Netshape Manufacturing of CuCr Medium Voltage Circuit Breakers

Claudia Kowanda1, Arno Plankensteiner2, Christian Grohs2, A. Schwaiger1, M. Hochstrasser1, F.E.H. Müller1 1 Plansee Powertech AG, Seon, Switzerland

2 Plansee SE, Reutte, Austria

Abstract- Continuum mechanics based modelling and finite element based simulation methods are commonly proved to represent cost efficient and timesaving tools for optimization of filling body geometries and tool designs. In case of adequate material descriptions they also allow to reduce the risk of flaws such as formation of cracks and low-density areas, respectively. By linking the simulation tools for compaction and sintering further enhancements are achievable such as minimized sintering distortions this way forming the basis for an advanced design tool of netshape (NS) manufacturing routes. The technical applicability and economic relevance of the proposed approach is demonstrated on the example of NS manufactured CuCr contacts for medium voltage circuit breakers as they are used in electric power distribution. The approach comprises modelling of powder compaction and sintering in an integrative manner via Drucker-Prager/Cap plasticity modelling and non-linear visco-elastic transversally isotropic sintering modelling, respectively. In particular, the careful calibration procedure for the material models used for the compaction and sintering regime with respect to the experimental findings of several types of mechanical green body tests as well as sintering experiments is described in detail. This way the presented approach claims and focusses on the first-in-class show case of a CuCr circuit breaker component being netshape manufactured.

I. INTRODUCTION

Over the past two decades, CuCr based contact materials have become established for use for circuit breakers, respectively, in vacuum interupters for the medium voltage range of 1 to 72.5/84kV. Depending on the application field of the contact component the Cr content typically varies between 20wt.% and 60 wt.%. The lower Cr content range is applied to operational conditions where high arc quenching and low current chopping plays a significant role and the higher Cr content range gets applied where high erosion resistance as well as a proper welding behavior comes into focus. Beside the choice for an adequate CuCr grade the geometrical features of the contact component also plays a significant role on the performance during switching operations.

II. PRODUCTION ROUTE

A. Classical PM production route

CuCr contact materials are produced by a commonly

established powder metallurgical (PM) processing route which includes mixing of the Cu and Cr powders, die compaction of the powder blend to a green body and sintering typically below the melting temperature of copper. The resulting microstructure of the contact material is mainly influenced by the grain size of the adopted Cr powder material. After sintering CuCr contact materials may exhibit a small amount of retained porosity. A further reduction of porosity after sintering can be achieved, i.e. by repressing. That leads to an increase of electrical, thermal and mechanical properties of the contact material, see also Ref. [1].

In order to get to the desired final shape of the contact component chipping and other types of mechanical treatment is necessary within the classical PM manufacturing appproach. This is mainly due to achieve the specified contact shape and to inhomogeneous spatial density distributions in the die pressed green body which governs the spatial gradient in sinter density. For the definition of the design of the pressing tools and pressing parameters the occuring sintering shrinkage along with sintering distortions have to be handled properly.

As long as mechanical treatment of the contact component is chosen for obtaining the final shape of the contact component, empirical knowledge on the interaction between material characteristics, process parameters, and the quality relevant parameters of the sintered product is widely being proofed to be sufficient for a successful product development. However, economically more challenging tasks such as minimization or even complete prevention of material to be machined off aim at reducing material and manufacturing costs as well as optimizing the time-to-market characteristics.

B. Netshape (NS) production route

Commonly such optimisation and cost reduction strategies are referred to as netshape manufacturing (NS), means the final shape of a CuCr contact component can be achieved by pressing and sintering without chipping or other types of machining. This can be achieved by intensively making use of advanced tools for numerical simulation of the NS process chain and advanced material characterization methods. The material characterisation methods are intended for

XXIVth Int. Symp. on Discharges and Electrical Insulation in Vacuum - Braunschweig - 2010

978-1-4244-8365-5/10/$26.00 ©2010 IEEE

extracting the characteristic material parameters for the material laws describing the constitutive behavior of powder materials and sinter materials under diepressing and sintering operations, respectively. The numerical simulation tools aim at performing efficiently numerical experiments of the NS process chain. Reduced overall costs are realized by establishing a numerically and experimentally verified prototype of the process chain.

