WO1993008952A1

WO1993008952A1 - Method for modifying the surface of an aluminum substrate

Info

Publication number: WO1993008952A1
Application number: PCT/CA1992/000465
Authority: WO
Inventors: Roland Sion Timsit
Original assignee: Alcan International Limited
Priority date: 1991-10-28
Filing date: 1992-10-23
Publication date: 1993-05-13
Also published as: AU2794592A; MX9206180A

Abstract

A method is described for modifying the surface of aluminum or aluminum alloy substrate. Also described are a coating mixture used in the method, a coated substrate surface and a modified surface product. The novel method steps comprise (a) applying as a coating to said surface a mixture of (i) a metal capable of forming in situ a eutectic with aluminum (ii) a flux material capable of removing an oxide layer from the aluminum and which melts below 600 °C and (iii) a surface modifying particulate material selected from particles of metal, a ceramic material and a refractory material, and (b) heating the surface and coating to a temperature above the melting point of both the flux and the brazable eutectic being formed in situ with aluminum to thereby remove oxide film on said surface, cause said eutectic forming metal to dissolve into the oxide-free aluminum surface and form therewith a eutectic alloy layer containing said surface modifying particles.

Description

Method for Modifying the Surface of an Aluminum Substrate Technical Field

This invention relates to a method for modifying the surface of an aluminum or aluminum alloy substrate, as well as to a coating mixture used in the method, a coated substrate surface and a modified surface product.

Background Art

Light metals, such as aluminum, are of great

commercial interest particularly because of their light weight. However, the surfaces of these metals often lack certain strength characteristics, such as good wear characteristics, good heat resistance, good corrosion resistance, etc. Accordingly, many efforts have been made to coat or modify the surfaces of light metal substrates to improve the surface properties.

For example, Japanese Published Application 59-219468 (Teikoku Piston Ring K.K.) describes a method for

imparting good sliding friction properties to an aluminum alloy substrate by coating the surface with a high silicon alloy. This is done by coating the substrate with either metallic silicon or Al-Si alloy, and then heating the coating by means of a laser, electronic beam or plasma arc. However, the heating of a hidden surface of a complex shape by lasers, electronic beams or plasma arcs is very difficult because these areas are not exposed to the outside and, moreover, the high reflectivity of aluminum surfaces leads to very low absorption of this energy.

A method for improving the corrosion resistance of an aluminum substrate is described in Showa, Japanese

Published Application 02-149679. In this process,

externally supplied Si or Ge is incorporated into the surface of an aluminum alloy by melting the surface by exposure to a laser beam. The use of laser energy for this purpose has the disadvantages mentioned above.

European Published Application EP-A 0 036 594 (Union Carbide Corporation), published September 30, 1981 describes a process for making porous the surface of aluminum substrates by coating the substrate with finely divided powder of aluminum alloy, aluminum-silicon alloy to act as the bonding component, and a flux, preferably of the potassium fluoaluminate type. This technique requires the use of the aluminum-silicon alloy binding agent which is a separate preformed aluminum brazing alloy powder.

In published Japanese Application 01-287279 (Toyota), published November 17, 1989, there is disclosed a process for reinforcing surfaces of light metals by placing a fluoride salt of the reinforcing metal on the surface to be reinforced and heating this. The use of fluoride salts of the metal represents a very expensive technique and, in fact, the surface to be reinforced is typically coated with an aluminum-silicon alloy, a K₂ZrF₆ powder, as well as with silicon carbide reinforcing whiskers and then heated to 600ºC.

Russian Patent No. 1,122,748, published November 7, 1984, describes a process for forming a tungsten-silicon layer on metal surfaces, such as steel and titanium, by coating the surface with 2-8% aluminum, 45-75% silicon, 6- 10% copper, along with A1F₃ and the balance tungsten oxide and heating to 850 to 900ºC. This is not used with aluminum substrates.

British Patent 234,969 (L.D. Hooper) teaches that any surface can be siliconized by making a preparatory deposit of a more receptive substance such as Pb, Sn, Cu, or Al in a known manner, or a flux may be incorporated with the silicon such as an alkali metal or its halide, aluminate, borate, oxide or hydroxide or aluminum or zinc halide.

Japanese Published Patent Application 61-321144

(Japan Light Metals Co.), published October 15, 1986, discloses a method for making a porous shape by sintering together particles of alloys of eutectic compositions such as Al-Sn, Al-Pb, Al-Si-Cu, Al-Mg-Si or Al-Zn-Mg, with the help of an Al-F-K salt as flux. It is the object of the present invention to develop a simple and inexpensive technique for modifying the surface of an aluminum or aluminum alloy substrate so as to improve its physical characteristics, e.g. wear resistance, heat resistance, etc.

