DENTAL CASTING AND SOLDERING ALLOYS

BY MELISHA WILSON

KEY TERMS

ALLOY A mixture of two or more metals or metalloids that are mutually soluble in the molten state.

CASTING

Casting is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures.

 METALIt is an element which ionizes positively in solution and which exhibits metallic bonding (through a spatial extension of valence electrons), opacity, good light reflectance from a polished surface and high electrical and thermal conductivity.

THE WIDE VARIETIES OF COMPLEX DENTAL ALLOY COMPOSITIONS CONSIST OF:

Dental amalgams containing the major elements such as mercury, silver, tin, and copper.

Noble metal alloys in which the major elements are some combination of gold, palladium, silver and important secondary elements including copper, platinum, tin, indium and gallium.

Base metal alloys with a major element of nickel, cobalt, iron or titanium and many secondary elements that are found in the alloy compositions.

NOBLE METAL Metals which are highly resistant to oxidation and dissolution in inorganic acids. Gold and platinum group metals (Platinum, palladium, rhodium, ruthenium,iridium and osmium).

BASE METAL A metal that readily oxidizes or dissolves to release ions.

PHYSICAL PROPERTIES AND EFFECT OF NOBLE METALS AND BASE METALS ON DENTAL CASTING ALLOYS

GOLD

Pure gold is a soft and ductile metal with a yellow “Gold” hue. It has a density of 19.3 gms/cm3 , melting point of 1063oC, boiling point of 2970 oC and CTE of14.2×10-6/°C. Gold has a good luster and takes up a high polish. It has good chemical stability and does not tarnish and corrode.

Gold content: Traditionally the gold content of dental casting alloys have

been referred to in terms of: • Karat : It is the parts of pure gold in 24 parts of alloys. For Eg: a)

24 Karat gold is pure gold b) 22 Karat gold is 22 parts of pure gold and remaining 2 parts of other metal. The term Karat is rarely used to describe gold content in current alloys.

• Fineness: Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fine. Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine

SILVER

It is sometimes described as the “Whitest” of all metals. It whitens the alloy, thus helping to counteract the reddish colour of copper. To a slight extent it increases strength and hardness. In large amounts however, it reduces tarnish resistance. It has the lowest density 10.4gms/cm3 and melting point of 961oC, boiling point of 2216 oC among the four precious metals used in dental casting alloys. Its CTE is 19.7×10-6/oC , which is comparatively high.

PLATINUM

It increases the strength and corrosion resistance. It also increases the melting point and has a whitening effect on the alloy. It helps to reduce the grain size. It has the highest density of 21.45gms/cm3 , highest melting point of 1769oC, boiling point of 4530 oC and the lowest CTE 8.9×10-6/oC among the four precious metals used in dental casting alloys. 

PALLADIUM

It is similar to platinum in its effect. It hardens as well as whitens the alloy. It also raises the fusion temperature and provides tarnish resistance. It is less expensive than platinum, thus reducing cost of alloy. It has a density of12.02gms/cm3. Palladium has a higher melting point of1552oC, boiling point of 3980o C and lower CTE which is11.8×10-6/oC, when compared to gold. 

IRIDIUM, RUTHENIUM

They help to decrease the grain size. They are added in very small quantities (about 100 to 150 ppm). IRIDIUM has a high melting point of 2454°C , boiling point of 5300°C , density of 22.5gm/cm3 and CTE 6.8×10-6/oC. RUTHENIUM has a melting point of 1966°C , boiling point of 4500 °C , density of 12.44 gm/cm 3 and CTE8.3×10-6/oC 

CHROMIUM

Its passivating effect ensures corrosion resistance.The chromium content is directly proportional to tarnish and corrosion resistance. It reduces the melting point. Along with other elements, it also acts in solid solution hardening. Thirty percent chromium is considered the upper limit for attaining maximum mechanical properties. It has melting point of 1875°C , boiling point of2665 °C , density of 7.19 gm/cm3 and CTE 6.2×10-6/ oC

COBALT

Imparts hardness, strength and rigidity to the alloy. It has a high melting point of 1495 degrees celcius, boiling point of 2900 degrees celcius and a density of 8.85 gm/cm3.

NICKEL

Cobalt and nickel are interchangeable. It decreases strength, hardness, modulus of elasticity and fusion temperature. It increased ductility. Bio-incompatibility due to nickel, which is the most common metal to cause Contact Dermatitis. It has a melting point of1453°C , boiling point of2730 °C , density of 8.9gm/cm3 and CTE 13.3×10- /C6 o

IRON,BERYLLIUM

They help to harden the metal ceramic gold – palladium alloys, iron being the most effective. In addition, beryllium reduces fusion temperature and refines grain structure . IRON has melting point of 1527°C , boiling point of 3000 °C , densityof 7.87 gm/cm3 and CTE 12.3 ×10-6/oC

COPPER

It is the principal hardener. It reduces the melting point and density of gold. If present insufficient quantity, it gives the alloy a reddish colour. It also helps to age harden gold alloys. In greater amounts it reduces resistance to tarnish and corrosion of the gold alloy. Therefore, the maximum content should NOT exceed 16%. It has melting point of 1083°C , boiling point of 2595 °C , density of8.96 gm/cm³ and CTE 16.5×10-6/°C

MANGANESE AND SILICON

Primarily oxide scavengers to prevent oxidation of other elements during melting. They are also hardeners. Manganese has melting point of 650°C , boiling point of 1107 °C , density of 1.74 gm/cm 3 and CTE 25.2 ×10- / C , where as SILICON has melting point of 1410°C , boiling point of 2480 °C , density of 2.33 gm/cm 3 and CTE 7.3 ×10-6/oC . 

