Hydrostatic extrusion process for the efficient production of magnesium structural components: The MAGNEXTRUSCO project. Index

Transcription

1 Hydrostatic extrusion process for the efficient production of magnesium structural components: The MAGNEXTRUSCO project J. Bohlen, D. Letzig, K.U. Kainer, W.H. Sillekens, P.J. Vet Index Abstract Introduction Project outline Process and alloy development Hydrostatic extrusion Magnesium alloys Development of lightweight magnesium structures Post-extrusion working techniques Structures and components Summary...9 Acknowledgements...9 References...10

2 Jan Bohlen The MAGNEXTRUSCO project Page 1 Abstract Magnesium alloys are topic of many research and development activities as promising metallic materials for lightweight applications. Especially the development of magnesium wrought alloys offers the possibility for an efficient production of structural components with a wide range of shapes and improved properties compared to cast components. More in particular, extrusion as shaping technology offers the possibility to produce such profiles. While the technology for processing magnesium alloys is available today, the industrial use is not well established because of technical and economical limitations of conventional extrusion processes. Within the project MAGNEXTRUSCO, research is conducted to overcome these limitations in the production of wrought magnesium components. The paper gives an introduction to the project regarding such general issues as motivation, goals and partnership. Further, the technical state of affairs will be reviewed. Magnesiumlegierungen werden derzeit als vielversprechender metallischer Leichtbau-Werkstoff in einer Vielzahl von Forschungs- und Entwicklungsaktivitäten untersucht. Insbesondere die Entwicklung von Magnesiumknetlegierungen ermöglicht eine effiziente Produktion von Strukturkomponenten mit erweiterten Formgebungsmöglichkeiten und verbesserten mechanischen Eigenschaften im Vergleich zu Gusskomponenten. Speziell der Strangpressprozess ermöglicht die Herstellung solcher Bauteile. Obwohl die Technologie zur Verarbeitung von Magnesiumlegierungen vorhanden ist, fehlt doch der industrielle Einsatz aufgrund technischer und ökonomischer Grenzen, die in den konventionellen Strangpressverfahren begründet sind. Im Projekt MAGNEXTRUSCO sollen diese Grenzen für die Produktion von stranggepressten Halbzeugen überwunden werden. Der Artikel führt in die Arbeit des Projektes ein und stellt die Motivation, die Zielsetzung und das Konsortium vor. Der Stand der Entwicklung wird diskutiert. 1 Introduction While in today s industrial use magnesium and magnesium alloys find their application as cast parts there is a general interest to broaden the use of magnesium alloys by also producing semi-finished products by wrought processes such as rolling or extrusion. Generally, the technology for processing magnesium alloys is available. Although a wider range of shapes together with improved properties compared to cast components could be achieved the present use of such components is limited. This is due to the fact that only few information about the detailed processing of magnesium alloys is available and indicates a high demand for research and development of such components along the complete process chain of the production of magnesium structural components.

3 Jan Bohlen The MAGNEXTRUSCO project Page 2 The most common processes used for shape-giving are the direct and indirect extrusion process. For the extrusion of magnesium and its alloys, technical limits are given due to the occurrence of hot-cracking which is caused by surface heating at the interface between billet and container and the forming heat during extrusion [1, 2]. This leads to the fact that the process window for the production of magnesium structural components using conventional extrusion is rather small, namely extrusion speeds are low and do not meet economic requirements. Within the project MAGNEXTRUSCO, the hydrostatic extrusion process is used in order to broaden the processing window for magnesium alloys and therefore pave the way for the realisation of lightweight, cost-effective magnesium structures based on extrusions. A consortium of 9 European partners from industry and research institutions is working on a complete process chain for the production of magnesium profiles. Table 1 gives an overview on the partnership. The consortium consists of five end-users, one extrusion company using the hydrostatic extrusion process, two research institutions and one university. More information on the MAGNEXTRUSCO project and the partnership is also available on the project s homepage [3]. Partner Location Type Audi; Aluminium-und Leichtbauzentrum (Coordinator) Neckarsulm (D) End-user Centro Ricerche Fiat Orbassano (I) End-user EADS; Centre Commun de Recherche Suresnes (F) End-user VÚKV Praha (CZ) End-user Rond Products Lienden (NL) End-user Boliden HME TNO Industrie; Afdeling Productieontwikkeling GKSS Forschungszentrum; Zentr. für Magnesiumtechnologie RWTH Aachen; Institut für Bildsame Formgebung Waalwijk (NL) Eindhoven (NL) Geesthacht (D) Extrusion company Research Centre Research Centre Aachen (D) University IBF Table 1: The MAGNEXTRUSCO Partnership 2 Project outline The objective of the project is to conduct research and development along the complete process chain for the production of magnesium structural components centred around the hydrostatic extrusion process [4, 5]. Table 2 gives an overview on important goals for the MAGNEXTRUSCO partnership. Beside of the use of commercially available magnesium alloys, an alloy development for the special needs of the hydrostatic extrusion process is a major objective of the project. The hydrostatic extrusion process itself is developed in order to increase extrusion speeds and lower

