2010 the Role of Process Parameters in Platinum Casting

By Dr. Ulrich E. Klotz & Tiziana Drago, Research Institute Precious Metals & Metals Chemistry (FEM)

The Role of Process Parameters In Platinum Casting

©2011 The Bell Group, Inc. All rights reserved.

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The Role of Process Parametersin Platinum Casting

Dr.UlrichE.KlotzTizianaDrago

ResearchInstitutePreciousMetals&MetalsChemistry(FEM)SchwäbischGmünd,Germany

1. IntroductionIn recent years several articles on casting properties of platinum have been published.1, 2Differentaspectssuchassuitablealloysforcasting,3-6 tree design7-10

and investment reactions11,12 have been treated. Articles from South African authors describe the effect of centrifugal casting parameters for different alloys and investments.12-14 95Pt5Co was identified as a very versatile casting alloy showing excellent form filling of filigree parts even for flask temperatures as low as100°C(212°F).95Pt5Ru,ontheotherhand,showedpoorformfillingoffiligreeparts for flask temperaturesbelow800°C (1472°F).12 Besides casting properties, functional alloy properties such as color, hardness, ductility and magnetic properties have to be taken into account for jewelry purposes. In this regard 95Pt5Ru is more versatile compared to 95Pt5Co or 95Pt5Cu and can be used for all jewelry purposes. 95Pt5Ru also offers higher hardness and finer grain size compared to 95Pt5Cu, which results in easier polishing and higher scratch resistance. In the present project the focus has been on 95Pt5Ru and 95Pt5Co as the most common alloys for jewelry purposes.

Casting is a process with many variables that can’t be controlled at will, and therefore has a somewhat chaotic nature.15 This requires many casting trials and a statistical analysis of the results obtained. It also appears very difficult to make simple recommendations about a specific set of working parameters.

The findings on platinum investment casting described in this paper are the result of a research project commissioned by the PlatinumGuild International,USA(PGI) in cooperationwith several industrialpartners. In the following sectionsthe properties of platinum alloys will be described as a basis for discussion of the observed casting behavior. Then experimental details will be described, followed by the casting results obtained with centrifugal and tilting casting. The paper will close with a summary of results and an outlook recommending topics for further research on platinum jewelry alloys.

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2. Properties of Platinum Alloys

2.1 Phase DiagramsPhasediagramsdescribethestabilityofthedifferentphases(forinstance,liquidand solid phase) as a function of temperature and composition. Phase diagrams of the Pt-Ru and Pt-Co systems are given in Reference 16. From the phase diagram the basic alloy properties given in Table 1 can be determined. However, the phase diagram describes the conditions in thermal equilibrium, which are most often not reached in technical processes such as investment casting. In ordertodescriberealcoolingconditions,theScheil-Gullivermethodwasapplied.Duringsolidification,segregationtakesplacewherecertainelementsareenrichedto melt and solid phases, respectively. In the case of Pt alloys, Ru and Co are segregating to the solid phase and melt, respectively. Comparable data and the amount of segregation are described in Reference 17. Such segregation, especially of impurities such as Si, strongly affects the behavior of the melting (meltingrange!)and investment reactions.Themelting rangeunderpractical conditionsincreasesremarkablybyafactorof2(Pt-Ru)or4(Pt-Co)asgiveninTable1(i.e.,the solidus temperature under real casting conditions is considerably lower than the value given in the phase diagram).

Table 1 Basic alloy properties of 95Pt5Ru and 95Pt5Co

95Pt5Ru 95Pt5Co

Alloy composition [mass%] 950Pt - 50Ru 950Pt - 50Co

Liquidustemperature[°C/°F] 1815/3299 1672/3042

Solidustemperature[°C/°F] 1797/3267 1654/3009

Meltingrange[°C/°F] 18/64 18/64

Meltingrange(Scheil)[°C/°F] 39/102 78/172

During melting and casting in silica-containing crucibles and investment, contamination of the melt with Si can occur. This can heavily affect the melting range of an alloy. Silicon is known to form a deep melting eutectic with platinum at830°C(1526°F)and4.2masspercent.16 The effect of Si content on the melting range of silicon-contaminated Pt-Ru and Pt-Co alloys was assessed using thermodynamic calculations with the ThermoCalc® software package and a database dedicated to precious metals (SNOB1). Results of Scheil-Gulliver calculations for two different silicon contents in the melt, 0.05 mass% Si and 0.2 mass% Si, are presented in Figure 1 and Figure 2. Even traces of silicon (0.05mass%) lower the solidus temperature by about 50°C (90°F) compared to thebinaryalloys.Higheramounts(0.2mass%)resultinareductionof150°C(270°F)and250°C(450°F)forPt-CoandPt-Ru,respectively.Theextensionofthemeltingrange is caused by the strong segregation of Si to the melt by a factor of about 20.

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Figure 1 Scheil calculation with ThermoCalc® software; influence of silicon contamination on solidus temperature

Figure 2 Scheil calculation with ThermoCalc® software; segregation of silicon to the melt


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2.2 Thermophysical Properties

The thermophysical properties of platinum and its alloys are the key for understanding the challenge in casting compared to other precious metals. Some important data, namely density, viscosity, surface tension and thermal conductivity, were compared to other precious metals. As far as available, data were taken from the Degussa Precious Metals Handbook.18 It remains mandatory to determine further data for jewelry alloys in order to obtain better understanding of casting properties and to allow casting simulation in the future.

The normalized density for gold and platinum and some of their alloys is plotted in Figure 3. The pure metals show a very large density reduction during freezing, resulting in high sensitivity to shrinkage porosity. For gold alloys this density reduction is much lower than for the pure metal, while platinum alloys show shrinkage comparable to pure Pt. Furthermore, the slope of the density-temperaturecurve isa factorof twohighercompared togold (i.e.,overheatingrequired during melting further increases the proneness to shrinkage porosity of platinum alloys).

