Advances in Science and Technology Online: 2008-09-02 ISSN: 1662-0356, Vol. 56, pp 170-175 doi:10.4028/www.scientific.net/ast.56.170 2008 Trans Tech Publications, Switzerland Integration of Piezoceramic Modules into Die Castings Procedure and Functionalities Matthias Rübner 1, a, Carolin Körner 1, b, Robert F. Singer 1, c 1 Institute of Science and Technology of Metals (WTM), University of Erlangen, Germany a matthias.ruebner@ww.uni-erlangen.de, b carolin.koerner@ww.uni-erlangen.de, c robert.singer@ww.uni-erlangen.de Keywords: Integration, high pressure die casting, piezoceramic sensor/actuator-module Abstract The complete integration of piezoceramic sensor/actuator-modules into metal components using high pressure die casting is a promising approach for the fabrication of multifunctional structural elements with enhanced properties. A technique providing stabilization and protection of the module during the highly dynamic mould filling is presented. Demonstration parts are produced which are fully capable to detect vibrations. An approach to characterize this sensory functionality of the adaptronic system is presented. Introduction Lightweight construction is a trend to save weight and hence to reduce fuel consumption. The use of light metals like aluminium or magnesium is one possibility to realize weight reduction. Unfortunately, this results in a vibration and noise increase [1] of light metal parts. The combination of functional modules, e.g. piezoceramic sensor/actuator-modules, and structural components is one opportunity to create parts with the ability of sensing and reducing vibrations [2]. A really simple way to connect sensor/actuator-modules with structural components is to bond them with an adhesive layer onto the surface of the component [3, 4]. However, there are several disadvantages using this method. In a rough environment as it appears in automotive applications, the module needs to be protected against external influences like humidity or foreign object damage. The state of the adhesive layer is also subjected by environmental influences, e. g. the temperature and the adhesive bonding is an additional production step which increases costs. In this contribution a new method for the integration of sensor/actuator-modules into light metal structural components is presented. The technology is based on high pressure die casting, which is established for mass production of light metal parts. The integration of the sensor/actuator-module proceeds during the part fabrication and no additional processing step is required. The modules basically consisting of a piezoceramic plate embedded in polyimide are commercially available. Amazingly, the polymer insulation is completely intact after the integration process although it got in contact with the liquid metal. In addition, the sensory functionality is still remained. Fabrication process High pressure die casting involves the injection of molten metal into a permanent mould at high pressure and velocity. Due the short cycle time it is the most commonly used process for mass production of thin walled light metal components requiring high dimensional accuracy and surface quality. Figure 1 shows schematically the procedure of high pressure die casting. First, the molten metal with a temperature between 600 C and 740 C is filled from an external crucible into a shot chamber using a feeding system. Secondly, the liquid metal is pressed into the mould cavity by moving the plunger with a velocity between 1 m/s and 6 m/s. Because of the small cross section at All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#69832669, Pennsylvania State University, University Park, USA-19/09/16,08:23:56)
