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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