Manufacturing of Composite and Hybrid Materials by Semi Solid Forming

Transcription

1 Solid State Phenomena Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Manufacturing of Composite and Hybrid Materials by Semi Solid Forming Dipl.-Ing. Kim Rouven Riedmüller 1a, Prof. Dr.-Ing. Mathias Liewald MBA 1b, Dipl.-Ing. Levente Kertesz 1c 1 Institute for Metal Forming Technology (IFU)/University of Stuttgart Holzgartenstraße 17, Stuttgart, Germany a kim.riedmueller@ifu.uni-stuttgart.de, b mathias.liewald@ifu.uni-stuttgart.de, c levente.kertesz@ifu.uni-stuttgart.de Keywords: Semi-solid forming, Metal-matrix composites, Hybrid components, Mould construction, Numerical analysis Abstract. Materials with good mechanical properties and low density are characteristic for lightweight constructions in automotive and aerospace application as well as in the building industry. For this reason, the manufacturing of composite and hybrid materials is in focus of academic and industrial research. Because of the complex and expensive manufacturing process, the application of composite and hybrid materials in many cases is confined to niche and custom-made products. Therefore, the Institute for Metal Forming Technology (IFU) is concerned with the development of new processes for the manufacturing of metal matrix composites and hybrid components by semi-solid forming. Fibre reinforced AlSi-alloys are produced by the application of laminates made of alternating metal matrix layers and carbon fibre fabrics. Hybrid components are manufactured by the integration of higher-strength materials in the semi-solid forming process of aluminium alloys. Here, the main challenge is the integration of the reinforcing components without damaging due to high thermal and mechanical loads that are affecting during the process. These research activities are not only interesting for the mechanical engineering, but also for the civil engineering as this paper will reveal. Introduction The Institute for Metal Forming Technology (IFU) is currently carrying out research in the field of forming lightweight metal alloys in their semi-solid material state to generate hybrid, intelligent design elements. The research work is carried out jointly with six other Institutes of the University of Stuttgart within the framework of the DFG research group FOR 981 "Hybrid Intelligent Construction Elements" (HICE). The aim of the sub-project, which is undertaken at the IFU, is to manufacture hybrid and adaptive elements to deflect or transmit forces. These elements consist of specified combinations of different materials, or classes of materials, in the sense of reinforced lightweight metal matrices. By means of such constitutive and structural material combinations, on the one hand, higher degrees of lightweight materials are obtained for structural elements and, on the other, intrinsic constitutive functions are produced, which visibly deviate from those of monolithic, metallic materials. In this way, the performance of the matrix's reinforcement is refined by means of appropriate volume fractions of fibres. Such a lever Fig. 1: Overall demonstrator "shell structure" (left); forcedeflector manufactured at the IFU (right) 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, (ID: , Pennsylvania State University, University Park, USA-12/05/16,07:14:49)

2 90 Semi-Solid Processing of Alloys and Composites XII element, which adapts to its application profile, is to be integrated into the overall demonstrator "shell structure" (Fig. 1). In order to achieve this aim, fundamental material investigations are initially performed using a testing tool at the Institute for Forming Technology and, based on this, two levers are simulated, structurally designed and then subsequently manufactured [1]. Metallographic material analysis and mechanical properties of reinforced light metal matrices To investigate the manufacturability of hybrid material systems and to determine the attainable mechanical properties, components in the form of plates were generated using an existing experimental tool [2, 3, 4]. Here, AlMg1 (EN AW-5005) wrought aluminium rolled sheets (s=0.280mm) were served as the matrix material whose mechanical properties were specifically changed by introducing woven carbon fibres and woven stainless steel wires (1.4301). During the test procedure, prepregs arranged as laminates are heated to the target temperature of 690 C in an infrared oven. At this temperature the aluminium alloy is above its liquidus line, whereby the reinforcing components are pre-infiltrated during the heating of the layer composition. To prevent an oxidation of the carbon fibres, which could occur at a temperature of circa 400 C and to avoid the flow of the matrix material, the prepreg is packed in a steel foil. This prepreg is subsequently introduced into the testing tool, by what the matrix material cools down to the semi-solid condition. Afterwards, the prepregs are loaded using a press capacity of 5,000 kn whereby the reinforcing components are impregnated by the semi-solid aluminium matrix. The component which is produced in this way is a 149x110x7 mm plate from which tensile specimens are machined for determining the material properties as well as samples for the metallographic examinations [5]. Fig. 2 depicts the schematic process for manufacturing the test components and an image of a cross-section of one of the components taken by a lightmicroscope. The Lightmicroscopic analysis of the material samples did not establish any flaws or sites exhibiting poor quality impregnation. As depicted in Fig. 2, the laminate Fig. 2: Schematic of the manufacturing process for plate shaped test components together with a microscopic image of a component's cross-section configuration and the orientation of the characterising components is maintained and can be clearly discerned. The aluminium matrix has impregnated the woven carbon fibres very well and the individual fibre filaments are completely embedded. In retrospect, only the interface adhesion proves to be insufficient between the AlMg1-Matrix and the 0.2 mm stainless steel wires, which are very thick in comparison to the individual fibres. Using the corresponding values of the matrix alloy as a reference, the resulting mechanical properties of the manufactured composite and hybrid materials increases by 45 percent in the composite's yield stress and by 30 percent in the Young's modulus, respectively, for a total specimen volume containing 20 percent woven carbon fibres. It was possible to more than double the fracture strain of the non-reinforced AlMg1 specimens by using the ductile material in the form of woven wires as the sole reinforcing component. On using a combination of both reinforcing materials, the carbon fibres proved to exert a dominating effect on the mechanical properties. On the one hand, this is attributed to the above mentioned inadequate interface adhesion between the AlMg1 alloy and the steel material and, on the other

