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Int J Adv Manuf Technol DOI 10.1007/s00170-009-2141-5

SPECIAL ISSUE - ORIGINAL ARTICLE

Investigation into reduction of die-cavity deflection in micro-hydroforming processes using FEA C. Hartl & G. Anyasodor & T. Ptaschlik & J. Lungershausen & S. Lippert

Received: 20 March 2009 / Accepted: 19 May 2009 # Springer-Verlag London Limited 2009

Abstract Within the field of micro-technology, merchandised products as well as research activities show an important demand for complex-shaped tubular microcomponents, for example, for medical devices or microfluidic applications. Concerning such micro-components made from metal materials, manufacturing techniques for the economic mass production of adequate tubular parts are often missing. Hydroforming, as a proven technology in the mass production of conventional-size components, offers miscellaneous advantages also for micro-part manufacture. However, due to the comparatively large forming loads involved, strategies for compensation of the elastic deflection of the forming tool elements resulting from these loads are particularly of interest when greater accuracy of the forming operation is required. Against this background, this paper presents a strategy to reduce elastic tool deflection in micro-hydroforming processes, verified by systematic finite element simulations. Keywords Micro-hydroforming . Micro-forming tools . Elastic tool deflection

1 Introduction The increasing integration of micro-system technology into modern products for electronics, telecom and medical devices, for an ever-growing market, requires efficient and time-saving production methods for micro-components. C. Hartl (*) : G. Anyasodor : T. Ptaschlik : J. Lungershausen : S. Lippert Cologne University of Applied Sciences, Betzdorfer Str. 2, 50679 Cologne, Germany e-mail: [email protected]

Concerning micro-components made from metal materials, the forming technology plays a decisive role in the mass production of these parts, as it can offer the demanded productivity and accuracy. For the mass production of hollow-shaped micro-components, this applies to hydroforming processes in cases where the manufacturing of such parts currently still relies largely on time-consuming techniques based on the removal of material, either by chemical or mechanical means. Merchandised products as well as research activities show an important demand for tubular micro-components. In addition, ongoing research projects indicate new innovative micro-products when the economic mass-produced of tubular components becomes available: a capacious market is represented by medical engineering. Elements for micro-fluidic chips, micro-needles and implanted microtubes for drug delivery are only a few examples. Nonmedical applications of micro-tubes concern, for example, elements for micro-heat exchangers, fluidic sensors and shafts for micro-motors or cameras. During the past few years, hydroforming has achieved a status where it permits the economic mass production of high-quality macro-components, e.g. for automotive or sanitary applications [1]. Complex-shaped automotive space frame components made from aluminium alloys [2– 5], stainless steel exhaust system elements [6] and chassis parts made from low-carbon steel [7, 8] are only some examples of the use of hydroforming technology in series production at the present time. Major advantages of hydroforming, which have led to this intensive application, consist in the possibility to form hollow complex-shaped components with integrated structures from single tubes, combined with improvements in stiffness and strength characteristics due to the reduction of welding seams and with reduced assembly costs [1].

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Recent experimental investigations with the overall objective of apply hydroforming to the mass production of micro-parts have already verified the feasibility of microforming processes [9] (Fig. 1) and delivered a first basis for adequate machine and tool design [10]. Within the context of this research work, investigations into measures for the optimisation of the accuracy of the parts produced accuracy are of utmost importance. Hydroforming is based on the application of pressurised liquid media to generate a defined workpiece shape from tubular materials with required pressures of up to 4,000 bar. Due to the comparatively high-forming loads, strategies for the compensation of elastic deflection of the forming-tool elements resulting from these loads are particularly of interest when high accuracy of the formation is required. However, when parts are scaled down to miniature-/ micro-size, it must be pointed out that the scaling process must account for the so-called size effects. These effects are defined as the change in the component or process behaviour when the geometrical dimensions are reduced whilst maintaining the overall shape [11]. Concerning the hydroforming of micro-tubes, the abovementioned influence of elastic tool deflection on the accuracy of the part produced is superimposed on, additionally, by the influence of size effects on the accuracy and repeatability of the forming process. These effects result, amongst others, from the low ratio of the tube wall thickness to the material grain diameter, as investigated in [12, 13] by theoretical means. In contrast to the research works on the influence of size effects on the accuracy of the micro-hydroformed components produced, currently, no investigations on the effect of elastic tool deflection exist as well as on measures to affect

it. Against this background, a strategy for control of diecavity deflection based on adjusted tool design and process parameters was developed making use of finite element analysis (FEA).

