GB1576933A

GB1576933A - Process and apparatus for coating metallic wires

Info

Publication number: GB1576933A
Application number: GB7804/77A
Authority: GB
Original assignee: Aluminium Pechiney SA; Societe de Vente de lAluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1976-02-25
Filing date: 1977-02-24
Publication date: 1980-10-15
Also published as: BE851809A; JPS572269B2; JPS52125425A; DE2707806A1; LU76833A1; DE2707806C3; FR2342350A1; IT1075283B; FR2342350B1; DE2707806B2

Description

(54) PROCESS AND APPARATUS FOR COATING METALLIC WIRES (71) We, SOCIETE de VENTE de L'ALUMINIUM PECHINEY, a body corporate organised under the laws of France, of 23, Bis, rue Balzac - 75008 Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a process for the continuous coating of metallic wires, of bars or of tubes of any cross-section and, in particular, of wires made from aluminium alloys with an external layer of a metallic alloy.

Processes for coating metallic wires with another metal to improve their surface properties such as appearance, resistance to corrosion, electrical contact resistance have been suggested. These processes are based on different theories such as metallisation by plasma, deposition in the vapour phase, chemical deposition, electrolytic deposition, coating and dipping in a metallic bath.

Dipping processes have been suggested wherein the wire to be coated is passed into a bath of molten metal, the temperature of the wire at the outlet being lower than the temperature of solidification of the molten metal. In this way a solidified layer of the metal from the bath would be formed on the wire, the thickness of the layer depending upon the speed of the wire and upon the temperature of the bath. Such a process is suggested in US Patent No. 2,294,750 which describes in particular the manufacture of galvanised steel wire by this process.

These coating processes would be most effective in situations where the coating metal is liable to react with the superficial layers of the metallic wire giving an intermediary diffusion layer which facilitates adherence.

If the coating metal does not form a solid solution with the metal which forms the wire, the adherence is sometimes less good since fixing may only be obtained by a fine interpenetration (on a microscopic scale) of the two metals. This would be the case, for example, when aluminium alloy conducting wires are coated with tin, with tin-based alloys, with cadmium or cadmium-zinc alloys.

We have sought to provide a process for coating metallic wires etc. by passing them through a molten metal bath, the process improving the qualities of adherence of the coating.

We have also sought to provide an apparatus for carrying out the process.

Accordingly the present invention provides in one aspect a process for coating a metallic wire, bar or tube with a metallic layer which comprises: passing the metallic wire, bar or tube through a shaving die; passing the shaven metallic wire, bar or tube through a molten metal bath with a layer of solidified metal from the bath; and drawing the coated metallic wire, bar or tube; and in another aspect an apparatus for coating a metallic wire, bar or tube which comprises the following in series: at least one shaving die; a bath arranged to contain a molten coating metal and a drawing die.

The word "wire" used throughout the specification includes wires of any cross-section.

Shaving the metallic wire may be carried out at ambient temperature by means of one or more shaving dies which remove the outer layer from the wire to be coated. This shaving removes the zone which is contaminated, for example by an oxide layer or by drawing lubricants from the surface, and produces a wire upon which the coating metal will adhere firmly.

It is very important that this shaving covers the entire external surface of the wire and that the surface which has become very reactive is not recontaminated or reoxidised before passing into the metallic bath. The wire which leaves the shaving die must pass into the molten metal bath directly, or after passing into a medium which is inert with respect to the metal or does not disturb the adherence of the deposit on the surface.

The temperature of the wire passing into the molten metal bath, is such that the molten metal from the bath solidifies on the external surface of the wire etc., forming a shell. Upon leaving the bath, the coated wire must therefore be at a temperature which is lower than that of the liquids of the bath.

The composite product composed of a core which is the metallic wire which may be made of aluminium alloy, and the solidified surface coating is passed through a drawing die.

The coated wire may be passed through a gauged pressurising tube prior to being drawn.

In one embodiment the present invention provides a process wherein the metallic wire bar or tube is passed through successive metallic baths having decreasing melting temperatures, the baths may be separated from one another by neutral medium cooling chambers, in order to coat the metallic wire, bar or tube with a plurality of metallic layers.

The present invention is further illustrated by reference to the accompanying drawings wherein: Figure 1 is a diagram showing a complete installation which enables a wire to be coated according to the process of the invention.

