinc: Acts as a deoxidizing agent (scavenger), and improves
hardness.
Iron - Tin - and Indium Used for metal-ceramic alloys

Question 3

Classification of Dental Casting Alloys

Answer 3

High Noble Alloys ,
Contain more than 40% gold and equal or more than 60% noble metal elements.

Noble Alloys, Contain equal or more than 25% noble metal
elements.

Predominantly Base , Contain less than 25% noble metal elements. Metals

Question 4

Heat Treatment of Gold Alloys

Answer 4

 Softening heat treatment or (solution heat treatment).

The casting is placed in an electric furnace for 10 minutes at temperature of 700˚
C and
then it is quenched in water at room temperature.

 Hardening heat treatment or (age-hardening).

The softened casting is placed (soaking) in an electric furnace at temperature generally
between 200˚
C - 450˚
C for 15– 30 minutes and then it is quenched in water at room
temperature. The proper time and temperature are specified by the manufacturer. It
increases strength, proportional limit and hardness, but it decreases ductility.
Type III, IV casting gold alloys are suitable for age-hardening.

Question 5

Phosphate-bonded investment
usually used because

Answer 5

it permits adequate thermal expansion of the mould.

Question 6

Advantages of titanium

Answer 6

1. High strength.
2. Light weight.
3. Biocompatibility.
4. Tarnish and corrosion resistance.
5. Can be laser-welded.

Question 7

Applications of titanium:

Answer 7

1. Metal-ceramic restorations, dental implants, dentures base.
2. Bar connectors for overdentures.
3. Bone augmentation, bone splints, and artificial hip joints.
4. Artificial heart valves parts and pacemakers.

Question 8

Requirements of alloys used for metal-ceramic restorations

Answer 8

1. Coefficient of thermal expansion of the alloy should be similar
   to dental porcelain.
2. The alloy should be rigid

3.The alloy must not deform or change the color of the porcelain
4.The alloy must form strong bonding with porcelain.

Question 9

Cobalt chromium alloys properties

Answer 9

1. Melting point:
    Type 1 : High-fusing alloys have melting point above 1300˚C.
    Type I1 : Low-fusing alloys have melting point below 1300˚C.
   Generally, the melting range of cobalt chromium alloy is 1250 to 1450˚C.
   Hence gypsum-bonded investment should not be used. Instead, phosphate-bonded or silicate-bonded investments may be used.
2. Casting shrinkage: 2.3%

Question 10

Applications of cobalt chromium alloy:

Answer 10

1. For construction of removable partial dentures.
2. For construction of metal-ceramic restorations.
3. Plates and screws for surgical repair of fractured bones.

Question 11

Stainless Steels are

Answer 11

iron-based alloys which contain less than 1.2% carbon. When chromium (12 to 30%) is added to steel, the alloy is called as stainless steel alloy.

Question 12

Stainless Steels classification

Answer 12

1. Ferritic alloys: They have good corrosion resistance, employed in the manufacture of
   instruments or equipment parts, provided that high strength is not required. Chromium
   content (11.5 to 27 %).
2. Martensitic alloys: because of their high strength and hardness, they are used or
   surgical and cutting instruments. They have lesser corrosion resistance. Chromium
   content (11.5 to 17 %).
3. Austenitic alloys: They are the most corrosion resistance, used most extensively or
   dental appliance in the mouth as orthodontic wires. Chromium content (16 to 26 %).

Question 13

Stainless Steels properties and applications

Answer 13

Properties:
1. Greater ductility and ability to undergo more cold-working without
breaking.
2. Tensile strength 300 x 103
3. Hardness 525 KHN.
4. Modulus of elasticity 26 x 106

Applications:
1- Orthodontic appliances (e. g. orthodontic wires) & production of dental
instruments.
2- Clasps for removable acrylic partial dentures.