The present invention relates to coated aluminium shapes, and more particularly to hollow aluminium shapes, e.g. tubes coated with a fluxless metallic layer, functioning as a soldering or low temperature brazing material when heated during assembling of the shapes to heat exchanging fins in the manufacture of heat exchangers, and to a process for coating of such aluminium shapes.

It is to be understood that the word aluminium as used in the present application is intended to mean aluminium and aluminium base alloys.

Contact between tubes and heat exchanging fins in heat ex¬ changers/radiators installed in cars is today ensured mainly by two methods - mechanical expansion of tubes, or brazing of tuoes to heat exchanging fins. The reliable contact achieved by the mechanical expansion of the tubes can be achieved only when using substantially round tubes, which means limited heat performance of the heat exchangers. Brazed flat copper or brass tubes are largely used in car radiators, providing a good performance when the inexpensive well-known brazing technique is used.

There is an increasing trend in the automotive industry to replace steel and copper with lighter materials like aluminium or magnesium.

Soldering and brazing of aluminium to aluminium or to other metals is, however, a difficult task because of the presence of an aluminium oxide layer formed instantly on the surface of aluminium members exposed to the atmosphere. This oxide layer prevents formation of a tight metallic bond between brazed members because of its poor wetting properties.

In practice it is therefore necessary to pretreat the aluminium surface prior to soldering/brazing in order to re¬ move the aluminium oxide. Different methods are used for re¬ moving the oxide layer, depending on the subsequent use of the aluminium members. A method called flux brazing is widely used where the aluminium members are immersed in a bath of molten salt and brazing flux or exposed to the brazing flux which removes the oxide layer from the preheated members. The method is quite efficient with regard to the oxide re¬ moval, but it is practically impossible to avoid that the flux is entrapped at the brazed joints representing a serious corrosion problem.

Moreover, the brazing material commonly used is an Al-Si alloy which is a high temperature brazing alloy. In practice when the units to be brazed are assembled with the parts in position on the coated tubes, the assembly is then placed in a vacuum furnace and heated. However, the melting temperature of this brazing alloy is around 565 to 595°C which is not far below the melting point of aluminium, and as a result the temperature must be very closely controlled. But the silicon from the brazing alloy vaporizes and tends to deposit on sur¬ faces within the heating- apparatus, including the temperature sensors, tnereby making control uncertain at best. Thus the process is not very reliable or efficient.

Fluxless brazing processes have been conducted successfully in vacuum or inert atmosphere and are described in US Patent Nos. 3,373,482 and 3,979,042.

The common drawback of these processes is that the necessity of cleaning/pretreating procedures is not eliminated and rather high temperatures are required in order to break down the oxide layer or to create cracking in this layer. These processes are therefore expensive and limited to special applications.

It is therefore an object of the present invention to provide coated aluminium shapes, particularly hollow shapes, e.g. tubes having a fluxless coating of soldering or brazing material for fluxless soldering/fluxless brazing at low temperatures, and to provide a process for direct coating of aluminium shapes having an oxide-free, non-pretreated surface.

Another object of the invention is to provide aluminium shapes precoated with soldering or brazing material ensuring improved corrosion, resistance of the shapes, such shapes be¬ ing thus particularly suited for brazed aluminium heat ex¬ changers. By shapes is meant elongated aluminium members having a cross-sectional shape such as an angle, an I or H beam, a flange, or any other of the many shapes in which elongated aluminium members are formed, and particularly hollow shapes, e.g. tubes with various cross-sectional shapes - round, flat oval etc., optionally provided with inner partition walls or multicavities.

The aluminium shapes according to the present invention pro¬ vided either by extruding the aluminium in the desired shape and dimension or by any other methods of forming the shapes from aluminium are coated with a metallic layer which con¬ stitutes a fluxless soldering or brazing material for the subsequent fluxless soldering or low temperature brazing of

the shapes to other structural elements, e.g. soldering or brazing of tubes to heat exchanging fins in the manufacture of heat exchangers. The soldering or brazing material applied as the coating layer consists of a zinc-base alloy.

The process of the present invention comprises the steps of forming aluminium in the desired shape, preventing contact between the surface of the formed shape and the atmosphere for avoiding oxidation of the surface prior to the subsequent coating of the outer surface with a metallic layer, and then carrying out coating of the outer surface of the shape with a metallic layer.

The invention will now be described in greater detail in con¬ nection with examples and the accompanying drawing. Fig. 1, which is a schematic representation of the process and the apparatus for carrying it out.