Figure 3 Density of precious metals and their alloys in the liquid and solid state

Platinum alloys have a high viscosity compared to gold or silver (Figure 4a).Alloying with Co and Cu reduces viscosity, but alloying contents typical for jewelryalloysaretoolow,allowingvaluescomparabletogoldalloys(Figure4b).Surface tension of platinum is about a factor of 1.5 higher compared to gold.18 Thermal conductivity of Pt is about one-third of Au and a factor of six lower than Ag. These three properties–surface tension, viscosity and thermal conductivity–are important factors influencing the filling of filigree items during casting. High surface tension and viscosity make it more difficult for the melt to flowsmoothlyintosmallcavitiesoftheflask.Lowthermalconductivityresults

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in inhomogeneous temperature of the melt and premature freezing of filigree parts, especially if the temperature difference of melt and flask is high as in the case of platinum. In practice, centrifugal casting is used to apply extra force and toenhance formfilling.Experimentswithcentrifugalandstatic (tilting)casting machines were made during the project to highlight the role of casting conditions.

a)

b)

Figure 4 Viscosity of precious metal melts and alloys


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3. Experimental Setup

3.1 Casting Machine and Machine ParametersMost of the casting experiments were made using a TopCast TCE10 casting machinewithinductionheating(Figure5).Formeltingitwasoperatedwithfullpower of 10kW. Metal temperature during heating and melting was registered bya computer-controlledquotientpyrometer (Maurer,modelQKTR1085)with100Hz acquisition rate. A typical heating curve is shown in Figure 6. At the melting point the heating curve reaches a plateau until the complete amount of alloy is liquid. Alloy weight used in the casting trials was 100 – 180g. Complete meltingwasobservedbythecasterandthemeltwasthenoverheatedfor5(±1)seconds before casting. During this time temperature increases linearly withtime. As the temperature increases very quickly, precise control of overheating is important. From the slope of the time-temperature curve the variation of casting temperaturecanbeestimatedtobe±40°Kduringtheone-secondreactiontimeofthe caster. Complete heating time until casting was only 30-40 seconds, depending on amount and type of alloy. Cooling time was measured by pointing the pyrometer on the metal button in the flask. In vacuum, cooling time of the melt button is by a factor of 2 longer compared with gas atmosphere (air/argon),explaining the occurrence of gas porosity in vacuum casting.

Figure 5 (a) Topcast TCE10 centrifugal casting machine with pyrometer and temperature data acquisition system; (b) detail of centrifugal arm,

heating coil and orientation of tree in casting machine

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Figure 6 Metal temperature during heating for two casting experiments. Drop of melt temperature in case of vacuum casting is due to manual operation.

The TopCast TCE10 machine allows rotation speeds up to 450rpm with adjustable accelerations up to 1000rpm/second (rpm/s). Before starting actualcasting experiments with platinum alloys, experiments with fine gold were made in order to determine optimum parameters to avoid material losses by crucible leftovers. A minimum speed is required to force the material to climb up the steep crucible wall and to leave the crucible through the nozzle. Results for different machine parameters are given in Figure 7. The crucible could be emptied with different centrifugal combinations. However, acceleration and speed have to meet a certain ratio; otherwise, the material might spill over the crucible wall. For the present machine two combinations, namely 300rpm–300rpm/s and 440rpm– 600rpm/s, were chosen.


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Figure 7 Effect of centrifugal speed and acceleration for TopCast TCE10 casting machine; results for fine gold (density 19.3 g/cm³)

Selected experiments were made using an Indutherm MC15 tilting casting machine. The machine contains a vacuum chamber with integrated induction- heated crucible (power 3.5kW) and flask holder. Crucible and flaskare oriented to each other under 90° angle.Melting is done with the crucibleinverticalposition.For casting, thecompletevacuumchamber is titledby90°.Form filling is assisted by argon over pressure immediately after casting. Metal temperature is controlled by internal thermocouples.

3.2 Tree DesignThe tree design was selected with the following considerations: main sprue and button should be lightweight in order to save material; heavy and lightweight filigree pieces should be included to study reactions with investment and form filling, respectively; standard samples such as ball rings were put on every ring in identical positions for comparison of different castings. Several tree designs were testedduring theproject.Most treesusedaDiabolo-type setup.10 Typical trees containingsomestandarditems(ballringsandgrid)togetherwithjewelrypiecesare shown in Figure 8. For special purposes other tree designs were used and are mentioned later.

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Figure 8 Example of two typical tree designs; tree #3AG (left) and tree #4AG (right) with Diabolo setup.

Duringcentrifugalcastingthreeforcesactonthematerial.First,inertiaofthemeltacts during the acceleration of the flask and results in increased form filling of the parts on the trailing side of the flask. Second, centrifugal force is constant for all positions on the tree depending on distance from the center. Third, gravity assists filling of bottom parts of the flask. Therefore, tree design plays an important role for optimization of casting results. All items were characterized by their position on the tree relative to the centrifugal direction as given in Figure 8. Duringplacementof the flask in the castingmachine, the flask is tiltedupsidedown.Therefore,position90°isonthetopoftheflaskduringcasting.Bestform filling is expected for positions 0° and 90° because of the combined action of inertia and gravity.

3.3 InvestmentFour different investments from different suppliers were tested during the project. Table 2 gives an overview on the properties of the investment and briefly describes the experience in working with them.


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Table 2 Investm

ent properties

Investment

Type

Base and

linerM

ixing tim

e [min]

Working

time [m

in]B

urnout tim

e [h]

Burnout tem

p. [°C/°F]

Rem

arks

No. 1

3-partPaper base and

liner30

10-1212.0

871/1600

Curing d

oes not start at room tem

perature; extend

ed w

orking time possible

No. 2

3-partPaper base and

liner20-25

10-1212.0

870/1598

Curing d

oes not start at room tem

perature; extend

ed w

orking time possible

No. 3

2-part/fiber

Rubber base

85-7

11.5900/

1652C

old w

ater required to reach upper w

orking tim

e limit; high viscosity

No. 4

2-part/fiber

Rubber base

85-7

10.0900/

1652C

old w

ater required to reach upper w

orking tim

e limit; high viscosity, risk of bend

ing of filigree parts, especially plastic parts

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Investments No. 1 and No. 2 are three-part investments. They do not cure at room temperature and therefore have sufficient working time. However, they require a paper base and liner to absorb excessive water during burnout. The paper base requires wax sealing and careful handling of the flask. Prior to casting, ash residues have to be removed from the hot flask.

Investments No. 3 and No. 4 are two-part and quickly cure at room temperature during the investing process. No. 4 is fiber reinforced; therefore, working time can be very short and viscosity increases during working. In some cases this caused filigree parts to bend during the investing process or caused gas bubbles to stick on the surface of the wax parts. Mixing with cold water allowed slight extension of working time. These two investments can be handled with rubber bases and cure sufficiently before actual burnout.

The burnout cycles of the investments are compared in Figure 9. No. 1 and No. 2 require several steps before reaching final burnout temperature. A first step ataround100°C (212°F) is required toevaporate thewater fromthe flask;waxmeltingtakesplaceataround200°C(392°F);andfinalburnoutandcuringoftheinvestmentduringheatingtoandholdingataround900°C(1652°F).No.4andNo.3becomesolidalreadyatroomtemperature.Therefore,onlyonestepat200°Ctomeltthewaxisrequiredbeforereachingtheburnouttemperatureof900°C.Allinvestmentsrequireburnouttimesof10-12hours(i.e.,overnightburnout).

