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Welding in Pediatric dentistry
- 1. Welding and Soldering Dr.Deepashree Paul
- 2. Joining Processes: Welding,Brazing, Soldering 1. Brazing and Soldering: Melting of filler only • Brazing: higher temperature, ~brass filler, strong • Soldering: lower temp, ~tin-lead filler, weak 2. Welding: Melting of base metals 3. Both: Join parts to form complex product sales.nordex-online.com
- 3. • Definition ofwelding proposed by the AWS is an • "operation which aims to get localized coalescence produced by heating to a suitable temperature, with or without applying pressure and filler metal, producing parts with strong welded union, nonporous and free of corrosion.”
- 4. Brazing • Steel basemetal + Brass filler rod is common • Lower temp than welding: retains heat treatment (if present), minimizes grain growth. • Strong but slow (careful preparation, cleanup) • Furnace brazing is easily automated
- 5. TYPES OF WELDING •Stick Welding • Metal Inert Gas Welding • Tungsten Inert Gas Welding • Spot Welding
- 6. Shielded Metal ArcWelding (SMAW): “Stick welding” • Older, simple technology • The electrode is also the filler rod • Only for steel • Strong welds if done properly (but often not) • Very high heat input: good for thick parts, bad for grain growth and distortion
- 7. Gas Metal ArcWelding (GMAW): “MIG” (Metal-Inert-Gas) • ~Complex mechanism but simple to perform and easy to automate • The electrode is also the filler rod, fed continuously from a spool. It melts in the arc. • For steel or aluminum • Low skill level can achieve good weld • Medium heat input: distortion and grain growth are significant
- 8. www.scenta.co.uk Gas Metal ArcWelding (GMAW): “MIG” (Metal-Inert-Gas) A fair/typical quality MIG weld (still hot!)
- 9. Gas Tungsten ArcWelding (GTAW): “TIG” (Tungsten-Inert-Gas) • The electrode is tungsten (not consumed) • The filler rod is separate and fed manually • High skill level required to achieve good weld • Difficult to automate • Low heat input and small weld bead: distortion and grain growth are minimized
- 10. Gas Tungsten ArcWelding (GTAW): “TIG” (Tungsten-Inert-Gas) • Typical good quality TIG welds
- 11. Resistance Spot Welding(RSW): “Spot Welding” • No filler rod: electrical current is passed through metal under pressure • Low skill level required • Easy to automate • Low heat input and no weld bead: distortion and grain growth are minimized
- 12. Distortion from WeldingProcesses • Non-uniform shrinkage of weld bead • Difficult to maintain alignments • Solution: Rigid fixtures, pre-compensate for warping, loose tolerances
- 13. Weld bead profile:Convex or Concave? • Solidification of molten bead leads to shrinkage • Shrinkage of a concave bead leads to tension on surface tends to crack • Shrinkage of a convex bead leads to compression on surface does not crack • Generally, slightly convex beads are preferred.
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- 15. Welding Flaws: Incomplete Penetration (notenough heat input)
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- 17. DEFINITION • Soldering involvesjoining two components of metal with an intermediate metal whose melting temperature is lower than the parent metal. It’s defined as the 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°C.
- 18. Silver solders •They arelow fusing –fusion temp-600-750°C •Used with stainless steel or other base metal alloys •Resistance to tarnish and corrosion is not as good as gold solders •But have strength comparable to gold solders •Composition •Silver -10-80 % •Copper -15-30% •Zinc -4-35% •with small amounts of cadmium, tin and phosphorus. •The formation of silver-copper eutectic is responsible for the low melting range of silver solder.
- 19. • HEAT SOURCE •The most common instrument used as heat source is gas- air or gas- oxygen torch. The type of torch depends on the type of fuel. • The fuels used are :- • Hydrogen -low heat content, so heating is slow. • Natural gas - heat content is four times that of hydrogen. • Acetylene - high flame temperature, but variation in temperature from one part of the flame to the other part is more than 100°C. So positioning of the torch is critical. It is chemically unstable gas, decompose to carbon and hydrogen. carbon can get incorporated in to nickel and palladium alloys. • Propane - is the best choice. Have highest heat content & good flame temperature. • Butane - has similar flame temperature and heat content. Both are readily available. Uniform in quality, virtually water free and burn clean.
