1. High Temperature Alloys

For this part of the work a range of casting patterns was selected that would enable an assessment to be made of the most important properties. For example signet rings weighing 40 grams were cast to assess metal-mould reaction, as this was considered to be most likely to occur with a combination of high temperature and large mass of molten metal. In contrast settings for gem stones, weighing as little as 0.5 gram, were cast to determine alloy fluidity. In addition, conventional tensile test pieces were cast and the mechanical properties of each alloy determined. Other properties evaluated were ability to reproduce detail, colour and hardness.

The alloy compositions chosen for evaluation included several of United Kingdom Hallmarking quality in addition to other casting alloys at present used in the United States of America (4.5 per cent ruthenium-platinum and 10.0 iridium-platinum) and in Japan (15.0 per cent palladium-platinum). All are listed in Table I, together with their mechanical properties.

As an oxidation resistant casting alloy would have clear advantages the first investigations were carried out on alloys of platinum with additions of only other platinum group metals. Some of these alloys demonstrated casting characteristics superior to those of 4.5 per cent copper-platinum, as shown in Table II. In particular, all castings produced in 1 per cent iridium-3.5 per cent palladium-platinum were free from the effects of metal-mould reaction and had smooth, bright and sound surfaces. The absence of an interfering oxide film was demonstrated by its ability to fill very fine patterns and to reproduce detail.

The quality of castings produced in the two binary iridium-platinum alloys was only marginally inferior. Some very slight surface erosion was observed in the large patterns through metal-mould reaction and in the case of the 10 per cent iridium-platinum alloy, a proportion of the fine sectioned patterns were difficult to fill, both of these effects probably being a function of the higher casting temperature required. Castings in 4.5 per cent palladium-platinum were also superior to those in 4.5 per cent copper-platinum.

All these alloys have limited use, however, in casting application, their poor mechanical properties as shown in Table I resulting in the items being easily distorted during the casting process and subsequent handling.

The surface appearance of the 15 per cent palladium-platinum buttons suggested that considerable gas was absorbed during melting and subsequently evolved during solidification. Large and medium sectioned patterns were consequently seriously affected by porosity and extensive surface roughness demonstrated metal-mould incompatibility.

Although the mechanical properties of 4.5 per cent ruthenium-platinum were of the order required for general purpose applications, most of the castings produced suffered from an intolerable degree of surface roughness and fine sectioned patterns were difficult to fill. Surface erosion was most extensive in the signet ring, an example of which is shown in Figure 3. Microprobe examination of the affected region revealed no chemical reaction and therefore the breakdown must have been purely mechanical.

As an extension of the all noble metal approach an alloy composed of 1 per cent rhodium-3.5 per cent gold-platinum was assessed. Both rhodium and gold have a hardening effect on platinum when present in binary alloys but the poor surface condition of the castings made this alloy unattractive.

Having explored thoroughly alloys containing only noble metals, base metal additions were next considered. Nickel, when added to platinum, imparts a significant increase in hardness and so in an attempt to retain the advantages gained in casting quality by the inclusion of palladium, two alloys, one containing 1 per cent nickel-3.5 per cent palladium-platinum and the other 2 per cent nickel-2.5 per cent palladium-platinum were evaluated. The product quality in 1 per cent nickel-3.5 per cent palladium-platinum was comparable to that of 1 per cent iridium-3.5 per cent palladium-platinum and there was no evidence of surface oxidation. The increase of strength and hardness was not, however, sufficient for the application. Conversely the mechanical properties of 2 per cent nickel-2.5 per cent palladium-platinum are ideal but the casting quality was no better than 4.5 per cent copper-platinum, and the colour of the castings was characterised by a dull grey appearance resulting from base metal oxidation. In the large sectioned pattern the surface topography was very irregular due to the metal-mould reaction while porosity was rather more prevalent than in other alloys.

These results suggested that a binary alloy where the addition element hardens platinum to a lesser degree than nickel and which is oxidation resistant, might fulfil our requirements. It seemed that cobalt could be such an additive, and subsequent trials revealed that castings in 4.5 per cent cobalt-platinum were comparable in quality to both 1 per cent iridium-3.5 per cent palladium-platinum and 1 per cent nickel-3.5 per cent palladium-platinum, while the mechanical properties were in the preferred range for an alloy to be used in general purpose applications.

2. Lower Melting Point Alloys

As previously stated the platinum alloys at present used for investment casting have melting ranges between 1725 and 1780°C, and therefore require a casting temperature of at least 2050°C to ensure satisfactory filling of fine sections. Even when the highest quality investment materials are used their properties are only just adequate for casting a few patterns at a time. To make the casting of platinum a less critical and more economic process, an improved investment material, with better high temperature properties, is therefore required. Alternatively or in addition to this, the development of a platinum alloy requiring a lower casting temperature would extend crucible life and permit more castings per button.

