An Investigation of Factors that Influence the Overall Magnitude of Cross-Sectional Distortions in Draw Bending

Abstract

European manufacturing companies are currently facing increased competition as a result of intensified globalization in the market and supply base. One strategy to meet this challenge is to develop and manufacture higher quality products at reduced cost. Metal forming is a typical manufacturing operation where improved technology can create advantages in the market place through higher value-added products. In the automotive industry, for example, improved shaping capabilities of profiles will improve product functionality, while reducing system cost due to reduction of part count and subsequent assembly operations such as welding. In addition, improved dimensional accuracy will provide benefits in terms of reduced quality costs and, sometimes, eliminating downstream processing steps such as calibration or machining. Rotary draw bending is typically used to manufacture profile-based shapes bent at tight radii with reasonable dimensional accuracy. The advantage of this process is low operational cost combined with relatively high flexibility, particularly with regard to bend angle. On the other hand, the disadvantage associated with the method is limited abilities to control local distortions of the cross section without taking special actions such as applying external stretching or complex tooling that ultimately would increase the investment and operational cost. The objective of the present paper is to identify the most important factors that influence cross-sectional distortions and quantify their impact on dimensional accuracy in draw bending, by performing a series of experiments in an industry-type draw bender. In order to accommodate different cross-sectional geometries, a flexible, modular tool concept was developed. AA6xxx aluminum alloy profile with different cross-sectional geometries (width, depth, thicknesses), single and multi-camber, were bent at different radii and bend angles, while measuring local distortions of the cross sections. The results have been presented in diagrams denoted flatness limit curves, showing the impact of various geometry (and material) parameters on local deformations of individual cross sectional members. The results show that the flange width (i.e. the free span between webs) is the main factor with regard to distortions, followed by wall thickness and bending radius and, finally, depth of the cross sections. Material parameters seem to have limited effect for the alloy tempers investigated. Attempts have been made to interpret the mechanisms associated with the development of local cross sectional distortions with the purpose of developing a practical design tool based on analytical calculations. The very first results show reasonably well agreement in the cases when local buckling of the internal compressive flange is less predominant.