The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

Transcription

1 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys Paolo Battaini Consulting Engineer 8853 SpA Milano, Italy Battaini Introduction Precious metal white alloys are playing an increasingly important role in the jewelry market and, in addition to white gold, platinum alloys have also gained interest for the consumer. In the case of platinum however, the price of the raw material limits its spread. The 950 palladium alloy seems to be a promising alternative to these materials due to a combination of two characteristic qualities: 1) its low specific weight and, 2) the lower metal price with respect to gold and platinum. To the consumer, palladium has a lot of platinum s characteristic qualities. For example, it is precious, white, sold in high-purity alloys (950 ), long-lasting and more. In the last two years, a remarkable increase in the use of palladium in jewelry production has been observed, due especially to the contribution of the Chinese market, and there has been a 55% global consumption increase in the period from 2004 to As a consequence, jewelry production has become the second main application of palladium worldwide. Palladium shows a low hardness in the raw-cast or soft-annealed condition. In fact, its Vickers hardness is equal to 39 HV 10/30. Furthermore, the yield strength is equal to 34.5 MPa and the rupture strength equal to 160 MPa. After work-hardening, the hardness can increase to about 100 HV, the yield strength to 205 MPa and the rupture strength to 325 MPa. These properties are similar to platinum. Metalworking operations are made easier if a material shows sufficiently high mechanical properties in the raw-cast condition and if its workhardening rate, the hardness and yield strength increase after working, is not too high. 2, 4 The properties of pure palladium in its as-cast condition are not ideal. In fact, its mechanical properties are not satisfactory in regards to the common use as a lost-wax casting material. Not even when using a work-hardened product does the palladium reach either a particularly high hardness or a satisfactory yield strength. Besides, it is practically impossible to obtain a finished piece of jewelry with the maximum work-hardening level. Hence, the final product can only be partially hardened. It should also be remembered that the higher the hardness and yield strength, September

2 the higher the resistance to surface abrasion. So it follows that palladium easily undergoes surface abrasion. Furthermore, in regards to tool wear, it shows the same working characteristics of platinum. In fact, its thermal conductivity is low (Table 1), 4 just like platinum. This means that the temperatures that could be reached at the contact points between palladium and mechanical tools could be higher than those reached while working with gold or silver alloys. As a consequence, microscopic fragments of the tool could become detached and solder to the palladium during working and vice-versa. Therefore, it is necessary to find palladium alloys with better mechanical properties and abrasion resistance consistently and with the 950 titer required in jewelry applications. This work aims to compare the mechanical workability of two distinct palladium alloys and to highlight their work-hardening properties and behavior after annealing. Table 1 Comparing the thermal conductivity of some pure elements. The characteristics of Pd are similar to those of Pt and completely different from those of Au and Ag. From Smithells 4. Pd Pt Au Ag Thermal conductivity at 20 C (Wm-1K-1) MATERIALS AND METHODS Since 1983, the use of high-palladium dental alloys (Pd > 750 ) has spread widely. These alloys contain elements like gallium, indium and copper in different ratios and show good mechanical properties. 5, 6 Moreover, some typical microstructural mechanisms are known to be responsible for their mechanical strength. 8 In this work, two Pd 950 alloys thought suitable for goldsmith work, and containing gallium, indium and copper in different amounts as well as other elements like ruthenium and aluminum, are compared. It is then useful to consider the binary phase diagrams Pd-Cu, Pd-Ga, Pd-In and Pd-Al (Figures 1, 2, 3 and 4) in this comparison. 20 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

3 Pd-Cu Both palladium and copper show a face-centered crystal lattice. They form a continuous series of solid solutions and, at low temperatures, ordered phases with compositions that are very close to Pd-Cu and Pd-Cu3. In regards to high-titer palladium alloys, an excellent mechanical workability along with a poor effect from copper on the hardness could be expected. As a matter of fact, the Vickers hardness of the binary alloy Pd950-Cu50 is equal to 50 HV 10/30. Battaini Figure 1 Binary Pd-Cu phase diagram (from Smithells 4 ) September

