Nd:YAG lasers have become an important tool in the industrial manufacturing of precious metal jewelry
Joshua M. Gold
Within the past 15 years, lasers have become accepted as a common tool for use in the industrial production of gold jewelry. The lasers most frequently used in the jewelry industry are lamp- and diode-pumped Nd:YAG with either fixed or scanning optics.
Spot welding with pulsed Nd:YAG lasers is the most widely used laser technology in industrial jewelry production where it is primarily used for the autogenous welding (without added special welding materials) of gold chain on automatic high-speed chain making equipment (see Figure 1). Alternatively, this type of laser source may be integrated into other types of robotic or automatically fed workstations (for the spot welding of earring posts to earrings) or even fitted out with glovebox style Class I benchtop enclosures for manual spot welding. This latter is used particularly for high-end jewelry production and repair where the jeweler must weld around precious stones (see Figure 2) or for correction of porosity created in the casting process. In these cases, the jeweler may add weld material of the same alloy being welded.
FIGURE 1. A laser welder is coupled with a high-speed chain making machine.
FIGURE 2. Glovebox style benchtop enclosures are useful for welding in proximity to precious stones.
Laser welding was adopted in the jewelry industry for the welding of precious metal chain on automatic chain making machinery because this process offers important advantages over traditional forms of welding precious metals. Because no welding material is usually employed and welding occurs by partial melting of the materials in the joint, no carat problems are created as a result of the welding process. And because no welding alloys or flux solder powders are used, aggressive products and/or solvents need not be used to clean the piece and therefore there is less risk to the jeweler and no waste disposal issues.
Because flux solder powders are not required to create a strong weld, the traditional process of running formed but un-welded chain through a nitrogen oven to weld the links has been eliminated, simplifying and speeding up production with almost no loss of links, previously a great concern for industrial chain manufacturers.
The combination of a laser with automatic assembly processes has simplified the process of making certain jewelry such as complex gold chain (for example box chain and bi-metallic chain), and laser welding has facilitated the export of these processes to countries lacking a history of industrial style jewelry production.
Autogenous welding enables joining of precious metals that differ from each other in terms of carats and alloy composition. Laser welded multicolor gold jewelry is easily recognized because, not having been subjected to furnace brazing, its different colors remain vivid, not darkened or mixed by the heating required for brazing (see Figure 3).
FIGURE 3. Laser welding on a two-color (white and yellow) 18k gold ring enables the colors to remain vivid.
In the jewelry industry, the first experiences with this type of laser welding date back about 15 years. Today, there are more than 3000 pulsed Nd:YAG lasers used worldwide for this application.
One example of the impact of lasers on the precious metal chain industry is in the production of “rope chain,” which was originally created in Italy and now is manufactured widely there, in the Far East, and in the U.S. This style dates back many years; however the recent introduction of laser spot welding into the manufacturing process allows this chain to be manufactured in a simple and automatic fashion not possible or previously available to industrial manufacturers.
Another chain style deeply affected by the introduction of laser welding is pendant chain, widely worn in the U.S. as typically a 16-inch necklace made with 0.005-inch diameter wire. Figure 4 indicates the extremely small dimensions of the links. Handling these chains from the chain making machine to the brazing furnace typically generated losses of 30 percent of the links, even for experienced goldsmiths. Welding the links by pulsed Nd:YAG laser directly in the machine almost completely eliminates these losses.
FIGURE 4. The close up photo of a welded pendant chain link shows the small dimensions.
Pendant chain production processes traditionally employed a solder filled wire, with a low-temperature melting core. With laser spot welding of the links, chain manufacturers have been able to replace this solder filled wire with stronger alloys (usually containing traces of titanium) having higher melting points. And these newer alloys, permitted by use of laser welding, increased the brilliance of the wire used.
Multiple link chains, such as triple, quadruple, or quintuple curb or cable chain with complex link structure can be laser spot welded, link by link, directly in the machine therebyavoiding the springing open of links by thermal effect during the subsequent braze welding operation. The same concept is applied to multiple strand chains (“Bismark”) such as ball chain (see Figure 5), which is now commonly combined using laser spot welding technology.
FIGURE 5. Ball chain strands are now commonly combined using laser spot welding technology.
These are all examples of the integration of laser technology and traditional furnace technology, in order to speed up the chain manufacturing process as a whole, mainly by the drastic reduction in scrap chain coming off the machine. It is true that certain manufacturers claim to be able to use lasers not just for mere spot welding, but to take fully welded chain from the machine. However, even then, the chain is subsequently placed in a furnace, without powders, in order to obtain a complete re-crystallization of the artifact, thereby homogenizing its mechanical characteristics.
Laser marking and engraving
Q-switched, diode-pumped Nd:YAG lasers, with average power up to 60 W, allow single-pass high-speed engraving, with penetration depth of several thousandths of an inch, and lighter etchings on flat and curved surfaces within the depth of field of the focusing lens.The typical set up of the laser system used for marking and engraving in jewelry manufacturing applications is the combination of the laser source with an X/Y beam scanning head, where the galvanometer mirrors are driven by the applicable laser marking software.
Laser marking and engraving technology is most typically employed for creation of ornamental effects, particularly for the deep engraving of carat stamps, branding, and personalized family jewelry, most often on the internal diameter of rings. This is so because high-brightness Q-switched lasers, with TEM00 mode qualities, can have an average spot size of 30 micron or less, making them capable of executing extremely refined engraving jobs in small sizes (see Figure 6). Logos or marks can also be “ensconced” and raised into engraved-out areas encompassing the logo, with straight wall sides of one millimeter (see Figure 7). This allows the jewelry product to be personalized “discretely” or, if desired, to encode particular information geared at the prevention of counterfeiting.
FIGURE 6. Small spot size lasers enable family ring engraving/personalization in gold.
FIGURE 7. Hallmarking on the internal diameter of a ring allows it to be personalized and prevents counterfeiting.
Particularly desirable effects involving multiple shades of brilliance can also be obtained on precious metal surfaces, such as medals, pendants, earrings, and so on by using the laser to roughen the shine of specified areas on an otherwise finished piece, a process previously done by traditional sandblasting methods (see Figure 8). With correct file optimization and laser control, it is also possible to get contrast bitmap/photo marks on gold (see Figure 9).
FIGURE 8. Previously done using sandblasting methods, surface contrast decoration, on an earring for example, is now accomplished with laser.
FIGURE 9. With correct file optimization and laser control it is possible to get contrast bitmap/photo marks on gold.
While one might assume that industrial laser applications within the jewelry application are specific to that industry, the author suggests that the methods of using lasers discussed above should be noted by manufacturers of other precision products requiring spot welding and marking and engraving because these methods are most definitely transferable to other manufacturing industries.
Joshua Gold (firstname.lastname@example.org) is CEO of Laservall in Pawtucket, RI.