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Dental casting alloys

Applied Dental Material
Second Year

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Dental casting alloys

  1. 1. Dental Casting Alloys Introduction Dental Cast Noble Alloys Dental Cast Base Metal Alloys
  2. 2. Revision:  Metal: It is an element which ionizes positively in solution.  Alloy: It is the combination of 2 or more metals. It allows combining of best properties of many metals for specific purposes. [Inlays, long span bridges, partial denture , etc.]  Shaping: Metals & alloys are shaped by the dental casting technology. It is the lost wax technique introduced by Taggart in 1907. It is the process of turning wax pattern of restoration into metallic one.
  3. 3. Revision:  Shaping:[Casting technology] 1. Create the wax pattern on the die. 2. Sprue the wax pattern. 3. Invest the wax pattern. 4. Burn out the invested pattern to create a mold. 5. Cast the molten metal. 6. De-investing. 7. Finishing & polishing. 8. Cementing.
  4. 4. Microstructure:[Cast structure] This means grains [crystals  regularity & repetition (crystalline)] & grain boundaries[No regularity & no repetition(Amorphous)]
  5. 5. Definition: Dental casting alloys are disordered substitutional solid solution. WHY?
  6. 6. Definition:  Solid solution alloy [S.S.] is a combination of 2 or more metals, which are completely soluble in each other in both liquid & solid states.  Properties:  It is stronger, harder but less ductile than the constituent metals due to the difference in atomic size crystalline structure of the alloy less than 15%. [solution hardening].  It is more ductile & resistant to tarnish & corrosion than other types of alloys as S.S. is 1phase & homogenous.
  7. 7. Dental applications: Inlay Onlay Metallic bridge Dental Implant Ceramometallic bridge
  8. 8. Requirements of the Metals:
  9. 9. 1- Biological requirements:  not allergic.  cause no health hazards.  resistant to tarnish & corrosion. This is achieved by : -Use of solid solution alloy [1phase= homogenous] -Use of noble alloy -Use of base metal alloy with the passive layer. It acts as corrosion barrier, because it is thin, non porous, adherent & transparent surface oxide layer protecting the underlying metal.
  10. 10. 1- Biological requirements:  Types of corrosion: Chemical Corrosion. Electrochemical Corrosion.  Requirements for Electrochemical Corrosion: Anode, Cathode, Circuit, Electrolyte.  Electrochemical Corrosion Categories: a.Galvanic Corrosion (macro-galvanic) b. Local Galvanic Corrosion (structure-selective corrosion) c. Concentration Cell Corrosion (crevice corrosion) d. Stress Corrosion
  11. 11. 2- Functional requirements: : a- all metallic restorations:  High yield strength:           to resist permanent deformation [Clinical ,Functional, failure] High cantilever bending strength: to resist cantilever bending. High transverse strength: to resist transverse bending. High fatigue strength: to resist cyclic loading. High impact strength: to absorb energy of fracture under sudden load. High modulus of elasticity:to resist elastic deformation; for equal stress distribution to preserve the supporting patient structures. High ductility: to facilitate burnishability High resilience: to absorb energy of elastic deformation so that not all the stresses will be transmitted to the supporting patient structures High toughness: to absorb energy of fracture. High hardness: to preserve the finished & polished surface. High sag resistance: to resist plastic deformation during soldering [joining of metallic parts by intermediate alloy]
  12. 12. 2- Functional requirements: a-Functional requirements of all metallic restorations: σ ε
  13. 13. 2- Functional requirements: b- ceramometallic restorations: • The metal substructure [Coping] is first casted & then used to support the brittle ceramic, which is built & fired on top. • This restoration provides strength & aesthetics.