Disclosure of the Invention

According to this invention, it has surprisingly been found that a technique derived from brazing can be used for carrying out the desired surface modification of aluminum or aluminum alloy substrates. The method

comprises: (a) applying as a coating to said surface a mixture of (i) a metal capable of forming in situ a eutectic with aluminum (ii) a flux material capable of removing an oxide layer from the aluminum and which melts below 600°C and (iii) a surface modifying particulate material selected from particles of metal, a ceramic material and a refractory material, and (b) heating the surface and coating to a temperature above the melting point of both the flux and the eutectic formed in situ with aluminum to thereby remove oxide film on said

surface, cause said eutectic forming metal to dissolve into the oxide-free aluminum surface and form therewith a eutectic alloy layer containing said surface modifying particles.

In this specification and claims, the terms "brazing" and "brazeable" refer to a process for covering a

substrate surface with or soldering to a substrate surface a layer of eutectic alloy which bonds to the surface.

Thus, brazing as used within the context of the present invention is not restricted to the joining of workpieces.

It is common practice to join aluminum components by disposing an aluminum brazing alloy between or adjacent the component surfaces to be joined, and heating the brazing alloy and the joining surfaces in appropriately assembled fashion to a temperature (brazing temperature) at which the brazing alloy melts while the components remain unmelted. Upon subsequent cooling, the brazing alloy forms a fillet or joint that bonds the joining surfaces of the components. For assured selective melting of only the brazing alloy in the heating step, it is commonly preferred that the melting point of the brazing alloy be at least 30º to 40ºC lower than that of the metal of the components. An example of a suitable aluminum brazing alloy is an Al-Si eutectic composition, which melts at about 577ºC.

In the process of the present invention, a eutectic is formed in situ with the aluminum by the application of the coating to the surface of the aluminum and then heating this coating material. The novel coating mixture may be applied either as a dry powder or as a slurry, preferably in a volatile liquid carrier, which may be water based or based on an organic substance, such as alcohol. It may also be applied as a paste made with an organic or other binder which is volatilized at the

temperature of brazing.

The preferred metal component of the coating mixture to provide the eutectic alloy is silicon but other metals such as zinc, copper or germanium may be used. In this specification and in the accompanying claims, the word "metal" refers to the elemental form of a metal, as commercially available in unalloyed form, which may include small concentrations of impurities which do not affect its characteristics. The main requirement is that there be present in the coating mixture a metal component which at brazing temperatures is capable of dissolving in the aluminum and forming in situ with the aluminum a layer of brazeable eutectic alloy.

The flux component of the coating mixture may be any material capable of removing the oxide layer and which melts below 600ºC. The preferred flux is a complex

potassium fluoroaluminate such as the Nocolok^® flux, other potassium fluoroaluminates such as KAlF₄, K₂AlF₅, K₃AlF₆, and their mixtures; and potassium fluoroaluminate mixed with one or more of cesium chloride, rubidium chloride. lithium fluoride, cesium fluoride and other alkali halide salts to reduce the melting point of the flux. Other known aluminum brazing fluxes are: mixtures of alkali and alkaline earth chlorides and fluorides, ammonium chloride, ammonium fluoride, potassium acid fluoride (KHF₂), sodium acid fluoride (NaHF₂), ammonium acid fluoride (NH₄HF₂), zinc chloride, mixtures of zinc chloride, potassium acid fluoride and ammonium chloride and potassium fluoro- zirconate (K₂ZrF₆).

The metal component and flux are typically present in the mixture in a ratio (wt/wt) of metal component to flux in the range of 0.1:1 to 5:1, preferably 0.25:1 to 0.5:1.

The metal component capable of forming a brazable eutectic with aluminum is preferably in the form of fine particles having sizes less than 10% of the thickness of the substrate, e.g. in the range of dimensions from about 1 to 1000 μm, preferably 4 to 80 μm, more preferably 5 to 50 μm, with a range of 10 to 20 μm being particularly preferred. The surface modifying particulate material component of the coating mixture is typically present in an amount of up to 1 part by weight per 1 part by weight of the eutectic forming metal of the coating mixture, but even higher and lower ratios may be used. This may be as high as 6 parts particulate material per 1 part eutectic forming metal.

According to a preferred embodiment, the coating mixture may also include a binder component. This binder may be selected from a variety of binder materials which are capable of volatilizing below the melting point of the flux and the eutectic alloy. Examples of suitable binder materials include a mixture polyethylmethacrylate and butylacrylate or 1-(2-methoxy-1-methyloxy)-2-propanol and propylene glycol as the carrier, or 2-methyl-2,4-pentanediol.

The amount of coating mixture applied to the surface is usually less than 130 g/m², with a range of about 30 to 100 g/m² being preferred. When a binder is included in the mixture, as much as 130 g/m² can be applied. However, a mixture without a binder should not be applied in an amount above 100 g/m².

