In particular, the present invention relates to a plant for brazing metals that makes use of a fluidized bed.

In general terms, brazing is a process wherein the metal that forms the parts to be welded melts at their interface, thus joining them.

In aluminium brazing, the pieces to be welded are coated with a layer made up of an aluminium/silicon (Al-Si) alloy, whose property is to have a melting point lower than that of"core"aluminium (i. e. the aluminium that forms the piece). Provided the pieces to be welded are heated to a temperature higher than the melting temperature of the coating but lower than that of the"core"alloy, the coating melts and creates a metallurgical bond between the parts, thus joining them.

In order for brazing to succeed, the pieces must have previously been coated with a suitable fluxant, usually in a aqueous solution. The fluxant has a three-fold function. First, it removes the oxide layer that normally covers the aluminium pieces and that, if not removed, would prevent them from brazing. Moreover, since the coating remains as a film on the pieces, even after brazing, it also acts as an antioxidant and,

finally, once molten, it wets the surfaces to be welded and allows the alloy to penetrate the pieces by capillarity.

Such brazing process is known as the NOCOLOK@process.

In order to obtain an acceptable brazing quality, there are some conditions to fulfil. For instance, both the dew point (a measurement of the percentage of water vapour in the furnace atmosphere) and the residual oxygen must be very low. Both species, in fact, disturb the brazing process.

Before the NOCOLOKs process was introduced, other aluminium brazing processes were in use; here below we briefly describe them.

In corrosive flow brazing, the pieces are dipped into a bath, mainly composed of chloride and some fluoride salts, that brings them to the brazing temperature while coating them with fluxant. Alternatively, the same process can be performed in a furnace, with the same procedure as in the NOCOLOKe process. In any case, the fluxant is both corrosive and hygroscopic. As far as corrosion is concerned, the fluxant layer has to be removed by water rinsing and subsequent chemical treatment. Due to hygroscopicity, then, furnace treatment has to be performed with both a very low dew point (<-40 C) and a large quantity of fluxant (150- 300 g/m2), so that the latter can be effective despite

water absorption.

Another known brazing process is vacuum brazing. By brazing in a vacuum, it is possible to avoid the use of fluxant with the related problems. In any case, vacuum brazing imposes very strict limits on piece cleaning, piece machining tolerances and the dew point in the furnace atmosphere, which must be lower than-60 C.

Although vacuum brazing has overcome the disadvantages of corrosive flow brazing, it is still a delicate process. The NOCOLOKs brazing process was developed with the aim of achieving the same advantages as fluxant-based brazing, while avoiding some of the disadvantages of the corrosive and hygroscopic fluxants. In fact, in the NOCOLOKs process a fluxant is employed that is both non hygroscopic, non corrosive and almost insoluble in water. The fluxant, which is manufactured according to a known process, is normally composed of various potassium fluoroaluminates. The percentage of each of the fluroaluminates varies according to fluxant temperature. However, by taking note of the range of variation of the stoichiometric coefficients, the composition can be written as K13AlF4-6.

The fusion temperature range for the NOCOLOKs fluxant is 565 C to 572 C. The fluxant appears as a fine white powder, whose grains have dimensions in the range 2 um

to 50 um. The powder is poured in water, often with the addition of surfactants, in order to make adhesion to the pieces easier. The needed quantity of fluxant is about 3-5 g/m2. The powder is not toxic and, before melting, chemically inert. It is virtually insoluble in water (0.2-0. 4% or less) and its shelf life is virtually infinite. At the end of the process, it remains on the pieces as a tightly bound film, with a thickness of 1-2 um. Such a film can be safely painted when needed and, as previously noted, has antioxidant properties.

In the NOCOLOK° brazing process, the following steps take place. a) The pieces to be treated are degreased, in order to remove any traces of the lubricant oils that were employed in the previous working processes. This can be achieved with vapour-based, thermal, water-based or chemical processes. Usually, due to environmental constraints, the preferred solution is washing with weakly alkaline solutions. b) The fluxant is then applied, usually in an aqueous emulsion, with concentrations in the range 5% to 25%.

The application method is dipping or spraying or, sometimes, by suitable electrostatic devices. The excess fluxant is then removed by an air curtain. c) One then enters the brazing furnace, which includes a

drying section (with temperatures in the range 180 C to 250 C), a heating section, where temperatures of about 600 C are reached, a brazing section and a cooling section. Cooling is usually first performed in a water jacket chamber (up to 200 C) and then in air (up to 40 C), so as to make piece manipulation easier.

