EP1649070A1

EP1649070A1 - Resistant alloy for heat exchangers

Info

Publication number: EP1649070A1
Application number: EP04763503A
Authority: EP
Inventors: Lothar LÖCHTE; Wolf-Dieter Finkelnburg; Pascal Wagner; Raimund Sicking
Original assignee: Hydro Aluminium Deutschland GmbH
Current assignee: Hydro Aluminium Deutschland GmbH
Priority date: 2003-07-25
Filing date: 2004-07-26
Publication date: 2006-04-26
Also published as: AU2004259849A1; JP2007500784A; KR20060030910A; EP1505163A2; EA200600211A1; WO2005010223A1; BRPI0412907A; EP1505163A3; CA2533428A1

Abstract

The invention relates to a cold-hardened aluminium alloy for heat exchangers, a method for production of a cold-hardened aluminium strip or sheet and an aluminium strip or sheet. According to the invention, a cold-hardened aluminium alloy for heat exchangers may be provided which permits an economic application of inert gas shielded welding for the production of heat exchangers and with high resistance after a natural hardening after the welding, whereby the aluminium alloy has the following alloy components in wt. %: Si ≤ 0.7%, 0.1% ≤ Mg ≤ 1, Fe ≤ 0.3%, 0.08% ≤ Cu ≤ 0.2%, Ti ≤ 0.2%, Mn ≤ 0.1%, Cr ≤ 0.1%, Zn ≤ 0.1%, unavoidable impurities individually max. 0.1%, in total max. 0.15% and remainder aluminium.

Description

High strength alloy for heat exchangers

The invention relates to a cold-hardenable aluminum alloy for heat exchangers, a method for producing a cold-hardenable aluminum strip and an aluminum strip or sheet.

Heat exchangers consisting of aluminum or aluminum alloys are increasingly being used in the automotive sector. The use of aluminum instead of the previously used non-ferrous metal heat exchangers with comparable size and performance has almost halved the weight of the heat exchangers. Heat exchangers made of aluminum or an aluminum alloy are today mostly used in motor vehicles for cooling the cooling water, oil and in air conditioning systems. Heat exchangers for motor vehicles are usually produced from aluminum strips or sheets, in that the individual prefabricated components of the heat exchanger, such as fins, pipes and distributors, are connected to one another by soldering. The loads which, in practical use, act on components manufactured in this way and installed in motor vehicles due to shock-like vibrations, longer-lasting vibrations, corrosion attack and the like are considerable. This applies in particular to the fins via which the heat is dissipated. Despite the considerable loads and increasing operating pressures of the heat exchangers in the motor vehicle, there is still a trend towards saving weight in the motor vehicle and thus further reducing the wall thickness of the heat exchangers. This leads, however, to further increasing strength requirements for the aluminum alloy of the heat exchangers, especially after soldering. Vacuum soldering without flux and protective gas soldering with non-corrosive flux are available for soldering the heat exchangers. The cold-hardenable aluminum alloys previously used in vacuum soldering of heat exchangers, for example the aluminum alloy AA6063 (AlMgO, 7Si), AA6061 (AlMglSiCu) or AA6951 (AlMgO, 6SiCu) have relatively high magnesium contents in order to prevent oxidation of the soldering process in a vacuum by "gettering" to prevent molten aluminum solder on the components to be soldered and thus to guarantee a perfect solder connection without flux and, on the other hand, to achieve high strength values of the soldered heat exchangers after a cold aging after soldering The alternative protective gas soldering (also called GAB) requires less effort from these points of view and also enables up to 20% shorter soldering cycles, but the use of the aluminum oils known from vacuum soldering is High magnesium content is not possible because the magnesium reacts with the non-corrosive flux during soldering. This can only be prevented by using expensive fluxes containing cesium. There is also the possibility of using high copper-containing aluminum alloy (Cu content> 0.5%), which, however, tends to crack when cast and therefore places increased demands on the casting of the ingots, which is critical from an economic point of view. In addition, with increased copper Considered the danger of sensitization for pitting or Grain boundary corrosion if copper is present in the structure in a correspondingly excreted form. Finally, an aluminum alloy with an intermediate plating can be used as a diffusion barrier layer in inert gas soldering, so that a cold-hardenable aluminum alloy with relatively high magnesium contents can be used as the core material. However, the intermediate plating with a diffusion barrier layer is associated with additional costs, so that economical production of heat exchangers cannot be achieved either.

The production of heat exchangers by soldering components consisting of the above-mentioned aluminum alloys is known, for example, from US Pat. No. 4,214,925.

Based on the prior art shown above, the present invention is therefore based on the object of providing a cold-hardenable aluminum alloy for heat exchangers, a method for producing an aluminum strip for heat exchangers and a corresponding aluminum strip or sheet which have high strength values a cold aging after soldering.

