Advanced high strength steels (AHSS) are becoming major enablers
for vehicle light weighting in the automotive industry. Crash
resistant and fracture-toughened structural adhesives have shown
potential to improve vehicle stiffness, noise, vibration, and
harshness (NVH), and crashworthiness. They provide weight reduction
opportunity while maintaining crash performance or weight increase
avoidance while meeting the increasing crash requirement.
Unfortunately, the adhesive bonding of galvanneal (GA)-coated
steels has generally yielded adhesive failures with the GA coating
peeling from the steel substrate resulting in poor bond strength. A
limited study conducted by ArcelorMittal and Dow Automotive in 2008
showed that GA-coated AHSS exhibited cohesive failure, and good
bond strength and crash performance. In order to confirm the
reliable performance, a project focusing on the consistency of the
adhesive bond performance of GA-coated steels of 590 MPa strength
level was initiated. The 590 MPa steels include 590R (complex
phase, high yield to tensile ratio), 590Y (dual phase (DP)), and
590T (transformation-induced plasticity (TRIP)). Adhesive bonding
was evaluated by lap shear tests at room temperature at Dow
Automotive and ArcelorMittal Global R&D-East Chicago. Bonds
were made with a crash-resistant structural adhesive, BETAMATE™
1488. Additionally, the 590 MPa steels were evaluated after bonding
with BETAMATE™ 1022DUS, a fracture-toughened structural adhesive,
and BETAMATE™ 73305GB, a conventional, structural, hem-flange
adhesive, to demonstrate the performance of adhesives with
different moduli of elasticity. Lap shear testing was also
conducted on GA-coated interstitial-free (IF), extra deep drawing
steels (EDDS), bonded with the three adhesives, to determine if lap
shear performance of GA EDDS can be improved with the new
generation adhesives. X-ray diffraction (XRD) was used to measure
the residual stress at the interface between the zinc coating and
steel substrate as an indication of the interface bonding. This
paper describes the results of this testing and demonstrates that
crash-resistant structural adhesives can provide consistently good
bonding for GA-coated AHSS and thus be used to reduce weight while
maintaining or improving crash performance.