ABSTRACT
Platinum casters in the U.S. have traditionally used 950Pt-Ru or 900Pt-Ir as the alloy of choice. However due to the high melting ranges of these alloys there have been frequent requests to introduce a new alloy for the investment caster with a reduced melting range. Although popular in Europe, 950Pt¬Co has not been widely successful as an alternative in the U.S. due to magnetic properties and a tendency to oxidize when heated. With this in mind, an alternative has been devel¬oped based on the platinum-gold alloy system. This alloy has substan¬tially reduced solidus-liquidus tem¬peratures, is not magnetic, does not oxidize and has mechanical proper¬ties similar to 950Pt-Ru and 900Pt¬Ir. 

DEVELOPMENT STEWART GRICE
There are numerous platinum alloys available for the jewelry investment caster. Pure platinum is an extremely malleable metal howev¬er is too soft for general jewelry applications. To increase the hard¬ness to acceptable levels, a small per¬centage of another element(s) can be added. The appeal of platinum alloys has often been the high percentage by weight of precious metal; if they are to be marked "platinum", con¬tent must be a minimum of 95%. There are four major alloys used in the U.S. and Europe for investment casting:
For the U.S. market, the most popular alloys for investment casting are 950Pt-Ru and 900Pt-Ir, the remaining two alloys being most popular in Europe, in particular the 950Pt-Co. The U.S. preferred alloys have several customer-specified drawbacks:

  • Both alloys melt at a higher temperature than pure platinum. As a consequence, a substantial amount of superheat is required when casting to ensure complete fill and progressive solidification.
  • The high casting temperatures result in exacerbated crucible degradation. It is not unusual to use a crucible for five or six melts only, making this a major consumable cost.
  • 950Pt-Ru can suffer from problems with shrinkage porosity.
  • 900Pt-Ir is relatively soft
  • 950Pt-Ru has high surface tension and so requires more force to completely fill the mold cavity.
  • Both alloys, particularly 950Pt-Ru, can exhibit a rough¬cast surface that requires an intensive finishing process.
  • •On the positive side, the alloys do not oxidize and are not magnetic, allowing them to be easily separated in bench scrap. Looking at the European-preferred alloys, there are again plus and minus points to consider:
  • 950Pt-Co offers greater fluidity and form filling capabilities than the U.S. preferred alloys. Cast pieces contain less porosity although 950Pt-Cu-Co has inferior fill properties to 950Pt-Co, and tends to generate a small amount of porosity.
  • The melting ranges of 950Pt-Co and 950Pt-Cu-Co are lower than both 950Pt-Ru and 900Pt-Ir. This can extend crucible life.
  • The presence of cobalt has been known to cause a reaction with ceramic crucibles. This can reduce crucible life and lead to detrimental inclusions being carried through to the castings. This is particularly relevant if there are residual acid salts remaining on recycled scrap.
  • Both alloys will oxidize when heating operations are performed.
  • 950Pt-Co is magnetic. This results in many difficulties when separating bench filings.

Upon joining Hoover & Strong Inc. in November 2000, there was an immediate requirement to develop a new platinum investment casting alloy, based on customer request. The criteria for the new alloy were:

  1. It must have a substantially lower melting range than both 950Pt-Ru and 900Pt-Ir.
  2. It must not oxidize when heated to welding soldering or annealing temperatures.
  3. It must be non-magnetic so the problem with bench filings encountered when using 950Pt-Co would not occur.
  4. It must have superior recycling abilities when compared to 950Pt-Ru, 900Pt-Ir and 950Pt-Co.
  5. Superior flow characteristics were required for complete fills. There must also be less porosity, particularly in prong settings.
  6. There must be a color-matched solder available to give invisible joints when sizing without welding.
    Around this time, Christopher Cart joined Hoover & Strong Inc. and was tasked with setting up a new investment casting facility. This would include casting platinum alloys. Christopher's experience in Europe had given him certain requirements for a platinum alloy, which he will discuss in the applica¬tion section of this paper.

PLATORO™:  THE PERFECT MARRIAGE
The Platinum S+® range, avail¬able from Hoover & Strong Inc., offer a good alternative to the tradi¬tional alloys. They have greatly reduced melting ranges, are non¬magnetic, however oxidize on heating and so fall out of the customer requested criteria.

