4- Metal Alloys and Casting (Dr. Akhtar)

Alloy

Define the following:

A mixture of 2 or more metals or metalloids that are mutually soluble in the molten state

1. hardness

2. strength

3. toughness

alloying elements are added to alter the _______ (3) of a metallic element, thus obtaining properties not found in a pure metal

1. Base metal alloys

2. Noble metal alloys

3. High noble metal alloys

Alloys may be classified as (3)

ALL

Which of the following are general requirements of a dental alloy?

A) Not produce harmful toxicologic or allergic effects

B) Chemical properties should provide resistance to corrosion and physical changes in the oral cavity

C) Physical/mechanical properties should meet certain minimum values

D) Technical expertise needed for fabrication and use should be feasible for the average dentist and skilled technician

E) The metals, alloys, and companion materials should be plentiful, relatively inexpensive, and readily available

Type 0

What type of dental metallic material (ISO 22674 classification) is the following for:

- Intended use: Low stress, single-tooth fixed restorations (small)

- Example: Small one-surface inlay

Type 1

What type of dental metallic material is the following for:

- Intended use: Low stress, single-tooth fixed restorations

- Example: Crown with minimal occlusal force

Type 2

What type of dental metallic material is the following for:

- Intended use: Single tooth, fixed restorations

- Example: Crown with normal occlusal force

Type 3

What type of dental metallic material is the following for:

- Intended use: Multiple tooth, fixed restorations

- Example: Fixed partial denture (bridge)

Type 4

What type of dental metallic material is the following for:

- Intended use: Thin sections under high force

- Example: Precision attachment

Type 5

What type of dental metallic material is the following for:

- Intended use: High stiffness and high stress

- Example: Removable partial denture clasp

Chemical

Noble and base metals are defined on the basis of their _____ properties

Noble metals

What type of metals are in the red boxes?

Base metals

What type of metals are in the red boxes?

Cost

Precious metals are defined based on ________

Semi precious alloys

________ was originally used for noble metal alloys containing significant amounts of silver

Nonprecious alloys

________ are composed of non-precious ingredients, with most being based on a combination of nickel + chromium

1. high noble alloys

2. titanium + titanium alloys

3. noble alloys

4. predominantly base metals

4 classifications of alloys for dental prosthesis (ADA)

High noble alloys

Which class?

Total noble content >60% where 40% is gold

Titanium and titanium alloys

Which class?

Titanium content >85%

Noble alloys

Which class?

Total noble content >25%

Predominantly base metals

(*I think the slide was incorrect🧐)

Which class?

Total noble content <25%

B - HARDNESS

All of the following are important considerations when choosing a cast alloy EXCEPT:

a. Noble metal content

b. Softness

c. Yield strength

d. Elongation

e. Fusion temperature

1. Thermal expansion

2. Bond strength

3. Composition

What 3 properties should we consider when choosing alloys for PFM restos?

Porcelain to metal bond strength

What ensures retention of porcelain to the metal both in the oral environment and during thermal processing?

some components can affect the color of the porcelain

why is composition important for alloy selection for PFM restos

small cross section

Yield strength is crucial due to the ________

Stress

What controls the minimum allowable dimensions of critical areas?

Elastic modulus

What allows the flexibility of a metal framework?

1. resistance

2. corrosion resistance

3. thermal stability

chemical stability of alloys affects what of PFM restos (3)

1. type I soft

2. type II medium

3. type III hard

4. type IV extra hard

4 classifications for dental casting gold alloys (ANSI/ASDA No. 5)

Type I

ID the class type:

- Soft (85% Au, ~20K)

- Use: Inlays (low stress)

- Yield strength: <140 MPa

- Elongation: ~18%

- Hardness: 60–90 VHN

Type II

ID the class type:

- Medium (75% Au, ~18K)

- Use: Inlays & onlays (moderate stress, cusp replacement)

- Yield strength: 140–240 MPa

- Elongation: ~18%

- Hardness: 90–120 VHN

Type III

ID the class type:

– Hard (70% Au, ~17K)

- Use: Crowns & short-span bridges (high stress)

- Yield strength: 201–340 MPa

- Elongation: ~12%

- Hardness: 120–150 VHN

Type IV

ID the class type:

– Extra Hard (65% Au, ~15K)

– Use: Long-span bridges, RPD frameworks (very high stress, high flexural demand)

– Yield strength: > 340 MPa

– Elongation: ~10%

– Hardness: >150 VHN (slide typo again?)

