casting alloys dental material

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casting alloys dental material

  1. 1. Casting AlloysCasting Alloys
  2. 2. Requirements of casting alloys I. Biologically 1. Casting alloys should not cause toxicity, allergy or even irritation in service or during the fabrication process (casting and finishing).  Beryllium-containing alloys could cause berylliosis if inhaled during finishing  The metal nickel is known to be allergenic 2. The alloy should resist the degradation in oral fluids
  3. 3. Requirements of casting alloys II. Interfacially and chemically 1. Casting alloys should have low surface energy to reduce the plaque attachment 2. If the alloy is going to be covered with porcelain should be able to form surface oxide layer 3. The alloy should be resistant to both tarnish or corrosion (Nobility and passivity) 4. Alloy surface should not be affected by the oral environment (show no pitting)
  4. 4. Requirements of casting alloys III. Mechanically 1. High strength (P.L, Y.S. & U.S.) to resist the permanent deformation or even fracture during service 2. Ductility is also required in certain situation where burnishing and marginal closure are needed 3. Alloys with higher hardness are difficult to be finished (Require sandblasting or electro-polishing) and could cause wear to the opposing natural teeth.
  5. 5. Requirements of casting alloys V. Practicability 1. Inexpensive and able to be soldered and repaired 2. Melting range  It is preferred to use alloys that fuses below 1000o C  Alloys with higher melting range require; 1. Either phosphate or silica-bonded investment 2.Special melting equipments (oxy acetylene gas torch or electric induction machine)
  6. 6. 3. The Density  Dense alloys are more easier in casting under relatively lower casting force  Lighter alloys requires more casting force and are more liable to casting defects (incompleteness and porosity) 4. The reactivity at the molten state  Many casting alloys (e.g. Titanium) are highly reactive at the molten state either to the surrounding atmosphere …….or Investment materials Requirements of casting alloys
  7. 7.  This fact could leads to 1. alloy oxidation, 2. Complication of the finishing procedure ….or even 3. Enhancement of alloy corrosion  To avoid these adverse effects, Proper selection of the following items is so important; 1. The alloy (Should be low-reactive …..such as gold alloys) 2. The investment (Should contain reducing agent) ………… and 3. The melting method (Proper usage of flame and using gas containing no or little amount of carbon) Requirements of casting alloys
  8. 8. 5. The casting shrinkage  All Metals expand on heating (↑ inter-atomic distances) and shrink on cooling (↓ inter-atomic distance and ↑ density)  Alloys with little solidification and cooling shrinkage are able to produce more accurate casting  Alloys with higher shrinkage rates require special support (Both the die and investment materials should show higher expansion rates) Requirements of casting alloys
  9. 9. A. HIGH NOBLE CASTING ALLOYS 1. Gold alloys 2. Low gold containing alloys B. NOBLE CASTING ALLOYS 2. Pd-Ag alloys C. BASE METAL CASTING ALLOYS 1. Co-Cr alloys 2. Ni-Cr alloys 3. Titanium alloys Types of Casting alloys
  10. 10.  Pure gold is yellow, soft (ductile & malleable) metal that welded (cohere) together and easily deformed under pressure.  Pure gold is used as direct filling material to restore small tooth cavities  Alloying the metal gold with Copper, Silver, Platinum, Palladium, Zinc and sometimes minute amount of Indium improves its mechanical properties to fit the requirements of different applications Gold Alloys
  11. 11. 1. Gold  ↑ alloy’s melting range (its MP = 1063o C)  ↑ the golden yellow color  ↑ the resistance to tarnish and corrosion  ↑ the ductility of the alloy ( FCC structure) 2. Copper  ↑ alloy’s melting range (its MP = 1083o C)  ↑ the reddish color  ↑ the strength and hardness (forms solid solution with gold  heat treatment)  ↓ the resistance to tarnish and corrosion  ↑ the ductility of the alloy ( FCC structure) Role of elements in the gold alloys
  12. 12. 3. Silver  ↓ alloy’s melting range (its MP = 960o C)  ↑ the whitish color that neutralize the red color of copper  ↓ the resistance to tarnish and corrosion  ↑ the ductility of the alloy ( FCC structure) 4. Platinum  ↑ alloy’s melting range (its MP = 1773o C)  ↑ the whitish color  ↑ the strength and hardness  ↑ the resistance to tarnish and corrosion  ↑ the ductility of the alloy ( FCC structure) Role of elements in the gold alloys
