Metal Alloys for Removable Partial Dentures (RPDs) and Clasps

For the last lecture 

Dentures rely on soft tissues while fixed pros don’t so it doesn’t matter if you use a viscous muco/compressive impression material

immediate casting for alginates

Elastomers vary on whether they are compressive or static based on high viscosity and low viscosity respectively

might use putty instead of green stick for border moulding

Polyether - hydrophilic

Deformation of metals requires what?

what happens in the metal in permanent deformations?

to deform metals you apply stress above yield stress

if it is lower than the yield stress, you get no deformation

you get movements of dislocations which then stop at the grain boundary

in the image there is an extra plane of atoms moving from A to B to D and keeps moving due to application of stress above the yield stress - as it moves it will eventually stop at the grain boundary - as between grain boundaries structure is amorphous where atoms are arranged randomly so cannot move beyond the crystalline structures (where atoms are arranged in organised structure). 

next time you deform the metal you require higher forces

when you bend/apply force the metal more dislocations are also made

eventually it will fracture

What is work (strain hardening)?

Application of shear stresses produces more dislocations, hinders the movement of dislocations, more difficult to deform the metal

What are other strengthening mechanisms? (6)

• Forging - drawing wire/rolling sheets

• Bending - denture clasp, orthodontic appliances

• burnishing - amalgam or Gold Inlay

• Solution hardening - different atomic radii of atoms within same crystalline structure limit the movement of slip planes (alloys)

• order hardening - super lattice (ordered structure) formation (in a solid solution)

• precipitation hardening - partial solubility varying with temperature Ag-Cu

in a solid solution the metals are completely soluble 

Describe this diagram 

heat above the pink line - liquid

fast cooling/quench/solid solution - 1 phase of metal in a random state (organised/ordered/crystalline structure but atoms in random structure) (atom position is disordered)

put the metal under heat treatment - not melting the metal - giving atoms energy and time to form ordered structure within the crystal = super lattice (atoms occupy specific repeating positions)

Explain this diagram?

Solid solution - 2 metals are completely soluble in each other so one phase

limit of solubility, if you cool metal you freeze structure in place (alpha) - supersaturated solution

if you slow cool and below limit of solubility - you get a phase rich in copper will start precipitating within the alloy - so you create obstacles that dislocations can’t move beyond that second phase

slow cooling allows the Cu to diffuse when the solubility decreases 

What are the 3 classifications of dental alloys? (3)

• High noble

• Noble

• predominantly base metal 

What is a Noble Metal?

• Must contain a Noble metal

• Containing least 60% noble metal of which 40% must be gold Au

• Other metals - Pt, Pd, Rh, Ru, Ir, Os

platinum, palladium, rhodium, ruthenium, iridium, osmium

Noble metals?

Must contain at least 25% noble metals

Predominantly base metals?

Contain <25% noble metal

What are the alloys used for removable partial dentures?

Initially 

1930’s

1970’s

other alloys (3)

Gold alloys

chrome-based developed

cobalt-chrome alloys

nickel chrome, type IV gold, titanium-based alloys

3 main categories for ideal requirements of Removable partial dentures alloys?

• Handling and fabrication

• Mechanical properties

• Biological and clinical

Handling and fabrication requirements includes: (7)

• Easy to cast (low melting point, high density)

High density = easier to fill mould completely and force air out of the mould

• Easily soldered or laser-welded

• Low shrinkage

• Easy to finish, polish and adjust

Mechanical properties requirements includes: (4)

• High rigidity (high modulus) - support and stability

• High yield stress and fatigue stress - avoids permanent deformations during function

• Good wear resistance

Biological and clinical ideal requirements include? (4)

• Biocompatible and corrosion resistant

• Comfortable (light weight thin sections)

• Aesthetic compatibility with acrylics and tissues

What type is high gold alloy? 1-4?

