A heat treatable alloy experiences a significant change in physical properties because of a specific method of thermal processing during manufacturing procedures. Platinum alloys, generally classified as heat treatable, are based on varying additions of tungsten (W), gold (Au), gallium (Ga), indium (In) or copper (Cu) according to industry sources. The response to heat treatment usually involves an increase in hardness or resistance to indentation. This affords greater wear resistance in service and an increase in yield strength, which makes the material exhibit superior spring properties. Because international hallmarking standards for jewelry require a minimum 95% Platinum content, the manipulation of physical properties available through alloying additions or combinations of cold working are limited. The simultaneous control of chemistry and thermal processing affords a means of manipulating physical properties over a greater range.

Properties of Heat Treatable Platinum
A variety of physical properties have been evaluated and are worthy of summary with comparison throughout to industry standard 95% Platinum 5% Ruthenium or 95% Platinum 5% Cobalt alloys.

Alloying additions required to impart response to heat treatment reduce the melting range of 95% Pt materials substantially (145ºC). The melting range is broad at 100ºC compared to the narrow 10-20ºC typical of most platinum alloys.

This causes a large slushy range during solidification that impairs the ability to feed volumetric contraction inherent to solidification during such processes as investment casting. Compensation with larger feeding gates and sprues is normally recommended to overcome this inherent property.

As-cast Physical Properties:
The initial starting hardness of heat treatable platinum is higher than conventional materials. Both strength and hardness can be increased substantially after the completion of assembly, but before gem setting and final polish by completing the aging process. The aging process produces a slight oxidation or cloudiness on the alloy surface that does not have any specific color. No chemical treatments have been attempted to remove this oxide layer. It is easily removed through conventional polishing techniques.

Wrought Physical properties:
Heat treatable platinum has a significantly different response to cold working compared to conventional materials. Properties are also affected by thermal processing.

In general, strengths of heat treatable platinum greatly exceed what can be obtained in conventional materials. These enhanced properties can be obtained through cold working, heat treatment alone or a combination of both. The elevated levels of strength attainable when cold work and aging are used in tandem are impressive.

Manufacturing Issues
Cold Working:
Platinum heat treatable alloys work harden faster than their conventional counterparts. Whereas 95%Pt-5%Ru can withstand 90% reduction in thickness during cold rolling, a heat treatable material will only accept 40-50% reduction prior to the onset of unacceptable gross fracture of the billet. The material behaves similar to a yellow gold easy solder that readily experiences edge cracking during fabrication because it's ductility has been compromised in favor of a much lower melting point. Numerous intermediate anneals (6X as many as regular material) are required to reach the same thickness. Each annealing must be done at a comparatively high temperature (1000-1100°C) preferably providing time for an extended soak (10 minutes per kg) and immediate water quench to maximize ductility for further working. All of these conditions are difficult to achieve with conventional handling equipment and 6000g
( 200 t.oz) billets. These conditions form a limitation towards achieving mass production volumes and economies attainable with regular platinum alloys.

These same attributes make jewelers bench handling potentially more difficult with heat treatable platinum. Hand rolling or cold forming will be hindered by the materials high stiffness and yield strength. All frequent anneals must raise the metals temperature into the bright orange to bright yellow color range, followed by a rapid quench to facilitate further forming. Purchasing the material fabricated as close as possible to final size from a supply mill will greatly reduce the effort required.

Hardening by Heat Treatment:
The material responds readily to hardening procedures over a broad range of temperatures and conditions achievable by both bench torch methods or mass production atmosphere furnaces. Both hardness and yield strength increase about 70% from fully soft values in response to a correct aging treatment. For bench work, a piece must be heated to a medium orange color (700ºC) with a torch or furnace and simply allowed to cool in air until no color can be seen before quenching. The treatment can be applied to either soft or partially worked material to boost hardness and strength. It is good practice to repeat the procedure if a torch is used to ensure complete aging. A thin layer of protective flux will aid in minimizing surface oxidation or cloudiness that can occur.

The longer soak time, more thorough heating and protective atmosphere afforded by a belt furnace will also harden heat treatable platinum. Deploying conventional hydrogen nitrogen mixed atmospheres, fixturing and belt speeds should be adjusted to allow components to soak at 700°C for 20-30 minutes. The traditional cooling experienced traveling through the water jacket cooled section of a furnace is sufficient to promote hardening. Correctly designed heat treatable platinum alloys are very responsive to hardening procedures with minimal need for close control of conditions. The hardening procedure is fully reversible by simply heating the component to 1000ºC followed by a rapid water quench to restore softness for additional work.

