WO2004080640A1

WO2004080640A1 - Aluminium layered brazing product and method of its manufacture

Info

Publication number: WO2004080640A1
Application number: PCT/EP2004/002659
Authority: WO
Inventors: Dieter Junkers; Johannes Alphonsus Franciscus Schade Van Westrum; Remco De Jong
Original assignee: Hille and Muller GmbH
Current assignee: Hille and Muller GmbH
Priority date: 2003-03-14
Filing date: 2004-03-12
Publication date: 2004-09-23
Anticipated expiration: 2005-09-14

Abstract

The present invention relates to a brazing product comprising an aluminium alloy core, a clad brazing alloy layer on at least one side of the core, and a metal coating layer on the outersurface of the clad brazing alloy layer for allowing fluxless brazing, and wherein the metal coating layer comprises a metal selected from the group of nickel, nickel alloy, iron, iron alloy, titanium, titanium alloy, cobalt and cobalt alloy, and said metal coating layer has an essentially columnar microstructure, and is devoid of a bonding layer between the clad brazing alloy layer and the metal coating layer. The invention relates further to a method of manufacturing such a brazing product.

Description

ALUMINIUM LAYERED BRAZING PRODUCT AND METHOD OF ITS MANUFACTURE

The invention relates to a method of manufacturing a brazing product, typically a brazing sheet product, comprising the step of coating a metal layer for allowing fluxless brazing onto a surface of a brazing product comprising an aluminium alloy core and a clad brazing layer on the aluminium alloy core, said surface being a surface of the clad brazing alloy, the method including a pre- treatment of said surface before the coating step. The invention also relates to a brazing product, typically a brazing sheet product, obtained by the method and to a brazed assembly comprising at least one component made of the brazing sheet product.

For the purpose of this invention brazing sheet is to be understood as a core sheet of aluminium or aluminium alloy, having on at least one side thereof a brazing alloy. Typical brazeable aluminium alloys useful as such a clad layer are the Aluminum Association ("AA") 4xxx-series-alloys, typically having Si in the range of 4 to 14%, such as for example AA4343 and AA4045. The brazeable aluminium alloys may be coupled to the core alloy in various ways known in the art, for example by means of roll bonding, cladding, semi-continuous or continuous casting processes, or thermal spaying or as a flux bonded powder. Controlled Atmosphere Brazing ("CAB") and Vacuum Brazing ("VB") are the two main processes used for industrial scale aluminium brazing. CAB requires an additional process step prior to brazing as compared to VB, since a brazing flux has to be applied prior to brazing. A brazing flux material for use in brazing aluminium alloys usually consists of mixtures of alkali earth chlorides and fluorides, sometimes containing aluminium fluoride or cryolite. CAB is essentially a continuous process in which, if the proper brazing flux is being used, high volumes of brazed assemblies can be manufactured. The brazing flux dissolves the oxide layer at brazing temperature allowing the clad alloy to flow properly. During the brazing cycle, corrosive fumes such as HF are generated. This puts a high demand on the corrosion resistance of the materials applied for the furnace and furnace scrubbers.

Ideally, a material should be available that can be used for CAB but does not have the requirements and drawbacks of the known brazing flux application. Such a material can be supplied to a manufacturer of brazed assemblies and is ready for use directly after forming of the assembly parts. No additional fluxing operations have to be carried out. Presently, only one process for fluxless brazing is used on an industrial scale. The material for this process can be for example standard brazing sheet made from an AA3xxx-series core alloy clad on one or both sides with a cladding of an AA4xxx-series alloy. The known method of achieving good brazing is to deposit a specific amount of nickel on the surface of the clad alloy. If properly applied, the nickel reacts, presumably exothermally, with the underlying aluminium.

Processes for nickel-plating in an alkaline solution of aluminium brazing sheet are known from each of US-A-3,970,237, US-A-4, 028,200, and US-A-4, 164,454.

