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Development of New 950Pd Investment Casting Alloys with Superior Properties

State of the Art

Palladium alloys were a topic at the Santa Fe Symposium mainly during 2006 and 2009 when the Pd demand in the jewelry section increased rapidly. Several papers on the palladium market [1], the investment casting process [2-4] and on the fabrication and manufacturing [5, 6] were published. The first investment casting study on 950Pd alloys was presented by Fryé [4]. Due to the fact that limited information was available, the core purpose of this paper was to gain a better understanding of the casting characteristics of 950Pd alloys used in the jewelry industry. The work of Battaini [2] aimed at presenting the main physical and chemical properties of palladium-based dental alloys and transferring the experience acquired in the dental field to the goldsmith’s.

In 2007 a study of the investment casting of 950Pd alloys was conducted by fem on behalf of Palladium Alliance International (PAI). The results were presented at the Santa Fe Symposium in 2008 and published in 2009 [3]. In the following section the main findings of these studies will be summarized.

2.1 Typical casting challenges with 950 Palladium alloys

Commercial 950 palladium alloys contain Ru, Ga or Co as main alloying elements. Table 1 provides a list of alloys with their most important properties and characteristics.

Table 1:                Overview on commercial palladium alloys taken from manufacturer’s alloy datasheets. AC = as cast, CW = cold worked, AN = annealed.

Ruthenium (Ru) is a platinum group metal with a white color that alloys well to palladium. Ru containing alloys show a higher melting temperature, because Ru increases the solidus and liquidus temperature according to the binary phase diagram. Ru has a limited solubility in palladium and therefore only 950Pd-Ru alloys are manufactured. The increase of the melting range by the addition of Ru requires higher casting temperatures compared to pure palladium. This results in higher thermal stress to the crucibles and investment materials during casting. The melting range of 950PdRu is very small, only a few degrees Celsius. Some values given in manufacturer’s data sheets for 950PdRu are contradictory to phase diagram information. This is attributed to further alloying additions that are not specified or to the difficulty in the determination of the real melting range.

During solidification dendrites are formed and Ru segregates to the dendrite core and the remaining melt enriches in Pd. Usually the melting range increases due to segregation, but this is very limited for 950PdRu. As a consequence the alloy shows a nearly isothermal freezing and therefore a very limited form filling during casting. This process was investigated in detail for 950PtRu [15].

Ru dissolves in palladium to form a solid solution. Pd-Ru alloys are relatively soft, because of the small difference in the atomic size of Pd and Ru. The typical hardness is about 100-120 HV1 in the annealed or as-cast condition. In order to improve the mechanical properties additions of Gallium (Ga) are frequently used.

The binary phase diagram of Pd with Ga is shown in Figure 1. Ga has a very low melting point (29°C) and its addition significantly lowers the solidus and liquidus temperature of Pd. The maximum solubility in Pd is ca. 8 mass% Ga. At higher Ga concentration many intermetallic compounds form in complex phase reactions. No systematic studies are reported on the precipitation hardening of Pd-rich alloys in the open literature. However, such studies are available for Pt alloys [16] and the results can be transferred to Pd.

Precipitation hardening is well-known and applied for 950Pt alloys [16, 17]. However, the solubility of Ga in Pd is higher than in Pt. Therefore, higher amounts of Ga are required in order to achieve the same hardness level – or for a given Pd content, e.g. 950Pd, the achievable hardness is lower. The hardening response of Ga alloyed Pt is reported to be unstable and therefore classified as non-viable for reliable hardening by some authors [16].

Figure 1:              Conventional approach of hard palladium alloys with higher Ga content. Section of the Pd-Ga system (left) compared to the Pt-Ga system (right) calculated using ThermoCalc and TCNOBL1 database.

Experimental trials and corresponding investigations in a previous study at fem [3] focused on two alloys, one with Ru/Ga and the other with Ag/Ga/Cu. Hence no general conclusion can be made about the suitability of alloys for palladium casting depending on alloy composition. On the basis of defect analysis on industrial castings, it seems that alloys having a relatively high Gallium content tend to have a higher susceptibility to formation of cracks in as-cast parts. Crack formation turned out to be a complex issue. In depth failure analysis revealed, that the underlying mechanism is related to particular casting conditions and properties of investment material. It should be noted that crack-free castings of the Ru/Ga alloy have been obtained in reproducible way during casting trials at fem and are also obtained in high quality and reproducible way by several industrial casters which cooperated in that project.