4 Pd-Ga Gallium has an orthorhombic crystal lattice. The phase diagram is rather complicated. The maximum gallium solubility in palladium 8 is about 18 atomic percent (at.%) at 1000 C (1832 F). Up to a weight percent concentration of 3.6, which is the highest attainable in the studied alloys, solid solutions without second phase precipitation are expected. Pd-In Figure 2 Binary Pd-Ga phase diagram (from Smithells 4 ) Indium has a tetragonal crystal lattice. The phase diagram is complicated and the highest solubility of indium into palladium 8 is about 20 atomic percent (at.%). In this case, solid solutions without second phase precipitation are to be expected as well at the indium concentrations established for the two prototype alloys. Figure 3 Binary Pd-In phase diagram (from Savitskii 8 ) 22 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

5 Pd-Al Aluminum has a cubic, face-centered crystal lattice. Even in this case, the phase diagram is complicated. The maximum aluminum solubility into palladium 8 is about 16 atomic percent (about 5% in weight) and it is reasonable to expect a decrease in the liquidus and solidus temperatures according to aluminum concentration in the alloy. Battaini Figure 4 Binary Pd-Al phase diagram (from Smithells 4 ) Taking into consideration the solubility of these elements into palladium and the information given by the phase diagrams, two prototype alloys, called Pd-Ga and Pd-Cu were prepared (their chemical compositions are given in Table 2). Other physical and mechanical data pertaining to the prototype alloys, as well as data belonging to comparison alloys, 9, 10 are reported in the same table. Note that the high hardness of the Pd-Ga alloy and its melting interval (Figure 5) are significantly lower than those of the Pd-Cu and the Pt alloys. September

6 Table 2 Chemical composition of the alloys studied in this work. The physical and chemical properties refer to the softened, annealed material. (*) Data related to other commercial alloys are reported for comparison. Battaini Alloys Pd Cu In Ga Al Ru Pt Other elem. Pd-Cu X - X - X Pd-Ga 950 X X 35.5 X - - X (*) Pd-Ru (*) Pt-Cu (*) Pt-Ru Specific Rupture Yield T solidus T liquidus weight HV strength strength ( C/ F) ( C/ F) (g/cm 3 ) 10/30 (MPa) (MPa) Pd-Cu 1470/ / Pd-Ga 1235/ / (*) Pd-Ru 1555/ / (*) Pt-Cu 1725/ / (*) Pt-Ru 1780/ / Figure 5 Melting interval of the Pd-Ga alloy measured using a Polymer Lab differential scanning calorimeter Different typical mechanical working operations were performed in order to evaluate the properties of the two prototype alloys. Square-section tubes were 24 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

7 obtained by drawing from a TIG-welded rolled strip. Further mechanical working operations were considered, such as wire production by rod milling and final drawing; production of wedding rings by blanking washers out of sheet and pressing into cones and rolling. 11, 12, 13 These operations cause the material to undergo stresses and deformations and, consequently, could reveal possible working problems. All the main working steps were studied by investigating the microstructural evolution of the material and the hardness changes. Microstructural investigations 14 were performed by a metallographic Leitz Metallux II optical microscope. Hardness measurements were performed with a Galileo Vickers durometer as well as a Shimadtzu Vickers microindenter. The behavior at laser welding and the brazing of the Pd-Ga alloy was also checked. Battaini METALWORKING Production of Pd-Ga Square-Section 2.7 x 2.7mm Tubes The process starts with casting an ingot with a thickness of 21mm. At this stage, the alloy microstructure is characterized by coarse, dendritic crystal grains as shown in Figure 6. The Vickers hardness is equal to 170 HV 10/30. The next step includes rolling with a reduction equal to 70% of the ingot thickness, subsequent annealing at 820 C (1508 F) for 20 minutes and further rolling to 1.25mm thickness. At this step, the Vickers hardness of the work-hardened material is 310 HV 10/30. Then an annealing treatment at 820 C (1508 F) is given to the sheet within a belt furnace for four minutes. After annealing, the alloy hardness is 190 HV 300, which is close to that of the as-cast condition, while the microstructure is typical of a re-crystallized material (Figure 7), with equiassic crystal grains having a mean size of 50 m. The coarse, dendritic grains (Figure 1) typical of as-cast material are no longer visible. Figure 6 As-cast Pd-Ga alloy microstructure representing coarse, dendritic grains September

