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
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