REQUIREMENTS OF THE ALLOYS

A casting alloy must be able to produce surface oxides for chemical bonding with dental porcelain.

A casting alloy should be formulated so its coefficient of thermal expansion is slightly greater than that of the porcelain veneer to maintain the metal porcelain attachment.

The alloy must have melting range considerably higher than the fusing range of the dental porcelain fired on to it.

The alloy must not under go distortion at the firing temperatures of the porcelain.

The first four requirements must be balanced with technicians need for ease of handling. A casting alloy should be biocompatible. 

CLASSIFICATION OF ALLOYS

BY FUNCTION Devised by the NATIONAL BUREAU OF

STANDARDS IN 1932. The gold based crown & bridge metals of that time were organized according to function into only four categories & described type 1, 2, 3, or 4 alloys. Alloys in each classification or type were arranged based on their gold &platinum group.

TYPE –1– SOFT USED FOR THE SMALL INLAYS TYPE –2– MEDIUMUSED FOR THE THREE QUARTER CROWN,THIN

BACKINGS. TYPE –3– HARD USED FOR THE FULL CROWNS, ABUTMENTS &

PONTICS TYPE –4—EXTRA HARDUSED FOR DENTURE BASE BARS, PARTIAL DENTUREFRAMEBARS, LONG SPAN FIXED PARTIALWORK.

BY USE Inlay Crown and bridge Prosthesis Post and core Removable partial denture implant

BY NUMBER OF METALS PRESENT Binary – formed by two metals Ternary – formed by three metals Quaternary – formed by 4 metals Complex alloys – formed by >4 metals

BY ELEMENT COMPOSITION Gold-based (Au) Palladium –based (Pd) Silver-based (Ag) Nickel-based (Ni) Cobalt –based (Co) Titanium-based (Ti)

PRINCIPAL THREE ELEMENTS COMBINATION

Au-Pd-Ag Pd-Ag-Sn Ni-Cr-Be

BY NOBILITY High noble Noble Base metal

DENTAL SOLDERING

SOLDERINGThe joining of metals by the fusion of filler metal between them, at a temperature below the solidus temperature of the metals being joined and below 450 degrees celcius.

BRAZINGSoldering procedures above 450 degrees celcius but below the solidus temperature of metals in alloy is known as brazing even though the names are used interchangeably in dentistry.

WELDINGThe joining of two or more metal parts through the application of heat or pressure or both with or without a filler metal to produce localized union across on the interface between the parts through fusion or diffusion.

CAST JOINING This is another type of metal joining

procedure in dentistry where two components of a fixed partial denture are combined by means of casting molten metal into interlocking region between invested components.This technique is prefered for base metal alloys because of the sensitivity of brazing or soldering these alloys

THE SOLDERING PROCESS

The soldering process involves the substrate or the parent metal to be joined, soldering filler metal (usually called solder), a flux and a heat source. All are equally important in the process and the role of each should be taken into consideration to solder metal components successfully.

COMPONENTS OF THE SOLDERED JOINT Parent metal Solder/filler metal Fluxes and Anti fluxes

PARENT METAL

This is also known as the substrate metal or base metal and is the metal or alloy to be joined.

The composition of the parent metal determines: Melting range Oxide that forms on the surface during heating Wetability of the substrate by the molten

solder Soldering should take place below the solids

temperature of the parent metal.

SOLDER/FILLER METAL

Classification of Solders Hard solders Soft solders Precious metal solders Non-precious metal solders

Hard solders have a high melting temperature and possess greater hardness and strength. Heating is done with gas torch or special devices. Two types of hard solders are used in dentistry.

Soft solders are a lead-tin eutectetic alloy with a low melting point. They have a low fusion rate of about 260 degrees celcius or less. Soft solders lack corrosion resistance so they are impractical for dental use.

FLUXES AND ANTIFLUXES

Flux is Latin word means flow. Dental do not flow or wet the metallic surfaces that have an oxide layer. The flux aid in the removal of the oxide layer so as to increase the flow of the oxide layer so as to increase the flow of the molten solder.

In addition the flux also dissolves the impurities and prevents the oxidation of the metals.

Fluxes used commonly are Borax glass– 55%. Boric acid --35% and Silica-- 10%.

Anti flux is a material that is used to confine the flow of the molten solder over the being joined.

The commonly used anti fluxes are pencil, graphite lines, iron rouge. 

SOLDERING TECHNIQUES

Investment soldering Used when accurate alignment is necessary The parts are placed on the master cast with a gap of at least

1 mm The parts are fastened with sticky wax before placing

soldering investment Antiflux is applied to confine the flow of the solder The investment is preheated to eliminate moisture Flux can be applied before or after heat treatment Soldering is carried out with a reducing flame of 750 to 870

degrees celcius The investment is cooled 5 mins before quenching Flux will cool to a glass which is removed by pickling

Freehand soldering Mainly used for hand soldering

orthodontic appliances Orthodontic torches are placed on a

bench so that both hands are used to hold the parts in position.

THE PROCESS

Substrate metals - basis

Filler metal - solder

Flux

Heat source

Also please see accompanying video

REFERENCES

Anusavice K.J.: Phillips science of dental materials, 10th ed. W.B. saunders Co.:111-555,19965. 

Messer R., Wataha J. Casting alloys Medical College of Georgia School of Dentistry, Augusta, GA 30912-1260, USA

Retrieved from http://survival-training.info/articles16/DentalCasting%20AlloysWataha.pdf on October 13, 2014

REFERENCES

Youtube video showing the DENTAL SOLDERING TECHNIQUE

https://www.youtube.com/watch?v=FO88qMWav94