Bonding Studies between Fracture Toughened Adhesives and Galvannealed Steels with Zinc Coating 2010-01-0434

Adhesive bonding technology is rapidly gaining acceptance as an alternative to spot welding. This technology is helping automobile manufacturers reduce vehicle weight by letting them use lighter but stronger advanced high strength steels (AHSS's). This can make cars safer and more fuel efficient at the same time. The other benefits of this technology include its flexibility, ability to join dissimilar materials, distribute stress uniformly, provide sealing characteristics and sound dampening, and provide a moisture barrier, thus minimizing the chance for corrosion. The lap shear work reported in the late 1980s and early 1990s has led to the prevalent perception that the galvannealed (GA) coating can delaminate from the steels, resulting in poor joint performance. However, the above work was carried out on steels used primarily in automobile outer body panels. The Dow Chemical Company and ArcelorMittal collaborated on a joint project to carry out comparative studies of bonding efficacies of fracture toughened adhesives (FTA's) with conventional versus AHSS's. Three GA steels, namely, extra deep drawing quality interstitial-free steel (EDDS-IF) and two high strength steels, namely, DP780 and TRIP 780 were tested with two different adhesives, one FTA with a modulus of ~ 1500 MPa and another conventional adhesive with a modulus of ~ 3500 MPa

The results of this joint study showed that the FTA, BETAMATE™ 1496, exhibits significantly better energy absorption/impact load than the conventional hem flange adhesive, BETAMATE™ 73305. Whereas EDDS-IF steel led to coating delamination, the AHSS's studied showed cohesive failure without any coating delamination. This underscores the importance of an FTA during a crash performance. Surface texture does not seem to be a factor in bonding in these systems. The AHSS's can take advantage of the adhesive bonding in automotive body structures to improve structural integrity and crash performance. Results of lap shear, impact peel, microscopy, as well as suitable analytical testing will be presented and discussed.

This initial work primarily dealt with lap shear and impact peel work carried out at ambient temperatures; it also included only a limited number of steels and adhesives. Future work planned includes GA steels covering a broader range of strengths as well as adhesives of broader modulus range. Also in plans is high speed impact work covering a broader temperature range following the procedure outlined in ISO 11343.