Casting Alloys

Requirements of casting alloysI. 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 finishingThe metal nickel is known to be allergenic

2. The alloy should resist the degradation in oral fluids

Requirements of casting alloysII. Interfacially and chemically

Casting alloys should have low surface energy to reduce the plaque attachment

If the alloy is going to be covered with porcelain should be able to form surface oxide layer

The alloy should be resistant to both tarnish or corrosion (Nobility and passivity)

Alloy surface should not be affected by the oral environment (show no pitting)

Requirements of casting alloysIII. Mechanically

High strength (P.L, Y.S. & U.S.) to resist the permanent deformation or even fracture during service

Ductility is also required in certain situation where burnishing and marginal closure are needed

Alloys with higher hardness are difficult to be finished (Require sandblasting or electro-polishing) and could cause wear to the opposing natural teeth.

Requirements of casting alloysV. Practicability

1. Inexpensive and able to be soldered and repaired

2. Melting rangeIt is preferred to use alloys that fuses below 1000oC

Alloys with higher melting range require;1. Either phosphate or silica-bonded investment2.Special melting equipments (oxy acetylene gas torch or electric induction machine)

Requirements of casting alloys

3. The DensityDense alloys are more easier in casting under relatively lower casting forceLighter alloys requires more casting force and are more liable to casting defects (incompleteness and porosity)

4. The reactivity at the molten stateMany casting alloys (e.g. Titanium) are highly reactive at the molten state either to the surrounding atmosphere .or Investment materials

Requirements of casting alloysThis fact could leads to 1. alloy oxidation, 2. Complication of the finishing procedure .or even3. 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)

5. The casting shrinkageAll 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

HIGH NOBLE CASTING ALLOYS1. Gold alloys2. Low gold containing alloys

B. NOBLE CASTING ALLOYS2. Pd-Ag alloys

C. BASE METAL CASTING ALLOYS1. Co-Cr alloys2. Ni-Cr alloys3. Titanium alloysTypes of Casting alloys

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 applicationsGold Alloys

1. Gold alloys melting range (its MP = 1063oC) the golden yellow color the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure)2. Copper alloys melting range (its MP = 1083oC) 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

3. Silver alloys melting range (its MP = 960oC) 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 alloys melting range (its MP = 1773oC) 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

5. Palladium alloys melting range (its MP = 1553oC) the whitish color the strength and hardness the resistance to tarnish and corrosion the ductility of the alloy ( FCC structure) the weight of the alloy6. Zinc alloys melting range (its MP = 788oC) the oxidation of the alloy (act as scavenger) the castability of the alloy ( flow during casting)Role of elements in the gold alloys

7. Indium grain size ( the mechanical properties)Responsible for forming the surface oxide layer in case of metal-ceramic alloys8. Tin and ironResponsible for forming the surface oxide layer in case of metal-ceramic alloys

Role of elements in the gold alloys

A. According to the gold contentsKarat systemThe gold alloys are divided into 24 partsKarat is the number of parts represent the pure gold in the alloye.g. 18 K gold alloy = 18 parts of gold, 6 parts of other metals

Fine systemThe gold alloys are divided into 1000 partsFineness is the number of parts represent the pure gold in the alloye.g. 750 F gold alloy = 750 parts of gold, 250 parts of other metals

Types of gold alloys

Types of gold alloys

B. According to the hardness and strength

Type I (Soft) used for small inlaysType II (Medium) used for large inlays & onlaysType III (Hard) used for crown and bridgeType 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

Types of gold alloys

What can you discover from the table ?The gold content Type I Type IVThe copper content Type I Type IVThe ductility& %elongation Type I Type IVThe hardness & strength Type I Type IVThe melting range Type I Type IVThe golden yellow color Type I Type IV

N.B.Heat treatment could alter these announced properties

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;Softening heat tttHardening heat ttt

Heat treatment of gold alloys

Softening heat treatment(Annealing)IndicationBefore hardening heat treatmentTo increase the workability of the alloyFor structured to be cold worked (shaped or ground)TechniqueHeating the alloy at 700oC for 10 min quenching Mechanism & outcomesAll the solid transformed into a disordered solid solution at the high temp., with rapid cooling the structure remains disordered, accordingly Strength & hardness P.L. & E Ductility

B. Hardening heat treatment (Age hardening)IndicationTo increase the strength of cold worked alloysTo decrease the workability of the alloy

TechniqueStep 1. Softening heat treatment (relief all stresses and start at disordered structure)Step 2. Heating the alloy at 700oC bench cooling Heating the alloy at 450oC bench cooling to from 450oC to 250oC quenching Maintain the alloy between 350-450oC for 15 min quenching

B. Hardening heat treatment (Age hardening)MechanismThe 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 solution2. Precipitation of super-lattices

Outcomes Strength & hardness P.L. & E Ductility

Phase diagram of gold-copper alloy system

Low gold containing alloysComposition

Low gold containing alloysCharactersIts Gold content = 45-50%The high % of silver & Palladium gives the characteristic whitish color of the alloyIts % of elongation = 2%