3 Solid State Phenomena Vols hand, to the high notch effect emanating from the transverse stainless steel wires. Since the fibres and wires, which lie transverse to the tensile direction in the tensile tests, exert no reinforcing effect, the actual volume fraction, i.e. the fraction which contributes to the laminate composite's reinforcement, is significantly lower. Thus for the woven carbon fibres (plain weave 3K/1K) only 75 % of the fibres' fraction lie in the tensile direction and just 50 % for the woven steel wires. By means of elevating the carbon fibres' volume fraction in the laminate composite, a further increase in the tensile strength can be expected in which the fibre orientation should correspond, as far as possible, to the applied stress state. Table 1 depicts the compositions of some analysed layer constructions. The diagrams in Fig. 3 and 4 present the appropriate results [6]. Table 1: Composition of the analysed layer constructions Carbon Fibre Steel Wire Hybrid I Hybrid II AlMg1-sheets 80 % 76 % 78 % 78 % Woven Carbon Fibres 20 % - 10 % 12 % Woven steel wires - 24 % 12 % 10 % Fig. 3: Stress-strain diagram of the investigated composite and matrix materials Fig. 4: Comparison of the tensile strength and fracture strain of the manufactured material specimens with the matrix material (AlMg1) Structural and simulated design of testing tools Based on the analyses presented above, two different test tools were simulated, structurally designed and then subsequently manufactured within the framework of the research work for manufacturing a hybrid, intelligent force-deflector. A forming tool possessing a closed die and semi-lockable cavities was developed for the preliminary investigations. The modular construction enables two-armed as well as three-armed levers to be manufactured. The tool's ejector pins were arranged such that location holes are integrated into the test component during the forming process. Based on the investigations using this preliminary tool, the second lever-tool for a three-armed lever geometry was designed having an open die. Since integrating the location holes by means of the ejector pins lead to good results for almost all the tests, these types of ejector pins were also implemented in the second test tool. The semi-solid forming tools employed within the scope of the research presented here are heated by means of high-power electrical cartridge heaters in order to reduce the heat flow from the test piece into the tool and to avoid premature solidification of the still incompletely formed component.

4 92 Semi-Solid Processing of Alloys and Composites XII Here, the specified tool temperatures lie between 250 C and 350 C, depending on the aluminium alloy used. The location of the heating elements must be previously designated such that the introduction of heat is concentrated in the forming tools' region, such as the punch and cavity, thereby generating a homogeneous temperature profile in the tool's cavity. In order to guarantee optimal tool heating, the design of the heating cartridge arrangement was aided by using the thermodynamic simulation program Ansys Workbench which numerically computes and displays the temperature distribution and the heat flows occurring in the tool. The results of the concluding simulations for both lever tools are depicted in Fig. 5. Fig. 5: Representation of the lever-tools' thermodynamic simulations Besides the numerical computation of the temperature distribution, the simulated material flow in the forming cavities was mapped (Fig. 6). For this purpose, the CFD program (CFD: Computational Fluid Dynamics) Flow-3D was implemented which employs a temperature and shear rate dependant viscosity model and is therefore well suited to numerically compute the material's flow in the semi-solid state [7]. In combination with the previously determined temperature distribution in the tool, the cavity's filling profile was investigated and, based on these results; the construction of the tool was optimised. Moreover, the overflows on the test components were defined by using the flow simulations and provided for in the tool. These overflows must be attached to the component such that the Fig. 6: Representation of the flow simulation of both manufactured lever types contaminating particles and the constituents of the lubricating media, which are taken up by the material being formed, can accumulate in them and be subsequently separated after moulding. For the components considered here, the overflows were attached to the ends of the lever's arms since oxidised and conta-minated material fronts converge here owing to the flow around the ejector pins, and must therefore be conducted away in order to avoid inclusions. To take into account the fluid dynamic properties specific to the material in its semi-solid state, material characteristic values, which were measured using rheological investigations prior to the computed simulations and were adopted during the numerical computations, served as parameters for the time and shear rate dependent viscosity model.

