Dental casting alloy

 DENTAL CASTING ALLOY - Dental material 

• Dental casting alloy was introduced in the year 1907 by taggart ,he described method of making gold inlay using lost wax technique.

AIM OF THIS POST : Will learn few historical significance,classification of dental casting alloy,ideal requirement ,mechanical properties,heat  treatment,soldering,brazing,welding and finally about the  difference between soldering and brazing .

• In 1932 : national institute of standard and technology classified dental material by vicker hardness number 

VHN TYPE 1 - SOFT
TYPE 2- MEDIUM
TYPE 3- HARD
TYPE 4- EXTRA HARD 

CLASSIFICATION 

1)BASED ON NOBLE METAL CONTENT:

• HIGH NOBLE: Noble content more than or equal to 60%
• noble- noble content more than or equal to 25% by weight .
• predominantly base-  less than 25% by weight of noble metal

2)BASED ON MECHANICAL PROPERTIES:

1. TYPE 1  - LOW (INLAY)
2. TYPE 2 - MEDIUM (INLAY,ONLAY)
3. TYPE 3 -  HARD (ONLAY,FULL CROWN)
4. TYPE 4 - EXTRA HARD (CROWN,BRIDGE)

3) BASED ON PRINCIPLE ELEMENT:

1. PALLADIUM-SILVER
2. COBALT -CHROMIUM
3. NICKEL-CHROMIUM - BERYLLIUM

4) BASED ON APPLICATION 

1. Alloy for metal and resin veneer
2. Metal ceramic restoration
3. Removable denture  

5) ACCORDING TO NUMBER OF ALLOY PRESENT:

1. BINARY
2. TERNARY
3. QUATERNARY

IDEAL REQUIREMENT:

1. Bio compatible
2. Tarnish and corrosion resistance
3. Thermal properties: melting range is low,must tolerate high processing temperature,sag resistance.
4. Porcelain bonding
5. Castable
6. Economic

MECHANICAL PROPERTIES:

1) ELASTIC MODULUS:

• FIXED PARTIAL DENTURE: resistance to flexure
• major connector of fixed partial denture: alloy must be rigid and should prevent flexure during placement and flexure.

• base metal alloy:elastic moduli twice as high as noble metal alloy 

2) YIELD STRENGTH:

Alloy should have high yield strength 

tensile yield strength above 300 MPa

3) DUCTILITY:

Clasp,margin adjustment and burnishing should have reasonable amount of ductility and malleability

4) Hardness:

High to resist:

scratching,abrasion and to maintain smoothness.

noble metal: lower hardness and greater ductility

5) FATIGUE RESISTANCE:

Cobalt chromium alloy exhibit highest fatigue resistance that greater than cobalt chromium alloy which is greater than type 4 gold ,greater than titanium 6 aluminum -4 vanadium Alloy 

HEAT TREATMENT OF GOLD ALLOY ;

GOLD:

 Pure gold : to soft and ductile and cannot be used for casting dental restoration

Type of gold alloy 

1. yellow gold alloy
2. white gold alloy
3. low gold alloy 

Measurement 

1. Karat: Part of pure gold in 24 part of alloy
2. Fineness: Part per thousand of pure gold

• HEAT TREATMENT OF GOLD ALLOY :increase the physical properties

Types :

1)SOFTENING :

INCREASES ductility
decreases strength,proportional limit,hardness

Indication: 

cold worked 

appliance that are grounded

method : casting in 10 minutes at 700 degree Celsius in a furnace and quenched in water 

HARDENING HEAT TREATMENT:

• Increases strength,proportional length,hardness
• decreases ductility
• indication: metallic partial denture,bridge.
• method: casting at 200 to 450 degree Celsius for 15 to 30 mutes and quenched and cooled slowly

SOLDERING:

joining 2 or more metal by heating them to a temperature below solidus temperature and filling the gap between them using molten metal with liqudius temperature

uses:

1. Assembling broken bridge
2. Building up of inlay,onlay
3. Joining orthodontic bracket

CLASSIFICATION:

1) HIGH SOLDER                                2) SOFT SOLDER 

• High fusion temperature                  lower fusion temperature
• higher strength                                              good mechanical properties 
• example:silver,gold                           tin-lead solder 

Requirements:

• Tarnish,corrosion resistance 
• fusion temperature
• flow properties

Flux: flow 

• Commonly used ; borax,fluoride
• Paint on substrate metal
• Fused on filler metal 

ANTI FLUX:

• Prevent flow of molten solder on area coated by it 
• example: graphite

HEAT SOURCE:

• Gas air or gas oxygen torch
• fuel used is hydrogen,natural gas,propane.

WELDING:

Process of fusing two or more metal part together the application of heat or pressure with or without filler metal

methods:

1) fusion welding

 2) pressure welding

BRAZING:

• Brazing provide strongest joint
• Require highest temperature
• Uses: repair of cast iron,wrought iron.

DIFFERENCE BETWEEN SOLDERING AND BRAZING 

SOLDERING                                         BRAZING 

• below 450 degree Celsius        heating temperature is above450
• both process involves metallic component ( substrate metal),filler metal ( solder) and flux and heat source   

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