III. NUMERICAL SIMULATION

The finite element method (FEM) is a cost efficient and timesaving tool for optimizing powder metallurgy (PM) netshape processing. With suitable material descriptions, FEM can open pathways how to reduce the formation of flaws such as low density areas and cracks. Furthermore, the prediction of sintering distortions, density distributions and residual stresses is possible when the compaction simulation is linked to sintering models. Consequently, modeling of the powder metallurgy process chain is a key for designing netshape PM processes. The designing of a netshape PM process for CuCr medium voltage vacuum interrupter, which are widely used in electric power distribution, was demonstrated by Ref. [2, 3].

A. Powder characterisation, tests and calibration

A prerequisite for the application of the described mathematical model is the identification of the involved constitutive parameters and their density dependence. This so called powder characterization is usually done on an experimental basis. In order to identify the mechanical response of the powder during die compaction uniaxial compaction tests using a standard or instrumented die are conducted, see Fig. 1a (1). These tests allow to indirectly obtain the hardening relationship between the triaxial compressive strength pb and the inelastic volumetric strain εv

pl or the density ρg, respectively.

For identification of the parameters related to the

shear failure surface fs at least two different types of experiments at different levels of stress triaxiality p/q have to be conducted. In this context most often the Brazilian disc test, see Fig. 1a (2), and the uniaxial compression test, Fig. 1a (3), are employed. These tests are conducted on cylindrical samples previously compacted to approximately the same density. The stress states found in the specimens can be calculated analytically from the applied load. Hence, from the maximum applicable loads the shear failure parameters can be derived. In order to determine the density dependence of the strength parameters identical sets of experiments must be conducted at various density levels ρ1, ρ2 … (shown in Fig. 1b) relevant for PM production purposes. For simulation purposes the obtained discrete values d1, d2, … and β1, β2 … will be fitted using an appropriate regression function, see Fig. 1b for the case of cohesion d.

B. Modeling of sintering

As sintering is a heat treatment process carried out at high temperatures that causes the consolidation of the powder particles in the porous green body. The volume of the pores between the powder particles gets reduced

Fig.2. Results obtained by sintering exepriments on two types of CuCr alloy – green density (ρg) in dependence of (a) sinter density ρs, (b) axial sinter shrinkage εz and (c) radial sinter shrinkage εr, CuCr alloy 1 - 75/25CuCr; CuCr alloy 2 - 60/40CuCr

Fig. 1. Mechanical powder characterization: (a) experimental determination of cap and shear failure surface parameter (b) density dependence of cohesive strength d.

during this process, therefore the material shrinks and the density increases.

With the used constitutive model which describes the mechanical behavior of a solid being sintered for usage in the context of simulations of the PM process chain it is possible to reproduce both, evolution of density and shrinkage as shown for two different CuCr alloys by means of results from sintering experiments, see Fig. 2.

Depending on the type of powder and chemical composition a more or less pronounced anisotropy (more precise: transversal isotropy) can be observed with respect to the sintering strains. This is shown for two CuCr powders in Fig. 2b and c, where sintering strains in compaction axis differ significantly from the ones in the plane normal to compaction. This effect is accounted for by a general anisotropic formulation of the viscoelastic model described in Ref. [4]. From sintering experiments one can determine the density ρs attainable through sintering (Fig. 2a) for which ρg < ρs = f(ρg) < ρth holds with ρth as the theoretical density of the (pore free) solid material and the sinter density depending on the green density ρg .

The micro-mechanical description of the occuring interparticle interactions is based on a purely pheno- menological model described by Ref. [5] and [6].

IV. NETSHAPE MANUFACTURED CUCR CONTACT

Within this work the nominal chemical composition for the CuCr contact material was chosen to be 75wt.-% Cu and 25wt.-% Cr, an established standard composition for vacuum chamber contacts with a theoretical density of ρth= 8.44g/cm3.

