6 - Precious Alloys Flashcards
how can the strength and hardness of pure gold be improved?
- cold working
- alloying
when two pieces of pure gold are pressed together, how are they welded together without the application of heat?
- metallic bonds are formed at their point of contact, and the gold is then welded together
cohesive gold: normally used in what form? with what method is the material condensed together? how to remove any adsorbed grease? why is it important to remove grease?
- form of a thin gold sheet/foil, approx 0.001mm thick.
- cold welding
- heat gold foil to 250deg in electric furnace or gas flame
- grease prevents efficient welding `
mechanical properties of gold depend on? degree of work hardening depends on?
what limitation does this bring?
- depend on the amount of cold working carried out.
- work hardening depends on the pressure applied during condensation, and length of condensation
- practical limitations of time and patient tolerance of pressure will restrict how hard the gold filling is
advantages and disadvantages of gold?
- perfectly corrosion resistant
- does not need cement lute for retention
- durable for protected cavities surrounded by tooth substance/low load bearing area
- in high load bearing areas, rigidity and elasticity are insufficient to resist distortion
gold - overworking at placement causes?
work hardening
traditional casting gold alloys - nobel metal content of at least how much?
75%
4 types of traditional casting gold alloys - how does nobility change from type 1 to 4?
it decreases.
traditional gold casting alloys: list and describe the use of each type?
type 1: low strength
- castings subject to slight stress
- inlays
type 2: medium strength
- castings subject to moderate stress
- inlays/onlays
type 3: high strength
- high stress situations
- onlays, thin cast backings, pontics, full crowns
type 4: extra high strength
- for castings thin in cross section
- saddles, bars, clasps, crowns, bridges, partial denture frameworks
nobility of gold alloys often indicated by which 2 values?
- carat value
- fineness value
carat value indicates? what is the carat value of a type 2 alloy with 75% gold?
number of parts by weight of gold in 24 parts alloy
- 75% of 24 is = 18.
fineness value indicates? fineness value of a type 2 alloy with 75% gold?
fineness value indicates the number of parts by weight of gold in 1000 parts alloy
- 75% of 1000 = 750
what increases with decreasing nobility in traditional casting gold alloys?
what can explain this effect?
- with the decrease in nobility comes the increase in hardness
- this is due to the solution hardening effect of the alloying metals, which all form solid solutions with gold
list 5 property changes that occur while moving up from type 1 to type 4.
increase in:
- hardness
- proportional limit
- strength
decrease in:
- ductility
- corrosion resistance
properties conferred by constituents of gold alloy: platinum and palladium?
- increases strength
- increases melting point
properties conferred by constituents of gold alloy:
zinc? what happens when zinc is used up (who takes over the function)? why is important to prevent all the zinc from oxidizing? how to prevent this from occurring?
- zinc acts as a scavenger
- scavenger function taken over by copper
- impairs physical properties of casting if Cu is used up
- always include fresh alloy in each alloy melt prepared for casting
heat treatment:
- possible for which types of alloy?
- results in? how?
- not possible for which types? why?
- possible for types 3 & 4
- results in further hardening by precipitation hardening of Ag/Cu and Au/Cu systems
- not possible for types 1 and 2
- lack of silver and copper required for the necessary atomic diffusions
heat treatment: how to achieve.
- melting alloy and cooling it results in?
- what influences the structure?
- how to avoid premature hardening?
- red color of sprue region indicates?
- once red colour is lost through cooling, what should occur to get a fine grain structure?
- new crystal structure
- length of time to cool.
- casting must be cooled rapidly from temperature above 600deg
- indicates temperature above 600deg
- cold water plunge, to cool down casting rapidly. disintegrates the investment and results in a fine grain structure
heat treatments in castings with Pt/Pd:
- cold water risks what?
- how to avoid the risks?
- what is the aim of homogenization heat treatment?
- cold water risks coring
- carry out homogenization heat treatment: heat to 700deg for 10 minutes, then quenching.
- aim is to eliminate coring
increase in the prices of gold have led to the increase in use of what? what is the main change in composition here? how does the change in composition affect the overall appearance of the alloy?
- increase in use of low gold content alloys
- gold was normally 45-50%, but now can be as low as 10%. palladium content is high
- this change in composition gives a whitish colour to the alloys
silver palladium alloys:
primary made out of?
decreased amount of?
- mainly Ag and Pd
- little or no Au
what are the factors that have led to the widespread use of low gold content alloys?
- casting techniques are equipment used are similar to the conventional gold alloys
- low cost
- good clinical performance
silver-palladium alloys:
how does its density compare to gold alloys? what does this affect? why?
- lower density than gold alloys, this affects castability. this is because there is a lower force generated by the molten alloy
silver palladium alloys:
alloys with large amount of palladium have a propensity for? how will this affect the casting of the alloy? how to avoid this effect?
- propensity for dissolving oxygen in molten state
- this may lead to porosity in the casting
- avoid overheating or oxidation of the melt during casting
