Every stamped part has springback. When sheet metal is plastically deformed into a part, the shape of the part always deviates somewhat from the shape of the punch and die after removal from the tooling. This dimensional deviation of the part is known as springback. Springback is caused by elastic recovery of the part, which can be illustrated simply on the stress-strain curves shown in Figure 1.
Unloading (by removing all external forces and moments) from the plastic deformation level A would follow line AB to B, where OB is the permanent deformation (plastic) and BC is the recovered deformation (elastic). Although this elastic recovered deformation at a given location is very small, it can cause significant shape change due to its mechanical multiplying effect on other locations when bending deformation and/or curved surfaces are involved.
In general, springback experienced in AHSS parts is greater than that experienced in mild or conventional HSLA steels, due in large part to the progressively higher initial yield strengths and the greater work hardening rate that significantly increases the yield strength during forming. In fact, springback for higher yield strength materials can be up to eight times that of traditional mild steels, as shown below in Figure 2.
The magnitude of springback is governed by the tooling and component geometry. When part geometry prevents complete unloading (relaxing) of the elastic stresses, the elastic stresses remaining in the part are called residual stresses. The part then will assume whatever shape it can to minimize the total remaining residual stresses. Creating a uniformly distributed residual stress pattern across the sheet and through the thickness will help eliminate the source of mechanical multiplier effects and thus lead to reduced springback problems.
Three common forms of springback are angular change, sidewall curl, and twist. Angular change is the angle created when the bending edge line (the formed part) deviates from the line of the tool. Sidewall curl is the curvature created in the side wall of a channel when sheet metal is drawn over a die/punch radius or through a draw bead. The primary cause is uneven stress distribution through the thickness of the sheet metal, generated during the bending and unbending process. Twist is caused by torsion moments in the cross-section of the part, developing because of unbalanced springback and residual stresses acting in the part to create a force couple, which tends to rotate one end of the part relative to another.
Die designers have engineered the dies to compensate for springback, and developed many effective countermeasures. They have applied the appropriate amount of restraining force to deform the material and configured the die geometry to lock in the plastic deformation that the material needs to form a quality panel. In the stamping operation, however, process variation can affect the forces applied to the material, resulting in unexpected springback, nonconforming parts, and potential fit-up issues during the assembly process. For these reasons, tool and die countermeasures and enhanced process recipe discipline are required for effective AHSS stamping productivity and quality.
BONUS! Dr. Stuart Keeler is a well-known name in the world of Metallurgy, having created the Forming Limit Diagram used widely today. Watch as Dr. Keeler talks about the origins of springback.