Based on the above described modeling, powder characterisation and sintering tests the pressing tools were designed, the density distribution of the CuCr contact as pressed and as sinterd as well as the shrinkage design were simulated.

Figure 3 shows the modeled design of the pressed CuCr contact. The contact shape is formed by a circular disc with a diameter and height of around 45mm and 9mm, respectively. The outer section is slightly inclined and all edges are rounded. In view of application requirements the part contains 4 radial slots of app.

3mm width each with a half cylindrical base. The formation of the radial slots is established using four core punches.

The die compaction was done on a multilevel press. Individual control of the inner and outer punches allows to uniformly compact the distinct sections of the contact component, thus reducing green density gradients.

The analysis of powder compaction and ejection also requires a mathematical representation of the employed tools. The latter are usually considered as rigid surface representations of the tools’ surfaces in contact to the powder.

Figure 4 shows the finite element model of the fill body and the toolset [6].

The distribution of the achievable relative density of

the green body regarding to the achievable theoretical density can be seen via a quarter model in Fig. 5. Therefore, a slightly lower density of the green body and the sintered material in the area around the center hole, as it results out of the numeric modeling, has to be taken into account.

Fig.3. Simulated distribution of absolute density of the NS manufactured CuCr contact after pressing Fig.5. Simulated relative density distribution in the green

body with respect to the theoretical achievable density

fill-body

upper outer punch

upper inner punch

center punch

lower inner punch

lower outer punch

die core punch

Fig.4. Finite element model of the fill body and tool set

V. RESULTS

Based on the shown simulated density distribution of the green body (see Fig. 5) the pressing parameters were adapted to achieve a minimized density gradient in the center area of the CuCr (75/25) contact.

The netshape pressed green body was sintered by a standard process for a classical PM processed CuCr contact material at a temperature below 1080°C and under reducing atmosphere. The measured sinter shrinkage was found to be in the predicted range. The required accuracy could be fulfilled concerning the requirements on dimensions and planeness.

No cracks or other defects could be observed on the CuCr (75/25) NS contact surface, see Fig. 6.

Some physical and chemical properties as well as the microstructure were investigated with respect to the application of the CuCr contact in a medium voltage circuit breaker. In Table 1 an overview is given about gained results for hardness (HV30), density (principle of Archimedes) and electrical conductivy (eddy current method) measurements, respectively, as well as the gas content analysis (hot gas extraction) of oxygen, nitrogen and hydrogen in the bulk material.

TABLE 1. Comparison of NS and classical PM CuCr contact

Units Netshape

(NS) Classical

(PM) Hardness HV30 72 ± 1.8 81 ± 2.4 Mean Density of 8 segments g/cm3

8.32 ± 0.09 8.33 ± 0.05

Full body density

8.2 ± 0.05 8.2± 0.05

Electrical Conductivity

MS/m 30.3 31.8

Gas Content Oxygen

ppm < 530 < 580

Nitrogen < 100 < 90 Hydrogen < 5 < 5

A comparison between the CuCr (75/25) NS contact

and a PM manufactured CuCr (75/25) contact, which

was in parallel produced by pressing, sintering, chipping and machining can be achieved with the data given in Table 1. The same powder specification was used in both cases, the NS and PM route.

The Vickers hardness measurements were carried out at 16 positions evenly distributed over the whole contact area. The hardness values of the NS contact are about 10 HV30 lower in comparison with the PM contact. As the PM contact was machined after the consolidation process to gain the final contact design, the hardness was increased effected by the deformation of the CuCr material in the surface area during chipping and machining. Earlier observations showed that this difference can be observed when measuring the hardness at the “as sintered” PM contact surface and the machined surface.

The gained data of the full body density measurement ended at 8.2g/cm3 for both production routes.

In particular the density distribution of the NS contact was of special interest as the numerical modeling predicted a lower density in the center area for nominal, i.e., non-optimized press plan. Therefore the contact was cut in 8 small segments (pieces of 0.5 – 1cm3) to be able to measure the density distribution from the outer diameter to the center area.

