Defence M. Ahmadi: "Deformation, Cracking and Formability of Zn-Al-Mg Coatings: Methodology, Optimization and Application"

Promotores: Prof. dr. Yutao Pei and Prof. dr. Bart J. Kooi

Abstract: Zn-Al-Mg coatings are important materials in coatings science and technology that are widely utilized in automotive and construction applications. These coatings which are mainly produced by the hot-dip galvanization process, offer excellent corrosion resistance and anti-galling performance for the steel substrates. In contrast, they currently exhibit a high cracking tendency when subjected to plastic deformation and forming processes. The formability properties of these coatings are limited due to the adverse and severe cracking. The generated cracks with a large opening on the coating surface enable the penetration of corrosive media into the coating and subsequently can facilitate the corrosion and degradation of the steel substrates. Therefore, the mechanical integrity and durability of Zn-Al-Mg coated steel sheets are strongly related to the fundamental understanding of the deformation, cracking behavior and formability of the coating. Only through such comprehension, it is possible to generate a scientific guideline to improve the cracking resistance of these coatings. The principal aims of the present thesis are therefore (1) to unravel the origin and mechanism of cracking in Zn-Al-Mg coatings, (2) to resolve the long-lasting cracking and formability problems associated with these coatings. To these ends, comprehensive evaluations on the structure-properties relationship are realized in this thesis. Step-wise and dedicated research works are carried out and in the light of the knowledge obtained, synergic solutions and strategies are applied in order to accomplish crack-resistant and highly-formable Zn-Al-Mg coatings.

Dissertation