Patent Application
20140182821
The invention relates to the field of brazing sheets for heat exchanger tubes made from aluminum alloys, particularly those used for cooling or optimization of engine efficiency, and heating or air conditioning of the passenger compartment.
This is particularly applicable to heat exchanger tubes subject to a severely corrosive environment such as charged air cooler tubes through which exhaust gas circulation takes place, and usually provided with a turbulator designed to improve heat exchange by increasing the exchange surface area and disturbing internal fluid circulation.
Aluminum alloys are used for the fabrication of most automobiles heat exchangers due to their low density which enables a weight saving, particularly compared with copper alloys, while giving good thermal conduction, ease of use and good resistance to corrosion.
All aluminum alloys referred to in the following are designated using the designations defined by the Aluminum Association in Registration Record Series that it publishes regularly, unless mentioned otherwise.
Exchangers comprise tubes for the circulation of internal fluid and fins or separators, and possibly a turbulator to increase heat transfer between the internal fluid and the external fluid, and they are fabricated either by mechanical assembly or by brazing.
In the case of Charged Air Coolers, known to those skilled in the art under the term CAC, the normal configuration is that the core sheet making up the tube (usually called the tube strip) usually made from AA3xxx aluminum alloy, is coated on its external and internal faces with an alloy called the brazing alloy, usually in the AA4xxx series. This has the advantage that it melts at a temperature below the core melting temperature and, through application of a thermal brazing cycle, can create a bond between two materials to be assembled, in other words brazed, or possibly to bond the tube strip onto itself so as to close the tube by brazing, the alternative being welding.
This configuration is shown in
An example is given in application EP 0283937 A1 by Nihon Radiator Co. Ltd.
At the moment there are engine designs in which exhaust gases are reinjected into the charged air cooler to be mixed with clean air and returned to the inlet, with the final purpose of reducing polluting emissions of vehicles.
In this configuration, exhaust gases that could condense can generate a particularly severe corrosive environment in the cooler, characterized particularly by a low pH (that can significantly be less than 3).
In the case of a tube configuration according to the state of the art as described above, a significant diffusion of silicon contained in the cladding AA4xxx alloy to the core alloy forming the core of the tube occurs during the brazing operation, thereby degrading its resistance to corrosion.
One solution known to those skilled in the art consists of inserting intermediate cladding during co-rolling, made from an AA1xxx or AA7xxx alloy, between the tube core alloy and its internal coating made from AA4xxx brazing alloy.
Such a configuration is diagrammatically shown in
Such a coating acts by limiting diffusion of silicon from the internal cladding to the tube core during brazing, thus improving its corrosion resistance.
A sacrificial alloy typically in the AA7xxx series, may also be used as the intermediate cladding alloy.
These “multi-clad” sheets are known to those skilled in the art and are disclosed particularly in applications JP 2003027166 A by Kobe Steel Ltd. Shinko Alcoa, JP 2005224851 A by Shinko Alcoa Kizai KK, WO 2006/044500 A2 and WO 2009/142651 A2 by Alcoa Inc, WO 2007/042206 A1 by Corus Aluminium Walzprodukte GmbH, US 2010/0159272 A1 by Novelis, etc.
The use of this type of “multi-clad” sheets in a charged air cooler through which exhaust gas passes is disclosed in application WO 2008/063855 by Modine Mfg Co.
Another solution disclosed in applications EP 1762810 A1 and US 2007/0051503 A1 by Behr America Inc., consists of favoring the creation of a “brown band” known to those skilled in the art, between the AA3xxx alloy core of the turbulator and its cladding composed of an AA4xxx alloy (typically AA4045), during the conventional type brazing inside the tube. The tube is also typically made from an AA3xxx alloy clad with AA4xxx on both sides. In one highly preferred embodiment, the tube and the turbulator are actually composed of the same material.
However, although such configurations can slightly improve the resistance of the tube to corrosion, they may be insufficient under particularly severe load conditions, which is the case for heat exchangers subject to recirculation of exhaust gases, characterized particularly by a low pH.
