Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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EXTRUSION Integrated processing <strong>of</strong> <strong>aluminium</strong> pr<strong>of</strong>iles subsequent to hot extrusion A. Jäger, N. Ben Khalifa, A.E. Tekkaya, Institute <strong>of</strong> Forming Technology and Lightweight Construction, TU Dortmund University, Dortmund, Germany Strategies <strong>of</strong> <strong>the</strong>rmo-mechanical processing <strong>of</strong> <strong>aluminium</strong> pr<strong>of</strong>iles subsequent to hot extrusion were developed and tested. By using <strong>the</strong> process heat <strong>of</strong> extrusion for subsequent forming and heat treatment steps, process chains for <strong>the</strong> production <strong>of</strong> graded <strong>aluminium</strong> pr<strong>of</strong>iles were developed and realized on an experimental scale. Due to <strong>the</strong> geometric limitations, <strong>the</strong> technology <strong>of</strong> realizing a grading in <strong>the</strong> geometry <strong>of</strong> open and hollow pr<strong>of</strong>iles is different. Therefore, two innovative concepts for <strong>the</strong> <strong>the</strong>rmo-mechanical processing <strong>of</strong> open and hollow sections were developed, allowing electromagnetic compression and rolling to become integrated within <strong>the</strong> process chains. Possible applications are seen in producing functionally graded products with locally adapted properties. Conventional hot metal extrusion is used to produce straight, semi-finished products in mass production (Laue and Stenger, 1976) with a constant cross section over <strong>the</strong> length and homogeneous mechanical and micro-structural properties (Hall and Mudawar, 1996). Aluminium sections are frequently used as construction elements. Generally, <strong>the</strong> loading conditions <strong>of</strong> structure elements in technical constructions differ locally. The constant cross Fig. 2: Concept <strong>of</strong> extrusion and subsequent forming by rollers (schematic) Fig. 1: Concept <strong>of</strong> extrusion with in-line electromagnetic compression (longitudinal section, schematic) section, which is attributed to <strong>the</strong> peculiarity <strong>of</strong> <strong>the</strong> extrusion process itself, mostly represents a compromise between <strong>the</strong> functionality and <strong>the</strong> component design, which e. g. involves a local oversizing and thus an excessive use <strong>of</strong> resources. Here, a local adaption <strong>of</strong> <strong>the</strong> geometry could be suitable in order to meet <strong>the</strong> locally specific demands on <strong>the</strong> structure properties. For this reason, <strong>the</strong> development <strong>of</strong> innovative forming technologies is indispensable in order to manufacture products with graded, locally adapted structural properties as well as varying geometric shapes. The increase <strong>of</strong> <strong>the</strong> formability <strong>of</strong> <strong>aluminium</strong> can be enhanced by heat treatment between <strong>the</strong> single forming steps. The integration <strong>of</strong> <strong>the</strong>rmo-mechanical processing after extrusion is a desirable substitute for additional heat treatment processes for several reasons: additional production steps lead to increased production time, energy consumption and production costs. Incorporating<strong>the</strong>rmo-mechanical processing immediately after extrusion will save <strong>the</strong> time and energy needed to reheat <strong>the</strong> pr<strong>of</strong>iles for o<strong>the</strong>r hot forming steps. In this paper, strategies for <strong>the</strong> <strong>the</strong>rmo-mechanical processing <strong>of</strong> <strong>aluminium</strong> pr<strong>of</strong>iles by hot forming subsequent to hot extrusion for <strong>the</strong> manufacturing <strong>of</strong> geometrically graded <strong>aluminium</strong> pr<strong>of</strong>iles are given. Due to <strong>the</strong> geometric limitations, <strong>the</strong> technology <strong>of</strong> subsequent processing <strong>the</strong> geometry <strong>of</strong> open and hollow pr<strong>of</strong>iles is different. Two concepts for <strong>the</strong> processing <strong>of</strong> <strong>aluminium</strong> pr<strong>of</strong>iles by applying hot extrusion combined with subsequent hot electromagnetic compression and alternatively combined with an integrated hot rolling concept were developed and tested on an