- 20. • FLAME Theflame can be divided in to four zones • Cold mixing zone (unburned gas) • Partial combustion zone (neutral zone) • Reducing zone • Oxidizing zone (burned gas). • The portion of the flame that is used to heat the soldering assembly should be the neutral or slightly reducing part, because this produces the most efficient burning process and most heat. • Improperly adjusted torch or improperly positioned flame can lead to oxidation of the substrate or filler metal and result in a poorly soldered joint. • If unburned portion of flame is used carbon may be introduced to the substrate or filler. • To prevent oxide formation the flame should not be removed once it has been applied to the joint area until soldering process has been completed.
- 21. PARENT METAL •The parentmetal is the metal or alloy to be joined. This is also known as a substrate metal or base metal. Soldering operation is the same for any substrate metal, but the ease of soldering is not same for any substrate metal. The composition of parent metal determines- •Melting range •Oxide that forms on the surface during heating • Wettability of the substrate by the molten solder. • Soldering should take place below the solidus temperature of the parent metal.
- 22. • Composition ofalloy determines • the oxides that form on its surface during heating. The flux used should be able to reduce these oxides, inhibit further oxidation and facilitate its removal. • the wettability of the substrate by the molten solder alloy. The solder chosen must wet the metal at as low a contact angle as possible to ensure wetting of the joint area. • Manufacturer of the alloy should provide guidance and instruction regarding the flux to be used with that alloy. • A low temp soldering is preferred rather than the high temp soldering for Stainless steel wire to prevent carbide precipitation and to prevent an excessive softening of the wire. So silver solders are generally preferred.
- 23. REQUIREMENTS OF ASOLDER • Fuse safely below the sag or creep temperature of the parent alloy. • Resist tarnish and corrosion. • Non pitting. • Free flowing. • Match the color of the parent metal.
- 24. JOINT DESIGN • Spacingbetween parts being joined greatly affects tensile strength • Strongest joints are obtained when parts use lap or scarf joints • Some joints can be designed so that the flux and filler metal may be preplaced • Joint preparation is very important
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- 26. • CLASSIFICATION OFSOLDERS • Soft solders • Hard solders • Precious metal solders • Non precious metal solders • SOFT SOLDERS They are lead- tin eutectic alloy with a low melting point. Sometimes called as plumbers solder. They have low fusion range of about 260°C or less. Soft solders lack corrosion resistance, so they are impractical for dental use. • HARD SOLDERS Hard solders have higher meting temperature & possess greater hardness and strength. Heating is done with gas torch or special devices. Two types of hard solders are used in dentistry
- 27. COMPOSITION OF SOLDERS •Gold • Silver • Copper • Tin • Zinc • Gold solders are designated by fineness.
- 28. SOLDERING FLUX • Fluxmeans flow. • Improve the flow of a metal • Chemicals that limit the flow of metals are called antifluxes
- 29. FUNCTIONS AND TYPESOF A FLUX • PROTECTOR It covers the metal surface and prevents oxide formation. • REDUCER It helps to reduce the oxides present on the metal surface. • SOLVENTS It dissolves any oxide presence and removes it. The resulting solution of oxides or other extraneous matter in flux constitutes “slag”.
- 30. COMPOSITION OF AFLUX • Borax glass • Boric acid • Silica • Fluoride According to their composition Borax fluxes Fluoride fluxes According to the pH of the flux •Acidic fluxes – SiO2 •Basic fluxes – CaO, lime CaCO3 LIMESTONE •Neutral – Fluorspar (Ca.F2),Borax (Na2B4O2)
- 31. • BORAX FLUXES •Borax from Persian burah • Also called sodium borate , or sodium tetraborate , or disodium Tetraborate. They are based on boric or borate compounds such as boric acid/boric anhydrate and borax. • It is usually a white powder consisting of soft colourless crystals that dissolve easily in water. • Borax has a wide variety of uses:- • It is a component of many detergents, cosmetics, and enamel glazes. • It is also used to make buffer solutions in biochemistry as a fire retardant as an insecticide as a flux in metallurgy. • They act as protective fluxes and reducing fluxes for low stability. oxides such as copper oxide and are used for noble metal alloys.