Figure 9 Investment burnout cycles according to manufacturer’s recommendation


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4. Casting Trials with Centrifugal Casting Machine

4.1 Experimental ConditionsA large number of casting experiments was carried out to analyze the influence ofcastingparameters(melttemperature,flasktemperature,castingatmosphere,casting machine type, alloy, and investment material) by using standard sample geometries such as ball rings and grids. The ball ring represents heavy section pieces. With the large ball acting as hotspot, it will provoke investment reactions and is prone to shrinkage porosity. The grid represents filigree items and will show form-filling ability under certain process parameters. The as-cast samples were evaluated in terms of surface quality and by metallographic investigation. A complete list of all casting trials is given in Table 3.

Table 3 List of centrifugal casting experiments with Topcast TCE10 machine

Trial

no.Alloy Tree Investment

Casting

temp.

[°C]

Flask

temp.

[°C]

AtmosphereSpeed

[rpm]

Acceleration

[rpm/s]

GPt001 95Pt5Ru 4A No. 1 850 Air 300 300

GPt002 95Pt5Ru 1A No. 1 850 Air 300 300

GPt003 95Pt5Ru 2A No. 1 850 Air 300 300

GPt004 95Pt5Ru 3A No. 1 850 Air 300 300

GPt005 95Pt5Ru 2A No. 1 850 Air 300 600

GPt006 95Pt5Ru 2A No. 1 850 Air 440 600

GPt007 95Pt5Ru 1A No. 1 850 Air 440 600

GPt008 95Pt5Ru 1A No. 1 850 Air 300 600

GPt009 95Pt5Ru 1 No. 1 850 Air 300 300

GPt010 95Pt5Ru 1 No. 1 850 Air 440 600

GPt011 95Pt5Ru 1A No. 1 950 Air 440 600

GPt012 95Pt5Ru 2 No. 1 950 Air 440 600

GPt013 95Pt5Ru 2A No. 1 950 Air 440 600

GPt014 95Pt5Ru 5 No. 1 950 Air 440 600

GPt015 95Pt5Ru 6 No. 1 950 Air 440 600

GPt016 95Pt5Ru 5 No. 1 950 Air 440 600

GPt017 95Pt5Ru 6 No. 1 950 Air 440 600

GPt018 95Pt5Ru 7 No. 1 950 Air 440 600

GPt019 95Pt5Ru 7 No. 1 950 Air 440 600

GPt021 95Pt5Ru 4AG No. 1 950 Vacum 440 600


GPt023 95Pt5Ru 3AG No. 1 950 Air 440 600

GPt024 95Pt5Ru 3AG No. 1 950 Vacuum 440 600


GPt026 95Pt5Ru 4AT No. 1 850 Vacum 440 600

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GPt027 Pt--5Co 4AG No. 1 850 Air 440 600

GPt028 95Pt5Co 4AG No. 1 850 Vacum 440 600

GPt029 95Pt5Co 4AS No. 1 850 Vacum 440 600

GPt030 95Pt5Co 4AS No. 1 850 Air 440 600

GPt031 95Pt5Co 4ATS No. 1 950 Air 440 600

GPt032 95Pt5Co 4ATS No. 1 950 Vacum 440 600

GPt033 95Pt5Ru 4ATS No. 1 950 Air 440 600

GPt034 95Pt5Ru 4ATS No. 1 950 Vacum 440 600

GPt041 95Pt5Co 13 No. 4 1947 850 Air 440 600

GPt042 95Pt5Ru 13 No. 4 2016 850 Air 440 600

GPt043 95Pt5Co 13 No. 4 1980 950 Air 440 600

GPt044 95Pt5Ru 13 No. 4 2059 950 Air 440 600

GPt045 95Pt5Co 13 No. 2 1989 850 Air 440 600

GPt046 95Pt5Ru 13 No. 2 2019 850 Air 440 600

GPt047 95Pt5Co 13 No. 2 1983 950 Air 440 600

GPt048 95Pt5Ru 13 No. 2 2065 950 Air 440 600

GPt049 95Pt5Co 13 No. 2 2180 950 Air 440 600

GPt050 95Pt5Ru 13 No. 2 2252 950 Air 440 600

GPt051 95Pt5Co 13 No. 2 1994 1050 Air 440 600

GPt052 95Pt5Ru 13 No. 2 2045 1050 Air 440 600

4.2 Platinum-Ruthenium Alloy

4.2.1 Filigree ItemsIn previous studies 95Pt5Ru showed higher hardness but worse form filling than 95Pt5Co. A set of experiments proving the form-filling ability of 95Pt5Ru was madeusingtreescontainingstandardgrids(5x12mesh)withaligamentof0.8x1.0mm.Inthesetests(GPt014–GPt019),investmentNo.1wasusedatconstantflasktemperatureof950°C(1742°F)withcastingparametersof440rpm-600rpm/s.Threedifferenttreesetupswereused(Figure10).OnTree#5allgridsareorientedperpendiculartocentrifugaldirectionwhiletheyareparalleltoitonTree#6.Tree#7usedashortmainsprueholdingallgridsinradialdirection.Theformfillingwas evaluated for each grid and was found to depend strongly on the type of treeandthegridposition.TheDiabolo-typetrees#5and#6showedmuchbetterformfillingthanTree#7withamainspruewheremaximumvaluesreachedonly about 80% fill (Figure 11). On all trees best form filling was achieved for positions 0° - 90° where for both Tree #5 and Tree #6, nearly 100% fill wasachieved.At positions 135° - 315° grid fill falls to 70-80%and 80-90% forTree#5andTree#6,respectively.Tree#7shows50-70%gridfillandisthereforenot suitable for filigree items. As a result of these tests, filigree parts should be mounted on a Diabolo-type tree with the long axis of the part parallel to centrifugal direction. It is expected that positions 0°-90° result in best form filling.


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Figure 10 Different tree setups (a = Tree #5, GPt014/016; b = Tree #6, GPt015/017; and c = Tree#7, GPt018/019) using standard grids to evaluate

the influence of position on tree. Centrifugal direction is to the right.

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Figure 11 Grid filling in casting trials GPt014 – 019 with 95Pt5Ru as a function of tree setup and grid position on tree; averaged values of two casting trials per tree setup

For the numerous tests, grid filling was evaluated for 95Pt5Ru as a function of casting parameters as given in Figure 12 – Figure 15. The averaged values for all grids per tree were chosen for evaluation, and results for 95Pt5Co are given for comparison.The45°positionwaspresentonalltrees,whileonsometreespositions135°,225°and315°werealsoused.Ingeneral, like inpreviousinvestigations,14 large scatter is observed for grid fill because not all casting parameters can be fully controlled.15 However, some clear tendencies can be determined from the grid filling results:

• Flasktemperature(Figure12)hasastrongeffectonformfilling.Higherformfillingisobtainedfor950°C(1742°F);inmostcasesgridfillof>60%isreached.For850°C(1562°F)themaximumvalueisbelow60%.