Silicon is a typical impurity that occurs in investment casting processes. If scrap material is used for remelting the removal of any investment residues is of utmost importance [2]. Such oxide residues might decompose during melting, especially under reducing conditions (forming gas: Ar/H 2 or N 2/H 2) that must be avoided. The released oxygen gets into solid solution inside the melt and evaporates during solidification to form significant gas porosity. Si forms a deep melting eutectic (Pd + Pd 3Si) at a temperature of 782°C. Such a low melting eutectic at the grain boundaries is responsible for hot tearing. An example of the catastrophic result of silicon impurities is shown in Figure 2 . The casting tree is completely embrittled. Many cracks in the parts cause multiple fractures that occur along the interdendritic grain boundaries. Position 4 in the lower right image shows increased Si concentration determined by EDX analysis.

Figure 2:              Hot cracking due to contamination with investment residues.

3 Development process

3.1 Identification of suitable alloying elements

Potential alloying elements have been selected from the periodic table. Some elements have to be excluded, because they are volatile, toxic, allergenic or radioactive, too reactive under the typical conditions of investment casting or insoluble. The main requirements to the new alloy were:

Sufficient melting range of min 25K

Medium Hardness (130-160HV1)

Fine grain structure

The alloy 950PtRu was defined as benchmark for the development of new 950Pd alloys. 950PdRu has a promising silver-grey color compared to the grey color of most 950Pd alloys. It contains 100% platinum group metals and does therefore not require protective gas during processing. However, the fluidity of the alloy is very low and some manufacturers do not recommend it for casting.

Only few elements remain as candidates. In order to overcome the poor casting properties of 950PdRu the following improvements are required:

Widening of the melting range  Addition of Co, Fe or Cu

Improvement of casting properties, especially form filling            Addition of Co

Optimization of the segregation, reduction of investment reactions        Addition of Sn

Improvement of color and hardness       Addition of Cr, Fe, B

Grain refining    Addition of Fe, W, Zr

Figure 3 shows the changing melting range of different 950Pd alloys were Ru is replaced by a third element (Me). On the left side of the figure we find the binary 950PdRu alloy and on the right the binary 950PdMe alloys. Some elements such as Au have hardly any effect on the melting range and the liquidus temperature. Other elements (Ag, Cu,Cr) have a medium effect on melting range and liquidus temperature. In case of Cu relatively high amounts are required to achieve an effect. The strongest effects are shown by the addition of Co and Fe. However, because of their tendency to oxidation the amount should be limited to maximum 2%.

Figure 3:              Effect of alloying additions to 950PdRu. Ru is replaced by a metallic third element (Me). The x-axis gives the amount of the third element in mass-percent. Calculated using ThermoCalc and SNOB3 database.

The effect of segregation during the solidification process can be simulated by so-called Scheil-Gulliver simulations. The effective melting range of an alloy usually increases, because the complete thermal equilibrium that is assumed in the equilibrium phase diagrams is not achieved during a relatively fast cooling process. This results in a continuous change of the chemical composition of the liquid phase as the solidification proceeds and this effect is what can be studied by the Scheil-Gulliver simulations . The effect of such non-equilibrium composition changes of the melt on the solidus temperature is shown in Figure 4 for a series of 950Pd-30Ru-Co,Fe alloys. The binary 950Pd-Ru shows a very narrow solidification range. The addition of 20 ‰ of Fe+Co reduces the solidus temperature and the segregation processes become more pronounced. The segregation of Fe and Co to the liquid phase results in a reduction of the effective solidus temperature and allows a melting interval of about 30-100 K. This appears promising in terms of improved form filling, better feeding (reduction of micro shrinkage) and reduced investment reactions.

Figure 4:              Scheil-Gulliver calculation. Segregation of 950Pd-30Ru alloys with varying Fe and Co content.

3.2 Investment casting trials

Based on the above described consideration a series of alloy compositions was derived as shown in Table 2. 950PdRu served as benchmark alloy and was purchased from C. Hafner, Pforzheim, Germany. The alloys were prepared by arc melting from pure elements with a purity of 99,9% or better (purchased from HMW Hauner Metallische Werkstoffe, Germany). The button shaped sample was cold rolled to sheet that was used for the centrifugal investment casting of typical trees according to Figure 5 with a mass of approx. 100 g. On these trees a basic characterization was made that comprised the determination of color, metal release, hardness, age hardening response and microstructure. The tree contained a series of typical jewelry parts that are prone to typical casting defects. The grid was used for the testing of form filling and the plate sample for color measurements and metal release tests. Ca. 35 alloy compositions were prepared and investigated. Table 2 provides a selection of these compositions. The most promising alloys were selected and modified in the following step.

