Key Engineering Materials Vols. 504-506

Abstract: This paper shows some product and process developments at the Institute of Forming Technology and Lightweight Construction of the TU Dortmund University supporting the lightweight construction. It presents the manufacturing of lightweight profiles by hot extrusion and their benefits as well as their design, material, and manufacturing potential for lightweight construction. Examples of process extensions in hot extrusion like curved profile extrusion, twisted profile extrusion and manufacturing of functional graded profiles and profiles with variable cross-sections during extrusion are shown. These procedures allow a flexible change of the profile geometry or contour in longitudinal axis and, therefore, support the shape lightweight construction. Other extensions like composite profile extrusion and energy efficient extrusion of profiles from scrap materials like chips support the material lightweight construction. The manufacturing and use of these profiles allow the realisation of diverse lightweight construction principles and promise to become a pillar of lightweight construction in future.

Abstract: Aluminum profiles are well-established components in lightweight constructions. However, these profiles have a small forming capability in comparison to steel profiles, which leads to a limitation in their application. Within this paper a new and innovative approach for the enhancement of the forming limit of aluminum profiles under bending load called Tailored Heat Treated Profiles (THTP) is presented. With THTP the mechanical properties of the material are locally modified by a short-term heat treatment. By this local modification the material flow during the following cold bending operation can be influenced. For the design of the heat treatment layout, the correlation between the heat treatment parameters and the material properties has to be investigated. Tensile specimens were cut out of the profile and were subsequently completely heat treated with a laser. The changes of the mechanical properties caused by the heat treatment were analyzed by tensile tests. However, with a complete softening of the profile, the formability could not be improved. To increase the formability a local heat treatment, which leads to partial softening of the profile, has to be investigated. In order to characterize the heat-affected zone (HAZ) of the laser treatment, thermal camera and microhardness measurements were carried out. Appropriate heat treatment layouts have to be found to enhance the forming limit. Different layout strategies were developed and afterwards validated by the heat treatment and forming of profiles. This paper will present the findings of this investigation and show that THTP can be used to improve the formability of aluminum profiles for bending operations.

Abstract: To prevent seizure in backward extrusion of holes for lightweight structural components, an extrusion method utilizing a punch having an internal channel for supplying liquid lubricant to the punch nose was applied to forming of pure titanium billet using a servo press. On the servo press, the punch was pushed into the billet in a manner combining pulsed and stepwise modes. The sufficient amount of the liquid lubricant was pulsating supplied to the cavity formed at the bottom of the hole. Appropriate punch ram motion and forming limit for preventing seizure are determined from the surface observation of the formed hole and the punch.

Abstract: In this study, conditions of metallurgical bonding between steel and aluminum in cold forging process is investigated. Two-layered cylindrical cup of the materials is produced in cold backward extrusion in five processing velocity conditions. Small tensile test specimens are cut off at the bonding boundary in the product using a wire-cutting machine and the bonding strength on the boundary is measured in tensile test using the specimens. Fractured contact surfaces are observed with an electron microscope for investigation of bonding. Finite element analyses for the backward extrusion are conducted and surface expansion ratio and interface pressure on the boundary are calculated. The influence of process conditions, extrusion velocity and surface expansion ratio and boundary pressure, on the bonding are investigated.

Abstract: The Commission of the European Communities aims for a reduction of new car CO2 emissions of 120 grams per kilometer in 2012. As a result of the omnipresent efforts of the automotive industry to hit these tighter emission standards innovative lightweight strategies, e.g. the use of lightweight materials are developed. This entails new joining techniques that are appropriated to the new lightweight materials. The die-less hydroforming process is a joining method for tubular joints which meets the new demands of lightweight strategies. Since there is no need for any additional connection elements or filling material, it is an interesting alternative to conventional welding and riveting processes. The present paper describes the basic principle of the die-less hydroforming joining technology with a special focus on form-fit connections. An analytical model, based on the membrane theory with an additional local consideration of bending stresses is developed. This analytic approach can be used to calculate the working fluid pressure, required to bulge the tube material into the groove of the outer joining partner. Taking into account the material parameters as well as the groove and tube geometry, this model allows a reliable process design. Additionally, validation of the model by experimental investigations will be provided.

Abstract: For a better understanding of the hydroforming process using magnetorheological fluids as a medium it is necessary to gain basic information for an analytical and numerical description of the fluid behaviour under process relevant conditions. Therefore an experimental setup is build up, which consists of a pressure chamber and a rotational symmetric outlet. Different parameters like feed rate and magnetic flux density are varied. Afterwards an analytical approach and a numerical model are developed. To get the relevant material input data for the fluid description an additional numerical simulation of the created magnetic flux density with flux lines perpendicular to the symmetry line of the outlet is performed. The leakage rates, which are observed for the experiments, are compared to the analytically calculated ones and to the results of the numerical simulation.