8 Figure 7 Pd-Ga alloy rolled to a 1.25mm thickness and annealed at 820ºC. Grains are equiassic with an average size of 50 m. The dendritic grains of Figure 1 are no longer visible. The next step is further rolling to 0.85mm sheet thickness, which corresponds to a thickness reduction of 37%. The micro-hardness increases to 280 HV 300. A further annealing at 820 C (1508 F) within a belt furnace for four minutes causes a micro-hardness decrease to 180 HV 300. Microstructure is still characterized by equiassic grains (Figure 8) with a mean size that is slightly higher than in the previous step. Figure 8 Pd-Ga alloy after a 35% thickness reduction and further annealing. Grains are still equiassic with an average size of 80 m. The obtained strip is 0.85mm. Its microstructure is suitable for roll-forming and subsequent, longitudinal TIG welding. 26 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

9 Slit strips of 37.5mm width are then obtained from the sheet using a linear shear. The strips are then roll-formed and TIG welded in the standard continuous practice (Figure 9) to obtain a raw tube of about 12mm in diameter. Battaini Figure 9 Longitudinal TIG welding aimed at obtaining a raw tube from the strip. The TIG-welded joint is defect-free both in the welded material, characterized by columnar and dendritic crystal grains and, at the interface, with the base metal (Figures 10 and 11). It is worthwhile to note that the Vickers micro-hardness of the welded material is still equal to 170 HV 300 quite close to that of the as-cast alloy. On the contrary, the base metal and the heat-affected zone show microhardness values of 220 HV 300 and 200 HV 300 respectively. Hence, the alloys have undergone a slight work-hardening due to the folding process which gave rise to a micro-hardness increase by 40 HV 300 with respect to the unfolded slit strip. Figure 10 Pd-Ga alloy. General aspect of the TIG-welded joint. The welded region shows the typical columnar dendritic grains. Both the welded zone and the base metal are defect-free. September

10 Battaini Figure 11 Pd-Ga alloy. Detail of Figure 5. Notice: a) the dendritic microstructure with columnar grains in the welded metal, c) the heat-affected zone (HAZ) pointed out by the arrows within the dashed lines and, b) the base metal. No defects are observed. The raw tube is subsequently drawn by several passes on an internal mandrel until it has an outer diameter of 8.5mm. During drawing, the alloy tends to stick to the mandrel, therefore it is necessary to lubricate the mandrel surface properly. An 820 C (1508 F) annealing is then carried out within a belt furnace for three minutes. Figure 12 shows the transversal section of the tube after this annealing. The micro-hardness is equal to 170 HV300, both for the base and the welded metal, even though the welded metal is still easily visible in the micrograph. This micrograph is particularly useful to understand the substantial deformations the material undergoes during drawing. In this regard, compare the width of the raw welded region (Figure 10) with that of the welded zone in Figure 7. Drawing causes a lengthening of the material along the drawing direction and a correspondent circumferential reduction highlighted by the decrease in width of the welded region from 2.5mm (Figure 10) to about 0.8mm (Figure 12). By a careful examination of Figure 12, it turns out that the base metal is undergoing re-crystallization, leaving the previous dendritic structure only partially visible. 28 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

11 Figure 12 Pd-Ga alloy. Microstructure of the tube drawn to an outer diameter of 8.5mm and subsequently annealed. The welded region has undergone re-crystallization even though some marks of the columnar grain still remain. Compare with the width of the welded zone of Figure 10. The width reduction gives the idea of the deformations undergone by the cane during drawing. The following drawing operations are accompanied with intermediate annealing processes until the outer diameter is equal to 4.6mm. This is done by means of a mandrel with a circular section that is 4mm in diameter. Figures 13 and 14 show the work-hardened microstructure of the transversal and longitudinal tube section. Figure 13 Pd-Ga alloy. Transversal section of the tube drawn to an outer diameter of 4.6mm, under work-hardened conditions. Neither cracks nor material ruptures are present. September

12 Figure 14 Pd-Ga alloy. Longitudinal section of the tube drawn to an outer diameter of 4.6mm under work-hardened conditions. Neither cracks nor material ruptures are present. Compare with Figure 8, which relates to the same tube. The comparison highlights the different deformation undergone by the grains along the two directions. A lengthening of the crystal grains along the drawing direction is observed. The alloy micro-hardness is now equal to 305 HV 300. A subsequent annealing at 820 C (1508 F) within a belt furnace for six minutes changes the alloy microstructure as shown in Figure 15. Note that the grain size is smaller, within 50 m from the inner and outer tube surfaces. This is due to the fact that, in these regions, the plastic deformation and the consequent work-hardening of the material have been higher thanks to the contact with the drawing die and the mandrel surface. The micro-hardness of the material is equal to 190 HV 300. The welded region is barely recognizable and the microstructure can be considered homogeneous throughout the whole tube section. 30 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