Other solutions consist of application of a surface treatment after brazing to improve the internal corrosion resistance of the tube. This is the case of the solution disclosed in application FR 2916525 A1 by Valeo Systèmes Thermiques, that recommends a coating based on resins. Another example of a surface treatment, in this case an electro-ceramic deposit, is given in application WO 2010/019664. Application FR 2930023 by Valeo Systemes Thermiques mentions the possibility of boehmiting the entire exchanger. Finally, application EP 1906131 A2 by International Truck discloses a solution consisting of applying a metal surface treatment based on Ni or Co on the inside face of the tube and on the turbulator.
Finally, other solutions lie in the use of a combination of different materials, particularly aluminum and stainless steel. Application WO2008/095578 by Behr GmbH & Co. thus claims the use of stainless steel for the disrupter, the tube being made from aluminum while application EPI906127 by International Truck discloses an aluminum tube with stainless steel clad on the inside face, still with a stainless steel turbulator.
However, such options are too expensive to provide an industrially satisfactory product.
The invention is aimed at optimizing the choice of materials or brazing sheets made from aluminum alloys designed for production of exchanger tubes with brazed turbulator to improve their resistance in a severe corrosive environment like that created by recirculation of automobile vehicle exhaust gases, without increasing the quantity of materials used or dimensions or the weight, and for which manufacturing conditions (ease of use and cost) starting from brazing sheets are at least equivalent to solutions according to prior art.
The purpose of the invention is an assembly of two brazing sheets, the first being composed of a AA3xxx clad on one face with on a AA1xxx alloy, and the second sheet being composed of an AA3xxx aluminum alloy clad on both faces with an AA4xxx alloy, the two sheets being assembled to each other by brazing, so as to form a closed channel or tube with an internal turbulator, inside which exhaust gases flow, particularly exhaust gases from an automobile vehicle, alone or in association with another fluid, typically air, the face of the channel exposed to these gases or to this mix being the face coated with the 1xxx alloy in the first sheet forming the channel, the second forming the internal turbulator.
According to one preferred embodiment, the second brazing sheet is composed of an aluminum alloy with the following composition (% by weight):
Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5, other elements <0.05 each and 0.15 total, the remainder being aluminum,
clad on its two faces with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, said two sheets being assembled to each other by flux-free brazing under a protective atmosphere.
According to an even more preferred embodiment, the second sheet forming the turbulator is composed of an aluminum alloy with the following composition (% by weight): Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum,
clad on its two faces with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, said two sheets always being assembled together by fluxless brazing under a protective atmosphere.
Furthermore, the first brazing sheet forming a channel or a tube, may be provided with fins or separators on its outside face, themselves made from an aluminum alloy core sheet with the following composition (% by weight):
Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum, clad on its two faces with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, and assembled on the channel or tube by fluxless brazing under a protective atmosphere.
Preferably, said fins or separators are made from a core sheet made from aluminum alloy with composition (% by weight):
Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum, clad on its two faces with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, and assembled on the channel or tube by fluxless brazing under a protective atmosphere.
According to another variant, the brazing sheet forming the channel or the tube is coated on the outside face of said channel or tube with a cladding layer made from an AA4xxx alloy and fitted with fins or separators on this coat, assembled by brazing.
Furthermore, the channel or tube may be made from a core sheet made from an aluminum alloy with the following composition (% by weight):
Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum, clad on its outside face with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, and fitted with fins or separators on this layer assembled by fluxless brazing under a protective atmosphere.
Also preferably, the channel or tube may be made from a core sheet made from an aluminum alloy with the following composition (% by weight):
Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum, clad on its outside face with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall, and fitted with fins or separators on this layer assembled by fluxless brazing under a protective atmosphere.
Finally, these brazing sheets may advantageously be used in the composition of turbulator tubes in a heat exchanger, particularly of the type known under the term EGRC (Exhaust Gas Recirculation Cooler) in which only the exhaust gases from an automobile vehicle flow, or of the type known under the term CAC (Charged Air Cooler) in an EGR (Exhaust Gas Recirculation) loop, inside which a mix of fresh air and exhaust gases from an automobile vehicle flows.
The invention also applies to this type of heat exchanger tube in which exhaust gases flow, particularly automobile vehicle exhaust gases, alone or associated with another fluid, typically air, made from said brazing sheets and also applies to a heat exchanger comprising at least one tube of this type.
