Microstructural investigation of galvanized coatings with prior flash coating of copper on DP steels

• Prior metallic flash coating of copper having thickness of 75 nm on DP steels.
• Inter diffusion of copper and iron during annealing.
• Formation of inhibition layer in the presence of metallic copper flash coating.
• Nodular shaped morphology of copper lean inhibition layer.
• Distribution of copper throughout the galvanized coating.

Advanced high strength steels have gained immense importance for typical auto body structures owing to their light weight and high performance. These steels are mostly alloyed with Mn, Cr, Al, Si, etc. that have a tendency to form oxides on the steel surface during annealing and result in uncoated spots during galvanizing. Various new coating processes and annealing atmospheres have been tried to improve the coatability of such steels. One such method is the use of a diffusion barrier on bare steel. The present investigation has been carried out with DP steel, having Mn content of 2 wt.% coated with copper following the replacement reaction of iron and copper. The annealing and subsequent galvanizing operations were performed using hot dip simulator. XRD and SEM analysis were done to examine the characteristics of deposited copper layer. GDOES analyses showed that inter diffusion of copper and iron took place during annealing. SEM investigations of annealed specimen showed copper depleted iron rich grain boundary regions. Different layers in the galvanized coating were investigated in great detail. The morphology of the inhibition layer of copper coated sample was found to be different compared to conventional galvanized steel although, no compositional variation was detected. The TEM analysis showed that the inhibition layer was lean in copper, as supported by the ThermoCalc Al–Fe–Cu ternary phase diagram showing negligible solubility of copper in Fe2Al5 crystals. The copper was found to be distributed in the overlay zinc coating. TEM elemental mapping revealed segregation of copper at different regions of overlay zinc. The bright field and dark field images confirmed the presence of nano size precipitates within zinc coating. Finally a schematic illustration of the probable mechanism for formation of inhibition layer and copper rich zinc coating in presence of a prior copper coating was formulated.