• Centrifugalspeed(Figure13)hasastrongeffectandpromotesformfilling.• Castingatmosphere(Figure14)hasamoderateeffectonformfilling.Form

filling is usually better in vacuum than in air casting. However, perfect fill can also be obtained with air casting and in most cases grid fill was above 60%.

• Castingtemperature(Figure15)hasaweakeffectonformfilling.Higherflask temperature tends to promote form filling. Casting temperature is difficult to measure and can only be controlled by the melting time and visual control of complete melting, which depends on the appraisal of the caster.

• Perfectformfillingwasalwaysachievedfor95Pt5Coindependentofcastingconditions. Therefore, this alloy is superior to 95Pt5Ru in terms of form filling ability.


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Figure 12 Grid filling as function of flask temperature for air casting (except value for 550°C); averaged values of all grids per tree

Figure 13 Grid filling as function of centrifugal speed for air casting; averaged values of all grids per tree

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Figure 14 Grid filling as function of casting atmosphere for centrifugal speed of 440rpm; averaged values of all grids per tree

Figure 15 Grid filling as function of casting temperature and atmosphere for centrifugal speed of 440rpm; averaged values of all grids per tree

4.2.2 Heavy Items (Ball Ring)A ball ring with ring shank diameter of 3.6mm, ball diameter of 9.5mm and total diameter of 24.5mm was used as the standard object in all casting trials. The ball ring was sprued either on the ball or on the ring shank opposite to the ball in order


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to test for form filling, surface appearance and shrinkage porosity under different casting conditions.

Form filling of the heavy ball ring was no problem under most casting conditions. Evenataverylowflasktemperatureof550°C/1022°F(Trial#GPt025),completefilling of the ball ringwas achieved.At a flask temperature of 1050°C/1922°F(Trial#GPt051and052withNo.2),theinvestmentbreaksdownandformfillingis incomplete. Maximum flask temperature for this type of investment should thereforenotexceed950°C–1000°C(1742°F–1832°F).

The surface of the ball ring is characterized by areas with glossy and matte surface. Table 4 gives a relative appraisal of surface quality, with the best surface quality indicated by +++, while the worst was indicated by +.

Table 4 Relative comparison of investment performance in terms of surface quality for air casting: +++ (best), ++ (medium), + (worst). Investment

No. 3 was only used for casting trials with Indutherm MC15 tilting machine.

Investment DevestingSurface quality of 95Pt5Co

@ TflaskSurface quality of 95Pt5Ru

@ Tflask

850°C 950°C 1050°C 850°C 950°C 1050°C

No. 1 + ++ ++ n.a. +++ +++ n.a.

No. 2 +++ ++ ++ + ++(+) ++(+) +

No. 4 ++ + + n.a. + + n.a.

No. 3 + +++ +++ n.a.+

(cracks)

+

(cracks)n.a.

The investment material has strong influence on surface quality. The three-part investments No. l and No. 2 show much better performance than the two-part investments No. 3 and No. 4. Of the three-part investments No. 1 appears slightly better with lower tendency to show fins. However, the difference between No. 1 and No. 2 is very small. For No. 1 and No. 2 there is no obvious influence of flask temperature or casting atmosphere. The ball always has a matte surface, while the ring shank is partially glossy. No. 4 shows a rougher surface for a flask temperatureof950°C(1742°F)comparedto850°C(1562°F).

SEM investigations of the ball and the ring shank showed the topology of the surface.ForNo.2andNo.1thesurfaceisfullydendritic(Figure16).Almostnoresidues of investment were found. On the matte part of the surface, the dendrites did not reach the surface of the ring cavity in the flask. This can be explained by premature freezing of the ring shank close to the sprue, preventing directional solidification and therefore complete form filling. In some cases the dendritic surface was restricted to one side of the sample, which was on the trailing sample side relative to centrifugal direction. In this case the liquid metal is forced to one side of the flask cavity by centrifugation, resulting in a glossy surface on this side and a matte, dendritic surface on the other side. In the glossy part of the surface

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dendritesarestillvisible(Figure16b),butthesurfaceappearsrelativelysmooth(i.e.,themeltwasindirectcontactwiththeflaskcavityduringsolidification).Nosigns were found that the matte surface was caused by investment reactions.

a) b)

Figure 16 SEM investigation of as-cast surface of trial 95Pt5Ru/No. 2/950°C/air. a) matte surface of the ball; b) glossy surface of the ring shank

With investment No. 4 the surface of the ball rings strongly depends on flask temperature. At 850°C (1562°F) the surface is similar to investment No. 2 or investment No. 1 (Figure 17). However, micro shrinkage and residues of investment material are frequently found in the matte areas. Even in the glossy parts the surface shows micro shrinkage and is less smooth compared to the other investments. At a flask temperature of 950°C/1742°F (Figure 18), the surface appears very rough (please note that Figure 18a was taken with the same magnification as the other pictures). Larger shrinkage pores and investment residues are sticking to the surface. In the glossy parts the surface is comparable to 850°C(1562°F),especiallyforsampleswiththinnercrosssection.Thesefindingsindicate that at 950°C (1742°F), investmentNo. 4mightbebeyond itsworkingtemperature for heavy parts such as the ball ring, where the ball acts as a hotspot, strongly heating up the investment and promoting investment breakdown. For filigree parts the surface is smoother at both flask temperatures and comparable to investment No. 1 and investment No. 2.


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a) b)

Figure 17 SEM investigation of as-cast surface of trial 95Pt5Ru/No. 4/850°C/ air. a) surface of the ball; b) surface of the ring shank close to sprue

a) b)

Figure 18 SEM investigation of as-cast surface of trial 95Pt5Ru/ No. 4/950°C/air. a) surface of the ball with dark inclusions of investment material;

b) surface of the ring shank close to sprue

The liquid metal can erode the investment during form filling, resulting in hard investmentparticlesembeddedinthecastsample(Figure19).EDXmeasurementshowed that such particles consist of SiO2, which is the main component of the investment. The particles were found at an inner shrinkage pore surface and were in intimate contact with the melt. However, no signs of reactions like rounding off of edges were observed for 95Pt5Ru. Nevertheless, eroded investment particles, which were observed for all investments, will act as hard spots during jewelry finishing.

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a)

b)

Figure 19 a) Hard investment particles (SiO2) embedded in 95Pt5Ru casting (Investment No. 1/850°C/vacuum); b) EDX

spectrum at position 4 showing Si and O signal


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A second type of very small, non-metallic inclusions with spherical shape was found more frequently independent from casting conditions or investment. The rounded edges indicate reaction with the alloy. The inclusions contained Si, O, Al, and Mg from the investment or crucible material. Because of their small size and low quantity, such particles are probably not critical in jewelry applications.

