Key Engineering Materials Vols. 554-557

Abstract: Abstract. The rather unintuitive and non-linear behavior of plastics melts is a well-known obstacle in the design and manufacturing cycle of profile extrusion dies. This is reflected, for example, in the so-called running-in experiments, in which the already manufactured die is modified up to 15 times until the final product, shaped by the die, matches the quality requirements. Besides a homogeneous outflow velocity and thus homogeneous material distribution, an appropriate die swell is a second design objective which complicates the reworking of the manufactured die. We are conducting work to shorten the manual running-in process by the means of numerical shape optimization, making this process significantly less costly and more automatic. From a numerical point of view, the extrusion process is not as challenging as high-speed flows, since it can be described by steady Stokes equations without major loss of accuracy. The drawback, however, is the need for ac- curate modeling of the plastics behavior, which generally calls for shear-thinning or even viscoelastic models, as well as for 3D computations, leading to large computational grids. The intention of this paper is to investigate the application of specific geometry features in extrusion dies and their influence on objective functions in an optimization framework. However, representative objective functions concerning die swell and the incorporation of known geometry features, as used by experienced die designers, into the optimization framework still remain a challenge. Hence, the topics discussed are the influence of the mentioned geometry features on existing objective functions as well as an outlook on an algorithmic implementation into the optimization process with regard to representative objective functions.

Abstract: Extruded aluminium profiles with a variety of different cross-sections are mainly used in lightweight structures for transportation means. Examples are stringer profiles in the fuselage of airplanes or profiles in chassis of trains and cars. Nowadays, the mass reduction of a lightweight structure is an important issue to achieve lower fuel consumption and CO2 emissions. With an increased portion of aluminium profiles the mass of structures can be reduced considerably in contrast to the application of steel parts, due to the lower density of aluminium. However, this is coupled with disadvantages such as the lower specific stiffness and strength of the material. One possibility to improve the mechanical properties of aluminium profiles without a considerable increase of their weight is the embedding of reinforcing elements during the extrusion process. Special porthole extrusion dies are used to feed reinforcing elements in form of high strength steel wires separate from the aluminium material flow. In the welding chamber of the die both materials bond together to a composite profile. To achieve a high advantage of the technology for lightweight applications a high reinforcing volume of aluminium profiles is targeted. A comparatively high reinforcing volume can be reached either by a high number of reinforcing elements or through a reduction of the profile wall thickness. A high number of reinforcing elements leads to a small distance between the single elements in the profile cross-section. The paper will show the results of an experimental and numerical analysis which were carried out to determine the minimum distance between the reinforcing elements as well as the minimum profile thickness. In the trials different arrangements of the elements in the profile cross-section and profile thicknesses were considered. Main parameters which have an influence on the process stability were analyzed and a process window for the manufacture of thin profiles with high reinforcing volume was deduced.

Abstract: Deep drawing is one of the most important processes applied in industrial production. Here the Finite-Element-Method (FEM) is an important tool in the development and optimization process. One aspect to optimize simulations is to consider real friction behavior. Thus the friction phenomenon has to be describable. In addition to contact normal pressure and velocity the surface topography and the lubricant amount have a great influence on friction. This paper illustrates the influence of surface evolution in real, inhomogeneous processes on the lubricant distribution. For this a rectangular cup with four different corner radii is used to evaluate local surface topographies and lubricant amounts in deep drawing. The lubricant amount is measured by fluorescence technique and the surface topography is evaluated by a confocal white-light microscope. Due to hydrodynamic effects the lubricant is squeezed out and displaced to adjacent regions. Further hydrostatic pressures built up in closed lubricant pockets force the lubricant to stay in the forming zone to bear a part of the load. In free forming zones without contact between the sheet and tool the surface roughens due to grain dislocations in the microstructure. This paper also presents the results of lubricant distribution and surface evolution by varying the initial lubricant amounts and drawing depth. It can be recognized that the different corner radii of the rectangle cup have a great influence on the surface evolution and lubricant distribution. Moreover it can be clearly seen that surface parameters correlate with the lubricant amount. By means of the described evaluation it is also possible to correlate these values with load histories consisting of contact pressures and strain evolution, evaluated in FEM. All the results contribute to a better understanding of the friction behavior in deep drawing and point out the inhomogeneous character of friction.

