The Science of Forming
More About the Laws of Nature
Hold an object in the air. The object has potential energy. A person holding the object in the air must contribute additional energy to continue maintaining the object at its potential-energy level. Release the object and the potential energy changes to dynamic energy. Upon hitting the floor, the dynamic energy causes deformation of the floor and heat. Now the floor must contribute the force to maintain the position of the object. The scientific explanation sounds complex, but examples in the real world are easy to understand.
In last month’s column, we covered why this law of nature forced specific forming modes—usually against our process-design intent. Examples included kinking when bending material, buckles forming under the blankholder, and unexpected flow of sheetmetal around draw beads. Unfortunately, forming the part in the die is only half of the problem. The law of least energy also applies when removing the stamping from the die and during all subsequent operations involving the stamping.
While still in the die, the stamping contains many elastic stresses—both tensile and compressive. The tooling (punch, die ring, blankholder, etc.) exerts external forces or stresses on the stamping to balance the internal elastic stresses created during forming. Remove the stamping from the die and the internal elastic stresses now are unbalanced and the part changes shape until the stresses rebalance. Common press-shop thinking says the stamping tries to return to its original flat blank. This would be true for stampings without permanent plastic strains or geometric features that constrain further material flow. However, useful stampings have strains and shapes that hinder the return of the part back to the blank geometry. Therefore, the stamping now attempts to undergo additional deformation to any shape that reduces the elastic stresses and minimizes the total remaining energy (residual stresses). The part will twist, oil can, curl, change angles or do anything to minimize the residual stresses. Springback is the technical term for this shape change. Press-shop personnel have other favorite names to describe this shape change.
Through years of trial and error, many tool shops have learned how to correct for springback for each stamping they make through tool compensation. These compensation techniques include initial stamping design, over-bending, over-crowning, specific die radii, draw beads, restrike dies, lubricant placement, and many other specifics of tooling and process design. Unfortunately, this approach applies mainly to average springback. The stamping-to-stamping variation causes all of the problems associated with dimensional variations.
Place the stamping on a table after springback has created the new shape for the stamping. One might assume that the stamping now is completely stable. Instead, the stamping contains residual stresses balanced against each other. These residual stresses are like thousands of interconnected springs. Disturb one spring and the whole set of springs now must readjust to a new balance for least total energy. Of course, the shape of the stamping responds accordingly.
The list of possible disturbances that can trigger additional springback and shape change is extensive. Here are a few examples:
• Offal is trimmed.
• Holes or cutouts are punched.
• Additional global forming is performed.
• Embossment or other local forming is added.
• Brackets are welded to the stamping.
• Stamping is stress-relieved.
• Part is dropped on the floor.
The unknowns (etc.) make springback prediction a very challenging activity.
Simply trimming the offal causes at least two changes in the stamping. First, trimming the offal removes a specific quantity of the residual stresses (springs) from the stamping. Second, the offal acting as reinforcement or a geometrical restraining feature around the stamping now is missing. Predicting how the remaining residual stresses and geometrical constraints will rebalance the remaining energy is extremely difficult.
The elastic stresses in the blank at the onset of plastic deformation are proportional to the yield stress. As the material is work hardened, the elastic stresses increase even further. Visualize higher-strength metal alloys as having springs that already have greater initial extension. This causes more springback as the springs attempt to return to their unloaded state.
Springback also is proportional to the elastic (Young’s) modulus of the material. This would be equivalent to having weaker or stronger springs. At the same stress, aluminum with only a 10 million modulus will have three times the springback compared to steel with a 30 million modulus. The lower-modulus aluminum springs will have to stretch three times further to develop the same stress. Unloading the springs also means three times the springback for the aluminum stamping as compared to the steel stamping.
Many press shops work only with lower-strength materials requiring reasonable amounts of springback compensation. However, some of the newer advanced higher-strength steels may reach deformed strengths exceeding 200,000 psi or eight times the strength of the common AKDQ steels. An eight-fold increase in springback and residual stresses requires not only a greater attention to springback compensation within the stamping but also an understanding of possible tooling and press deflection. Those stampings are full of very big and strong springs just waiting to give you trouble. MF
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