Type 4 (IV)

3 and 4 further divided into?

cast and hard

Which have the better property?

hard is more hard and higher tensile strength the 4 has better properties Don’t have hard type 1 and type 2 - hard - not enough copper in the system for ordered structure 

Properties of type IV gold for removable partial dentures (not used anymore): adv(7) dis(4)

Adv:

• Low melting point

• Easy to cast

• easy to solder

• can be heat hardened (order hardened)

• easy to finish/polish

• biocompatible

• corrosion resistant

dis:

• High density

• Low yield stress

• low modulus 

• expensive

high density and low modulus - needs to be in thick cross sections - not comfortable for patient  

Typical composition of Co-Cr alloys

what is the maximum amount of Chrome?

other metals that can be involved? (3 main and 4 others)

Co - 50-65%, Cr - 25-30%

Ni 0-30% (some without) (replaces some of the Cr)

Mo 4-6% or C 0.2-0.5%

Small amounts of: Mn, Si,Fe,W

Co and Cr forma a solid solution alloy up to 30% Cr - limit of solubility of Cr in Co

What are the functions of Co, Ni, Cr, Mo, Fe, C, Mn in the Co-Cr metal framework?

Cobalt: 

• hardness and strength

Ni:

• hardness and strength (in Ni-Cr)

• increases ductility and decreases hardness (C to Co in Ni-Cr alloys)?

Cr:

• hardens the alloy by solution hardening,

• corrosion resistance by passive oxide layer

Mo+Fe:

• reduce grain sizes

• hardens by solution hardening

C:

• forms carbides which precipitates on slow cooling to give strength and hardness (excesses results in brittle alloy - therefore added in small amounts)

Mn:

• improve fluidity and castability

Adv of Co-Cr? (7)

• Low cost compared to gold alloys

• hard

• abrasion resistant

• High modulus - can be used in thinner sections

• low density - light weight 

• high yield stress - less likely to become permanently deformed

• Ni free are biocompatible 

Dis of Co-Cr ? (5)

• High casting temperature

• casting shrinkage - 2%

• Limited ductility - clasps may fracture if adjusted

• rapid work hardening - limited chairside adjustment

• Ni sensitivity 

• difficult to finish/polish

• Electroyle polishing of fit surface?

Ni-Cr alloys composition (4 main) small amounts of other metals? (6)

Ni, Cr, Mo, Be

Al, C, Co, Cu, Mn, Ti

The purpose of Ni?

Cr?

what to note about C addition?

Ni:

• Strength and hardness

Cr:

• Hardens solution by solution hardening

• Corrosion resistance - passive oxide layer

• must also limit carbide precipitation for strength

Properties of Ni-Cr alloys similar to those of?

CO-Cr

Ni-Cr has largely been replaced by Co-Cr and Ti alloys

Titanium and alloys 2 types?

Commercially pure and alloys

What can titanium and alloys be used for? (4)

Dental implants

Crown

Bridges

Partial dentures 

Generally:

High density - low MP

Difficult to cast - high MP - low density - high shrinkage

Titanium and alloys properties? (8)

• Biocompatible

• Low density 

• Excellent corrosion resistance - passive O layer

• High melting point

• Well defined fatigue limit - highest for alloys

• difficult to cast

• high casting shrinkage - 3.5%

• can react with investment material 

How can commercially pure titanium be classified, what elements might be included (5) and % of titanium in CpTi?

4 grades containing +99% Ti, N, C, H, Fe, and O

Grade 1 - 4

How does increasing O affect the titanium material?

increases X and decreases Y?

Increasing O increases strength and decreases ductility

Titanium alloys:

alloyed with what metals?

Al or Nb (niobium) or V (vanadium)

2 forms of Ti?

alpha - low temp

Beta - high temp form

What is the form found in Cp titanium? what elements are need to stabilise this form?

Cp titanium alpha form is stabilised by O, C,N

What forms are found in Ti6AL4V alloys and what stabilises it?

• Alpha and beta - V stabilises the beta form, Nb in TiAL7Nb will stabilise the beta

What is the problem with V?

vanadium is toxic in elemental form, can be replaced by Nb - Ti6Al7Nb

Titanium framework are now made by what methods ? (2)

Digital dentistry:

Computer numerical control (CNC) milling

Laser sintering SLM for RPDs

Properties similar to ?