experimental scale. By process integration <strong>the</strong> process heat <strong>of</strong> extrusion is used for <strong>the</strong> successive forming operation. Description <strong>of</strong> <strong>the</strong> new concepts For <strong>the</strong> processing <strong>of</strong> hollow pr<strong>of</strong>iles a combination <strong>of</strong> hot extrusion and electromagnetic compression was developed. Hot metal extrusion is used to produce tubular semi-finished products continuously by pushing a billet through a die (DIN 8583-6, 2003). In typical industrial hot <strong>aluminium</strong> extrusion, pr<strong>of</strong>ile exiting speeds <strong>of</strong> up to 50 m/min are common (Ostermann, 2007). In contrast to this, electromagnetic compression is used to reduce <strong>the</strong> cross section <strong>of</strong> a workpiece locally (Harvey and Brower, 1961). The applied magnetic pressure can achieve values <strong>of</strong> up to several hundred MPa and accelerates <strong>the</strong> workpiece to typical strain rates <strong>of</strong> about 10 4 s -1 completing <strong>the</strong> entire forming process in approximately 50 to 200 μs. Due to <strong>the</strong> favourable relation between resulting exit speed in extrusion and processing time in electromagnetic compression, a longitudinal translation be- 40 ALUMINIUM · EAC CONGRESS 2011 Images: IUL
Fig. 3: Experimental setup for extrusion with subsequent electromagnetic compression tween <strong>the</strong> workpiece and <strong>the</strong> tool coil during <strong>the</strong> compression process can be neglected. For example, with an exiting speed <strong>of</strong> 50 m/min, and 100 μs for <strong>the</strong> electromagnetic forming process, a relative movement between <strong>the</strong> extrudate pr<strong>of</strong>ile and <strong>the</strong> tool coil <strong>of</strong> about 160 μm results, which meets <strong>the</strong> geometrical tolerance field <strong>of</strong> macroscopic forming processes. Therefore, <strong>the</strong> application <strong>of</strong> electromagnetic compression subsequent to extrusion is possible without a compensation <strong>of</strong> <strong>the</strong> relative speed between <strong>the</strong> workpiece and <strong>the</strong> tooling. To integrate both processes, a tool coil for compression was positioned behind <strong>the</strong> die exit and coaxial to <strong>the</strong> extrudate in order to reduce <strong>the</strong> workpiece cross section locally (Fig. 1). Additionally, a counter die in <strong>the</strong> shape <strong>of</strong> a mandrel can be mounted to <strong>the</strong> mandrel <strong>of</strong> a porthole extrusion die, which extended into <strong>the</strong> tool coil (Jäger et al., 2009). By this, besides achieving a more defined geometry in comparison to a free forming operation, an increase <strong>of</strong> <strong>the</strong> geometrical complexity <strong>of</strong> locally compressed areas can be achieved. For processing open sections, like L-, T and double-T-shaped pr<strong>of</strong>iles, a rolling process was developed. By executing <strong>the</strong> subsequent forming operation as a rolling process, <strong>the</strong> continuous run out <strong>of</strong> <strong>the</strong> pr<strong>of</strong>ile can be taken into account. Fig. 4: Dimensions <strong>of</strong> <strong>the</strong> mandrel for electromagnetic forming A special rolling stand was developed and mounted behind <strong>the</strong> extrusion platen <strong>of</strong> an extrusion press, allowing <strong>the</strong> continuous forming <strong>of</strong> <strong>the</strong> outgoing extrusion pr<strong>of</strong>iles. The rollers mesh toge<strong>the</strong>r with sinusoidal teeth, creating a corrugated contour on <strong>the</strong> web <strong>of</strong> a continuously fed I-beam-shaped pr<strong>of</strong>ile. The contour <strong>of</strong> <strong>the</strong> forming dies stores <strong>the</strong> desired geometry <strong>of</strong> <strong>the</strong> workpiece. As <strong>the</strong> exit speed <strong>of</strong> <strong>the</strong> extrudate varies, compensation strategies have to be provided. For this, <strong>the</strong> rolling stand was swivel-mounted and <strong>the</strong> rotational speed <strong>of</strong> <strong>the</strong> rollers was adapted permanently (Fig. 2). Extrusion and electromagnetic forming A test rig for evaluating <strong>the</strong> concept <strong>of</strong> <strong>the</strong> integration <strong>of</strong> an electromagnetic compression step subsequent to