According to a first teaching of the present invention, the object derived and shown above for an aluminum alloy is achieved in that the aluminum alloy has the following alloy components in% by weight: Si <0.7%, 0.1% <Mg <1%, Fe <0.3%, 0.08% <Cu <0.2%, Ti <0.2%, Mn <0.1%, Cr <0.1%, Zn <0.1%, unavoidable accompanying elements individually max. 0.1%, in total max. 0.15% and the rest aluminum.

Surprisingly, it has been shown that heat exchangers consisting of an aluminum alloy containing the above-mentioned alloy components after cold aging at room temperature after soldering have the strength required for use in motor vehicles, in particular the yield strength RPo, _{2 /} , without the need for further heat treatments. The reason for this is the combination of the Si and Mg contents according to the invention, which form finely divided precipitates of the Mg ₂ Si type in the aluminum alloy according to the invention and lead to an increase in strength through cold aging at room temperature. This increase in strength due to cold aging is further improved by the addition of copper in the claimed range from 0.08% by weight to 0.2% by weight.

Limiting the Fe content to a maximum of 0.3% by weight ensures that Si is present in the aluminum alloy in the dissolved state. Furthermore, the low Cu contents of a maximum of 0.2 wt. that the increase in strength during cold aging can be increased; on the other hand, this limitation of the Cu content reduces the sensitivity of the strength of the aluminum alloy to the cooling rate after soldering. To limit the dependence of the strength of the aluminum alloy on the cooling rate after soldering, the Mn content must be limited to a maximum of 0.1% by weight. Cr contents of at most 0.1% by weight, on the other hand, increase the strength and the corrosion resistance of the aluminum alloy according to the invention. In addition, a maximum Ti content of 0.2% by weight has a positive effect on the resistance of the aluminum alloy according to the invention to corrosion, since the alloying element Ti contributes to the grain refinement of the structure of the aluminum alloy and thus smoothes out a corrosion attack. To avoid the negative effect of zinc on the corrosion of the aluminum alloy according to the invention, the Zn content must be limited to a maximum of 0.1% by weight.

According to a first advantageous embodiment, the strength of the aluminum alloy according to the invention can be further increased by cold aging after soldering in that the aluminum alloy contains Si, Mg and Cu as main alloy elements.

In order to avoid softening of the components of a heat exchanger to be soldered during the soldering, it is advantageous to carry out a perfect soldering process if the solidus temperature of the aluminum alloy does not fall below 610 ° C, since usually soldering at temperatures up to 600 ° C he follows. This is achieved according to the invention in that the sum of the alloy proportions of Si, Mg and Cu does not exceed 1.2% by weight. Alloy elements generally bring about a lowering of the solidus temperature, whereby Si causes a lowering of the solidus temperature of the aluminum alloy by a factor of 1.2 than Mg and Mg in turn causes a lowering of the solidus temperature by a factor of 3.5 more effectively than Cu.

This does not apply to the alloy element Ti, so that an increase in the solidus temperature of the aluminum alloy according to the invention can be achieved in that the aluminum alloy has Ti as an alloy component.

If the upper limit of the claimed alloy for magnesium is exhausted, the heat exchangers made from this alloy are preferably soldered by vacuum soldering. Protective gas soldering is also possible to a limited extent when using fluxes containing cesium. Protective gas soldering when using cesium-containing flux is simplified in particular by the fact that the magnesium alloy content does not exceed 0.8% by weight.

In addition, with a low Mg content of up to a maximum of 0.3% by weight, the aluminum alloy according to the invention is readily suitable for shielding gas soldering with non-corrosive fluxes, since there is only a slight reaction with the fluxes and the use of expensive cesium-containing ones Flux can be dispensed with. The aluminum alloy according to the invention is particularly advantageous in that the aluminum alloy has particularly high strength values after processing and soldering and after about 30 days of cold aging at room temperature. This material property ensures a particularly cost-effective manufacturing process, since the cold aging during the transport process already ensures very good strength without any further measures.

According to a second teaching of the present invention, the object derived and shown above is achieved according to the method in that

a billet is cast from an aluminum alloy according to the invention in the conventional ingot casting process,

- The rolling bar is homogenized at 500 to 600 ° C for more than 6 h, in particular for more than 12 h, and is hot rolled to a strip at at least 400 ° C, preferably 450 ° C, the final temperature during hot rolling being at least 300 ° C is

- The hot-rolled strip is cold-rolled to its final thickness and then subjected to soft annealing at at least 300 ° C, preferably 350 ° C.

