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Dental Ceramics & Miscellaneous
Joining metal parts together by melting a filler metal between them at a temperature below the solidus temperatrue of the metal being joined. Here the fusion temperature of filler material is greater than 450°C.
Joining metal parts together by melting a filler metal between them at a temperature below the solidus temperature of the metal being joined. Here the fusion temperature of filler material is less than 450°c.
Joining two or more metal pieces by applying pressure (Cold welding) or heat (Hot welding) or both.
Spot welding is used to join orthodontic components.
- Joining of metal before porcelain veneering is called pre-soldering and join metals after porcelain veneering is called post soldering.
- Dental solders are alloys that are used as intermidiary or filler material to join two or more metallic parts.
Gold and silver solders are commonly used in dentistry.
Silver solders are low fusing solders and are used for soldering stainless steel or other base metal alloys.
- Composition of Solders:
- The fusion (flow) temperature of the solder should be at least 56°C lower than the solidus temperature of the substrate (parent) metal.
- For a solder to wet and flow properly, the parent metal must be free of oxides.
- "Flux" is used to remove oxide coating on the parent metal and to protect the metal surface from oxidation during soldering procedures.
Borax and boric acid fluxes are used with noble metal alloys. Fluoride fluxes are used with base metal alloys, as they help to dissolve the more stable chromium, nickel oxides.
The composition of flux includes Borax glass (55%), Boric acid (35%) and Silica 10%.
Fluoride fluxes contain boric acid and potassium floutide in 1:1 ratio.
- "Anti-flux" is used to limit the follow of molten solder over metals being joined.
Eg.: Graphite, lead, rouge (Iron oxide), etc.
- The most commonly used heat source for melting solder is gas air or gas oxygen torch.
- The various "fuels" used are hydrogen, natural gas, acetylene and propane.
Propane is the best choice because of its highest flame temperature and flame heat.
Acetylene has highest flame temperature but it is unstable and the temperature varies from one part of the flame to the other.
- Zones of flame from inner to outer space.
|1||Cold mixing zone||Unburned gas||Not used|
|2||Combustion zone (Green)||Oxidizing||Not used|
|3||Reducing zone (Blue)||Hottest zone||Used for soldering and melting alloys|
|4||Oxidizing zone||Burned gas||Not used|
- There should be optimum gap between metal parts to be joined. If the gap is too narrow, strength is limited and if gap is too wide, the joint strength will be controlled by the strength of the solder.
- Pure titanium can be laser welded in an argon atmosphere.
- Cast joining is a process of joining two components of fixed partial denture by means of casting molten metal into the interlocking region between the invested components.
- Orthodontic Spot Welding:
Heat and pressure are the two basic principles involved in orthodontic spot welding.
It is very important that the passage of current at the weld spot may be of very short duration i.e., not more than 1/10th of a second. If the current is passed for longer duration, it will result in weld decay due to precipitation of carbides from metal.
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