A wide range of different powdered materials may be used as the modifying material. These are typically metals, ceramic materials or refractory materials, but preferably comprise powders selected from the group consisting of nickel, iron, manganese, cobalt, chromium, copper, titanium, boron nitride, nickel aluminide, metal borides, metal suicides such as tungsten silicide or chromium silicide, metal carbides such as tungsten carbide or silicon carbide, metal nitrides or metal oxides. These are preferably used in the form of powders and typically have particle sizes in the range of 1 to 150 μm.

The joining procedure of the invention is preferably carried out at a temperature in the range of 420 to

650ºC. When silicon is used, the temperature required is between 580 and 650ºC. The quality of the brazed layer depends upon the relative Si/flux content in the brazing mixture and on the surface coverage by that mixture and the length of time held at the brazing temperature.

The in situ formation of the eutectic Al-Si alloy occurs through the complementary actions of the flux material and the silicon. For instance, at 600ºC the flux is molten and dissolves or removes the native oxide film on the aluminum surfaces being treated, exposing fresh aluminum to the silicon powder. Because of the high solubility of silicon in aluminum at this temperature, the silicon dissolves rapidly into the exposed aluminum surface, forming a surface layer of liquid Al-Si alloy with a composition believed to be close to the Al/Si eutectic of 89% Al/11% Si in the aluminum substrate.

Since the melting point of this eutectic is only 577ºC, the silicon-containing aluminum surface melts and flows in the temperature range 577 to 650ºC. It is believed that the molten flux reduces the surface tension of the molten Si/Al eutectic alloy so that the molten alloy forms around the modifying particles. Because the formation of the

Al/Si eutectic alloy depends on diffusion of silicon into aluminum, the brazing process must be carried out at the above temperature for a time interval sufficiently long for Si-diffusion and for the ensuing alloy-forming to occur. This time interval typically ranges from about 0.1 to 10 minutes, preferably 1 to 7 minutes.

Brief Description of the Drawings

In the drawings which illustrate the present invention: Fig. 1 is a photomicrograph of a flux:silicon:nickel modified surface;

Fig. 2 is a photomicrograph of a

flux:silicon:tungsten carbide modified surface;

Fig. 3 is a photomicrograph of a flux: silicon:niobium modified surface and

Fig. 4 is a photomicrograph of a flux:silicon: silicon carbide modified surface.

Description of the Preferred Embodiments

By way of further illustration of the invention, reference may be had to the following specific examples:

Example 1

A substrate alloy consisting of AA1100 aluminum alloy was sheared into 25 mm × 3 mm × 1 mm coupons. The coupons were cleaned by swabbing in acetone followed by caustic etching for 10 seconds in 5 % w/w, 65ºC sodium hydroxide solution, desmutted in 50% nitric acid, water rinsed and forced air dried. The slurries were prepared at 50% total solids in isopropyl alcohol with a silicon: flux powder ratio of 1:3:0.65. Silicon powder 50 μm median particle size and Nocolok^® KC 100 flux, a potassium fluoroaluminate flux, were used to prepare the slurries. A variety of different powders were added and the coated coupons were brazed horizontally in a brazing furnace using a rapid heat-up to 605°C and soaked at 605ºC for 5 minutes,

resulting in a total braze time of 16 to 18 minutes.

Table 1 lists the loading of each component on the various coupons.

After brazing, the coupons were metallographically prepared for examination of each sample in cross-section. It was found that each of the modifying powders combined with the in situ formed Al-Si eutectic alloy resulted in a unique surface. In many instances the modifying powder appeared to be totally embedded in the Al-Si eutectic and this can be seen in Figure 2 where tungsten carbide was used as the powder. In other instances the modifying powder reacted in part to form intermetallics and this is shown in Figure 1 where nickel powder was used as the metal powder. Niobium was almost completely embedded in the Al-Si eutectic alloy as shown in Figure 3 and

magnesium did not incorporate into the eutectic but it did not interfere with the formation of the eutectic. As seen in Figure 4, silicon carbide powder combined with the Al-Si alloy resulting in spheres being formed on the surface. The spheres, although not evident from the

photomicrograph, are not loose on the surface, but are held or are embedded in a eutectic alloy layer and cannot normally be rubbed or scraped off.

Wear tests were also performed on some of the

modified surfaces using a reciprocating friction measuring apparatus (Spherical Pin on Moving Flat). The initial contact stress was 450 MPa with a normal load of 30 g on the ball. The travel distance in one direction only was 4 mm at a speed of 60 μm/sec. The steel spherical pin was made of 52100 steel, had a diameter of 1.5 mm, and had a Grade 3 surface finish. A drop of motor oil spread out over the surface was used in lubricated wear tests. All wear tests were performed at 20% relative humidity. The results of the tests are shown in Table 2 below:

Generally, a lower friction coefficient corresponds to a more wear resistant surface. However, the modified surfaces are also characterized by the amount of wear on the steel pin and the substrate. For example, the Mn and Fe modified surfaces have friction coefficients of 0.1- 0.15 and 0.3 respectively. However, the amount of substrate wear on the Mn surface ranges from light to severe whereas substrate wear on the Fe surface is undetectable and consequently is more wear resistant.