Advances in Science and Technology Vol. 56 171 the gate the melt is accelerated to a velocity of far above 100 m/s. After the die is filled in less than 50 ms a dwell pressure of several 100 bar is applied to reduce porosity and compensate shrinkage [5]. At last removing from the mould takes place after less than 60 s depending on the component geometry. Fig. 1: Scheme of a die casting machine. The plunger injects the liquid metal with high pressure and velocity into a permanent mould. The die casting unit FRECH DAK 450-54 with a locking pressure of 450 tons is used for the experimental work. In addition, a vacuum system is integrated to minimize the air pressure in the cavity in order to reduce gas porosity in the casting [6]. For the experiments quadratic components are produced with an edge length of 178 mm and a thickness of 3.5 mm. They consists of the standard aluminium die casting alloy 226D (AlSi9Cu3(Fe)). The used sensor-/actuator-modules are piezoelectric d 31 patch transducers (brand name: DurAct, distributor: PI Ceramic GmbH, Lederhose, Germany) which are commercially available (figure 2). The main element is an electroded monolithic PZT plate of 28 x 14 x 0.2 mm 3. The electrical connection guarantees a flexible copper mesh on both sides of the ceramic. For thermal and electrical insulation the ceramic as well as the electrical connection are fully embedded into a polyimide matrix. Additionally, the polymer structure acts as mechanical stabilizer for the ceramic. Fig. 2: Schematic assembly of a piezoceramic sensor-/actuator-module (left) [7]. The electroded ceramic together with the electrical connection is completely embedded into polyimide. Macroscopic view of a commercially available sensor-/actuator-module (right). Support technique The temperature of the liquid metal and the dynamic die filling effectuate a high mechanical and thermal load to the module. Therefore, the integration of sensor/actuator-modules can only be realized by using a supporting structure to avoid damage. There are two main requirements to the supporting structure. First, the modules need to be fixed in the cavity without any direct contact to the die wall, because a complete encasing with aluminium is
172 Emboding Intelligence in Structures and Integrated Systems desired. Secondly, positioning of the modules outside the neutral axis of the cast component needs to be possible for adaptronic purposes. Our approach to secure the modules against the loads in the die cavity is to surround them with expanded metal consisting of pure aluminium [8]. During die filling, the melt penetrates the expanded metal on both sides without deforming it. Thus, the modules are completely encased by the metal and kept at a certain position. The concept is shown in figure 3. Fig. 3: The package of the sensor/actuator-module and the surrounding expanded metal is applied onto ejector pins in the opened die. The contacts shear off when the die is closed. For holding the package consisting of module and expanded metal in the die cavity it is attached to two ejector pins protruding into the die cavity. If the package thickness is marginal larger than the cavity thickness, an additional clamping force appears after the die is closed which contributes to the fixing of the package. An excentric placement of the sensor/actuator-modules can be realized by using geometric different types of expanded metal. The thickness of these types defines the distance between the die wall and the module. Figure 4 shows schematically the arrangement for centric and excentric positioning of the module. As expanded metal is commercially offered in a multiplicity of geometries, this technique can be adopted to a large variety of thin walled castings. Fig 4: Scheme of the support technique for centric (left) and excentric (right) placement of the module. By using geometrical different types of expanded metal on both sides of the module the excentric placement can be realized. For the experiments discussed in this paper 3.5 mm thick castings with centric (a=b=1.5 mm) and excentric (a=2 mm, b=1 mm) placement of the module are produced. As reference samples act castings with modules bonded onto the surface. Module integration In figure 5 a section of a casting with integrated module is shown. For contacting the module a small range of the casting edge has to be dismantled to lay open the connecting wires. The expanded metal is completely infiltrated with aluminium. On the surface just areas are visible where the expanded metal is in contact with the die wall during the integration process.
