Flux-free joining of aluminium composite materials

Abstract

Use of an aluminium composite material in a thermal joining method, said material consisting of at least one aluminium core alloy and at least one external brazing layer consisting of an aluminium brazing alloy provided on one or both sides of the aluminium core alloy, wherein the aluminium brazing layer has a pickled surface. Reduced costs and a lower environmental impact is achieved by using an aluminium composite material in which the pickled surface of the aluminium brazing layer had been pickled by pickling with an acid, aqueous pickling solution containing at least one mineral acid and at least one complex-forming agent or a complexing mineral acid, wherein the removal of material in the pickling is between 0.05 g/m.sup.2 and 6 g/m.sup.2, the aluminium composite material is used in a flux-free, thermal joining method, and the joining method is carried out in the presence of a protective gas.

Claims

1. A method, comprising the step of: utilizing an aluminium composite material in a thermal joining method, wherein the aluminium composite material is manufactured from a strip-shaped aluminium composite material consisting of at least one aluminium core alloy and at least one external brazing layer consisting of an aluminium brazing alloy provided on one or both sides of the aluminium core alloy, which is produced by roll bonding or simultaneous casting followed by rolling, wherein the aluminium brazing layer has an acid pickled surface, wherein the pickled surface of the aluminium brazing layer has been pickled with an acid aqueous pickling solution containing: H.sub.2SO.sub.4: 0.1% to 20 wt. %, HF: 20 ppm to 1000 ppm, and at least one surfactant, wherein the removal of material in the pickling is between 0.05 g/m.sup.2 and 6 g/m.sup.2, wherein the aluminium composite material is used in a flux-free, thermal joining method and the thermal joining method is carried out in the presence of a protective gas.

2. The method according to claim 1, wherein the aluminium composite material is used in a flux-free CAB brazing method.

3. The method according to claim 1, wherein the surface was pickled with a mineral acid and fluorides as complex-forming agent.

4. The method according to claim 1, wherein the aluminium core alloy is an aluminium alloy of the type AA1xxx, AA2xxx, AA3xxx, AA5xxx or AA6xxx, wherein the Mg content in the specified aluminium core alloys is in each case at most 1.0 wt. %.

5. The method according to claim 1, wherein the aluminium brazing alloy has the following composition in wt. %: 6.5%Si15%, Fe1%, Cu0.3%, Mg2.0%, Mn0.15%, Zn0.15%, Ti0.30%, the remainder Al and unavoidable impurities individually in an amount of at most 0.05%, totaling at most 0.15%.

6. The method according to claim 1, wherein the aluminium composite material was soft annealed or re-annealed or solution annealed before the pickling.

7. The method according to claim 1, wherein at least one aluminium brazing alloy layer has a mean thickness of at least 10 m.

8. A method for producing a strip-shaped aluminium composite material consisting of at least one aluminium core alloy and at least one external brazing layer consisting of an aluminium brazing alloy provided on one or both sides of the aluminium core alloy, in which a strip-shaped aluminium composite material is produced by roll bonding or simultaneous casting followed by rolling, and the aluminium brazing layer of the strip-shaped aluminium composite material is then pickled with an acid pickling solution, wherein the aluminium composite material is pickled with an aqueous pickling solution that contains 0.1% to 20 wt. % of H.sub.2SO.sub.4, 20 ppm to 1000 ppm of HF, and at least one surfactant, wherein the removal of material in the pickling is between 0.05 g/m.sup.2 and 6 g/m.sup.2.

9. The method according to claim 8, wherein the residence time of the strip-shaped aluminium composite material in the pickling solution is 1 to 20 sec.

10. The method according to claim 8, wherein the temperature of the pickling solution is 40 C. to 80 C.

11. The method according to claim 8, wherein the residence time of the strip-shaped aluminium composite material aluminium composite material in the pickling solution is 2 to 8 sec.

12. The method of claim 8, wherein the aluminium core alloy is an aluminium alloy of the type AA1xxx, AA2xxx, AA3xxx, AA5xxx or AA6xxx, wherein the Mg content in the specified aluminium core alloys is in each case at most 1.0 wt. %.

