Key Engineering Materials Vols. 651-653

Abstract: The author presents an approach to study an innovative manufacturing process developed to produce composite parts with new geometrical possibilities. The aim of this first study is to understand the motions of fibers during the forming of this kind of composite parts and to explore the feasibility of this forming process by an experimental analysis and FEM simulations with Forge3©. The reliability of the simulation tool and its potential, never exploited in this area, will be evaluated.

Abstract: A structural concept in multi-material design is used in the automotive industry with the aim of achieving significant weight reductions of conventional car bodies. In this respect, the specific use of steel foils and continuous fiber-reinforced thermoplastics represents an interesting material combination for the production of hybrid parts in sandwich design. This contribution deals with the experimental and numerical analysis of a conventional sheet metal forming process using a composite material based on Polyamide 6 (PA6) with unidirectional endless glass fiber reinforcement and HC220Y+ZE steel foil. A unidirectional composite plate is positioned between two steel foils in sandwich design and formed under appropriate temperature conditions. For the numerical analysis of the forming process the software LS-DYNA is used.

Abstract: The properties of fiber reinforced materials are depending on the fiber direction. During draping processes - which are necessary to form complex structures - the fiber direction and therefore the resulting properties of the final part are changing.To ensure that the fibers in the final complex structure are placed exactly in the direction needed, a new approach is investigated.The idea is to define the orientation of the reinforcement fibers based on the distribution of forces in a complex structure under certain loading determined by a structural simulation. Best lightweight behavior is achievable in the final complex structure. A three-dimensional mesoscopic model of the directed fibers is created using FEM-software. In reverse draping simulations the three-dimensional fibers are formed from complex shape to a two-dimensional flat sheet.In manufacturing the two-dimensional patches can be created using the tailored fiber placement process. With this process it is possible to place the fibers orientated to the required paths. The patches are formed to the necessary three-dimensional shape by a real draping process. The relative sliding behavior of crossing fibers can be achieved by varying the stitch during the TFP process.Using that approach it is possible to create lightweight structures in which fibers are orientated directly along the load paths of the three-dimensional application.

Abstract: A short overview of different locking mechanisms is presented, in which the more recent locking phenomenon tension locking is placed. Tension locking is found during composite forming simulations and occurs when the element edges are unaligned with the fiber directions. Discontinuities in composite forming simulations show similarities with other weak discontinuities such as material interfaces and shear bands found during shear localization. Two enrichment strategies are explored to resolve tension locking in linear triangular elements.

Abstract: Compression molding of near net-shaped rib-stiffened plates has been performed for a parametric investigation on the filling behavior of chopped woven flake reinforcements. The experimental investigation showed that different aspect ratios of ribs can be filled completely within the tested maximum ratio of flake size to rib opening width of 6.25 and a maximum consolidation pressure of 15 bar. However, defects such as voids, non-impregnated regions and fiber matrix separation may arise depending on the combination of parameters and a mechanical jamming effect caused by the woven architecture of the flakes. A tendency for a limiting consolidation pressure is observed based on the fiber matrix separation. The ability to re-use thermoplastic prepreg cutting waste has been demonstrated.

Abstract: This paper presents the applications of advanced CVD Tungsten Carbide coating to extend the life of tooling used for forming abrasive and corrosive materials.Hardide nanostructured Tungsten Carbide coating combines high hardness (70-77Rc) with excellent toughness. Unlike other hard coatings Hardide can produce a conformal coating layer on complex-shaped tools, including internal surfaces of extrusion die cavities and moulds. In ASTM G65 test the Hardide coating abrasion resistance exceeded WC/Co (9%) cemented carbide by a factor of 4X, and D2 tool steel by 10X. Thus the coating can significantly increase the life of D2 steel tooling used for forming abrasive materials and by maintaining better dimensional tolerances and surface finish of the tool it will manufacture better quality products.The Hardide coating has enhanced resistance to corrosion and aggressive media, including acids; this makes the coating especially suitable for the tooling used in forming uPVC, PTFE and other corrosive materials.The Hardide coating has been tested on extrusion and pelletizing dies processing abrasive and corrosive slurries and typically showed a 3X increase in the life of the tooling. Similar results were achieved by the coating of powder compaction punch/die sets for pharmaceuticals tableting.