Advances in Science and Technology Vol. 56 173 Fig. 5: Cast part (thickness: 3.5 mm) with integrated sensor/actuator-module. The module is integrated in the indicated area. The non-destructive testing by x-raying an integrated module (figure 6) with an ndt-analyser of the phoenix x-ray company shows, that the contacts with the soldered wires are still interconnected with the PZT-plate by the flexible copper mesh. The dark spots in the area of the contacts are spillings of the soldering joints. The time of appearance of these spillings is not known up to now. Fig. 6: X-ray images of a casting with integrated module. In spite of the high mechanical load the PZT-ceramic is still in a good order. The right image shows the contacts and the flexible coppermesh with spillings of melted solder. The cross section of the active area and the connecting wire in the integrated state is shown in figure 7. No damage of the module is viewable. For insulation the wire is also surrounded by two or three layers of a thin polyimide film. Fig. 7: Cross section of an active area and a connection wire of an integrated module. No damage of the thermal and electrical polyimide (PI) insulation is visible. Surprisingly, the polyimide encapsulation for both ceramic and wire is still retained after the integration. Electrical measurements confirm the preservation of the electrical insulation of modules and connecting wires against the metal matrix. A dielectric breakdown can not be provoked by
174 Emboding Intelligence in Structures and Integrated Systems applying the maximum operation voltage of 400 V. This is precondition for the functionality because of the electrical conductivity of the metal matrix. Sensory functionality For measurement of the sensory functionality of the integrated sensor/actuator-modules the castings are fixed in a clamping unit at one end and excited to vibration by an impulse added at the free end of the cast parts. The measuring setup and the developing of the voltage signal versus time are shown in figure 8. The measurement is performed without repolarisation of the modules after the integration process. Fig. 8: Measuring setup and resulting voltage signal for analysing the sensory functionality of an integrated module. A variety of small mechanical impulses with an excitation time of 100 ms is applied. The first peak of the voltage signal represents the first bending of the casting. The maximum of this peak is taken as parameter for the evaluation of the sensory functionality depending on the placement of the module inside the component. The effects of impulse variation and position of the module are shown in figure 9. Fig 9: Sensory functionality of the structural components with the integrated modules depending on impulse strength and module placement (bonded: lever arm = 2 mm, excentric: lever arm 0.5 mm, centric: lever arm 0 mm).
Advances in Science and Technology Vol. 56 175 A linear correlation between strength of impulse and voltage signal for both bonded onto the surface and integrated is visible. An impulse increase about 20% results in a 20% increase of the voltage signal. The excentric placed modules generate a voltage which is several times higher than the voltage of the centric placed modules. Due to the parallel developing an equation for the estimation of the real lever arm of the integrated modules x integrated can be formed x integrated U integrated = x bonded. (1) U bonded x bonded : lever arm of the bonded module, U integrated : voltage signal of the integrated module, U bonded : voltage signal of the integrated module. With this equation a lever arm of 0.55 mm for the excentric placed modules can be calculated (theoretic: 0.5 mm). As the centric integrated modules also generate a small voltage signal it s possible to estimate a lever arm of 0.08 mm. Hence, an exact positing of the modules, e.g. centric is almost impossible because the packages consisting of module and expanded metal are currently produced manually. Consequently, the maximum deviation from the desired module placement can be mentioned as round about ±0.1 mm. Conclusion High pressure die casting is a suitable process for the integration of piezoceramic sensor/actuatormodules into light metal structural components. The development of an adapted support technique using expanded metal consisting of aluminium ensures a complete integration of the modules and provides the possibility of a module placement outside the neutral axis. Astonishingly, commercially available sensor/actuator-modules with their polyimide embedding are still electrical insulated after the integration process. The sensory functionality is still remained in the integrated state without repolarisation of the PZT-ceramic. However, an excentric placement of the module is important to get a significant voltage signal. Acknowledgement The authors thank the German Research Foundation (DFG) for its financial support within the transregional collaborative research center SFB/TR 39 production technologies for light metal and fiber reinforced composite based components with integrated piezoceramic sensors and actuators. References [1] Y.H. Guan, T.C. Lim, W.S. Shepard: Jour. of Sound and Vibration Vol. 282 (2005) p. 713-733 [2] J.-C. Lin, M. H. Nien: Jour. of Mater. Processing Technology Vol. 189 (2007) p. 231-236 [3] G. Caruso, S. Galeani, L. Menini: Simul. Model. Practice and Theory Vol. 11 (2003) p. 403-419 [4] K.Y. Kim et al.: Sensors and Actuators A Vol. 120 (2005) p. 123-129 [5] M.S. Dargusch et al.: Jour.of Mater. Processing Technology Vol. 180 (2006) p. 37-43 [6] X. P. Niu, B.H. Hu, I. Pinwill, H. Li: Jour. of Mater. Process. Tech. Vol. 105 (2000) 119-127 [7] Invent GmbH: data sheet DurAct type P1, Braunschweig, 2007 [8] Patent application DE 10 2005 016 402.1-24, 2005