Longchang Tong

Modeling for the FE-Simulation of Warm Metal Forming Processes

Better formability, less forming force and satisfactory quality are the most important characteristics of warm forming processes. However, the material models for either cold forming or hot forming cannot be directly adopted for the numerical simulation of warm forming processes. Supplement and modification are necessary. Based on the Zener‐Hollomon formulation, additional terms are proposed in the presented work to describe the softening effect observed during warm forming processes as well as the strain hardening effect. The numerical simulation provides detailed information about the history and distribution of both deformation and temperature, the phase transformation can then also be evaluated, provided the experimental data are available.

Zero Failure Production Methods Based on a Process Integrated Virtual Control

Although the virtual methods are nowadays fully established as a widely used tool in the planning and optimization of forming processes they are still completely omitted for a direct, “intelligent” process control in the later production of the parts. The paper presents a proposal for a Process-Integrated-Virtual-Control (PIVC) considering the real process perturbations induced by deviations of material properties as well as by further time dependent process parameters like the tool temperature. For the detection of the material deviations an in-line eddy-current measurement method and the appropriate evaluation method for the definition of the stochastic yield curves will be presented. The paper closes with a virtually taught control system modifying the blank-holder forces in dependency of thermal conditions and material deviations. The goal of this PIVC coupled to in-line process controls is to achieve a Zero Failure production even under alternative time dependent process conditions.

Forming Limit Prediction of Metastable Materials with Temperature and Strain Induced Martensite Transformation

Stainless steels as well as TRIP and TWIP steels show a hardening behavior, which can be described only in dependency on the deformation and temperature history during the real forming process. Because the hardening behavior is the determinate factor for the necking phenomenon, the prediction of rupture becomes also deformation path and temperature dependent. As a consequence, the common FLC‐method, using a single curve for the prediction of the failure state is not accurate enough. In this paper, a temperature dependent Forming Limit Surface (FLS) is presented.

An Improved Modeling of Friction for Extrusion Simulations

Realistic representation of friction is important in extrusion simulations. Purposefully designed multi‐hole die aluminum extrusion experiments showed that the conventional friction models, like the Coulomb and the shear friction models, are deficient to represent the boundary phenomena that occur during aluminum extrusion. Based on the observations, phenomenological and implementational improvements are made in the friction modeling.

An ALE Based FE Formulation for the 3D Numerical Simulation of Fineblanking Processes

Fineblanking is a manufacturing process which allows the mass production of blanked products with superior surface quality. The 3D numerical simulation of this particularly precise process is however challenging. This is because quality-critical tool features such as the die clearance and the shape of the cutting edges have dimensions up to two orders of magnitude smaller than the average part dimensions. If conventional Updated Lagrange codes are used, a very high FE mesh resolution becomes a must in order to accurately represent the surface evolution along the edge, which in turn makes the computation unfeasible. The methodology presented in this paper makes use of the Arbitrary Lagrangian Eulerian FE Formulation in order to keep control over the mesh region in contact with the tools. This way an optimal FE mesh can be guaranteed throughout the computation. This not only reduces the computational cost considerably, but also avoids mesh distortion along the cutting edge, allowing an accurate representation of the tool features. This approach will be used in conjunction to the stress limit criterion delineated in order to predict material failure in fine blanked products. Numerical results will be validated against the experiments carried out with a specially designed fineblanking tool in use at our institute.

Stress Limit Model With Deformation Dependent Damage For Failure Prediction In Bulk Forming Processes

Based on the experimental results, a model for the description of damage in materials during bulk forming processes is presented. Unlike the most of the existing models which suppose no damage occurs under compressive stresses, the presented model considers all deformations as the cause for damages. The damages are strongly oriented according to the deformations. Therefore, it cannot be described using just a scalar value. However, failure appears only when the tensile stress exceeds the limit value in the material. In order to perform the failure prediction in FE‐simulation of bulk forming processes, an ellipsoid in the stress space is introduced to describe the limit stress. The ellipsoid, or a sphere for an isotropic material, changes its shape according to the deformation applied on the material. The stress state works as the criterion for the rupture in the material.

Failure Prediction in Fine Blanking Process with Stress Limit Model

Extremely small blanking clearance and nearly sharp edges of blanking tool are the characteristics of fine blanking that produces near net shape parts. The extreme forming conditions make the failure prediction for fine blanking more difficult than for ordinary forming processes. Stress Limit Criterion (SLC) is adopted in this work to perform the failure prediction of 3D fine blanking process. In comparison with the stress triaxiality diagram, SLC is not sensitively affected by complex nonlinear deformation paths and can perform the task as well. However, the parameters that support the model have to be obtained with combination of dedicatedly designed experiments and numerical simulation. The FEM simulation must also be able to provide stable and reliable results.

Constitutive Modeling and Numerical Simulation of Super Plasticity Forming Processes

Up to now the power function σ = A? m is commonly used to evaluate the flow stress to describe the material behavior in the numerical simulation of Super Plastic Forming (SPF) processes with the assumption that the m-value is a material constant. However, experiments revealed that the m-value is not a constant. It varies as the strain rate changes and reaches its maximum value at the ideal super plastic strain rate. Considering this fact, a mathematic expression is presented in this work. The variation of the m-value is taken into account in this model. Comparison with the measurement verified the method and simulation examples are shown as well.

Constitutive Modelling of Dynamic Strain Aging Effect under Various Loeading Conditions

Various analytical rules of mixture are commonly used to take into account heterogeneous features of a material and to derive global properties. But, with such models, one may not be able to fulfil the requirements for separating appropriately the different lengthscales. This might be the case for some issues such as strain localisation, surface effect, or topological distributions. At an intermediate lengthscale, which we refer to as the mesoscopic scale, one can still apply continuum mechanics. So why not perform calculations using the finite element method on volumes of material to obtain the response of Representative Elementary Volumes (R.E.V.). The construction of digital microstructures for such calculations is performed in two steps. First, a series of R.E.V.s with statistics of features of real materials should be defined. Then, finite element meshes should be produced for these R.E.V.s and updated when calculations involve large strains. Powerful automatic three-dimensional mesh generators and remeshing techniques prove necessary for this latter task. This strategy is applied to create digital R.E.V.s which match statistical features of forgings. Measurements provide micromechanical parameters of each subvolume. As an example of calculations, numerical simulations provide the anisotropic fatigue properties of forgings.