extrusion was built (Fig. 3). A tool coil was positioned behind <strong>the</strong> die exit in order to reduce <strong>the</strong> workpiece cross section locally. To adapt <strong>the</strong> tool coil and <strong>the</strong> extrudate geometry, a field shaper made out <strong>of</strong> a CuCrZr-alloy, electrically insulated with a polyimide foil, is used. Beyond shaping and concentrating <strong>of</strong> <strong>the</strong> electromagnetic field, <strong>the</strong> field shaper also prevents an overheating <strong>of</strong> <strong>the</strong> tool coil by <strong>the</strong>rmal radiation <strong>of</strong> <strong>the</strong> processed hot extrudate. To assure a uniform air gap between <strong>the</strong> field shaper and <strong>the</strong> workpiece guiding rollers made out <strong>of</strong> brass are ar- EXTRUSION ranged pairwise at <strong>the</strong> inlet and run-out side <strong>of</strong> <strong>the</strong> tool coil. For heat treating <strong>of</strong> <strong>the</strong> extruded and subsequently hot deformed workpieces made <strong>of</strong> heat treatable alloys, an additional quenching setup was positioned behind <strong>the</strong> tool coil. Atomizing nozzles are located around <strong>the</strong> press axis providing air atomized water mist streams. To use <strong>the</strong> press heat <strong>of</strong> extrusion for <strong>the</strong> subsequent <strong>the</strong>rmo-mechanical processing steps efficiently, <strong>the</strong> whole setup is designed compactly. From <strong>the</strong> die exit to <strong>the</strong> quenching, <strong>the</strong> whole setup has a length <strong>of</strong> less than one meter. Experimental trials were performed using a 250-ton direct extrusion press (Collin PLA250t) and a pulse power generator (Maxwell-Magneform series 7000). Solenoid coils fabricated from copper and embedded in a fibre-reinforced resin were used as tool coils (Poynting GmbH). Using an EN AW-6082 <strong>aluminium</strong> alloy, a porthole extrusion die and a squared field shaper, a squared hollow pr<strong>of</strong>ile (30 x 30 x 1.8 mm) was extruded with an exiting speed <strong>of</strong> about 16 mm/s while compression by electromagnetic forming was applied periodically in intervals <strong>of</strong> approximately 10s. Semi-finished products with local bulges arranged around <strong>the</strong> circumference <strong>of</strong> <strong>the</strong> squared tube can be produced. In order to expand <strong>the</strong> range <strong>of</strong> geometrical shapes with cross-sections o<strong>the</strong>r than that <strong>of</strong> <strong>the</strong> extruded <strong>aluminium</strong>, a mandrel can be used to define <strong>the</strong> cross section geometry. A squared mandrel for <strong>the</strong> processing <strong>of</strong> a round tube was chosen and specially designed to prevent force fit between <strong>the</strong> workpiece and <strong>the</strong> die (Fig. 4). By using a squared field shaper, with an aperture <strong>of</strong> 43.4 x 43.4 mm and a length <strong>of</strong> 27 mm <strong>of</strong> <strong>the</strong> concentrator, in combination with <strong>the</strong> extrusion <strong>of</strong> a round tube (Ø 40 x 2 mm) fur<strong>the</strong>r trials were performed. In this combination <strong>of</strong> field shaper, workpiece and mandrel an adapted air gap and pressure distribution along <strong>the</strong> workpiece circumference can be achieved which helps to prevent a force fit between <strong>the</strong> extrudate and <strong>the</strong> mandrel. In order to character- ize <strong>the</strong> forming results when a mandrel was used as a form defining tool, an experimental study was performed at charging energies <strong>of</strong> 6.4 kJ, 8 kJ and 8.8 kJ. To measure <strong>the</strong> high speed current pulse in <strong>the</strong> tool coil a Rogowski coil was used. The best forming result/geometrical accuracy could be achieved at a charging energy <strong>of</strong> 8 kJ. For this a maximum coil current amplitude <strong>of</strong> 62 kA and frequency <strong>of</strong> <strong>the</strong> discharging current <strong>of</strong> 4.5 kHz were detected. To prevent damage by overheating <strong>of</strong> <strong>the</strong> tool coil and <strong>the</strong> electrical isolation, <strong>the</strong> field shaper was cooled by compressed air ALUMINIUM · EAC CONGRESS 2011 41
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Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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