  14. 14. 2- Functional requirements: b- ceramometallic restorations:  Melting temperature of metal > Firing temperature of ceramic  to resist sag or melting of metal.  The coefficient of thermal expansion & contraction of metal slightly >α of porcelain to bring the ceramic in state of compression which closes the cracks at the interface.  To strengthen ceramic. * To obtain compressive bonding between both of them.  Metal must form surface oxides layer  which react chemically with ceramic. [chemical bonding]   Metal must have surface roughness  mechanical bonding with ceramic.  High modulus of elasticity - No flexure  Increase the fracture resistance of porcelain. - Use in thin section  Enough space for ceramics.  Metal must not discolor porcelain
  15. 15. 3- Working requirements:
  16. 16. Dental Casting Alloys
  17. 17. 1- General classification:  Definitions  Noble metal is the one which retains a lustrous metallic surface resisting tarnish & corrosion during casting, soldering & service. i.e. It is stable in its elemental form. Its nobility indicates the inertness of the element in relation to the standard EMF series.  Noble metals: gold [Au] + platinum group. The Platinum group: -light group: -Ruthenium [Ru], rhodium [Rh], palladium [Pd]. -heavy group: -Osmium [Os], iridium [Ir], platinum [Pt].  Precious metals: noble metals + silver [Ag].  The base metal: the one which is reactive to atmosphere.
  18. 18. 1- General classification:
  19. 19. 2-Mechanical properties:  Classification: Type % Elongation [Ductility] Type I [Soft] 140 MPa Low hardness. 18% minimum. Inlay Type II [Medium] 140-200 MPa Medium hardness. 18%minimum. Onlay Type III [Hard] Strength & hardness from type I-IV Yield stress Hardness Restoration 200-340 MPa High hardness. 12%minimum. Crown Short bridge. Type IV [Extra hard] 340- 500 MPa Extra high hardness. 1o%minimum. Crown Long bridge Post & Core Partial denture Ductility % Elongation from type I-IV
  20. 20. 2-Mechanical properties:  This classification is more clinically relevant, because:  The yield stress indicates -Clinical [functional] failure of the restoration.  The hardness indicates -The ease of finishing & polishing. -The ability to retain the finished & polished surface.  The % elongation [Ductility] indicates -The ability of the restoration for adjustment & burnishing.
  21. 21. 3-Specfic use: Alloys for all metal restorations.  Alloys for metal-ceramic restorations.  Alloys for frameworks for RPD.
  22. 22. Gold Restorations  Types: 1. Direct gold restorations.[Pure gold] 2. Indirect gold restorations.[Gold alloy  Dental cast noble alloy]
  23. 23. Direct Gold Restorations  Definition: Direct gold restoration is one of the oldest restoration.  Supplied form: Sheets or pellets of 99.99% pure gold.  Indications: Small lesions in non stress bearing areas.  Properties: 1. It is soft 25HV ,% elongation 45%,30MPa. 2. It can be easily cold worked during filling the cavity . 3. Gold foil is an example of cohesion in dentistry. 4. Disadvantages: It needs skill. Time consuming. Expensive.
  24. 24. Indirect gold restorations  General characteristics of gold[Au]:  Weak, soft, flexible, ductile & malleable.  Resist tarnish & corrosion.  Yellow color.  Melting temperature:1083ºC.[Relatively low]  Density:19g/cm3.  Coefficient of thermal expansion:14 Χ 10-6 /ºC.
  25. 25. Alloying elements in gold alloys for all metallic restorations:  strength , hardness & stiffness; ductility .·. Add to gold:  Platinum, Palladium, Silver & Copper Solution Hardening due to difference in atomic size [less than15%]  Copper [Cu] Hardening Heat Treatment  Iridium ,Ruthenium ,Rhodium Grain Refiner Coarse grains Less grains Less grain boundaries Fine grains More grains More grain boundaries
  26. 26.  Color: .·. Add to gold  Platinum Palladium & Silver  Copper To produce white color. To enrich yellow color.  Cast-ability:  Silver  Ag in the molten state occludes gases  porous casting. Cu in the gold alloy will prevent this gas occlusion.  Ag reacts with sulfur  tarnish layer on gold alloy. Pd in the gold alloy prevent this reaction. [3:1]  Palladium  Pd in the molten state absorbs hydrogen  porouscasting.  Zinc [Zn]  It acts as scavenger for oxides during manufacturing.  It increases fluidity of molten alloy by decreasing its surface tension  improving cast-ability.
  27. 27. Alloying elements in gold alloys for ceramometallic restorations:  Melting temperature. Coefficient of thermal expansion & contraction.  Platinum , Palladium [of high melting temperatures]  To obtain surface oxides.  Iron, Tin, Indium   Strength, hardness & stiffness.  Platinum , Palladium  Solution hardening due to difference in atomic size less than15%.  Iron  Reacts with platinum forming intermetallic compound, which precipitates within the solid solution.[Precipitation hardening]  Neither Silver or especially Copper is added, because they produce greenish discoloration to the ceramic.