13. The method of claim 8, wherein the aluminium brazing alloy has the following composition in wt. %: 6.5%Si15%, Fe1%, Cu0.3%, Mg2.0%, Mn0.15%, Zn0.15%, Ti0.30%, the remainder Al and unavoidable impurities individually in an amount of at most 0.05%, totalling at most 0.15%.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The invention will now be described in more detail hereinafter with the aid of exemplary embodiments in conjunction with the drawings, in which:

(2) FIG. 1 is a perspective view of the brazing experimental geometry for determining the brazing abilities of the aluminium composite materials,

(3) FIG. 2 is a side view of the brazing experimental geometry,

(4) FIG. 3 is a photograph of a brazed exemplary embodiment with the use according to the invention of the aluminium composite material,

(5) FIGS. 4A and 4B are two light microscopy transverse sections of an exemplary embodiment of a brazed construction using Mg-containing aluminium core alloys,

(6) FIG. 5 is a schematic sectional view of an exemplary embodiment of a method for the production of a strip-shaped aluminium composite material, and

(7) FIG. 6 is a sectional view of an exemplary embodiment of a brazed construction according to the invention in the form of a heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

(8) In order to investigate the advantages of the use according to the invention of the aluminium composite material, a large number of experiments were carried out with a specific brazing experimental arrangement, as illustrated in perspective view in FIG. 1. In principle the brazing experimental arrangement consists of a total of three parts, a sheet 1, an angle sheet 2 and a supporting sheet 3 for the angle sheet 2. The angle sheet 2 lies with its closed end 2a on the supporting sheet 3 arranged on the sheet 1. Both arm ends 2b on the other hand lie on the sheet 1, so that, as illustrated in the side view in FIG. 2, a varying gap is formed starting from the supporting point of the arm ends 2b of the angle sheet 3 up to the supporting point of the closed end 2a on the supporting sheet 3. The brazing joint clearance or gap 4 becomes increasingly larger starting from the angle ends 2b up to the closed end 2a of the angle sheet. Owing to the increasing brazing joint clearance 4 it is possible to evaluate to what extent the brazing properties of the aluminium composite material of the sheet 1 change under different surface treatments. In particular, in the brazing results the wetting of the prepared brazing gap was given an evaluation ranging between (1) for very good and (6) for unsatisfactory, in which connection the ability to fill the gap together with the manifestation of the brazing neck were particularly pertinent. The experiments that showed an almost complete wetting of the brazing joint clearance and a broad brazing neck were evaluated as very good (1). The experiments that led to no brazing of the structural parts were evaluated as unsatisfactory (6).

(9) The sheet 1 consists in the present exemplary embodiment of the respective tested aluminium composite material, which comprises a roll-bonded aluminium brazing alloy layer. The length of the arm of the angle piece 2 was in each case 50 mm, the opening angle of the angle sheet being 35. The supporting sheet 3 has a thickness of 1 mm, so that the height difference from the closed end of the angle sheet to the arm end is 1 mm. The thickness of the angle sheet 2 was kept constant and was 0.6 mm in each case. The angle sheet 2 as well as the supporting sheet 3 are not provided with an aluminium brazing layer.

(10) In general, the brazing ability is, apart from the use of brazable materials, also always a function of the design of the structural part, for example the geometry, gap size, etc., and also of the furnace atmosphere. Here the oxygen partial pressure and the moisture content of the atmosphere play a role. The illustrated brazing results were carried out in a batch furnace under a nitrogen flow. These brazing results were also obtained in industrial production runs using a tunnel furnace.

(11) In the brazing experiments marked by L1, two different, roll-bonded sheets were investigated. The first sheet V has an aluminium core alloy layer of the type AA3005 as well as an aluminium brazing alloy layer of the type AA4045 plated on one side and an external aluminium alloy layer of an aluminium alloy of the type AA1050 plated on the opposite side. The overall thickness of the sheet V was 1.5 mm, the thicknesses of the plated aluminium alloy layers being on average 112 m an for the aluminium brazing alloy layer and on average 82 m for the opposite roll bonded aluminium alloy layer of the type AA1050.

(12) The second investigated material R likewise consisted of a core consisting of an aluminium alloy of the type AA3005, which was roll bonded on both sides with an aluminium brazing alloy layer of the type AA4045. The overall thickness of the sheet R was 0.5 mm, so that the aluminium brazing alloy layers have on average in each case a thickness of 57 m, about 11.5% of the overall thickness.