Figure 2 is a diagram showing the drawing die of Figure 1 in greater detail.

Figure 3 is a diagram showing a heating device for the outlet die of Figures 1 and 2.

Figure 4 is a diagram showing two transverse sections of the heated vessel of the liquid bath.

A supply of wire to be coated 1 comes from a spool 2 upon which the wire is wound. If necessary, it is gauged by means of a draw-plate 3, then straightened, for example on a roller bench 4. The wire is then shaved, that is, the thin contaminated superficial layer, for example of 1 to 10 hundredths of a millimetre, is removed with a special "shaving" die. This operation may take place in one or two stages.

Two shaving dies are shown in the diagram by 5 and 6. One of the two dies is cooled externally by circulating water 7. In order to avoid contamination and reoxidation of the surface of the wire which has been cleaned by shaving, the wire enters the bath either directly after shaving or by passing first through a chamber which is filled with a neutral medium 8 which may be for example a gas such as nitrogen, argon or carbon dioxide which is non-oxidising with regard to the metal to be coated, or a liquid, or a solid such as flux. The wire enters the bath below the level of the free surface of the metal by means of a gasket 9. The wire passes through a coating metal bath 10 which is maintained at the desired temperature by any known heating system 11, equipped with a control which is not shown. Insulating surfaces 12 may be provided to thermally insulate the molten metal.

After passing through the furnace and without leaving the bath, the wire enters directly or after passing through gauged tube 13 which has the shape of the coated product, a drawing die 14 the neck of which may be cooled by a circulating fluid 15. Throughout the specification, the word "gauged" when used in association with the word "tube" or the expression "pressurizing tube", means that the tube or pressurizing tube in question has a constant and determined diameter. According to the dynamic equations of the liquids used in the present invention it is possible to calculate said diameter with regard to the operating conditions used e.g. the diameter of the thread and its velocity. The characteristics of this drawing die are more fully shown in Figure 2.

Upon leaving the die 14, the coated wire is cooled, for example by a spray of water 16: it may be gauged or re-drawn through die 17.

Finally it is wound onto a winding machine 18 which also serves to draw the wire by traction.

The speeds at which the wire passes may vary from 10 m/minute to 300 m/minute or even higher. In fact, it is the shaving operation which restricts the passing speed.

Figure 2 shows a particular design of the outlet die 14 in detail, shown as a cross-sectional view passing through the axis of the die. Like the drawing plates, it is composed of a working cone 19 in which the cross-section of the wire is gradually reduced;in current practice, the total opening angle for drawing aluminium alloys is 12". This angle has been adopted, but it is obvious that this angle may be more open or more closed (and even almost zero) without departing from the scope of the invention. The cylindrical or very slightly conical neck 20, which serves to gauge the cross-section and to guide the flow into the working cone, is situated downstream of the working cone.

The length of the neck is not very critical since neck lengths of 1.44 mm to 14 mm have been used. However, too long a neck which is difficult to construct and to use perfectly, damages the uniformity of the coating. Outlet cone 21, which prevents scaling of the die and packing by accumulation of the coating, is situated after the neck.

This die has two significant features: - the working cone is hot enough for the coating metal not to solidify on the interior of this cone, since this would cause the wire to be blocked and broken; - the throat neck is cold enough for the coating metal not to heat up too much.

This die may in fact operate in two distinct manners: the first method consists of a true drawing of the composite wire, the diameter of its solidified coating being greater than the diameter of the neck.

In the second method of operation, the external diameter of the wire which is coated with a solidified film is smaller than the diameter of the neck. In this way, a wedge of liquid metal is formed between the throat of the die and the wire, and, by means of a hydrodynamic effect, the movement of the wire creates a pressure in the liquid metal wedge which may become considerable. Although the diameter of the wire, including the covering is slightly smaller than the diameter of the die, the presence of the wedge of liquid metal is possible since, at the level of the die, which is cooled at least to the level of the bearing, there is always at least partial solidification of the covering metal, and this ensures a sufficient tightness.Although the diameter of the wire is smaller than that of the die, the path of the wire across the die is not completely free and the hydrodynamic wedge of liquid metal is always formed. Accordingly to this particular method, three zones can, in fact, be distinguished: (I) the zone situated above the bearing of the die where the wire has a diameter which is smaller than that of the die; (II) the contact zone with the bearing of the die where the covering has a liquid and/or solid structure occupying the whole cross-section of this part of the die; and (III) the exterior zone where the covering is solid as a consequence of the cooling influence of the die and/or the surrounding environment.