  28. 28. Indirect gold restorations Mechanical Properties Gold Content Description Classification
  29. 29. According to gold content: Carat in24 parts. Number of parts of gold by weight Fineness in 1000 parts. It is used to describe dental solder. •This classification is not suitable for dentistry; as it refers to the amount of gold only, & not to the other alloying metals which may have a pronounced effect over the properties of the gold alloy.
  30. 30. According to mechanical properties:  Classification: Type Gold Copper Silver Type I Soft 87 4 11 Type II Medium 76 8 12 Type III Hard 70 10 14 Type IV Extra hard 65 15 9 N.B.: Type III and Type IV can be heat treated into soft and hard forms, as their Gold/Cupper allow heat treatment.
  31. 31. According to description: N.B: In general it should be kept in mind that the higher the gold content, the better is the fit and marginal adaptation of the restorations.
  32. 32. Heat treatment of gold alloys: AuCu3 AuCu Ordered crystal lattice
  33. 33. • Heat Treatment of Gold Alloys: Au:Cu ≥88:12 [Types III & IV] Disorder –Order Transformation Softening heat treatment REVERSIBLE  Procedures: Heat the alloy to 700ºC for 10 minutes then quench.  Effect:  Proportional limit, tensile strength & hardness. Hardening heat treatment[Age hardening]  Procedures: Heat the alloy to 424ºC for 2minutes then cool to 250ºC over 30 minutes, then quench.  Effect:  Proportional limit, tensile strength & hardness.  Ductility. Modulus of elasticity is not affected.  Ductility. Modulus of elasticity is not affected. • Indicated for adjusting, burnishing & • Indicated for service as oral restoration polishing. • Microstructure: Disordered face centered cubic substitutional solid solution. delivered to the patient. • Microstucture: Precipitation of ordered face centered tetragonal superlattice in disordered face centered cubic substitutional solid solution.
  34. 34. • Heat Treatment of Gold Alloys: Au:Cu ≥88:12[Types III & IV] Disorder- Order Transformation:  Explanation: The thermal treatment provide the atoms with sufficient energy for atomic diffusion. This permits for regional ordering & precipitation of these superlattice in the alloy structure  Structural discontinuities  Strengthening.  Factors affecting the amount of transformation: The process depends on : 1. Composition of the alloy. 2. Temperature. 3. Time.
  35. 35. 38
  36. 36. Properties of gold alloys: 1.   Biological properties: Biocompatibility: Gold alloys are biocompatible due to their noble content. Resistance to tarnish & corrosion resistance: Gold alloys are resistant to tarnish & corrosion due to their noble content.
  37. 37. 2. Mechanical properties: Yield strength MPa Vickers hardness number %Elongation Type I Soft 100 - 120 50 - 90 35 Type II Medium 150 - 180 95 - 120 30 Type III Hard S 200 H 310 S 120 H 170 S 35 H 20 Type IV Extra hard S 275 H 510 S 150 H 250 S 30 H8
  38. 38. Mechanical properties:  Strength & hardness increase from Type I t o Type IV gold alloys.  Ductility [%elongation] decreases from Type I to Type IV gold alloys. 2.  When compared to base metal alloys, gold alloys are:  Slightly weaker [Lower yield &ultimate strength]  Less stiff [Flexible, Lower modulus of elasticity] Used in thick section to flex with the same degree as base metal alloys.  More ductile [Higher% elongation] More burnishable  Accurate fit restoration.  Less hard  Easier to be finished & polished.
  39. 39. Physical properties & casting :  Color:  Gold alloys are either yellow or silvery white according to the alloying elements.  Melting temperature: 870˚C – 1050˚C.  Gold alloys can be easily melted by the reducing zone of gas air torch in carbon crucible.  Casting shrinkage : 1.6%.  Use -Gypsum bonded investment. -Phosphate bonded investment mixed with H2o.  Density: 15.2 – 16.8gm/cm3.[high density]  Use air pressure or centrifugal casting machine. Better cast-ability, when compared to base metal alloys.  Cooling:  It is performed according to the type of the alloy. -Slow or rapid cooling for Types I & II gold alloys. -For Types III & IV cooling is done regarding to soft or hard condition. 3.