13 Figure 15 Pd-Ga alloy. Longitudinal section of the tube drawn to an outer diameter of 4.6mm after annealing at 820 C (1508 F). Compare with Figure 9, relevant to a condition prior to annealing. A finer grain is observed both at the inner and outer cane surface. The tube could be drawn further, even to square sections. A proper choice of the reduction percentages and the annealing processes allows us to obtain a finished product with very interesting hardness properties in regards to applications. For example, Figure 16 shows a portion of the longitudinal section of a square tube with a 2.7mm side and a Vickers micro-hardness equal to 220 HV 300. Figure 16 Pd-Ga alloy. Microstructure of the longitudinal section of the tube finished at a square-section of 2.7 x 2.7mm. The alloy is partially work-hardened and its Vickers micro-hardness is equal to 220 HV 300. September

14 Production of Pd-Ga Wedding Rings Battaini This production starts with casting a 21mm thick ingot. After casting, the alloy microstructure is characterized by coarse, dendritic grains as shown in Figure 6. The next step is rolling to a 70% reduction of the ingot thickness followed by annealing at 680 C (1256 F) for 20 minutes and subsequent rolling to a thickness of 1.9mm. The final thickness depends on the desired wedding ring model. Under the above work-hardened condition, the Vickers hardness of the alloy is equal to 280 HV10/30. Washers with a 10mm inner diameter and a 19mm outer diameter are blanked out from the strip (the diameters will depend on the desired ring model size). The washers are then annealed at 820 C (1508 F) for 20 minutes and rapidly water-cooled. After this treatment, the alloy s hardness is equal to 180 HV10/30. The washers are then subjected to the usual four-phase procedure of bending, deep-drawing, pressing and sizing to obtain the ring blanks (Figures 17 and 18). The ring blank turns out work-hardened and has an average Vickers micro-hardness equal to 230 HV 300. The microstructure of the transversal section of the ring blank is visible in Figure 19. A fibrous microstructure is observed, especially at the edges. The work-hardening of the material is not homogeneous. Neither cracks nor exfoliation are present. Crystal grains are deformed by the mechanical working (Figure 20). The ring blanks are then annealed at 820 C (1508 F) for 20 minutes and regain a Vickers micro-hardness of 180 HV 300. Figure 17 Pd-Ga alloy. Bended (1) and deep-drawn (2) washers 32 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

15 Figure 18 The deep-drawn washers are pressed and sized (thickness is adjusted by a press) so that the raw blank shown in this figure is obtained. Figure 19 Pd-Ga alloy. Transversal section of the raw blank of Figure 18 under work-hardened conditions. The fibrous structure of the material is clearly visible, especially at the edges where the plastic deformation is higher. Neither cracks nor ruptures are present. September

16 Figure 20 Pd-Ga alloy. Detail of Figure 19. The plastic deformation of the alloy is clearly visible and appears as grain deformation. At this point, the ring blank microstructure has undergone a re-crystallization that has partially undone the effects of the previous work-hardening (compare Figures 21 and 22 with 19 and 20). Finally, the blank is rolled between a pair of rollers (Figure 23) in order to obtain the raw wedding ring with the desired size. After this treatment, the material shows a fibrous work-hardened microstructure (Figure 24). The alloy s Vickers micro-hardness is about 260 HV 300, with this value depending on the desired final diameter. Prior to final sizing, a partial annealing is performed to facilitate the operation and maintain a good final hardness of the finished product. After this annealing 700 C (1292 F) for 15 minutes, for example the final microstructure is obtained (Figure 25). The material still shows a fiber-like microstructure, however re-crystallization marks are very clear. After sizing, the wedding ring, ready for polishing and final finishing, has a micro-hardness of about 220 HV The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

17 Battaini Figure 21 Pd-Ga alloy. Transversal section of the blank after annealing at 800 C (1472 F) for 20 minutes. Re-crystallization is evident. Compare with Figure 19. The material is defect-free. Figure 22 Pd-Ga alloy. Detail of Figure 16. The effect of annealing on the grain morphology and size is clear. Compare with Figure 20, relevant to the condition prior to annealing. September