2 Process and tool design Figure 2 represents the principle of hydroforming processes. At the beginning of the process, the base tube is placed into a die cavity, which corresponds to the final shape of the component. The dies are closed with the closing force Fc, whilst the tube is internally pressurised by a liquid medium with the internal pressure pi being sufficient to effect the expansion of the component. The tube ends are compressed axially by sealing punches with an axial force Fa to seal the workpiece and to force material into the die cavity. The component is formed under the simultaneously controlled action of internal pressure and axial force. The industrial production of hydroforming parts requires, in general, several additional manufacturing steps besides the hydroforming process itself, depending on the component design [1]. A typical process chain for the manufacture of hydroformed components is shown schematically in Fig. 3. A decisive factor for the magnitude of the elastic tool deflection is the level of applied internal pressure pi. In general, at the end of the process, the internal pressure is increased up to the calibration pressure pk to form the tube wall into the corner radii of the die cavity, which area may not have been formed during the main expansion of the workpiece. The necessary level of internal pressure is influenced significantly by the local yield stress σY of workpiece and the wall thickness t of the formed component, as well as by the component shape, along with the minimum corner radii rc of the die cavity resulting from this. Miscellaneous conditional equations are available for the determination of pk, e.g. [14–16], predominantly based on the correlation: pk ¼ f ðs Y ; t; rc Þ

Fig. 1 Micro-hydroformed prototype parts (material: AISI 304, base tube outer diameter, 0.8 mm; wall thickness, 0.04 mm)

Fig. 2 Principle of the hydroforming process

ð1Þ

Int J Adv Manuf Technol Fig. 3 Typical process chain for hydroforming part manufacture

The level of applied internal pressure affects, besides the sealing force Fa, particularly the required force Fc to close the top and bottom die during the forming process. Commonly, the opening and closing of the hydroforming tool is implemented by a press [1, 10], predominantly hydraulically driven. The minimum closing force required, which has to be applied by this press to avoid the loss of contact between the two die halves during the forming process, can be estimated according to [4, 16] with: Fc min ¼ pi Ap

ð2Þ

with the projected area of the formed component Ap being perpendicular to the closing direction. It is obvious from this correlation that the greater is the internal pressure pi, the greater is the required force Fc. Hence, in addition to the deflection caused by pi, the deflection due to Fc has to be taken into consideration. A schematic drawing and an example of microhydroforming tooling for mass production with its essential components are represented in Figs. 4 and 5. The tool inserts, which contain the die cavity, are located within the basic tool blocks. Adjusting plates, covering the contact areas between the basic tool blocks and the integrated tool inserts, serve to adjust the correct position of the die-insert elements to each other and to the axis of the sealing punches. The distribution of the contact pressure between the top and bottom die, induced by the closing force, is commonly adjusted by matching plates, which are arranged around the joint face of the die cavity. It is known from practical experience that the elastic tool deflection in macro-hydroforming processes caused by the acting loads can be about 0.5 mm [17]. Adjustments in the design of the forming die cavities enables a reduction in errors resulting from this factor [18] but only within certain

Fig. 4 Schematic built up of a micro-hydroforming tool cross-section (guiding and ejectors are not shown here)

limits. Scatter in the process loads as well as in the tube dimensions and material characteristics induces additional unsteady deviations in the dimensions of the hydroformed part. Typical accuracies for conventionally hydroformed macro-parts are within the range of 0.1 up to about 0.6 mm as reported in [17] for the example of a hydroformed subrame made from a tube with an outer diameter of 52 mm. Details of the elastic deformation behaviour of the cavities of hydroforming dies under load are shown schematically in Fig. 6a and b when—apart from controlling the amount of closing force Fc—no further measures for the reduction of deflection are made. However, it is obviously that only the deflection in the direction of the closing force can be influenced through this. A fundamental idea to control the deflection behaviour perpendicular to the closing direction consists in the superposition of bending stresses within the tool. This can be achieved, for example, by an adapted design of the joint face between the top and bottom tool (parameter b) as well as of the matching plates (parameter a), as shown schematically in Fig. 6c.