The porosity was assessed by metallographic sections of as-cast rings. The section was cut through the complete ring from sprue to ball. An overview of typical examplesof these sections is shown inFigure 20 for 850°C (1562ºF) and950°C(1742ºF), respectively. Porosity was evaluated qualitatively in terms of ‘low’(acceptable,with either noporosity visible in the shanks or small pores in the heavyparts)or‘high’(withlargeporesoraccumulatedfineporesintheheaviestpartofthecasting)or‘veryhigh’(unacceptable,withlargeporesnearthesurfaceofthecastinginitsheaviestsection)(Table5).Inthecaseof95Pt5Ru,astronglydendritic solidification with large, intersecting dendrites was observed. The chaotic formation of a three-dimensional dendritic network by the growing crystals prevents the flow of the liquid metal. Typically, this results in numerous small pores distributed in the ball or ring shank. As shown later, 95Pt5Co showed a different freezing behavior, resulting in few but larger pores.

a) b)

c) d)

Figure 20 Metallography of 95Pt5Ru ball rings. a) No. 1/950°C(1742°F)/air; b) No. 4/950°C (1742°F)/air; c) No. 2/950°C (3749°F)/air/melt temperature 2065°C

(1742°F); d) No. 2/950°C (1742°F)/air/melt temperature 2252°C (4086°F)

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Table 5 Relative comparison of porosity in metallographic sections of 95Pt5Ru casting trials depending on investment: +++ (low = best) ++ (high = medium) + (very

high = worst)

95Pt5Ru No. 1 No. 4 No. 2

850°C/air n.a. +++ ++

950°C/air ++ +++ ++

850°C/vacuum + n.a. n.a.

950°C/vacuum ++ n.a. n.a.

The most common defect was shrinkage porosity in the ball and/or in the ring shank. Generally, porosity was low in the shank sections, while it was worse in the heaviest part of the casting (ball and sprue sections). This is caused bypremature freezing of the ring shank. Under the casting parameters investigated it was not possible to achieve complete directional solidification, not even if the ring was sprued on the ball. The centrifugation forces the liquid metal to flow to one side of the ring. As a consequence, the shrinkage porosity is concentrated on the opposite side. This is illustrated in Figure 21, where the large shrinkage pore in the ball is asymmetric to the ball center. On the lower side the ball surface is rough and dendritic, while the opposite side is smooth because the liquid metal is forced to the upper side by centrifugation, and the frozen ring shank prevented directional solidification. This was the typical behavior observed for investments No. 1 and No. 2. The same principle is valid for No. 4, but this investment showed the best casting results for the ball ring in terms of porosity. Shrinkage pores did not accumulate to one big shrinkage hole but instead were scattered throughout the complete ring.


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a) b)

c) d)

Figure 21 Effect of melt temperature on porosity, 95Pt5Ru/No. 2/950°C/air; a), b) Melt temperature 2252°C; c), d) Melt temperature 2065°C

Casting conditions have a small influence on shrinkage porosity. No influence was observed for centrifugal speed and acceleration in their investigated range (330-400 and 440-600rpm/s). Casting atmosphere also had no clear influence on porosity. The process is controlled by the geometry of the sample and the thermal conditions. Therefore, higher flask temperature should result in directional solidification and reduced porosity as shown in previous work on silver casting.19 However, it was not possible to increase flask temperature above 950°C – 1000°C (1742ºF – 1832ºF) without the risk of investment breakdown.Besides sprue geometry optimization, only an increase of melt temperature might change solidificationbehavior and reduce shrinkageporosity. In casting trial #GPt050, melt temperature was increased by 200°C (360ºF) compared to trial #GPt048(2065°C/3749ºF).Figure22showsthecomparisonofthecastingresults. In both cases comparable shrinkage porosity was found in the ball, whereas porosity in the ring shank was lower at higher casting temperature. However, such extreme melt temperatures will result in high crucible erosion under practical casting conditions.

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a) b)

Figure 22 Casting trial GPt012: 95Pt5Ru/No. 1/950°C/air. a) Position 90, ball sprue; b) Position 180, ring sprue

Gas porosity was merely observed under the chosen casting conditions and for the investment materials used. Figure 20 shows a comparison of the three investments for a flask temperature of 950°C (1742ºF) at air casting. Melt temperaturewasabout2060°C(3740ºF)exceptforFigure20d,wherethemeltwasoverheatedby200°C(360ºF).Scatteredgasporeswerefoundinallcastingswitha tendency for increased gas porosity with increasing flask temperature and for vacuum casting compared to air casting. Even in the case of an overheated melt, no significant gas porosity was found for the No. 2 investment. No. 4 investment showed numerous pores after casting. However, high magnification reveals that these pores are, according to their shape, scattered shrinkage pores rather than gas pores. Therefore, gas porosity was no major issue for 95Pt5Ru even under extreme casting conditions.

The 95Pt5Ru casting results for heavy items can be summarized as follows:

• Thecastingdefectsobservedwerecausedbynon-directionalsolidificationand premature freezing of the ring shank or sprue.

• Shrinkageporesaccumulatedbycentrifugationwerelocatedintheball(inthecaseofshanksprue)oratthetransitionfromballtosprue(ballsprue).

• Defect-freecastingofballringscouldnotbeobtainedwiththechosencastingparameters.LowestshrinkageporositywasobservedforinvestmentNo.4at850°C(1562ºF)flasktemperature.

• Highflaskandmelttemperaturetendtoreduceshrinkageporosity,but chosen values are already close to investment breakdown temperature.

• Gasporositywasmerelyobservedforallinvestmentmaterialsevenunderextreme casting conditions.


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4.3 Platinum-Cobalt Alloy

4.3.1 Filigree Items95Pt5Co was recommended in earlier studies for filigree items because of its much better form filling.12, 14 As shown in Figures 12-15, grid fill was always 95-100% independent of casting conditions. Therefore, just considering form filling ability, 95Pt5Co is the alloy of choice for filigree items.