Claims

Claims:

1. A method for modifying the surface of an aluminum or aluminum alloy substrate which comprises:

(a) applying as a coating to said surface a mixture of (i) a metal capable of forming in situ a eutectic with aluminum (ii) a flux material capable of removing an oxide layer from the aluminum and which melts below 600ºC and (iii) a surface modifying particulate material selected from particles of metal, a ceramic material and a

refractory material, and

(b) heating the surface and coating to a temperature above the melting point of both the flux and the brazable eutectic being formed in situ with aluminum to thereby remove oxide film on said surface, cause said eutectic forming metal to dissolve into the oxide-free aluminum surface and form therewith a eutectic alloy layer

containing said surface modifying particles.

2. A method according to claim 1 wherein the

eutectic forming metal of the coating is selected from silicon, copper and germanium.

3. A method according to claim 2 wherein said eutectic forming metal has particle sizes in the range of 1 to 1000 μm.

4. A method according to claim 3 wherein the surface modifying particulate material is a powder selected from the group consisting of nickel, iron, manganese, cobalt, chromium, niobium, copper, titanium, boron nitride, nickel aluminide, metal borides, metal suicides, metal carbides, metal nitrides or metal oxides.

5. A method according to claim 4 wherein the metal silicide is tungsten silicide or chromium silicide, and the metal carbide is tungsten carbide or silicon carbide.

6. A method according to claim 3 wherein said particles of eutectic forming metal have sizes less than 10% of the thickness of the substrate.

7. A method according to claim 3 wherein said metal has particle sizes in the range of 4 to 80 μm.

8. A method according to claim 3 wherein said metal has particle sizes in the range of 5 to 50 μm.

9. A method according to claim 3 wherein said metal has particle sizes in the range of 10 to 20 μm.

10. A method according to claim 3 wherein the coating mixture is applied to the surface in an amount of up to 100 g/m².

11. A method according to claim 3 wherein the ratio wt/wt of said metal to flux in the coating is in the range 0.1:1 to 5:1.

12. A method according to claim 3 wherein the ratio wt/wt of said metal to flux in the coating is in the range 0.25:1 to 0.5:1.

13. A method according to claim 4 wherein the eutectic-forming metal is silicon.

14. A method according to claim 13 wherein the surface modifying particulate material has particle sizes in the range of 1 to 150 μm.

15. A method according to claim 14 wherein the coating mixture contains up to six parts by weight of particulate material per part by weight of the eutectic forming metal.

16. A method according to claim 3 wherein the flux is a potassium fluoroaluminate flux.

17. A method according to claim 1 wherein the surface and coating are heated to a temperature in the range of 420 to 650ºC.

18. A method according to claim 17 wherein the heating is continued for 0.1 to 10 minutes.

19. A method according to claim 1 wherein the mixture of metal, brazing flux and surface modifying particulate material is applied as a dry powder.

20. A method according to claim 1 wherein the

mixture of metal, brazing flux and surface modifying particulate material is applied as a slurry.

21. A method according to claim 20 wherein the slurry includes a volatile liquid carrier.

22. A method according to claim 1 wherein the coating mixture also includes a binder capable of

volatilizing at a temperature below the melting point of the flux and the eutectic alloy.

23. A method according to claim 22 wherein the coating mixture is applied to the surface in an amount of up to 130 g/m².

24. A coating mixture for use in modifying the surface of an aluminum or aluminum alloy substrate, said coating mixture comprising (i) a metal capable of forming in situ a eutectic with aluminum (ii) a flux material capable of removing an oxide layer from the aluminum and which melts below 600°C and (iii) a surface modifying particulate material selected from particles of metal, a ceramic material and a refractory material.

25. A coating mixture according to claim 24 which also includes a binder capable of volatilizing at a temperature below the melting point of the flux and the eutectic alloy.

26. A coating mixture according to claim 24 in the form of a slurry in water.

27. A coating mixture according to claim 24 in the form of a slurry in organic solvent.

28. A coating mixture according to claim 25 in the form of a slurry in water.

29. A coating mixture according to claim 25 in the form of a slurry in organic solvent.

30. A coating mixture according to claim 25 wherein the mixture is in the form of a paste.

31. A non-heat treated composite comprising an aluminum or aluminum alloy substrate surface coated with a layer of coating mixture as claimed in claim 24.

32. A product according to claim 31 wherein the coating mixture also includes a binder capable of

33. A surface modified aluminum or aluminum alloy substrate obtained according to the method of claim 1.