According to these documents, nickel is deposited in combination with lead. The lead addition is used to improve the wetteability of the aluminium clad alloy during the brazing cycle. An important characteristic of these plating processes is that the nickel is preferentially deposited on the silicon particles of the aluminium clad alloy and furthermore that a relatively thick layer of nickel is deposited. To obtain sufficient nickel for brazing, the surface of the aluminium clad alloy should contain a relatively large number of silicon particles to act as nuclei for the nickel deposition. It is believed that in order to obtain sufficient nucleation sites a part of the aluminium in which the silicon particles are embedded should be removed before pickling by chemical and/or mechanical pre-treatment. This is believed to be a necessary condition to obtain sufficient silicon coverage to serve as nuclei for the plating action of the brazing or clad alloy. On a microscopical scale the surface of the Si-containing cladding of the brazing sheet is covered with nickel-lead globules. However, the use of lead for the production of a suitable nickel and/or cobalt layer on brazing sheet has several disadvantages. The use of lead for manufacturing products, such as automotive products, is undesirable.

The international application no. WO-00/71784, by J.N. Mooij et al. discloses a brazing sheet product in which there is provided a plated bonding layer comprising zinc or tin between the AlSi-alloy clad layer and the nickel layer in order to improve the bonding of the applied nickel layer.

A drawback of the known brazing sheet products having a layer comprising nickel is the limited corrosion life of brazed products in a SWAAT-test in accordance with ASTM G-85. Corrosion lifetimes without perforations are typically in the range of 4 to 6 days when having an AA3003-series core alloy.

According to the international application WO-02/060639, by A.J. Wittebrood et al., the corrosion performance of Ni-plated brazing sheet products can be improved considerably by the introduction of selected alloying elements to the molten filler alloy, in particular dedicated amounts of tin.

Other disadvantages of plating technology for the application of Ni layers on a brazing sheet product are the fairly low line-speeds even on an industrial scale line, the use of Ni-containing salts in the plating baths requiring skillfull handling, the need for expensive effluent treatment.

In one aspect of the invention there is provided in a method characterised by the features of claim 1.

In another aspect of the invention there is provided a brazing product characterized by the features of claim 14.

Preferred embodiments of both the method and product according to the invention are set forth in de description and the claims. This achieves one or more of the following advantages:

The use of PVD allows for the production of brazing products having a metal coating layer to allow fluxless brazing in the absence of an intermediate metal bonding layer, in particular those bonding layers of zinc or tin, which PVD applied metal coating layer has an adhesion at least equivalent to a electro-plated metal layer of similar composition;

The avoidance of a metal bonding layer overcomes several processing steps, such as dedicated cleaning of the surface before and after applying the bonding layer using chemical solutions, and the step of applying the bonding layer;

PVD allows for a much higher productivity compared to plating techniques, since industrial line-speed for PVD coating technique is up to 120 feed/min and possibly even more;

There is no need for effluent treatment, which is a significant advantage from an environmental and cost point of view;

For PVD the need for the handling of toxic Ni-containing salts as for plating baths has been overcome;

Each of PVD and CVD allow in a less complicated manner for the application of thin metal coating layers on one side only of the brazing product;

Each of PVD and CVD allow for the application of thin metal coatings on both sides of the brazing product, whereby one side has a different composition or thickness than the other side;

In an embodiment of the method according to the invention the metal coating layer is applied via electron-beam physical vapour deposition ("EB-PVD"). In a further embodiment of the method according to the invention the physical vapour deposition is performed at a pressure in the range of 1.10^"4 to 1.10^"6 mbar (=1.E-4 to 1.E-6 mbar). EB-PVD results into a metal coating layer having an essentially columnar microstructure. In another embodiment of the method according to the invention the metal coating is applied via plasma-activated physical vapour deposition ("PA-PVD") resulting in an essentially pore-free microstructure.