4.3.2 Heavy Items (Ball Ring)As expected from the results with 95Pt5Ru, form filling of heavy items such as the ball ring was no problem with 95Pt5Co as well. 95Pt5Co always showed some reaction with investment materials. This is obvious from the dark blue color of theinvestmentincontactwiththealloy(Figure23).Theinvestmentmaterial incontact with the alloy shows a deep blue color, while the fresh fracture surface of the investment appears white. The reaction is clearly visible around the 95Pt5Co particle embedded in the investment. Local composition measurement in thescanning electronmicroscope (SEM) found a composition close to themineralCo2SiO4, which is known for its blue color. The surface reaction is promoted by the segregation of Co during solidification17 and depends on casting parameters. The metallographic cross section of the reaction products shows SiO2 particles from the investment sticking to the alloy surface (Figure 23c, position 10). TheEDXmeasurementoftheseparticlesshowsonlySiandO(i.e.,thequartzparticlesfromtheinvestment).TheEDXspectrumatposition11alsocontainsPandtracesof Al and Mg together with Co, indicating the formation of Co silicate with the binder phase of the investment. The blue color is much more pronounced for air casting and on heavy parts. With increasing melt and flask temperature, the reactionincreases(i.e.,oxidationofCoduringmeltingandcastingpromotesthereaction with the investment material).

Figure 23a

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Figure 23b

Figure 23c


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Figure 23d

Figure 23 a) Reaction between 95Pt5Co alloy and investment material indicated by the dark blue color of investment in contact with the alloy; b) EDX measurement showing

Co from the alloy and Si, O, Mg, Ca, and P from the investment; c) metallographic cross section through reaction products; d) EDX spectrum at position 11

The surface of 95Pt5Co ball rings usually appears rougher compared to 95Pt5Ru. No glossy parts can be found on the ring shank or sprue but the surface appears matte all over the ring. Table 4 gives a relative comparison of surface quality of as-cast parts, showing that results with 95Pt5Co were slightly worse than with 95Pt5Ru. Investment No. 2 showed the smoothed surface similar to No. 1, while parts cast in No. 4 had the roughest surface. On filigree parts, such as the grid, it was very difficult to remove the investment completely, while it fell off heavy parts.

SEM investigations of ball and ring shank showed a dendritic structure in most parts of the surface. Results for investment No. 2 are given in Figure 24 showing the transition from ring shank with dendritic structure to the sprue connection where the surface is smooth. The dendritic surface is attributed to non-directional solidification. A smooth surface only appeared directly at the sprue. The surface is partially covered with black-appearing reaction products of the alloy and No. 2 investment. For the No. 4 investment these black residues were found to a much larger extent between the dendrites (Figure 25). Practically the complete interdendritic space shows a film-like layer of reaction product. EDX measurements (Position 3) showed high concentration of Co from the alloyandO, Si, P andMg from the investment.TheEDX spectrum is similar to the measurementonthebluelayeroftheinvestmentmaterial(Figure23).Athinlayerof a second reaction product of the 95Pt5Co alloy was found on the dendrite tips (Position2inFigure25).EDXmeasurementsshowedhighconcentrationsofCoand O, while the Pt signal is coming from the alloy matrix. Co forms several black oxides, amongwhich cobalt(II) oxide (CoO) forms a lowmelting eutecticwithcobaltat1451°C(2644ºF).TheformationofCoOispromotedbythesegregationof

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Co to the interdendritic areas. A thin layer of CoO probably forms on the surface during casting, and this reacts with the investment to form the blue Co silicate layer. For investment No. 4 this reaction is much more pronounced compared to No. 2 or No. 1. Additives of the investment may play an important role in these reactions. Casting in vacuum or protective atmosphere prevents Co oxidation and investment reaction.

a) b)

Figure 24 SEM investigation of as-cast surface of trial GPt049 (905°C/air/95Pt5Co). a) Transition from sprue to ring shank. b) Backscattered image at higher magnification.


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a) b)

c)

d)

Figure 25 SEM investigation of as-cast surface of trial GPt041 (No. 4/850°C/air/ 95Pt5Co); a) Surface of the ball with

interdendritic residues of investment; b) Backscattered electron image; c), d) EDX point measurements at Positions 2 and 3, respectively.

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Figure 26 provides an overview of the metallographic cross sections of 95Pt5Co castings depending on casting parameters and investment. Table 6 shows the evaluation of porosity depending on investment. Usually there is one big shrinkage hole formed close to the trailing side of the ball relative to centrifugal direction. No major porosity was found on the ring shank or sprue, which is different from 95Pt5Ru, where scattered shrinkage pores were usually found in the whole ring. The different morphology of the shrinkage pores is controlled by the thermophysical properties of the alloy.

a) b)

c) d)

Figure 26 Metallographic investigation for gas porosity in 95Pt5Co alloys; a) No. 1/850°C/air; b) No. 1/950°C/air; c) No. 1/950°C/vacuum; d) No. 4/950°C/air

Table 6 Relative comparison of porosity in metallographic sections of 95Pt5Co casting trials depending on investment:

+++ (low = best) ++ (high = medium) + (very high = worst).

95Pt5Co No. 1 No. 4 No. 2

850°C/air + + +

950°C/air + + +




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As observed above, 95Pt5Co shows some reaction with investment, manifested by Co silicate formation. It was investigated whether such reaction causes gas porosity close to the ring surface. Metallographic results are given in Figure 26. The ball ring is an object susceptible to gas porosity because of its high mass and the strong heating of the investment. The most critical position where the investment heats up most during solidification is the transition from ball to sprue on the inner side of the ring. All three investments were found to show practically nogasporosityat850°C(1562ºF)/air.At thehigher flasktemperatureof950°C(1742ºF)/air, there was very slight increase, while the situation was worst for 950°C/vacuum. However, gas porosity was a secondary issue for the heavy section ball ring in these casting trials. Compared to 95Pt5Ru, gas porosity was less for 95Pt5Co, probably because of the lower alloy temperature.

The 95Pt5Co casting results for heavy items can be summarized as follows:

• 95Pt5CoshowsinvestmentreactionsmanifestedbyabluelayerofCo silicate. Such reactions increase with increasing flask temperature and are stronger in air casting.

• Shrinkageporosityiscausedbynon-directionalsolidificationand premature freezing of the ring shank or sprue. Flask and melt temperature had no remarkable influence on shrinkage porosity.

• Defect-freecastingofballringscouldnotbeobtainedwiththechosen casting parameters. The investment material showed little influence on shrinkage porosity.

• Gasporositywasmerelyobservedforthechosencastingparametersandinvestment materials.

5. Casting Trials with Tilt Casting MachineThe Indutherm MC15 casting is an induction-heated machine with a maximum power of 3.5kW. It consists of a vacuum chamber where crucible and flask are mountedina90°orientation.Forcastingthevacuumchamber is tiltedandtheliquid metal is poured into the flask. Immediately after casting argon pressure is applied to improve form filling. Experiments with this machine were made for comparison of static casting vs. centrifugal casting. The investment used in these tests was No. 3, which is a two-part investment similar to No. 4. All casting tests were made under vacuum in order to have improved form filling at flask temperatures of 850°C (1562ºF) and 950°C (1742ºF).Alloys usedwere 95Pt5Coand 95Pt5Ru with details given in Table 7. For reasons of flask size and machine power, the tree was much smaller compared to the centrifugal casting trial, containing only two ball rings and some industrial samples.

