Page 12 - MetalForming July 2019

Tech Update
Springback Prediction Yields
Automotive Parts Without Costly Tool Recuts
Family-owned Atlas Tool Inc., Roseville, MI, provides tool building and low-volume stamping with its impressive stable of 40 mechanical and hydraulic presses. Through the use of advanced simulation software from AutoForm, the company’s engi- neers were able to accurately predict springback when stamping an auto- motive rear cab reinforcement part from 1.3-mm-thick DP600 advanced high-strength steel (AHSS), saving significant time and money as com- pared to a trial-and-error approach.
Increasing applications of light- weight sheet materials, mainly AHSS and aluminum alloys, to meet weight reduction requirements, bring new challenges to numerical modeling of sheet-material forming processes, explains Yurdaer Demiralp, senior application engineer at AutoForm Engineering USA, Inc., and Mark Broadworth, tool engineer at Atlas Tool Inc. Demiralp and Broadworth go on to explain these challenges and how simulation software can overcome them, then detail how Auto- Form’s software proved effective in predicting springback on this Atlas Tool automotive part.
Of chief concern when forming these lightweight materials is spring- back—undesired shape changes in a formed part due to recovery of elastic deformation after the tooling opens. In AHSS, springback results from high yield stresses, and in aluminum alloys from a lower young modulus.
Fig. 1—These diagrams describe the Bauschinger effect, where sheet material flowing on the tooling interface undergoes complex strain-path changes such as tension to compression, or vice versa, during several loading and unloading cycles. Simulation that successfully addresses springback account for this effect. (Left illustration courtesy of D. Banabic et al., Sheet Metal Forming Processes: Constitutive Modelling and Numerical Simulation, 2010, p. 122 and p. 126. Right illustration courtesy of T. Yoshida et al., “Material Modeling for Accuracy Improvement of the Springback Prediction of High-strength Steel Sheets,” Nippon Steel Technical Report No. 102, January 2013.)
The accuracy of results from sheet metal forming simulation strongly depends on the material model characteristics that include the elastic property range and plastic property deformations, represented by the hardening curve and the yield sur- face, according to Demiralp. In addition, the stress and strain state during the forming process and after tool opening must be simulated accurately in order to achieve reliable springback prediction.
In typical metal forming processes, the material experiences complex strain- path changes as well as repeated reversal of the stress state. Such reversals occur due to unloading after bending, and also due to cyclic and repeated bending and unbending of metal over bead and die radii. Isotropic hardening commonly is used to model the hardening of material under plastic deformation. This model assumes that material properties—essen- tially yield stress and Young’s modulus—
which mainly drive springback results, stay the same during cyclic tension-to- compression deformations. However, extensive research has shown that these two material properties will not stay the same during cyclic loading, Demiralp explains.
Strain hardening of sheet material dif- fers under cyclic tension and displays a behavior commonly referred to as the Bauschinger effect, also known as kine- matic hardening (Fig. 1). Consideration
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Fig. 2—Simulation of the forming of an automotive rear cab reinforcement part, produced from DP600 advanced high-strength steel, reveals how the use of kinematic hardening modeling accurately predicts springback. Performing this simulation early in tool design enabled Atlas Tool to produce tooling without the need for expensive tool recuts.