In general, metal-joining operations are usually divided into three categories: soldering, brazing, and welding.
WHAT IS SOLDERING?
Soldering is a method of joining two or more cast or wrought pieces using another alloy called a solder.
Purpose of soldering:
It is used for joining metal components produced in two or more parts formed from different alloys and cast individually,
It is used also to overcome distortions that may occur during cooling for metal frameworks for long span bridge work. These casting defects can be corrected with soldering.
WHAT IS THE DIFFERENCE BETWEEN SOLDERING AND BRAZING?
In dentistry the terms soldering and brazing are used interchangeably.
In industrial metal working, a distinction is made between soldering, in which the filler metal has a melting point below 450° C, and brazing, in which the filler has a melting point above 450° C.
Both soldering and brazing require a filler metal usually called a solder.
However, in normal metallurgical terms the two processes, although trying to achieve the same goal, are carried out at different temperatures, brazing being a process carried out at higher temperatures.
For some unknown reason in dentistry the term soldering is used to describe the joining of metal components using a filler metal. This is technically incorrect as all metal joining in dentistry is generally done above 450°C which is the process of brazing.
The requirement of a brazing material for noble alloys is that it be capable of forming a strong, corrosion resistant joint between the two components being joined without changing their structure or properties.
Ideal Properties of a Dental Solder
1. Lower fusing: 50°C-100°C lower than melting point of the components, so as to prevent the distortion and melting.
2. Free-flowing: it is important that the brazing material has a flow temperature which is well below the melting point of the alloys being joined —- The flow temperature is defined as the lowest temperature at which the filler material is fluid enough to flow into the gap and to wet the surface of the metallic parts.
3. Strong: should be as strong as the components being melted.
4. Corrosion resistant: so as to prevent tarnishing and discoloration.
5. Same color: the color of the solder should match that of the alloy
6. Solder must be compatible with the metal, but it does not necessarily have a similar composition.
Solders for joining noble metal components are formulated from mixtures of gold, silver and copper designed to have low fusion temperatures. Base metal alloys can be difficult to solder because they oxidize; this must be controlled with special fluxes.
Dental solders are supplied in a variety of shapes, such as strips, rods, wires, or cubes, each of which is convenient for certain operations.
Soldering investments should not expand as much as casting investments. They are similar in composition to casting investments. Units must be correctly gapped so that they do not touch. When the work units are allowed to touch, distortion and porous inadequate joints result. Alternatively, excessive gap spaces cause undersized restoration widths because of solder solidification shrinkage.
Solders are susceptible to oxidation during the melting/softening procedure and the resulting oxides can weaken the soldered joint. Furthermore, the metal components being joined are often coated with a thin oxide film which can limit the ability to achieve proper jointing.
Fluxes are employed to break down the surface oxide layers on metals and to prevent oxidation of the solder. Fluxes commonly used are fluoride salts and borax. Proper application of fluxes provides a protective coating that dissolves surface oxides and allows the melted solder to wet and flow onto the adjoining alloy surfaces.
For soldering to be successful basic practical steps should be followed:
- Ensure the surface of the alloy is free from dirt and oxides.
- Place the components as close together as possible without touching.
- If using soldering investment use the smallest amount possible. This will maintain the proper position of the parts during the procedure.
- Place flux in the joint to be soldered.
- Select a suitable solder which melts 50°C-100°C below the melting point of the components.
- Heat the components evenly by either using the reducing zone of the flame or a furnace.
- When the flame is evenly cherry red, apply the solder.
- The parts being joined are not melted during soldering but must be thoroughly wettable by liquefied solder.
- The careful and skillful use of the soldering torch flame is important to a high quality soldered joint. A well-defined, not-too-large pointed flame is advisable for the final heating of solder in a localized area, but a larger, less well defined flame of the “brush” type may be used for the initial heating.
- The operation should be completed in the shortest time possible to avoid oxidation of the base metal ingredients of the alloys involved and to prevent damage to the microstructures of the alloys.
The Soldering Process:
A. Assembling parts
B. Parts invested
Note: Eye protection is essential during soldering process
C. Torch soldering
D. Clinical evaluation
- The connector is carefully inspected. If signs of an incomplete joint are evident (e.g., visible porosity in the solder), they are removed by grinding with a fine disk; the units are then re-invested and re-soldered.
- The joints must be tested for strength. Any connector that can be broken by force of hand will not serve adequately in the mouth.
The difference between soldering (brazing) and welding is that welding does not require a “third” low fusing alloy (solder) as additional material for joining two pieces together.
The material used for welding is always the same composition and the same color as the alloy to be welded.
Welding is also generally done at higher temperatures than soldering.
About Laser Welding:
A laser generates a high intensity pulse of light that can be focused. By selecting the duration and intensity of the pulse, metals can be melted in a small region without extensive damage to the surrounding areas.
In laser welding, the beam is focused at the joint to melt the opposing surfaces. The two liquid surfaces contact and form a weld on solidification.
Components are carefully aligned in the laser welding unit. The joining procedure is monitored with high-magnification video.
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