CA1050832A

CA1050832A - Continuous metal coating process and apparatus

Info

Publication number: CA1050832A
Application number: CA268,980A
Authority: CA
Inventors: Joseph A. Kovacs
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-02-12
Filing date: 1976-12-31
Publication date: 1979-03-20
Also published as: DE2705420A1; DE7703827U1

Abstract

INVENTOR:
JOSEPH ALEXANDER KOVACS
TITLE OF INVENTION:
CONTINUOUS METAL COATING APPARATUS
ABSTRACT OF THE DISCLOSURE:
Apparatus to coat a moving strip of metal with uniform layers of metal on one or both sides. Molten metal is sprayed on the surface of the moving strip. The coating is subsequently compacted into a ductile non-porous layer having desired physical and chemical properties.

Description

~ 5~3Z
S P E C I F I C A T I O N
_ _ _ _ ._ _ _ _ _ _ _ _ _ SUMMARY OF THE INVENTION:
My invention relates to apparatus to coat a movlng strip with uniform coatings of metal on one or both sides. The substrate to be coated is cleaned, chemically treated and preheated before entering the coating chamber.
The molten metal is atomized by the combined effect of pressure and ultrasonic vibration or by pressure alone and sprayed using an ~;
inert gas on the cleaned base metal. The inert propellant gas serves also to provide a controlled atmosphere within the coating chamber.
~here both sides of the moving metal strip are coated3 different coating metals and different thicknesses of coat may be applied.
Postconditioning provides additional control of the physical and chemical characteristics of the final product, with the coated metal strip passing between compacting rollers~ The inert propellant gas may be recovered for reuse.
The coated substrate resulting from use of my apparatus and pro-cess~ has the advantageous properties of several conventional products, while eliminating the disadvantages inherent in the way of manufactur-ing the conventional products.
There are several presently known methods to coat metaL strip with metal in a continuous process. All of these methods have lim-; itations which limit their use and makes them unsuitable for uni-versal application~
Coating of metal on an other metal is applied for many purposes, such as decorative~ anticorrosive and antifrictionD
Some of the conventional techniques hitherto employed are hot `~
dipping, electroplating, flame spraying, detonation gun spraying~
plasma spraying, fused coating and vacuum deposition. ~-For purposes of comparison, and to indicate the advantages of my invention, it is desirable to describe briefly the conventionally 83;Z
practiced methods of coating and thelr limitatlonsO
~ lot dipplng is one of the most commonly practiced coating methods.
A metal strip is pulled through a pot of molten metal under controlled conditions, the excess metal removed by mechanical means or an air knife and cooled. Some of the disadvantages oE this method are:
1. It is difficult to apply thin coating.

2, The coating is often brittle because of the formation of inter-facial alloy layer between the coating and the substrate.

3. It is difficult and uneconomical to apply coating on one side only.

4. It requires longer dwell time in the molten metal pot to apply thick coating, which in turn results in a more brittle coating, due to thicker interfacial alloy formation, Electroplating is also widely practiced process. Some of the limitations of this process are:
1. High equipment cost as compared to the rate of deposition.
2. High electrical energy cost as compared to the rate of depo-sltion.
3. Difficulty and cost of depositing coatings comparable in thick-ness to the hot dipping process.
. The complicated arrangement required to deposit metal on one slde only. -Flame spraying, a conventional process, is performed with metal wire or powder which is melted by high temperature flame, such as an .,:;
acetylene torch, and propelled toward the substrate. Some of the dis-advantages of this method are:
1. High porosity of the resultant coating.
2. Slow deposition rate.
3, The requirement of mechanical roughening, such as sand-blast-ing of the surface coated to achieve satisfactory adhesion of the coating.

:'~
''-.'.' ~
.. . ...
, ~: : . . ... . . . .