Table 2:                Alloy compositions tested in small scale trials (selection)

1.	Three-ball ring
2.	Single gate ring 
3.	Light signet ring
4.	Plate
5.	Grid
6.	Solitaire ring
7.	Double gate ring
8.	Heavy signet ring

Figure 5:              Casting tree setup and cast parts

Casting required a sophisticated process control in order to guarantee reproducible and reliable casting conditions (Figure 7). The casting machine was the model TCE10 from Topcast, Italy that allowed melting and casting within 40-60 s from the beginning of the heating process. A high quality quartz based crucible of type „KGZ“ from Porzellanfabrik Hermsdorf, Germany was used for all casting trials. This type of crucible was proofed as suitable for platinum alloys in a previous study. The metal temperature was controlled during melting and casting with a thermal imaging camera. This allowed a detailed evaluation of the metal temperature that is superior to the pyrometer integrated into the casting machine. Even the flask temperature could be controlled by thermocouples mounted onto the tree or close to the interior flask surface to document investment overheating. However, such measurements require very high effort and where therefore used only in a very limited amount of casting trials.

The flask temperature was selected depending on the size and shape of the parts and was 650°C in most casting trials. This temperature showed the best compromise of high form filling and low shrinkage porosity. In order to reduce investment reactions as far as possible a two-part phosphate bonded investment powder was used (Ransom&Randolph Platinum). After casting, the parts underwent nondestructive testing by computer tomography and by conventional metallography.

Figure 6:              Casting machine and process control (description see text)

Optimized form filling requires a suitable tree-setup. Based on the experience of previous casting projects with platinum, the parts were mounted on the leading side relative to the spinning direction of the casting machine. Figure 7 illustrates the casting setup, the acting forces and an example of the simulation of the form filling process. Due to the mounting of the parts on the leading side the metal is forced to flow to the tip of the tree. The parts are then filled gradually from the tip towards the ingate of the tree. Details on the investment casting process and the casting simulation can be found in [18, 19].

Figure 7:              Casting conditions in centrifugal casting. Orientation of the parts on the leading side of the tree. relative to the acting forces. Blue arrows indicate the rotation direction. Arrows indicate the acting forces: orange (inertia), red (gravity) green (resulting force). Simulation of the form filling process.

After casting the tree was cut and documented as shown in Figure 8. The surface quality was assessed using the appearance of the plate sample. The grid sample provided information about the form filling, which was given in percent of the filled node points of the grid. Metallographic inspection was made on the single and double gate rings that are prone to shrinkage porosity. The metallographic section of the plate sample was used for color measurement before and after a metal release test in artificial saliva. The results were categorized into three categories that are given in Figure 9 for form filling, investment reaction and porosity. The microstructure and grain size were determined using scanning electron microscopy. Possible defects such as cracks, chemical inhomogeneity or inclusions were investigated ( Figure 10).

Figure 8:              Casting results and evaluation routine (description see text)


Figure 9:              Evaluation criteria for form filling, surface quality and porosity

Figure 10:            Microstructure in the as-cast condition of selected alloys

Table 3:                Main results of selected alloys in full-size casting trials

Table 3 provides the results of some selected alloy compositions. 950PdRu shows good basic properties with medium grain size and only very few intergranular cracks. The addition of Co reduces grain size and helps to completely avoid cracking. Fe additions are promising in reducing grain size, but cause issues with heavy gas porosity. Despite that fact Fe as kept as potential alloying element. Cu brought no real advantages and reduced hardness resulting in distorted rings already during devesting. This led to the exclusion of Cu. Adding Cr resulted in too strong reactions with the investment and heavy cracking hence Cr was excluded from the study. The addition of Sn significantly changed the grain morphology, which was rated as disadvantageous. However, the form filling was very good and the overall performance was good which kept Sn on the list of promising alloying elements.

Finally, the alloys marked in green were studied further while those marked in red were excluded from the study. Based on this evaluation the most promising alloys were selected and optimized in further steps by adding additional alloying elements. All alloys showed low hardness at this stage of development, therefore the focus of further improvement was on increasing the hardness.

In order to improve hardness the literature provided boron (B) and aluminum (Al) as promising alloying elements [20, 21]. However, both elements are not easy to add due to their high reactivity. This required the preparation of pre-alloys with carefully adjusted amounts of Al and B. Using such pre-alloys enabled the caster to prepare alloys that contain Al and B without the risks of oxidation during initial melting of the alloy. It further enabled the manufacturer to control the amount of alloying addition very precisely. Figure 11 provides results at different levels of B and Al. Small additions of 1‰ B (alloy PD1502) increase strength and hardness significantly while maintaining ductility. Additions of Al have a similar effect, but much higher amounts of Al are required. An approximately linear increase of strength and hardness was found.