18 Battaini Figure 23 Rolling of the ring blank. The blank is rolled until the required diameter is attained. Figure 24 Pd-Ga alloy. Microstructure of the transversal section of the work-hardened, rolled ring blank. The fibrous structure is typical of the material work-hardened by rolling. Notable defects are absent. 36 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

19 Figure 25 Pd-Ga alloy. Microstructure of the transversal section of the rolled ring blank annealed at 700 C (1292 F) for 15 minutes. A fibrous microstructure is still visible, together with incoming re-crystallization. An incomplete re-crystallization is useful to carry out the mechanical working and preserve a high hardness. Compare with Figure 24, relevant to the material before annealing. Production of Pd-Ga Wires In this case, the procedure starts with casting a 20 x 20mm square-section bar. By alternating between rod milling and annealing at 820 C (1508 F), the bar is reduced to a 1.25 x 1.25mm square-section wire. After the last passage into the rod mill, the material becomes work-hardened (Vickers micro-hardness equal to 320 HV 300 ) and has a clear slip-band accumulation within the grains close to the wire edges (Figures 26 and 27). The subsequent annealing at 820 C (1508 F) for 40 minutes, followed by rapid water-cooling, causes re-crystallization (Figures 28 and 29) and a Vickers micro-hardness of 190 HV 300. The procedure continues with drawing the wire, using wire drawing dies with diamond nibs, to a diameter of 0.7mm. The hardness is 310 HV 300 and the microstructure is typical of a work-hardened material (Figures 30 and 31) with neither cracks nor exfoliation. A further annealing at 800 C (1472 F) for 30 minutes causes re-crystallization (Figures 32 and 33) as well as the recovery of workability and a decrease in micro-hardness, which is now equal to 180 HV 300. A partial annealing then produces a slightly work-hardened material as well as better mechanical properties. The average grain size is about 50 m, with larger grains towards the center of the wire section. The smaller grain size at the outer layer of the section (Figures 32 and 33) is due to the higher work-hardening level undergone by the material in this zone. September

20 Tension tests are performed on the wire and the rupture strength of the annealed material with an average grain size of 50 m is equal to 880 MPa, while the yield strength is equal to 650 MPa (see Table 2). Lubrication of the wire during drawing is very important. Figure 26 Pd-Ga alloy. Transversal section of the wire rod milled to a section of 1.25 x 1.25mm. Work-hardened material. The accumulation of plastic deformation is observed at the edges of the rolled product as well as, generally, along the section diagonals. Cracks are not observed, nor are ruptures. 38 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

21 Figure 27 Pd-Ga alloy. Longitudinal section of the wire rod milled to a section of 1.25 x 1.25mm. Work-hardened material. Grain deformation is manifest as well as twins along directions that intersect each other in an X-like pattern with respect to the rolling direction (horizontal in the image). Compare with Figure 26, relevant to the same wire and to the transversal direction. Cracks are not observed, nor are ruptures. Figure 28 Pd-Ga. Transversal section of the wire rod milled to a section of 1.25 x 1.25mm and annealed at 820 C (1508 F) for 40 minutes. Compare with the microstructure before annealing (Figure 26). Neither cracks nor ruptures are observed. September

22 Figure 29 Pd-Ga alloy. Longitudinal section of the wire rod milled to a section of 1.25 x 1.25mm. Annealed at 820 C (1508 F) for 40 minutes. Compare with Figure 27, that is prior to annealing. Neither cracks nor ruptures are observed. Figure 30 Pd-Ga alloy. Transversal section of the work-hardened wire drawn to a diameter of 0.7mm. Neither cracks nor ruptures are observed. Crystal grains are highly deformed and a high density of twins is visible. 40 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

23 Figure 31 Pd-Ga alloy. Longitudinal section of the work hardened wire drawn to a diameter of 0.7mm. Neither cracks nor ruptures are observed. Grains are elongated along the wire-drawing direction. Compare with Figure 30, relevant to the transversal section. Figure 32 Pd-Ga alloy. Transversal section of the wire annealed at 800 C (1472 F) for 30 minutes after wire drawing to a diameter of 0.7mm. Neither cracks nor ruptures are observed. The material has undergone re-crystallization, with smaller re-crystallized grains at the surface. September

