Dental Casting Alloys

1. Dental Casting Alloys Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics

2. Dental Casting Alloys Casting is the most commonly used method for the fabrication of metal structures (inlays, crowns, partial denture frames. A pattern of the structure is first made in wax. This is then surrounded by an investment material. After the investment hardens, the wax is removed (burnt out) leaving a space or mould. Molten alloy is forced into this mould. The resulting structure is an accurate duplication of the original wax pattern.

3. TERMINOLOGY Alloy : An alloy is defined as a metal containing two or more elements, at least one of which is metal, and all of which are mutually soluble in the molten state.

4. Classification of alloys According to Use : 1) Alloys for all metal and resin veneer restorations" (e.g. inlays, posts, resin and composite veneered crowns & bridges) 2) Alloys for metal ceramics restorations

5. According to Yield Strength and Percentage Elongation (ADA Sp. 5 Classification) Type I Soft Type II Medium Type III Hard Type IV Extra hard

6. According to Nobility (ADA 1984) A. High noble metal alloys Contains greater or equal 40 wt% Au and greater or equal 60 wt% of noble metals B. Noble metal alloys Contains greater or equal 25 wt% of noble metals C. Predominantly base metal Contains less than 25 wt% of noble metals

7. According to Major Element 1- Gold alloys 2-Silver alloys 3-Palladium alloys 4-Nickel alloys 5-Cobalt alloys 6-Titanium alloys

8. According to the Three Major Elements 1-Gold-palladium-silver 2- Palladium-silver-tin 3- Nickel-chromium-molybdenum 4- Cobalt-chromium-molybdenum 5-Iron-nickel-chromium 6- Titanium-aluminum-vanadium

9. According to the Number of Alloys Present 1) Binary-two elements 2) Ternary-three elements 3) Quaternary (and so forth)-four elements

10. General Requirements of Casting Alloys All cast metals in dentistry have some basic common requirements 1)They must not tarnish and corrode in the mouth. 2) They must be sufficiently strong for the intended purpose. 3)They must be biocompatible (non-toxic and non-allergic). 4)They must be easy to melt, cast, cut and grind (easy to fabricate). 5)They must flow well and duplicate fine details during casting. 6)They must have minimal shrinkage on cooling after casting. 7)They must be easy to solder.

11. Not all of them meet all the requirements. Some have shown a potential for allergic reactions (nickel containing alloys) and other side effects when used without proper precautions. Some are quite difficult to cast. Some are so hard (base metal alloys) that they are difficult to cut, grind and polish. All alloys shrink on cooling. Some (base metal alloys) show more shrinkage than others. The shrinkage cannot be eliminated but it can be compensated for (investments). Besides these general requirements, alloys intended for a certain specific use must meet requirements for that. For example metal ceramic alloys must have additional requirements in order to be compatible with porcelain.

12. ALLOYS FOR ALL METAL RESTORATIONS These alloys were among the earliest alloys available to dentistry. The early alloys were mostly gold alloys. Since they were intended for all metallic and later for resin veneered restorations, they just had to meet the basic requirements. No special requirements are needed for veneering with resin. Currently, the use of these alloys are slowly declining because of the: 1) Increased esthetic awareness has reduced the trend for metal display. 2)Increasing popularity of all ceramic and metal ceramic restorations. 3)Reducing popularity of resin and composite as veneering material. Resing facings have a number of disadvantages. - They wear rapidly (soon wear resistance) - They may change color (color instability and stain absorption) - They are porous. They tend to absorb food material and bacteria. This makes it unhygienic and gives it a bad odor.

13. Classification (As mentioned before this 1934 classification was originally intended for gold alloys and were based on hardness. Since 1989, it was relaxed to include any dental alloy as long as they met the new yield strength and percentage elongation criteria). Type I Soft Small inlays, class III and class V cavities which are not subjected to great stress. They are easily burnished. These being highly malleable and ductile have self-sealing margins like gold foil filling. Type II Medium Inlays subject to moderate stress, thick 3/4 crowns, abutments, pontics, full crowns, and sometimes soft saddles. They are less resistance to tarnish and corrosion than type I they are not self-sealing like type I gold alloys. Type III Hard Inlays, crown and bridges, situations where there may be great stresses involved. They usually can be age hardened. Type IV Extra Hard Inlays subjected to very high stresses, partial denture frameworks and long span bridges. They can be age hardened. These are less resistant to tarnish and corrosion than type I,II and III alloys.