The tube core is referenced as mark 3, the outside coating made from an AA4xxx alloy is referenced as mark 4, the inside coating also made from an AA4xxx alloy is referenced as mark 1, and the intermediate coating made from an AA1xxx or AA7xxx alloy is referenced as mark 2.
The invention consists of selecting the most appropriate aluminum alloys forming the brazing sheets used to make channels or tubes of a heat exchanger with a turbulator, more particularly adapted to exchanger tubes subjected to a severe corrosive environment particularly such as tubes through which exhaust gases from an automobile vehicle recirculate.
The turbulator placed inside the tube will be designed precisely to disturb internal fluid circulation and to increase heat exchange with the outside medium.
This turbulator typically consists of a sheet corrugated along its length similar to an exchanger fin or separator, at least within the scope of the invention, this embodiment being well known to those skilled in the art, as mentioned in the “State of the art” section.
Still for elements considered within the scope of the invention, both the tube and the disrupter are made by bending and assembly of brazing sheets, using usual methods known to those skilled in the art.
The turbulator is assembled inside the tube by brazing. The brazing sheet forming the turbulator is made from a core sheet made from an AA3xxx alloy, coated on its two faces with a cladding layer made from a AA4xxx brazing alloy, for this purpose.
The tube itself is composed of another brazing sheet consisting of a core sheet made from an AA3xxx alloy, and the invention consists particularly of forming a lining by cladding on the tube inside face, by selecting an AA1xx alloy for this purpose.
According to the invention, this tube may be coated on its outside face with a cladding layer made from an AA4xxx alloy to enable brazing of uncoated fins, also called separators, on said outside face of the tube or possibly brazing of the tube on itself.
The different alloy clads making up the brazing sheets may typically be made by co-rolling, a frequently used method known to those skilled in the art.
Patent EP 1687456 B1 issued by the applicant discloses a brazing sheet composition consisting of a core sheet made from an AA3xxx alloy coated on one or both faces with an AA4xxx brazing alloy making fluxless brazing possible under a controlled atmosphere (also called a protective atmosphere), typically of nitrogen and/or argon, in a furnace used in a standard manner for use of the Nocolok® process.
This brazing sheet is composed of a core sheet made from an aluminum alloy with the following composition (% by weight):
Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum,
coated on at least one face with an aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall.
An even more preferred composition for the core alloy is as follows (% by weight): Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, the remainder being aluminum, and the coating(s) remaining unchanged.
According to two preferred embodiments of the invention, this first type of sheet and more advantageously the second type coated on both faces, is selected for making the turbulator, that can thus be assembled by fluxless brazing under a protective atmosphere inside the tube, thus resulting in a considerable cost saving and preventing any risk related to the possible entrainment of residual flux into the circuit.
Similarly these two alloy compositions in the same coating configuration on two faces, can advantageously be used for making fins or separators and their assembly by fluxless brazing on the uncoated, outside face of the tube or channel, in other words directly on the core AA3xxx alloy of the tube, in contact with the AA4xxx alloy of the fins according to the above-mentioned preferred compositions.
Obviously, still according to the invention, it would also be possible to coat the outside of the tube or channel with a cladding layer of AA4xxx brazing alloy, so that clad or unclad standard fins or separators could be assembled to it by brazing.
Furthermore, the two types of sheets with the above-mentioned compositions may advantageously be used for making the tube or channel itself, always coated on the inside face with a cladding layer made from an AA1xxx alloy and on its outside face with an AA4xxx brazing alloy enabling fluxless brazing of standard fins under a protective atmosphere.
Obviously, the invention is very useful particularly for heat exchangers comprising this type of tube and more particularly heat exchangers known to those skilled in the art under the name EGRC (Exhaust Gas Recirculation Cooler), in which only the exhaust gases from an automobile vehicle flow or of the type known as Charged Air Cooler (CAC) in an EGR (Exhaust Gas Recirculation) loop inside which a mix of fresh air and automobile vehicle exhaust gas mix flows, in both cases particularly corrosive media.
Details of the invention will be better understood after reading the following examples that are in no way limitative.