Abstract: Welding properties and formabilies of a tailored blank which consists of dissimilar metals are interesting because of the possible benefits for lightening of materials. Especially, the combination of mild steel and aluminum or aluminum alloy is expected as a representative of such a light hybrid material. Until now, some successful examples about welding of steel and aluminum using friction stir welding and laser roll welding have been reported. Recently, our research team has developed a butt laser welding method using CO2 laser. It was already confirmed that, by using this method, it was possible to obtain SPCC/A1100 O-tailored blanks with almost equal joint strength to the tensile strength of base aluminum. However, it has not been reported what was made in welded interfaces of the hybrid sheets. In this study, the welded interface of SPCC/A1100-O tailored blank obtained by butt laser welding was observed and analyzed using FE-SEM and EDS. Furthermore, fractured site in some formability tests were also observed, and it was identified where fracture occurred. In the experiments, the welded interface of steel/aluminum tailored blank, which has joint strength of 80MPa-90MPa, was observed. In the observation of low magnification, no clear intermetallic compound layers were seen. In the observation of higher magnification, it was confirmed that some intermetallic layers existed. From results of EDS analysis, it was indicated that this intermetallic compound layer consisted of FeAl3, Fe2Al5, FeAl, Fe2Al and Fe3Al. Thin Fe2Al5 layers of about 3 micro meters mainly existed between steel side and aluminum side. From this layer, some dendritic FeAl3 layers appeared into the aluminum side. In the steel side, relatively wide FeAl layers and Fe3Al layers were seen in turn. According to references, because welded sheet of steel and aluminum with good mechanical properties has intermetallic layers less than 4 micro meters, it was confirmed that butt laser welding method developed by our team had possibility to provide good steel/aluminum tailored blanks. In the fractured site by Erichsen test, Fe2Al5 layered were left in both steel side and aluminum side. Thus, it was indicated that a crack spread along the Fe2Al5 layer. From results of these experiments, it was confirmed that butt laser welding method developed by our team provided steel/aluminum tailored blanks with thin Fe2Al5 layers of about 3micro meters. And, it was indicated that this Fe2Al5 layer brought about early fracture.

Abstract: The Finite Element Method (FEM) is today the most widely used in numerical simulation of forming processes, due essentially to the continuous improvement of the FEM over the years and the simplicity of its implementation. However, this method has some limitations such as the distortion of elements under large inelastic deformation and the influence of the mesh on the results in several applications. The simulation of metal forming process with large plastic strain is a classical example where the successive remeshing is often the proposed solution in this case. But the remeshing raises the problems of precision and computing time. In this context and in order to avoid the remeshing process, a Meshless method is experimented in the solving of an elastoplastic problem coupled to the isotropic ductile damage. An Element Free Galerkin (EFG) method based on Moving Least Square (MLS) concept is considered in this proposal. A two-dimensional Mechanical problem was studied and solved by a Dynamic-Explicit resolution scheme where the material behaviour is based on an isotropic hardening fully coupled to ductile damage model. In a first step a parametric study is conducted in order to find the most influent parameters on the accuracy of the results. The effect of the number of nodes, of support nodes, of quadrature points, the effect of the time-step and the support domain size are analysed and optimal values are found. In a second step, the meshless results are compared with those of the finite element method and some concluding remarks relative to the accuracy and the computing time are given.

Abstract: Friction stir spot welding (FSSW) as a variant of the linear friction stir welding is implemented in automotive industry as a partial replacement of resistance spot welding for aluminium. FSSW as a solid state joining technology, primarily takes advantage of severe thermoplastic deformation, to achieve the joining between two parts, which can be from the same material or even dissimilar. In this paper, the coupled thermo-mechanical viscoplastic finite element formulation is presented based on the character of FSSW. The model was calibrated by comparing temperature history obtained from the simulation with experimental data and subsequently used to investigate the effective strain distribution in the weld zone as well as the material flow and the shape of the stir zone.

Abstract: The contribution deals with the simulation of sheet metal forming processes by means of a recently developed hexahedral solid-shell finite element. In contrast to this earlier work, we pursue here explicit integration. The element formulation has the following features. In order to avoid undesired effects of locking an enhanced assumed strain (EAS) concept using only one EAS degree-of-freedom has been implemented. In addition, by means of the assumed natural strain (ANS) method an excellent performance in bending situations is obtained. A key point of the element formulation is the construction of the hourglass stabilization by means of different Taylor expansions. This procedure leads to the important advantage that the sensitivity of the results with respect to mesh distortion is noticeably reduced. Further the hourglass stabilization is in this way designed that locking is eliminated and numerical stability guaranteed. The finite strain material model for plastic anisotropy and non-linear kinematic and isotropic hardening is motivated by a rheological model including Armstrong-Frederick kinematic hardening. The element formulation has been implemented into the academic code FEAP. Some standard benchmark examples are computed.