Casting alloys Flashcards
What is an alloy?
An alloy is a mixture of two or more metallic elements (binary, ternary or quaternary)
What is the benefit of an alloy over pure metals?
usually has better/ improved properties
e.g.
- cost (i.e. cheaper than pure gold but keeping similar properties)
- biocompatibility (resistance to corrosion and tarnish)
- mechanic properties (strength, stiffness, hardness)
- ease of casting/fabrication (melting point, shrinkage & density)
Why are very light (not very dense) materials not good for casting?
Force is not sufficient to get all the detail
Which alloys are used commonly in dentistry?
- Amalgam
- Steel
- Nickel-titanium
- Gold
- Cobalt chrome
- Titanium
When are amalam alloys used?
dental fillings
What is amalgam an alloy of?
Mercury, tin, silver and copper
What are steel and nickel-titanium alloys used to make?
Instruments and endodontic/orthodontic wires (shape memory)
What are steel alloys made of?
Iron and carbon
What are gold and cobalt chrome alloys used for?
Crowns, inlays, onlays and denture bases
What are titanium alloys used for?
Implants and fixed/removable partial dentures
What different fabrication processes of alloys are there?
- Forming
- Amalgamation
- Casting
- Powder metallurgy
What is forming?
Plastic deformation, including hot or cold working (e.g. forging, rolling, extrusion and drawing)
What is amalgamation?
The blending of mercury with other metals
What is casting?
Pouring molten metal into a mould
What is powder metallurgy?
Sintering, injection moulding (for metal and polymers = mix in polymerising chamber under pressure = injected into mould) & 3D printing
What is the current problem with 3D printing?
Coarse particles used = thick layers and needs polishing etc. afterwards
What is the typical method for dental alloy casting?
Lost wax mould casting
What are the typical steps in lost wax mould casting?
- preparation of the dention
- preparation of an impression
- pouring of a model
- waxing of the desired shape
- investing the wax pattern
- burn-out and heating the mould
- melting and casting the alloy
- finishing and polishing
- heat treatment
What is the history of alloys?
1907: lost wax technique
1932-1948: standardisation of dental casting alloys
1950s -1960s: development of PFM alloys (adding palladium and platinum to gold lowers the coefficient of thermal expansion sufficiently to ensure the physical compatibility between porcelain and the metal substructure)
1970s: gold placed on the free market = increased prices = search for low gold and base metal alloys
What are the general (dental) requirements of a casting alloy?
Physical properties:
- reasonably low melting point (flow)
- moderately high density (castibility)
- low coefficient of thermal expansion
Chemical properties:
- chemical corrosion (tarnish) resistance
- electrochemical corrosion resistance
Mechanical properties:
- high modulus (stiffness)
- moderately high yeild strength (resistance to plastic deformation)
- hardenable by heat treatment (retention of polish)
Biological properties:
- biocompatible: no toxic soluble phases
- non-reactive in the oral environment
What are the two classes of dental casting metal alloys?
- Noble (precious metal) alloys
- Base metal casting alloys
Which alloys are classed as noble (precious metal) alloys?
Gold alloys
Silver-palladium
Which alloys are classed as base metal alloys?
Cobalt-chromium
Nickel-chromium
Titanium
What is a noble metal?
Elements with good metallic surfaces that retain luster in clean dry air
= resists oxidation, tarnish and corrosion during heating, casting and soldering
Which metals count as noble metals?
Gold
Platinum group (6 metals) =
- platinum,
- iridium.
- osmium,
- palladium,
- rhodium,
- ruthenium
-> n.b. sometimes these are mixed together to make a very good alloy
Silver (? less good tarnish resistance than the others but expensive)
Why should we avoid the old term of precious and semiprecious metals?
Indicates how expensive a metal is based on supply and demand = should be avoided because they are imprecise terms
What are the fundamental properties of Gold (Au)?
