Coating processes – Immersion or partial immersion – Molten metal or fused salt bath
Reexamination Certificate
2001-03-28
2004-07-13
Barr, Michael (Department: 1762)
Coating processes
Immersion or partial immersion
Molten metal or fused salt bath
C427S434200, C427S434600, C427S436000, C427S008000, C427S598000
Reexamination Certificate
active
06761935
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of producing a metallic coating on an object emerging from a bath of molten metal. The invention also relates to a device applying the said method.
It has a particularly interesting application in the field of the manufacture of electrode wire for spark erosion. For this purpose, first a metallic coating is made, in zinc for example, on a metallic wire, of copper or steel for example, then the coated wire is placed in a heat-treatment furnace so as to obtain diffusion of the zinc into the metal wire.
It is also possible to make a coating of tin on a core of steel or of copper, and the product obtained is then intended to undergo drawing operations.
The invention can also find applications in other fields such as the production of a metallic coating for protecting a non metallic core, for example an optical fibre.
The general principle of manufacture of electrode wire for spark erosion is described extensively in the prior art, in particular in documents U.S. Pat. No. 4,169,426 and EP-A-0 811 701, in which a conducting wire passes vertically through a bath of molten metal and is then subjected to treatments for the purpose of being drawn. The complex and costly process described in document U.S. Pat. No. 4,169,426 relates to a pretreatment for cleaning metal wire before the latter passes through the bath of molten metal and undergoes rapid cooling. Document EP-A-0 811 701 describes two electrodes in contact with the metal wire, respectively upstream and downstream from the bath of molten metal, so that the part of the metal wire between the two electrodes is heated by the Joule effect, by passing a current through these electrodes.
One of the principal characteristics in the production of a coating is the thickness of the outer layer obtained. Theoretical results relating coating thickness to the speed of travel of the metal wire and to the hydrodynamic properties of the molten metal were established in particular by L. Landau and B. Levich in an article in Acta Physicochimica U.R.S.S. Vol. XVII, No. 1-2, 1942: “Dragging of a Liquid by a Moving Plate”. This article gives an equation relating, in first order, the coating thickness—which is assumed constant—to a capillary number that is a function of the hydrodynamic properties of the molten metal, provided that the molten metal is a liquid that wets perfectly and the object being coated is a plate.
Now, on the basis of the aforementioned theoretical results, the thickness obtained is often too great for coating applications in which a fine thickness is desired. Accordingly, various forms of wiping, i.e. of reducing the thickness of the coating formed, have been proposed, such as techniques of pneumatic wiping (action of air knives forming a back-pressure on the free surface of the metallurgical product emerging from the liquid bath), techniques of mechanical wiping (action of rollers that “lick” the metallurgical product by means of asbestos pads) and finally, techniques of magnetic wiping, the present invention belonging to this last-mentioned category.
The magnetic wiping techniques make use of the Lorentz forces that are generated in the coating liquid by a magnetic field, static or alternating, fixed or sliding. The action of a magnetic field on a liquid metal is known and is described in particular in document U.S. Pat. No. 4,324,266. This document discloses a device for accomplishing the confinement of a jet of liquid metal by creating an overpressure by means of a coil encircling the jet and carrying an alternating current whose frequency is below a given value. In general, many techniques of magnetic wiping are included in the state of the art, in particular patent EP 0 720 663 B1 of the present applicant, in which an inductor, arranged around an exit channel of the bath of molten metal, produces a transverse, alternating electromagnetic field of quite low frequency, and sliding, the movement of the galvanized product taking place along a horizontal axis. The device thus embodied makes it possible to determine the conditions for which the Couette lengths associated with the flow of the coating liquid respectively in the container and in its exit channel remain below critical values, above which the flows become decidedly turbulent. These conditions require accurate dimensioning in the vessel containing the liquid metal and make it possible, in the case of horizontal drainage, to keep the molten metal inside the exit channel. The thickness is controlled according to a formula similar to that employed in the hydrodynamic model of Landau and Levich, the references for which were cited above. However, the method described in this document EP 0 720 663 B1 cannot relate to products of small thickness, as the design of the inductor means that its air gap is too large for the sliding field created by the said inductor to be able to act effectively on the said products.
Document U.S. Pat. No. 4,228,200 describes a method of controlling the metallic coating on a wire emerging vertically from a bath of molten metal. The thickness is controlled by means of a single-coil device creating a fixed, alternating electromagnetic field of very low frequency, applied at the point of exit or below the point of exit of the wire. The electromagnetic field thus created expels the molten metal from the zone of highest flux density towards zones with a lower flux density. Coating thickness is adjusted by altering the amplitude of the electromagnetic forces exerted by the field generated by the electromagnetic device. However, as can be seen in
FIGS. 3A and 3B
of document U.S. Pat. No. 4,228,200, the device saturates for a frequency above 300 Hz for example. The magnetic field created no longer exerts an influence on the thickness of the coating. In addition, this saturation must be strongly dependent on the type of metal used, since each metal has a different saturation level.
There is a known method of magnetic wiping, developed by M. Malmendier, J-F. Noville and S. Wilmotte of the Metallurgical Research Centre (Centre de Recherches Métallurgiques, CRM) of Liege, and disclosed in the “Conference Proceedings” with the title “Improvement of control of the zinc loading in the hot-dip galvanizing process”, pages 407-412, 27-29 May 1997. This method employs a magnetic field created by means of an alternating current, acting on the thickness of the coating already formed. However, the said method requires the use of high power, and involves an excessive temperature rise of the coating.
SUMMARY OF THE INVENTION
The present invention aims to remedy the aforementioned drawbacks and relates to a method of making a coating in which the coating thickness is controlled accurately, by taking into account all of the parameters involved in the production of the said coating.
Another object of the invention is the production of a coating of small thickness, typically of the order of a micrometer on small objects, with low energy consumption and limiting the temperature rise of the coating.
Yet another object of the present invention is a device in which the vessel containing the bath of molten metal is suitably dimensioned so as to permit efficient control of coating thickness regardless of the type of drainage of the object (vertical, slanting or horizontal).
The aforementioned aims are achieved with a method of producing a metallic coating on an object emerging from a bath of molten metal, in which a magnetic field is created near the point of exit of the object. According to the invention, the object leaves the bath of molten metal through an exit channel containing a meniscus of the said bath of molten metal, and the thickness of the metallic coating is controlled as a function of a second derivative of the curve of the meniscus and of a capillary number Ca representing the ratio between the viscous forces of the molten metal and the forces of surface tension at the surface of the molten metal.
This characteristic can be represented in the form of an equation:
e
0
·
ϕ
zz
=
1.3
Delot Jose
Sanchez Gérald
Barr Michael
Delot Process
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