- ; ~

Detonat$on gun spraying, ~ co~v~h~ional process, is performed by mixing oxygen and fuel gas with the metal in powder form and igniting by a spark to propel the particles with very high velocity. It results in a more dense coating than flame spraying. Some of the disadvantages of this method are: ~
1. Slow deposition rate. ~;
2. The requirement of mechanical roughening, such as sand-blasting to achieve satisfactory adhesion of the coating, Plasma spraying, a conventional process, is performed with loni~ed, high temperature gases which melt and propel metal particles toward the substrate at high velocity. Some of the disadvantages of this method are:
; 1. Slow deposition rate.
2. High equipment cost as compared to the rate of deposition.
3. Substrate requires cooling to prevent deformation.
Fused coating, a conventional process, is performed by applying metal powder on the substrate and then heating the substrate above the melting point of the metal powder in order to fuse~ Some of the disadvantages of this method are:
1. The substrate may deform at the melting point of the coating metal.
- 2. Interfacial alloy layer forms which may result in brittle coating.
Vacuum deposition~ a conventional process, is performed by evapor-ating metal in vacuum, and condensing the metal vapor on the substrate.
Some of the disadvantages of this method are:
1. Very high equipment cost as compared to the rate of deposition. ; ;~
2. Very slow deposition rate.
3. Poor adhesion of the coating unless the substrate is heated to high temperature. ~
None of the described conventional methods is suitable without major modification to deposit a metal coating on a moving, flat metal ~: . - . . . , ~

3~
strlp with the versatility of my invention.
The main advantages of my invention are:
1. Varying thickness of metal can be deposLted on one or on both sides of the moving metal strip.
2. Different metals can be deposited in one process.
3. The rate of deposition is infinitely variable without propor-tional increase in equipment cost.
4, The quality of deposition is varlable~ The thickness of the interfacial alloy layer is controllable. Ductile or brittle coating can be produced on the same equipment.

5. The porosity of the coating can be controlled by rolling and consequent compacting to the desired degree.

6. The adhesion of the coating can be readily controlled by chemical ;
means, as is done in the hot dipping process.
70 Due to the flexibility of the temperature ranges of the process, substrate deformation is minimized.
8. The process can be adopted, with minor modification~ on some of the existing metal coating equipment.
BRIEF DESCRIPTION OF T~E DRAWING:
The objects and features of the invention may be understood with reference to the detailed description of an illustrative embodiment of the invention, taken together with the accompanying drawings in which:
FIG. 1 is a side elevation view of the coating apparatus;
FIG. 2 is a sectional view taken through line II-II of FIG. l;
FIG. 3 is a sectional view taken through line III-III of FIG. l; and FIG. 4 is a sectional view taken through line IV-IV of FIG, 1 DESCRIPTION OF THE PREFERRED EMBODIMENT:
.
Turning now descriptively to the drawLngs, in which similar reference characters denote similar elements throughout the several views, FIGS.
1-4 illustrate the substrate 10 in the form of a continuous metal strip, ~-~ being coated in the apparatus utili~ing an airless spraying system.

.. . . . .. . ..

The substrate strLp 10 is initially chemically cleaned, rinsed, optionally annealed and temper rolled, pickled and flux washed and pre-heated prior to entering the apparatus through entering rotating rotls 33 mounted across an opening in the housing 12 which is preferably made of asbestos board. The coated metal strip 10 exits the apparatus through similar rotating rolls 23.
The atmosphere within the confines 11 of the apparatus is an inert gas, preferably nitrogen, maintained at a pressure above that of the ambient air surrounding the housing 12, and at an appropriate temperature ranging up to 200 degrees F above the melting temperature of the coating metalO The housing 12 forms a relatively air-tight enclosure about the confines 11 in which the metal is sprayed.
Gas is drawn out of the interior 11, through pipe 21 by pump 43 and forced through heat exchanger 20 whlch is gas fired or electrically en-ergized~ and returned into the system. Valves 22 in pipe 21 control the flow of reheated gas.
Make-up nitrogen is replaced through line 24 by pump 53 and preheated through heat exchanger 51 with valve 52 controll~ng flow of make-up nitrogen into the interior 11.
The coating metal is melted in pot 15 by heaters 16. The molten metal is kept at 50 degrees - 500 degrees F above its melting point in pot 15. Line 55, spraying guns 14 and pump 17 are also kept close to the temperature of the molten metal to avoid clogging of the system~
with use of exterior insulation, or by additional heating elements, if required.
A pump 17 pumps the molten metal in each pot through lines 55 to spraying guns 14 where the molten metal is atomized by the combined effect of pressure and ultrasonic vibration or by pressure alone. The spraying guns 1~ are mounted above and below the substrate sheet 10~
The pressure requirement is determined by the specific gravity and viscosity of the molten metal and the degree of atomizing requlred, and .:

,-, :

, ~, ' : ': ,, ::

\
3~
ranges to 5000 pounds per inch srluare.
The molten metal is kept as noted at 50 degrees to 500 degrees F
above its melting point in pots 15, with one pot 15 supplying the guns 14 above the substrate sheet 10~ and the other pot 15 supplying ~he guns 14 below the sheet 10.
The temperature of the substrate 10 is maintained at 0 degrees to 800 degrees F below the melting point of the coiating metal~ Lower tem-perature of the substrate will reduce the fonmation of the interfacial alloy layer, while higher temperatures of the substrate will increase its formation.
Higher temperature will also result in a more dense, less porous and better adhering coating but undesirable substrate deformation can also be the result.
Lower substrate temperature will result in a more ductile and flex- ;
ible coating.
The end use of the product will determine the proper substrate tem-perature selection.
By closing valve 57 and opening valve 19 the molten metal is re~
circulated through line 55 and by-pass line 56.
FIG. 4 illustrates the arrangement of the molten metal lines and spray-ing guns. By opening or closing valves 18, individual lines of spraying ; guns 14 can be connected or disconnected to vary the width of substrate to be coated. The overspray is caught by asbestos boards 28 shown in FIGS. 2, 3 and 4, which are positioned at a g5 degree to 105 degree 25 angle to the substrate sheet 10 and are movable sideways as the width -of the substrate sheet 10 varies.
These asbestos boards 28 do not alloy with the molten metal. Since the temperature in the coating area is higher than the melting point of the coating metalg the overspray slowly flows down on the boards 28 ; 30 and is collected and reused. ~
: . '.
:

:'~

".

3;~ :~
The molten metal is preferably sprayed in a V or U-shaped flat spray.
Electrostatic spray will result in a more uniform coating.
The sprays are overlapping. The nozzles 14 are positioned in a staggered fashion in several rows for more uniform and thicker coating.
The guns 14 for spraying above the substrate sheet 10 may be offset from guns 14 below the sheet 10 so that the top coat of metal initlalty sprayed on the sheet 10 which then passes through compacting rollers 49A
and 49B prior to passing over the lower guns 14 that spray the bottom coat of the metal.
The coated substrate initially passes between compacting rollers 49A
and 49B. Roller 49B is a back-up roller and turns with the same surface speed as the speed of the substrate, while roller 49A, located to roll newly deposited coating, is coated with a hard, smoth, non-alloying, `~ high temperature resistant metal coating of refractory metal and turns at a 1-10% lower surface speed than the speed of the substrate~ The ," .
function of roller 49A is to compact the partially solidified, but still formable metal coating, and smoothen the surface~ The function of roller ~ ;
49A is also to eliminate pinholes and discontinuities in the metal coat- ~ ~ `
ing. The metal coating must be solid enough not to be removed from the 2~ substrate during the compacting process.
After the bottom coat has been sprayed, the coated sheet 10 passes through a second set of compacting rollers 49A and 49B and exit rollers 23.
The pressure on compacting rolls is variable. Higher pressure wiil give more dense coating. The surface temperature of rollers 49A and 49B
is kept 0 degrees to 800 degrees F below the melting point of the coating metal to prevent metal removal from the substrate, yet hot enough to produce a uniform and non-porous coating.
If uniform characterist~s of the coatings on both sides are cri ti-cal, the temperature of the molten metal in the second stage of coating should be higher than in the first.

. :. . . . :

33;~ -If different metals~ having very different meltlng points are to be coated on each side, then two completely separated coating areas are required with separate controls~ The higher melting point metal will always be coated in the first zone. Each coating zone will be con-structed the same way as described above, with optional substratetemperature ad~ustment between the coatlng zones.
The coating unit comprises also the necessary drives, rollers, guides, furnaces, cleaning, pickling, rinsing and fluxing equipment, which are not described here, since they are not part of this invention.
Post conditioning of the coated sheet 10 may include heat treating, temper rolling and pack diffusion, and occurs after the coated sheet 10 has left the exit rollers 23, either before or after cooling coated ;
sheet 10 to ambient temperature.
The main advantage of this apparatus is the flexlbility of the equipment with the ability to coat different thicknesses of the same or different metals on the top and bottom of the substrate sheet~ at a continuously variable deposition rate, with coating porosity controlled by rolling.
Pumps, spraying nozzles, connecting pipes, rollers, housing~ melting pot, etc., are constructed of su~h material as does not alloy itself with the molten metal, has sufficient structural ~ ~ and resist- ;
ance at working temperatures so as no~ to corrode.
Materials suitable for such equipment use include refractory metals, such as ~ ~ and tungsten, ceramics, ceramics lined steel, and asbestos for the housing.
Air-spray or airless spray guns can be used, in combination with .
- other known techniques to acieve the maximum degree of atomizing of the molten metal. The smaller the size of the molten metal droplets are the more uniform and controllable the coating will be~
The pressure required in the spraying guns is determined by the viscosity and specific gravity of the molten metal. The higher the ' ~:' "

