Electric Pulse Aided Bending: An Experimental Study

Sheet Metal Forming is used in major manufacturing like Automotive and Aerospace industries for producing various parts. Materials with low forming limit and high springback cannot be formed using this process without inducing failure and geomet- rical inaccuracies into the parts. Whereas materials with higher formability can be deformed to a larger extent before failure as compared to lower formability materials. The elastic recovery of the sheet metal after the forming process commonly known as springback which causes geometrical inaccuracies. Materials like Ti-alloys, with higher strength possess low formability and higher springback but also they provide advantage of higher strength to weight ratio over ferrous alloys. Many researchers are trying to develop new forming techniques to form these materials. A new approach to improve formability and reduce springback of materials using electrical current has been studied and is found to provide greater advantage over the conventional sheet metal forming processes. Reduction in flow stress due to electric current flow during deformation process is defined as Electroplastic Effect. This effect is related to the interaction between flowing electrons and moving dislocations. It was observed that decrease in force required for plastic deformation is directly proportional to the difference between drift speed (electron flow speed in particular material) and dislocation motion speed. Sheet metal bending experiments are conducted and the effect of electric current on springback is observed. To conduct bending experiments bend fixture is designed. Proper electrical insulation is included in design to ensure safe operation and to pre- vent electric flow in machine. Material selection is done on the basis of maximum stress developed in deformable tool and die in Finite Element Analysis results. Mate- rial is selected in such a way that the yield strength (including factor of safety ’3’) of the material must always be greater than produced maximum stress in FE analysis. After finalizing design and material for each part of the bend fixture, fabrication is done. FE analysis is carried out for sheet metal bending process and results are compared with published results to validate FE model. After validation, FE model is used with provided material (HSQ steel) properties to compare simulation and experimental results. It is observed that FE model is predicting springback close to experimental results. Sheet metal bending experiments on HSQ (High Strength Quenched) steel (pro- vided by TATA Steel) are performed. Electric current, pulse frequency, duty cycle, tool radius, feed rate and dwell time are considered as factors for electroplasticity experiments. Full factorial design of experiments with 2-levels is carried out to see complete effect. Total 64 experiments are carried out to analyse complete design of experiments. Whole sheet bending process is recorded using a digital camera. From recorded video, images are extracted at the end of deformation and after unloading. For springback measurement, radius of curvature in bend region is calculated using points traced from both the images and the difference between radius of curvature before unloading and after unloading is defined as springback. This methodology is used to eliminate the error induced due to initial curvature present in sheet speci- mens. Because of initial curvature, it is observed that measuring bend angle after springback is not accurate method. It is observed from electroplastic bending experiments that springback and punch force are decreased compared to conventional bending process. From DOE analysis it is concluded that the parameters which are affecting springback are mainly current, pulse frequency and tool radius. Temperature measurement is also carried out and maximum temperature reached during each experiment are noted down. It is con- cluded that increase in temperature (200o C) is not significant enough to cause any change in grain structure levels.