MXPA06002762A - Soldering work piece, soldering method and heat exchanger - Google Patents

Abstract

The invention relates to a soldering work piece made of aluminium and/or aluminium compounds, to a soldering method, in addition to a heat exchanger soldered in said manner. One surface of the soldering work piece is provided with an artificially applied oxide layer.

Description

WELDING WORKPIECE, WELDING METHOD AND HEAT EXCHANGER Field of the Invention The present invention relates to a welding workpiece made of aluminum and / or aluminum compounds, to a welding process and to a heat exchanger welded in this manner. BACKGROUND OF THE INVENTION In order to join two metal workpieces by means of a welded joint, it is necessary that the oxide layer formed on the surfaces of the work pieces be at least partially removed before the welding operation, and not be formed again during the welding process. This is generally achieved by using special welding agents for metal oxides known as fluxes. At present, for example for tanning aluminum components of heat exchangers, as it is used in the automotive industry, it is a custom to use special welding processes, in particular what is known as the "Nocolok" welding process using fluxes to potassium fluoroaluminate bases. Even the application of the fluxes is often very complex and expensive. In addition, the components are covered with fluxes after the welding operation, often requiring complex cleaning treatment and / or other surface treatments after the welding process in order to meet industry requirements for the desired properties of the components, such as resistance to corrosion, hydrophilic and / or bonding surfaces and / or to comply with aspects such as cleaning, design or appearance of the product. For example, the flux that is commercially available under the trade name "Nocolok" remains on the surface after welding of the aluminum components, covering it with a crystalline layer which has to be subjected to additional cleaning and conversion treatments, depending on the intended use. In addition, the use of fluxes has an adverse effect on the environment and on the equipment and machines used which are exposed to high levels of wear, which result in a short service life. In addition, it is difficult to use materials that contain zinc and / or magnesium, since zinc or magnesium react with the flux, thus increasing the consumption of flux and affecting the properties of the materials. To avoid these drawbacks, it is desirable to provide a process for welding aluminum, which the surface of a welding workpiece made of aluminum or aluminum compounds, no longer needs to be supplied with a flux. The elimination or free cracking of flux of the oxide layer formed on the metal surface, can be achieved through a pre-treatment of the associated work piece by applying a welding coating, for example the process known as nickel-aluminum welding , or by means of evaporation elements, for example magnesium, outside the base material or the solder coating during a vacuum welding. In the case of free tanning of flux and welding at high temperatures in specially developed vacuum furnaces, which are heated electrically, the vacuum acts as a reducing atmosphere. This makes it possible to achieve metallic-glossy surfaces for the solder to wet the components to be bonded and to prevent the growth of metal oxide layers. However, the drawbacks of vacuum welding are technically complex and therefore require expensive welding facilities and costly pretreatment to clean the workpieces that will be welded. SUMMARY OF THE INVENTION Accordingly, as an alternative to flux welding, the present invention is based on the object of providing a welding work piece and a welding process for flux-free solder, which can be carried out in the form of solder. economic and without a large-scale industrial investment. This object is achieved through the welding workpiece having the characteristics according to claim 1, through the process having the characteristics according to claim 10 and claim 11, and through the heat exchanger having the characteristics according to claim 15. To form a stable welded joint, there is the provision of the weld that will be allowed to flow between the work pieces which will be joined together from the work piece or from the external part. Accordingly, it must be possible to crack or at least partially remove the oxide and / or hydroxide layer covering the welding work piece, so that the welder can penetrate into the non-homogeneities, such as notches, pores, cracks, or similar, which have formed in the oxide and / or hydroxide layer and which can ensure a reliable welded joint between the work pieces that will be joined. The base material of the workpiece that joins the oxide and / or hydroxide layer in this case, has a thermal expansion different from that of the oxide and / or hydroxide layer itself, obtaining as a result, taking into account the heating during the welding operation, that tensions accumulate in the oxide and / or hydroxide layer and in the boundary region between the base material and the oxide and / or hydroxide layer. The native oxide and / or hydroxide layer, which is formed on all surfaces of workpieces made of aluminum or aluminum compounds that are exposed to ambient air, typically has a thickness of less than 10 nm and may also have a thickness up to 20 nm in the case of humid air. Taking into account this small thickness, the oxide and / or hydroxide layer has a flexibility, so that the stresses within the oxide and / or hydroxide layer can be dissipated. A core concept of the present invention is one that deliberately selects the thickness, such as the average thickness of the oxide and / or hydroxide layer, such that the thermal stresses can no longer be dissipated due to the flexibility within. of the oxide and / or hydroxide layer. During heating at welding temperatures, the stresses become greater than the non-homogeneities, in particular pores and / or cracks that form in the oxide and hydroxide layer, particularly in the vertical direction with respect to the surface of the piece of welding work, and so that the oxide and / or hydroxide layer under certain circumstances is at least partially separated. The liquid solder material can subsequently penetrate into the cracks or between the workpiece and the oxide and / or hydroxide layer. The object of the present invention is therefore achieved by virtue of the fact that the thickness of the oxide and / or hydroxide layer adjusted on a surface of the welding material is greater than the thickness of an oxide layer and / or native hydroxide. A thickness greater than 25 nm is desirable and a thickness greater than 50 nm is particularly preferred. If the thickness of the oxide and / or hydroxide layer is particularly greater, the desired crack formation becomes more difficult taking into account the internal stability of the oxide and / or hydroxide layer, and accordingly, a thickness of the layer less than 1,000 nm, preferably less than 500 nm. With the standard welding conditions for aluminum materials, for example, temperatures between approximately 500 ° C and 660 ° C, a thickness of the oxide and / or hydroxide layer between 80 nm and 250 nm, has proved to be particularly convenient for the free welding of flux of the workpieces, in particular the parts of a heat exchanger made of aluminum or aluminum compounds. As is known, chemical, or electrochemical or physical processes are usually used to produce an oxide layer before welding. A suitable chemical production process is, for example, the boehmite process, obtaining as a result that the oxide and / or hydroxide layer comprises predominantly in a convenient boehmite, with an exposure time of from 1 minute to 1 hour, in particular from 1 minute to 15 minutes, and a temperature of from 80 ° C to 150 ° C, the temperature being selected in particular from 100 ° C to 120 ° C . In this case, it is customary to use water, which may also contain chemicals. A temperature between 15 ° C and 80 ° C is also suitable for a workpiece temperature of up to 555 ° C. Alternatively, the surface of the workpiece can also be treated electrochemically in order to form an oxide layer. By way of example, for this purpose the material known as Eloxal is used, which remains for the electrolytic oxidation of aluminum, with an exposure time of from 1 minute to 1 hour, in particular from 1 minute to 10 minutes, a temperature of from 20 ° C to 50 ° C and a voltage of 40 V.