Gold (Au)

ID the alloy in dental gold alloys:

- Specific gravity: 19.32

- Melting point: 1,063°C (1,945°F)

- Atomic diameter: 2.88 Å

- Space lattice: Face-centered cubic

- Chemical activity: Inert

- Color: Yellow

- Approximate content: 50-95%

- Density (specific gravity): Increases markedly

- Effect on color: Lends yellow color

- Melting: Raises melting point mildly

- Tarnish resistance: Essential to good tarnish resistance

- Heat hardening: Contributes importantly with Cu

Platinum (Pt)

ID the alloy in dental gold alloys:

- Specific gravity: 21.45

- Melting point: 1,769°C (3,224°F)

- Atomic diameter: 2.77 Å

- Space lattice: Face-centered cubic

- Chemical activity: Inert

- Color: White

- Approximate content: 0-20%

- Density: Increases markedly

- Effect on color: Whitens slowly; ~12% required; not pure white

- Melting: Raises melting point fairly rapidly

- Tarnish resistance: Contributes importantly

- Heat hardening: Increases with Cu

Palladium (Pd)

ID the alloy in dental gold alloys:

- Specific gravity: 12.0

- Melting point: 1,552°C (2,829°F)

- Atomic diameter: 2.74 Å

- Space lattice: Face-centered cubic

- Chemical activity: Mild

- Color: White

- Approximate content: 0-12%

- Density: Lowers slightly

- Effect on color: Whitens rapidly (as little as 5%)

- Melting: Raises melting point rapidly

- Tarnish resistance: Increases (less than Au and Pt)

- Heat hardening: Some increase with Cu

- Gas absorption: Rather high for hydrogen

- Castability: Effects not critical

Copper (Cu)

ID the alloy in dental gold alloys:

- Specific gravity: 8.96

- Melting point: 1,083°C (1,981°F)

- Atomic diameter: 2.55 Å

- Space lattice: Face-centered cubic

- Chemical activity: Very active

- Color: Red

- Approximate content: 0-17%

- Density: Lowers

- Effect on color: Lends red color; dark plate high in Cu

- Melting: Lowers melting point (even below its own)

- Tarnish resistance: Contributes to tarnish (flame/sulfur)

- Heat hardening: Essential if alloy heat hardens

- Castability: Effects not critical

Silver (Ag)

ID the alloy in dental gold alloys:

- Specific gravity: 10.49

- Melting point: 961°C (1,761°F)

- Atomic diameter: 2.88 Å

- Space lattice: Face-centered cubic

- Chemical activity: Active

- Color: White

- Approximate content: 0-20%

- Density: Lowers

- Effect on color: Whitens slowly; counteracts Cu; can create green gold

- Melting: Slight effect; may raise or lower mildly

- Tarnish resistance: Tarnishes with sulfur

- Heat hardening: Increases with Cu

- Gas absorption: Rather high for oxygen

Zinc (Zn)

ID the alloy in dental gold alloys:

- Specific gravity: 7.31

- Melting point: 420°C (787°F)

- Atomic diameter: 2.66 Å

- Space lattice: Close-packed hexagonal

- Chemical activity: Very active

- Color: White

- Approximate content: 0-2%

- Density: Lowers

- Effect on color: Too low % to affect

- Melting: Lowers melting point rapidly (in solders)

- Tarnish resistance: Will tarnish (low % minimal effect)

- Heat hardening: Slight with Cu

- Gas absorption: Good deoxidizer

- Castability: Decreases surface tension; increases fluidity

Iridium (Ir)

ID the alloy in dental gold alloys:

- Specific gravity: 22.4

- Melting point: 2,443°C (4,429°F)

- Atomic diameter: 3.32 Å

- Space lattice: Face-centered cubic

- Chemical activity: Active

- Color: White

- Approximate content: 0.005-0.1%

- Density: Increases slightly

- Effect on color: White

- Melting: No effect

- Tarnish resistance: Increased

- Heat hardening: No effect

- Gas absorption: No effect

- Castability: No effect

High gold alloys

ID the alloy:

- Gold content between 78% to 87% by weight

- Total noble metal content about 97%

- Tin, indium, and iron are added for strength and to promote good porcelain bond to metal oxide

- Light yellow in color

- Excellent hardness, tensile strength, and corrosion resistance

Gold-palladium-silver alloys

ID the alloy:

- 20% to 30% palladium content

- 10% to 15% silver content

- Higher elastic modulus and less susceptibility of dimensional change during the porcelain baking when compared to high-gold alloys

- Good clinical working characteristics and corrosion resistance

- Silver can change the color of porcelain

Palladium-silver alloys

ID the alloy:

- 50% to 60% palladium, with the most of the balance being silver

- Physical and chemical properties are comparable to other noble metal alloys

- Good corrosion and tarnish resistance

- Can affect the color of porcelain

gold-palladium alloys

ID the alloy:

- 50% gold

- 40% palladium

- Favorable yield strength and hardness

- Higher elastic modulus when compared to high-gold alloys

- Excellent corrosion resistance

- Thermal expansion is incompatible with some higher-expansion porcelain

palladium-copper alloys

ID the alloy:

- 70% to 80% palladium

- Up to 15% by weight of copper

- 9% gallium

- High hardness and low elastic modulus

- Sag resistance is a contra-indication for large span fixed partial dentures

Palladium-Cobalt Alloys

ID the alloy:

- 88% palladium

- 4% to 5% cobalt by weight

- Have high coefficient of thermal expansion

- Forms a dark oxide that may be difficult to cover

- More susceptible to hot tearing and embrittlement from carbon when no silver is present

Base Metal Alloys

ID the alloy:

- Based mostly on nickel and chromium, with a few cobalt-chromium and iron-based alloys available

- Corrosion resistance is based on chemical properties

- In case patient is allergic

- Manufacturing recalls

What are important about these stickers?

