Flaking behaviour of GA IF and GA AHSS and its correlation with the adhesion strength and intefacial residual stress

Abstract

Lap Shear test results showed that GA AHSS possess significantly higher adhesion strength than GA IF. SEM
examination of the Lap-Shear-tested GA IF specimens revealed adhesive failure at the coating/steel interface
with multiple steel grains pulled out from the IF steel substrate. Conversely, SEM results showed all Lap-Sheartested
GA AHSS specimens failed cohesively (failed within the adhesive) and exhibited very little to no GA
coating separation from AHSS substrates. A newly developed XRD procedure was used to measure the residual
stress on both sides of the coating/steel interface. The gamma layer of GA coating in all GA IF and GA AHSS
exhibited a distinct tension residual stress, which matches with theoretical predictions. GA AHSS showed a
distinct compressive residual stress on the steel substrate side. However, for most GA IF steels there was no
obvious compressive residual stress on the steel substrate side. The lack of compressive residual stress in the
GA IF steels is likely due to weakening of the steel grain boundaries by liquid zinc penetration. It is consistent
with the Lap Shear test results. Two main differences in the interfacial structure between GA IF and GA AHSS
are (a) weak steel grain boundaries likely due to apparent liquid Zn penetration of IF steel grain boundaries
and (b) relatively smoother coating/steel interface of GA IF. These two factors contribute significantly to the
poor flaking resistance of GA IF.