EP1446511B1 - Aluminium alloy strips for heat exchangers - Google Patents

Abstract

Aluminum alloy strip with a thickness of less than 3 mm for the fabrication of brazed heat exchangers has the following composition, by wt %: Si less than 1.0; Cu less than 0.5; Fe less than 0.7; Mg less than 0.1; Mn : 0.8 - 1.5; Zn less than 2.0; In less than 0.2; Sn less than 0.2; Bi less than 0.2; Ti less than 0.2; Cr less than 0.25; Zr less than 0.25; and other elements less than 0.05 each and less than 0.15 in total. The aluminum alloy strip has a corrosion potential difference of at least 10 mV between its surface and its mid-thickness, measured with respect to a calomel saturated electrode according to the ASTM G69 Standard. An Independent claim is also included for a method for the fabrication of this aluminum alloy strip.

Description

Field of the invention

The invention relates to the field of thin strips (thickness <0.3 mm) of aluminum alloy for the manufacture of heat exchangers, in particular those used for engine cooling and air conditioning of the passenger compartment of motor vehicles. The aluminum alloy strips for exchangers are used either bare or coated on one or both sides of a solder alloy. The invention relates more particularly to uncoated strips used for fins or spacers attached to tubes or elements in contact with the cooling fluid.

State of the art

Aluminum alloys are now very widely used in the manufacture of heat exchangers for the automobile because of their low density, which allows a weight gain, especially compared to copper alloys, while ensuring good thermal conduction, ease of implementation and good resistance to corrosion. These exchangers comprise tubes for the circulation of the internal heating or cooling fluid and fins or spacers to ensure the heat transfer between the internal fluid and the external fluid, and their manufacture is done either by mechanical assembly or by soldering. In addition to their thermal transfer function, the fins or spacers must provide protection against galvanic perforation of the tubes, ie by providing for the fins an alloy having a lower electrochemical corrosion potential than for the tubes, so that the fin plays the role of sacrificial anode. The most commonly used alloy for the tubes being alloy 3003, an alloy of the same type is usually used for the fins with an addition from 0.5 to 2% zinc. The composition of alloy 3003 recorded at the Aluminum Association is as follows (% by weight):
If <0.6 Fe <0.7 Cu: 0.05 - 0.2. Mn: 1.0 -1.5 Zn <0.1.
The strips in this type of alloy are generally obtained by semi-continuous casting of a plate, homogenization of this plate, hot rolling, then cold rolling with optionally an intermediate annealing and / or a final annealing. They can also be obtained by continuous casting of strips between two belts ("twin-belt casting") or between two cooled rolls ("twin-roll casting"). It is known that with the latter technique, in order to obtain a fine grain structure in Al-Mn alloys, homogenization of the blank is applied which eliminates the segregations resulting from the casting, which leads to a good compromise between the mechanical strength and formability. These properties are described in particular in patent EP 0039211 (Alcan International) for alloys between 1.3 and 2.3% of manganese, and in US Patent 4,737,198 (Aluminum Company of America) for alloys containing from 0.5 to 1.2% iron, less than 0.5% silicon and 0.7 to 1.3% manganese, which can be used for the production of exchanger fins.
Applicant's patent application WO 98/52707 describes a process for manufacturing aluminum alloy strips containing at least one of Fe (from 0.15 to 1.5%) or Mn (from 0.35 to 1.9%) with Fe + Mn <2.5%, and optionally containing Si (<0.8%), Mg (<0.2%), Cu (<0.2%), Cr (<0 , 2%) or Zn (<0.2%) by continuous casting between cooled cylinders and shrink wrapped to a thickness between 1 and 5 mm, followed by cold rolling, the force applied to the casting rolls, expressed in tonnes per meter of bandwidth, being less than 300 + 2000 / e, where e is the thickness of the band expressed in mm. The use of these strips for the production of brazed exchanger fins is mentioned.
The patent application WO 00/05426 of Alcan International describes the manufacture of aluminum alloy fin strips of composition: Fe: 1, -2 - 1.8%, Si: 0.7 - 0.95%, Mn: 0.3 - 0.5%, Zn: 0.3 - 2%, by continuous casting of strips with a cooling rate greater than 10 ° C / s.
The patent applications WO 01/53552 and WO 01/53553 of Alcan International also relate to the manufacture of finned iron alloy strips containing up to 2.4% iron by continuous casting and very rapid cooling. The goal is to obtain a more negative corrosion potential.