Abstract: In cold extrusion of aluminum alloys adhesive wear can be prevented by an excessive lubrication of the process. While this causes additional process steps also environmental risks have to be addressed. Hence, dry metal forming, i.e. avoiding lubrication by means of coatings and topography modifications is highly desirable. In this paper first results concerning the behavior of tailored surfaces under dry metal forming conditions for pure aluminum are presented. Different surface treatments (laser polishing and Mo₂BC coating) of the tool steel AISI H11 are tested in a compression-torsion-tribometer under conditions adapted from cold extrusion. Normal stresses six times higher than the initial yield stress of the tested workpiece material pure aluminum (AA1050-O) are applied. Furthermore, a strategy for the characterization of aluminum adhesions to the tool is introduced. The influences of different topographies and the presence of a coating on the loss of material due to adhesive wear are investigated.

Abstract: Ecological aims and political requirements today are increasing demands on lubricants in sheet metal forming and their impact on environment. For that reason, metal forming industry wants to reduce the amount of lubricants containing polluting additives with a long-term goal of avoiding lubrication entirely. Additionally, dry metal forming will reduce the cleaning steps after the forming operation. This paper shows a new tribological system in which lubrication is replaced by CO₂ in a liquid state. Here, CO₂ is expanding directly into contact area between workpiece and tool surface and changes its state from gas to solid. The combination of this particular dry ice as well as the pressure of approximately 57 atm affects resulting friction coefficient significantly. After forming operation, CO₂ medium vaporizes and a dry component can be used immediately for the next process steps. In this case, the lubricant is applied directly into the contact area. Therefore, laser drilled micro holes are located in the contact area of the tool. Very first gained experimental results disclose such feasibility, the effects and the potential of this new lubrication system at that moment is based on strip draw tests. Different numbers of micro holes are examined to support blank holder pressure ranging between 5 MPa and 6 MPa. In this investigation a mild strength steel DC04 is used as sheet material. This knowledge is aimed to be used for further investigation and later transfer into real deep and stretch forming processes.

Abstract: Tools sustainability and reliability is a key axis for economic competitiveness of companies in the field of cold heading of steels. This durability is currently limited by the damage occurring at the contact surfaces.The main objective of this study is to propose an energy based approach to understand the mechanisms of deterioration of the WC-Co carbide tools.Firstly a finite element simulation of an industrial cold heading process is run in order to identify the contact condition at the tool workpiece interface. Main results are the stress, strain and temperature distributions in the near surface of the tools. A particular attention is paid to the location of critical areas that may limit the tool life.Jointly, characterizations of the morphology of the worn surfaces are performed. SEM observations added to EDS and roughness measurements are done from midlife to end of life of industrial tools. Friction tests are performed with the Upsetting-Sliding Test involving contactors extracted from real worn tools to identify friction coefficients in order to provide the evolution of the friction coefficient according to the wear state of the tools. Finally, the correlation between the numerical analysis and the experimental measurement is discussed to attest to the relevance of the energy fracture based model to explain the deterioration of the tribological conditions.

Abstract: This paper presents recent investigations in the field of lubricant escapes from asperities. This phenomenon, named Micro Plasto Hydrodynamic Lubrication (MPHL), induces friction variation during metal forming processes. A better understanding of MPH lubrication would lead to a better management of friction, which is a central element in most sheet metal forming processes. To fulfil that goal, experiments were conducted in plane strip drawing using a transparent upper tool in order to observe lubricant flow around macroscopic pyramidal cavities. These experiments were then numerically reproduced with two complementary Finite Element models. The numerical results are discussed in this paper and show good agreement with experimental measurements.