Over the past several years much has been written about the processes of fusion welding, laser weld¬ing and brazing for bonding platinum to itself and other metals. In this paper I would like to talk about a different method for bonding platinum: diffusion weld¬ing.

Diffusion Welding
The Merriam-Webster Dictionary defines diffusion as: "the process whereby particles of liquids, gases, or solids intermingle as the result of their spontaneous movement caused by thermal agitation and in dis¬solved substances move from a region of higher to one of lower concentration." In diffusion welding two or more surfaces are joined by the diffusion process.

The quality of this bond depends on a number of vari¬ables including how good a solvent one metal is for another, how intimate the contact between surfaces is, and how clean/free of oxides the surfaces are. With care and attention to detail, it is possible to make very strong bonds between most of the metals used in jew¬elry work by diffusion.

Mokume Gane: an Example of Diffusion Welding
The diffusion welding process is central to the work that we do in my studio. All of our work is made of patterned laminated metal. This technique dates back to the 17th century when the Japanese swordsmiths developed a means of laminating and patterning non¬ferrous metals that were similar to the patterns they achieved in forge welded iron and steels. They called this technique of patterning, mokume gane, which translates to wood eye metal in English.

In the style of mokume gane, sheets of metal are thoroughly cleaned with detergent and abrasives and then rinsed in water to remove oxides and other foreign elements. These cleaned sheets are clamped between a pair of thick metal plates and bolt¬ed tightly together. This provides for intimate contact between the layers during lamination. This stack is then placed in an oxygen free environment, a kiln. In the kiln, the stack is raised to a temperature that is between 50 and 80 percent of the solidus of the metal alloys that are being laminated and allowed to soak at this temperature while the diffusion process is occurring. After soaking long enough for significant diffusion to occur, the stack is removed from the kiln and allowed to cool. Once cool, the laminated stack is removed from the laminating fixture, and

it is then processed by one or more standard metalworking techniques to develop patterns in the laminate.

We work with a variety of alloys in both precious and base metals that we have learned to laminate. 

For this article I decided to try something different from our typical laminates. For several years I have thought it would be interesting to laminate platinum to iron, because of the color and textural differences that can be achieved between the two. I thought it would be a beautiful laminate. This seemed to be a good chance to experiment with it. 

Diffusion Welding Platinum to Iron
The steps listed below for this laminate are basically the same as would be used for any of the typical jewel¬ry metals and alloys. The main difference is that the time and temperature must be adjusted for whatever combination is being bonded.

An early decision was to use very low carbon steel for the laminate since modern steels have a number of alloying elements in them. For the platinum portion of the laminate, the 950¬platinum 50-palladium alloy was chosen. If there is too great a difference in hardness exhibited during the rolling of a laminated stack, the softer layer will end up extruding out of the laminate and result in a much thinner layer of the soft material in the billet.

The Process
The initial stack consisted of 8 layers of platinum and 7 layers of iron. The metal sheet must be free from surface defects, dings, heavy scratches or other gross imperfections. The edges of the sheet need to be de-burred so that the sheets will stack evenly. 

Cleaning
Thorough cleaning of the surfaces to be bonded is extremely important. The sheets are cleansed by wet scrubbing with ScotchBrite abrasive pads in a series of three baths. After the final bath, each sheet is dried with a cool, high velocity, low-pressure air stream and set aside.

Once all the sheets are cleaned and dried, the sheets are stacked in a clamping fixture also known as a torque plate. A hydraulic press is used to compress the fixture and stack. The bolts are then torqued down to hold the stack in com¬pression so that the layers remain in intimate contact during the diffusion process.

The torque plate assembly is now ready for firing but protection from the highly oxidizing environment in the kiln must be provided. We make a protective enve¬lope from a stainless steel foil. The foil, because of its low thermal mass, will heat up very rapidly and capture much of the oxygen inside the bag on its surface. It will also act as an effective barrier that will keep the oxygen in the air in the kiln from coming inside and making contact with the stack. Filling the bag with granular charcoal will provide a reducing atmosphere in the bag.

Firing temperatures need to be between 50% and 80% of the solidus of the metals being laminated. Firing times will depend on the size and thermal characteris¬tics of the bag, torque plate stack, and the kiln but are typically in the 1hour to 8hour range.

Once the bag is removed from the kiln and cooled to room temperature, the diffusion-welded billet can be removed from the torque plates and other operations can begin. In this case the laminate was reduced to a 10 mm x 10 mm square section bar and then rolled and drawn to a 6 mm diameter rod before the pattern¬ing process was begun.
After patterning was completed, a ring blank was cut and soldered and then an inner lining of 950 Pt Ru was installed. After cleanup the ring was etched and polished. The light grey of the platinum against the blue grey of the iron exhibits a beautiful, subtle contrast.
 
Other Applications of Diffusion Welding
Mokume gane is not the only way to utilize the diffu¬sion welding process in studio jewelry manufacturing. By developing the proper fixtures, it is possible to use diffusion welding to assemble jewelry without solder.

CONCLUSION
Diffusion welding is certainly not the answer for all platinum bonding situations. However, it does allow for the production of precisely aligned multi-part assem¬blies without the use of solder and its associated clean up. It also allows for the production of bonds that can be cold worked in ways that cannot be duplicated with solder. I believe that it has a place in the process repertoire of the studio jeweler.

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Diffusion Bonding of Platinum
James Binnion
BINNION METAL ARTS, LLC.

This is an abbreviated version of the original work. For full technical details, please consult the original paper.