In order to develop an alloy with the required properties, a clean design sheet was required. Several possible binary alloy systems were investigated, the one showing most potential was platinum-gold. Gold can harden platinum significantly if present in sufficient quantities, or if introduced locally to the surface as an impurity, but if added in small percentages during alloying, it is quite acceptable. The alloy has an extended melting range and as such requires rapid cooling to prevent porosity formation. The gold addition was maximized whilst keeping the alloy single phase, and then a third element added as a "filler". This element must also retain a single-phase microstructure, and if possible decrease the liquidus and melting range; whilst the liq¬uidus of the 5% alloy is still high, it is lower than 950Pt-Ru and 900Pt-Ir. A third element was eventually identified that would fulfill these requirements and the alloy "PlatOro"™ was born. PlatOro™ has some unique properties:

  • It has a substantially reduced flow point of 2965°F when compared to 950Plat/Ru (3235°F) and 900Plat/Ir (3270°F).
  • The density of PlatOro™ is comparable to 950Pt-Ru and Pt-Co, and superior to 900Pt-Ir, resulting in lower product weight and cost.
  • It has high flow characteristics, resulting in maximized usable product and minimized non-fills per tree. The high flow characteristics also keep porosity to a minimum and result in a premium cast surface, which requires minimal finishing.
  • It has excellent recycling properties with increased tolerance to scrap usage.
  • The as-cast color of PlatOro™ is a bright white color.
  • It can be welded using both torch and laser.
  • It does not oxidize after investment casting, welding, soldering or annealing.
  • It has a hardness value superior to 900Plat/Ir.
  • It does not age harden or require any special heating and/or cooling processes during manufacture.
  • It is extremely ductile with maximum reductions possible between anneals.
  • It has excellent machining characteristics and gives a premium final polish for a superior finish.
  • It has good corrosion resistance, with no discernable effects after extensive in-house corrosion testing.
  • The wear characteristics of PlatOro™ are comparable with those of 950Plat/Ru.
  • The price of PlatOro™ is comparable with 900Pt-Ir.
  • It meets the requirements of the most popular European platinum hallmark, making it suitable for export. Further in-house tests were per¬formed to ensure that the alloy met all requirements and was suitable as a premium grade sale product.

PlatOro™ makes an excellent mill product and can be manufactured into both wire and sheet form. Offer¬ing high ductility and malleability with good wear resistance excellent machining and finishing properties.

For the final part of the study, a new solder was required to make invisible joints. The solidus of PlatOro™ shows that 1700 and 1600 grade platinum solders cannot be used. Traditional platinum solders up to 1500 grade may be used, but these solders do not perform well and generally result in visible joints. A 900 quality plumb platinum solder was developed specifically to comple¬ment this alloy. The solder has an excellent color match with compara¬ble finishing properties, making joints as invisible as they can be with a solder.

CASTING APPLICATION – CHRISTOPHER CART

Having cast platinum/cobalt for three years while in the UK, I had no desire to return to casting platinum/iridium or platinum/ruthenium in the U.S. However, as Stewart stated in the first half of this presentation, when we polled our customer base, platinum/cobalt met extreme resistance. Subsequently, we decided to develop PlatOro™ for the cast product at Hoover & Strong Inc.

At the outset, we forwarded 2000 dwt to Daniel Coghlan, North American Jewelers, Linus Drogs, and Au Enterprises for casting tests. North American cast the PlatOro™ on a Schultheiss PPC 2000 platinum casting machine and Au Enterprises cast on an Ecco high frequency induc¬tion centrifugal casting machine. This made certain that the metal would be cast in at least two different methods.

Initially, Au Enterprises and North American Jewelers were instructed to cast 100% fresh metal, reducing this by 25% increments in subsequent melts to 25/75. Following this, they were then instructed to cast 100% scrap and to recycle that metal by casting it three times with¬out any fresh metal additions. Essentially, they recycled the PlatOro™ at 300% scrap. North American Jewelers found no notice¬able detriment to the cast product after polishing, while Au Enterprises experienced some gas porosity by the second 100% scrap re-melt. This may have been due to the metal not being clean, as there is no con¬stituent that will readily absorb gas in this platinum alloy. All trees cast had duplicate waxes to ensure that the tests were replicated from one cast to the next.

Samples from each tree were then put through all phases of manufac¬ture (finishing, soldering, welding and stone setting) to review the working characteristics of this new platinum alloy.

The samples were further ana¬lyzed in micro-sections (up to X1200) and no evidence of gas porosity or oxides were present. Some shrinkage porosity was evident, however, this was minimal and more due to design and process.

The overall consensus was that we had a platinum alloy that was superior to the platinum/cobalt for casting. PlatOro™ did not oxidize, was not magnetic and cast at sub¬stantially lower temperatures than all common platinum casting alloys. The latter being one of the major advantages of PlatOro™ is that the melting crucibles last 300% longer due to its lower melting range.

Is this platinum alloy the panacea for platinum casting problems? No, you can still have gas and shrinkage porosity, pipe cavitation at the sprue and non-fills. These issues have to be dealt with in process as with all other platinum alloys. However, the posi¬tive points remain that PlatOro™ is a high purity, low melting platinum alloy that doesn't oxidize, it is not magnetic and is sufficiently hard enough for jewelry manufacturing.

 

V10N2

 

PlatOro™: The Perfect Marriage
by Stewart Grice, Mill Products Director and Christopher J. Cart, Casting Director
HOOVER & STRONG, INC.

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