In another embodiment of the method according to the invention the metal coating is applied via evaporation by levitation at reduced pressure, and in particular using an alternating electromagnetic field, the alternating electromagnetic field being generated with the aid of a high-frequency alternating current. A particular embodiment of such a method is disclosed in the international application

WO-03/071000, incorporated herein by reference. In an embodiment of the method according to the invention the pre-treatment comprises the step of ion-etching at a reduced pressure or sputter-etching at a reduced pressure or magnetron etching at a reduced pressure or by glow-discharge etching at a reduced pressure in order to provoke the adhesion of the applied metal coating layer. The pressure applied would typically be in the range of 1.10^"3 to 1.10^"5 (=1.E-3 to 1.E-5) mbar and using an inert gas, e.g. Argon or Helium.

In another embodiment of the method according to the invention the pre- treatment comprises the step of plasma cleaning or plasma activation in order to further improve the adhesion of the applied metal coating layer.

In an embodiment of the method and brazing product according to the invention the metal coating layer or the clad brazing alloy comprises a wetting agent, preferably in a range of 0.01 to 1 wt.%. The wetting agent is added to improve the wettability of the molten metal filler during the brazing operation. Preferably, the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, and any mixture thereof. Optionally, the wetting agent can be applied as a separate layer onto or below the metal coating layer, e.g. applied as a Bi-flash in case Bi as wetting agent is used.

In an embodiment of the method and of the brazing product there is provided a further metal layer onto or underneath the metal layer of Ni, Fe, Ti or Co. Such a further layer can be applied in a similar manner as the metal layer itself, and such further metal layer is to improve on specific properties of the brazing sheet product. An example is the application of a layer of tin or tin-alloy onto under underneath the layer of Ni or Fe to improve on the corrosion performance after brazing, as is known for the Ni from international application WO-02/060639, in which document also alternative metal for tin are proposed. In an embodiment of the method and of the brazing product according to the invention the clad brazing alloy comprises an aluminium-silicon alloy containing silicon in an amount in the range of 4 to 14% by weight.

In an embodiment of the method and of the brazing product according to the invention the metal coating layer has a thickness of not more than 2.0 μm, and preferably of not more than 1.0 μm. A suitable lower limit for the metal coating thickness is 0.2 μm.

In a preferred embodiment of the method and of the brazing product is elongated aluminium alloy stock, such as sheet, strip, wire or rod, resulting for example in brazing sheet products in accordance with the invention.

In a preferred embodiment the method and of the brazing product is a brazing sheet product comprising a core sheet made of an aluminium alloy coupled and on at least one surface of said core sheet a clad brazing alloy layer, the clad brazing alloy layer being made of an aluminium AA4xxx-series alloy comprising silicon in the range of 4 to 14% by weight, preferably in the range of 7 to 14%), and a metal coating layer for allowing the fluxless brazeability capabilities of the product applied on the outersurface of said clad brazing alloy layer such that taken together said clad brazing alloy layer and all layers exterior thereto, e.g. the metal coating metal, form the filler metal for a brazing operation. Preferably the aluminium alloys for the core are selected from the group consisting of AA2xxx, AA3xxx, AAδxxx, and

AAΘxxx-series alloys. In a further aspect of the invention there is provided a method of manufacturing a brazed assembly using the brazing product, ideally a brazing sheet product, in accordance with the invention comprising the steps of:

(a) shaping parts of which at least one is made from the brazing product according to the invention; (b) assembling the parts into an assembly;

(c) brazing the assembly in an inert atmosphere in the absence of a brazing-flux at elevated temperature, preferably at a temperature in the range of 490 to 600°C, for a period long enough for melting and spreading of the molten filler;

(d) cooling the brazed assembly to below 100°C, typically with a cooling rate of at least 20°C/min, preferably of at least 40°C/min.

Depending upon the aluminium alloy of the core the process may include the further processing step (e) of ageing of the brazed and cooled assembly in order to optimise the mechanical and/or corrosion properties of the resultant assembly.