14. Types These alloys will be discussed under the following categories High noble (gold alloys) Noble (Silver palladium alloys) Base metal Cobalt chrome alloys-Titanium and its alloys Aluminum bronze alloys

15. GOLD ALLOYS (FOR ALL METAL RESTORATIONS) Synonyms: Traditional gold alloys, Au-Ag-Cu alloys.

16. Why do We Alloy Gold? Pure gold is a soft and ductile metal and so is not used for casting dental restorations and appliances. Dental casting golds are alloyed commonly with copper, silver, platinum, nickel and zinc. Alloying gold with these metals not only improves its physical and mechanical properties but also reduces its cost. Earlier, people liked to display the yellow color of gold probably as a symbol of social status. Currently, the trend is to avoid display of metal. At the same time, increase in the platinum, palladium and silver content of modern alloys have resulted in whiter colored gold alloys. Thus, there are 'yellow gold alloys' and 'white gold alloys'. The rise in gold prices have also led to the availability of alloys with low gold content. These are the 'low golds

17. GOLD CONTENT Traditionally, gold content of dental casting alloys was called: 1)Karat 2)Fineness. Karat It refers to the parts of pure gold present in 24 parts of alloy, e.g. - 24 karat gold is pure gold. - 22 karat gold is 22 parts pure gold and 2 parts of other metal. " Fineness Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fine. Thus, if 3/4 of the gold alloy is pure gold, it is said to be 750 fine.

18. Function of Each Element (gold alloys) Gold : Provides tarnish and corrosion resistance and has a desirable appearance. Also provides ductility and malleability. Copper : It is the principal hardener. It reduces the melting point and density of gold. If present in sufficient quantity, it gives the alloy a reddish color. 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 percent.

19. Silver : It whitens the alloy, thus helping to counteract the reddish color of copper. It increases strength and hardness slightly. In large amount, however, it reduces tarnish resistance Platinum : It increases strength and corrosion resistance. It also increases melting point and has a whitening effect on the alloy. It helps to reduce the grain size.

20. Palladium : Similar to platinum in its effect. It hardens and whitens the alloy. It also raises fusion temperature and provides tarnish resistance. It is less expensive than platinum, thus, reducing the cost of the alloy.

21. The Minor Additions Zinc : It acts as a scavenger for oxygen. Without zinc, the silver in the alloy causes absorption of oxygen during melting. Later during solidification, the oxygen is rejected producing gas porosities in the casting.

22. Indium, Tin and Iron : They help to harden the ceramic gold- palladium alloys, iron being the most effective. Calcium : It is added to compensate for the decreased CTE that results when the alloy is made silver free (The elimination of silver is done to reduce the tendency for green stain at the metal-porcelain margin).

23. Iridium, Ruthenium, Rhenium They help to decrease the grain size. They are added in small quantities. Note All modern noble metal alloys are fine grained. Smaller the grain size of the metal, the more ductile and stronger it is. It also produces a more homogeneous casting and improves the tarnish resistance. A large grain size reduces the strength and increases the brittleness of the metal. Factors controlling the grain size are the rate of cooling, shape of the mould, and composition of the alloy.

24. PROPERTIES OF GOLD ALLOYS Color : Traditionally the gold alloys were gold colored. The color of modern gold alloys can vary from gold to white. It depends on the amount of whitening elements (silver, platinum, palladium, etc.) present in the alloy.

25. Melting Range : Ranges between 920 to 960°C. The melting range of an alloy is important. It indicates the type of investment required and the type of heating source needed to melt the alloy. Density : It gives an indication of the number of dental castings that can be made from a unit weight of the metal. In other words, more number of cast restorations per unit weight can be made from an alloy having a lower density, than one having a higher density. Gold alloys are lighter than pure gold (19.3 gms/cm'). Type III - 15.5 gm/cm:‘ Type IV - 15.2 gm/cm' The castability of an alloy is also affected by density. Higher density alloys cast better than lower density alloys.

