Compensating for material springback during the forming process can be challenging. During forming, stress is applied to the material making it conform to the die geometry. The material will strain or displace across the die geometry, which consists of complex shapes, creating numerous strain patterns throughout the part. These multiple strain patterns trap residual stresses within the sheet metal, resulting in distortion and springback once the forces are released.
New Advanced High-Strength Steels are characterized by yield strengths several times that of typical mild steels. They require higher forming forces to reach the “yield” strength, allowing permanent deformation to occur in the shape of the die. Modifications to die geometry or process parameters are required to correctly compensate for these residual stresses, otherwise the stamping facility will battle repeatability issues, which results in scrap or tool and die downtime. Typical reaction to this variation is to ask for tolerances to be increased and forcing operations further downstream to address these issues. The use of part stiffening beads, stake beads, and die geometry compensation will help to correct springback issues.
Strategically placing geometric shapes along flange walls, flange angles and sidewalls can lock in residual stresses, reducing the angular changes or twist created by springback. These geometric changes must be approved through product design. Commonly used shapes are darts, beads, step flanges and offsets.
Step flanges (example 1) can be added to flange walls to compensate for angular changes due to springback. The added stiffness created by the part geometry locks in the residual stress, which would create a curl or twist in the flange. Stiffening darts (example 2) are also used to reduce the angular changes in sidewalls or flanges. Adding an offset (example 3) horizontally along a wall will reduce twist and add stiffness to the part and creating higher strain patterns, which are introduced at the bottom of the forming cycle.
The use of stake beads when forming Advanced High-Strength Steel is a common practice to manage the stamping’s residual stresses. The stake beads are added to the part addendum and come into play at the bottom of the stroke, resulting in increased sidewall strain; the effect is to lock in the plastic deformation, reducing the curl and angular change in the sidewall. Figure 2 illustrates an open hat section with and without stake beads.
Training die makers to understand springback associated with Advanced High-Strength Steel, and how to manage springback through proven countermeasures enables more consistent stamping performance. The AHSS Application Guidelines Version 6.0 has a great deal of information on this topic. Section 3.C.3 (starting on Page 3-53) is devoted to Springback Management, addressing it in depth. If you haven’t already downloaded a copy, it is available free of charge at worldautosteel.org
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Contributions made by Phoenix Group.