3 FE model and simulation results An analysis was carried out for an example cross-section of the micro-hydroforming tool shown in Fig. 5, which was developed for the forming of a cylindrical camera shaft. This selected section represented a die-cavity diameter of d= 1.04 mm, where a tube made from stainless-steel AISI 304 in the soft-annealed condition, with an initial diameter of 0.8 mm and a wall thickness of 0.04 mm, is expanded. An internal pressure pi of 4,000 bar was applied in the

Fig. 5 Example of a micro-hydroforming die half

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Fig. 6 Elastic deformation behaviour of hydroforming tools, schematically (a excessive level of pi, b excessive level of Fc; c superposition of bending stresses)

simulations, according to the maximum value of the pressure required in initial experiments. The commercial system Abaqus was used to perform FE simulations. Figure 7 shows the prepared half-symmetrical model of the tool containing approximately 5,000 elements. Fournode bilinear, reduced integration plane strain elements (CPE4R) were selected, assuming elastic, isotropic material behaviour, with a Young’s modulus of 216,000 MPa and a Poisson’s ratio of 0.3. Contact between the top and the bottom tool was specified by using the surface-tosurface contact capabilities of the software to consider compressive-stress transmission between the tool halves as well as possible local loss of the contact. The tool and machine components bordering to the here-analysed model were assumed to be rigid in order to simplify the simulation. The level of pressure acting normally to the inner surface of the die cavity due to the contact with the expanded and pressurised tube was assumed to be identical with the internal forming pressure pi: The difference between the contact pressure and pi due to the tube wall was treated as negligible. Based on this model, FE simulations with systematic variations of closing force Fc and tool parameters were carried out to investigate the applicability of the herepresented idea of tool-deflection reduction by the superposition of bending stresses. The width of the joint face b and the amount of bending deformation, defined by the gap a, have shown to be decisive tool parameters. Measured values to evaluate the influence of these parameters were the deformed cavity diameters in the x- and y-directions, dx and dy, as well as the gap of the joint face e arising from the cavity deflection (see also Fig. 6). Figure 8 shows the result of the investigations conducted, developed from 72 single computations, from

which it appears that without the superposition of bending stresses (a=0), neither dx nor dy could be kept constant at the nominal size of 1.04 mm. A maximum deviation of about 3µm was determined. Additionally, it was found that, in comparison to the simulations with a>0, the gap of the joint face e was influenced more significantly by the closing force Fc as well as by the width of joint face b. In the here-presented example, e reached a maximum of almost 2µm when the closing force Fc was kept to the minimum required force to close the die halves according to Eq. 2, which is about 11,000 N for the investigated microcomponent. The smaller the joint face width b, the smaller is the deflection e. The reason for this is the increase in contact pressure within the joint face with decreasing value of b. As can be seen from Fig. 8, the superposition of bending stresses by the increase in a>0 enabled the retention of dx and dy at the nominal values of 1.04 mm and e=0. However, the closing force Fc and the gap a have to be adapted to the selected value of b, and a compromise has to be made regarding the accuracy in the x- and ydirections. As an example, for a selected joint-face width of b=0.78 mm (which is 75% of the die-cavity width) and a selected closing force of Fc =14,300 N (which is 130% of the minimum required force), the deflection is zero in the xdirection and about 0.1% of the die-cavity diameter in the y-direction, when a=0.02 mm.