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Table 7 List of tilting casting experiments with Indutherm MC15 machine

Trial no. Alloy Tree InvestmentCasting temp.[Cº]

Flask temp.[Cº]

Atmosphere

GPt035 95Pt5Ru MC15 No. 3 1850 950 Vacuum



GPt038 95Pt5Co MC15 No. 3 1928 950 Vacuum



5.1 Surface of cast partsFigure 27 shows the surface of as-cast parts for both alloys and flask temperatures. The surfaces are smooth for both alloys and usually smoother than the centrifugal cast parts. With 95Pt5Ru a non-metallic, glassy surface layer formed around the ball, which shows large dendrites on the surface. The glassy layer seems to stem from melting of the investment materials around the ball. The 95Pt5Co ball ring has a smooth, clean surface without dendritic structure. SEM investigation of the surface showed smooth, slightly dendritic surface for both alloys (Figure 28). In the case of 95Pt5Ru, surface cracks along the grainboundaries are present, which formed during spontaneous cracking of the ring at room temperature. The 95Pt5Co ring shows no cracks but some residues of investment.

a) b)

Figure 27 Tilting casting machine results; a) surface detail of 95Pt5Ru/950°C/vacuum; b) 95Pt5Co/950°C/vacuum


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a) b)

Figure 28 SEM investigation of as-cast surface (tilting casting machine); a) 95Pt5Ru/950°C/vacuum; b) 95Pt5Co/950°C/vacuum

5.2 Metallographic InvestigationMetallographic cross sections along the ring’s plane are shown in Figure 29 for both alloys and flask temperatures. 95Pt5Ru shows similar form filling compared to centrifugal casting experiments, with scattered shrinkage holes in the ball and the sprue. No remarkable difference in porosity between 850°C (1562ºF) and950°C(1742ºF)flasktemperaturewasobserved.

95Pt5Ru 95Pt5Co

850°C/ Vacuum

a) b)

950°C/ Vacuum

c) d)Figure 29 Metallography of as-cast part from tilting casting machine

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At 950°C (1742ºF) the 95Pt5Ru shows several grain boundary cracks perpendiculartotheringshank(Figure30),whicharepartiallyfilledwithSi-richinclusions. This is a typical situation of hot tearing due to dissolution of Si from the investment and formation of a low melting eutectic16 at the grain boundary. Cracking was induced by thermal stresses, occurring during devesting of the sample when the sample was already at room temperature. The fracture surface shows fully interdendritic, brittle fracture.

a) b)

Figure 30 Metallographic section through cracked 95Pt5Ru ball ring (950°C/vacuum); a) grain boundary crack; b) silicon-rich inclusion at grain boundary

With 95Pt5Co a large shrinkage hole was present close to the center of the ball at850°C(1562ºF)flasktemperature.Comparedtocentrifugalcastingtheporeissphericalandcloser to thecenterbecausenocentrifugal forcewasacting.Verygood casting resultswereobtainedwith a flask temperatureof 950°C (1742ºF).The ball ring is filled completely without any shrinkage pores. Cracks were not observed in any of the 95Pt5Co castings.

5.3 SummaryCasting results with a tilt casting machine were strongly dependent on alloy. 95Pt5Ru showed similar form filling compared to centrifugal casting but hot tearing due to long and intensive investment reactions because of longer process times and high metal temperature. 95Pt5Co showed perfect form filling and smooth surfaces for a flask temperature of 950°C. Form fillingwas better thanwith centrifugal casting. Whether the good form filling can be attributed to the different tree design or the applied overpressure remains open for discussion.


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6. Summary and ConclusionsPlatinum alloys are a challenge for defect-free investment casting because of their inherent thermophysical properties, which are high melting point, high viscosity of the melt, high shrinkage during solidification, and low thermal conductivity compared to other precious metals alloys. In this experimental casting project numerous casting trials were conducted with a variation of:

• Castingparameters(meltandflasktemperature,atmosphere,investment)• Treesetup(positionontree,light-andheavyweightpattern)• Castingmachine(centrifugalandtiltingmachine)• Platinumalloy(95Pt5Ruand95Pt5Co)The performance of alloys and investments was assessed qualitatively as shown in Tables 4, 5 and 6. The main results of the project are summarized as follows:

• Shrinkageporositywasthemainissueforbulk,heavypatternsaswellasfor filigree, lightweight patterns, if directional solidification is not possible. The effect of casting parameters and the position on tree are relatively low. 95Pt5Co shows few but large pores while 95Pt5Ru often shows scattered pores built by intersecting dendrites. Investment material influences shrinkage porosity. For instance, lowest levels of shrinkage porosity were achieved for No. 4. This is probably an effect of thermal conductivity of the investment, which is assumed to be lower for two-part investments than for three-part investments.

• Formfillingisacriticalissueforfiligreeitems.95Pt5Cohassuperior form-filling ability over 95Pt5Ru. Form filling increases considerably with increasingcentrifugalspeedandflasktemperature,whichshouldbe950°C(1742ºF)forfiligreeitems.InvestmentNo.4showedbestformfillingforcomparable casting parameters, probably because of the higher gas permeabilityoftheinvestment.Vacuumcastingand,inthecaseof95Pt5Ru,overheating of the melt allowed complete filling of filigree items with three-part investments, while at the same time promoting investment reactions.

• Theinvestmentsusedrequiredifferentworkingconditions.Thetwo-partinvestments can be handled with a rubber base and cure quickly at room temperature, resulting in very short working time. In the case of centrifugal casting, No. 4 resulted in rougher surface of the as-cast parts. The three-part investments have sufficient working time but require furnace curing and therefore paper base and liner. Complete form filling was difficult with these investments unless working under vacuum or with overheated melt. The surface quality was usually superior over No. 4.

• Investmentreactionswereobservedfor95Pt5Coindependentofcastingatmosphere and resulted in a blue layer of Co silicate. 95Pt5Ru did not show any investment reactions despite its considerably higher casting temperature.

• Alloypropertiesdifferindendritemorphology,segregation,andmeltingtemperature,whichisabout150°C(270ºF)higherfor95Pt5Ru.Thephase diagramsgivethesamemeltingrangeof18°C(32.4ºF)forbothalloys,but due to segregation the melting range of 95Pt5Co is about twice that of 95Pt5Ru, which is probably the reason for the better form filling ability of

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95Pt5Co. Co segregates to the melt, promoting oxidation of Co and investment reactions, even for vacuum casting.