Purpose of the invention

If the fins or spacers must play a role of galvanic protection of the tubes, they must not however be too deteriorated by corrosion during the life of the exchanger. Indeed, it is necessary to maintain a sufficient integrity of the material, because if it perforates too quickly, the heat exchange will be less effective due to the loss of useful area. It could even occur a separation of the fin and the tube, which would block the thermal conduction between these components. The object of the invention is therefore to obtain strips for fins or spacers of heat exchangers made of aluminum alloy intended in particular for the automotive industry, having both good mechanical strength, good formability and good resistance to wear. perforating corrosion while having a sacrificial anode role.

Object of the invention

The subject of the invention is aluminum alloy strips with a thickness <0.3 mm, intended for the manufacture of heat exchangers, of composition (% by weight): Si <1.5 Fe <2.5 Cu <0.8 Mg <1.0 Mn: <1.8 Zn <2.0 In <0.2 Sn <0.2 Bi <0.2 Ti <0.2 Cr <0.25 Zr <0.25 Si + Fe + Mn + Mg> 0.8, other elements <0.05 each and <0.15 in total, remaining aluminum, having between the surface and mid-thickness a difference in corrosion potential, measured with respect to a saturated calomel electrode according to ASTM G69, d. at least 10 mV.
The invention also relates to a process for producing such strips by continuous casting under conditions favoring the formation of segregations in the core of the strip, possibly hot rolling, cold rolling with possibly one or more intermediate annealing (s) or final from 1 to 20 hours at a temperature between 200 and 450 ° C.

Description of figures

• FIG. 1 represents the evolution of the corrosion potential, measured with respect to a saturated calomel electrode, of a strip according to the invention of alloy of example 1, as a function of the depth with respect to the surface.
• FIG. 2 represents in the same way the evolution of the corrosion potential of an alloy strip of example 2.

Description of the invention

The Applicant has found that by using, for alloys of type 3000 (Al-Mn) or type 8000 (Al-Fe) with possible zinc addition, continuous casting under particular casting conditions and with a range of transformation. adapted, bands with a corrosion potential gradient in their thickness, and that this property favored a lateral propagation rather than perpendicular to the surface of the corrosion, which ensured the sacrificial effect while avoiding the perforation, and therefore deterioration of the fin or interlayer over time. This potential gradient is at least 10 mV. According to a hypothesis emitted by the inventors, this difference could be linked to the presence, for the particular casting conditions selected, of segregations at the center of the band, a phenomenon that is usually sought to be avoided, and which leads to compositional differences. in solid solution in the thickness of the strips.
The zinc content varies according to the alloy used for the tubes, so as to obtain an electrochemical potential difference between the tubes and the fins both sufficient to allow the fin to perform its role of sacrificial anode , and not too high to prevent it from deteriorating too fast. To lower the corrosion potential of the fin or spacer, indium, tin and / or bismuth can also be added up to a level of 0.2%. For 3003 alloy tubes, the zinc content is preferably between 1.0 and 1.5%. For Al-Mn alloy tubes loaded with copper, such as the alloys with more than 0.4% copper described in the applicant's patent application EP 1075935, the zinc content should rather be kept below 0.8%.
The copper content is preferably maintained below 0.5%. The optional addition of titanium up to 0.2%, zirconium up to 0.25% and / or chromium up to 0.25% makes it possible to improve the heat resistance ("SAG resistance") of the alloy.

