برنامه ریزی فرآیند به کمک تشکیل لیزر

Laser forming has emerged as a viable means of assisting conventional forming processes with geometrical accuracy-related problems. By combining the incremental nature of laser forming with conventional processes such as brakeforming which forms material by a single continuous movement of the tooling, the exact specified bend angle and radius of curvature of the bent component may be approached. This may be achieved by sequential or simultaneous application of the conventional tooling and the laser beam. The laser beam may be applied once to the forming zone or multiple laser beam scans may be used. The combined process allows the forming of highly accurate sheet metal products in a cost effective way, through the possibility to make corrections to the bend angle in a controlled way. Furthermore, the combined process makes it possible to form intricate products that cannot be bent on a press brake due to collision problems or problems emanating from spring-back. Consequently there are new implications for process planning in brakeforming when a laser beam is used in combination. These implications are discussed for some primitive applications.

Laser forming is a non-contact forming process realised by introducing thermal stresses into the surface of a workpiece with a laser beam to induce plastic strains that results in forming as shown in Fig. 1. It may proceed by two primary forming mechanisms which includes out-of-plane or in-plane strain. The mechanism activated is dependent on the laser processing parameters employed, the geometry of the workpiece and the material properties. Out-of-plane deformation is induced by the introduction of a steep thermal gradient to the sheet via a rapidly scanning laser beam applied to the surface of the material to be processed. Initially, the sheet bends away from the irradiated surface and then towards the laser beam. A material with a low thermal conductivity and a relatively small second moment of area (e.g. a sheet material) coupled with a rapid heat input facilitates this. Out-of-plane strain results in bending (see Fig. 2). Full-size image (6 K) Fig. 1. A laser forming set-up. Figure options Full-size image (1 K) Fig. 2. Out-of-plane bending. Figure options In-plane strain occurs when the heat generated from the laser beam is fully penetrative through the sheet thickness, the heated width is large compared to the sheet thickness and when the geometry (e.g. a box section material) resists out-of-plane bending. In-plane strain results in shortening (refer Fig. 3). Full-size image (2 K) Fig. 3. In-plane shrinkage. Figure options The laser is useful as a power source for forming as: 1. Mechanical contact between the workpiece and the forming tool (the laser beam) is not required. 2. Potential for accuracy and controllability of the amount of forming is great. 3. Material may be formed remotely. Large-scale applications of laser forming may be found in ship building and it has been applied successfully to the alignment of miniature structures in the micro-electronics industry [1]. It is not likely that laser forming will replace batch or mass production forming techniques for large sheet metal components. This is because laser forming is still in its infancy in terms of some of these more complicated forming tasks, and only recently has progress been made towards symmetrical three-dimensional laser forming [2]. Compared to laser forming, conventional forming processes such as brakeforming or swing-type forming are fast and large deformations can be obtained in one process step. However, process control can present problems, as discussed in [3] for air bending. One of the main problems is the direct influence of material properties on the bend angle. Even when adaptive in-process control is used, variations in spring-back angle cannot be addressed in such a way that their effect is eliminated completely. Near-net-shape forming has been gaining increasing popularity due to improved processes, high material utilisation, suitability for high value-added and/or low-volume production and superior mechanical material properties. As a result, brakeformed sheet metal components have on average become increasingly intricate. In this paper, a simple rectangular box is used as an example. This suffices to demonstrate the problems of conventional forming and the advantages of hybrid processing. However, these problems and advantages become even more profound in the case of intricate sheet metal components.

This paper has shown the potential to combine the benefits of two radically different forming techniques to improve geometrical accuracy and flexibility in sheet metal part processing. The collision problems can be largely circumvented by underbending the component and then finishing the forming in the final stages by using a laser. It was discussed how the same laser beam could be used to weld the seams of the box closed, and to cut features from the box, which may have shifted out of tolerance had they been pre-cut or punched before the forming operations. The incremental nature of the laser adjustment process was shown experimentally with a CO2 laser, at least to the limit of the accuracy of the bend angle measuring device. The geometry of the box itself and the proportion of the laser beam that was able to strike the correction zone served as process control parameters. The decreasing bending rate with increasing the number of laser scans over the same track was shown to be beneficial for approaching an exact angle.