Brazing furnaces can either be of the batch type or continuous, although the latter type is more common.

Heating times, of course, vary according to the different pieces, but times in the range 15-60 min (including the brazing phase) are common. The brazing phase takes about 3 minutes. Typically, heating gradients have a value of more than 20 C per minute.

The brazing furnace has an atmosphere composed of an inert gas (typically nitrogen), with a dew point smaller than-40 C and an oxygen concentration smaller than 500 ppm (parts per million). Nitrogen is typically injected at the centre of the brazing chamber and goes out of it at its sides, so that contamination from the outer atmosphere is avoided. Temperature uniformity in the brazing chamber must be better than 2-3 C. Finally, since in the brazing process small quantities of hydrogen fluoride (HF) are produced, due to KalF4 reacting with water, both the furnace and the initial drying section send their fumes to a filtering hood

(the"scrubber"), typically equipped with an active alumina filter with an efficiency of about 90%. HF concentration is then brought to levels compatible with the environmental regulations (2 mg/m3 in Europe).

The approaches traditionally followed for fluxing aluminium parts to be brazed with the NOCOLOK° process are the following.

In the dip fluxing approach, the pieces are dipped into the water/NOCOLOK@ powder emulsion and, in such a way, coated with fluxant. An air curtain, then, gets rid of the excess fluxant, which mainly accumulates downwards due to gravity. Applicant has observed that the disadvantage of such a method is that fluxant is wasted, since most it drips from the piece and is then lost, unless a suitable recycling system is provided.

In the spray fluxing approach, the pieces to treat are covered with the water/NOCOLOKX powder emulsion by spraying. Applicant has observed that such an approach does not lead to the large waste one has in dip fluxing since, by this approach, it is possible to apply just the right quantity of fluxant. However, in order to optimize process efficiency different spray geometries are needed for different piece typologies. For instance, in a typical system the spray nozzles are able to vary their distance according to piece dimensions. However, this feature makes plant layout

more complicated. Finally, it is difficult for the fluxant to reach the holes and/or recesses that are not lined up with one of the nozzle jets.

Moreover, electrostatic fluxing systems, wherein the.

NOCOLOKe powder is sprayed inside a suitable electric field that causes its granules to be polarized, are also known. The granules, following their trajectory, impinge on the piece. If the piece is grounded, the granules stick to it by electrostatic attraction.

However, such systems are quite complex from the technological point of view and have the same disadvantages as the spray fluxing systems.

According to present invention, Applicant has realized a metal piece brazing system, wherein the fluxant is applied on the pieces to be brazed by a fluidized bed.

The pieces, which have been previously wetted, are dipped into the fluidized bed, wherein such powder is fluidized by air injected into it (possibly mixed with water vapour). Since the piece is wet, the powder sticks to it. Like in any fluxing system, the fluidized bed is followed by a thermal dryer that causes water to evaporate, forming the above-mentioned fluxant film that melts during brazing and, at the same time, removes the aluminium oxide layer-from the piece, makes welding easier by capillarity and, outside the joints, sticks to the piece and acts as an anti-oxidant.

An embodiment of present invention relates to a plant for brazing metal pieces, including a device for washing the metal pieces to be brazed, a device for applying fluxant on said pieces and a brazing furnace, characterized in that said device for fluxant application is a fluidized bed, including at least a chamber wherein a powder, made up of solid particles, is placed, at least a flow distributor and an inlet duct, aimed at injecting a fluidizing gas into said chamber and fluidizing said powder, at least a lateral opening for inserting such metal pieces.

A further embodiment of present invention relates to a device for applying fluxant on metal pieces to be brazed, characterized in that it includes at least a fluidized bed, including at least a chamber wherein a powder, made up of solid particles, is placed, at least a flow distributor and an inlet duct, aimed at injecting a fluidizing gas into said chamber and fluidizing said powder, at least a lateral opening for inserting such metal pieces.

Further objects and advantages of the present invention shall become clear from the description below and appended drawings, provided purely as a non-limiting explanatory example, in which: figure 1 is a schematic lateral view of a fluidized bed in a plant for metal piece brazing, according to

present invention; figure 2 is a schematic front view of a fluidized bed in a plant for metal piece brazing, according to present invention.