(13) The sheets were then treated with the following six different pickling solutions, the duration of the treatment on account of the laboratory experimental arrangement being between 10 seconds and 300 seconds. The aqueous pickling solution composition was made as follows: No. 1: 0.73 wt. % H.sub.2SO.sub.4, HF: 300 to 400 ppm, surfactant No. 4: HNO.sub.3 13 wt. %, HCl: 12.5 wt. % HF: 2.2 wt. % No. 6: HNO.sub.3 25 wt. % No. 7: H.sub.3PO.sub.4 10 wt. % No. 8: Citric acid 10 wt. % No. 9: H.sub.3PO.sub.4 5 wt. %, citric acid: 5 wt. %.

(14) The experiments specified in Table 1 were carried out respectively with the sheet V and the sheet R and the brazing results were evaluated corresponding to the wetted length of the brazing joint clearance 4 and the width of the brazing neck. With the brazing, the samples were heated corresponding to the experimental arrangement illustrated in FIG. 1 in a batch furnace for six minutes at a brazing temperature of 595 C. to 610 C. and brazed without flux.

(15) Surprisingly it was found that, as can be seen from Table 2, two conditions have to be met in order to obtain a good brazing result. First of all, a certain pickling removal must be achieved, and thus it can be seen from Table 2 that the samples without or with nearly no pickling removal, L1-5 and L1-6 or L1-11 to L1-14, exhibited a very poor brazing result. However, the brazing ability cannot be predicted simply on the basis of the pickling removal; thus, the samples L1-3 and L1-4 with a shorter action time and lower pickling removal show a better brazing result compared to L1-1 and L1-2.

(16) If, however, a combination of a mineral acid and a complex-forming agent, such as for example in No. 1, No. 4 and No. 9, is used, then a significant difference in the brazing results is found. The brazing results show in addition that, on account of the high complex formation constant of the fluoroaluminates themselves, very small amounts of HF as complex-forming agent in conjunction with a mineral acid are sufficient in order to achieve a very good brazing result in a CAB method without flux.

(17) TABLE-US-00001 TABLE 1 Duration (sec) No. Pickling 10 30 60 120 300 1 H2SO4/HF/Surfactant X X 4 HNO3/HCl/HF X X 6 20-30% HNO3 X X 7 10% H3PO4 X X X 8 10% Citric acid X X X 9 5% H3PO4 + 5% Citric X X X acid

(18) TABLE-US-00002 TABLE 2 Pickling Pickling Time removal Brazing Experiment No. Sheet No. (sec) (g/m.sup.2) result L1-1 Inv. V 1 120 No data 1 L1-2 Inv. R 1 120 1.46 2 L1-3 Inv. V 4 30 No data 1 L1-4 Inv. R 4 30 0.54 1 L1-5 Comp. V 6 120 No data 6 L1-6 Comp. R 6 120 0.03 6 L1-7 Inv. V 7 120 No data 3 L1-8 Inv. R 7 120 0.90 3 L1-9 Inv. V 7 300 No data 2 L1-10 Inv. R 7 300 2.38 3 L1-11 Comp. V 8 120 No data 6 L1-12 Comp. R 8 120 0.00 6 L1-13 Comp. V 8 30 No data 6 L1-14 Comp. R 8 30 0.01 6 L1-15 Inv. V 9 120 No data 3 L1-16 Inv. R 9 120 0.64 2 L1-17 Inv. V 9 300 No data 1 L1-18 Inv. R 9 300 1.55 2 Inv. = according to invention; Comp. = comparison

(19) Not only HF but also H.sub.3PO.sub.4 and citric acid have complex-forming properties in relation to aluminium, which can be seen immediately from the brazing results. H.sub.3PO.sub.4 has produced good to satisfactory brazing results with the scores 2 and 3. A combination of 5% phosphoric acid with 5% citric acid with reaction times of 120 and 300 seconds already showed good brazing results. In addition, as can be seen from Table 2, it was found that the removal of material of 0.01 g/m.sup.2 or 0.03 g/m.sup.2 is not sufficient in order to lead to good brazing results. The experiments in which a removal of material of at least 0.05 g/m.sup.2 was achieved, for example L1-4 with 0.54 g/m.sup.2, show that very good brazing results can be achieved if the corresponding removal of material of at least 0.05 g/m.sup.2 is established and a complex-forming agent is used.