- 32. • They areavailable in • Liquid form : Solution of borax/boric acid in water. Indicated for soldering of orthodontic appliances and bridges in which minimum amount of flux is required. • Paste form : Formed by mixing borax with petroleum jelly. Required when fluxes are needed in large quantity. • Powder form : Contains a mixture of borax, basic acid, silica flour and finely divided charcoal. Charcoals reducing agent and silica holds molten flux in surface of hot metal. This is usually used for casting operation.
- 33. • FLUORIDE FLUXES •Composition:- • Potassium fluoride – 50-60% • Borax glass - 6-8% • Potassium carbonate – 8-10% • As the choice of flux is dictated by the type of alloys to be soldered, the fluoride flux is used with alloys containing base metals even if a gold/silver solder is used. Some fluoride containing fluxes involve toxic fluorides when heated, so inhalation of fumes should be avoided.
- 34. SUPER FLUX •A combinationof high melting salts is used as fluxes to combine the good characteristics of each ingredient and create superior flux. •A formula for efficient flux is • Borax glass – 55 parts •Boric acid – 35 parts •Silica - 10 parts •The ingredients may be fused together and then crushed to fine powder.
- 35. • Too littleflux tends to burn off and will be ineffective. • Excess flux remains trapped within filler metal and cause a weakened joint. • Flux combined with metal oxides forms a glass during soldering process that is difficult to remove completely. A two step method for removing residual flux • Blast joint immediately after removal from investment with alumina abrasive particles followed by boiling in water for about 5 minutes.
- 36. ANTIFLUX • Used torestrict the flow of the metal. • Most common anti-fluxes used is graphite. • Better fluxes like Rouge in chloroform can be used. • It is applied on the surface of specific area where the solder should not flow into. It is applied before applying flux or solder. • E.g.: Graphite in the form of lead pencil. Disadvantage of graphite is that it can burn off on prolonged heating at high temperature. • In such cases whiting (CaCO3 in alcohol and water suspension) is used.
- 37. QUALITIES OF ANIDEAL SOLDER • FILLER METAL/SOLDER • Ease of flow at relatively low temperature. • Sufficient fluidity to freely flow when melted. • Ability to wet substrate metal. • Strength compatible with that of the structure being joined. Resistance to tarnish and corrosion. • Acceptable colour to give an inconspicuous joint. • Resistance to pitting during heating.
- 38. The joining areashould be three times the thickness of the thinnest joint member.
- 39. When building upa surface, alternate the direction of each layer
- 40. Best types offlame to use: •Air acetylene •Air MAPP •Air propane •Any air fuel-gas mixture
- 41. SOLDERING AND BRAZING METHODS Groupedaccording to method of applying heat: • Torch • Infrared • Laser • Furnace • Induction • Dip • Oven
- 42. TORCH SOLDERING • Solderingis done under direct flame. • A gas air torch is used for this purpose. • The torch flame has two parts – the reducing part (is at a higher temperature) and the soft brush part.
- 43. • The soldershould be melted using the soft brush flame. • The flame should be constantly swiped over the solder for a period of 4 -5 minutes. • At no point of time the flame should be held in a stationary position.
- 44. INFRARED SOLDERING • Usedfor low fusing connectors. • Good accuracy. • Similar strength as conventional soldering. • Protective eyewear is necessary.
- 45. LASER SOLDERING • Doneto join Ti components of dental crowns, bridges and partial denture frame works. • Pulsed high power Neodymium lasers are used. • Low thermal influence preferred in dentistry. • Superior joint strength.
- 46. FURNACE SOLDERING • Advantagesof using a furnace: • Furnace brazing is a semi-automatic process • Temperature control • Controlled atmosphere (Common atmospheres used include: inert, reducing or vacuum atmospheres all of which protect the part from oxidation) • Uniform heating • Mass production • Disadvantages of using a furnace: • Size • Heat damage
- 47. Furnace brazing permitsthe rapid joining of parts on a production basis
- 48. SOLDERING TECHNIQUE • Thedesign of the connector is determined while fabricating the wax pattern. • All the solder connectors require about 0.25mm parallel spaced between the parent components. • The smaller units are inserted separately in the mouth.
- 49. • A thickmix of quick setting plaster is molded over the inserted units. • Once the plaster sets, it is removed along with the inserted units. • Plaster index is inverted. • Components of the prosthesis will be visible.
- 50. • A triangularpiece of utility wax should be placed to index restoration in order to shape the soldering assembly. • The units are invested and the investment is allowed to bench set. • Invested wax is eliminated using boiling water or chloroform.