Abstract: The use of PVD and CVD coatings has increased significantly thanks to the improved tribological performances they offer in many metalforming processes. Nevertheless the proper coating selection for a specific forming operation is not well established yet, being mainly based on trails and error approaches. The use of FEM-supported analyses may represent an effective support in the optimization of process parameters, but the need of testing procedures and reliable models to describe the mechanical and tribological phenomena at the interface between the dies and the workpiece is still significant. The paper presents a novel experimental set-up for the evaluation of the wear resistance of dies coatings in sheet metal forming operations. A progressive stamping process was taken as reference case and analyzed by numerical analyses. Contact pressures, temperatures and tangential loads at the tools-blank interface were evaluated in each deformation step. TiAlN and CrN were selected as reference coatings and deposited via magnetron sputtering technique. The first part of the research was focused on the design of the novel set-up capable to carry out controlled wear tests in laboratory environment, performed with the parameters obtained from the numerical simulation. The results of such experiments were compared to the ones from standard laboratory tests and with industrial trials, though measurements of loads, of surface roughness evolution and by surface investigations trough Scanning Electron Microscope observations.

Abstract: A new upsetting-extrusion type tribometer has been developed to investigate the performance of the lubrication coating on the side surface of a billet in the multi-stage cold forging. In this tribometer, the lubrication coating is first destroyed by the upsetting process due to the free expansion of the billet surface and then evaluated by the extrusion process. The frictional shear factor of the lubrication coating is obtained by plotting the measured extrusion load and the position after the extrusion of a centerline drawn on the billet in advance on the calibration curve obtained by FEM. Experimental results using a zinc phosphate coating and a dry in-place type coating showed that the reduced peak height Rpk is more appropriate than the maximum height Rz to express the effect of the surface roughness of tool on galling generation. When no galling occurs, the frictional shear factor hardly varies with the type of the lubrication coating and the surface expansion ratio. The anti-galling ability of the dry in-place type coating is greatly improved by a two-stage shot blast before the lubrication coating and reaches a level better than the zinc phosphate coating.

Abstract: The main objective of this research is study the wear of a mould, made in easy machining material; aluminium EN AW-6082 T4 and the surface characterization of mould cavity, used in a polymeric manufacturing process, after several injection cycles. To evaluate the effectiveness of this materials in the plastic injection process, a test part with different thicknesses and shapes was designed and some inserts of aluminium were made. In the way of plastic material, a 30 % fiberglass reinforced polyamide PA6 has been employed in the experimental procedure. A reusable injection mould was used and surface measurements of mould cavity have been performed after 9200 cycles. The surface topography has been measured by using optical interferometry profiling technology and there have also been obtained roughness and surface parameters according to ISO 25178 and EUR15178N. In order to surface characterization, different areas of the mould cavity with different geometries, has been analysed. The behaviour of wear rate has been analysed by the study of the evolution of the surface parameters and functional parameters in different points of the surface mould.

