Zinc coatings for hot sheet metal forming: Comparison of phase evolution and microstructure during heat treatment

The production of high strength steel parts via hot forming of manganese boron steels is an appropriate solution satisfying the strictest lightweight demands in modern body in white engineering. Since cathodic corrosion protection is essential for automotive parts, manufacturer demands zinc based coatings suitable for hot sheet metal forming. For that reason, microstructure and phase evolution during and after typical heat treatment, prior to hot sheet metal forming, of zinc and zinc based coatings have been investigated by glow discharge emission spectroscopy, scanning electron microscopy with EDX analysis, X-ray diffraction and nanoindentation. Therefore, characteristics of the heating behavior dependent on the different surfaces have been studied. It was hereby possible to compare the alloying behavior of hot dip zinc coating to that of electro-plated intermetallic zinc–nickel coating at different temperatures. Surface analysis revealed a ZnO layer formation on the heat treated Zn–Ni coating. Moreover, the present study proves a significant stabilization of the alloying process and the resulting phases due to the presence of nickel. Hence, electro-plated uniphase, intermetallic Zn–Ni coatings are highly suitable for hot sheet metal forming applications owing to their good thermal stability when compared to hot dip zinc coatings.