CpTi

Which has better properties, the titanium alloy or the commercially pure titanium 

Improved properties in titanium alloys comparedto CpTi

comparison of alloys used for RPDs

2 methods of making clasps?

• Wrought clasps attached by soldering

• Casting with framework 

Which materials are used for cast clasps and wrought clasps (attached by soldering?), properties of each, 1 adv and 1 dis for each?

Rapid work hardening in Co-Cr - hence fracture if bent

Easily adjusted so better fatigue 

flexible - better for undercuts 

Aesthetic and tooth-coloured clasps - use which materials? (3)

Acetal resin (Polyoxymethylene)

Polyamide (Nylon)

PEEK/PEKK (polyetheretherketone polymers)

What are the adv of aesthetic and tooth-coloured clasps?

• tooth coloured - improved aesthetic 

• Flexible - engage deeper undercuts

• Biocompatible - metal-free

• can be used with metal or acrylic frameworks

3 dis of aesthetic and tooth-coloured clasps?

• lower rigidity - limited support

• may discolour over time 

• harder to polish than metal 

Flexible partial dentures:

what materials are commonly used?

• Polyamide (Nylon) - e.g valplast

• Acetal resin - polyoxymethelene

• Peek/pekk - polyetheretherketone 

Fabrication method? (2)

• Injection moulding - traditional

• CAD/CAM milling or 3D printing (modern)

Adv of flexible partial dentures? (4)

• Flexible - engage undercuts without stress

• lightweight and comfortable

• aesthetic - no visible metal

• biocompatible - no residual monomer

Dis of flexible partial dentures? (3)

usually used as what?

• Not as rigid - limited support and stability

• Prone to staining or warping overtime

• Difficult to repair or reline

• Usually used as interim or provisional solution

What is soldering How does it work?

The joining of metals by a filler metal (solder)

solder melts and wets the surface and joins 2 metals together

What must be used first before the solder?

• Flux, as it dissolved the surface oxides 

• it wets the surface 

• and solder displaces the flux 

what happens to the metal once passive oxide layer is removed?

metals have high surface energies

attract the molten solder to form bonds

Fluxes are commonly what?

give examples used with gold alloys (4)

and one example used with base metals alloys (1)

• Borates

• gold alloys: borax/sodium tetraborate/borax glass, sodium pyroborate

• fluoride fluxes are used with base metal alloys

Hard dental solders need to be (4)

• Corrosion resistant

• Have a high fusion temp - 50 degrees less than alloy (Don’t want it to change shape/melt)

• be as strong as the alloy

• have good flow

2 types of Solder

what is one of them typically used with?

which has the higher melting range?

Gold solders

Silver solders - stainless steel 

What is welding?

Joining metals without the use of a solder

3 types of welding?

• Pressure welding - metals are heated to recrystallisation temperature under pressure

• Electric spot welding - electric current generates heat at contact points

• laser welding (modern technique)- uses a focused laser beam to melt and fuse metals precisely

Pressure welding is used for and not used for?

• used for orthodontic wires not RPD frameworks

Electric spot welding is limited to?

thin metal sheets or wires

Laser welding is suitable for which materials used for RPD, 2 adv?

• gold, Co-Cr, SS, Ti

• minimal heat distortion and high accuracy

CAD/CAM workflow for RPD frameworks: main steps? (4 main)

• Scanning

• CAD

• CAM:

• - milling

• -SLM/DMLS selective laser melting or direct melting laser sintering

• creating pattern then conv casting

• Post-processing - heat tx, polishing and surface finish

ADV of CAD/CAM? (3)

short time production

digital storage and easy remakes

Difference between selective laser melting and direct metal laser sintering?

what type of manufacturing technique are they?

both use what?

main difference

• additive

• metal powder in layers

• In SLM - melted

• In DMLS - sintered