Analysis of damage and fracture formulations in cold extrusion

In forming processes, components generally undergo large deformations. This induces the evolution of damage, which can influence material and product properties. To capture these effects, a continuum damage mechanics (CDM) model, based on the work of Lemaitre [8] and Soyarslan [13, 14] as well as different fracture criteria according to Cockcroft and Latham [2], Freudenthal [4] and Oyane [10] are implemented and in- vestigated. While the CDM theory considers the evolution of damage and the associated softening, fracture criteria do not affect the results of the mechanical finite element (FE) analysis. However, a coupling is generally possible via element deletion, but material softening cannot be depicted in the simulation. Tensile tests with notched specimens are performed in order to obtain the material parameters associated with these models by inverse parameter identification processes. The optimized set of parameters is finally ap- plied to the damage and fracture models used for the FE simulations of a cold extrusion process, which are investigated in terms of damage evolution and material failure. It is demonstrated that the CDM model predicts the evolution of damage observed for differ- ent process parameters in cold extrusion quantitatively. The prediction of the failure by the fracture criteria does not agree well with the experiments

Part-optimized forming by spatially distributed vaporizing foil actuators

Electrically vaporizing foil actuators are employed as an innovative high speed sheet metal forming technology, which has the potential to lower tool costs. To reduce experimental try-outs, a predictive physics-based process design procedure is developed for the first time. It consists of a mathematical optimization utilizing numerical forming simulations followed by analytical computations for the forming-impulse generation through the rapid Joule heating of the foils. The proposed method is demonstrated for an exemplary steel sheet part. The resulting process design provides a part-specific impulse distribution, corresponding parallel actuator geometries, and the pulse generator’s charging energy, so that all process parameters are available before the first experiment. The experimental validation is then performed for the example part. Formed parts indicate that the introduced method yields a good starting point for actual testing, as it only requires adjustments in the form of a minor charging energy augmentation. This was expectable due to the conservative nature of the underlying modeling. The part geometry obtained with the most suitable charging energy is finally compared to the target geometry

Experimental and numerical analysis of the influence of burst pressure distribution on rapid free sheet forming by vaporizing foil actuators

Vaporizing Foil Actuators (VFA) can be employed as an innovative, extremely fast sheet metal forming method. An ultimate goal in forming technologies is generally to be flexible and rely on as few part-specific tools as possible. Therefore, various realizable VFA pressure distributions were investigated with a focus on the free forming result. Fundamental experiments including laser-based dynamic velocity measurements were conducted to discuss some key forming characteristics of the process. To compare more complex pressure distributions in a well-defined way, a numerical model was built. The strain rate dependency of the blank material was identified experimentally and incorporated in the model. It is shown that there are some VFA free forming capabilities in terms of creating certain part shapes, but only to a limited degree because relevant inertial forces can be present in regions where displacements would actually be either undesirable or wanted. Potential solutions to this are given at the end

Consequences of large strain anisotropic work-hardening in cold forging

The influence of anisotropic work-hardening on the component properties and process forces in cold forging is investigated. The focus is on the material behaviour exhibited after strain path reversals. The work-hardening of three steels is characterized for large monotonic strains (equivalent strains up to 1.7) and subsequent strain path reversals (accumulated strains up to 2.5). Tensile tests on specimens extracted from rods forward extruded at room temperature reveal an almost linear work-hardening for all investigated steels. The application of compressive tests on extruded material gives insights into the non-monotonic work-hardening behaviour. All previously reported anisotropic work-hardening phenomena such as the Bauschinger effect, work-hardening stagnation and permanent softening are present for all investigated steels and intensify with the pre-strain. Experimental results of 16MnCrS5 were utilized to select constitutive models of increasing complexity regarding their capability to capture anisotropic work-hardening. The best fit between experimental and numerical data was obtained by implementation of a modified Yoshida-Uemori model, which is able to capture all observed anisotropic work-hardening phenomena. The constitutive models were applied in simulations of single- and multi-stage cold forming processes, revealing the significant effect of anisotropic hardening on the predicted component properties and process forces, originating in the process-intrinsic strain path reversals as well as in strain path reversals between subsequent forming stages. Selected results were validated experimentally

Manufacturing of integrated cooling channels by Directed Energy Deposition for hot stamping tools with ball burnished surfaces