The density distribution measured from the outer to the inner radius of the contacts showed, surprisingly no significant differences between the NS and PM manufactured contacts. The standard deviation was calculated to be smax.= 0.09 in the case of the NS contact and smax.= 0.05 for the PM contact. It can be pointed out that the adaption of the pressing paramaters being based on the previously simulated density distribution of the green body for the nominal press plan lead to a satisfying homogeneous density distribution over the whole CuCr contact.

The electrical conductivity of CuCr contact material is a quality criterion for the applicability in medium voltage circuit breakers. Herewith the evaluated data were, for both types above 30MS/m, which is a sufficient value for the electrical conductivity.

The amount of entrapped gas in the CuCr bulk material can be critical and cause an inacceptable arcing resistance based on a decrease of the vacuum atmosphere in the vacuum chamber of the circuit breaker after switching. This is why the gas content has to be monitored consequently by CuCr contact component producers.

It was found that the gas content of oxygen, nitrogen and hydrogen was not affected to higher values by the NS manufacturing process versus the classical PM process route as can be seen in Table 1.

Nevertheless, for a successful introduction of NS manufactured CuCr contact material the microstructure, in particular a homogeneous distribution of the Cr phase in the Cu matrix is crucial to increase the electrical strength and the resistance against welding of the contact surfaces during switching operations. The following investigation of the microstructure was

Fig. 6. Net shape manufactured CuCr (75/25) contact

carried out by metallographic cross sections and light microscopy. Herewith the focus was put on the investigation of the microstructure of the NS contact as the microstructure of classical PM contacts is well known. Today evenly distributed Cr particles in the Cu matrix is state of the art and can be figured out in Ref. [1]. The Cr grain size distribution is mainly driven by the inserted Cr powder specification, its grain size distribution and particle shape.

In Fig. 7a,b the micrographs of the microstructure perpendicular and parallel to the pressing direction of the CuCr (75/25) NS contact bulk material are presented.

Looking into the microstructure perpendicular to the pressing direction a tendency can be seen that unidirectional elongated Cr particels are oriented perpendicular to the pressing direction in the Cu matrix. It is obvious that these observed orientation was caused by the die compaction process through further plastic deformation of the Cr particles and the pressing direction.

The Cr particles in the micrograph taken parallel to the pressing direction are evenly distributed and almost no unidirectional shaped Cr particles can be seen (Fig. 7b). It can be mentioned that the investigated NS contact microstructure leads to the conclusion that no significant difference compared to conventionally manufactured can be observed.

As the predicted lower density in the center area could not be confirmed by density measurements a look into the microstructure is of central interest. The micrographs of the CuCr microstructure around the inner edges of the center area of the NS contact is given in Fig. 8. No significant volume of pores can be seen. This obseravtion confirms the overall measured and sufficient density values. By means of the executed adaption of the pressing parameters based on the simulation results the predicted density gradient in the center area could be decreased succesfully.

Only in small regions around the inner edge the Cr particle orientation follows the flux lines caused by the movement and deformation of the initial Cr grains during the die compaction process. It can be said that the Cr particle distribution is more or less homogeneous over the whole contact material. That indicates a comparable material characteristic of the NS contact versus the classical manufactured PM contact material.

To approve this comparability first external performance tests were carried out with a NS manufactured contact surface based on the same material specification as discussed in this work. These performance tests resulted in a surprisingly good manner that the conclusion can be drawn that the introduction of NS manufactured CuCr contacts is an intersting possibility to decrease costs by reducing the inserted material volume as well as avoiding addidional production steps after the sintering process of the NS pressed green body.

VI. SUMMARY AND CONCLUSION

Within this work the influence of powder characteristics and tool design were assessed and a proper adjustment of compaction parameters could be derived and proven by manufacturing a CuCr (75/25) NS contact component for medium voltage circuit breakers in a vacuum chamber.

