Analysis and Optimization of Springback in Sheet Metal Forming

Sheet metal forming processes are widely used in the automotive industry to fabricate many components such as body panels, the structural members of the chassis and so on. The forming process involves many stages. There are many defects that might occur on a work piece during or after each set of processes and one of the most challenging of these is associated with the phenomenon of springback; that is, the distortion in specimen geometry due to the elastic recovery and other effects. The integration of springback into the design of the forming process represents a significant challenge due to difficulties associated with its prediction.
There are several factors that control the magnitude and direction of component distortion causing by springback. The primary aim of the present study is to evaluate the influence exerted on springback by the main parameters that affect the forming process. This will provide guide lines to create new CAE methods that can be used to predict the amount of springback within sheet metal forming processes. Two common forming processes will be investigated within this work, the so called L-bending and U-drawing processes, since these underpin many of the more complex forming operations. A forming test rig has been designed and manufactured that replicates each of these processes under controlled and repeatable conditions. Process parameters that can be controlled are the die and punch profile radii and clearance between the punch and die, and the normal clamp load applied on the work piece by the blank holder. In parallel, finite element models capable of simulating the L-bending and U-drawing bending processes were developed and validated for four different blanks materials: high and low strength steel, and high and low strength aluminium alloy.
Material characterization for four different blanks was conducted to derive required parameters for the simulation analysis. Also, friction coefficients were measured between each blank material and the forming tools using a pendulum tribometer.
Mesh sensitivity studies were firstly conducted to provide a mesh that represents an appropriate compromise between accuracy and consuming time. Results from the numerical analysis were compared to those from the experiments and good agreement was generally found, except for the high strength steel where the galvanised coating (not modelled in the analysis) affected the results.
The model was then used to conduct parametric studies on the effect of certain parameters on the amount of the springback i.e. the blank holder load, die and punch radii and the radial clearance. Finally, an optimisation scheme was developed to derive the optimum combination of parameters to minimise springback. These results and the general methodology could form the basis of a reliable CAE system to control springback in common metal forming operations.