In a first variant of the invention, the alloy used is a type 3003 alloy with a zinc content of up to 2%, that is to say an alloy of composition (% by weight):
If <1.0 Fe <1.0 Cu <0.8 Mg <1.0 Mn: 0.8 - 1.8 Zn <2.0 In <0.2 Sn <0.2 Bi <0.2 Ti <0.2 Cr <0.25 Zr <0.25 other elements <0.05 each and <0, 15 in total, remains aluminum.
The addition of silicon, preferably above 0.5% and up to 1% contributes to increasing the solidification range of the alloy, which promotes the appearance of segregations in the casting. Beyond 1%, it is possible to reach the burning temperature of the alloy during the soldering operation of the exchanger.
In a second variant of the invention, an alloy of the 8000 series of composition (% by weight) is used:
If: 0.2 - 1.5 Fe: 0.2 - 2.5 Cu <0.8 Mg <1.0 Mn: <1.0 Zn <2.0 In <0.2 Sn <0.2 Bi <0.2 Ti < 0.2 Cr <0.25 Zr <0.25 If + Fe> 0.8, other elements <0.05 each and <0.15 in total, remain aluminum.
A particularly suitable composition domain is the following:
If: 0.8 - 1.5 Fe: 0.7 - 1.3 Mn <0.1 Cu <0.1 Mg <0.1 and, preferably, Si: 1.0 - 1.3 and Fe: 0.9 -1.2.
The method of manufacturing the belts according to the invention comprises forming the alloy from an adjusted filler to obtain the desired alloy composition. The metal is then continuously cast in the form of a strip of thickness between 1 and 30 mm, either by casting between belts between 12 and 30 mm, or, preferably, by casting between two rolls cooled and shrunk, at a distance of thickness between 1 and 12 mm. In contrast to the teaching of the patent application WO 98/52707, casting parameters are chosen that favor the appearance of relatively large segregations in the core of the cast strip.
In the case of casting between rolls, it is necessary that the contact between the metal and the cooled rolls is the best possible, so as to increase the thermal gradient on the surface of the metal during casting, which promotes segregation. The various parameters on which one can act are in particular the length of the arc of contact between the metal and the cylinders, the force exerted by the rolls during the casting and the temperature of the rolls of the rolls. A high contact arc, preferably greater than 60 mm, is favorable to the formation of segregations. It is likewise with a high force, preferably greater than 100 + 2000 / and / m cast strip width, e being the thickness of the cast strip expressed in mm. Finally, the temperature of the hoop must be as low as possible, preferably less than 100 ° C.
The cast strip is optionally, in the case of the casting between belts, hot rolled, and then cold rolled. On the other hand, the cast-rolled strip is directly cold-rolled. If the final thickness is rather small, it is necessary to provide at least one intermediate anneal at a temperature between 200 and 450 ° C. If the metal is to be delivered in the annealed state, the annealed strip is subjected to annealing at a temperature between 200 and 450 ° C. on the rolled strip to the final thickness. In the case where the metal is delivered in the hardened state, the transformation range is adapted so that the reduction rate is adjusted to the target work hardening rate. The strips according to the invention make it possible to produce fins or interleaves of heat exchangers having a high mechanical strength, which makes it possible to reduce the thickness with respect to a fin or a spacer according to the prior art, while keeping good formability. In use, the fin or spacer plays its sacrificial role, but corrosion progresses laterally parallel to the surface, which prevents or delays perforation, ensures the integrity of the tube-fin assembly, and therefore a heat exchange continued. The strips have a coarse-grained microstructure, favorable to the hot resistance during brazing.

Example Example 1

An alloy of composition (% by weight) was prepared in the melting furnace: Yes Fe Cu mn mg Cr Or Zn Ti 0.80 0.55 0.10 1.0 0,069 0,002 0.005 1.4 0,015

A 5 mm thick strip was cast on a Jumbo 3Cm ™ continuous casting plant from Pechiney Rhenalu, at a width of 1420 mm, with a force between the rolls of 780 t, a contact arc of 70 mm and a temperature of cylinders frets 70 ° C. The strip was then cold rolled in one pass to the thickness of 0.7 mm, then subjected to an intermediate annealing of 12 h in an air oven programmed at 520 ° C to bring the metal to a temperature of 1 mm. order of 380 ° C, and cold rolled in three passes up to 130 μm.
A first part of the strip underwent a 2 hour restoration annealing at 350 ° C and then a rolling up to 100 μm. A second part was recrystallized for 2 hours at 400 ° C. and then rolled up to 100 μm. Finally, a third part underwent the same annealing, but a rolling up to 75 microns. For comparison, zinc alloy strips 3003 were made of composition: Yes Fe Cu mn mg Cr Or Zn Ti 0.22 0.57 0.12 1.15 - - - 1.4 - according to the same manufacturing range, but starting from a vertical semi-continuous casting process, with a 2-hour restoration annealing at 350 ° C, and a rolling up to 100 μm.
The static mechanical characteristics were measured on these bands: elastic limit R _0.2 , tensile strength R _m and elongation A. The results are shown in Table 1: Range Thickness (μm) R _0.2 (MPa) R _m (MPa) AT (%) CC Rec. 350 ° C 100 235 248 3.2 CC Rec. 400 ° C 100 188 197 2.4 CC Rec. 400 ° C 75 213 227 1.8 CV Rec. 350 ° C 100 158 162 1.5 * CC = continuous casting CV = vertical semi-continuous casting

It is found that the metal obtained by continuous casting has both a better mechanical strength and a better elongation than the metal from conventional casting.
In the 75 μm thick strip, the evolution of the corrosion potential in the thickness was measured with respect to a saturated calomel electrode according to the ASTM G69 standard. We can see in the figure the presence, under the surface, and on a depth of about 15 μm from an area in which the potential is rapidly evolving from -890 mV to -870 mV.