This phenomenon which is similar to the creation of the film of oil in lubrication problems has the effect here of centering the wire, that is, of encircling it with a uniform casing of liquid or solidifying metal. In fact, if the clearance between the wire and the neck which is filled with liquid metal, is smaller on one of the generatrices of the wire and greater on the opposite generatrix, the hydrodynamic pressure which is created on the side where the clearance is smaller is higher than on the side where the clearance is greater. The force which results from this pressure difference tends to bring the wire to the centre of the throat.

The cooled liquid metal casing solidifies at the outlet of the die.

In order to regulate the temperature of the die accurately, the die is provided with both a heating device and a cooling device. The heating device is not shown in Figure 2, but in Figure 3.

It is formed, for example, by a hollow block made of copper or similar conducting metal 34 provided with electrical heating resistances 35 inside which the die is fixed. The heaters may also be arranged inside the body of the die, but the system as described simplifies the operations of assembly and disassembly. As is usually the case, the cooling device is formed by a fluid circulating preferably in an annular direction, close to the neck to be cooled. In Figure 2, the water circulated through an annular chamber 22 equipped with a feed pipe 23 and a delivery pipe 24. A recess 25 allows a thermocouple to be placed close to the neck of the die and thus allows the heating of the die and the flow of the cooling fluid to be regulated to obtain the desired neck temperature.

Figure 3 also shows a vertical section passing through a preferred embodiment of the device of the invention.

Reference number 26 designates the support of the single shaving die or of the last shaving die before the bath. The shaved wire passes through chamber 27 which is filled with a neutral medium (for example nitrogen, or argon), or enters the bath directly. The die support 26 and chamber 27 are supported by frame 28 which forms a sleeve. The shaved.

wire then penetrates a bath of molten metal 29 via a gasket 30. This gasket may for example be made of polytetrafluoroethylene in the case of coating with alloys having low melting points such as tin. The molten metal bath may be thermally insulated from the section of the device which is upstream (shaving die end) by a plate made of an insulating material which forms a thermal break 31. It is maintained at a constant temperature by a heating device such as heating resistances 32. The wire which is still submerged in the molten metal enters die 33 which is enclosed in a block of copper 34 heated by electrical resistances 35. It should be noted that the outlet die serves as a sealing ring, on the wires outlet end, and that the upstream surface is therefore in contact with the liquid metal.An intermediary part 36 may be placed between the die and the furnace wall to simplify the operations for disassembling the die/heating block assembly. This intermediary part is not indispensable. If it has a diameter which is close to that of the wire, it serves as a tube for pressurising the liquid metal hydrodynamically.

We have also found it advantageous, but not essential to the invention, to construct the heated vessel of the liquid bath according to the diagram in Figure 4. This diagram shows two transverse sections of the vessel. In these two sections, the transverse section of the wire is labelled with 1, the vessel wall by 37 and the level of the metal by 38. The vessel may rotate around a longitudinal axis 39 which is parallel to the wire 1. In the section A, the furnace is shown in the normal operating position. In the section B, the vessel is starting to empty by being pivoted around the axis 39; this arrangement enables the level of the bath to be varied in relation to the wire, the immersion and emergence of the wire and, finally, the suitable drainage of the bath contained in the vessel.

The process apparatus of the present invention may be used for coating any metallic wires with any metallic coating providing that the melting point of the coating metal or the residence time of the wire is small enough for the wire which is passing through the bath not to exceed a temperature at which its properties are insufficient to bear the tractive forces required for shaving and drawing at the outlet.

In order to avoid having to transmit the tractive force through the entire length of the wire, it has been found advantageous to insert devices which aid the drawing of the wire, for example a pushing device between the shaving dies 5 and 6 or a pulling device between 15 and 17.

We have also discovered that shaving the wire improves the thermal exchanges between the wire and the bath of molten metal. The wire is heated more rapidly than it would be without shaving, which allows the coating of the wire and a thermal treatment such as solution heat treatment, annealing, recovery and ageing to take place simultaneously.

As an example, if a cold worked A5/L wire is coated, a wire which is at least partially recovered is obtained after the coating treatment.