Several sheets of 3916 core alloy and AA4045 brazing alloy were cast with AA1050 alloy sheets and one sheet of each alloy 3920 and 4945.
The 3916 alloy had the following composition (% by weight):
Si: 0.18 Fe: 0.15 Cu: 0.65 Mn: 1.35 Ti: 0.08 other elements <0.05 each and 0.15 total, remainder being aluminum,
The 3920 alloy had the following composition (% by weight):
Si: 0.5 Fe: 0.15 Cu: 0.5 Mn: 1.65 Mg: 0.5 Ti: 0.08 other elements <0.05 each and 0.15 total, remainder being aluminum.
The 4945 alloy is an AA4045 alloy that also contains 0.15% of Bi.
Assemblies were made from these sheets to obtain the cladding percentages (% total thickness) indicated in table 1 below, at the end of the transformation.
These assemblies were hot and then cold rolled to produce 0.40 mm thick clad strips for the tubes and 0.20 mm thick clad strips for the turbulators. A 2 h restoration treatment at 280° C. was then applied to these strips after a temperature rise at a rate of 45° C./h.
Tube mockups were fitted with turbulators brazed using the Nocolok® process for references 1 to 3 in table 1 and fluxless brazed for reference 4, with a temperature increase phase up to 600° C. at a rate of about 40° C./min, constant temperature for 2 min at 600° C. and then a temperature reduction at a rate of about 50° C./min, all under nitrogen with a flow of 8 l/min.
The tested configurations are shown in table 1 below.
Configuration 1 applies to a simple tube made from a 3916 core alloy coated on its two faces with an AA4045 brazing alloy and a turbulator made from a AA3003 alloy.
Configuration 2 is identical except for an additional and intermediate layer of AA1050 alloy between the core and the AA4045 internal brazing alloy.
Configuration 3 according to the invention uses a tube made from a 3916 core alloy coated on the outside with an AA4045 brazing alloy and on the inside with an AA1050 brazing alloy and a turbulator made from a 3916 core alloy coated on both sides with an AA4045 brazing alloy.
Brazing for these three cases was done using the Nocolok® process with flux.
Configuration 4 according to the invention uses a tube made from a 3916 core alloy coated on the outside with an AA4045 brazing alloy and on the inside with an AA1050 alloy and a turbulator made from an AA3920 core alloy coated on both faces with a 4945 brazing alloy (containing 0.15% of Bi) in accordance with the above-mentioned preferred compositions.
In this case, brazing was done fluxless under a protective atmosphere in accordance with the patent deposited by the above-mentioned applicant.
The resistance of tubes to corrosion was tested by means of a corrosion test alternating immersion and emersion in order to reproduce the conditions applied to CAC (Charged Air Cooler) type exchangers in an EGR (Exhaust Gas Recirculation) low pressure loop. The detailed test conditions used are described in tables 2 and 3 below.
In practice, the test cycle consists of two repetitions of steps 1, 2 and 3 and then three repetitions of steps 4, 5 and 6 and the complete cycle is repeated four thousand times.
Sulfate ions were introduced in the form of sulfuric acid, nitrate ions in the form of acetic acid and chloride ions in the form of hydrochloric acid.
For each tested configuration, the resistance to corrosion was evaluated by metallographic observation on a cross-section of the tube and of the turbulator.
The results obtained from characterizations made on samples after 300 h are summarized in table 4 below.
The tubes in configurations 1 and 2 have severe corrosion that can result in perforations and a significant deterioration of some brazing joints between the tube and the turbulator. The unclad turbulators associated with this type of tube are only slightly corroded.
On the other hand, configurations 3 and 4 according to the invention show very little tube corrosion, often limited to cladding. Corrosion of brazing joints is also limited. Turbulators associated with this type of tube are significantly corroded. However this situation is considered to be significantly better.
A single perforation on a tube causes a leak in the complete heat exchanger circuit. A leak on a turbulator causes a reduction in the heat exchange quality of the exchanger.
Therefore it is easy to understand that it is more important to maintain the integrity of the tube than the integrity of the turbulator.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1101559 | May 2011 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2012/000177 | 5/3/2012 | WO | 00 | 2/5/2014 |
