Welding TechniquesVarious joining techniques are theoretically

available to the platinum jeweller, including tradi-tional brazing with solder alloys, fusion welding,friction welding, arc welding and electron beamwelding (1). Until recently, only brazing and fusionwelding were accessible to the small- or medium-sized jeweller. In the past decade, laser weldingunits have become small enough and inexpensiveenough for application in the jewellery workshop(13). Even more recently, miniature electric resis-tance spot welding machines have been developedspecifically for the jewellery studio (4).

Traditional welding of platinum is carried out byinserting a thin over-sized sheet of identical metalbetween the pieces to be joined. Heat is applied tothe projecting sheet, fusing and sealing the joint (5).This contrasts with traditional brazing, using analloy of lower melting point, as is commonly thepractice in gold- or silver-smithing (1). Laser weld-ing uses high-intensity, focused laser light beams toapply energy to very small areas, resulting in veryrapid and efficient local melting (2). The positionof the weld spot is located using a stereomicro-scope with cross-hairs. Very little heat is generated,

and laser welds can be made on complicated parts,between dissimilar metals, and close to set stoneswithout damaging them (3). The new spot weldingunits use an electrode to create a high-intensityelectric spark, either to melt the parent metal or athin wire, to effect the weld (4). Both laser and spotwelding take advantage of the fact that platinumhas a low thermal diffusivity when compared withsilver or gold jewellery alloys (3). This means thatthe focused application of a small spot of intenseenergy can cause localised melting, without signifi-cant heating of the surrounding metal. This resultsin a comparatively small heat affected zone aroundthe weld, and the mechanical properties of the bulkof the workpiece remain unchanged.

Fusion welding, laser welding and spot weldingare now the three most commonly used joiningtechniques in platinum jewellery manufacture(13). We set out to compare these three tech-niques on three different platinum alloys, usingequipment actually in use in jewellery workshops.

Alloys and Sample PreparationThree different platinum alloys were tested: the

commonly-used platinum-5 wt.% ruthenium (Pt-

23

Welding of Platinum Jewellery AlloysCOMPARISON OF FUSION, LASER AND SPOT WELDING

By Duncan Miller, Katyusha Vuso, Penny Park-Ross and Candy Lang*Centre for Materials Engineering, University of Cape Town, Rondebosch 7701, South Africa; *E-mail: clang@ebe.uct.ac.za

The relatively recent application of laser welding and spot welding to platinum has supplementedthe traditional joining techniques of conventional welding and brazing with a gas torch. Severalrecent publications (13) have promoted the use of laser welding, because of the superiorstrength of the joins compared with that of joins by conventional welding and brazing. Inthis paper we compare welds in three different platinum alloys, produced with three differentwelding techniques: conventional welding, laser welding and spot welding. The welds joiningcold-rolled bars were performed by jewellers operating in their own workshops. The extentsof the heat affected zones and consequent decrease in hardness were assessed in our laboratory.Laser and spot welding produced very narrow heat affected zones, with correspondinglynarrow regions of diminished hardness, while the conventional welding resulted in samplesbeing annealed and softened for their full lengths. Complete joining was difficult to achieveby laser and particularly spot welding, which could be problematic in joining thicker sections.With this knowledge, jewellers can design appropriately to take advantage of the novel joiningtechniques.

Platinum Metals Rev., 2007, 51, (1), 2326

DOI: 10.1595/147106707X171954

5% Ru) and platinum-5 wt.% copper (Pt-5% Cu),and a novel heat-treatable platinum-3 wt.% vanadi-um (Pt-3% V) alloy (currently the subject of apatent application). Rectangular strips 1.2 mmthick were prepared by cold rolling. The Pt-5% Ruand Pt-5% Cu were hardened by cold work, andthe Pt-3% V specimens by heat treatment. Theresulting initial microhardness values were 256 HVfor Pt-5% Ru, 270 HV for Pt-5% Cu, and 457 HVfor Pt-3% V.

Joining was performed in all cases by abuttingflat surfaces which had been prepared by sawingthrough the bars transversely. Most of the sampleswere prepared without bevels or notches at the sur-faces to be joined. This was to test the penetrationof the laser and spot welding.