4 Conclusions The FE-based simulations presented herein show that the elastic deflections of the micro-hydroforming tools—and through these the accuracy of the component produced— can be controlled by suitable tool design and load selection. As an overall strategy to reduce the tool deflection in micro-hydroforming processes for practical applications, it is recommended to implement a tool design, which enables the superposition of bending stresses in conjunction with an adapted joint-face design and a controllable closing force,

Fig. 7 Finite-element model of the investigated micro-hydroforming tool cross-section

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Fig. 8 Elastic tool deflection as a function of load and tool parameters

where the parameter optimisation is conducted by FE analyses. Fine adjustment within the first experimental tryouts can be carried out by adapting the closing force and the level of tool bending. However, it should be pointed out that besides the consideration of an adequate tool design, influences from size effects and the elastic spring-back behaviour of the formed component also has to be taken into account when designing a micro-hydroforming processes. Acknowledgments The present research work is part of the project MASMICRO, which is supported by the European Commission within the 6th Framework Programme.

References 1. Hartl C (2005) Research and advances in fundamentals and industrial applications of hydroforming. J Mater Process Technol 167:383–392. doi:10.1016/j.jmatprotec.2005.06.035 2. Schulze T (1999) Seam-welded aluminium tubes for lightweight components. Tube Int 181(11), pp 497–503 3. Leppin A, Boucke B (2001) Hydroforming of aluminium extrusions. Proc Int Conf Hydroforming, Stuttgart, pp 311–322

4. Boehm A (2004) Development and mass production of the BMW 5 and 6 series lightweight aluminium front end. In Proceedings of North Am Hydroforming Conf, Waterloo, Ontario 5. Schuster C, Loretz C, Klaas F, Seifert M (2005) Potentials and limits with hydroforming of aluminium alloys. In Proceedings of Int. Conf Hydroforming, Stuttgart, pp 113–136 6. Mertes P, Schroeder M (2000) Anwendung des Innenhochdruckumformens in der Automobilindustrie. In Proceedings of EndForm2000, Paderborn, pp. 47–66 7. Prelog H, Nottrott A (1999) 4 Jahre Erfahrung in der Grossserienfertigung von Rahmenstrukturen aus IHU-Bauteilen. In Proceedings of the Int. Conf. on Hydroforming, Stuttgart, Germany, October 12/13, pp. 1999 8. Hartl C (2003) Case Studies in Hydroforming. In Proceedings of the Int. Conf. Recent Development on Tube and Sheet Hydroforming, Columbus, OH, USA 9. Hartl C, Lungershausen J, Biedermann H, Conzen J (2006) Study of hydroforming processes for the production of micro-components. In Proceedings of 1st Jubilee Scientific Conf. Manufacturing Engineering in Time of Information Society, Gdansk 10. Hartl Ch, Lungershausen J, Eguia J, Uriarte G and Lopez Garcia F (2007) Micro hydroforming process and machine system for miniature/micro products. In Proceedings of Int Conf 7th euspen, Bremen, vol. 2, pp, 69–72 11. Vollertsen F, Schulze Niehoff H, Hu Z (2006) State of the art in micro forming. Int J Mach Tools Manuf 46:1172–1179. doi:10.1016/j. ijmachtools.2006.01.033

Int J Adv Manuf Technol 12. Kang SJ, Kim HK, Kang BS (2005) Size effect on hydroforming formability. J Mater Process Technol 160(1):24–33. doi:10.1016/j. jmatprotec.2004.02.035 13. Zhuang W, Wang S, Cao J, Lin J, Hartl C (2008) Hydroforming of micro tubes: Crystal Plasticity FE modelling. In proceedings of 12th Int- Conf. Metal Forming 2008, Krakow, PL 14. Koc M (1999) Development of guidelines for tube hydroforming. Doctoral dissertation. Columbus, OH 15. Birkert A (1997) Abschaetzung der Kalibrierdruecke beim Innenhochdruck-Umformen. Blech Rohre Profile 9:102–107

16. Hartl C (1999) Theoretical fundamentals of hydroforming. In Proceedings of Int Conf Hydroforming, Stuttgart, 1999, pp. 23– 36 17. Vollertsen F (2000) Accuracy in process chains using hydroforming. J Mater Process Technol 103:424–433. doi:10.1016/S0924-0136(00) 00507-0 18. Böhm A, Hartl C (2001) Aspects of quality and die life in tube hydroforming. In Proceedings of Int. Conf. on New Developments in Tube Hydroforming Technology. Michigan, USA