(20) On account of the complex-forming properties of oxalic acid in relation to aluminium it is assumed from this that a combination of oxalic acid with a mineral acid achieves equally good brazing results.

(21) In addition it can be seen from the experimental results from Table 2 that with layer thicknesses of the aluminium brazing alloy of 51 m and 112 m no difference could be found as regards the brazing results. It is therefore assumed from this that, depending on the design of the structural part and quality of the furnace atmosphere, very good brazing results are achieved with brazing layer thicknesses of more than 25 m and 30 m.

(22) In FIG. 3, the exemplary embodiment L1-4, which was treated with the pickling solution No. 4, shows that a very good brazing result was achieved. As can be seen, almost the whole brazing joint clearance 4 between the angle sheet 2 and the angle sheet 1 had been wetted.

(23) FIGS. 4A and 4B show light microscopy images of a transverse section of a further exemplary embodiment brazed flux-free in the CAB method, consisting of an aluminium core alloy material of the type AA3005 and aluminium brazing alloys of the type AA4045 provided in each case on both sides. As can be seen from FIG. 4B, in the exemplary embodiment the aluminium material was brazed with itself. In FIG. 4A it can very clearly be seen that, despite the Mg-containing core, the core alloy having an Mg content of 0.3 wt. %, there is a very clean formation of the brazed seam in the region of the overlap joint. Mg-containing aluminium core alloys could hitherto be brazed only with difficulty in a flux-free manner in the CAB method.

(24) A similar, very good result was for example also achieved with an aluminium alloy of the type AA6063 as core alloy layer, in which a one-sided aluminium brazing alloy layer of the type AA4045 with a mean thickness of about 100 m was applied as cladding to the core alloy layer. Also the aluminium alloys of the type AA6063 with an aluminium brazing alloy cladding showed very good brazing results if this was used with the aluminium brazing alloy layer pickled in the method according to the invention in a flux-free CAB method.

(25) In addition further experiments were carried out, in which the picking solutions A to D were used. With these further experiments, the aim was to investigate to what extent the content of HF influenced the brazing result and whether the degree of removal by picking was an important factor.

(26) TABLE-US-00003 TABLE 3 Pickling solution A 0.73 wt. % sulphuric acid + surfactant B 450 ppm fluoride + 0.73 wt. % sulphuric acid + surfactant C 1000 ppm fluoride + 0.73 wt. % sulphuric acid + surfactant D 1000 ppm fluoride

(27) As aluminium composite material, a sheet of the type R of thickness 0.5 mm was used, which comprised an aluminium core of a core alloy of the type AA3005 and aluminium brazing alloy layers of an aluminium alloy AA4045 applied as cladding on both sides, which have a mean thickness of ca. 57 m, about 11.5% of the overall thickness.

(28) On the one hand, the pickling solution A simply contained sulphuric acid and a surfactant, so that no complex-forming agents, for example HF, were present in the pickling solution. The other picking solutions B, C and D contained fluoride in each case, wherein the content was raised from 450 ppm to 1000 ppm. The pickling solution D contained only HF in an amount of 1000 ppm.

(29) The brazing experiments arranged corresponding to the experimental arrangement in FIG. 1 were in a drum furnace The brazing results were evaluated beforehand as in the tables as unsatisfactory (6) to very good (1) on the basis of the resultant brazed seam length.

(30) TABLE-US-00004 TABLE 4 Pickling Pickling duration Experiment solution (sec) 10 30 60 180 L2-1 A Comp. 6 6 6 6 L2-2 B Inv. 3 2 2 2 L2-3 C Inv. 2 2 1 1 L2-4 D Inv. 3 2 2 1

(31) Table 4 clearly shows that the brazing experiments L2-2 to L2-4 with material according to the invention produced an excellent brazing result, in which with increasing pickling duration the result still improved from good to very good. The aluminium alloy composite materials treated only with sulphuric acid and a surfactant showed, independently of the picking duration, i.e. the removal of material by pickling, no brazing ability when brazed in a batch furnace with nitrogen flow without using fluxes and under a protective gas. The brazing temperature in the brazing experiments for 6 minutes was in the range from 595 C. to 607 C.