  40. 40.  Finishing & Polishing :  Acid pickling [immersion in warm hydrochloric acid solution] to remove surface oxides.  Finishing & polishing are done by regular methods.  Recast:  Gold alloys can be recast.  Microstructure:  Small equi-axed grains.  Joining of metals  By soldering, using - Hard high fusing solder [gold solder] - Boric acid flux -Anti-flux  N.B.: Gold alloys cannot be spot welded ,as they are characterized by high electrical conductivity  No localized heating for the area to be joined
  41. 41.  1. 2.  1. 2. 3. 4. 5. 6. 7. 8. Other Noble Alloys Types: Depending on the relative content of silver & palladium, there are: Palladium silver. [60% - 30%] Silver palladium.[70% - 25%] Properties: The recommended ratio [3 : 1] between silver & palladium is followed to avoid silver sulfide formation. Palladium is essential for high sag resistance & lowering α. Silver is used to balance the low α to be slightly higher than that of ceramic in ceramometallic restoration. Presence tin & gallium is essential to provide surface oxides. Their tendency to gas absorption during melting produces porous casting. Their lower density requires greater centrifugal force for casting. Their lower ductility results in lower burnishability. They are substitutes for gold alloys Types III& IV.
  42. 42. Dental Cast Base Metal Alloys  Alternatives to gold alloys: Due to continuous increased price of gold, alternative dental casting alloys has been introduced for Types III & IV gold alloys such as:  Economy gold alloys.  Silver palladium alloys.  Palladium silver alloys.  Base metal alloys. [Further cost reduction]
  43. 43. Dental Cast Base Metal Alloys  Definition: They are substitutional solid solution alloys which do not contain any noble metals. They are substitutes for gold alloys Type III & IV.  Applications in dentistry: 1. Removable partial denture framework. 2. Full denture bases. 3. Crown and bridge. 4. Dental implants.
  44. 44. Dental Cast Base Metal Alloys  Introduction in dentistry: Cast base metal alloys are known since1930’s. But they have been only widely used in dentistry since 1970’s.  Types: 1. Nickel chromium alloy. 2. Cobalt chromium alloy. 3. Titanium and titanium alloys.
  45. 45. Types: 2. Nickel chromium alloy: Composition Major elements 90% by weight Minor elements 10% by weight 1. Nickel: 70-80% 2. Chromium: 12-20%. 1. 2. 3. 4. Molybdenum: 3- 6% Silicon and Manganese. Aluminum: 2-6% Beryllium: 0.5%. It is a substitute for Type III gold alloy
  46. 46. Types: 1. Cobalt chromium alloy Composition Major elements 90% by weight Minor elements 10% by weight 1. Molybdenum: 3- 6% by 1. Cobalt: 35-65% by weight. weight. 2. Chromium: 28- 30% by weight. 2. Silicon and Manganese. 3. Nickel: 0-30% [ It is used interchangeably with Cobalt). 3. Carbon: 0. 2% by weight. It is a substitute for Type IV gold alloy.
  47. 47.  Role of each element in cobalt chromium & nickel chromium alloys:  Major elements: 1- Cobalt 2- Nickel Strength. Hardness. Modulus of elasticity. Strength. Hardness. Modulus of elasticity. Ductility. Nickel is allergic .[female>male] gingival discoloration, swelling or redness .˙. Nickel free base metal alloys has been introduced. 3- Chromium  Tarnish &corrosion resistance by passive layer. Passive layer is an oxide layer , which is thin, uniform, nonporous, adherend & transparent.
  48. 48. Role of each element in cobalt chromium & nickel chromium alloys:  Minor elements: To increase strength, hardness & decrease ductility: -Molybdenum as grain refiner. -Carbon 0.2% as discontinuous precipitate in the grain boundaries. -Aluminum reacts with nickel forming intermetallic compound which precipitates inside the solid solution alloy  precipitation hardening  To improve cast-ability: -Silicone & Manganese -Increase fluidity of molten alloy -Act as deoxidizer -Beryllium -Decreases the melting temperature Beryllium vapor is carcinogenic and may lead to fibrosis of the lungs. .·. Many alloys are free of beryllium. 