• Acomparisonoftwotypesofcastingmachines,acentrifugalandatiltingmachine, showed that form filling of filigree items was superior with centrifugal casting. Both machines provided comparable results for heavy itemscastin95Pt5Coalloy.Defect-freecastingsoftheballringwereobtainedat950°C(1742ºF)/vacuum,whichwasnotpossiblebycentrifugalcasting under comparable conditions. 95Pt5Ru was difficult to cast in the tilting casting machine because of the low heating rate in the specific model used, which resulted in hot tearing of the parts. Machines with higher power and sufficiently short melting time may enable the successful casting of 95Pt5Ru also.

6.1 Recommendations for further workIn order to solve the main problem of shrinkage porosity, sprue design and tree setup aremost important. Directional solidification has to be assured. Typicalmeasures such as increase of flask temperature are limited by the thermal stability of the investment materials. However, optimization of casting behavior solely by experimental means remains challenging.

In recent years casting simulation proved to be a valuable tool for gold and silver casting.19-23 Sophisticated software packages are available on the market to determine form filling and shrinkage porosity depending on alloy, tree setup, and melt and flask temperature, allowing optimization by computer simulation. In case of platinum this would pay off even more, because of high material price and extreme casting and flask temperatures.

Detailedknowledgeof the thermophysical properties of alloys and investmentis required and, as such data are scarce, they have to be determined in suitable experiments. Benchmark experiments with sophisticated thermal recording during the centrifugal casting process have to be performed to calibrate the casting simulation results. Investment materials were found to play an important role in form filling and shrinkage porosity. It is assumed that properties such as gas permeability and thermal conductivity are responsible for that behavior. Therefore, the influence of water:powder ratio, burnout cycle, flask temperature and casting atmosphere requires further investigation to understand how the physical properties of the investments can be tailored.


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ACKNOWLEDGMENTSTheauthors aregrateful for financial supportbyPlatinumGuild International,with special thanks to JurgenMaerz. C. Hafner GmbH, Germany enabled theproject by providing platinum alloys, which is kindly acknowledged. The companiesRansom&Randolph,LaneIndustriesandSpecialistRefractoryServices(SRS) are acknowledged for allocation of investmentmaterials and InduthermGmbH, Germany for casting experiments with their MC15 casting machine.Special thankstoDieterOtt forfruitfuldiscussionsof theprojectresultsandtothe staff members of the metallurgy department at FEM, especially to Franz Held and Ulrike Schindler for casting trials and metallography.

REFERENCES1. N. Swan, “Improvements in Platinum Casting,” Jewellery in Britain 19 (December2004):5-16.

2. N. Swan, “Casting Platinum Jewellery –AChallenging Process,”Platinum Metals Review51,no.2(2007):102.

3. G.Ainsley,A.A.BourneandR.W.E.Rushforth,“PlatinumInvestmentCasting Alloys,”Platinum Metals Review22,no.3(1978):78-87.

4. J. Huckle, “TheDevelopment of PlatinumAlloys toOvercome Production Problems,”The Santa Fe Symposium on Jewelry Manufacturing Technology 1996, ed.DaveSchneller(Lafayette,CO:Met-ChemResearch,1996):301-326.

5. J.Maerz,“PlatinumAlloyApplicationsforJewelry,”The Santa Fe Symposium on Jewelry Manufacturing Technology 1999,ed.DaveSchneller(Lafayette,CO: Met-Chem Research, 1999): 55-72.

6. J.Maerz,“PlatinumAlloys:FeaturesandBenefits,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2005, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2005): 303-312.

7. J. Maerz, “Casting Tree Design and Investing Technique for Induction PlatinumCasting,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2002,ed.EddieBell(Albuquerque:Met-ChemResearch,2002):335-352.

8. A. Nooten-Boom II, “Controlled Flow = Controlled Solidification,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2004, ed. Eddie Bell (Albuquerque:Met-ChemResearch,2004):315-344.

9. A. Nooten-Boom II, “Dynamics of the Restricted Feed Tree,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2006, ed. Eddie Bell (Albuquerque:Met-ChemResearch,2006):387-419.

10. J.Maerz,“PlatinumCastingTreeDesign,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2007, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2007): 305-322.

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11. H.Frye,M.YasrebiandD.H.Sturgis,“BasicCeramicConsiderationsforthe LostWax Processing of HighMelting Alloys,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2000, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2000): 101-116.

12. P.Lester,S.TaylorandR.Süss,“TheEffectofDifferentInvestmentPowders andFlaskTemperaturesontheCastingofPtAlloys”The Santa Fe Symposium on Jewelry Manufacturing Technology 2002, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2002): 321-334.

13. D. Miller, T. Keraan, P. Park-Ross, V. Husemeyer and C. Lang, “Casting Platinum JewelleryAlloys. TheEffects ofCompositiononMicrostructure,” Platinum Metals Review49,no.3(2005):110-117.

14. D. Miller, T. Keraan, P. Park-Ross, V. Husemeyer and C. Lang, “Casting PlatinumJewelleryAlloys.PartII:TheEffectsofCastingVariablesonFilland Porosity,”Platinum Metals Review49,no.3(2005):174-182.

15. D.Ott,“ChaosinCasting:AnApproachtoShrinkagePorosity,”Gold Bulletin 31(1997):13-19.

16. T. B. Massalski, Binary Alloy Phase DiagramsVol.1and2(MetalsPark,Ohio: American Society of Metals, 1986).

17. J. C. McCloskey and S. Aithal, “Microsegregation in Pt-Co and Pt-Ru Jewelry Alloys,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2006, ed. EddieBell(Albuquerque:Met-ChemResearch,2006):363-376.

18. G. Beck, H.H. Beyer, W. Gerhartz, J. Haußelt and U. Zimmer, Edelmetalltaschenbuch2nded.(Hüthig-VerlagHeidelberg,Germany:1995).

19. J.Fischer-Bühner,“ComputerSimulationofJewelryInvestmentCasting:What CanWeExpect?”The Santa Fe Symposium on Jewelry Manufacturing Technology 2006,ed.EddieBell(Albuquerque:Met-ChemResearch,2006):193-216.

20. J.C.Wright,“ComputerSimulationandJewelleryProduction,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2005, ed. Eddie Bell (Albuquerque:Met-ChemResearch,2005):521-535.

21 J. Fischer-Bühner, “Advances in the Prevention of Investment Casting DefectsAssistedbyComputerSimulation,”The Santa Fe Symposium on Jewelry Manufacturing Technology 2007, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2007): 149-172.

22. M. Actis Grande, L. Porta and D. Tiberto, “Computer Simulation of the Investment Casting Process: Widening of the Filling Step,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2007, ed. Eddie Bell (Albuquerque:Met-ChemResearch,2007):1-18.

23. J.Fischer-Bühner,“ImprovementofSterlingSilverInvestmentCasting,”JTF - Jewellery Technology Forum 2006 (Vicenza,Italy:2006):122-145.

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