These results were determined by tensile testing of rods that were cast into copper molds. Such casting results in fast cooling and might not be representative for investment casting. Therefore, further tests were done by investment casting. Strength levels and hardness are maintained by investment casting, but ductility is significantly lower. This is typical and is usually an effect of the coarser and columnar grain structure after investment casting.

Figure 11:            Mechanical properties of selected alloys based on 950PdRu with additions of B and Al

Further optimizations in additional casting trials resulted in the compositions provided in Table 4. The boron content was reduced for safety reasons, because higher level of B might cause hot cracking under inappropriate cooling conditions. A combination of Al and B provided the best and reliable properties in repeated casting trials. The combination of metals was more effective than single additions of even higher amounts. Alloy variations using Fe and Sn provide the benefit of grain refinement and improved form filling, respectively.

Table 4:                Compositions of optimized alloys meeting the hardness requirements

A comparison with other high caratage white alloys (Figure 12) shows some specific benefits of the newly developed 950Pd alloys. Compared to 950PdRu the hardness is significantly increased to levels of 140-160 HV1, which is considered optimum. Higher hardness might be beneficial for improved scratch resistance but compromises the formability of the material during stone setting. The alloys show some age hardening response that might be used, if higher hardness is required. The comparison with state of the art 950 platinum alloys (green columns) shows superior properties over 950PtRu, but lower hardness than 950PtRuGa, which is sometimes considered as too hard. The hardness is also comparable to 18k Pd white gold alloys that contain zinc for improved hardness (yellow column).

Further properties that should be considered are color and density of the alloys. For white alloys the yellowness index (YI D1925) is the accepted standard for color assessment [22]. YI values below 18 are considered as “premium white”, which means that the alloys do not require rhodium plating. 950PdRu based alloys show YI values of below 10, which is comparable to 950Pt alloys. The color difference between 950PdRu and 950PtRu is hardly visible by a human observer. In contrast, premium 18k Pd white gold alloys with a YI value around 18 appear much more yellow. The density of 950PdRu based alloys is close to 12 g/cm³, which is 60% of 950Pt alloys and 75% of 18k white gold alloys. The lower density enables the manufacturer to produce bulkier jewelry at the same weight or light jewelry, e.g. earrings or pendants. The combination of the properties labels the newly developed 950PdRu alloys as “light, bright and strong”.

Figure 12:            Comparison with commercial alloys (typical properties according to [23])

4   Summary and Conclusions

The present paper describes the development of 950Pd alloys with improved properties for jewelry applications. Promising alloy compositions were selected based on thermodynamic calculations, which were then melted and cast by centrifugal investment casting. Typical crucibles and investment materials that proofed to be suitable for platinum alloys were used. These materials were found to be suitable for 950Pd alloys as well. The melt was overheated by ca. 80°C prior to casting. The flask temperature was 650°C in most casting trials. Alloys containing boron can be sensitive to flask quenching. Therefore, the flasks should be cooled slowly to room temperature before devesting.

The new alloys are based on 950PdRu and contain additional alloying elements to widen the melting range (Co, Sn, B), to reduce grain size (Fe) and to increase hardness (Al, B). Typical liquidus temperature of such alloys was 1560-1570°C, which is slightly lower than 950Pd50Ru. The addition of the above mentioned alloying elements increases the hardness from ca. 100HV1 of the soft binary alloy 950PdRu to 140-160 HV1. This hardness range is assumed to be the ideal hardness for stone setting and finishing that provides sufficient scratch resistance during jewelry wearing. Such hardness is also reached for medium to hard 950Pt alloys of Ni-free 18k white gold alloys. The color of 950Pd alloys is comparable to 950Pt. Both groups of alloys show a yellowness index of about 1, which is significantly whiter than Premium white gold alloys (YI <18). The density of 950Pd alloys is ca. 40%and 25% lower compared to 950Pt and 18k Pd white gold, respectively. The low density is an advantage for light or bulky jewelry items.

The binary alloy 950PdRu suffers from the low hardness. The new 950Pd alloys show superior hardness while maintaining good form filling ability, low crucible and investment reactions and sufficient resistance to hot cracking wit suitable process conditions. They might therefore enable 950Pd alloys to become an option for jewelry applications.

5   Acknowledgements

Norilsk Nickel, Russia, is acknowledged for financial support of this work. Linus Drogs (AuEnterprises, USA) is acknowledged for supporting this project and consulting during its realization. The colleagues at fem are kindly acknowledged for their contribution to SEM investigation, metallography and chemical analysis.

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