Similarly, if a 4/4 hard, cold worked A-GS/L wire is treated, the wire will be partially recovered and aged at the outlet.

The process of the present invention is particularly suitable for coating aluminium or conducting aluminium alloy wire, for example A-GS/L, with tin, a tin alloy and, more generally, with any other alloy whose melting point is below approximately 450 C.

In fact, domestic electric wire must satisfy certain conditions: - it must be able to be connected by the same methods as copper, with a low contact resistance, - it must be able to be soldered (soft solder), - it must have a high resistance to corrosion.

We have shown that aluminium wires coated with a conducting film of tin or of a tin alloy in accordance with the present invention possess these qualities and are consequently particularly suitable for this use.

By using the process and the apparatus described above, various coating thicknesses may be obtained; 1 to 100 micrometres in the case of depositing tin alloys on aluminium alloy wire.

The specifications and figures relate to a particular apparatus designed for carrying out the process, which is arranged horizontally. But it is obvious that this arrangement is not essential for carrying out the process. For example, the entire apparatus, or only a part of it may be arranged vertically, the changing directions of the wire being assured by means known per se such as guide pulleys inserted at a suitable point in the apparatus.

After the first coating then cooling in a neutral medium, it is possible as mentioned above to effect one or more successive coatings according to the same principle, but without intermediary shaving, the melting points of the metals of the successive layers decreasing in order to prevent them from re-melting completely.

For this, it is sufficient to place a chamber such as 8, filled or traversed by a neutral cooling medium after the outlet die of the first bath, then a second vessel containing the metal of the second coating at the outlet of this chamber and so on, the successive coating baths being separated by chambers which are filled by neutral cooling media.

The following examples illustrate the inven tion by showing in more detail the application of various metallic coatings on to A-GS/L con ducting alloy wires.

In all the examples, the process and the apparatus described above were used. The nature of the coating and, sometimes, the diameter of the wire which was always A-GS/L, were the only details which changed.

Also, the length of the metallic bath was 350 mm in all the examples.

EXAMPLE 1 - initial diameter of the wire : 2.40 mm - diameter of the wire after shaving : 2.37 mm - coating alloy : zinc 95% aluminium 5% - melting point of the : 382 "C coating (liquidus) - temperature of the bath : 390 "C - outlet die (diameter of the neck) : 2.39 mm (temperature of the neck) : 390 "C - speed at which wire passes, :1 50 metres/ minute - passing time in the bath : 0.12 seconds - thickness of the coating : 0.009 mm layer EXAMPLE 2 - initial diameter of the wire : 2.40 mm - diameter of the wire after shaving : 2.37 mm - coating alloy: composition Zn = 83% Bi =10% Cd= 4% Sn =2.5% Pb =0.5% temperature of the bath : 385 to 400 OC - outlet die: - diameter ofthe neck : 2A1 mm - temperature of the neck : 385 to 400 OC - passing speed : 40 to 1 50 metres/min.

- passing time in the bath : 0.4 to 0.12 seconds - thickness of the coating : 0.005 mm to 0.012 mm EXAMPLE 3 - initial diameter of the wire : 2.44 mm - diameter of the wire after shaving : 2.39 mm - coating alloy: composition Sn = 91% Zn= 9% melting point : 198 OC (eutectic) temperature in the bath : 230 OC - outlet die: diameter of the neck : 2.41 mm temperature of the neck : 198 OC - passing speed : 150 metres/ minute - passing time in the bath : 0.12 seconds - thickness of the coating : 0.01 mm EXAMPLE 4 - initial diameter of the wire : 2.40 mm - diameter of the wire after shaving : 2.37 mm - coating alloy: composition:Cd = 67% Zn = 33% melting point : (liquidus 3000) temperature of the bath : 340 C - outlet die diameter of the neck : 2.41 mm temperature of the neck : 280 OC - passing speed : 65 metres/ minute - passing time in the bath : 0.3 seconds - thickness of the coating : 0.009 mm WHAT WE CLAIM IS: 1. A process for coating a metallic wire, bar or tube with a metallic layer which comprises: passing the metallic wire, bar or tube through a shaving die; passing the shaven metallic wire bar or tube through a molten metal bath to coat said metallic wire, bar or tube with a layer of solidified metal from the bath; and drawing the coated metallic wire, bar or tube.