- Soft, (most) malleable and ductile
- Tarnish resistant in air and water at any temp
- Attacked by only a few of the most powerful oxidizing agents
- Insoluble in sulfuric, nitric or hydrochloric acids
- Soluble in a mixture of nitric acid and hydrochloric acid (aqua regia)
- Relatively low strength/hardness
- Small amounts of impurities (i.e. lead, mercury. base metals have a pronounced and usually detrimental effect on its properties)
- * Fusion temp = 1063 degrees C
- * Density 19.3g/cm cubed
- * Thermal coefficient of expansion = 14.2 x10^-6/ degree C
- * Youngs modulues (MOE) = 80 GPa (very close to dentine)
What are the fundamental properties of Platinum (Pt)?
- Tough, malleable and ductile
- High corrosion resistance
- Very high cost (usually replaced by Palladium in most modern alloys)
- Higher melting temp than porcelain
- * Fusion temp = 1755 degrees C (>Au)
- * Density = 21.37 g/cm cubed (>Au)
- * Thermal coefficient of expansion = 8.9 X 10^-6 per degree C (Au)
What are the fundamental properties of Palladium (Pd)?
- Not used in the pure state in dentistry (properties aren’t good enough)
- Has replace platinum in dental casting alloys (cheaper)
- Helps prevent corrosion of silver in the oral environment
- Absorbs hydrogen gas when heated improperly (properties worsen)
- * Fusion temp = 1555 degrees C (>Au)
- * Modulus of elasticity = 112 GPa (>Au)
What are the fundamental properties of Silver (Ag)?
- Malleable and ductile
- Harder than gold
- Best known conductor of heat and electricity (bad= damages pulp = sensitivity)
- unaltered in clean dry air BUT combines with Sulfur, Chlorine and phosphorus = severe tarnish in the oral environment
- occludes large quantities of oxygen in molten state (= defects -> evolves during solidification = pits and porosities and affects properties)
- * Fusion temp = 960.5 degrees C (Au) (>Pt) (>Pd)
- * Modulus of elasticity = 120 GPa (>Au) (Pd)
What are the two minor alloying elements?
Iridium (Ir) and Ruthenium (Ru)
What so the two minor alloying elements do?
Grain refining (stops crystals growing so much by producing as many boundaries as possible)
-> addition of as little as 50ppm (0.005%) of Ir and Ru = 100 X increase in the no. of grains per unit volume
- = increased alloys tensile strength and elongation by >30%,
- increases tarnish resistance and
- slightly increases yield strength -> does not appreciably affect hardness
Tell me about gold casting alloys:
- used as a dental material for last 2500 years
- pure form too soft to maintain shape = strength and hardness improved by alloying
- n.b. pure gold can be used in fillings = cohesive gold; alloys of gold used for cast restorations
What are the typical compositions of gold alloys?
What is a Karat, Carat (K)?
Parts of pure gold per 24
e.g. 12k = 50%, 18k, 24k
What is fineness?
Parts of pure gold per 1,000
e.g. a 650 fine alloy has a gold content of 65% n.b. this is primarily used for gold solders
How does gold content/ nobility vary from type I gold alloys to type IV?
Gold content/ nobility decreases from type I to type IV
What does adding copper do to the properties of a gold alloy?
- Strengthening (order-hardening),
- reducing melting temperature
BUT increases tarnish, adds reddish tint
What does adding silver do to the properties of a gold alloy?
- Slightly strengthening,
- counteracts the reddish tint of the copper
What does adding platinum and palladium do to the properties of a gold alloy?
- Strengthening,
- increasing melting temperature
What does adding Zinc do to the properties of a gold alloy?
A scavenger during casting = prevents oxidation and improves castability
What is class IV (hardened) gold alloy?
Post heat treated
What are the comparitive properties of gold alloys?
When are Type I casting gold alloys used?
Inlays that are well supported with no large masticatory
High ductility allows burnishing = improves hardness and fit
When are Type II casting gold alloys used?