The fusion welding on all three sample materi-als was carried out by the conventional method ofinserting a thin sheet of the identical alloy betweenthe two parts to be joined, and heating the sheetlocally with an oxygen/propane flame until itfused. Laser welding was performed using a RofinStarWeld YAG model SWL-Y 65 laser weldingmachine, in air. Owing to limited availability of thePt-3% V material, spot welding was carried outonly on the Pt-5% Ru and Pt-5% Cu samples,using a Lampert PUK2 microwelder, with an argonstream to exclude oxygen.

ResultsThe welded specimens were sawn longitudinal-

ly with a jewellers saw, cold mounted in resin, andground and polished for microscopic examinationin reflected light. The fusion welds exhibited thebest penetration, whereas all the laser and spotwelds had median gaps, of various sizes up to halfthe specimen width, which were evident in thesawn sections (Figure 1).

Microhardness was measured with a Zwickmicrohardness tester and Vickers indenter. Figures2 and 3 show the effects of welding on the micro-hardness of Pt-5% Ru and Pt-5% Cu, respectively.These alloys showed similar patterns, with signifi-cant but narrow drops in microhardness in theweld zones for both laser and spot welding, andextensive softening due to recrystallisation alongthe length of the samples for fusion welding. All

three samples started out with very similar micro-hardnesses, so the overall softening due to fusionwelding was very obvious (Figures 2 and 3).

Etching showed that for Pt-5% Ru and Pt-5%Cu, both the laser and spot welds had local recrys-

Platinum Metals Rev., 2007, 51, (1) 24

Fig. 1 Polished section through the spot weld in Pt-5%Ru, showing incomplete joining

Fig. 2 Plot of microhardness (HV) against distancealong the length of welded bars of Pt-5% Ru, showingloss of hardness restricted to the weld zone of laser andspot welding, but an overall drop in microhardness alongthe entire length of the fusion weld specimen

Fig. 3 Plot of microhardness (HV) against distancealong the length of welded bars of Pt-5% Cu, showinglocalised loss of hardness in the weld zone in both laserand spot welding samples. The fusion weld sample hadthe same initial hardness as the other samples, andshowed reduced post-weld microhardness along its entirelength

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tallisation only in the immediate vicinity of thewelds. The cold worked microstructure, with flat-tened and elongated grains, was preserved withoutheat-affected alteration to within 2 to 3 mm of theweld line, whereas the heat affected zone of thefusion weld samples extended the full length of thespecimens. They were completely recrystallised.

Figure 4 shows the effect of laser and fusionwelding on the microhardness of Pt-3% V. Thelaser welded sample showed a steep but narrowdrop in microhardness across the weld zone. Afteretching, it was clear that local recrystallisation hadtaken place only in the immediate vicinity of theweld. The fusion welded Pt-3% V sample, whichstarted out with a microhardness of about 457 HV,showed a significantly decreased microhardness ofabout 200 HV across the specimen after welding,due to recrystallisation along the entire length ofthe sample, as evident from light microscopy(Figure 5).

Scanning electron microscopy with a Kevexenergy-dispersive X-ray analysis system on a LeicaS440 microscope was carried out principally toassess the effect of welding on alloy composition,which was determined at closely spaced pointsalong the lengths of the polished sections of select-ed welded specimens. Detectable loss of thealloying element was observed only in the case offusion welding of the Pt-3% V specimens. No lossof vanadium was detected in the weld zones of thelaser welded specimens, nor was alloying elementlost in the Pt-5% Ru and Pt-5% Cu specimensjoined by all three techniques.

DiscussionIt is clear from the results that traditional fusion

welding is capable of producing a good join in plat-inum jewellery alloys, but that the degree ofheating required causes extensive recrystallisation,which in turn reduces the overall hardness. Anyfusion welding towards the end of the manufactur-ing process will compromise the aim of theplatinum smith to increase the final hardness ofjewellery through work hardening or controlledheat treatment.

In our tests neither laser welding nor spot weld-ing produced good joins. All the welds wereincomplete, with internal voids of varying extent,and with welding only effective on the outer mar-gins. This is consistent with the results for buttwelding obtained by Volpe and Lanam in an exper-imental study comparing fusion welding withconventional brazing using solder (1). Their studyhad more success with laser welding bevelledjoints, and undoubtedly our welds would havebeen better if all the pieces to be joined had beenbevelled. The jeweller conducting the spot weldtests reported great difficulty in fusing and joiningthe flat ends of the platinum workpieces. The rec-ommended practice for microwelding is to melt athin wire of the same metal into the groove creat-ed by a bevelled joint (5).

