Effect of zinc coatings, Materials for Automobile Bodies

In the author’s experience IZ shows a similar performance to mild steel sheet, the lubricity being enhanced slightly by absorption and retention of the lubricant in the fissures typical of the coating. IZ is notorious for appearing to be deficient in mill applied lubricant but coating weight tests suggest it is present, an effect attributed to the absorbent nature of the network of cracks. The natural tendency of the fissured structure is to powder and even flake – although recent modifications to the substrate (Nb/Ti now being preferred to Ti) and optimization of thickness (45 g/m2 recommended) have improved the tendency for ‘pimpling’. This effect, when zinc-rich particles deposit on the punch and are impressed through the sheet to give a shallow mound, only shows on painting or stoning which means a high number of panels requiring rework are produced before the defect is recognized. This is now less of a problem with EZ coatings as process disciplines, which reduce particle generation, e.g. use of side-trimmed strip, regular die cleaning and blank washing, have been progressively introduced. FLDs have also been constructed in an attempt to predict coating behaviour under various strain regimes. Reverting to coating lubricity, electrozinc coatings have been slightly beneficial but the press performance of drawn parts such as sparewheel wells and door inners has definitely been improved by the use of hot-dip coatings, although tools should be inspected regularly for signs of pick-up.

Materials for Automobile Bodies
Geoff Davies F.I.M., M.Sc. (Oxon)
Butterworth-Heinemann
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
200 Wheeler Road, Burlington MA 01803
First published 2003