As an example of physical production of a defined oxide layer, it is possible to use the PVD process (physical vapor deposition) where a slope voltage of -40 V and a coating pressure of 0.1 mbar to 1 mbar are preferably set . Another option is to form mixed oxide layers consisting of aluminum oxides and / or other oxides, and they can be produced through a chemical reaction. Taking into account the formation of an oxide and / or hydroxide layer that allows the formation of non-homogeneities of the welding material to be introduced, such pre-treatment is especially suitable for the work pieces that will be joined, especially for a production of large series standardized. A concept of the original core of the present invention is to help break and / or flake the oxide and / or hydroxide layer by influencing its morphology in a directed manner, and if appropriate, penetrating the aluminum surface of the piece. of work to allow it to be particularly well dampened by the welding material. According to a convenient embodiment, this is achieved with the aid of non-homogeneities, such as for example notches, pores and / or cracks or the like, in the oxide and / or hydroxide layer. These non-homogeneities are preferably introduced into the oxide and / or hydroxide layer through chemical and / or thermal and / or mechanical treatment of the work piece. For example, preferably during or after the oxidation operation, a lubricant, which conveniently contains halogen, is applied to the welding workpiece, that is, to the oxide and / or hydroxide layer. During the subsequent heating step as part of the welding operation, the modification of the oxide layer implemented in this way promotes the formation of non-homogeneities and the aluminum surface that are not covered, are activated through the halogen compounds , so that it can be moistened with the welding material. The elimination of a process step for applying flux allows a thermal grease extraction operation and a welding operation to be carried out at the same time in a welding furnace, for example in a continuous furnace. It is particularly convenient that the two steps of the process are carried out during a single heating operation. A heat treatment is preferably integrated in a grease extraction process, which may be required in any case and is carried out for the removal of the lubricants. It is particularly preferable that the heat treatment, and if appropriate, the removal of thermal grease, is integrated into the welding operation, so that only a single heating operation is required. This allows the investment of the process to be further reduced. To simplify the sequence of the process, it is preferable that the application of the halogen-containing lubricants in particular, preferably take place during the preceding machining processes that are carried out on the material, such as deep extraction, cutting, perforation and the like, which for engineering reasons of the process are required for lubrication. In addition, reoxidation in places where the oxide layer breaks, needs to be suppressed without significant additional investment. For this purpose, halogen-containing lubricants contain additives that break down when they are laced and constituents that have an affinity for oxygen, so that they bond to oxygen in the immediate surroundings of the work pieces that will be joined, thus improving the atmosphere of welding and the flow of welding material. Within this context, carboxylic acids, amines, sulfur compounds and / or phosphorus compounds can be conveniently used in the form of additives or oxygen-binding constituents. The welding operation itself, it can also be influenced in a positive way by the additives and / or constituents. Under a gas protection atmosphere, magnesium is retained in relatively large amounts, ie, it does not evaporate to the same degree as under vacuum, and can therefore facilitate the distribution of the liquid solder and contribute to improving the strength properties of the solder. the work piece after the welding process. On the other hand, the partial evaporation of the magnesium can help the breakdown of the oxide layer described above, and the reoxidation of the aluminum surface can be suppressed when the magnesium diffuses towards the surface as a result of its reaction with the residual oxygen, which is found to form magnesium oxide (MgO). Therefore, a base material made of aluminum with a high content of aluminum for welding is necessary. In a particularly convenient configuration of the process, a base material made of aluminum with a magnesium content greater than 0.2%, in particular greater than 0.5% or particularly less than 2%, is used. A gas protection atmosphere that has a reduction action over the entire temperature range makes an important contribution to ensuring that the aluminum surface does not oxidise again and that the oxide layer still does not grow additionally. Therefore, during heating and welding, gas protection is used, conveniently hydrogen, argon or nitrogen; Nitrogen is particularly not expensive. In addition, the use of a continuous gas protection furnace allows a high degree of automation for this manufacturing process. The process described is used for the welding of flux-free aluminum components, such as for example tubes, plates, fins or semi-finished products, such as strips