26. Yield strength: The yield strength for : Type III – 207 MPa. Type IV- 275 MPa Hardness : The hardness for Type III- 121 MPa Type IV- 149 MPa. The hardness indicates the ease with which these alloys can be cut, ground or polished. Gold alloys are generally more user friendly than the base metal alloys which are extremely hard.

27. Elongation It indicates the ductility of the alloy. A reasonable amount is required especially if the alloy is to be deformed during clinical use, e.g. clasp adjustment for removable partial dentures, margin adjustment and burnishing of crowns and inlays. Type I alloys are easily furnished. Alloys with low elongation are very brittle. Age hardening decreases ductility: Type III - 30 to 40% Type IV - 30 to 35%.

28. Modulus of Elasticity This indicates the stiffness/flexibility of the metal. Gold alloys are more flexible than base metal alloys Tarnish and Corrosion Resistance Gold alloys are resistant to tarnish and corrosion. This is due to their high noble metal content. Noble metals are less reactive.

29. Casting Shrinkage All alloys shrink when they change from liquid to solid. The casting shrinkage in gold alloys is less (1.25 to 1.65%) when compared to base metal alloys. The shrinkage occurs in three stages. 1)Thermal contraction of the liquid metal. 2)Contraction of the metal while changing from liquid to solid state. 3)Thermal contraction of solid metal as it reaches room temperature. Shrinkage affects the fit of the restoration. Therefore, it must be controlled and compensated for in the casting technique.

30. Biocompatibility Gold alloys are relatively biocompatible. Casting Investment Gypsum-bonded investments are used for gold alloys because of their lower fusion temperature.

31. HEAT TREATMENT OF GOLD ALLOYS Heat treatment of alloys is done in order to alter its mechanical properties. Gold alloys can be heat treated if it contains sufficient amount of copper. Only type I and type IV gold alloys can be heat treated. There are two types of heat treatment; 1) Softening heat treatment (solution heat treatment) 2)Hardening heat treatment (age hardening).

32. Softening Heat Treatment: Softening heat treatment increases ductility, but reduces strength, proportions limit, and hardness. Indications : It is indicated for appliances that are to be ground, shaped or otherwise cold worked in or outside the mouth. Method :The casting is placed in an electric furnace for 10 minutes at 700°C, and then it is quenched in water. During this period, all intermediate phases are change to a disordered solid solution, and the rapid quenching prevents ordering from occurring during cooling. Each alloy has its optimum temperature. The manufacturer should specify the most favorable temperature and time.

33. Hardening Heat Treatment (or Ageing) Hardening heat treatment increases strength, proportional limit, and hardness, but decrease ductility. It is the copper present in gold alloys which helps in the age hardening process. Indications For strengthening metallic dentures, saddles, bridges, and other similar structures before use in the mouth. It is not employed for smaller structures such as inlays.

34. Method It is done by "soaking" or ageing the casting at a specific temperature for a definite time, usually 15 to 30 minutes. It is then water quenched or cooled slowly. The ageing temperature depends on the alloy composition but is generally between 200C and 450°C. During this period, the intermediate phases are changed to an ordered solid solution (The proper time and temperature for age hardening an alloy is specified by its manufacturer). Ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment in order to relieve all strain hardening (stresses which occurs during finishing). Starting the hardening treatment when the alloy is in a disordered solid solution allows better control of the ageing process.

35. low Gold Alloys Also known as "economy golds". They are crown and bridge alloys having gold content below 60% (generally in the 42 to 55% range). However, gold must be the major element. The low gold alloys were developed because of the increase in gold prices. However, reducing gold content increased tarnish and corrosion. This problem was overcome by two discoveries. 1)Palladium made the silver in gold alloy more tarnish resistant. 1% palladium was required for every 3% of silver. 2)The silver-copper ratio had to be carefully balanced.

36. Advantages Because of this research numerous low gold alloys were introduced into the market. Thus, these alloys were not only less expensive but also had good tarnish and corrosion resistance. Their properties are comparable to Type III and IV gold alloys.

37. SILVER-PAllADIUM AllOYS These alloys were introduced as a cheaper alternative to gold alloys. It is predominantly silver in composition. Palladium (at least 25%) is added to provide nobility and resistance to tarnish. They may or may not contain copper and gold. They are white in color.