4 Jan Bohlen The MAGNEXTRUSCO project Page 3 extrusion temperatures as compared to conventional extrusion on one hand and ensure a safe quality of the extruded profiles on the other hand. Area Feedstock Extrusion process Working techniques Demonstrators Specification New alloys/billets with improved as-extruded mechanical properties (yield strength MPa and elongation 20 30% in tension) Hydrostatic extrusion technology with exit speeds comparable to conventional aluminium extrusion ( m/min) Post-extrusion processes (forming, welding) with shaping possibilities similar to aluminum Designs and prototypes of selected structural components with lower weight than conventional structures (20 30% lighter) Table 2: Research and development objectives for magnesium extrusions Post-extrusion technologies like bending and hydro-forming at elevated temperature are being developed for extruded profiles as well as joining technologies for the use in mixed material applications. The complete research and development activity of the project is carried out on selected profiles that could be used in industrial applications as shown in terms of demonstrators by the end-users. Thus, a review on possible shapes and a redesign for the special needs of magnesium alloys is conducted. 3 Process and alloy development 3.1 Hydrostatic extrusion During hydrostatic extrusion, a metal billet is put into a container and surrounded by a fluid medium which forces it through a die opening. The principle of the process is shown in Fig. 1. The main difference compared to direct and indirect extrusion is that there is no direct contact between the billet and the container and therefore friction is almost avoided. Furthermore, the fluid medium itself leads to a better lubrication of the billet-die interface. The process itself finds some industrial application focussing on the better cold workability of brittle metals. The reason is that using hydrostatic pressure has a positive effect on the ductility of the material. At present further applications using warm and hot working are also found including copper tubes, copper-clad aluminium wire and superconductors. Beside of the enhanced workability by using hydrostatic extrusion, shapes have to be kept rather simple compared to conventional extrusion. One important constraint is a cross-sectional symmetry of the profile. Single-hollow sections like tubes can be produced by forming the billet around a mandrel, requiring billets with a bore.

5 Jan Bohlen The MAGNEXTRUSCO project Page 4 Fig.1: Principle of the hydrostatic extrusion process Table 3 shows experimental data for the extrusion of some test profiles. Simple shapes such as rods, tubes, angles and dumb-bell profiles were extruded by using rather high extrusion speeds compared to conventional extrusion where a range of 1 20 m/min is reported [1]. Alloy R [ ] α D [ ] ROD: T b [ C] v [m/min] p R [ ] α D [ ] T b [ C] TUBE: v [m/min] p M ZM AZ ANGLE: DUMB-BELL: ZM AZ Table 3: Experimental data for hydrostatic extrusion of magnesium test profiles (R: extrusion ratio; α D : die semi-cone angle; T b : set-point billet temperature; v: nominal exit speed; p: steady-state extrusion pressure) The reason why the process window for hydrostatic extrusion is larger than for conventional extrusion is essentially as follows [2]. Lubrication conditions in hydrostatic extrusion are nearly ideal, with billet-tooling contact being limited to the die cone. For conventional extrusion, on the other hand, friction and internal shearing is much more extensive and adding considerably to the overall required mechanical work. Thus, the heat originating from the dissipation of work and therefore the temperature rise in hydrostatic extrusion is lower. Effectively, this means that the extrusion speed can be higher (that is, more adiabatic-like) without initiating incipient melting of the material and hence hot cracking.