In an embodiment of the method of manufacturing a brazed assembly in step (a) at least one of the parts to be joined by brazing is made of the brazing product in accordance with the invention set out above, ideally a brazing sheet product, and at least one other part is selected from the group consisting of titanium, plated or coated titanium, bronze, brass, stainless steel, plated or coated stainless steel, nickel, nickel-alloy, low-carbon steel, plated or coated low-carbon steel, high- strength steel, and plated or coated high-strength steel. In a further aspect of the invention there is provided a brazed assembly manufactured in a fluxless Controlled Atmosphere Brazing process and comprising at least one component made of the brazing product of this invention or manufactured in accordance with the invention. Typical examples of such brazed assemblies are heat exchangers, such as radiators and condensers, oil coolers, fuel cells and solid fuel cells.

Example

On a pilot-scale of testing a coil of aluminium brazing sheet consisting of an AA3003 core alloy roll clad on both sides with an AA4045 clad alloy and having a total thickness of 0.5 mm and a clad layer thickness of 50 microns on each side has been coated on one side with a uniform layer of nickel-bismuth alloy with a thickness of 0.8 micron. The bismuth content was about 0.04 wt.%>. The nickel- bismuth layer was applied onto the clad layer without the use of an intermediate metal bonding layer comprising of zinc or tin. The brazing sheet strip was degreased by neutral electrolytic degreasing using ChemTec 30014 (a commercial available bath) at 50°C and a current density of 5 A cm². The coating process used was electron-beam physical vapour deposition ("EB-PVD"), the line speed at the pilot-line was 3 m/min and the pressure was 3.3x10^"5 (3.3E-5) mbar. Just prior to applying the metal layer the sheet was pre-treated via sputter-etching with Argon at a pressure of 2x10^"3 (2E-2) mbar.

The resultant brazing sheet product has been tested for the adhesion of the applied Ni-layer and for its brazeability. For comparison reasons the PVD-produced material has been compared with Ni-plated material manufactured as set out in WO-02/060639, Table 1 , for Sample 1 , incorporated herein by reference and having a bonding layer of zinc.

All samples have been tested for adhesion of the Ni layer using the Erichsen dome test, and the T-bend test. An overall value assessment is then given to the adhesion where: (-) = poor, (±) = fair, and (+) = good.

On a laboratory scale of testing the brazing tests were carried out in a small quartz furnace. Small coupons of 25 mm x 25 mm were cut from the coated sheets. A small strip of a bare AA3003 alloy measuring 30 mm x 7 mm x 1 mm was bent in the centre to an angle of 45° and laid on the coupons. The angle-on-the-coupon samples were heated under flowing nitrogen, with heating from room temperature to 580°C, dwell time at 580°C for 1 minute, cooling from 580°C to room temperature. The brazing process was judged on possible formation of wrinkles, capillary depression and fillet formation. An overall assessment was given where:

(-) = poor brazeability, (-/+) = fair brazeability, (±) = good brazeability, and (+) = excellent brazeability.

The Ni-plated coating having the bonding layer of zinc had a good adhesion

(an "+" rating) and the Ni-PVD coating had an adhesion at least equivalent to the Ni-electro-plated coating (an "+" rating).

Both the Ni-plated coating and the Ni-PVD coating had an excellent brazeability (an "+" rating).

When applying a bonding layer of zinc for example via a zincate treatment, there is also the formation of an Al-Zn or Al-Zn-O based impurity which preferably needs to be removed in a proper pre-treatment prior to applying the Ni-layer via electro-plating. If such pre-treatment is not carried out, it may adversely affect the brazeability or the corrosion behaviour after brazing the final product. Such pre- treatment is an additional treatment adding to the overall cost of manufacturing the product. Furthermore, such pre-treatment using chemical solutions provides an environmental. disadvantages to the whole process when the Ni-layer is applied via electro-plating. All the disadvantages associated with electro-plating and in particular the use of a bonding layer are overcome with the process according to this invention as no bonding layer is required in order to obtain a sufficiently good bonding of the metal layer, e.g. Ni. As demonstrated above the Ni-PVD coating had an adhesion at least equivalent to the Ni-electro-plated coating

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as hereon described.