A soldering/brazing coating provided on aluminium shapes according to the present invention is a zinc-base alloy, preferably ZnAl alloy, comprising from 1,0-10,0 wt % Al and from 0 to 3,0 wt % Cu, the balance being zinc. This alloy composition allows conducting of low temperature coating, approximately at 385-420°C, and consequently low tempera¬ ture and low cost brazing of the coated tubes to heat ex¬ changing fins during assembling of heat exchangers. The addi¬ tion of Al, preferentially from 3,0 to 6,0 wt %, improves the wetting of aluminium by molten zinc and in combination with Cu results in a sufficiently ductile coating of high tensile strength of the shapes in cold state.

Example 1

A drawn aluminium tube was mechanically cleaned of its sur¬ face oxides in a protective atmosphere of nitrogen and then passed directly, i.e. while still in the nitrogen protective

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atomosphere, into a molten bath of zinc alloy comprising 5,8 wt % Al, 1,0 wt % Cu and the balance mainly zinc. The zinc bath temperature was kept at 450°C in order to avoid an excessive solidification of zinc on cold aluminium. After a 3 sec. contact time the tube was removed from the zinc bath and then cooled by water spray. A coherent layer of zinc coating was achieved on the thus treated drawn aluminium bath.

The apparatus for carrying out the particular coating method according to the present invention as shown in Fig. 1, com¬ prises a forming apparatus, e.g. an extruder (1), for forming an aluminium member (2) in a desired form, especially that of a tube, from aluminium (3) supplied thereto. Surrounding the exit from the extrusion die (4) of the extruder and in sub¬ stantially gas-tight relation with the end of the extruder from which the die extends, is an oxidation preventing chamber (5) to which an oxidizing preventing gas is supplied through an inlet (6), e.g. an inert gas such as nitrogen. It is sufficiently short so that the extruded shape does not cool below the desired coating temperature before it passes out of the chamber. From the oxidation preventing chamber the extruded aluminium shape is passed into a means for coating the shape, e.g. a bath (7) of soldering or brazing metal without being exposed to the atmosphere, i.e. so that it is prevented from being oxidized. In a specific embodiment of the apparatus of the invention, this is done by attaching the bath directly to the outlet end of the oxidation preventing chamber (5). In this specific embodiment the thus coated aluminium shape can then be passed through a conventional wiper (8) to wipe excess coating metal therefrom, and then cooled, either by being cooled in the ambient atmosphere or passed through a cooling means such as a water quenching apparatus (not shown).

This means for applying the coating is here shown as a bath (7) through which the aluminium shape (2) is directly passed.

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However, a variety of known coating processes such as dipp¬ ing, spray-coating, and flash vaporization can be used within the scope of the present invention. Care must be taken that the extruded shape is not exposed to the ambient atmosphere or other oxidizing atmosphere between the time it leaves the extruder and the time it is coated.

The extruded shapes in status nascendi, i.e. the nascent state, after leaving the extrusion die are free of oxide on all surfaces and remain at a temperature of approximately 400-425°C so that an additional heating of the shapes is not required. The resulting coated aluminium shapes offer a corrosion resistant, leak-proof tubing particularly suited for heat exchangers.

A further advantageous characteristic which is given by the present method to the extruded shapes as compared with con¬ ventional coating methods is improved workability. In the prior art process when the extruded aluminium shape is pre¬ heated prior to application of brazing flux or immersed in a bath of molten salt and brazing flux in order to remove the oxide coating the aluminium is heated sufficiently, which causes grain growth in the aluminium, thereby reducing the workability. In the method of the present invention, since coating takes place immediately after extrusion without an intermediate heating step, there is insufficient time for any significant grain growth.

Example 2

A hollow aluminium tube having a flat, oval cross-section is extruded from a conventional extruder at a temperature of about 510 C, and is passed directly, i.e. without being exposed to the ambient atmosphere, into an oxide preventing chamber connected in substantially gas-tight relation to said extruder around the extrusion die thereof. The chamber is

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filled with nitrogen and is about 70-100 cm long. At the end of the chamber the tube has cooled to about 400°C, an excellent temperature for applying a soldering or brazing material thereto. The thus cooled tube is then passed directly, i.e. without being exposed to the ambient atmos¬ phere, into a bath of a Zn-Al fluxless soldering alloy com¬ posed of Al in an amount of about 3,5 to 4,5 wt %, Cu in an amount of about 2,5 to 3,5 wt %, Mg about 0,05 wt % and the balance Zn, which is at a temperature of from 380-382 C.

Thereafter the thus coated tube is passed through a conven¬ tional wiper in which the excess coating metal is wiped from the tube, and then the tube is cooled in the atmosphere.

The thus completed tube can then be easily fabricated into a structure by having pieces soldered thereto. For example, where the tube is to form the core tube of a radiator, the fins can be positioned on the tube and the tube simply passed through a heating chamber at about 400°C, whereby the fins will be soldered to the tube. No soldering flux need to be used and the soldering temperature is very low compared to the prior art which means low cost assembling of the radiators.

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