2. A process as claimed in claim 1, including the step of passing the shaven metallic wire, bar or tube through a neutral medium prior to the coating stage.

3. A process as claimed in claim 1 or 2, wherein the coated metallic wire, bar or tube is passed through a gauged (as herein defined) pressurizing tube prior to being drawn.

4. A process as claimed in any of claims 1 to 3, wherein the metallic wire, bar or tube is passed through successive metallic baths having decreasing melting temperatures, in order to coat the metallic wire, bar or tube with a plurality of metallic layers.

5. A process as claimed in claim 4, wherein the successive metallic baths are separated from one another by cooling chambers containing a neutral medium.

6. A process as claimed in claim 1 substantially as described with reference to the accompanying drawings.

7. A process as claimed in claim 1 substantially as described with reference to any one of the Examples.

8. A coated metallic wire, bar or tube whenever produced by a process as claimed in any one of claims 1 to 7.

9. An apparatus for coating a metallic wire, bar or tube which comprises the following in series: at least one shaving die; a bath arranged to contain a molten coating metal and a drawing die.

10. An apparatus as claimed in claim 9, wherein a chamber arranged to contain a neutral medium is located between the shaving die and the bath.

11. An apparatus as claimed in claim 9 or 10, wherein a gauged (as herein defined) pressurizing tube is located between the bath and the drawing die.

12. An apparatus as claimed in any of claims 9 to 11, wherein the drawing die consists of an inlet cone and a neck.

13. An apparatus as claimed in claim 12, wherein the drawing die is supplied with a heating system for the inlet cone and a cooling system for the throat.

14. An apparatus as claimed in claim 9 substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

temperature in the bath : 230 OC - outlet die: diameter of the neck : 2.41 mm temperature of the neck : 198 OC - passing speed : 150 metres/ minute - passing time in the bath : 0.12 seconds - thickness of the coating : 0.01 mm EXAMPLE 4 - initial diameter of the wire : 2.40 mm - diameter of the wire after shaving : 2.37 mm - coating alloy: composition: Cd = 67% Zn = 33% melting point : (liquidus 3000) temperature of the bath : 340 C - outlet die diameter of the neck : 2.41 mm temperature of the neck : 280 OC - passing speed : 65 metres/ minute - passing time in the bath : 0.3 seconds - thickness of the coating : 0.009 mm WHAT WE CLAIM IS: 1.A process for coating a metallic wire, bar or tube with a metallic layer which comprises: passing the metallic wire, bar or tube through a shaving die; passing the shaven metallic wire bar or tube through a molten metal bath to coat said metallic wire, bar or tube with a layer of solidified metal from the bath; and drawing the coated metallic wire, bar or tube.
2. A process as claimed in claim 1, including the step of passing the shaven metallic wire, bar or tube through a neutral medium prior to the coating stage.
3. A process as claimed in claim 1 or 2, wherein the coated metallic wire, bar or tube is passed through a gauged (as herein defined) pressurizing tube prior to being drawn.
4. A process as claimed in any of claims 1 to 3, wherein the metallic wire, bar or tube is passed through successive metallic baths having decreasing melting temperatures, in order to coat the metallic wire, bar or tube with a plurality of metallic layers.
5. A process as claimed in claim 4, wherein the successive metallic baths are separated from one another by cooling chambers containing a neutral medium.
6. A process as claimed in claim 1 substantially as described with reference to the accompanying drawings.
7. A process as claimed in claim 1 substantially as described with reference to any one of the Examples.
8. A coated metallic wire, bar or tube whenever produced by a process as claimed in any one of claims 1 to 7.
9. An apparatus for coating a metallic wire, bar or tube which comprises the following in series: at least one shaving die; a bath arranged to contain a molten coating metal and a drawing die.
10. An apparatus as claimed in claim 9, wherein a chamber arranged to contain a neutral medium is located between the shaving die and the bath.
11. An apparatus as claimed in claim 9 or 10, wherein a gauged (as herein defined) pressurizing tube is located between the bath and the drawing die.
12. An apparatus as claimed in any of claims 9 to 11, wherein the drawing die consists of an inlet cone and a neck.
13. An apparatus as claimed in claim 12, wherein the drawing die is supplied with a heating system for the inlet cone and a cooling system for the throat.
14. An apparatus as claimed in claim 9 substantially as described with reference to the accompanying drawings.