24 Figure 33 Pd-Ga alloy. Longitudinal section of the wire annealed at 800 C (1472 F) for 30 minutes after wire drawing to a diameter of 0.7mm. Neither cracks nor ruptures are observed. The material has undergone re-crystallization, with smaller re-crystallized grains at the surface. Production of Pd-Cu 3.25 x 3.25mm Square Section Tubes This production starts with casting a 21mm thick ingot. After casting, the alloy microstructure is characterized by coarse, dendritic grains as shown in Figure 34. The Vickers hardness is 70 HV 10/30. The ingot is then rolled to a thickness of 2 mm, corresponding to a 90.5% reduction. After rolling, the Vickers hardness is equal to 170 HV 10/30 and the microstructure shows the typical fiber-like features due to grain lengthening along the rolling direction (Figure 35). The rolled product is given a 20-minute annealing at 680 C (1256 F) followed by rapid water-cooling. A Vickers hardness of 82 HV 10/30 is thereby obtained. The next step is rolling to a thickness of 0.55mm with a 72.5% reduction this obtains a Vickers hardness of 170 HV 10/30. The strip is then annealed at 680 C (1256 F) within a belt furnace for two minutes. The final Vickers micro-hardness is equal to 120 HV 300. (the alloy microstructure at this stage is shown in Figure 36). Recrystallization has produced an average grain size of 35 m. The strip is then rollformed, drawn and TIG welded in the standard continuous practice in order to obtain a raw tube with a diameter of 8mm. The welding is free from defects (Figure 37) and shows coarse crystal grains that are columnar in the region of contact with the base metal (Figure 38). In this case, the heat-affected zone is clearly visible in the base metal close to the welded region (HAZ in Figure 37). The heat-affected zone is visible due to the previous annealing at 680 C (1256 F), which did not allow a complete re-crystallization. The material close to the welded region re-crystallizes further due to the heat supplied by welding 42 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

25 Figure 34 Pd-Cu alloy in the as-cast condition. Microstructure is characterized by coarse dendritic grains. Figure 35 Pd-Cu alloy. Work-hardened, rolled strip. Typical fibrous microstructure with grains elongated along the rolling direction. September Battaini and develops grains with a size twice that of the grains of the base metal (compare Figure 36 to Figure 38). The Vickers micro-hardness of the welded metal is equal to 80 HV300, while that of the HAZ is 87 HV300. The HAZ width is about 0.5mm. The raw tube is drawn to 5.4mm diameter using a 5mm mandrel. A microstructure typical of the work-hardened material is observed (Figure 39). As a consequence of drawing, the HAZ width is reduced to about 0.3mm. Likewise, the width of the welded region is reduced also. In fact, the tube circumference has decreased, whereas its length has increased. The material s micro-hardness increases up to 170 HV200, both in the welded and in the base metal.

26 Figure 36 Pd-Cu alloy. Microstructure of the strip rolled to a thickness of 0.55mm and annealed at 680 C (1256 F). Neither cracks nor ruptures are observed. Annealing at 680 C (1256 F) leads to a partial re-crystallization that produces relatively small grains. In the heat-affected zone, re-crystallization will continue causing further grain growth (see Figure 37). Figure 37 Pd-Cu alloy. General aspect of the TIG-welded joint. Welding is accomplished without any deposit material and the welded region shows the typical columnar dendritic grains. No defects are seen in the welded region nor are they seen in the base metal. The heat-affected zone (HAZ) of the base metal is characterized by larger grains, due to the further re-crystallization created by the heat brought in during welding. 44 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

27 Battaini Figure 38 Pd-Cu alloy. Detail of Figure 37 at the interface between the base metal (on the right) and the welded metal (on the left). The welded metal is characterized by columnar dendritic grains. No defects are observed at the joint. Figure 39 Pd-Cu alloy. Microstructure of the transversal section of the tube drawn to an outer diameter of 5.4mm. The image shows the interface between the heat-affected zone (HAZ) and the base metal (on the right). September

