Key Engineering Materials Vols. 611-612

Abstract: This paper presents results from application of discrete state spacebased feedback control of the blank holder force (BHF) magnitude and distribution in the drawing ofa rectangular workpiece. By means of monitoring the flange draw-in during the punch stroke, the controller makes continuousadjustments to the BHF magnitude and distribution by individuallycontrolling the pressure of a hydraulic fluid in a number of cavitiesembedded in the blank holder plate. The adjustments are conducted onlineduring the punch stroke to minimize the deviation of the achieved flange draw-in from a predetermined reference.

Abstract: Single point incremental forming (SPIF) of sheet metal is a promising process to produce small batch production and prototyping. This process consists of a controlled process of displacement performed on a three-axis CNC milling machine. In former work, the most critical factors which affected single point incremental forming process were found to be formed shape, tool size, material type, material thickness and incremental step size. The present work is focused on an optimization strategy of (SPIF) process determined by a numerical study based on finite element analyses (FEA) according to a Box-Behnken Design of Experiments. Two types of hardening behaviour laws of material are used: isotropic and combined isotropic-kinematic hardening behaviour. To do so, a set of numerical simulations are carried out for an aluminum truncated cone as geometry of a benchmark model. The simulation results include some decisions about the mechanical resistance and geometrical quality of the parts such as the thickness distribution and the magnitude of springback. In this paper, the main objective is to present an overview of multiobjective design optimization of process parameters in single point incremental forming operation in order to minimize the sheet thinning rate and the springback simultaneously. In this investigation, the steps of optimization procedure include the using of Box-Behnken experimental design for sample producing, response surface model for coarse fitting and a developed Multiobjective Genetic Algorithm (MOGA) for exact solving of fitness functions. The results show that these methods are able to determine all the best possible compromise with respect several antagonistic objectives as well as generate the approximate Pareto optimal solutions. So these will make it possible to choose the appropriate process parameters according to the objectives functions to be minimized and consequently the improvement of the products formed by the process of incremental forming.

Abstract: Due to the development of corrosion-resistant lightweight, todays automotive manufacturers typically use zinc coated sheet metals in the forming process. However, zinc abrasion in industrial presses decreases the process stability and often causes interruption of the whole process. The application of high strength steels leads to a significant increase of the temperature due to the plastic work. So far a detailed, quantitative analysis of the relation between temperature and zinc abrasion is not available. Therefore, this paper examines the impact of the temperature on abrasion behaviour in sheet metal processes. To achieve this, a progressive die was built. The deep drawing stage of this tool is connected to a cooling / heating system in order to obtain a constant temperature during the forming process. A variety of different galvanized sheet metals compared to commonly used tool materials has been tested. For each combination of materials five experiments at different temperatures were performed to determine the effect of the temperature on the zinc abrasion. Applying the method of total reflection x-ray fluorescence (TXRF) the quantity of zinc abrasion was measured. A relation between low temperatures and reduced zinc abrasion can be clearly observed. Industrial experiments revealed that temperature exerts a high influence on the zinc abrasion. The new insights into the impact of the temperature show a significant way to lower the zinc abrasion and therefore increase the process stability in deep drawing processes.

Abstract: In this work, the multi-breakage effect has been studied by means of an experimental campaign and finite element analysis. We suggest that large radius bending (XL-bending) consists of three phases that are distinguishable according to the type of contact of the plate with the tool: 1-point, surface and 2-points. In the experimental investigation the high-strength steel Weldox 1300 and a 40 mm radius punch were used. The authors created a camera setup to film the multi-breakage effect. Additionally, finite-element calculations were performed to confirm the hypothesis of the three phases of the bending process. For the springback and the bending force evaluation, the difference in the moment distribution for each phase has been calculated in the case of a beam. It shows that the multi-breakage effect must be taken into account to obtain a good accuracy for the springback and the bending force calculation.