  13. 13. 5. Palladium  ↑ alloy’s melting range (its MP = 1553o C)  ↑ the whitish color  ↑ the strength and hardness  ↑ the resistance to tarnish and corrosion  ↑ the ductility of the alloy ( FCC structure)  ↓ the weight of the alloy 6. Zinc  ↓ alloy’s melting range (its MP = 788o C)  ↓ the oxidation of the alloy (act as scavenger)  ↑ the castability of the alloy ( ↑ flow during casting) Role of elements in the gold alloys
  14. 14. 7. Indium  ↓ grain size (↑ the mechanical properties)  Responsible for forming the surface oxide layer in case of metal-ceramic alloys 8. Tin and iron  Responsible for forming the surface oxide layer in case of metal-ceramic alloys Role of elements in the gold alloys
  15. 15. A. According to the gold contents Karat system  The gold alloys are divided into 24 parts  Karat is the number of parts represent the pure gold in the alloy  e.g. 18 K gold alloy = 18 parts of gold, 6 parts of other metals Fine system  The gold alloys are divided into 1000 parts  Fineness is the number of parts represent the pure gold in the alloy  e.g. 750 F gold alloy = 750 parts of gold, 250 parts of other metals Types of gold alloys
  16. 16. Types of gold alloys 18K gold alloy ??F gold alloy = Pure gold contains 24 K gold Pure gold contain100 0 F gold 18K gold alloy = 750 F gold alloy =
  17. 17. B. According to the hardness and strength Type I (Soft)… used for small inlays Type II (Medium)… used for large inlays & onlays Type III (Hard)… used for crown and bridge Type IV (Extra-hard)… used for denture frameworks The content of both gold and copper are the most effective in this classification (See the table of alloys’ composition) Types of gold alloys
  18. 18. Types of gold alloys Alloy Au % Cu % Ag % Pt % Pd % Zn % VHN M R I. Soft 87 4 9 0 0 0 50-90 943- 960o C II. Medium 76 8 13 0 2.5 0.5 90- 120 924- 960o C III. Hard 70 10 15 1 3 1 120- 150 924- 960o C IV. Extra- Hard 66 15 12 2 3 2 >150 871- 921o C
  19. 19. What can you discover from the table?  The gold content  ↓ Type I Type IV  The copper content  ↑ Type I Type IV  The ductility& %elongation  ↓ Type I Type IV  The hardness & strength  ↑ Type I Type IV  The melting range  ↓ Type I Type IV  The golden yellow color  ↓ Type I Type IV N.B. Heat treatment could alter these announced properties
  20. 20.  The mechanical properties of gold alloys could be altered through the solid state reactions (Heat treatment)  The ability of heat treatment presents only in types III & IV due to the higher % of copper and silver  2 types of heat treatment could be carried out; 1. Softening heat ttt 2. Hardening heat ttt Heat treatment of gold alloys
  21. 21. A. Softening heat treatment (Annealing) Indication 1. Before hardening heat treatment 2. To increase the workability of the alloy 3. For structured to be cold worked (shaped or ground) Technique Heating the alloy at 700oC for 10 min  quenching Mechanism & outcomes All the solid transformed into a disordered solid solution at the high temp., with rapid cooling the structure remains disordered, accordingly 1. ↓ Strength & hardness 2. ↓ P.L. & E 3. ↑ Ductility
  22. 22. B. Hardening heat treatment (Age hardening) Indication 1. To increase the strength of cold worked alloys 2. To decrease the workability of the alloy Technique Step 1. Softening heat treatment (relief all stresses and start at disordered structure) Step 2.  Heating the alloy at 700o C  bench cooling  Heating the alloy at 450o C  bench cooling to from 450o C to 250o C  quenching  Maintain the alloy between 350-450o C for 15 min  quenching
  23. 23. B. Hardening heat treatment (Age hardening) Mechanism The solid at the start has a disordered structure, slow cooling or even maintaining the temperature for sometime helps the diffusion of atoms that leads to; 1. Rearrangement of atoms ordered solid solution 2. Precipitation of super-lattices Outcomes  ↑ Strength & hardness  ↑ P.L. & E  ↓ Ductility
  24. 24. Phase diagram of gold-copper alloy system 1083 o C 1063 o C 410 o C 390 o C 0%Au------------------------------ 40------60---70---90--100% Au 100% Cu ----------------------------------------------------0% Cu 1083 o C 1063 o C 410 o C 390 o C 1083 o C 1063 o C 410 o C 390 o C 0%Au------------------------------ 40------60---70---90--100% Au 100% Cu ----------------------------------------------------0% Cu
  25. 25. Alloy Au% Cu% Ag% Pt% Pd% Other Conventional 75 10 9 2 2 2 Low-Gold 44 12 36 0 6 2 Low gold containing alloys Composition
  26. 26. Low gold containing alloys Characters 1. Its Gold content = 45-50% 2. The high % of silver & Palladium gives the characteristic whitish color of the alloy 3. Its % of elongation = 2% <<< Gold alloys = 20% 4. Other properties are similar to those of type III and IV gold alloys 5. Has good clinical performance and cast using the same equipments required for regular gold alloys

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