5 Solid State Phenomena Vols Integrating reinforcing components and sensor elements into the test components The aim of the research work presented here is to generate a lever which firstly adapts to its application profile by means of integrated reinforcing components and secondly, is able to detect working loads by means of "forged-in" sensor elements. On the one hand, carbon fibres and, on the other, brass sleeves were chosen as the reinforcing components. Prior to the actual forming process, these brass sleeves are positioned on the ejector pins described above and can also serve as fixings for wound, endless carbon fibres as shown in Fig 7. On the macroscopic level, the initial results of the manufactured test components are satisfactory. No pores, cavities or other optically visible flaws or contaminations could be seen along the interfaces. However, the microscopic images show the existence of a non-uniform distribution of the liquid phases both in the core as well as at surface region. Furthermore, α-solid solutions with enclosed eutectics exist in both cases. Apart from the primary α-solid solutions, secondary α-solid solutions can be observed in the core region. This indicates that besides the eutectic phase, parts of the α-solid solution were also already melted during the heating. As shown in Fig. 8, it was possible to obtain a significant improvement by varying the press' parameters and by using different aluminium alloys [6]. Fig. 7: Prepreg with brass sleeves as fixings for wound carbon fibres Fig. 8: Metallographic samples and microscopic images of a forged-in brass sleeve Brass sleeves were successfully integrated into the test components for both for the "lever-tool 1" as well as for the "lever-tool 2". Moreover, as the metallographic and computer tomographic investigations have shown, it was possible to obtain good results for introducing the endless fibres as a reinforcing element. In further tests, the impregnation of reinforcing components is to be continuously optimised. Since an attempt to integrate insulated resistance wires; and thereby a sensory function, into the component failed owing to the breakage of the wire's insulation due to too high temperatures, too high mechanical loading and possibly due to chemical reactions with the liquid aluminium matrix, additional piezo ceramic flat transducers used as sensor elements were forged-in as part of the tests using the "lever-tool 2". Here, different ceramic materials as protective sheaths were investigated and successfully employed to insulate the piezo elements and the connecting wires (see Fig. 10). With regard to this, more tests are to be carried out.

6 94 Semi-Solid Processing of Alloys and Composites XII Fig. 9: CT image of a force deflecting element possessing integrated carbon fibres as reinforcing components Fig. 10: CT image of a force deflecting element possessing an integrated piezo sensor Summary and perspectives The presented investigations show that an adaption of the mechanical properties like the tensile strength and the fracture strain of aluminium alloys to a specific application profile of a light-weight construction by the integration of reinforcing components using the semi-solid forming technology is possible. In future research activities at the Institute for Metal Forming Technology at the University of Stuttgart in this regard, the achievable mechanical properties are to be increased and to be applied to different construction elements. Therefore, the forming parameters like heating curves, the velocity profile of the press, the tool temperature and the relation between solid phase and liquid phase have to be variegated and to be optimized. In addition further light-weight materials and filamentary reinforcing components will be investigated in this context. Acknowledgements The authors would like to thank the German Research Foundation (DFG) for their financial support for the presented work within the scope of the Research group FOR 981 "Hybrid Intelligent Construction Elements". References [1] Kertesz, L.; Liewald, M.: Thixo-Forming Der Weg zum schlauen Bauteil, Journal Umformtechnik, 3/2011, pp [2] Siegert, K.; Unseld, P.; Baur, J.; Arzt, E.; Kauffmann F.; v. Niessen, K.:Thixoforging of continuous fiber-reinforced AlSi/AlMg-alloys, International Journal of Machine Tools and Manufacture; Volume 46, Issue 11, September 2006, pp [3] Liewald, M.; Unseld, P.: Bauteil auf Basis eines Hybridwerkstoffes, Europäische Patentanmeldung, Anmeldedatum: ; Patentnummer: [4] Liewald, M. Unseld, P.: New Potentials for Lightweight Part Design of MMC utilising Semi- Solid Forming Technology, International Conference on Product Design and Manufacturing System, PDMS 2007, Chongqing, China, 12-15, October 2007 [5] Unseld, P.: Ein Beitrag zur Herstellung metallischer Verbundwerkstoffe durch teilflüssige/thixotrope Formgebung, Dissertation at the University of Stuttgart; 2009 [6] Kertesz, L.; Liewald, M.; Abt, F.: Teilflüssige Formgebung hybrider Werkstoffsysteme, UTF- Science, 2011 [7] Kertesz, L.; Liewald, M.; Unseld, P.: The Centre of Competence in Casting and Thixoforging an example of integrated research. Liewald, M. (Editor), New developments in massive forming, MAT INFO Werkstoffinformationsgesellschaft mbh, Frankfurt/M., 2009, pp

7 Semi-Solid Processing of Alloys and Composites XII / Manufacturing of Composite and Hybrid Materials by Semi Solid Forming / DOI References [2] Siegert, K.; Unseld, P.; Baur, J.; Arzt, E.; Kauffmann F.; v. Niessen, K.: Thixoforging of continuous fiberreinforced AlSi/AlMg-alloys, International Journal of Machine Tools and Manufacture; Volume 46, Issue 11, September 2006, pp /j.ijmachtools