Hot stamping tools require cooling channels, preferably with a high positioning flexibility. Conventionally, these are machined. This represents a disadvantage because of the limited accessibility for milling tools and the low flexibility. By means of the Directed Energy Deposition (DED) process, a flexible design of the cooling channels is possible. Different geometries of cooling channels can be manufactured by DED in order to control the heat balance in the hot stamping tool. In this context an agreement between the additive producibility and the surface fraction of the cooling channels, which contributes to the effective heat at the tool surface, is important. Experimental and numerical analyses demonstrate that a possible configuration in this field is the drop shaped cooling channel. To reduce the surface roughness after the DED process, the tool surfaces are ball burnished subsequently. The resulting roughness and the waviness of the tool surface are reduced but not leveled completely. Texturing of the surface can be applied to in fluence the material flow in the hot stamping process which is implemented by DED. The combination of the described methods allows for manufacturing hot stamping tools with near-surface cooling channels and a global or local adjustment of the surface properties of the tools.Presshärtewerkzeuge erfordern Kühlkanäle, vorzugsweise mit einer hohen Positionierungsflexibilität. Diese werden üblicherweise gefräst. Dies stellt aufgrund der eingeschränkten Zugänglichkeit für Fräswerkzeuge und der geringen Flexibilität einen Nachteil dar. Mit Hilfe des Laserpulverauftragsschweißens (LPA) ist eine flexible Gestaltung der Kühlkanäle möglich. Mittels LPA können unterschiedliche Kühlkanalgeometrien hergestellt werden, um den Wärmehaushalt im Presshärtewerkzeug zu steuern. In diesem Zusammenhang ist eine Abstimmung zwischen der additiven Herstellbarkeit und dem Oberflächenanteil der Kühlkanäle, der zur effektiven Wärme an der Werkzeugoberfläche beiträgt, wichtig. Experimentelle und numerische Analysen zeigen, dass eine mögliche Konfiguration in diesem Bereich der tropfenförmige Kühlkanal ist. Um die Oberflächenrauheit nach dem LPA-Prozess zu reduzieren, werden die Werkzeugoberflächen anschließend glattgewalzt. Die resultierende Rauheit und die Welligkeit der Werkzeugoberfläche werden reduziert, aber nicht vollständig eingeebnet. Die Texturierung der Oberfläche, welche durch das LPA-Verfahren realisiert wird, kann zur Beeinflussung des Werkstoffflusses im Presshärteprozess eingesetzt werden. Die Kombination der beschriebenen Verfahren ermöglicht die Herstellung von Presshärtewerkzeugen mit oberflächennahen Kühlkanälen und eine globale oder lokale Einstellung der Oberflächeneigenschaften der Werkzeuge

Production and subsequent forming of chip-based aluminium sheets without remelting

Bent components and deep drawn cups are produced by direct usage of aluminium chips without melting following a new process chain: hot extrusion of aluminium chips to a cylindrical open profile, flattening, subsequent rolling and bending or deep drawing. The properties of the hot extruded chip-based AA6060 sheets are examined by tensile tests and microstructural investigations and the results are compared with those obtained from material extruded from conventional cast billets. The chip-based sheets were used to form components by bending or deep drawing. No significant differences between the bent components or deep-drawn cups made of chips and those from cast material are observed regarding their capability for further plastic forming operations. This makes the new process route a resource-efficient alternative for the production of aluminium sheet products

Interaction of process parameters, forming mechanisms, and residual stresses in single point incremental forming

The residual stress state of a sheet metal component manufactured by metal forming has a significant influence on the mechanical properties, and thus determines the time until the component fails, especially for dynamic loads. The origin of the resulting residual stress state of incrementally formed parts with regard to the forming mechanisms of shearing, bending, and the normal stress component is still under investigation. The relationship between the process parameters, the forming mechanisms, and the resulting residual stress state for a complex part geometry manufactured by single point incremental forming (SPIF) is presented in this publication. For this purpose, a validated numerical process model is used to analyze the influence of the step-down increment Δz for truncated cones on the characteristics of the forming mechanisms and the resulting residual stress state. For the first time the forming mechanisms are evaluated numerically on both sides of the formed component. A relationship between the process parameters, forming mechanisms, residual stresses, and the mechanical properties of an incrementally formed component is shown. Shearing-induced hardening is identified as a relevant influence on the residual stress state of cones