28 The tube is now annealed within a belt furnace at 850 C (1562 F) for three minutes. The material re-crystallizes as shown in Figure 40. Re-crystallization takes place even in the welded material and the heat-affected region disappears. The material micro-hardness is now equal to 72 HV 200, both in the base and welded material. At this point, the tube can be drawn using square-section drawing dies of decreasing sizes. A square-section copper mandrel is then inserted into the tube. Figure 41 shows the typical microstructure of the cane finished at a 3.25 x 3.25mm section after extracting the copper mandrel. Under these conditions, the Vickers micro-hardness of the tube is 120 HV 200. The inner tube surface is rough due to the deformation of the copper mandrel. Figure 40 Pd-Cu alloy. Microstructure of the transversal section of the tube drawn to an outer diameter of 5.4mm and annealed at 850 C (1562 F). The image shows the interface between the heat-affected zone (on the left) and the welded metal (on the right). Re-crystallization is present in the welded metal as well. 46 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

29 Figure 41 Pd-Cu alloy. Transversal section of the tube, finished at a section of 3.25 x 3.25mm. Neither cracks nor ruptures are observed. The inner surface of the tube is not flat, due to the deformation undergone by the copper mandrel. However, this defect is hardly significant for this kind of tube. Laser Welding and Brazing of the Pd-Ga Alloy Laser welding of the Pd-Ga alloy was experimented with by using a pulsed, Nd:YAG laser. The results were encouraging even though the microstructural analyses showed that the process parameters must be carefully identified in order to avoid any defects in the welded joint. Figure 42 shows the longitudinal section of a welded wire. The alloy microstructure in the welded region is the typically composed of coarse, dendritic grains (see the detail in Figure 43). It can be observed that the microstructure of the wire, partially work-hardened by wire drawing, is not influenced by the welding process. In fact, the grains do not undergo re-crystallization, even close to the welded metal. The microstructure of the laser-welded material seems very similar to that of the TIG-welded metal (see Figure 10, for example). However, it is extremely unstable, probably due to the high solidification rate. In fact, a thermal treatment at 820 C (1508 F) for 20 minutes of the wire, welded under the conditions of Figure 42, causes material re-crystallization as shown in Figure 44. This behavior suggests annealing after laser welding, aimed at reducing the stresses within the welded region and stabilizing the microstructure. September

30 Figure 42 Pd-Ga alloy. Microstructure of the longitudinal section of a laser-welded wire. Notice the absence of the heat-affected zone in the base metal, where the grains are still elongated in the drawing direction. In the laser-melted region, the grains are typically dendritic and coarse. Figure 43 Pd-Ga alloy. Detail of the melted zone in a laser-welded joint. The melted metal is typically dendritic with coarse grains. 48 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

31 Figure 44 Pd-Ga alloy. Microstructure evolution of the laser-welded joint after annealing at 820 C (1508 F) for 20 minutes. The alloy with the microstructure, as in Figure 42, re-crystallizes and the previous melted zone is barely visible due to some elongated grains. As already highlighted, it is necessary to identify the proper process parameters in order to avoid any drawbacks in the laser welding of this alloy. The Pd-Ga alloy is susceptible to solidification cracking (hot cracking). Hot cracks 15 are solidification cracks that occur in the fusion zone near the end of solidification (Figure 45). They result from the inability of the semi-solid material to accommodate the thermal shrinkage strains associated with weld solidification and cooling. Cracks then form at susceptible sites (such as grain boundaries and inter-dendritic regions) to relieve the accumulating strain. Solidification cracking requires a sufficient amount of mechanical restraint. In a laser welding process, under rapid solidification and cooling, the rate of strain accumulation is rapid, leading to the increased possibility of cracking. The practical approach to minimizing the problem is to reduce the overall weld restraint through joint design (keep the joint gap to a minimum) and the appropriate choice of welding parameters (ramping down the power slowly may be useful). September

32 Figure 45 Pd-Ga alloy. Detail of the laser-melted alloy showing hot cracking (arrows). Hot cracking is due to excessive thermal shrinkage strains. To avoid this defect, a good joint design and an appropriate choice of the welding parameters are needed. Welding by using an oxygen/propane torch gave good results, too. By using an 18K white gold wire with nickel as filler metal, it was possible to carry out both joint brazing and welding. This white gold alloy has a liquidus temperature of 965 C (1769 F). By moderately heating the filler material and the joint, brazing is obtained (shown in Figure 46). In this case, the filler diffuses around the Pdalloy wire and no mixture of the two alloys occurs. If the heating supply is higher, the process is equivalent to welding, as shown in Figure 47. In this case, there is a wide region where both the deposit material and the base metal liquefy, giving rise to the mixture of the two alloys (melted zone in Figure 47). The heat-affected zone in the base metal is also observed where, in a part of it, the alloy temperature exceeds that of the solidus. 50 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

