2010 the Role of Process Parameters in Platinum Casting

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process parameters in pt castings

Text of 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.

    800.545.6566 riogrande.com

  • 287May 2010

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    The Role of Process Parametersin Platinum Casting

    Dr.UlrichE.KlotzTizianaDrago

    ResearchInstitutePreciousMetals&MetalsChemistry(FEM)SchwbischGmnd,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 as100C(212F).95Pt5Ru,ontheotherhand,showedpoorformfillingoffiligreeparts for flask temperaturesbelow800C (1472F).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 cant 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 at830C(1526F)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 50C (90F) compared to thebinaryalloys.Higheramounts(0.2mass%)resultinareductionof150C(270F)and250C(450F)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 propertiessurface tension, viscosity and thermal conductivityare 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 temperaturecanbeestimatedtobe40Kduringtheone-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 ex

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