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Base Metal Casting alloys Dr Ambreen
- 1. Dental Casting Alloys Cast Base Metal Alloys Presented by: Dr Ambreen Azam (MSc, UK)
- 2. Objectives To classify base metal alloys used in dentistry To understand the composition of base metal alloys To compare between gold alloys type III and IV and base metal alloys. To predict the properties of base metal alloys when used for removable and fixed prosthesis Discuss titanium alloys for dental implants
- 3. Defined as alloys which do not contain noble metals. They are alternative for gold alloys type III & IV. What are base metal alloys?
- 4. Uses in Dentistry 1. Removable partial denture framework. 2. Full denture bases. 3. Crown and bridge. 4. Dental implants.
- 5. BaseMetalAlloysUsedinDentistry Cobalt chromium alloy Nickel chromium alloy Commercially pure titanium (Cp) and titanium alloys
- 6. CompositionandRoleof EachElement 1- Cobalt Strength, Hardness Modulus of elasticity 2- Chromium 12-30 %: Passive layer (CrO): a- Thin b- Uniform c- Non porous d- Adherent 30% sigma phase ppt of chromium, embrittle the alloy
- 7. 3- Nickel: Strength, Hardness Modulus of elasticity Ductility Nickel is a well-known metal to produce allergic reaction in some patients. In such cases it is advised also to use a Nickel-free Co-Cr alloys. 4- Molybdenum: Strength (grain refiner) 5- Silicon and Manganese: Fluidity of the molten alloy thus improve castability, Act as deoxidizers CompositionandRoleof EachElement
- 8. 6- Carbon 0.2%: Discontinuous Carbides phase appears at the grain boundaries at high melting temperature during casting Strength, Hardness Modulus of elasticity 7- Aluminum: Strength With Nickel Ni3Al nickel aluminide (intermetallic compound) CompositionandRoleof EachElement
- 9. 8- Beryllium: Fluidity of the molten alloy thus improve castability Lowers the Tm of alloy by 100°C Beryllium vapor is carcinogenic and may lead to fibrosis of the lungs. Therefore many alloys are available now free of beryllium. 9- Titanium: Tarnish and corrosion resistance CompositionandRoleof EachElement
- 10. Comparison with Cast gold alloys Property Cobalt- chromium Nickel- chromium Gold type III IV Comment Resistance to tarnish & corrosion Excellent Excellent Excellent All well Resistant Color Lustrous silvery white Yellow or white Used alone/ porcelain fused Fusion Tm 1400°C 1300°C 800 - 1050°C Different melting/ investment use
- 11. Comparisonwith Cast goldalloys Property Cobalt- chromium Nickel- chromium Gold type III & IV Comment Density 7-8 gm/cm3 15- 18gm/cm3 Difficult to produce defect- free casting in Co/Ni/Cr alloys casting shrinkage 2.3% 1.6% Compensated by correct investment material Modulus of Elasticity E (MPa) 250X103 200X103 100X103 Co/Cr most rigid for equivalent thickness; advantage for connectors; disadvantage for clasps Yield strength (MPa) 600 - 700 300-500 Adequate for the applications
- 12. Comparisonwith Cast goldalloys Property Cobalt- chromium Nickel- chromium Gold type III & IV Comment Hardness (VHN) 3 5 0 VHN 250 VHN Co/Ni/Cr more difficult to polish but retains polish during service % Elongation 1 – 2% 8-20% large values suggest that burnishing is possible Casting Carbon & Nitrogen No technique sensitivity difficult to produce defect- free casting in Co/Ni/Cr alloys Investment Carbon-free phosphate or silicate investment with vents Gypsum & phosphate Correct choice effects cast quality
- 13. Comparison with Cast gold alloys Property Cobalt- chromium Nickel- chromium Gold type III & IV Comment Casting ring Rubber ring Metallic ring Rubber rings allow more expansion to compensate for the casting shrinkage Melting crucible Ceramic Carbon or ceramic High Fusion Temp Melting Oxygen acetylene Electrical melting Flame gas air torch Better melting Casting machines Centrifugal + pressure Air pressure casting machine Different requirements Cooling Bench cool Rapid Cool High Fusion Temp