We did not carry out any strength tests on ourexperimental joints. Laser welding has been report-ed to produce consistently stronger joints in bendtests than does brazing, especially when using awide 60 bevel joint in the laser welds (1). This is

Platinum Metals Rev., 2007, 51, (1) 25

Fig. 4 Plot of microhardness (HV) against distancealong the length of welded bars of Pt-3% V, showinglocalised softening associated with the weld zone in laserwelding. The fusion weld sample had the same initialhardness as the laser weld sample, and showed reducedpost-weld microhardness along its entire length

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easily done when joining simple components suchas bars or two sides of a ring shank, but requiressome ingenuity in making the bevel when joiningthicker sections to surfaces, for instance attaching aring shank to a bezel setting.

The results of the microhardness tests showedthat both laser welding and spot welding causedminimal recrystallisation, and that was restricted toa narrow zone in the immediate vicinity of theweld. This left the overall hardness of the work-piece unchanged. The implications formanufacture include retaining prior hardnessachieved through cold work or controlled heattreatment when welding in late-stage assembly orin repairing jewellery.

Fusion welding in air of the novel heat-treatablePt-3% V alloy resulted in an appreciable loss ofvanadium. Laser or spot welding would be the onlyeffective ways of joining such an alloy.

ConclusionsThis study has shown that laser and spot weld-

ing can be used to weld a variety of platinum alloys,producing a very narrow heat affected zone, andthus significantly limiting the extent of annealingand softening associated with the weld. This is instrong contrast to conventional fusion welding,where the heat generated tends to anneal large vol-umes of metal, if not the entire workpiece.

It is important to prepare the sections to bejoined by laser or spot welding appropriately. Ifpossible there should be a 60 notch or bevelbetween them, in order to get good penetration of

energy and to fill the gap with molten metal. Thismay require some ingenuity in design.

The primary advantage of using laser or spotwelding is the retention of desirable hardeningcaused by prior cold work or low temperature heattreatment.

AcknowledgementsWe thank Oro Africa and Sitali Jewellers in

Cape Town for performing the welds, and MirandaWaldron of the Electron Microscope Unit at theUniversity of Cape Town for assistance with thescanning electron microscopy. Chumani Mshumipolished the sectioned samples in the Centre forMaterials Engineering at the University of CapeTown, which provided laboratory and office facili-ties. This research was funded by a grant from theInnovation Fund, administered by the SouthAfrican National Research Foundation.

Platinum Metals Rev., 2007, 51, (1) 26

References1 C. Volpe and R. D. Lanam, Laser Welding or

Conventional Soldering, 1999 Platinum DaySymposium, Vol. 5, Platinum Guild InternationalUSA, Los Angeles, U.S.A., 1999; http://www.pgi-platinum-tech.com/pdf/V5N6.pdf

2 J. E. Gervais, Making Soldering a Technique of thePast, 1999 Platinum Day Symposium, Vol. 5,Platinum Guild International USA, Los Angeles,U.S.A., 1999; http://www.pgi-platinum-tech.com/pdf/V5N8.pdf

3 J. C. Wright, Platinum Metals Rev., 2002, 46, (2), 664 C. Patrich, Changing Ring Widths and Saving

Time, GZ Art + Design, 2005, 4, 345 Practical Platinumsmith, 4th Edn., Rhle-

Diebener-Verlag, Stuttgart, Germany, 2000, p. 21

The Authors

Duncan Miller is an HonoraryResearch Associate at theUniversity of Cape Town(UCT) and a member of theteam in the Centre forMaterials Engineeringstudying novel platinumjewellery alloys.

Katyusha Vuso is a recentgraduate from the Universityof Cape Town, who carriedout a research project onwelding of platinum jewelleryalloys.

Penny Park-Ross is aResearch Assistant in theCentre for MaterialsEngineering (UCT), and isresponsible for thelaboratory analysis of alloys.

Candy Lang is Professor inthe Department ofMechanical Engineering(UCT). She is leader of theteam developing novelplatinum alloys for thejewellery industry.