With reference to the above-mentioned figure, a plant for metal piece brazing is shown. Such plant implements the above-mentioned phases a), b) and c), according to the NOCOLOKs brazing process and essentially includes a device for washing said pieces in water, a device for applying fluxant on said pieces and a brazing furnace.

The device for piece washing and the brazing furnace may be of a traditional type and, accordingly, will not be described any further.

The device for fluxant application according to present invention includes a fluidized bed and a thermal dryer.

The pieces, after being wetted by said device, are dipped into the fluidized bed, where a NOCOLOKf powder is fluidized in the bubble regime by air injected into the bed (perhaps including water vapour). Since the piece has been wetted, the powder sticks to it. A thermal dryer that causes water to evaporate and forms the above-mentioned fluxant film that, in turn, melts during the brazing process in the above-mentioned furnace, follows the fluidized bed.

The metal pieces to be brazed are driven into the fluidized bed by a suitable moving device, such that

the pieces go in and out of the bed through two opposite lateral openings or alternatively, for a batch process, through a single opening.

Said fluidized bed includes a container 2 of fixed length, partitioned into a plurality of sections, each one including a chamber 21,22, 23,24 and 25, wherein a powder, made up of solid particles, is placed, a flow distributor 31,32, 33,34 and 35 and an inlet duct 41, 42,43, 44 and 45 for the fluidizing gas. Such fluidizing gas may be, for instance, compressed air, injected into said duct by a compressor.

In the example shown in figure 1, five sections are shown; equivalently, the fluidized bed may include both one and one only section or a larger number of sections, according to the desired conditions and the metal pieces to be treated.

Said plant also includes at least two opposite lateral openings 51 and 52, including, for example, a device that generates a laminar flow that prevents the powder to escape without hindering piece entrance. The aim of said openings is to allow insertion of the pieces to be brazed into said chambers, through which said moving device 5 operates. The latter may be realized, for example, by a conveyor belt, suitably driven by an electric motor.

A fluidized bed operates in the following way.

The gaseous flow, for example compressed air injected into such ducts, after being driven into suitable flow- dynamic conditions by said flow distributor, sets the solid particles in the powder into random motion and the whole assembly behaves as a liquid, in the laminar or turbulent regime. The intimate contact between particles and fluid can be exploited for various applications.

If, for example, the abrasive action of the powders is exploited, the fluidized bed can be employed for such operations as painting, de-painting or, in the case of present invention, mechanical piece washing and, more generally, degreasing.

The flow-dynamic regime inside the fluidized bed includes different phases, according to the fluidizing gas flow rate, all other parameters, such as fluidized bed dimensions and particle physical and geometrical properties, being equal.

Such phases are known as :"fixed bed","bubbling regime","slugging regime","turbulent regime","fast fluidization","pneumatic conveying".

The transition from a regime to the next one takes place when gas velocity, and consequently vacuum degree, increases. Thus, from a regime wherein the powders act as a"plug"for the fluid, one meets a regime wherein the particles are just expelled from the

fluidized bed by the flow. Both extreme regimes, of course, are of no practical interest, while there is interest for the intermediate regimes.

The minimum fluidization regime is characterized by a threshold velocity called, accordingly,"minimum fluidization velocity". Such a velocity is related to the gas throughput that is able to balance particle weight and, accordingly, allows the particles to float.

The regime that is achieved after the minimum fluidization regime, as throughput grows, is the bubble regime. According to the different kinds of particles employed, one has a transition field of different width between the two regimes. In the case of the bubble regime, the fluidized area is characterized by a two- phase structure, composed of the emulsion and the bubbles. The emulsion is the dense phase and is made up of particles in the minimum fluidization regime. The second phase is made up of bubbles. The bubbles are formed when the gas throughput exceeds the minimum fluidization condition. The excess gas passes through the emulsion in the shape of bubbles and is then freed at the upper surface. The fluid/powder emulsion, instead, stays at a constant height, depending on the operating conditions. The flow distributor is aimed at generating said regimes.

Thanks to the modular structure of the previously

described fluidized bed, it is possible to adopt standard assembly procedures for the single modules and just supply the number of modules needed for the desired production rate in a given plant, thus realizing a plant that can be easily upgraded and such that, some modules only can be switched on when needed.

