Electric Pulse Aided Draw Bending

Singh, Saurabh and Reddy, N Venkata (2019) Electric Pulse Aided Draw Bending. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

Sheet metal forming is widely used in automobile and aerospace industries. In current era, fuel efficiency is the main concern which requires materials having light weight and high strength. Although the new generation materials are having these required properties but most of these materials are difficult to form. Along with lower formability, higher springback is also a challenge for these materials. Currently hot forming is used for these materials but hot forming comes with the disadvantages of degraded mechanical properties and quality of formed component. For this reason, other alternatives are being explored to form these materials without affecting the mechanical properties significantly. Among all these alternatives, one is the use of electric pulses during deformation. It is reported that high strength materials can be formed at low temperatures by applying electric pulses during deformation. Application of electric pulses during deformation is called Electric Pulse Aided Deformation (EPAD). The pure effect of introduction of electric pulses is called electro-plastic effect for which different theories are proposed and it is concluded from majority of the theories that this effect is either due to the interaction of moving electrons and dislocations or due to localized joule heating. Other effect of applying electric pulses during deformation includes joule heating which causes the bulk heating of the material which is being exposed to the electric pulses. Studies reported that application of electric pulses during deformation has resulted in significant reduction of forming loads compared to hot forming at similar temperatures. Other advantage of applying electric pulses during deformation is the reduction of spring-back due to decrease in forming forces and reduction in residual stresses. Existing works are making use of this technique by applying electric pulses throughout work- piece the deforming material which causes increase in overall joule heating. In the present work draw bending setup is designed and fabricated to apply the electric pulses only in the deformation zone and by doing so overall joule heating is reduced. Setup is designed and fabricated to pass current through main deformation zone (die arc radius) as well as through the length of specimen with a provision to change the polarity of current. Design process started with finite element simulations of draw bending process to identify major regions of deformation and to analyse residual stresses. Ti-6Al-4V is considered for the present work. Initially uniaxial tensile testing with electro-pulsing is carried out on a custom designed Universal Testing Machine (UTM)to understand the mechanical behaviour when subjected to electric pulses. Different process parameters like current density, frequency of pulsing, pulse period etc. are taken into consideration while performing experiments. It is observed that when multiple pulses are applied during experiments, room temperature strength of the material can be achieved after each single pulse application with a proper selection of deformation rate and pulse period. Application of electropulsing during the test has decreased the percentage of elongation. Effect of frequency is also studied by keeping the same duty cycle and it is observed that total instantaneous drop for a single pulse is reduced with increase in frequency and simultaneously permanent softening is increased With the similar set of parameters used in tensile test, experiments are carried out on draw bending setup by passing the current through the length of specimen. From experiments it is found that when using the electrical parameters equivalent to tensile testing studies in drawbending there is not much reduction in punch force and springback. Higher current by keeping other parameters same is used for draw bending operation. It is observed that because of application of electric pulses retained height of the specimen increases to 5.2 mm from 3.9 mm (33.33 % reduction in spring-back) vi of room temperature with current of 440 A, pulse duration 0.2 s and pulse period 10 s. Experiments are carried out by passing current through die arc region (main deformation zone) to see the effect on punch force. It is observed that, there is less reduction in punch force in comparison to the case of passing electric pulses through length. It is analysed that the reason for this less punch force drop may be because of less amount of material which is being exposed to the electric pulses or the variation in the current density distribution in the sheet as it is moving down while deformation. To verify the same finite element simulations for current density distribution are done at different punch depths and it is proved that current density distribution is varying with respect to the punch travel. Finally, the effect of electropulsing on properties of the formed components are studied through investigations on microstructure, hardness and residual stresses. Sample which has shown more springback and punch force reduction is taken into consideration for carrying out the above studies. It is found that there is no visible change in the microstructure occurred due to electropulsing. Vickers hardness is measured in the wall region of formed component and it is observed that hardness is reduced only by 10 compared to room temperature forming. Residual stress measurement using XRD technique indicated that electro-pulsing during deformation reduces the residual stresses in the formed components, i.e., residual stress of 84.54 MPa is retained in the component formed using electropulsing compared to a residual stress of 146.69 MPa in the component formed at room temperature. This observation indicates that electropulsing has resulted in stress relaxation compared to room temperature forming, which is the major reason for springback reduction.

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