: . . ~ , , ~ ................ : -~. . , . : ; . ~ . , .

~5431l33;~
specific gravity and viscosity the higher the pressure will be required.
~ here equipment and conditions permit, the use of electrostatic spraying equipment is des$rable for more uniform coatillg.
If air spray is used, the propelling gas such as nitrogen, must be inert to prevent the oxidation of substrate or coating metal. Since the coating and compacting is done also in an inert atmosphere, the re-covery and recycling of the nitrogen gas is important for economy, and can easily be done by recirculating pumps.
If airless spray is used, the pump must create enough pressure to atomize the molten metal.
Ultrasonic atomization of the molten metal in the spraying device will greatly reduce the pressure required in the spraying system. The ultrasonic vibrator must be also coated or made of a corrosion resistant ceramic or refractory material or the life of the vibrator will be re-duced.
Other advantage of the ultrasonic atomization is that uniform sizedroplets will be sprayed out which will result in a more uniform coat-ing.
The higher the frequency of the ultrasound is the smaller the dia-meter of the droplets will be, Since obvious changes may be made in the specific embodiment of theinvention described herein, such modifications being within the spirit and scope of the invention claimed~ it is indicated that all matter contained herein is intended as illustrative and not as limiting in scope.

. -' -. ' ' , ' , . " , . , ,. . , ~ -

Claims

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. Apparatus for spraying a coating of molten metal on a continuous sheet comprising an enclosed housing which is of relatively air tight nature, means for continuously feeding a continuous sheet of metal into said housing and out of said housing, means in the enclosed housing to spray molten metal onto the first side of said sheet, and roller means in the enclosure to compact the sprayed metal coating onto the sheet, said roller means comprising a pair of rollers mounted so as to apply pressure and to roll the sheet after it has been sprayed, with the sheet between the rollers of the pair, said rollers rotatably linked together so that the rollers rotate at different surface velocities, with a first roller, in contact with a first side of the sheet on which a metal coating has been sprayed, rotating with a surface velocity that differs from the surface velocity of the sheet so as to move the coated surface of the sheet in contact with said roller in relation to said sheet to compact the sprayed coating in contact with said first roller, with the second roller rotating at a surface velocity sub-stantially that of the surface velocity of the sheet.

2. The combimation as recited in claim 1 together with additional means in the enclosure to spray molten metal to the second side of the sheet, together with a second set of such roller means in the enclosures to compact the sprayed metal coating on said second side of the sheet.

3. The combination as recited in claim 1 in which the spray means in the enclosure incorporates an ultra-sonic atomization vibrator.

4. The combination as recited in claim 1 in which temperature control means are mounted in the housing to regulate the temperature in the housing adjacent the spray means and sheet to provide a temperature which is up to 200° F. higher than the melting point temperature of the molten metal being sprayed.

5. The process of continuously spraying a metal coating on a moving metal sheet utilizing the apparatus described in claim 1, said process incorporating the steps of a) feeding a metal sheet into an air-tight enclosed housing, b) spraying a metal coating on one side of said sheet inside the housing, c) compacting the sprayed metal coating by rolling the sheet through a pair of rollers, with the surface of the roller in contact with the sprayed coating moving in relation to the coated surface of the sheet, d) feeding the coated metal sheet out of the air-tight en-closed housing.

6. The process as recited in claim 5 in which the tempera-ture inside the housing is maintained up to 200° F. higher than the the melting point temperature of the coating, with the surface temperature of the rollers which compact the coating and sheet maintained at a temperature which is 0° F. to 800° F. lower than the melting point temperature of the coating.