6 Jan Bohlen The MAGNEXTRUSCO project Page Magnesium alloys With respect to the production of semi-finished parts the material properties are of special importance for the final use as a structural component as well as for the behaviour of the material during extrusion. Therefore the combination of alloy development together with the process development is an important issue of the MAGNEXTRUSCO project. Beside of the selection of promising commercially known alloys such as AZ31 and ZM21, an emphasis was laid on the selection of new and modified alloys to optimise the feedstock quality. It is a concern that additions to the alloy composition stay inexpensive and furthermore that the castability of the alloys allows an industrial production of billets; e.g. by DC casting. Different alloys lead to a significant difference in the microstructure of billets. Emphasis is laid on grain refinement and microstructural homogeneity for both the billet material and the extruded profile. Material properties that have to be fulfilled deal with the objectives of the material development like shown in Table 2 in accordance with the needs of the end-user profiles. A special emphasis is laid on an enhanced ductility of the material. a b c Fig. 2: Micrographs (longitudinal sections) of cast billets; a) AZ31, b) ZM21, c) new alloy

7 Jan Bohlen The MAGNEXTRUSCO project Page 6 Fig. 2a shows a typical cross-section of an AZ31 billet in an as-cast condition with a grain size of µm. Fig. 2b shows the same for ZM21 also showing a similar coarse microstructure. In comparison to this the billet microstructure of a new alloy composition with a finer grain size of µm is shown, promising a more homogeneous microstructure as well as profile properties due to an increased formability. Fig. 3 shows the resulting typical microstructure in longitudinal sections of some hydrostatically extruded rods. In Fig. 3a an example for the commercial alloy AZ31 is given, showing a rather inhomogeneous microstructure containing large long grains oriented in extrusion direction as a remain of a partial recrystallisation during extrusion [6]. The recrystallised regions show an average grain size of 12 µm. An example for a more homogeneous microstructure is given in Fig. 3b for ZM21. An average grain size of approximately 15 µm is revealed. In comparison to this a micrograph from a new alloy is shown in Fig. 3c showing a homogeneous and fine-grained microstructure with an average grain size of approximately 6 µm. a b c Fig. 3: Micrographs of extruded rods; a) AZ31, b) ZM21, c) new alloy (extrusion direction )

8 Jan Bohlen The MAGNEXTRUSCO project Page 7 Resulting mechanical properties of some commercial and new alloys are shown in Table 4. Compared to AZ31, ZM21 and M1 the new alloys #1, #2 and #3 show an improvement on the yield behaviour and strength and especially on the elongation. Also the anisotropy in tensile and compressive yield strength is less pronounced in the new alloys. Alloy Tension Compression TYS UTS Elongation [%] CYS UCS M ZM AZ # # # Table 4: Mechanical properties of extruded rods (longitudinal direction) 4 Development of lightweight magnesium structures 4.1 Post-extrusion working techniques Semi-finished products such as extruded profiles often require further working in order to become a finished part. This is realised by using post-extrusion technologies such as forming and joining which therefore are of significant importance for the successful development of magnesium wrought applications in high-volume industrial products. More in particular, specific technology for (rotary-draw) bending and hydromechanical forming is developed in order to bring the shaping capabilities of magnesium tube to a satisfactory level. Different from conventional forming, these processes are conducted at elevated temperature; that is, at typically a few hundred degrees Centigrade [7, 8]. Thus, there are certain requirements on the heating and isolation of the tooling and working media, as well as on process control. Some examples of products are shown in Fig. 4. Results demonstrate that formability is greatly enhanced by raising temperature above a certain threshold value. This is often required for realising considerable shape alterations and/or forming specific details (for instance, the diameter expansion in hydro-forming and the curvature in bending, as shown in the picture). Still, operations that do not require excessive plastic deformations to be imposed can be done at ambient temperature (for instance, curvatures with moderate radii).