Dental waxes

What has the following characteristics?

- One of several esters of fatty acids with higher alcohols (usually monohydric alcohols)

- Combinations of various types of waxes compounded to provide the desired physical properties

- Uses include patterns for inlays, crowns, pontics, partial and complete dentures, and for bite registrations

- Classified according to their applications into pattern, processing, and impression waxes

spruing

What has the following characteristics?

- The "channel or hole through which plastic or metal is poured or cast into a gate or reservoir and them into a mold"

- It forms a mount for the wax pattern and fix the pattern in space so a mold can be made

- It creates a channel for elimination of wax during burnout

- This channel allows for molten metal to enter such mold

sprue button

Define the following:

"The excess material remaining at the base of the mold after dental casting"

Sprue designs for casting

What do all of these show?

B - Should attach to the BULKIEST PART

Which of the following is FALSE about spruing?

A) The sprue must be large enough so it remains open until the casting solidifies and short enough to allow rapid filling of the mold

B) It should always attach to the thinnest part of the pattern

C) Wax sprues are the most common

6mm

What is the minimum height at the blue box?

Investment Materials

Define the following:

Materials consisting principally of an allotrope of silica and a bonding agent (gypsum or phosphates and silica)

1. gypsum-bonded

2. phosphate-bonded

3. ethyl silicate-bonded

3 types of investment materials

gypsum-bonded investments

Define the following:

Most commonly used in the casting of dental gold alloys with liquids temperatures no higher than 1080ºC

Gypsum-bonded investments

What has the following characteristics?

- Normally used for gold inlays, crowns, and fixed and removable partial dentures

- Not suitable for high-melting alloys

Phosphate-bonded investments

What has the following characteristics?

- Available as a two-component system (powder and an aqueous solution of stabilized colloidal silica)

- Ingredients in the powder include powdered ammonium dihydrogen phosphate, calcined magnesium oxide powder, particles of refractory materials (quartz and cristobalite), and other metal oxides

Ethyl silicate-bonded investments

What has the following characteristics?

- Supplied as powder and liquid

- Powder consists of refractory particles of silicas and glasses, along with calcined magnesium oxide and other refractory oxides

- Liquid is a stabilized alcohol solution of silica gel, or on two liquids systems, ethyl silicate and an acidified solution of denatured ethyl alcohol

Lost wax technique

(Inexpensive and convenient to use even on complicated shapes)

What is one of the oldest casting techniques available?

0.1%

What is the desired accuracy % for casting?

Investment expansion

Shrinkage from wax and alloy can lead to ________

1/3

_____ of new gold alloy (by weight) must be used for each new casting

True

T/F: Care should be taken to avoid alloy contamination during metal melting

Natural gas/air torch

Gold alloys can be melted via ________/________

Gas/oxygen

Base metal alloys can be melted via ________/________

Mixing zone

ID zone of torch flame:

Consists of unburnt gases and is not hot enough to melt gold

Combustion zone

ID zone of torch flame:

- Partial Combustion of air & gas takes place here. Green in Color

- Zone is oxidizing &should always be kept away from molten alloy during fusion.

Reducing zone

ID zone of torch flame:

• Blue with burning gases and forms a reducing atmosphere

• Best for melting or soldering, because it is the hottest zone of the flame

Oxidizing zone

ID zone of torch flame:

Contains burnt gases, forms an oxidizing atmosphere and should be avoided

Reducing zone (hottest zone)

What zone of torch flame is best for melting metal?

True

T/F: Electric resistance, induction, and electric arc melting are also used for gold alloys, cobalt-chromium, and titanium alloys

Before!

Cleaning and finishing of a gold crown should happen _______ cementation

Soldering and welding

All of the following are indications for what?

A) Gold alloy fixed partial denture soldering

B) Adding proximal contacts

C) Repairing casting voids

D) Breaking solder joints

E) Pre-veneer metal-ceramic alloy soldering (pre solder)

F) Post-veneer metal-ceramic alloy soldering (post-solder)

Pre-veneer soldering

What has the following characteristics?

- During the fabrication of a fixed partial denture, the metal framework is often returned without porcelain application for try-in

- Soldering the metal framework uses a torch as the source of heat

Pre-veneer soldering

What is the most reliable method for soldering?

Post-veneer soldering

What has the following characteristics?

- The completed restoration (metal and porcelain) have been returned and a misfit is found during the delivery appointment

- Source of heat is a vacuum oven to allow precise heat temperature delivery

- Not as reliable for soldering as pre-veneer scenario

Welding

Define the following:

To unite or fuse 2 pieces by hammering, compression, or by rendering soft by heat with the addition of a fusible material

bonding

during welding, _________ will occur between 2 metallic surfaces if they are placed in contact and are free of surface films, oxides, and roughness

1. Spot welding

2. Pressure welding

3. Laser welding

What are 3 methods of metal to metal welding?