Identify the causes of coating dusting when stamping hot-dip galvanised sheet metal parts

Abstract

Dusting the zinc coating when stamping car bodies results in coagulated dust particles (lumps) that are imprinted on the bodywork surface. A visual check is always carried out after the last stamping operation to prevent such a defect. If a defect is detected, the entire stamping line is stopped and the die is cleaned or, even worse, replaced. In order to clarify the causes of dusting, the chemical composition X-ray microanalysis of dusting particles was performed, 3D dimensions of these particles were determined and they were compared with the rough surface dimensions of the automotive sheet. It is shown that the hypothesis about occurrence of dusting particles as a consequence of shearing roughness of the zinc coating during stamping should be considered unlikely. Another mechanism for dusting zinc coating was considered. A study of surface roughness formation during uniaxial deformation showed that these pro-cesses of bare metal and zinc-coated metal differ significantly. These differences can provoke another type of zinc coating failure as a process of loosening the metal-zinc coating boundary, which has not been considered so far. This type of fracture can be avoided if the discussed experimental dependences of micro topographic characteristics on elongation in the coating and in the substrate lack such radical difference