Ceramic dies for hot extrusion of NF metals (Reprint)

• T Böhm, Didier Werke AG, Wiesbaden, Germany
• BH Neimeier, Zircoa, Inc., Solon, Ohio, USA

Partially stabilized zirconia provides the required hot strength together with high deformation resistance experienced during hot extrusion.

Conventional extrusion die materials are given a brief examination before fully explaining zirconia ceramics. The nature of this ceramic, its chemical and physical properties together with applications in the extrusion of nonferrous metals, is thoroughly presented.

Hot extrusion

By using the hot extrusion process it is possible to reach the final product dimensions using as few process steps as possible. Thus the required number of cold drawing and reheating operations can be minimized.

The dimensional accuracy of the extruded product using a given tool design is influenced by the billet temperature, the reduction ratio and the lubrication.

In practice an adjustment of the extrusion parameters to control the required tolerances is not possible because the dimensions of the extruded product vary along its length due to unstable ram speed and deformation of the die. These are in turn caused by high thermo-mechanical stresses.

Usually a cold drawing operation (calibration) is applied afterwards to achieve closer dimensional tolerances. [1]

Conventional extrusion die materials

During hot extrusion the dies are subjected to very high thermo-mechanical stresses such as pressure and temperature.

Conventional materials for extrusion dies are traditional tool steels or hot working steels (even stellites) which suffer a considerable, permanent deformation during use.

Poor dimensional accuracy and unacceptable surface finish of the product -- even when using lubricants -- results. Metal dies hence require frequent reworking; therefore costs are increased by: [2]

• Additional press downtime
• Larger die inventories
• Short die life

Thus, an extrusion die material is required to have high strength and good thermal stability, to retain its hardness at high pressures and temperatures, and enable the manufacture of products with improved surface finish and consistency in product dimensions.

These requirements are fulfilled by partially stabilized zirconia (PSZ). [3]

Introduction to zirconia-ceramic

Depending on temperature and pressure, zirconia can be present in three different crystalline forms as indicated in Figure I.

Figure 1. Crystal forms of zirconia

The monoclinic phase is present at room temperature and, as the temperature increases, the crystal structure changes, first to tetragonal and then to the cubic high temperature form.

This t → m phase transformation takes place without diffusion and change in chemical composition, which means a martensitic transformation as is well-known in steel (Fe - C phase diagram) [4] by rapid cooling or quenching:

Table 1. Properties of Zircoa-ceramic extrusion dies

 

It is only by the addition of certain amounts of stabilizers, such as the alkaline earths, MgO and CaO, as well as rare earth oxides (Y₂0₃, etc), and using appropriate conditions during manufacture, that it is possible to transform a part of the microstructure into the metastable cubic form.

The resulting lattice also shows some tetragonal crystal phase remaining, transformable between monoclinic and tetragonal above and below the phase change temperature respectively.

If the martensitic transformation of a crystal is restrained at the given thermodynamic temperature by a constraint of the matrix, this crystal remains metastable. [5]

Further addition of stabilizer (10 - 15 mol%) results in a 100% cubic phase at room temperature. Such a fully stabilized zirconia not only exhibits high electric conductivity but also low strength and poor thermal shock resistance.

The strength and fracture toughness that PSZ-ceramics exhibit depends on the transformation potential of the metastable tetragonal zirconia particles, specifically on their size and on the temperature curve during sintering.

The volume expansion of particles transforming from tetragonal to monoclinic is used positively in PSZ to increase strength and fracture toughness, [6] and transformation toughening by microcracking. [7, 8]

Microcracking

By cooling down from the sintering temperature to below the transformation temperature the transformation t → m takes place with a high volume expansion.

Due to good dispersion, only non-critical micro-cracks develop around the tetragonal particles in the cubic matrix.

During application of stress and temperature, the PSZ-ceramic seeks to reduce stresses by dissipating the elastic energy in microcracking and branching of a propagating crack, Figure 2.

This results in an improved fracture toughness (K_1C factor).

