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Re: Welding precious metals
Follow UpsPost FollowupBramblebush ForumsFAQ Posted by Tony Truong on January 03, 1998 at 22:43:47: In Reply to: Re: Welding precious metals posted by Valerie on September 05, 1997 at 17:06:30: : : Brian, this is a two-part question and I'll answer them briefly first and then elaborate later on the weldability. : : What's required of a metal that it is weldable? Poor heat conductivity, I guess. : There are quite a few considerations when you use the term "weldable". We can pretty much weld any metal, it is just that some properties obviously make one metal harder to weld than another - but with proper techniques you can get the job done. The trick is knowing the properties of the metal to be welded and then to figure out a weld procedure that will make a joint which stands up in service. Yes, if a metal has a high thermal conductivity (heat is sucked out of the joint quickly by the mass of the metal itself), it is harder to weld since it takes a high, fast heat (electric) or immense quantities of low heat (oxyfuel). Some also need a special atmosphere (such as argon or helium) to be properly welded. : This topic is called the weldability of metals. There are special concerns for each type of metal and for different types of applications. I think that I will spread out this inforamtion over time because all of it at once is -how shall I say - pretty heavy stuff. : As far as welding, what is it? Welding is when we melt the base material and have the material fuse or flow together. It may be done with or without a filler material. Some processes don't even use heat - such as ultrasonic (sound wave) welding! : As far as welding the precious metals (by most books - gold, silver, platinum and palladium), they can all be welded. The oxyfuel process can be used for each of these metals. There is one problem with welding these materials, though: usually we are talking about relatively small objects. When you are working with a small mass of metal, it is easy for the material to get to the molten temperature. The heat is easily distributed through the piece. This makes the time to get the material molten fairly rapid. So, you are joining the pieces, but the rest of the material is awfully close to the melting temperature (which in pure metals is a melting *point*). In other words, WHAM, the piece melts down. There is no real color indication, either, as there is in steel turning red to let you know that you are close to temperature. : So, the weldability handbooks that I have say that these metals CAN be welded. For Gold and Silver oxyfuel welding, use a reducing flame. This means that there is a slight feather left at the end of the blue "cone" of the flame as you adjust your oxygen at the torch. This reduces the oxides at the surface since this type of flame uses more oxygen from the atmosphere. : For platinum and palladium, a slighly oxidizing (excess oxygen) flame is used. This means that you open the oxygen a bit more than needed to get that small feather at the end of the blue cone to disappear. This prevents carbon from the flame from becoming part of the weld joint. : For silver, a flux made of equal parts borax and boric acid mixed with equal parts water an alcohol is suggested (by American Welding Society Welding Handbook, Vol 4, 7th Edition). : Gold can also be joined by Gas Tungsten Arc Welding (GTAW, TIG, Heli-Arc are common shop terms), Plasma Arc Welding or Electron Beam Welding. Platinum can also be welded using thes techniques. : I would suggest a heat-sink material (clay-like, heat-absorbing material) to support the metal near the weld zone. This has really helped me in joining small parts without the worry of melt-down! : Preferable to welding these metals is the brazing process (also called silver braze, hard solder, silver solder, nickel silver in shops). With brazing, we can still get the same strength as a weld but we don't have to get the base metal molten! So we can work at a bit lower temperature and not worry so much about the parts melting down in the process. Filler metal used for braze welding has been engineered for perfect color match to all sorts of metals. At the temperature that brazing is done, the base metal will allow the braze material to join. No real intermetallic melting occurs, but in a microscope used for metals, we can see that the base metal has "opened up" a bit and allowed good bonding at the surface. : A flux is always used in brazing. It "wets" the surface, allowing the braze material to flow easily over the surface. It also lifts minor oxide layers and allows for a clean surface. These flux residues should be cleaned off after brazing since they usually are a mild corrosive (since they can clean off oxides!) : The trick with good braze welding is to first pick a brazing filler material. Jewelry suppliers and specialty weld filler metal suppliers have a wide assortment. Next most important is to have your metal clean of grase, oxides and any other surface contaminants. The flux can only clean off so much!! Brazing usually uses an oxyfuel torch. Preheat the joint and get it to a temperature at which the flux can flow. This is usually 100 degrees Fahrenheit lower than the melting temperature of the filler material. If the filler material beads up, it means that the metal is not hot enough. WIpe these beads off and start over: if you try to just puddle them around with the torch you will never get good bonding. The flux is usually your temperature indicator. : For production techiques, there are dip brazing processes, furnace brazing and also pastes which contain both the flux and the powdered filler material (dispensed in syringes to the joint for fine control!) : Brazing needs a tight joint, flux in the joint and proper temperature. The filler material flows into the joint by capillary action (same action as water pulled up into a paper towel). Filler may be added from the side of the joint and the flux and tight joint will pull it right in. : We have tested brazed joints here and found them to be equally as strong in tension as a welded joint in which the base metals were molten and fused together. Good luck!!! >>>> Hi, enjoyed the detailed explanation above on brazing. I have a question going in the reverse direction. How do you unsolder or unbraze a silver-soldered joint between stainless steel and brass without destroying optical components housed within?
>>> Hi, enjoyed the detailed explanation above on brazing. I have a question going in the reverse direction. How do you unsolder or unbraze a silver-soldered joint between stainless steel and brass without destroying optical components housed within? : " /> Follow UpsPost FollowupBramblebush ForumsFAQ |
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