Through the homogenization of the rolled ingot cast in the conventional ingot casting process at temperatures of 500 to 600 ° C for more than 6 hours, in particular for more than 12 hours, is achieved that even diffusion-inert elements such as manganese and chromium are finely dispersed when the melt cools. By hot rolling at least 400 ° C with regard to the formability and

Corrosion resistance produces an optimized structure of the hot strip, whereby the final rolling temperature during hot rolling must be at least 300 ° C, on the one hand to ensure sufficient deformability of the rolling ingot and on the other hand to optimize the structure during hot rolling. The hot strip end thicknesses can be, for example, less than 9 mm. In order to facilitate the forming of the strip produced by the process according to the invention into prefabricated components for the heat exchanger, for example fins, pipes or distributors, the strip cold-rolled by cold rolling to a final thickness of at most 2 mm is preferably given a final soft annealing at at least 300 ° C 350 ° C subjected.

By combining the alloy composition of the aluminum alloy in conjunction with the process features described above, heat exchangers can be produced on the basis of conventional alloy elements (Mg, Si, Cu), which after an inert gas soldering and a cold aging of approx. 30 days at room temperature, yield limits of RP ₀ , ₂ ≥ 65MPa and are therefore particularly well suited for the enormous loads in the motor vehicle. In addition, shielding gas soldering without the use of cesium-containing fluxes can be used to manufacture the heat exchangers be made, so that an economical production is possible.

If the hot rolling and / or the cold rolling takes place reversibly or unidirectionally on single or multi-stand rolls, the method according to the invention can be carried out with conventional means and devices with regard to the reducing rolling.

A particularly high level of process reliability when soldering the heat exchanger can be achieved by cladding the rolled bar with an aluminum solder after homogenization. The aluminum strip produced from this rolled ingot has a uniform layer of aluminum solder, which leads to particularly homogeneous and uniform solder connections during soldering, for example between the fins, tubes and distributors of the heat exchanger. If only one side of the aluminum strip according to the invention is clad with an aluminum solder, the other side can be clad or coated, for example, with an alloy serving as corrosion protection.

An aluminum alloy with a silicon content of 6 to 13% by weight, in particular an AlSi7 or AlSilO alloy, is preferably used as the aluminum solder, which alloy soldering has a particularly good wetting capacity with regard to the wetting of oxide layers remaining in a non-oxidizing atmosphere have on the components of the heat exchanger to be soldered with aluminum solder. Finally, according to a third teaching of the present invention, the object derived and shown above is achieved by an aluminum strip or sheet for the production of heat exchangers, which is produced by the method according to the invention. As already stated, an aluminum strip or sheet produced by the method according to the invention has improved strength values, in particular yield strength, after cold aging after soldering, so that the wall thicknesses of the heat exchangers can be further reduced. In addition, shielding gas soldering with non-corrosive fluxes without the use of cesium-containing fluxes can be used to manufacture the heat exchangers.

The aluminum strip or sheet according to the invention preferably has a maximum thickness of 2 mm, in particular 1 mm. Due to the higher strength compared to conventional materials, the strip thickness can be reduced further when the aluminum strip according to the invention is used, thus saving material in the production of heat exchangers and a further reduction in the weight of the heat exchanger. The operational reliability of the heat exchanger is not impaired, even at higher operating pressures, due to the higher strength of the aluminum alloy.

There are now a large number of possibilities, the cold-hardenable aluminum alloy for heat exchangers according to the first teaching of the invention, the method for producing a cold-hardenable aluminum strip for heat exchangers according to the second teaching of the invention and the aluminum strip or sheet according to the invention Manufacture and develop production of heat exchangers according to the third teaching of the invention. For this purpose, reference is made, for example, on the one hand to the claims subordinate to claims 1, 5 and 9, on the other hand to the description of an exemplary embodiment of a method for producing a cold-hardenable aluminum strip for heat exchangers according to the second teaching of the invention in conjunction with the drawing.

In the drawing, the single figure schematically shows a production route for realizing an exemplary embodiment of a method for producing a cold-hardenable aluminum strip for heat exchangers according to the second teaching of the invention.

In a first step, the production route shown in the single figure comprises the ingot casting 1 made of an aluminum alloy. The aluminum alloy of the exemplary embodiment has the following alloy components in% by weight

0.60% <Si <0.70%, 0.12% <Fe <0.30%, 0.08% <Cu <0.20%, 0.04% <Mn <0.08%, 0, 12% <Mg <0.30%, Cr <0.05%, Zn <0.05%, 0.08% <Ti <0.20%, B <50 ppm, unavoidable accompanying elements max. 0.03%, in total max. 0.1% and the rest aluminum.

