Sheet Metal 2007

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Authors: Kari Mäntyjärvi, Markku Keskitalo, Jussi A. Karjalainen, Anu Leiviskä, Jouko Heikkala, Jarmo Mäkikangas
Abstract: When sheets of high-strength (HS) and ultra-high-strength (UHS) steels are bent by a press brake the process suffers from large bending forces, considerable springback, and eventual cracks. Additionally, some unpredictable effects, such as lost contact to the punch, caused by strain hardening may occur producing a bend with erroneous radii. The strain hardening of the bending line may make further processes, such as forming or welding, more complex. One solution to these problems is to anneal the bending line with a laser in advance. Of course, it is also possible to utilise other types of heat sources, but the laser can offer the most precisely controlled heat treatment. The proper process parameters depend on the material, and it has been noticed that inadequate process parameters may harden the material instead of annealing. In this work some experiments on bending sheet metal samples of HS or UHS steel with previously laser-annealed bending lines have been carried out and the outcome analysed. The results show that the annealing produces better bending results compared to the conventional procedure. This includes lower springback, less hardening in the bending line and more precise geometry of the bend. It can be even suggested that proper annealing with strain hardening in bending will produce the original material structure. Obviously, more theoretical and experimental work is required to optimise the process parameters including the laser power and speed for each pair of material strength and thickness.
Authors: Masaaki Otsu, Yasuhiro Ito, Akira Ishii, Hideshi Miura, Kazuki Takashima
Abstract: Pure titanium foils were bent by laser forming and the effect of c-d transformation and history of heat treatment of specimen on bending angle was investigated. The thickness of specimens was changed from 40 to 100om, the length of them was 20mm and the width of them was 10mm. The specimens were annealed at 600-1100oC for 30 minutes in argon atmosphere. A 20W YVO4 laser was employed and laser power was changed from 2 to 16W. From the experimental results, when laser power was increased, bending angle also increased and it was dramatically changed at the laser powers occurring c-d transformation and melting. Bending angle increased as grain size increased and it jumped up when grain size exceeded the foil thickness and then became constant. Bending angle decreased by annealing after forming and degree of decrease was greater when the annealing temperature before forming was lower.
Authors: Peter Groche, Jens Ringler, Dragoslav Vucic
Abstract: Due to the high effort involved, bifurcated constructions in mass market products made from sheet metal remained largely unused. Extruded profiles with cross-sections containing bifurcations show the possibility to increase the stiffness and allow modern lightweight design using load optimized structures as well as in box strap, sandwich and stringer constructions or different profiles. The two new forming processes linear flow splitting and linear bend splitting developed at the PtU enable the production of bifurcated profiles in integral style made of sheet metal without joining, lamination of material or heating of the semi-finished product. These forming processes use obtuse angled splitting rolls and supporting rolls to transform the sheet metal at ambient temperature. Whereas the linear flow splitting process increase the surface of the band edge and forms the band into two flanges. At linear bend splitting a bended sheet metal as semi finished product is used. Thereby bifurcations at nearly any place of a sheet metal can be produced. Both processes induce high hydrostatic compressive stresses in the local forming zone during the process which leads to an increased formability of the material and thereby to the realization of large strains. Parts produced are characterized by increased stiffness, high surface hardness and low surface roughness. Experimental investigations have shown an increasing of the band edge surface at maximum splitting depth up to 1800%. By a following forming process new multi-chambered structures and integral stringer construction can be realized with thin walled cross-sections from steel of higher strength.
Authors: Mario Marobin
Abstract: In the machine tool sector, for production purposes, flexibility and automation are elements that play conflicting roles because greater automation generally means less flexible production. As a result, it is not easy to choose between technologies that have different degrees of automation or may even be manual. We are therefore proposing a comparative method of analysis that can be used to evaluate one machine tool with respect to another, which is used as a reference. This combination leads to the identification of breakeven points on productivity, flexibility and automation that, when taken together, provide criteria for evaluating the economic suitability or unsuitability of the choice in hand. This comparative model rotates around evaluation variables that have long been consolidated and recognized by sheet metal machine tool manufacturers and users.
Authors: Branko Grizelj, Branimir Barisic, Miljenko Dino Math
Abstract: The paper is concerned with the numerical method of determination bending force and calibration force in plate bending. For numeric procedure the finite element method is used. Calibration force is determined when bending force and calibration coefficient are known. Significant factors for determination of bending force are: material of the circular plate, bending radius circular plate, diameter of the circular plate, thickness of the circular plate and method of loading of the circular plate. The calibration coefficient is determined by experiment. The analysis of bending plate is limited to the facts and figures used so far in the fabrication of spherical tanks, i.e. for deformations up to 1 %.