of a heat exchanger, in particular in the automotive industry. The welding workpieces according to the present invention preferably have a layer of welding material consisting of an aluminum compound. The advantages that can be achieved by the present invention are, in particular, that before the gas protection welding, the supply with the use of fluxes in the welding is possible, taking into account that a surface of a piece of Welding work made of aluminum and / or aluminum compounds is supplied with an oxide and / or hydroxide layer and taking into account that some halogen-containing lubricants in particular, are applied during or after oxidation. This absence of flux leads to reduced environmental pollution and reduced equipment wear, and consequently to a longer service life of the equipment. In addition, it is possible, or at least easier to use materials containing zinc and / or magnesium for gas protection welding. Within this context, the application of the halogen-containing lubricants makes it possible to assist the breaking and / or flaking of the oxide and / or hydroxide layer during the welding operation and also cause the aluminum surface to be activated to assist to an improved wetting of the welding material; reoxidation of the uncoated aluminum surface becomes more difficult, in particular by the additives or constituents of the lubricants having oxygen affinity. The gas protection atmosphere, which has a reduction action over the whole temperature range, also contributes to this last characteristic. Consequently, the non-homogeneities formed during the heating can be filled with welding material in such a way as to allow the formation of welded joints in fixed or permanent form. The result is a particularly economical and relatively simple process for the free welding of aluminum flux of the aluminum compounds, which ensures the formation of particularly stable welded joints, for example aluminum components for heat exchangers, with reproducibly quality constant, as is required in particular for large-scale industrial use, especially for mass production in an automotive industry. BRIEF DESCRIPTION OF THE DRAWINGS In the following, an exemplary embodiment of the present invention will be explained in more detail, with reference to the drawings, in which: Figure 1 shows a welding work piece according to the present invention. Figure 2 shows a welding work piece according to the present invention; and Figure 3 shows a welding work piece during a welding process according to the present invention. Detailed Description of the Invention Figure 1, which is not to scale, shows a welding workpiece 10 having a base body 20 made of an aluminum alloy, which has been supplied with a coating of welding material 30 of an aluminum alloy, which has a thickness of approximately 0.1 mm. A layer of boehmite 40 having a thickness of approximately 100 nm has been applied to the surface of the coating of the welding material 30. A lubricant 50 serves to facilitate the cutting or drilling processes before the welding process. The lubricant 50 can be applied during or after oxidation to produce the boehmite layer. Figure 2 shows a welding workpiece 110 having a base body 120 and a welding liner 130 which has been covered with a non-homogeneous oxide and / or hydroxide layer 140. The oxide and / or hydroxide layer 140 has notches, pores and / or cracks 160, wherein the coating of welding material 130 is covered only with a layer of native oxide and / or hydroxide 170 with a thickness of about 1 nm to 5 nm. These non-homogeneities 160 may occur, for example, during a thermal grease extraction operation, in particular when a halogen-containing lubricant is removed. At high temperatures, halogens effect and / or promote the formation of non-homogeneities of this nature. The locations of the welding of two work pieces that have been pretreated in this way, and which will be joined, are suitably placed in such a way that it is already known and therefore not illustrated. Subsequently, the entire arrangement is introduced into a gas protection furnace, in particular, for reasons of automation, into a continuous gas protection furnace, in which the previously treated workpieces are heated. This leads on the one hand (Figure 1) to stresses in the boehmite layer 40 and in the region of the boundary between the coating of the welding material 30 and the boehmite layer 40. In the heating at the welding temperature, if it is With the aid of the halogen-containing lubricant 50, these stresses lead to the formation of notches, pores and / or cracks in the boehmite layer 40, preferably in the vertical direction with respect to the surface of the welding coating 30, since the layer of boehmite 40 is partially separated from the coating of the welding material 30. On the other hand (Figure 2) similarly, the oxide and / or hydroxide layer 140 is partially separated from the welding liner 130, due to the notches, pores and / or cracks 160. As illustrated in Figure 3, the welding material 230 of the welding liner, during the welding operation, flows and / or rinses around the separated fragments. 80 of the boehmite or oxide and / or hydroxide layer. As a result, the liquefied solder material 230 can wet the base body 220, consisting of aluminum, of the workpiece 210, resulting in the formation of desired solder joints.