The possibility of developing platinum casting alloys with a significantly reduced melting range has now been explored, the two major objectives being to satisfy the 1973 Hallmarking Act and to obtain the properties required by the manufacturing jeweller.

Initially the effect of additions such as silicon and boron were evaluated. Molten platinum readily reacts with these elements to form low melting point eutectics, and therefore stringent precautions are generally taken to prevent them coming into contact. The eutectic silicon-platinum alloy, at 4.2 per cent silicon, melts at 830°C and could consequently be air/fuel gas torch melted and cast in standard low temperature investments without undue difficulty; similarly the 2.1 per cent boron-platinum alloy with a melting point of 789°C could also be easily cast. Predictably the hardness of these alloys, 440 H_v and 327 H_v, respectively, coupled with their extreme brittleness rendered them unsuitable for subsequent fabrication. Figure 4 shows the dark coloured, hard, interdendritic constituents that occurred in the microstructure of an as-cast eutectic alloy.

Attempts to modify the structure by ternary and quarternary additions while retaining the reduced melting range, did not reduce the overall hardness of these materials below about 300 H_v and the castings were therefore unacceptable.

The relevant phase diagrams were examined for an element that, on alloying, could substantially reduce the melting point of platinum without exceeding the 5 per cent addition limit. Gallium appeared to do this, although not to the same extent as silicon and boron, and it offered the possibility of forming alloys having mechanical properties within the range that might be acceptable to manufacturing jewellers. The relevant part of the phase diagram is shown in Figure 5. A series of exploratory binary alloys covering the range 2 to 5 per cent gallium-platinum were therefore evaluated, the melting range and as-cast hardness values being given in Table III. Increasing the gallium content resulted in lower melting temperatures, but also increased the hardness values to such an extent that alloys containing 3.5 per cent or more gallium were unusable.

Some further optimisation of the casting and mechanical properties was required and a series of ternary alloys based on the gallium-platinum system was evaluated; several combinations appeared to show a reasonable melting range while retaining good ductility, the relevant figures being given in Table IV. Small scale centrifugal casting trials using a fine claw setting dress ring pattern provided a critical test of fluidity and general casting characteristics, and revealed the good casting qualities of the solid solution gold-gallium-platinum alloys. Larger scale trials using production equipment determined the minimum casting temperature required to fill a standard production flask consisting of a button of six dress rings. In these the 3 percent gold-2 per cent gallium-platinum, the structure of which is shown in Figure 6, could be successfully cast at a lower temperature than alloys containing 2.5 or 3 per cent gallium despite having the higher melting range. The results are given in Table V, together with comparable data for 4.5 per cent copper-platinum. This suggested that the gallium-containing alloys were susceptible to the absorption of oxygen, with a consequent reduction in fluidity. Further trials, in which the melt was protected by a reducing atmosphere, showed a significant improvement in the fluidity of these alloys thus enabling the casting temperature required to fill the six-ring button to be reduced to about 1900°C. Gas analysis of the castings produced showed a significant decrease in the oxygen content when a reducing atmosphere of 10 per cent hydrogen/nitrogen had been used to protect the melt; the results are given inTable VI.

As an alternative approach the effects of adding a deoxidant to the alloy at the make-up stage or to the melt prior to casting, were evaluated. A small addition of yttrium incorporated in the alloy was found to be very effective in increasing alloy fluidity without promoting metal-mould reaction, and any casting scrap could be re-cast providing 25 per cent virgin alloy, containing the deoxidiser, was added to each charge. While a small gain in alloy fluidity could be obtained when a deoxidant such as calcium boride was added to the melt prior to casting, the associated metal-mould interaction was considerable and made the castings unacceptable.

To summarise, 3 per cent gold–2 per cent gallium-platinum can offer an advantage in casting temperature of between 90 and 150°C, depending on casting conditions, over the existing casting alloy, see Table VII.

Conclusions

The work reviewed in this paper has shown that the high temperature alloy 4.5 per cent cobalt-platinum is superior inall aspects to the existing 4.5 per cent copper-platinum alloy for investment casting. The versatility of this alloy forcasting the whole range of patterns from fine filigree to heavy section items has been clearly demonstrated. This together with its amenability to all the usual fabricating and finishing operations applied to such castings gives the jeweller the opportunity to improve both the quality of his product and the efficiency of his manufacturing process.

A significant decrease in casting temperature has been achieved with the 3 per cent gold-2 per cent gallium-platinum alloy, although it has not been possible to produce a platinum alloy that can be cast at temperatures comparable with those used for gold alloys. The advantage of this lower casting temperature is likely to assume even greater importance if the anticipated improvement in investment durability is achieved.