A novel superplastic dieless drawing process of ceramic tubes

doi:10.1016/j.cirp.2017.04.118

CIRP Annals

Volume 66, Issue 1, 2017, Pages 265-268

https://doi.org/10.1016/j.cirp.2017.04.118 Get rights and content

Abstract

A novel superplastic dieless drawing was performed for fabrication of ceramic tubes. The reduction in area of tube obtained through the drawing was controlled by changing feeding and drawing speeds. Some tested tubes showed extremely low drawing stress of 15 MPa and high value of strain rate sensitivity parameter of 0.42. Furthermore, the maximum reduction in area of 84% was accomplished through single pass drawing. Thus, the ceramic tubes showed a good superplasticity in the proposed process. The conclusion is that the proposed superplastic dieless drawing can be widely used for efficient fabrication of various ceramic tubes.

Introduction

Recently, a compact solid-oxide fuel cell (SOFC) was proposed for down-sizing and higher efficiency of the fuel cell with aim to innovate car driving system [1]. The compact SOFC consists of micro ceramic tubes expecting high performance (see Fig. 1) [2]. These tubes are required to accomplish further miniaturization. Mother ceramic tubes are manufactured through sintering after sharping processes such as powder paste extrusion, powder injection molding or press forming of powder to half tubes and bonding them to round tube. However, their size is not so small, therefore, an appropriate mass production process to reduce their size and make into micro ceramic tubes are required to extend the proposed SOFC.

Normally, the ceramics are not subjected to plastic forming due to hard and brittle characteristics. Meanwhile, the ceramics show superplastic deformation at the high temperature range from 1400 to 1700 °C, reported by Wakai et al. [3]. However, dies and tools cannot be practically safe in such high temperature. Therefore, the ceramics cannot be delivered, in general, to the practical plastic forming processes.

On the other hand, Weiss and Kot proposed the dieless drawing by introducing local heating of metal bars [4]. The authors proposed a superplastic dieless drawing for fabrication of microtubes and the effectiveness of the process was demonstrated for manufacturing superplastic metal tubes [5], [6], [7].

In this study, the authors intend to propose a novel superplastic dieless drawing for effective and expectable mass manufacturing of micro ceramic tubes SOFC used in fuel cell car. For this final goal, a superplastic dieless drawing apparatus with acetylene burner for obtaining ultra-high temperature over 1700 °C has already designed for 3Y-TZP tubes in our previous study [8]. The performance of designed apparatus such as load and temperature distribution measurements in the dieless drawing were evaluated. However, detailed deformation behaviour of zirconia ceramic tubes in the superplastic dieless drawing has not been clarified adequately.

In this paper, the fundamental deformation behaviour of zirconia ceramic tubes were investigated in the superplastic dieless drawing by using designed apparatus. Geometry of cross-section and surface roughness of drawn tubes and necessary drawing stress were made clear in a single pass drawing. The effectiveness of the proposed drawing method was verified. Furthermore, the possibility of manufacturing micro zirconia ceramic tube which could be applied to SOFC was demonstrated by applying the multi-pass drawing.

Section snippets

Basic formula of superplastic dieless drawing

The superplastic dieless drawing as shown in Fig. 2 was performed by a combination of local heating and drawing, that is tensile elongation. The local heating zone is fixed and the tube moves through the heating zone. Concurrently, the tube was subjected to tensile force and resulted elongation due to the difference in speed between drawing V₁ and feeding V₂. The reduction in area R is expressed by $R = 1 - A_{2} / A_{1} = 1 - V_{2} / V_{1},$

based on the volume constancy law, where A₁ and A₂ are initial and deformed

Material and experimental procedure

In this study, as a representative superplastic ceramic, 3Y-TZP was employed and its tubes with outer and inner diameter of D = 6 mm and d = 4 mm were subjected to the experiment. Fig. 3 shows the schematic illustration of the proposed superplastic dieless drawing designed by authors [8]. As a heating source, an acetylene burner was introduced to obtain ultra-high temperature over 1500 °C. This heating method was effective for achieving superplastic deformation of 3Y-TZP tubes. The heating temperature

Fundamental features of superplastc dieless drawing

Fig. 4 shows the deformation features of 3Y-TZP tubes captured by a camera. The deformation mode changes from initial non-steady deformation to steady states deformation during drawing. There are the results obtained under the condition of V₂ = 0.05 mm/s and reduction in area of 70.5%. As shown in the figures, the outer diameter of the tube is reducing gradually during non-steady stage (Fig. 4(b)). In the steady stage from Fig. 4(c) to (d) (final stage), the deformation zone becomes stable. Thus,

Conclusion

In this paper, fundamental deformation features of 3Y-TZP tubes for next generation SOFC in a novel dieless drawing process were investigated. The following results were obtained. (1) The reduction in area R and/or the outer diameter of drawn tubes is controlled by changing feeding and drawing speeds. (2) The smooth surface of drawn tubes can be obtained by introducing fine grained of mother tubes. (3) Extremely low tensile stress is enough for actual drawing. (4) High value of strain rate

Acknowledgement

This study was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) Grant number 24560888.

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