The flow distributor in a fluidized bed is made up of a metallic drilled plate (or, more simply, with a finely meshed net). Such a distributor on one hand creates a physical barrier against powders falling down when the bed is not working and, on the other hand, causes pressure to drop, thus lowering the Reynolds number and turning the inlet flow into the laminar regime, thus smoothing the turbulence caused by the gas supply compressor. Experimentally, it has been found that a drilled plate distributor is not enough to achieve a non-turbulent flow. Better results can be obtained with a flow distributor made up of a porous polymeric material plate. Even better results have been obtained by Applicant by using two porous polymeric material plates, with aluminium wire sandwiched in between.

Applicant points out that a certain non-ideality in the fluidized bed behaviour remains, even in the bubble regime. In particular, even by making the fluidized bed as high as the theory prescribes, some powders still leak out of it. A remedy to this has been found by

"plugging"the bed with a metallic net of the same kind as those used as flow distributors in fluidized beds.

Normally, a fluidized bed is equipped with a conventional loading system (either of the batch or continuous type, according to the needed production rate).

According to present invention, the pieces are wetted in water before introduction into the fluidized bed, so as to allow the NOCOLOK° powder to stick to them. Such a procedure can be implemented by dipping the pieces to be treated in water. In such a case, there would certainly be a certain waste of fluid due to piece dripping but the fluid would just be water, as opposed to conventional systems, where powder would be wasted together with water. Preliminary tests, performed by Applicant with a prototype fluidized bed, have shown that, by dipping a piece in water and then plunging it into a fluidized bed loaded with glass spheres having roughly the same diameter as the NOCOLOKe powder (in order to simulate its behaviour), the piece is compactly and uniformly coated, at least up to a few centimeters from its upper edge.

A possible alternative is to use a spray pre-fluxing system. With such a system, as in conventional spray fluxing systems, uniform piece coating with optimized water and fluxant consumption should be achieved. A

further alternative might be a fluidized bed with an embedded shower. In other words, inside the fluidized bed there would be a shower, through which the pieces would be wetted by sprayed water.

Another possible approach might be that of employing a fluidized bed wherein the fluidizing gas consists of air, mixed with a suitable amount of water vapour. In this case, water vapour would condense on the pieces, thus wetting them in a uniform way while they are hit by the fluxant particles.

As previously mentioned, the optimal NOCOLOK° fluxant load is around 3-5 g/m2. By suitably tuning the transit time in the fluidized bed and perhaps also powder size, Applicant estimates that such an optimal level can be achieved even without placing an air curtain at the bed exit. However, it is not possible to rule out the eventuality that, as in conventional fluxing systems, the pieces coated with the NOCOLOKe emulsion have to be exposed to a hot air jet (an air curtain) aimed at both removing the excess flow and making the coating more uniform. Such a curtain might be equipped with a fluxant recycling system, should this be economically advantageous.

The drying system that follows fluxant application in the fluidized bed brings the pieces to a temperature that is normally comprised between 180 C and 250 C;

this typically happens by exposing the travelling pieces to an air jet at the desired temperature for a suitable time. While the flux distribution system described in the previous paragraphs would presumably be designed independently on its associated furnace, the same does not hold for the drying system, which is typically part of the brazing furnace. However one cannot, a priori, rule out the eventuality that a drying section connected to the fluxing system, as opposed to the brazing furnace, be built.

Plant performance, according to present invention, is essentially equivalent to that of a dip fluxing system, due to the uniform coating achievable and the possibility to reach holes and/or recesses that would otherwise be hardly reachable in the object to be treated.

However, one does not have the disadvantages related to dip fluxing systems, namely the large amount of wasted fluxant due to piece dripping and the need to provide a fluxant recycling system, with the related technological and economical complications, so as to alleviate, at least partially, the problem.

An advantage of the system is modularity. On the side of the plant manufacturer, in fact, it is possible to employ fluidized bed modules which are standardized as far as designing and building are concerned and join

them for any reasonable length, according to the user requests. On the side of the plant users, it is possible to order a system made up of just the number of modules needed for their production needs, thus optimizing costs while leaving room to the option of increasing productivity by purchasing more modules.

Alternatively, it is possible to keep some modules switched off, should the production needs be below the peak rate, thus saving on energy.

By suitably tuning the treating times and the size of the NOCOLOKX powder granules, Applicant envisages the possibility of optimizing the amount of water needed for fluxing, thus making it smaller than the amount needed in conventional systems. The energy needed for drying the pieces before brazing would then be smaller than in conventional systems; accordingly, the related drying times would be smaller, as well, thus saving on energy.

The selling price of the system is sensibly smaller than that of conventional fluxing systems.