Example 2

An alloy of composition (% by weight) was prepared: Yes Fe Cu mn mg 1.2 1.1 <0.1 <0.1 <0.1

A 6.1 mm thick strip was cast on a Davy ™ continuous casting plant from Pechiney Eurofoil, at a width of 1740 mm, with a force between the rolls of 550 t, a contact arc of 60 mm and a cylinder roll temperature of 42 ° C. The strip was then cold rolled to a thickness of 80 μm to obtain a H19 type metallurgical state.
The mechanical characteristics of this band are as follows: R _m (MPa) R _0.2 (MPa) AT% 311 256 7.3

It is found that this metal, produced by continuous casting, has an excellent compromise mechanical strength / elongation.
A typical soldering cycle was then applied to the metal, in a furnace under a nitrogen atmosphere, with a 2 minute dwell at 600 ° C.
The mechanical characteristics obtained after this treatment are as follows: R _m (MPa) R _0.2 (MPa) AT% 135 53 13.2

The elastic limit after brazing, R _0.2 , equal to 53 MPa is significantly greater than that obtained for conventional alloying strips 3003 used, obtained by conventional casting (of the order of 40-45 MPa).

From the point of view of the corrosion resistance, we find on these alloys 8xxx, as can be seen in Figure 2, and always in connection with the casting process used, a change in the corrosion potential in the thickness of the metal, whose beneficial character has been explained above for 3xxx alloys.
In order to adapt the corrosion potential to that of the alloys used for the tubes to which the spacers will be coupled, it is possible to achieve an addition of zinc, which has very little influence on the mechanical characteristics or the thermal conductivity.

Claims (15)

1. Aluminium alloy strips with thickness < 0.3 mm, to be used in the manufacture of brazed heat exchangers, with composition (% by weight):
   Si < 1.5; Fe < 2.5; Cu < 0.8; Mg < 1.0; Mn = < 1.8; Zn < 2.0; In < 0.2; Sn < 0.2; Bi < 0.2; Ti < 0.2; Cr < 0.25; Zr < 0.25; Si + Fe + Mn + Mg > 0.8, other elements < 0.05 each and < 0.15 in total, with a difference of corrosion potential between the surface and the mid-thickness, measured with respect to a saturated calomel electrode according to ASTM standard G69 equal to at least 10 mV.
2. Strips according to claim 1, characterised in that the zinc content is between 1.0 and 1.5%.
3. Strips according to claim 1, characterised in that the zinc content is below 0.8%.
4. Strips according to one of claims 1 to 3, characterised in that the copper content is below 0.5%.
5. Strips according to one of claims 1 to 4, characterised in that they are made from an alloy with the following composition: Si < 1.0; Fe < 1.0; Cu < 0.8; Mg < 1.0; Mn = 0.8 - 1.8 Zn < 2.0; In < 0.2; Sn < 0.2; Bi < 0.2; Ti < 0.2; Cr < 0.25; Zr < 0.25; other elements < 0.05 each and < 0.15 in total, the remainder being aluminium.
6. Strips according to claim 5, characterised in that the silicon content is between 0.5% and 1%.
7. Strips according to one of claims 1 to 4, characterised in that they are made from an alloy with the following composition: Si = 0.2 - 1.5; Fe = 0.2 - 2.5; Cu < 0.8; Mg < 1.0; Mn = < 1.0: Zn < 2.0; In < 0.2; Sn < 0.2; Bi < 0.2; Ti < 0.2; Cr < 0.25; Zr < 0.25; Si + Fe > 0.8, other elements < 0.05 each and < 0.15 in total, the remainder being aluminium.
8. Strips according to claim 7, characterised in that they are made from an alloy with the following composition: Si = 0.8 - 1.5; Fe = 0.7-1.3; Mn < 0.1; Cu < 0.1; Mg < 0.1.
9. Strips according to claim 8, characterised in that the silicon content of the alloy is between 1 and 1.3%.
10. Strips according to either claim 8 or 9, characterised in that the iron content is between 0.9 and 1.2%.
11. Method for manufacturing strips according to one of claims 1 to 10, by continuous casting at a thickness of between 1 and 30 mm under conditions that promote the formation of segregations in the strip core, possibly hot rolling, cold rolling possibly with one or more intermediate or final annealing(s) lasting for 1 to 20 h at a temperature of between 200 and 450°C.
12. Method according to claim 11, characterised in that continuous casting is made between two cooled and shrinked rolls.
13. Method according to claim 12, characterised in that the force applied by the rolls during casting is more than 100 + 2000/e t/m of width of the cast strip, where e is the thickness of the cast strip expressed in mm.
14. Method according to either claim 12 or 13, characterised in that the contact arc between the metal and the rolls is longer than 60 mm.
15. Method according to one of claims 12 to 14, characterised in that the temperature of the shells is less than 100°C.

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