- 51. • Area ofthe restoration surrounding the joint should be coated with anti-flux. • After coating the flux and the anti-flux, the assembly is preheated in a burnout furnace. • Connectors are soldered using a torch or a furnace.
- 52. • The partsto be joined are fixtured and the brazing compound applied to the mating surfaces, typically in slurry form. Then the assemblies are dipped into a bath of molten salt (typically NaCl, KCl and other compounds) which functions both as heat transfer medium and flux. • Advantages of dip processing: • Mass production • Corrosion protection • Distortion minimized • Disadvantages of dip processing: • Steam explosions • Corrosion • Size • Quantity
- 53. CERAMIC/ACRYLIC VENEERING • Theceramic or acrylic veneers in metal resin or metal ceramic restorations are usually added after soldering. • Ceramic veneering is done in 3 steps 1. METAL PREPARATION. 2. PORCELAIN APPLICATION. 3. PORCELAIN FIRING.
- 54. SOLDERING INVESTMENT • Theseare silica –bonded investments that are fused quartz. • Fused quartz is used because it is the lowest thermally expanding form of silica.
- 55. SOLDERING FOR METALCERAMIC RESTORATION • It is usually done prior to ceramic application hence it is also known as pre ceramic soldering. • Post ceramic materials are also available.
- 56. • Pre ceramicsoldering is done at a temperature of 1075 to 1120 degree celsius whereas post ceramic soldering should be done at a temperature of 920 degree celsius because ceramic may begin to sag at higher temperatures.
- 57. SOLDERING INVESTMENT • Theseare silica –bonded investments that are fused quartz. • Fused quartz is used because it is the lowest thermally expanding form of silica.
- 58. SOLDERING FOR METALCERAMIC RESTORATION • It is usually done prior to ceramic application hence it is also known as pre ceramic soldering. • Post ceramic materials are also available.
- 59. • Post ceramicsoldering should be done after ceramic firing. • Ceramic portions should be finished only after soldering.
- 60. ADVANTAGES OF PRECERAMIC SOLDERING • The metal framework can be soldered and tried in prior to ceramic build up. • Minor casting errors can also be patched up during ceramic build up.
- 61. DISADVANTAGES OF PRECERAMIC SOLDERING • Difficult to build ceramic on already soldered units.
- 62. ADVANTAGES OF POSTCERAMIC SOLDERING • Porcelain can be properly build up due to better access.
- 63. DISADVANTAGES OF POSTCERAMIC SOLDERING • The metal and porcelain may sag at high soldering temperatures. • It is more technique sensitive. • The solder should be re-glazed and re- fired.
- 64. OVEN SOLDERING • Performedunder vacuum or air.Performed under vacuum or air. • A piece of solder is placed in the jointA piece of solder is placed in the joint space and it is heated to a standardspace and it is heated to a standard temperature in the furnace.temperature in the furnace. • Superior joint strength.Superior joint strength.
- 65. DISADVANTAGES • The parentmetal will sag or melt ifThe parent metal will sag or melt if heated for a long time.heated for a long time.
- 66. SOLDERING FAILURES • Failureto clean the parts to be joined • Improper fluxing • Poor flow of solder • Over heating of the solder can lead to pitted joint of low strength Besides porosities and brittleness from oxides, gases, or foreign matter resulting from the soldering procedures as factors for increasing the incidences of failure of soldered joints. • Creep, corrosion, stress corrosion cracking, corrosion-fatigue, and corrosion-erosion. • Gas embitterment can also be generated by gases formed from electrochemical processes. Hydrogen embitterment from corrosion is a very well-known phenomenon that occurs with some material-solution combinations and at temperatures comparable to physiologic conditions.
- 67. CONCLUSION • The choiceof solder material has extreme importance in determining the properties of the soldered joints. Silver solders are popular because of their lower fusion temperatures and easy-handling characteristics. Soldering is still a useful and needed procedure for the joining of metallic parts. The choice of whether free-hand or investment, torch or oven, or pre or post ceramic soldering techniques are used, as well as variations in gap distance and high- or low-fusing soldering alloys, is in many instances up to the discretion of the dental surgeon and technician. The breakage of soldered components is one of the pitfalls associated with this joining procedure. Even though following guidelines should theoretically prevent them from occurring, solder failures are not rare.
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