Figure 2. Microcracking

Properties of PSZ-ceramics

Two different PSZ-ceramics, commercially known as Zircoa Composition 2016 and Composition 5027, will be described, as shown in Table 1.

The material Composition 2016 has relatively good thermal shock resistance due to the evenly distributed monoclinic content of 40-50% between inter- and intracrystalline sites.

The monoclinic phase content of the material Composition 5027 is precipitated mainly on grain boundaries. Strength and fracture toughness are increased, but the thermal shock resistance in the region of application temperature is reduced.

Application of ceramic extrusion dies in PSZ

Each application requires consideration of the fact that ceramics can only withstand limited tensile and shear stresses.

The introduction of PSZ-ceramic extrusion die material may be described by a typical "learning curve". The potential of this material cannot be totally exploited until ceramics and their characteristics are familiar and production adjustments have been made.

The successful use of ceramic extrusion dies very much depends on the manufacturing personnel. The correct -- i.e. adjusted to ceramics -- shrink-fit, careful handling, and appropriate press operations are critical. The resulting optimization of product and process may be as follows:

If possible, the press should run on a three-tool cycle or, circumstances permitting, cycling only two dies alternately. If a ceramic extrusion die is not cycled, premature failure of the whole assembly (ceramic extrusion die and metal die holder) will result.

Excessive thermal load on the metal die holder drastically decreases the strength of the hot working steel and results in a warped steel case. The compressive shrink-fit stress on the ceramic extrusion die is therefore lost.

The cycling of four dies is possible if the extrusion speeds are fast, otherwise the temperature drop between each push can subject the ceramic to thermal shock. [9]

An abrasive fiber cloth can be used to reduce the amount of NF-oxides built up on the ceramic die surface. If necessary it can be used after each push.

Only very good control of the process allows the employment of PSZ-ceramic extrusion dies to their full potential. The prevention of water flow-back against the ceramic die is also of great importance.

Table 2. Case study information

 

Use for copper and copper-alloys

PSZ-ceramic extrusion dies have proved their usefulness for the hot extrusion of:

Copper:		SF-Cu, E-Cu, OF-Cu
and
Copper alloys:		brass CuZn (MS58), argentan CuNiZn and CuNi
as well as precious metal alloys.

Using the hot extrusion process, tubes and rods are formed. Round cross-sections, hexagonal, square cross-sections and more complicated forms are also made.

Application advantages of ceramic extrusion dies in PSZ

Using PSZ-ceramic as an extrusion die material results in the following technical and economic advantages, and is also described in Table 2.

PSZ is not wetted by nearly all extruded NF-alloys and thus exhibits a good lubricity, even without an added lubricant.

With a 50% lower coefficient of friction compared to hot working steel, it is possible to increase the ram speed.

The non-abrasive surface and highly polished surface of PSZ leads to improved surface quality of the extruded product together with prolonged life of the ceramic extrusion die as well as the metal casing.

Unlike conventional metal dies, PSZ-ceramic dies do not require re-machining.

As PSZ has a lower thermal conductivity than hot working steel, Table 3, less heat is transmitted to the metal die holder. Therefore the metal casing can retain its hardness longer and deformation is limited.

 

Table 3. Comparison of thermal conductivity

 

In case study A, Table 2, permanent control was implemented at the press to monitor tube surfaces and direct die changes when the surface finish began to deteriorate. As a result of the successful introduction of Zircoa-extrusion dies an increase in life of 100% compared to metal dies (stellite Co-based) together with a higher surface quality of the extruded product resulted.

This calculation includes two to three reworks of the metal dies in the machine shop. If the reduction of the reject rate of the final inspection (heat exchanger) is taken into account, a further significant cost saving is evident.[2]

Metal extrusion dies tend to "turn in" due to the high stresses during hot extrusion and therefore the outer diameter of the extruded shell becomes slightly smaller. The wall thickness of the shell gradually decreases and the length increases. Even after removal and opening up of the metal dies after the first two or three pushes and then thirty pushes afterwards, this cannot be avoided.

