An Experimental Investigation of Parameters Influencing Dimensional Accuracy in Rotary Stretch Bending

The rapid application of alternative materials is vital to the environmental and sustainable development of the European manufacturing industry. In many cases, companies seek using ‘old’ materials, such as aluminum alloys, magnesium, titanium, composites and plastics, for new applications in order to improve customer value; i.e., the perceived product benefits subtracted its price. For a subset of manufacturing companies, one of the most viable strategies is to offer weight savings at an affordable price by replacing steel with aluminum alloys, typically providing added value in the 0.25 - 0.5 kg/€ range, depending on application. Hence, weight savings shave to come with minimum cost penalty, which calls for a balanced optimization of manufacturing process and material consumption. For formed products, the latter optimization is driven by the performance characteristics of the final product as well as its quality capabilities relating to dimensional accuracy. The former is closely related to the attributes of the particular forming method. In this study, an industry-like rotary stretch bending set-up has been equipped with two different modular die inserts to determine how process characteristics impact dimensional accuracy of the product, as represented by local cross-sectional distortions and global elastic springback in industrial practice. A number of cross-sectional profile geometries with different wall thickness, depth, width and number of internal chambers have been simultaneously stretched and bent into the shapes provided by two die different configurations. The extruded profiles were made in different heat treatable alloys within the AA6xxx and AA7xxx series and formed according to industrial practices, which for the latter included solution heat-treatment immediately before forming. The results show that the slenderness of the external flange of the cross section is the main parameter with regard to the magnitude of local cross-sectional distortions. In practice, therefore, wall-thickness is the critical quality control parameter. The material’s yield characteristic proved to have limited impact on local distortions, although this is usually a major concern with regard to springback variations. The results from the experimental series have been structured into a design relationship for assessment of local distortions where the problem is reduced to the use of a proposed dimensionless bendability parameter as input.