What is the metallurgic phase?
A physically distinct, homogeneous mechanically separably portion of a system with a unique crystal structure and physical properties. A phase is separated by other phases by a boundary.
What is the difference between soldering and brazing?
In soldering, the filler metal has a liquidus temp. lower than 450 and the solidus temp of the substrate metal. Brazing has a temp higher than 450 but still below the solidus temp.
What is the purpose of soldering flux?
To prevent/remove the oxide layer on a metal surface and allow the solder to wet the substrate.
What is the difference between type I, II, and III flux mechanisms?
I: Protects substrate from O2 access.
II: Reduces oxides and exposes clean metal
III: Dissolves oxides and corrosion products
What are the types of fluxes for nobel metal alloys and what are they?
Borax, boric acid, and boric anyhdride act as type I and II fluxes. (used with Au solders).
What are the types of fluxes for base metal alloys and what are they?
Fluoride acts as a type III flux and may contain borate as glass formers (protective). Used with Ag solders.
What is an "anti-flux" and what does it do?
Graphite or rouge in chloroform. It prevents solder flow onto adjacent areas.
What is the flow temperature of an ideal solder?
At least 55.6 degrees C below solidus temp of the substrate metal.
What are the other aspects of the ideal solder?
-Wets the substrate metal
-Sufficient fluidity at flow temp.
-Adequate hardness and strength
-Tarnish and corrosion resistance
Why don't we use soft solders in dentistry?
Because of their poor corrosion properties. They do have good mechanical properties though.
What is the melting point of dental solders? What are its properties?
>450 degrees C.
Better mechanical properties than soft and with fair to good corrosion properties.
What is the composition of silver solders?
Alloys of Ag, Cu, Zn (Cd, Sn, P)
-Ag-Cu eutectic (low Mp)
The lower the gold content of a solder, the __________, ___________, and __________ ductile the alloy.
harder, stronger, less
What happens if the solder flow temperature isn't at least 55.6 degrees C below the substrate solidus temperature?
1. Melting or sag of the substrate
2. Recrystallization (change in grain size)
3. Damage to porcelain.
If you have a high fineness then you have a:
___ TS, ____ flow, ____ corrosion, ___ brittleness.
Why do you want to use reducing atmosphere to heat?
It eliminates oxides, keeps the correct part of the flame on the area until done, and the solder is drawn toward the heat.
What happens if you over-heat the solder?
1. Changes the metal substrate
2. Pitting of the solder joint: boil Sn or Zn.
3. Alloying of solder and substrate.
4. Distortion or meltdown of the substrate.
What happens if you underheat?
1. Lack of solder flow: weak joint
2. Pitting of the solder joint: flux inclusions
3. Poor wetting of the substrate: weak joint
What is postsoldering?
Process of brazing or soldering two or more metal components of a prosthesis after the metal substructure has been veneered with a ceramic.
What is presoldering?
Process of brazing or soldering two or more metal components of a prosthesis before a ceramic veneer is fired or hot pressed onto the structure.
What do some dental technicians do to circumvent presoldering in ceramics?
They avoid the soldering process by using a cast-joining procedure instead.
What is cast-joining?
Process of joining two components of a fixed partial denture (bridge) by casting molten metal into the region between the invested components.
When is cast-joining preferred?
In PB alloys (especially Ni-Cr) due to technique sensitivity and variation in quality of the solder joint and sometimes in ceramics to circumvent presoldering.