Application of the Phase Field Method to the Solidification of Hot-Dipped Galvanized Coatings

  1. Prof. Yves Bréchet
  1. A. Semoroz,
  2. S. Henry and
  3. M. Rappaz

Published Online: 19 DEC 2005

DOI: 10.1002/3527606157.ch5

Microstructures, Mechanical Properties and Processes - Computer Simulation and Modelling, Volume 3

Microstructures, Mechanical Properties and Processes - Computer Simulation and Modelling, Volume 3

How to Cite

Semoroz, A., Henry, S. and Rappaz, M. (2000) Application of the Phase Field Method to the Solidification of Hot-Dipped Galvanized Coatings, in Microstructures, Mechanical Properties and Processes - Computer Simulation and Modelling, Volume 3 (ed Y. Bréchet), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG. doi: 10.1002/3527606157.ch5

Editor Information

  1. Institut Nat. Polytechnique de Grenoble, L.T.P.-C.M. ENSEEG, BP75, Domaine Universitaires, 38402 Saint Martin D'Hères Cedex, France; Tel.: 0033–76–82 6610; Fax: 0033–76–82 6644

Author Information

  1. Laboratoire de Métallurgie Physique, Département des Matériaux, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

Publication History

  1. Published Online: 19 DEC 2005
  2. Published Print: 20 APR 2000

Book Series:

  1. EUROMAT 99

ISBN Information

Print ISBN: 9783527301225

Online ISBN: 9783527606153

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Keywords:

  • microstructures;
  • computer simulation;
  • phase field method;
  • solidification of hot-dipped galvanized coatings;
  • zinc coating

Summary

Zn coatings of steel sheets are used to protect them against corrosion. The coating is deposited on the steel substrate by the hot-dipping process. It is a continuous process where the steel sheet (typical thickness: 0.5 mm) passes through a bath of liquid Zn. Some liquid is taken away with the steel and solidifies later on to form the coating (typical thickness: 20 µm). The Zn used to galvanize steel sheets contains typically 0.2 wt pct Al and 0.04 wt pct of Pb or more, plus some other impurities. Within a single Zn grain called spangle, different regions can be distinguished simply by visual inspection. The “shiny region” of the grain corresponds to long primary dendrite arms with visible secondary and tertiary arms, while only an array of dots is visible in the “dimpled region” of the same grain. Moreover, the concentrations of Pb and Al on the shiny surface are much lower than those measured on the dimpled surface. The surface roughness also differs: shiny regions are smooth, while dimpled ones are rough.

The large size of spangles has since long fascinated scientists but has still not been explained satisfactorily. Among other hypotheses, Fasoyinu and Weinberg have proposed that wetting effects on the boundaries increase the growth velocity of the dendrites. Wetting phenomena considered here and all along this article occur at the triple points, junctions of the solid dendrite, the liquid melt and the boundary (either the steel substrate covered by an intermetallic layer or the oxide layer at the free surface). Wetting of a growing solid phase on a boundary has already been observed in directional solidification of transparent substances such as succinonitrile: a dendrite growing along a good wetting boundary has a triangular tip characterized by a small wetting angle, while a non-wetting boundary maintains the dendrite tip away (large wetting angle). In a Bridgman configuration, the wetting and non-wetting dendrites grow at the same imposed velocity of the isotherms, but the wetting dendrite grows ahead of, i.e. at smaller undercooling than, the non-wetting one. Therefore, for a fixed undercooling, a dendrite wetting a boundary would grow faster, as proposed by Fasoyinu and Weinberg, since the sharper tip can reject solute more easily.