Page 25 - MetalForming February 2010

mation, a circumferential band of mate- rial around the punch became warmer than the remainder of the dome. As the deformation continued, the zone became warmer and warmer with increasing amounts of strain until fail- ure occurred within that heated zone.
Dr. Nine also conducted two addi- tional studies using the same test equip- ment. First, the rate of deformation was increased. A slow punch speed showed less heating in the previous critical zone but a wider band of heating (trending toward isothermal heating). The heat created in the critical zone had time to dissipate to neighboring material. A fast punch speed showed increasing heating within a narrower band. The heat did not have time to dissipate (trending towards adiabatic heating). The second experiment involved a change in lubricant. A water-based lubricant tended to carry heat away from the critical zone into the punch, while an oil-based lubricant tended to thermally isolate the heating from the punch.
Strain gradients are hard to control and are a major cause of variable spring- back and dimensional inconsistency. Making the problem worse, add the variability of the thermal effect to the strain-gradient variability and the prob- lem becomes more severe. And the problem becomes more complex when heating interacts with the temperature response of different lubricants. To min- imize the problem of localized heating in the stamping, dies are being built with cooling-fluid channels to deliver cooling to the punch surface in contact with the strain gradient in the stamping.
Even some of the new steels becom- ing popular today have different thermal characteristics. Friction tests and press shop forming trials of the dual-phase advanced high-strength steels (AHSS) have indicated that these steels become hotter during forming than do other steels of comparable strength. In the press shop, these steels can react differ- ently in the die, create different thermal
gradients, develop increased forming severities, and require new trou- bleshooting skills. A laser temperature gun is rapidly becoming an instrument of choice for analyzing forming prob- lems. An interesting thermal trou- bleshooting case study entitled “The Die Has a Fever?” was the topic of the
Science of Forming column in Metal- Forming magazine’s April 2004 issue.
For too many years, the sheetmetal itself was blamed for most forming problems. Today, strain gradients and thermal effects are being identified as the root causes for many of the reject- ed stampings. MF
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METALFORMING / FEBRUARY 2010 23