  49. 49. Role of each element in cobalt chromium & nickel chromium alloys:  Carbon content is very critical:  If it is more than 0.2% .·.Continuous carbide precipitation at the grain boundaries  Strength & Hardness  Ductility i.e. Extreme brittle The alloy can not be used in dentistry.  Avoid carbon pick up by avoid using: -Carbon containing investment. -Carbon crucible for melting. -Improper adjusted oxyacetylene flame. -Ordinary casting atmosphere. .·. We should use: -Carbon free investment. -Ceramic crucible for melting. -Proper adjusted oxyacetylene flame. -Vacuum casting atmosphere.
  50. 50. Types: 3.  Titanium & Titanium alloys: Introduction: Titanium is named after the Titans, the powerful sons of the earth in Greek mythology.   Application in dentistry: Commercially pure titanium. [CpTi] There are 4 grades of CpTi according to oxygen [0.18- 0.4%] & iron [0.2- 0.5%] with different properties & applications.
  51. 51. Titanium & Titanium alloys:  Application in dentistry:  Titanium alloys: Titanium exists in 2 allotropic transformations according to the temperature.
  52. 52. Titanium & Titanium alloys:  Application in dentistry:  Titanium alloys: Each phase has its own properties, which can be stabilized by the addition of stabilizing elements. Aluminum is α stabilizer, while copper, palladium & vanadium are ß stabilizers. .˙. Titanium alloys are introduced to stabilize either α,ß or both to get different properties.
  53. 53. Properties: Biological properties:  Biocompatibility: All base metal alloys are biocompatible due to the presence of passive layer except base metals containing: - Nickel, as it causes allergy in females > males, because the females have been sensitized to nickel by wearing inexpensive jewelry. - Beryllium, as its vapor can cause lung carcinoma to the technician or dentist. .·. Proper ventilation of lab & clinic is necessary. 1.
  54. 54. Properties: Biological properties:  Resistance to tarnish & corrosion: All base metal alloys are resistant to tarnish & corrosion due to the presence of the passive layer.  N.B.: Only CpTi & its alloys can repassivate in nanoseconds after scratching their passive layer. 1.
  55. 55. Properties: Mechanical properties:  N.B.: Mechanical properties of CpTi are similar to gold alloy Types III & IV. While those of titanium alloys are similar to cobalt chromium and nickel chromium alloys.  When compared to gold alloys, the base metals are:  Slightly stronger [Higher yield & ultimate strength]  Double stiffer [Double modulus of elasticity]  Used in thin section & still retain their rigidity.  Less ductile [Lower % elongation]  Less burnishable.  Harder [1/3] Difficult to be finished & polished but retain their surface for longer time. 2.
  56. 56. 3.         Physical properties & casting: Color: Silvery white. Melting range: ~1400ºC. They need complicated & expensive technique for melting e.g. Melting in ceramic crucible by oxygen acetylene flame or electric induction. They are very reactive to the atmosphere. .·. Vacuum casting is required. They are of high sag resistance, an advantage for soldering & ceramometallic restoration. Casting shrinkage:~2.3%[High casting shrinkage] Use dental carbon free investment with stable oxides. Phosphate bonded investment mixed with special liquid. Silicate bonded investment with vents.
  57. 57. Physical properties & casting:  Density: 4-8gm/cm3 [Low density]  They need high casting force to obtain complete casting. They should be cast using special designed casting machines to produce high casting force.  However, they are useful in construction of bulky restorations [e.g. upper denture] due to the light weight, which aids in retention of the appliance & comfortable feeling for the patient.  Cooling: Bench cooling.  Finishing & polishing:  Acid pickling should be avoided, as it attacks the passive layer. They are difficult to be finished & polished due to their high hardness. Sandblasting & electrolytic etching are essential. 3.
  58. 58. Properties: 3.         Physical properties & casting: Recast: They can not be recast due to their high technique sensitivity, which can affect both their microstructure & properties. Microstructure: Coarse grains. Heat treatment: The mechanical properties of cobalt chromium alloys are not improved by heat treatment. The ductility of nickel chromium alloys can be improved by dissolution of carbides at the grain boundaries. Heat treatment of titanium alloys is complicated.[Out of scope]
  59. 59. Physical properties & casting:  Joining of metals:  By soldering using: -Hard low fusing solder -Fluoride containing flux [capable of removing the passive layer at the site of soldering] -Anti-flux  N.B.:  The lab work of dental base metal alloys is very difficult due to: - High melting point. - High reactivity to atmosphere. - Low density. - Difficult in finishing & polishing. - Inability to recast. - Poor burnishability. - High casting shrinkage. 3.
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