The low boron content of maximum 50 pp improves the recyclability of the aluminum alloy. The rolled ingots cast from the aluminum alloy just described in the DC process are then homogenized in a homogenization stage 2. Particularly good results with regard to the homogenization of the billet were achieved at a temperature of 575 ° C. for more than 6 h, in particular 12 h. Subsequent to the homogenization, the rolled bars are hot rolled on a tandem stand 3a to a thickness of, for example, 7 mm, the final temperature during hot rolling in particular having to be more than 300 ° C., preferably 330 ° C., in order to achieve an optimized structure during hot rolling guarantee. Alternatively, the hot rolling can also be carried out on a reversing stand 3 and wound on a reel (not shown) and the hot rolling in the tandem stand 3a can be dispensed with. The subsequent cold rolling to a final thickness of approximately 1 mm is carried out on single or multi-stand rolls 4. Like hot rolling, cold rolling can also alternatively take place on a reversing stand. A final soft annealing at about 350 ° C. in a chamber furnace 5 brings the aluminum strip into a state of low strength and high elongation in order to facilitate subsequent forming work in the manufacture of the components of the heat exchanger.

As an alternative to the exemplary embodiment of the method according to the invention for producing a After the homogenization in the homogenization stage 2, the strip for heat exchangers can be plated with an aluminum solder, for example made of an AlSi7 or ALSilO alloy, in order to avoid the subsequent application of an aluminum solder before soldering by the heat exchanger produced from the strip according to the invention. For this purpose, the billet must be heated to a rolling temperature of at least 400 ° C, preferably 450 ° C, before hot rolling. When soldering heat exchangers made from the aluminum strip or sheet according to the invention, especially in the case of shielding gas soldering, without using cesium-containing fluxes at temperatures up to 600 ° C. and typical cooling speeds of 30 ° C./min from 600 ° C. to 200 ° C. and a cold aging of approx. 30 days at room temperature after soldering, particularly high strength values of the heat exchanger, in particular values for the elongation limit of RPo, ₂ ≥ 65 Mpa, can be achieved. The cooling from 200 ° C to room temperature does not have to be exactly defined.

Claims

P AT ENT TO SAYING

1. Cold-hardenable aluminum alloy for heat exchangers d a d u r c h g e k e n n z e i c h n e t that the aluminum alloy has the following alloy components in% by weight:

Si <0.7%, 0.1% <Mg <1%, Fe <0.3%, 0.08% <Cu <0.2%, Ti <0.2%, Mn <0.1%, Cr <0.1%, Zn <0.1%, unavoidable accompanying elements individually max. 0.1%, in total max. 0.15% and the rest aluminum.

2. Cold-hardenable aluminum alloy according to claim 1, d a d u r c h g e k e nn z e i c h n e t, that a s s contains the aluminum alloy Si, Mg and Cu as main alloy elements.

3. Cold-hardenable aluminum alloy according to claim 1 or 2, characterized in that the sum of the alloy proportions of Si, Mg and Cu does not exceed 1.2% by weight.

, Aluminum alloy according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t, that the aluminum alloy has as an alloy component Ti.

5. Aluminum alloy according to one of claims 1 to 4, d a d u r c h g e k e n n z e i c h n e t, that the alloy proportion of Mg does not exceed 0.8 wt .-%.

6. Aluminum alloy according to one of claims 1 to 4, d a d u r c h g e k e n n z e i c h n e t, that the alloy proportion of Mg does not exceed 0.3 wt .-%.

7. Aluminum alloy according to one of claims 1 to 6, d a d u r c h g e k e n n z e i c h n e t, that the aluminum alloy has particularly high strength values after processing and soldering and after about 30 days of cold aging at room temperature.

8. A method for producing a cold-hardenable aluminum strip for heat exchangers made of an aluminum alloy according to one of claims 1 to 7, d a d u r c h g e k e n n z e i c h n e t, d a s s

- a billet is cast in the conventional ingot casting process, the rolling bar is homogenized at 500 to 600 ° C. for more than 6 h, in particular for more than 12 h,

the hot rolled bar is hot rolled into a strip at at least 400 ° C., preferably 450 ° C., the final temperature during hot rolling being at least 300 ° C.,

9. The method according to claim 8, d a d u r c h g e k e n n z e i c h n e t that hot rolling and / or cold rolling takes place reversibly or unidirectionally on single or multi-stand rolls.

10. The method according to claim 8 and 9, d a d u r c h g e k e n n z e i c h n t, that the rolled ingot is plated after homogenizing with an aluminum solder.

11. The method according to claim 10, d a d u r c h g e k e n n z e i c h n e t, that as an aluminum solder an aluminum alloy with a silicon content of 6 - 13 wt .-%, in particular an AlSi7 or AlSilO alloy, is used.

12. Aluminum strip or sheet for the production of heat exchangers consisting of an aluminum alloy according to one of claims 1 to 7, in particular produced by a method according to one of claims 8 to 11.

13. Aluminum strip or sheet according to claim 12, characterized in that the aluminum strip or sheet has a maximum thickness of 2 mm, preferably 1 mm.

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