Most widely used alloys for inlays = better mechanical properties than Type I but less ductile
When are Type III casting gold alloys used?
Areas where less support and opposing stress
e.g. Class II cavities in molars, all inlays, onlays, full-coverage crowns and short-span bridges
When are Type IV casting gold alloys used?
Extensive use of construction of partial denture components, long-span bridges (stiffer and stronger)
What classifies a gold alloy as a low content gold alloy?
<65%
(some as low as 10% but typically 45-50%)
What is the characteristic colour of high palladium content?
Whiteish colour
What are the properties of a low content gold alloy?
They vary but similar to type III & IV casting alloys except for ductility = significantly lower
Which casting technique is used for low content gold alloys?
Same as for gold alloys
What are low content gold alloys used for?
Widespread use on grounds of acceptable properties, good performance,
same equipment requirements and lower cost when compared to coventional gold alloys
What are Silver-palladium alloys made from?
- Silver
- A minimum of 25% Palladium
- With small quantities of copper, zinc, indium and occasionally gold
What are base metal casting alloys?
Alloys which contain no gold, silver platinum or palladium
What are Nickel chromium alloys used for?
Crown and bridge casting
n.b. some concern about biocompatibility of nickel
What are cobalt chromium alloys used for?
Partial denture framework casting
What are titanium and titanium aluminium vanadium alloys used for?
Implants
What are the properties of Cobalt/Nickel chromium alloys?
- * Melting point = 1300-1500 degrees C
- * Modulus of elasticity = 250 GPa (>Au) = STIFF
- * Vickers hardness number 350-400 (>Au = 250-300)
What is the composition of cobalt/Nickel chromium alloys?
- Chromium (30% = difficult to cast and brittle)
- Cobalt and Nickel (65-78%)
- = pretty interchangeable
- Ni alloys have decreased strength, hardness, modulus of elasticity, fusion temps and increased ductility & % elongation compared to cobalt
- Minor alloying elements = control the majority of the physical properties
- Carbon (0.1-0.5%) = increases strength, hardness and brittleness -> if increased by 0.2% = too hard and brittle for dental use, if decreased by 0.2% = decreases yield strength and ultimate tensile strength to unacceptable levels
- Molybdenum (3-6%) = refines grain size = increases strength, hardness and % elongation
- Aluminium 4-5% = forms a Nickel Aluminium in NiCr alloys = contributes to precipitation hardening = increased tensile and yield strength
- Beryllium (0.5-2%) = decreases the fusion temp by approx 100 degrees C, increases fluidity during casting, allows for electrolytic etching (with resin bond prosthesis)
- -Manganese (5%)
- Silicon (0.5%) = increases fluidity and cast-ability of molten alloy
- (+ Boron = deoxidizers = essential in Ni containing alloys)
- Iron and copper = increase hardness
N.b. composition changes slightly dependant on the company that manufactures them
What are the properties of titanium and titanium alloys?
- Light weight (Density: 4.5 g/cm cubed n.b. not ideal for casting = why we often use alloy)
- Strong (>100MPa)
- Corrosion resistant (dynamically formed titanium oxide on the surface = most important reason for increased use in dentistry
- Biocompatible - Low cost
What is commercially pure titanium (cp Ti) used for?
- Dental implants
- Crowns
- Partial and complete dentures
How do we improve osteointegration of Titanium implants?
Sandblast or etching surface left
Cells = like growing on titanium oxide
Which Titanium alloy is more widely used?
Ti-6Al-4V
= stronger (225MPa), smaller diameter implants
-> concern with aluminium (not scientifically proven) but never actually contacts tissue directly as oxide layer does
What are the problems with casting titanium alloys?
- High melting point (approx 1700 degrees C)
- Chemical reactivity (reacts with gaseous elements easily especially at high temps (>600 degrees C) = needs a well controlled vacuum in processing (inert gas atmosphere & centrifugal force applied)
= technology required makes casting titanium expensive
