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The Art & Science of Silver Soldering |
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Originally published January 2007
The Art & Science of Silver Soldering
Silver soldering is a common procedure in any orthodontic laboratory that produces expanders and Herbst®-type appliances. This skill is probably easier to learn than a lot of other lab procedures, and most technicians are fairly adept at making the process work for them. Besides the skill and craftsmanship requirements, there is also quite a bit of science at work, and the more you know about these unchangeable forces of nature , the more consistent and reliable your joining procedures will be.
Most solders for orthodontic purposes contain over 50% silver, plus the addition of copper, zinc, and tin. The high silver content makes this alloy pricey, but it is a great material for bonding stainless steels as well as many other dissimilar metals. Silver solder when properly applied is active at a low enough temperature to prevent alteration of the force and strength properties of wires and other metal components. Orthodontic silver alloys also have the capability of filling larger joint clearances with little, if any, sacrifice in strength.
Always make sure that the alloy you select for soldering is cadmium-free. Cadmium is nasty for the technician in creating hazardous fumes when melted and is considered toxic for use in the patients’ mouth.
Filler alloys used in silver soldering have two critical temperatures: (1) the point at which they change from solid to liquid (melt point) and (2) the temperature at which they reach complete fluidity and chemical activity (flow point / or liquidus). Being aware of these two critical temperatures is just one aspect of successful silver soldering. Here are a few more:
Good fit in joint clearances.
The integrity of the silver solder joint depends on successful capillary action – the process by which the filler metal at flow/ liquidus temperature is drawn through the entire joint by the uniform clearances between the components being bonded. The strength of the completed joint is directly related to the space between the two components. For example, a silver soldered joint created with a joint clearance of .04mm can have a tensile strength exceeding 130,000 psi. This same joint created with a joint clearance of .5 mm will result in a tensile strength of less than 30,000 psi. Maintaining .04-.06 mm joint clearances creates optimum strength joints. Besides joint clearance, capillary action is also affected by surface finish of the components being bonded. Unlike “soft” soldering, a mechanical metallurgical action happens between the components being bonded and the silver filler material. The filler at flow temperature actually permeates the surface of the components and creates a mechanical “grip”. If the material we are bonding is highly polished (like wire and crowns), the surface is more resistant to this flow and grip, and the metallurgical bond is weakened.
Flux
Flux does a number of important things in the brazing process:
• It is an oxide preventative.
• It is an oxide absorbent.
• It is a temperature indicator.
Flux is not a surface cleaner. You absolutely need to clean the surfaces to be bonded prior to applying flux. A good degreasing solvent is essential to have in your soldering area. Combine a polished surface with grease or oil (even from your fingers), and flux will not adhere properly. Oils get you twice – (1) by not allowing even coverage of flux, causing oxide formation that inhibits flow and (2), oils carbonize during heating, preventing flow. You will easily recognize an oily part if you apply flux and it balls up.
Most technicians understand that flux protects the metal surface from oxide formation when heating. It does more. In spite of flux covering the joint, oxides are continually being formed during heating. Flux absorbs these oxides. The amount of flux applied is directly proportional to the amount of oxides able to be absorbed. The amount of oxides created is directly proportional to the amount of heat applied and the duration of the heating process. The obvious conclusion here is apply enough flux, and get your heating process done quickly. The more oxides the flux absorbs, the harder it is to remove. The green (and then black) color of flux when cooled is an indication that it is getting saturated with oxides.
Flux is also a great temperature indicator, particularly for the soldering alloys used in orthodontics. Just by observing the state of the flux while heating, the technician can get a very accurate idea of how hot the assembly is becoming. Here are some critical points in the heating process of the general purpose flux we use:
212° F – Water boils off.
600° F – Flux becomes puffy and starts a dry bubbling.
800° F – Flux flattens against surface of components with a milky appearance.
1100° F – Flux liquefies to clear appearance, exposing clean metal underneath. You’re moments away from applying your filler alloy.
Heating the joint
Fortunately, stainless steel is a lousy conductor of heat, so temperatures are relatively easy to keep localized during the soldering process. Also, it is good news that most often the materials we are heating are similar metals, with similar heating characteristics (the only potential glitch being differences in mass). It is important that both components reach the same temperature (liquidus) at the same time, and that applying the silver alloy and completing the joint happens as quickly as possible. Besides the flux issue mentioned above, here is another reason: Stainless steel is made stainless primarily by its chromium content. During manufacturing, 300 series stainless (austenitic) is heated to a very high temperature and cooled rapidly to maintain the dispersion of the metallurgical components in a specific order. By heating stainless steel in the range of 800-1600° F, that dispersion is altered, allowing the chromium to diffuse away from the grain boundaries. This little phenomenon results in stainless steel that is not particularly stainless where it was heated, and is subject to corrosion. Using a silver alloy with a liquidus temperature of around 1200° F, this situation is unavoidable, however it can be minimized by getting your heating phase done ASAP.
Peter Sildve / General Manager
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2009 Laboratory Equipment and Products (3962)