soldering in dentistry
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    DEFINITIONS    SOLDERING: It’s defined as the joining of metals by the fusion of filler metal between them, at a temperature below the solidus temperature of the metals being joined and  below 450°C.      BRAZING : It’s a term used industrially. Soldering operations at or above 450°C is generally termed brazing. Most dental soldering procedures are actually brazing, but the names are used interchangeably in dentistry.    Brazing is defined as joining of metals by the fusion of a filler metal between them, at a temperature below the solidus temperature of metals being joined and above 450°C.      LIQUIDUS TEMPERATURE: the temperature at which metals of an alloy system  begin to solidify on cooling or become totally liquid on cooling.    SOLIDUS TEMPERATURE: the temperature at which metals of an alloy system become completely solidified on cooling or start to melt on heating.      WELDING: The joining of two or more metal parts by applying heat, pressure or both, with or without a filler metal, to produce localized union across the interface through fusion or diffusion.      CAST JOINING Another type of metal joining procedure in dentistry. It is the process of combining two components of a fixed partial denture by means of casting molten metal into interlocking region between invested components. This is preferred for base metal alloys because of technique sensitivity of brazing or soldering these alloys.      Soldering / Brazing : The diff. is between the liquidus temp. of the filler metal.    Soldering & Brazing / Welding : Possible absence of the filler metal & the partial fusion of the parts joined by welding.   SOLDERING    HISTORY    The soldering technique has been known to man for hundreds, maybe thousand years. However, with the industrial revolution in Europe as well as North America, the need for higher temperatures and more user-friendly tools emerged. The old methods of heating metals with coal fires etc. were no longer practical.    It was discovered that when the vapor from heated alcohol was ignited over a burning wick, it burnt with a very concentrated flame of high temperature very suitable for different heating purposes.     Many different designs emerged using this technique and these heating tools were generally called blow pipes. The first known patent is from France and is dated January 7, 1791.      During the early 1900`s, a great variety of blow lamps specially designed for different  purpose and applications came on the market.    After the Second World War, the propane gas emerged as a cleaner and safer fuel for different heating purposes. The introduction of propane caused a lot of changes in the  blow lamp industry world-wide.      The first appliances used had a metal frame work. The attachment of axillaries to bring about the different type of tooth movements required soldering of these parts. Welding in orthodontics became popular after the arrival of spot welders. It became popular because of the short time required, the ease of welding and the absence of elaborate equipments      Soldering is often used in construction of dental appliances. Large partial dentures are frequently cast in parts that are soldered together after carefully fitting them to master cast. In orthodontics soldering is used for joining wires, bands springs etc    The soldering process involves the substrate or the parent metals to be joined, soldering filler metal (usually called solder), a flux, and a heat source. All are equally important and the role of each must be taken in to consideration to solder metal components successfully .      COMPONENTS OF SOLDERED JOINT    Parent metal    Solder/filler metal    Fluxes and Anti fluxes   PARENT METAL    The parent metal is the metal or alloy to be joined.    This is also known as a substrate metal or base metal.    Soldering operation is the same for any substrate metal,     but the ease of soldering is not same for any substrate metal.    The composition of parent metal determines-    Melting range     Oxide that forms on the surface during heating    Wettability of the substrate by the molten solder.    Soldering should take place below the solids temperature of the parent metal.      Composition of alloy determines the oxides that form on its surface during heating. The flux used should be able to reduce these oxides, inhibit further oxidation and facilitate its removal.    Composition of alloy determines the wettability of the substrate by the molten solder alloy. The solder chosen must wet the metal at as low a contact angle as possible to ensure wetting of the joint area.    Manufacturer of the alloy should provide guidance and instruction regarding the flux to  be used with that alloy.    A low temp soldering is preferred rather than the high temp soldering for Stainless steel wire to prevent carbide precipitation and to prevent an excessive softening of the wire. So silver solders are generally preferred.   FLUX    In Latin flux means ―to flow‖ .    Purpose of flux is to remove any oxide coating on the substrate metal surface when the filler metal is fluid and ready to flow into place.    They protect the alloy surface from oxidation during soldering and dissolve metallic oxides as they are formed.    The resulting solution of oxides or other extraneous matter in flux constitutes ―slag‖.   CLASSIFICATION OF FLUX    1.According to their primary purpose / activity o   Surface protection type: - This type of flux covers the metal surface and prevents access to oxygen, so that no oxides can form. o   Reducing agent type: - This type reduces any oxides present and exposes clean metal. o   Solvent type: - This type dissolves any oxides and drives them away.     The composition of most commercial fluxes is formulated to accomplish two or more of these purposes.      2.According to their composition    Borax fluxes    Fluoride fluxes    3. According to the pH of the flux    Acidic fluxes  –   SiO2    Basic fluxes  –   CaO, lime CaCO3 LIMESTONE     Neutral  –   Fluorspar (Ca.F2),Borax (Na2B4O2)   BORAX FLUXES    Borax from Persian burah    Also called sodium borate , or sodium tetraborate , or disodium    Tetraborate.    They are based on boric or borate compounds such as boric acid/boric    anhydrate and borax.    It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.    Borax has a wide variety of uses:-    It is a component of many detergents, cosmetics, and enamel glazes.    It is also used to make buffer solutions in biochemistry    as a fire retardant    as an insecticide    as a flux in metallurgy    They act as protective fluxes and reducing fluxes for low stability.
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