Abstract: The general framework of this paper is in the field of numerical simulation of asperity crushing. Different material forming processes, such as strip-rolling and deep drawing, imply mixed lubrication. In this lubrication regime, two types of contact are present at the same time: a direct contact between the two solids at the asperity level and also valleys filled with pressurized oil. Theses contact conditions have a large influence on friction and wear taking place during the upsetting process. As this mixed type of contact is not yet fully understood from the physics point of view, numerical models are essential to achieve a better understanding. For example, semi-analytical asperity crushing models have been developed by Wilson&Sheu [1] and Sutcliffe [2] to take into account the influence of bulk plastic deformations on asperity crushing. The finite element method has also been used to model asperity crushing. Ike&Makinouchi [3] studied the behavior of 2D triangular-shaped asperities under different boundary conditions. Krozekwa et al. [4] modeled 3D triangular asperities behavior, for various bulk strain directions. More recently, Lu et al. [5] compared experimental results of pyramid-shaped asperity and ridge-shaped asperity crushing with finite element simulation results. As in the three former references mentioned above, it has been decided, to study the interaction between a rigid plane and a simplified geometry asperity without lubricant. In this article, numerical asperity crushing results obtained with Metafor[6], a home made large strains software, will be presented. Those results will illustrate the influence of boundary conditions, contact pressure, large bulk strain and geometry of asperities on the evolution of the contact area. As the asperity crushing behaviour is known to be very sensitive to the boundary conditions, in this article, we will also present results using boundary conditions from a cold rolling model named MetaLub. MetaLub [7-8] is a software developed at the University of Liege in partnership with ArcelorMittal R&D center. It iteratively solves the equations resulting from the discretisation using the slab method of the strip coupled to a mixed lubrication model at the interface. This lubrication model takes into account the evolution of the oil film thickness as well as the asperity crushing along the roll bite. We will compare the evolution of the relative contact area obtained with MetaLub to the results obtained with finite elements simulations using the same boundary conditions. [1] Wilson, W.R.D and Sheu, S. Real area of contact and boundary friction in metal forming. Int. J. Mech. Sci. 1988, 30(7), 475-489. [2] Sutcliffe, M.P.F Surface asperity deformation in metal forming processes. Int. J. Mech. Sci., 1988, 30(11), 847-868. [3] Ike, H. and Makinouchi, A. Effect of lateral tension and compression on plane strain flattening processes of surface asperities lying over a plastically deformable bulk. Wear, 1990, 140, 17-38. [4] Korzekwa, D.A., Dawson, P.R. and Wilson W.R.D., Surface asperity deformation during sheet forming. Int. J. Mech. Sci., 1992, 34(7), 521-539. [5] Lu, C., Wei, D., Jiang, Z., and Tieu, K., Experimental and theoretical investigation of the asperity flattening process under large bulk strain, Proc. Inst. Mech. Eng. J. 222 (2008), 271–278. [6] LTAS-MN2L. ULg. http://metafor.ltas.ulg.ac.be/. [7] Stéphany, A., Contribution à l’étude numérique de la lubrification en régime mixte en laminage à froid. PhD dissertation (in French), Université de Liège (2008) [8] Carretta, Y., Stephany, A., Legrand, N., Laugier, M., and Ponthot, J.-P., MetaLub – A slab method software for the numerical simulation of mixed lubrication regime. Application to cold rolling. In Proceedings of the 4th International Conference on Tribology In Manufacturing Processes (ICTMP), 2010,799-808.

Abstract: In order to better understand superficial abrasive wear and metal processing sclero-topometric investigations have been developed and applied to spheroidal cast iron at boundary lubrication regime with new, waste and regenerated oils. Recycling of waste oils is the major challenge of worldwide society today, specifically for petroleum resources, and therefore, for derivatives like transformer oil. The contradictory characteristics from physical approach of transformer oil, requiring and insuring simultaneously a high thermal conductivity as well as an electrical resistivity, are specific for their applications. During the efficient lifetime, these characteristics progressively decrease, due to complex pollution (divers pollutions, water absorption, Polychlorobiphenyl (PCB), etc) and degradation (acids, resins, polluted oils, biological invasions, wear debris's du to local frictions, fretting, etc…), ultimately making the oil unsuitable for the initial application. [1] The strategy to upgrade the waste oil is investigated in the presented work using improved sclerometric and topometric characterisations [2,3] on deliberately selected heterogeneous material - ductile cast iron (with spheroidal graphite precipitations) widely manufactured for various transport industry[4].

Abstract: Abstract: A new concept of incremental equal channel angular pressing (I-ECAP) with converging billets is proposed and simulated numerically. It follows a recently introduced process of ECAP with converging billets, in which the contact surface between converging billets plays the same role as a movable die wall in the output channel of classical ECAP and thus reduces friction and the process force. The process productivity is doubled and material pickup, especially problematic in the output channel, avoided. However, ECAP with converging billets, as any ECAP-based process, suffers from a limited length of the billets it can process. This paper proposes an incremental version of ECAP with converging billets, which enables processing very long billets. Additionally, a new option for ECAP or I-ECAP with converging billets is considered, which assumes their separation with a movable tool. This tool can also be used to apply a back force. FEM simulations of all these processes enable their comparison in terms of strain distribution and the force required.

Abstract: An AZ31B wrought magnesium alloy was processed by incremental equal channel angular pressing (I-ECAP) using routes A and B_C. Despite the fact that the measured grain size for both routes was very similar, the mechanical properties were different. Tensile strength was improved using route A comparing to route B_C, without ductility loss, while tension-compression anisotropy observed for route A was significantly suppressed when using route B_C. Moreover, billet shape evolution resulting from subsequent passes of I-ECAP was studied. Significant distortion after processing using route B_C and no occurrence of such effect for route A were observed. Results of a finite element analysis showed that non-uniform strain rate sensitivity might be responsible for different billet shapes. The conclusion is drawn that processing route has a strong influence on the billet shape and mechanical properties when processing magnesium alloys by I-ECAP.