Abstract: Since the last few years several researches have focused in proposing new approaches to decrease the energy impact of the hot sheet stamping process. Among them, one possibility is to use steel grades characterized by a lower austenitization temperature in order to reduce the heating energy required for the process, but still maintaining the steel quenchability under the investigated process conditions. In the paper, the behaviour of two new steel grades developed by ArcelorMittal and devoted to hot stamping is investigated in terms of transformation kinetics and microstructural behaviour. An extensive dilatometric analysis, performed by using a modified axial extensometer, permitted to evaluate the steel kinetics of phase transformation as a function of the applied cooling rate, amount of strain and strain rate. The micro-hardness of water and die quenched samples of the investigated steel grades was measured and their microstructure analyzed by means of optical and scanning electron microscopy to evaluate the characteristics of the formed martensite and the possible phenomenon of autotempering.

Abstract: The presented study applied the state of the art in roller hemming simulation on a car body assembly. Corner areas with changing flange lengths are always challenging in case of reaching the quality demands. Thus, the numerical results like e.g. springback, hem thickness, roll-in and hemming geometry are compared to experimental results. It is shown that the quality of prediction depends on the system stiffness of the roller hemming device, the geometrical contour of the car body assembly and the consideration of steps in the process chain like stamping and flanging. Finally, the gained cognitions point out challenges for future research in this topic.

Abstract: Flexible sheet metal forming processes represent a big challenge, which involved a number of researchers all over the world in the last decades. Among these, Incremental Sheet Forming (ISF) process is one of the most investigated and promising due to its simplicity, cheapness and applicability. Furthermore, the possibility to increase the process velocity makes the ISF more suitable than in the past; as a consequence, its application potential is surely increased. It was already highlighted that high speed significantly raises the process temperature, improving the workability of Titanium alloys. In this process configuration, no further heating source is strictly required because the temperature increase is generated due to the plastic deformation and the friction conditions at the interface between the punch and the sheet. While the process feasibility has been already investigated, a lack of knowledge in the literature is present focusing on the analysis of the process impact on the material properties. Accordingly, an experimental campaign on Ti6Al4V sheets has been performed, considering a punch speed two orders of magnitude higher than the conventionally used one. The obtained surfaces have been compared to sheets worked by traditional velocity in order to accurately analyze the impact of high speed. Furthermore, microstructural analyses have been carried out confirming the high speed suitability. All the details are reported in the manuscript

Abstract: The lightweight metal alloy Ti-6Al-4V is widely used in the aeronautical industry due to its excellent mechanical properties. However, it is known the difficulty to deform Ti-6Al-4V sheets at room temperature because of its microstructure conditions. The present work focuses on the evaluation of formability of Ti-6Al-4V sheets using hot single point incremental forming (SPIF) process which it seems appropriate to produce small batches of parts due to its flexibility as it allows a significant reduction of costs and lead times. In order to characterize the SPIF of Ti-6Al-4V under hot forming conditions, a set of forming trials evaluation tests was carried out. The obtained results have allowed identifying the key process features and have demonstrated the potential of the proposed approach to hot form of small amounts of Ti-6Al-4V parts.

Abstract: Short Cycle Stretch Forming (SCS) is a new stretching process patented by the Institute for Metal Forming (IFU) of the University of Stuttgart in 2006 [. It was mainly developed for outer car panels like doors [ and roofs [ with the aim of:

Abstract: Hot forming processes are becoming a successful solution when complex geometrical components with high mechanical properties are desired. In fact, automotive structural components with tensile strengths higher than 1500MPa are being nowadays industrially produced. The technology is based on the forming and quenching of the sheet inside the forming tool using boron steels. Aiming at boosting the advantages of this technology, car manufacturers have started to demand structural components with different mechanical behavior areas in order to improve the impact response of the auto-motive passenger compartment: the so called tailor tempered components. The basic idea is to obtain final parts with different properties like it has been successfully done using tailored welded blanks. Although different solutions exist, one of the most common strategy is to use partially heated tooling, which influences the cooling of the sheet and consequently the local properties. At the present work, a special tooling with independent heated and cooled areas has been developed in order to evaluate the final properties achievable in the tailored tempering process. Furthermore, high and low conductivity alloys have been used to find the process limits. Hardness values, Ultimate Tensile Stresses and microstructures are shown for different steels, tool temperatures and contact pressures