In case study B, such behavior was observed for four metal dies in rotation. Each extrusion was different, length could vary by 6%.

In contrast PSZ-ceramic extrusion dies gave a very consistent outer diameter and the extrusion lengths were almost exactly the same.

The surface finish of the extrusions tended to be brighter and smoother, but the difference in the reduction of extrusion flaws and the improvement in drawability for the first draw was not very significant.

A cost analysis, Table 4, in 1990 indicated a clear economic advantage in favor of PSZ-ceramic extrusion dies. PSZ-extrusion dies are ~ 20% more expensive but provide 33.4% more pushes than metal extrusion dies. This resulted in an annual saving of US$10,000.

However, the greatest saving was in the reduced amount of machine shop work required, as metal dies require final machining and frequent rework during their life cycle, whereas ceramic dies do not.

A machine shop labor saving of 3,351.2 man hours/year was achieved, resulting in a further annual cost reduction of US$83,780.[9]

Conclusions

Due to its unique structure, PSZ-ceramic has the ideal combination of properties:

• Excellent thermal stability
• Good thermal shock resistance
• Low thermal expansion coefficient
• Reduced thermal conductivity
• High fracture toughness

These are required to withstand the demanding conditions during use as an extrusion die material for hot extrusion without showing the brittle fracture behavior typical of other ceramics.

This results in a reduction of the operational costs due to increased productivity by:

• Increased life of the ceramic extrusion die
• Lower costs for reworking
• Improved surface finish and
• Better dimensional control of the extruded product

Table 4. Cost analysis: machine shop saving

 

This combination of properties makes it possible to extrude larger billets and/or to increase the ram speed, resulting in an increase in productivity of the hot extrusion process.

Experience shows the importance of close cooperation between the extruder and the ceramic die and metal casing manufacturers.

References

[1]	Moik M, Rethmann W, Einfluß der Preβparameter auf die Eigenschaften von Produkten aus Kupfer und Kupferlegierungen, Vortragsband Symposium Strangpressen DGM Eschhorn 1970.
[2]	Gulati S T, Hansson J N, Helfinstine J D, Zirconium Oxide: A new die material for hot extrusion, Metal Progress, February 1984.
[3]	Gulati ST, Hansson J N, Helfinstine J D, Marlarkey CJ, Ceramic dies for hot metal extrusion, Tube International, March & June 1985.
[4]	Horstmann D, Das Zustandsbild Fe-C, Verlag StahlEisen mbH, Düsseldorf 2 Auflage 1961.
[5]	Stevens R, An introduction to zirconia and zirconia ceramics, Magnesium Elektron Ltd. 1986.
[6]	Claussen N, Umwandlungsverstärkte Keramische Werkstoffe, Z Werkstofftechn. 13, 138 - 147, 185 - 196, l982.
[7]	Garvie R C, Hannik R H, Pascoe R T, Ceramic Steel?, Nature 258, 703 - 704, 1975.
[8]	Claussen N, Erhöhung des Riβwiderstandes von Keramiken durch gezielt eingebrachte Mikrorisse, Ber. Dtsch. Keram. Ges. 54, 420 - 423, 1977.
[9]	Buchanan T, Conreaux R E, Hintz J J, An experience using ceramic dies in the extrusion of copper tubes, Conference on Emerging methods for the production of tubes, bars and shapes, 1992.

Didier Werke AG, Specialty Ceramics, Abraham-Lincoln-Str.1, D-65189
Wiesbaden Germany (Fax +49-611-7335-558)

Zircoa, Inc., 31501 Solon Road, Solon, OH 44139,
USA (Fax +1 216 349 7209)

The above appeared in Tube International, January 1996. Reprinted with permission of Mack Brooks Publishing Ltd., Forum Place, Hatfield Herts, UK.

For more detailed information on available compositions, chemical analysis and physical properties, please visit our Ceramic Extrusion Dies Homepage (www.zircoa.com/dies).

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