It could be shown that with the help of finite element

Fig. 8. Microstructure at the inner edge region of the CuCr (75/25) NS contact, Magn. x 50

Fig. 7. a) Micrograph perpendicular and b) parallel to the pressing direction of the CuCr (75/25) NS contact, Magn. x 100

a

b

Cr

Cr

Cu

Cu

based modeling particularly based on constitutive modeling for powder compaction and sintering with corresponding powder characterization by mechanical testing and free sintering experiments the NS production process of CuCr contact components with a non trivial surface shape can be described sufficiently. This allows designing optimized pressing tool geometries and pressing kinematics by specifically linking the density distributions in as pressed and as sintered conditions with sintering shrinkage and distortion.

By NS manufacturing a CuCr (75/25) contact the defined production parameters could be established as the physical properties (density, hardness,…) and chemical composition, in particular the critical gas contents in the NS contact material showed satisfying results in comparison to the specified values for a classical PM contact.

The hardness of the PM contact added up to be around 10HV30 higher in relation to the NS contact, caused by the deformation of the surface area by menas of chipping and machining.

The density measurement results were in the same range of a classical PM manufactured CuCr contact. The density values were found to be ρs-full body= 8.2g/cm3

and mean ρs-segments ~ 8.3g/cm3. The gas content analysis of O2, N2 and H2 showed

reasonable and satisfying low values, on a reasonable level which is known to pass validation tests.

The simulated lower density distribution in the center area of the contact could not be confirmed by density measurements of the NS manufactured prototype using the priciple of Archimedes. Reason therefore is the adaption of the pressing parameters as a reaction on the simulated density distribution, which was carried out based on the initial nominal press plan .

Nevertheless the investigation of porosity by means of a microstructure analysis by light microscopy showed no evidence of a significant pore volume overall the contact material nor a significant pore volume in the critical areas could be observed.

The microstructure showed a homogeneous Cr particle distribution in the bulk material compareable to a PM contact microstructure. In the center area, in regions of the inner edges a few areas could be observed where the Cr particles showed an orientation parallel to flux lines, caused by the powder flux during the pressing process by means of the multi level press. The successful link between the theoretical numerical description and modeling of the material and process parameters and the needed design and the conversion into production could be demonstrated. With the established data base it is possible to design the proper pressing tool and the process parameters to manufacture different types of CuCr contacts e.g. AMF, RMF by means of NS technology.

ACKNOLEDGMENT

The authors thank the engeneering team of PTM in Füssen Germany supporting the project with their expertise in Near netshape and Netshape production as well with their equipment. The authors thank goes also to Ms. Z. Olcay who carried out all measurements and prepared the cross sections and micrographs with her usual accuracy.

REFERENCES

[1] “Copper Chromiun (CuCr) Contact Materials for Vacuum Interrupters”, Technical Information, Plansee 7000764-TI-E018E09.08, Plansee SE, Reutte, Austria, 2008

[2] C. Feist, R. Oberbreyer et al.., „Near netshape manufacturing of CuCr vacuum switching contacts without prototyping“, 17thPlansee Seminar 2009, Reutte, Proceedings, Vol. 3, pp. WS 15/1 – WS15//12.

[3] A. Plankensteiner et al., " Finite Element Based Optimization of a Near Net-Shape Manufacturing Process Chain for CuCr Medium Voltage Circuit Breakers”, PM 2009 Copenhagen, 2009, Vol. 3, pp. 299-304.

[4] K. Korn et al., Proceedings 4th International Conference on Science, Technology and Applications of Sintering, D. Bouvard Ed., Grenoble, 2005, pp. 260-263

[5] H. Riedel, Proceedings Ceramic Powder Science III, G.L. Messing et al. Eds., American Ceramic Society, Westerville/OH, pp. 619-630, (1990).

[6] H. Riedel, and D.-Z. Sun, Numerical Methods in Industrial Forming Processes, Numiform 92, J.-L. Chenot et al. Eds., Balkema, Rotterdam, 1992, pp. 883-886

[7] C. Grohs, “Simulation des “near netshape” Matrizenpressens eines Schaltkontakts aus CuCr”, Unpublished Internal Investigation Report, February 2009, Plansee SE, Austria

E-mail of authors: claudia.kowanda@plansee.com