Melting:
Previously alloyed stock can be readily melted using all of the materials and equipment inherent to platinum investment casting. Despite the lower melting range (1650-1550ºC) compared to conventional materials, the sluggish flow must be overcome using more superheat (200ºC) than is usually required. This ensures that high temperature fused quart/ crucibles and induction or oxy-hydrogen heat sources are required for small melts. The tendency towards oxidation of the alloying additives can be reduced by providing a protective cover gas of neutral argon. Avoid reducing conditions that promote the formation of brittle platinum phosphides and silicides. Primary melting and alloying practice must take care to preserve low melting point additions with protective cover gas, while casting a thin enough section to allow subsequent cold working. This is a major challenge that requires additional study.

Machining:
Preliminary machining studies indicate that heat treatable platinum exhibits substantially different behavior than conventional alloys. Equivalent speeds and feeds during lathe cutting produced large continuous swarf more indicative of machining gold than the small broken chips of platinum. Tool forces and wear appeared to be much lower despite using the same lubricants.

The machined surface of the heat treatable platinum is smeared with little or no tool marks, compared to the conventional material. The heat treatable platinum lathe chip also has noticeably different slip plate buildup density. The dislocated platelets of material sheared during machining are much more densely packed with the ruthenium based material. Corresponding hardness mapping of the machined face and chip cross section revealed hardness increased 15-20% from a nominal 230HV to 270HV with the heat treatable platinum. A much higher rate of work hardening in shear was noted with conventional platinum. Hardness increased from 170HV to 270HV (+50-60%). These results suggest that alloying additives in heat treatable platinum reduce the high rate of strain hardening during shear that contributes to conventional platinum alloys renowned poor machining performance. The possibility of manipulating the size and distribution of the second phase in heat treatable alloys to enhance machinability requires further research. The potential for a highly machinable soft alloy that can be subsequently hardened to the range of 350HV for superior consumer wear resistance to scratching and indentation is a very real possibility with platinum heat treatable alloys.

Applications
Investment Casting:
It is possible to investment cast a broad range of jewelry articles with heat treatable platinum.

Findings and Hardware:
Heat treatable platinum has significant potential where enhanced spring properties will increase holding power or the number of cycles during service. This includes bracelet closure clips, omega clips and butterfly earring clasps. The increased holding power of these items is substantial after correct heat treatment to increase the yield strength. Numerous other hardware applications involving the stamping of strip or the forming of wire exist. Wherever improved spring properties are required, heat treatable platinum may have an application.

Seamless Bands:
Lathe swarf consists of long thin ribbons more typical of gold machining. The potential for improved surface finish and enhanced tool life exists.
95% Platinum Braze or "solder" for Joining:

The depressed melting range of the heat treatable platinum alloy makes it an ideal candidate for joining operations where the 95% platinum content of an article cannot be compromised by conventional platinum "solders."

Conclusions & Areas for Further Study

•  Heat treatable platinum has a higher ascast hardness than conventional materials. This property can be      increased with heating to 700°C followed by slow air cooling. Yield strength also increases in roughly the same proportion.
•  Work hardening through cold working occurs at a much faster rate with heat treatable platinum alloys. They can achieve a higher hardness and yield strength than conventional alloys.
•  Correct aging heat treatments increase hardness and yield strength about 60% above the fully annealed or solution treated state. Final properties are double the strength of conventional materials.
•  A broad range of heat treatment conditions as simple as torch heating followed by air cooling will cause a significant increase in physical properties.
•  Manufacturing procedures require extensive high temperature anneals with a rapid quench to promote softening for further cold working. These conditions are difficult to achieve with billets in excess of 6000g. This limits the size of wire and strip coils.
•  Investment casting heat treatable platinum into a large variety of jewelry articles is possible.
•  Performance during machining operations requires more study. The possibility that tool life can be enhanced from inherently lower strain rates during machining with heat treatable platinum requires full exploration.
•  Improved methods of casting that can control solidification in large weight billets
( >3000g) with a thickness of less than 9.6mm require development. Such thin sections will enhance production capabilities for thin light weight strip sizes required for stamping.
•  Continued study of the microstructure, phase relationships and heat treatment parameters may improve the production methods for these difficult to handle alloys.

V5N7

Understanding Heat Treatable Platinum Alloys
Greg Normandeau and David Ueno
Imperial Smelting & Refining Co. of Canada Ltd.

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