- 14. Property Cobalt- chromium Nickel- chromium Gold type III & IV Comment Finishing & polishing Sand blasting for finishing Electrolytic polishing (act as anode) Polishing by rubber cup & paste Acid pickling in warm HCl for Au but Not in Co/Ni/Cr alloys because it attack the passive layer Recasting No No Yes Co/Ni/Cr alloys tech. sensitive Microstructure Complicated coarse grain with cored structure Fine equiaxed grains Effect properties Heat treatment Not Possible Possible Type IV gold heat hardened Uses Co/Cr for partial denture frameworks, Ni/Cr for crown & bridge Can be used for both Base metals alloys economical than gold Comparisonwith Cast goldalloys
- 15. Base metal alloys for removable appliances •For partial denture connectors high modulus and high proportional limit is required to resist deformation •In addition low density is preferred in denture base connectors for a lighter weight in thin sections •For clasps high flexibility and ductility is required to be resilient and adjustable. •Co/Cr alloys are more suitable for connectors while gold alloys for clasps •Ideal to solder both alloys but corrosion at the joint is inevitable •In practice both connectors and clasps are cast with same alloy •Co/Cr alloys most commonly used for denture framework alternating gold alloys in properties and cost
- 16. Base metal alloys for removable appliances •Increased use of Ni/Cr alloys for partial denture framework castings. This is due to the relative ease of finishing and polishing compared with Co/Cr as a consequence of the lower hardness value
- 17. Base metal alloys for fixedprosthesis oNi/Cr preferred for fixed prosthesis over Co/Cr provided patient is not nickel allergic oNi/Cr rarely used for all metal cast restoration while widely used for PFM restorations oHigh rigidity with lower hardness of Ni/Cr is of advantage for long span bridges oCast gold is superior in terms of casting shrinkage, marginal fit and accuracy oNi/Cr casts can be ill fitting due to casting tech sensitivity oMarginal adaptation is less effected by polishing oHigh hardness in Ni/Cr limits Occlusal adjustments compared to cast gold crown
- 18. Biocompatibility oBeryllium is a known animal carcinogen and poses a potential threat to lab dental personnel's oFor the patient, the most immediate biocompatibility risk concerns nickel and the risk of allergic contact dermatitis oMore than 1% of Nickel is labelled oWarning for 0.02% beryllium or other hazardous element
- 19. TitaniumandTitaniumAlloys oIt was named Titanium after the Titans of Greek mythology (the God of enormous strength) oCommercially pure titanium is never present as pure element in nature o Contains Oxygen, nitrogen, carbon and hydrogen, which are always present between the titanium atoms oCommercially pure titanium occurs in 4 grades, grade I II III IV, according to oxygen content (0.18% to 0.40 %) & iron content (0.20 to 0.50 wt %)
- 20. Titaniumand TitaniumAlloys oCommercially pure titanium (cp Ti) or its alloys are used for dental implants, surface coatings, and crowns, partial and complete dentures, endodontic files and orthodontic arch wires. oTi-6Al-4V alloy is the most widely used. oForms a very stable oxide layer, repassivates in seconds therefore called bio-inert
- 21. PureTitaniumandTitaniumAlloys: Composition: Oxygen 0.18- 0.4% Iron 0.2- 0.5% Form stiff form flexible & ductile 885°C Stabilizer is aluminum Stabilizers are copper, palladium and vanadium aluminum
- 22. TitaniumCasting oProblems include: High melting point (~ 1700°C) o Chemical reactivity Reacts with gaseous elements easily, esp. at high temp (>600°C) Need a well-controlled vacuum in processing The technology required makes casting Ti expensive.
- 23. Properties of Titanium oMechanical properties of (commercially pure Titanium) are similar to gold alloy type III and IV. oWhile those of Titanium alloy are similar to Cobalt Chromium and Nickel chromium alloys. oShaped by milling the surface oSpark erosion for the fitting surface using electric discharge machine
- 24. References McCabe Applied dental materials 9th Edition Phillips science of dental materials 12th Edition.