33 Figure 46 Pd-Ga alloy. Microstructure of a brazed joint obtained with an oxygen/propane torch and a wire of 18K white gold alloy containing nickel as filler metal. There is no mixture of the two alloys and the Pd-Ga alloy undergoes only a slight re-crystallization. Figure 47 Pd-Ga Alloy. Oxygen/propane welding of a wire. As in Figure 41, the same 18K white gold wire was used as filler metal. In this example, the temperature was higher than in the situation of Figure 46 and melting of the base metal occurred. The melted zone consists of a mixture of the filler metal and the base Pd-Ga alloy. The heat-affected zone (HAZ) portion of the base metal was heated above the solidus temperature. Discussion The different mechanical operations performed demonstrate that mechanical working, involving a considerable plastic deformation, can be carried out on both the Pd-Ga alloy with a higher hardness and the Pd-Cu alloy. These alloys show a workability behavior similar to that of the Pt-based alloys. In both cases, the TIG welding gave excellent results and the presence of a heat-affected zone does not damage the base material. After proper cycles of work-hardening and annealing, the welded region disappears and the material becomes homogeneous. The Pd-Ga alloy shows the best mechanical and hardness properties, September

34 reaching hardness values of 310 HV in the work hardened condition and 180 HV in the annealed one. By properly tuning the work hardening and annealing treatments, even the Pd-Cu alloy reaches hardness values that are quite high if the material is partially annealed (150 HV). The mechanical strength of drawn and annealed wires of Pd-Ga alloy are unquestionably interesting and better than those of some Pt alloys. This is of great interest in regards to chain production. V. Conclusion and Further Development The Pd alloys examined lend themselves to many uses and different mechanical working operations. This present work is a reference for setting up the working cycles. Laser welding tests on the Pd-Ga alloy have given good results, but more work is necessary to set up the procedure for different Pd alloys. Suitable brazing alloys are under consideration as well, even if tests with an 18K white alloy have given good results on the Pd-Ga alloy. Further studies will be necessary in order to produce Pd alloys for investment casting. References 1. Platinum Interim Review. November 2005, Johnson Matthey 2. Grimwade, Mark, Working, Annealing and deformation processes? Proceedings, Santa Fe Symposium on Jewelry Manufacturing Technology Wright, John C., Mechanical Properties and Jewelry Proceedings, Santa Fe Symposium on Jewelry Manufacturing Technology Smithells Metals Reference Book, edited by E.A. Brandes & G.B. Brook, 1992 Seventh Edition, Butterworth-Heinemann 5. Battaini, Paolo, Investment Casting for Oral Cavity Applications: Peculiarities and Problems La Metallurgia Italiana, Vol. 10, 2004, p Wataha, J.C., Alloys for Prosthodontic Restorations The Journal of Prosthetic Dentistry, 2002; 87, p Guo W.H., Brantley W.A., Clark W.A.T., Xiao J.Z., Papazoglou E. Transmission Electron Microscopic Studies of Deformed High-Palladium Dental Alloys Dental Materials 19, 2003, p Savitskii Ye, M. Handbook of Precious Metals, Hemisphere Publishing, Maerz, Jurgen J. Platinum Alloy Applications for Jewelry Proceedings, Santa Fe Symposium on Jewelry Manufacturing Technology Normandeau, G., Ueno, D. Understanding Heat-Treatable Platinum Alloys Proceedings, Santa Fe Symposium on Jewelry Manufacturing Technology Metalworking, Bulk Forming ASM Handbook, Volume 14A, ASM International 12. Dieter, G.E. Mechanical Metallurgy 1986 Third edition. McGraw-Hill 52 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys

35 13. Hosford, W.F., Caddell R.M. Metal Forming, Mechanics and Metallurgy 1983 Prentice-Hall International 14 Metallography and Microstructures ASM Handbook, Volume 9, 2004 ASM International 15. Welding, Brazing and Soldering. ASM Handbook, Volume 6, 1993 ASM International. Battaini September

36 54 The Working Properties for Jewelry Fabrication Using New Hard 950 Palladium Alloys