| With the aim to improve dent resistance on the new Ford Mustang, Ford engineers embarked on an alternative materials investigation for the vehicle’s outer body panels. Ford’s design team had specific goals in mind, with customer satisfaction at the top of the list, closely followed by the need to reduce warranty claims, in-plant damage, reduce weight and avoid a cost penalty as a result of a material change. |
Ford formed a cross-functional 6-sigma team that involved steel and Tier 1 suppliers as well as internal departments, all charged with finding solutions to immediately and quickly improve panel dent performance This included potential solutions to increase material thickness, add beta patches to the panels and test higher strength steel grades.
Dual Phase (DP) 500, 0.8 mm steel was successfully tested and implemented for the body panels. No significant manufacturability issues were encountered, with no major stamping welding or hemming issues. DP 500 produced panels with good surface quality and paint appearance. And dent resistance was better than that achieved by body panels made of BH210, 0.9 mm steel that used a beta patch. It was determined that the body panels were feasible for implementation, and they were approved for production. Use of DP 500 enabled a change in gauge that helped achieve a weight savings of 2.3 lbs. per vehicle.
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Ford Mustang Stamped (Unpainted DP500) |
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Some stamping and forming issues were encountered during development. Following are observations on these issues and how each was solved: |
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Draw die modifications to eliminate buckle lines |
Forming Issues
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Double draw line control
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Gentle buckle lines
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Cracking in mirror pocket area
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Intermittent Edge cracks at stretch flange
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Stamping Issues
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Double-draw/skid lines noted along the belt line
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Vertical buckle lines observed on the front upper panel
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Edge splits noticed in the trim/flange operations in stretch Flange area on RH panel
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Draw die balance block adjustments to eliminate skid lines. |
Draw-Die Modifications
Adjustments were made to the Balance Blocks located at the front of the draw die to eliminate double-draw and skid-line conditions. The height of the blocks was decreased to increase the binder pressure.
To eliminate the buckle lines, the radii on the lower and upper draw dies were polished and smoothened to avoid zinc build up. The binder beeds were also polished and smoothened for better material flow.
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Trim-Die Modifications
Some edge cracking occurred in the mirror flag edge area. Trim-die modifications were made to re-nest (weld) the sail area to provide support to the metal and avoid burrs on the trim edge and edge cracking.
To eliminate edge cracking in the flange area, trim upper and lower dies were sharpened to obtain a clean flange edge. In addition, pads were built underneath the die to support the metal for a cleaner trim.
Trim edges were also sharpened and polished for a clean cut and to reduce burrs on rear side of door panels.
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Trim die balance to eliminate flange edge cracks. |
| Flange-Die Modifications |
| The punch was re-contoured in the flange upper die by increasing the radii, providing better material flow and eliminating edge splits. Flange lower die edges were cleaned to prevent burrs that cause edge splits. |
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Flange lower die clean edges |
Flange upper die increase in radii |
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Other general forming aspects addressed during development were as follows:
Press loads. DP 500 exhibits a higher work hardening behavior, which is important to understand for estimating the press capacity needed for a part, as well as for down gauging. FEA is informative for making this estimation.
Forming in the draw die. DP 500 has a higher n-value for the high strength level of the steel, and there is a higher envelope for a safe formable region. However, these issues are similar to that of conventional steels, and the Ford team found nothing unique to DP 500.
Die wear. Local contact loads can be very high for features with small radii. In parts prone to wrinkling, die wear can be an issue because of the increased load response. Giving attention to the die process to eliminate wrinkling can be very important.
Post-forming operations. There are factors affecting the edge condition after trimming, influenced by the clearance and die wear. Attention should be given to the proper support of the panel during the trim operation. This will eliminate bad burrs after trimming and avoid edge cracking and tearing. For stretch flanges, aspects should be implemented that will reduce tensile stress levels at the flange edge. These could include reducing the local flange length or engineering metal gainers during the development process. Change in the length of the line should also be compensated—the transition of the flange length should be gradual. Abrupt changes in flange length act as stress raisers locally leading to edge cracks. Sharp notch features should be avoided in curved flanges.
Springback. Springback on the Ford Mustang body panels was not a major issue since it was compensated by the over bend of the flange.
In summary, development of the AHSS body panels for the Ford Mustang identified stamping issues that were subsequently addressed, resulting in DP500, 0.80 mm door outer panels currently in production for this vehicle. Production undergoes continual monitoring for the die and die surface impact and die wear, and, of course, quality of the panels. |