9 Jan Bohlen The MAGNEXTRUSCO project Page 8 Fig. 4: Examples of hydro-formed, bent and welded magnesium tube (ZM21) 4.2 Structures and components Following the requirements of end-user applications together with the results of a feasibility study for the shaping and the production of extruded profiles, current components were re-designed by the end-users and validated as demonstrators for the application of magnesium wrought components. Table 4 gives an overview of different categories of selected components and related requirements. Area Component Type Functional requirement Car Body and hang-on parts Stiffness, crashworthiness Aircraft Interior structural parts Strength, corrosion Tram Interior fittings Stiffness, strength Bicycle Forefork interior parts Strength Table 5: Demonstrator outline As compared to the current designs in steel or aluminium, weight reduction is the primary reason for considering magnesium. Actually, two categories can be distinguished between these different applications: those that call for semi-finished material with medium strength in combination with high ductility, and those that call for high strength as such. While some demonstrator components feature rather basic

10 Jan Bohlen The MAGNEXTRUSCO project Page 9 profile shapes such as tube, others are more of a challenge with respect to the hydrostatic extrusion process. Fig. 5 gives an overview on the demonstrators like selected by the end-users. CENTRO RICERCHE FIAT Inner upper door frame Cockpit command panel Rond Products BV Bicycle fork Steerer tube Air Tube Damping Tube Audi Demonstrator Tramcar interior fittings Bumper Bar Fig. 5: Demonstrator components 5 Summary MAGNEXTRUSCO is an Research and Technical Development (RTD) project within the European Community that uses hydrostatic extrusion for an efficient production of magnesium structural components. Hot hydrostatic extrusion is an attractive process for the production of such components, enabling the manufacture of sections at increased production rates. Initial applications are anticipated for basic shapes that are of limited size. In particular for complicated sections such as multihollows, it is no feasible alternative for conventional extrusion. The newly developed alloys exhibit excellent strength and ductility, allowing to better benefit from the lightweight potential for structural and possibly crash-relevant applications. Threedimensional shapes can be produced from magnesium sections by bending and hydro-forming at elevated temperature. Demonstrators are shown following the major objective of weight saving by using magnesium. Acknowledgements MAGNEXTRUSCO is conducted as a GROWTH project within the 5 th Framework Programme of the EC under contract number G5RD-CT The financial support is gratefully acknowledged. The authors also appreciate the contributions of the project partners to this presentation and the consent for publication.

11 Jan Bohlen The MAGNEXTRUSCO project Page 10 References [1] J. Swiostek, J. Bohlen, D. Letzig, K.U. Kainer, Proceedings of the 6th International Conference and Exhibition on Magnesium Alloys and Their Applications (2003), [2] W.H. Sillekens, J.A.F.M. Schade van Westrum, A.J. den Bakker, P.-J. Vet, Materials Science Forum 2003, Vols (2003), [3] [4] W.H. Sillekens, J. Bohlen, Proceedings of the 6th International Conference and Exhibition on Magnesium Alloys and Their Applications, (2003), [5] A. den Bakker, W.H. Sillekens, J. Bohlen, K.U. Kainer, G. Barton, Proceedings of the 6th International Conference and Exhibition on Magnesium Alloys and Their Applications (2003), [6] J. Bohlen, S.B. Yi, J. Swiostek, D. Letzig, H.G. Brokmeier, K.U. Kainer, Scripta Mat. (2004), submitted for publication [7] W.H. Sillekens, P.P.H. Veltmans, N.A.P.M. Lamboo, P.J.C.M. Rozier, Proceedings of the 10th International Conference on Sheet Metal (2003), [8] W.H. Sillekens, P.P.H. Veltmans, M.H.F.M. van Hout, 12 th Magnesium Automotive and End User Seminar (2004), this volume