38. Ag-Pd (non-copper) Ag - 70 to 72% Pd - 25% (Properties are like Type III gold alloys) Ag-Pd-Cu (Ag - 60%) (Pd - 25%) ( Cu - 15% ) Properties are like Type IV gold alloys

39. The properties of the silver-palladium alloys are similar to the type III and IV gold alloys. However, they have lower ductility and corrosion resistance. They also have a significantly lower density than gold alloy. This may affect its castability. A major difference between type III and IV Ag-Pd alloys is that, the latter can be significantly age hardened, because of its gold and copper content.

40. NICKEL·CHROME AND COBALT·CHROMIUM ALLOYS These are known as base metal alloys and are extensively used in many of the developing countries. In India, because of their relatively low cost many of the laboratories use these alloys along with resin facings. These metals are very strong and hard. Because of this they are generally difficult to work with (cutting, grinding, polishing, etc).

41. TITANIUM AND TITANIUM ALLOYS Though these metals can be used for all-metal and metal ceramic restorations, as well as partial dentures.

42. ALUMINUM BRONZE ALLOY Bronze is an alloy known to man since ancient times. Traditional bronze is copper alloyed with tin. The ADA approved bronze does not contain tin. The composition is as follows: Copper (81 to 88%) , Aluminum (8 to 10%) Nickel ( 2 to 4%) , Iron ( 1 to 4%). Being relatively new, the information on these alloys is relatively scanty.

43. METAL CERAMIC ALLOYS Metal ceramic alloys are those alloys that are compatible with porcelain and capable of bonding to it. A layer of porcelain is fused to the alloy to give it a natural tooth­ like appearance. Porcelain being a brittle material fractures easily, so these alloys are used to reinforce the porcelain.

44. Several types of alloys are used to cast sub-structures for porcelain fused to metal crowns and bridges. They may be noble metal alloys or base metal alloys. All have coefficient of thermal expansion (CTE) values which match that of porcelain. Note CTE has a reciprocal relationship with melting point, i.e. the higher the melting point of a metal, lower is its CTE.

45. Synonyms Porcelain-fused-to-metal (PFM), Ceramometal alloys, porcelain-bonded-to-metal (PBM). The preferred term however, is metal ceramic or PFM.

46. EVOLUTION OF METAL-CERAMIC ALLOYS The metal-ceramic alloys evolved from resin-veneered crown and bridge alloys. Resin facing faced the problem of gradual wear and had to be replaced over time. Besides resin could not be used on the occlusal surface. To retain a resin veneered restoration undercuts had to be provided. The early metal-ceramic alloys were high gold alloys (88% gold). They were not strong enough for use in bridges. In those early days before porcelain-metal bonding was clearly understood, porcelain had to be retained by mechanical means with the help of undercuts. Later, it was discovered that adding 1% of base metals like iron, tin, indium, etc. induced chemical bonding by the formation of an oxide layer. This significantly improved the bond strength between the porcelain and the metal

47. REQUIREMENTS OF ALLOYS FOR PORCELAIN BONDING In addition to the general requirements of alloys mentioned earlier, metal ceramic alloys have some special requirements in order to be compatible with porcelain veneering. 1)It's melting temperature should be higher than the porcelain firing temperature. 2)It should be able to resist creep or sag at these temperatures. 3)Its CTE should be compatible with that of porcelain. 4)They should be able to bond with porcelain. 5) It should have a high stiffness (modulus of elasticity). Any flexing of the metal framework may cause porcelain to fracture. 6) It should not stain or discolor porcelain.

48. USES OF METAL-CERAMIC ALLOYS 1)As the name implies these alloys are intended for porcelain veneered restorations (crowns and bridges). 2)They can also be used as an all metal restoration.

49. TYPES (CLASSIFICATION) OF METAL CERAMIC ALLOYS Alloys for metal ceramics restorations may be categorized as: 1)High noble (commonly referred to as gold alloys) a) Gold-palladium-platinum alloys b) Gold-palladium-silver alloys c) Gold-palladium alloys

50. 2)Noble (commonly referred to as palladium alloys) Palladium-silver alloys Palladium-gallium-silver alloys Palladium-gold alloys Palladium-gold-silver alloys Palladium-copper alloys Palladium-cobalt alloys 3)Base metal Nickel-chromium alloys Nickel-chromium-beryllium alloys Cobalt-chromium alloys Pure titanium Titanium-aluminum-vanadium