Claims (15)

1. CLAIMS 1. A welding workpiece made of aluminum and / or aluminum compounds, having an oxide and / or hydroxide layer fitted on a surface of the welding workpiece, characterized in that the thickness d of the layer Oxide and / or hydroxide is greater than the thickness of the native oxide and / or hydroxide layer.
2. 2. The welding workpiece according to claim 1, characterized in that it has 25 nm < d < 1000 nm, in particular 500 nm < d < 500 nm, in particular 80 nm < d < 250 nm.
3. 3. The welding workpiece according to any of the preceding claims, characterized in that the oxide and / or hydroxide layer consists predominantly of boehmite.
4. The welding workpiece according to any of the preceding claims, characterized in that the oxide and / or hydroxide layer includes non-homogeneities, in particular notches, pores, and / or cracks.
5. 5. The welding workpiece according to any of the preceding claims, characterized in that the homogeneities are introduced into the oxide and / or hydroxide layer through chemical and / or thermal and / or mechanical treatment of the workpiece of welding.
6. 6. The welding workpiece according to any of the preceding claims, characterized in that the welding workpiece is supplied with a halogen-containing lubricant in particular.
7. The welding workpiece according to any of the preceding claims, characterized in that the lubricant includes additives or constituents such as carboxylic acids, amines, sulfur compounds and / or phosphorous compounds.
8. The welding workpiece according to any of the preceding claims, characterized in that the welding workpiece has a welding layer having a layer of welding material comprising an aluminum compound.
9. The welding workpiece according to any of the preceding claims, characterized in that a base material of the welding workpiece has a magnesium content greater than 0.2% or, in particular greater than 0.5%, preferably less than 2%.
10. 10. A welding process for joining at least two workpieces together, characterized in that at least one workpiece is used as described in any of the preceding claims.
11. 11. A welding process, in particular the welding process according to claim 10, wherein pre-machining processes are carried out on at least one work piece, in particular deep extraction, cutting and / or drilling, characterized because a particular halogen-containing lubricant is applied to the workpiece during the previous machining processes.
12. 12. The welding process according to any of the preceding claims, characterized in that the lubricant includes additives or constituents such as carboxylic acids, amines, sulfur compounds and / or phosphorus compounds.
13. The welding process according to any of the preceding claims, characterized in that the thermal grease extraction and the welding operation are carried out together, in particular during a single heating operation.
14. The welding process according to any of the preceding claims, characterized in that the protection of gas, in particular hydrogen, argon or nitrogen, is used for heating and / or welding.
15. 15. A heat exchanger, in particular for a motor vehicle, characterized in that the heat exchanger is at least partially welded using the process according to any of the preceding claims. The present invention relates to a welding workpiece made of aluminum and / or aluminum compounds, to a welding method, in addition to a heat exchanger welded in this manner. A surface of the welding workpiece is supplied with an oxide layer applied artificially.