(32) The difference between the use of a complex-forming agent such as HF without and with mineral acid, without sulphuric acid as in the present example, is clearly demonstrated. The combination of mineral acid and complex-forming agents in the pickling solutions B and C showed very good results, in which the results corresponding to amounts between 450 ppm and 1000 ppm differed only marginally.

(33) Since HF acts in relation to aluminium both as an acid and at the same time as a complex-forming agent, a pickling solution D containing exclusively HF also enables very good brazing results to be achieved, though a clear dependence on the pickling duration can be seen.

(34) In principle the amount of HF in the pickling solution should however be kept as low as possible, since handling HF in a production environment requires strict safety measures. The combination with a mineral acid thus enables the concentration of HF to be minimised, and therefore preferably 20 ppm to 1000 ppm or 20 ppm to 600 pm, particularly preferably 300 ppm to 600 ppm or 300 ppm to 480 ppm HF, are used.

(35) In order to determine the minimal removal of material by pickling, a sheet of the type R was sprayed with a pickling solution and the contact time was varied. After measuring the removal of material, brazing experiments were carried out and the brazing results were evaluated as previously. The results are shown in Table 5. An aqueous solution containing 300 ppm fluoride and 0.73 wt. % sulphuric acid was used as pickling solution.

(36) TABLE-US-00005 TABLE 5 Contact time (sec) Removal of material (g/m.sup.2) Brazing result Comp. 0 0 6 Inv. 1.5 0.05 3 Inv. 3 0.10 3 Inv. 6 0.17 2 Inv. 9 0.24 2 Inv. 12 0.31 1 Inv. 15 0.38 1 Inv. 18 0.47 1

(37) It can be seen that below 0.05 g/m.sup.2 removal of material, the brazing results are obviously significantly worse. The best brazing results were observed starting with a removal of material of ca. 0.3 g/m.sup.2.

(38) FIG. 5 shows an exemplary embodiment of a method for producing a strip-shaped aluminium composite material. In the production step A the aluminium composite material is produced by simultaneous casting of different melts or by roll bonding. Then a cold rolling B to the end thickness can for example be carried out, in which during the cold rolling at least one intermediate annealing can be performed. Subsequently, the aluminium composite material is for example soft annealed in the process step C. In the process step D at least the aluminium brazing alloy layer undergoes a surface treatment. The process step D is then illustrated for a strip-shaped aluminium composite material.

(39) The aluminium composite material wound on a coil 5 optionally undergoes a degreasing step 6. Subsequently, the aluminium composite material passes through the pickling step 7 in which the material is for example passed through a bath containing an aqueous, acid pickling solution, which contains apart from a mineral acid also a complex-forming agent, so that removal of material on the aluminium brazing alloy surface takes place. Preferably the bath consists of an aqueous sulphuric acid solution of 10% to 40% concentration, optionally at least one surfactant and a HF content of 20 ppm to 600 ppm, preferably 300 ppm to 600 ppm or 300 ppm to 480 ppm.

(40) After a rinsing and drying step 8, the surface-treated aluminium composite material is wound on a coil 9. The described surface treatment step D can however also be carried out on non-strip material or directly at the outlet of the production process, i.e. at the outlet of the cold rolling or for example the soft annealing, as long as a continuous furnace is used.

(41) FIG. 6 shows in plan view an exemplary embodiment of a construction thermally joined according to the invention in the form of a heat exchanger 10.

(42) The fins 11 of the exchanger 10 normally consist of blank aluminium alloy strip or aluminium alloy strip coated on both sides with an aluminium brazing material. The fins 11 are brazed bent in a meander pattern onto tubes 12, so that a large number of brazing joints is required. It is therefore particularly advantageous to use the aluminium composite material according to the invention since the particularly good brazing results in the CAB method can also be achieved without using fluxes. The absence of flux residues have, compared to heat exchangers brazed with fluxes, a positive effect on the operation of the heat exchangers.

(43) The experimental results showed in particular that the use of an aluminium composite material that has an acid pickled surface of an aluminium brazing alloy layer, in which the pickling was performed with a combination of a mineral acid and a complex-forming agent, provides very good properties in relation to its brazing ability in a flux-free thermal joining method carried out under a protective gas, for example a CAB brazing method.