Claims

1. A method of manufacturing a brazing product, comprising the step of coating a metal layer for allowing fluxless brazing onto a surface of a brazing product comprising an aluminium alloy core and a clad brazing layer on the aluminium alloy core, said surface being a surface of the clad brazing alloy, the method including a pre-treatment of said surface before the coating step, characterised in that the metal layer comprises a metal selected from the group of nickel, nickel alloy, iron, iron alloy, titanium, titanium alloy, cobalt and cobalt alloy, and the metal coating is applied via physical vapour deposition or chemical vapour deposition, and the metal coating is applied onto the clad brazing layer in the absence of a bonding layer.

2. A method according to claim 1 , wherein the metal coating is applied onto the clad brazing layer in the absence of a bonding layer comprising zinc or tin.

3. A method according to claim 1 or 2, wherein the metal coating layer has a thickness of not more than 2.0 μm, and preferably of not more than 1.0 μm.

4. A method according to any one of claims 1 to 3, wherein the metal coating layer is applied via electron-beam physical vapour deposition, and preferably at a pressure between 1x10^"4 to 1x10^"6 mbar.

5. A method according to any one of claims 1 to 3, wherein the metal coating layer is applied via plasma-activated physical vapour deposition.

6. A method according to any one of claims 1 to 3, wherein the metal coating layer is applied by evaporation by levitation at reduced pressure.

7. A method according to any one of claims 1 to 6, wherein the pre-treatment comprises one or more steps carried out at a reduced pressure and selected from the group comprising ion-etching, sputter-etching or magnetron etching and glow-discharge etching.

8. A method according to any one of claims 1 to 7, wherein the metal coating layer or the clad brazing alloy comprises a wetting agent.

9. A method according to any one of claims 1 to 7, wherein there applied a further layer comprising a wetting agent onto or below the metal coating layer.

10. A method according to claim 8 or 9, wherein the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, and any mixture thereof.

11. A method according to claim 8 or 10 wherein the clad brazing alloy or the metal coating layer comprises a wetting agent in a range of 0.01 to 1 wt.%.

12. A method according to any one of claims 1 to 11 , wherein the clad brazing alloy comprises an aluminium-silicon alloy containing silicon in an amount in the range of 4 to 14%> by weight.

13. A method according to any one of claims 1 to 12, wherein the brazing product is elongated stock.

14. A brazing product comprising an aluminium alloy core, a clad brazing alloy layer on at least one side of the core, and a metal coating layer on the outersurface of the clad brazing alloy layer for allowing fluxless brazing, characterised in that the metal coating layer comprises a metal selected from the group of nickel, nickel alloy, iron, iron alloy, titanium, titanium alloy, cobalt and cobalt alloy, and said metal coating layer has an essentially columnar microstructure, and is devoid of a bonding layer comprising zinc or tin between the clad brazing alloy layer and the metal coating layer.

15. A brazing product according to claim 14, wherein the metal coating layer or the clad brazing alloy comprises a wetting agent, preferably in a range of 0.01 to 1 wt.%.

16. A brazing product according to claim 14 or 15, wherein the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, and any mixture thereof.

17. A brazing product according to any one of claims 14 to 16, wherein the clad brazing alloy comprises an aluminium-silicon alloy containing silicon in an amount in the range of 4 to 14% by weight.

18. A brazing product according to any one of claims 14 to 17, wherein the metal coating layer has a thickness of not more than 2.0 μm, and preferably of not more than 1.0 μm.

19. An assembly of components joined by brazing, at least one said components being a brazing product according to any one of claims 14 to 18.