Authors: Gianluca D'Urso, Giuseppe Pellegrini, Giancarlo Maccarini
Abstract: Angle control in air bending is achieved either by exploiting direct angle measurement (adaptive forming) or by controlling punch displacement. In general, the desired angle is supplied as input to CNC press brakes and the choice of punch stroke relies on either analytical or empirical models. Process geometry and material properties affect the outcome, therefore, full knowledge of these values is critical. Since a major source of inaccuracies is due to errors in material description (sheet thickness is critical as well), material data (or, more generally, sheet behavior information) collection in process is advisable. In air bending, process control can be improved by studying the total load as a function of punch displacement. This approach becomes more and more interesting since devices for load measurement are now available on the market. The aim of this work is to analyze some experimental load measures, collected in actual working conditions, to evaluate the accuracy of such technique and its potential for in process applications. Several sheet materials (ferrous and non-ferrous alloys) are studied through both bending and tensile tests; the resulting material properties (tensile and bending) are evaluated and compared. After data treatment of punch force signal, the ability of predicting punch displacement needed to reach a defined bending angle (after springback) is discussed.
Authors: Peter Groche, Michael Henkelmann
Abstract: During the last half century roll forming has become a highly productive metal forming technology, well-established in the industry for the manufacturing of mass products. About 8 % of the annual world production of steel is processed by roll forming mills. Roll forming technology enables the manufacturing of profile-shaped products with an extremely wide spectrum of geometrical shapes. In lightweight construction, the utilization of roll-formed structures of high and ultra-high-strength steels has increased remarkably in the recent years. However, the application of those types of steel entails some disadvantages resulting in a decreasing forming capacity and enormous efforts to reach the required dimensional accuracy. Until a profile leaves the roll forming machine with the target quality, it is mostly necessary to align the forming rolls several times. Sometimes even design changes are required. This is the result of unreliable process planning. Furthermore, typical profile failures such as twist, flare and spring-back occur even stronger compared to mild steels. Nowadays, it is usual to control the dimensional accuracy of the profiles after the last forming stand. This kind of quality control has the following disadvantage: manufacturing errors are detected very late. Therefore, a continuous quality control process and an active manipulation during the forming process promise a large potential for an improvement of the dimensional accuracy and an increase of roll forming productivity.
Authors: Junichi Endou, Shunji Katoh
Abstract: In the proto-type bending machine reported in the previous paper, the Bending Roller was fixed in the same position during bending. Therefore, almost uniform curvature in longitudinal direction was obtained. We improved the machine as the Bending Roller can be moved by a servo motor. By this improvement, we can bend aluminium space-frame into varied curvature in longitudinal direction. The curvature of a bent aluminium space frame is different from the setting curvature calculated from the moving position of the bending roller, because so called spring back occurs. We found out that the abrupt increase of objective curvature causes collapse of the specimen and we cannot obtain the objective curvature. And the sudden decrease of the objective curvature by sudden decrease in position of bending roller cannot be realized because of the outbreak of gap between the bending roller and the bent space-frame. If we avoid the abrupt change of the objective curvature, we can compensate the spring-back and we can get the objective curvature.
Authors: Reimund Neugebauer, Eberhard Kunke, Hans Bräunlich, Angela Göschel
Abstract: Today's automotive manufacturers are required to meet ever greater demands for increased flexibility due to decreasing batch sizes. Solutions to meet these demands will bring about far-reaching changes to the mass productions methods which currently dominate automotive manufacturing. In addition to the current need for sheet metal components, such trends will also have an effect on assembly and joining techniques used. The paper describes the challenge for production engineering resulting from current and future market demands.
Authors: Seijiro Maki, Atsushi Hamamoto, Shouichi Saito, Kenichiro Mori
Abstract: The feasibility of hot stamping and press quenching of ultrahigh strength steel sheet using resistance heating was investigated by demonstrating experiments. In the experiments, a rectangular blank of high strength steel sheet, SPFC980Y, 1.2mm thick, 20mm wide and 130mm long was used and it was resistance-heated with a pair of parallel electrodes placed 120mm away from each other, using an AC power supply with a function of input energy control. The relationship between input energy and heating temperature was examined and then hot stamping and press quenching performances were examined. The press quenching was also experimented in hot hat-shaped bending, and the influences of the heating temperature on spring-back and hardness property were investigated. The results of the experiments revealed that hot stamping and press quenching using resistance heating are highly feasible.

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