Forming technology of small parts in semi solid state.

Aisman, David ; Jirkova, Hana ; Masek, Bohuslav 等

1. INTRODUCTION

The principle technology of thixoforming has been known very well for years, but is not often used for treating steels in industry. This is mainly because of the technological complexity of the process. Its main advantage is that complex shapes can be created during one forging operation, which is not possible using conventional technology. It is also possible to obtain excellent surface quality and durability, which minimizes costs for finishing. This technology can be used to process difficult to work and hard to machine materials too. Microstructure after thixoforming is distinctively different from casting micro structure (Cezard & Sourmail, 2008).

The thixotropic behaviour of a material is used when forming metals in the semi-solid state. If material with solid and liquid fractions is exposed to a shearing load, it shows apparent viscosity which vanishes during the beginning of its deformation. The forming force is relatively small compared to the force needed for conventional forming processes, and the temperature of material treatment is lower than for conventional casting processes. This means that the temperature straining of the die is reduced.

The thixoforming process itself usually means heating the material to the temperature between liquid and solid, the forging operation usually carried out in the die cavity, and air cooling.

2. EXPERIMENT

The steel X210Cr12 (Tab.1) was selected for the experiment. This material is difficult to work and hard to machine using conventional methods. The steel is used for cold forging tools and has good abrasion resistance, high compression strength and small deformation during heat treatment (Puttgen et al., 2007). The microstructure consists of a ferrite matrix with globular cementite and primary carbide chrome in annealed state (Fig. 1)

The curve for the dependent fraction of solidus and liquidus for this steel was calculated using the JMatPro program (Fig. 2). The fusing point is 1225[degrees]C. According to numerical calculation, a temperature interval of 1290--1330[degrees]C was found to be suitable for thixoforming. In this temperature interval, around 40-60% of the fluid fraction should be obtained, which is sufficient for achieving thixotropic behaviour (Omar et al., 2009). In order to verify these results, a model was designed proceeding with various heating temperatures between 1200-1320[degrees]C followed by compressive deformation without the use of a die. The selected temperature interval covered the range from the initiation of the fluid fraction to the existence of 60% of the fluid fraction. Strength was measured during deformation and the profile and size of burrs which appear in the middle of the specimen were observed. The specimens were metallographically evaluated.

At 1200[degrees]C, the temperature at which melting initiates, a compressive force higher than 14Mpa had to be achieved. With increased heating temperature, compressive force dropped and at 1320[degrees]C a value of only 5.9Mpa was reached (Fig. 3).To complete the experiment, a tensile test was done at 1200[degrees]C, but the strength for fracture was very low and impossible to measure on ordinary equipment.

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The collected data was used to propose a process and die for thixoforming small semi products. Forming with cross extrusion was chosen. High speed power was used for heating which enabled heating a semi product in the cavity of the mould before the start of the forging process. The temperature was continually controlled by thermocouple and corrected in the control unit. Before a hollow form could be filled, the process had to be corrected several times and its parameters optimized (Rassili et al., 2006)

The metallographic analysis showed us that the microstructure after thixoforming consists of polyhedral austenite grains which are bordered by allotriomorphic carbide structure after deformation. The content of residual austenite was measured at 96 % by x-ray diffraction. The content of ferrite reached 4%. The high content of retained austenite at room temperature is possible through high temperature, high speed and pressure in cooling (Dizllach et al., 2008). The microstructure in the semi-product was homogenous, apart from a dendritic layer on the side burr. Creation of dendrites was caused by sweating of the melt and the different cooling rates at the edge and in the centre of the semi product (Fig.6).

3. CONCLUSION

The results of our work validated the usefulness of thixoforming. It is possible make a product with a complicated topography in one forging operation. The sizes of semi-products were about 20 mm and wall thickness under 2 mm. Tool steel X210Cr12 was used. An appropriate temperature interval for thixoforming was proposed. The temperature interval was used in thixoforming pilot semi-products. The resulting structure contains polyhedral austenite grains and fine carbide mesh. Austenite content was cca. 96% and ferrite content less than 4%.

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4. ACKNOWLEDGEMENTS

This paper includes results obtained within the project 1M06032 Research Centre of Forming Technology.

5. REFERENCES

Cezard, P. & Sourmail T. (2008). Thixoforming of Steel: A state of the Art from an Industrial Point of View, 10th International Conference Semi-Solid Processing of Alloys and Components, Hirt, G. (Ed.), pp 25-35, ISBN 3-90845159-0, Aachen, September 2008, Trans Tech Publications LTD, Zuerich

Dizllach S.; Puttgen W. & Bleck W. (2008). Development of Adapted Heat Treatments for Steels out of the Semi-solid State after Thixoforming, 10th International Conference Semi-Solid Processing of Alloys and Components, Hirt, G. (Ed.), pp 25-35, ISBN 3-908451-59-0, Aachen, September 2008, Trans Tech Publications LTD, Zuerich

Omar, M; Atkinson, H; Howe, A. et al. (2009). Solid-liquid structural break-up in M2 tool steel for semi-solid metal processing. Journal of Material Science, Vol. 44, pp 869-874, ISSN 0022-2461 (Print) 1573-4803 (Online)

Puttgen W.; Hallstedt B.; Bleck W. & Uggowitzer P.J. (2007). On the microstructure formation in chromium steels rapidly cooled from the semi-solid state. Acta Materialia, Vol. 55, No. 3, February 2007, pp 1033-1042, ISSN 1359-6454

Rassili, A; Robelet, M. & Fischer, D.(2006). Thixoforming of carbon steels: Inductive heating and process control, 9h International Conference on Semi-Solid Processing of Alloys and Composites, pp: 717-720, ISBN: 978-3-90845126-6, 2006, Busan, Korea

Tab. 1. Chemical composition of X210Cr12

 C Cr Mn Si Ni P S

 1.9 12 0.35 0.35 max max max
 0.5 0.03 0.035