GB2357300A

GB2357300A - Flux for brazing an aluminium heat exchanger based on fluorides

Info

Publication number: GB2357300A
Application number: GB0028039A
Authority: GB
Inventors: Gerald Adam Grab; Matthew John Zaluzec; Timothy Van Evans; Wendy L Hale
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1999-12-14
Filing date: 2000-11-17
Publication date: 2001-06-20
Also published as: GB0028039D0; KR20010062366A; DE10044454A1

Abstract

A flux for CAB brazing an aluminium heat exchanger includes caesium fluoride, lithium fluoride and potassium aluminium fluoride. The caesium fluoride is present in a range of approximately 0.1 wt.% to approximately 14 wt.% and the lithium fluoride is present in a range of approximately 0.5 wt.% to approximately 15 wt.% and the potassium aluminium fluoride being a balance thereof. A method of brazing an aluminium heat exchanger is also disclosed. As illustrated the flux is used for brazing an aluminium heat exchanger.

Description

2357300 FLUX FOR CAB BRAZING ALUMINIUM HEAT EXCHANGERS The present

invention relates generally to heat exchangers and, more specifically, to a flux for CAB brazing an aluminium heat exchanger for a motor vehicle.

It is known to provide motor vehicles with heat exchangers such as condensers, evaporators, heater cores and coolers. These heat exchangers have alternating rows of tubes or plates with convoluted fins made of a metal material such as aluminium or an aluminium alloy.

Typically, the heat exchangers are pre-assembled and brazed together. Previously, the heat exchangers have been brazed in a vacuum furnace. Recently, a process known as "controlled atmosphere (CABP, furnace brazing has been used is with non-corrosive fluxes. CAB furnace brazing has been preferred over vacuum furnace brazing due to improved production yields, lower furnace maintenance requirements and greater braze process robustness.

It is also known that the CAB furnace brazing currently used to manufacture aluminium heat exchangers requires the use of fluxing agents, either chloride based or fluoride based, applied to the assembled heat exchanger prior to entering the CAB braze furnace. Preferably, the flux is a potassium aluminium fluoride flux also known as Nocolok.

The use of fluxing agents with conventional aluminium heat exchangers promotes the dissociation and disruption of the native aluminium oxide and magnesium oxide layers present on the surface of the aluminium heat exchanger to promote wetting of the molten clad layer between mating components and provides a robust braze joint. Flux application is accomplished by applying the flux to pre-assembled aluminium heat exchangers by suspending the flux in water, applying the flux to the exterior of the heat exchanger, and then thoroughly drying the heat exchanger in an oven to remove excess water.

The mechanism for reducing the native aluminium oxide is well established for the Nocolok' flux braze process. When 2 low magnesium content (>0.25 wt.% Mg) braze sheets are used to fabricate aluminium heat exchangers, Nocolok can effectively reduce the oxide and promote clad flow and braze filleting. Higher magnesium content aluminium braze sheets do not have the same surface oxide, instead, they have a more complex oxide containing magnesium. The presence of magnesium not only changes the composition of the oxide layer to that of a complex aluminium/magnesium spinel, but it is well known that magnesium poison's the Nocolok flux, rendering it inactive as an oxide reducing agent. To counter act these effects, the addition of LiF to Nocolok effectively increases the chemical activity of the flux alloying the complex oxide to be reduced. However, when LiF is added to the Nocolok flux, it raises the melting point of the flux above 5950C which is above the melting point of a clad layer on the aluminium braze sheet.

Although the above Nocolok flux has worked in brazing aluminium heat exchangers, it suffers from the disadvantage that the flux works effectively on only low magnesium content aluminium braze sheets. As a result, this flux does not work on higher strength aluminium braze sheet alloys containing greater than 0.25 wt.% magnesium as an alloying element.

Also, the flux suffers from the disadvantage that it is not environmentally friendly. Therefore, there is a need in the art to provide an environmentally friendly flux that can effectively braze aluminium heat exchangers containing a high magnesium content during CAB brazing.

Accordingly, the present invention is a flux for CAB brazing an aluminium heat exchanger including caesium fluoride, lithium fluoride and potassium aluminium fluoride.

The caesium fluoride is present in a range of approximately 0.1 wt.% to approximately 14 wtA and the lithium fluoride is present in a range of approximately 0.5 wt.% to approximately wt.% and the potassium aluminium fluoride is a balance thereof.

One advantage of the present invention is that a flux is provided for CAB brazLng aluminium heat exchangers for a motor vehicle that has a composition containing caesium fluoride, lithium fluoride and potassium aluminium fluoride to promote oxide layer breakdown. Another advantage of the present invention is that the flux is environmentally friendly. Yet another advantage of the present invention is that the flux can effectively braze aluminium heat exchangers fabricated from aluminium braze sheet containing a high magnesium content (<0.25 wt.%). A further advantage of the present invention is that the flux is based on a controlled composition of fluoride additions that effectively reduce the complex aluminium/magnesium surface oxides (also known as spinels) on the surface of the aluminium braze sheet, thus allowing for a robust leak free braze.

FIG. 1 is a perspective view of a heat exchanger; and is FIG. 2 is an enlarged perspective view of a tube- manifold assembly brazed by a flux, according to the present invention, for the heat exchanger of FIG. 1.

Referring to the drawing and in particular FIG. 1, one embodiment of a heat exchanger 10, according to the present invention, is shown. In this example, the heat exchanger 10 is a condenser for an air conditioning system (not shown) of a vehicle such as a motor vehicle (not shown). It should be appreciated that the heat exchanger 10 may be a parallel flow condenser, serpentine evaporator, heater core, or transmission oil cooler.

The heat exchanger 10 includes a pair of generally vertical, parallel manifolds 12 (only one shown) spaced apart a predetermined distance. The heat exchanger 10 also includes a plurality of generally parallel, flat tubes 14 extending between the manifolds 12 and conducting fluid such as a refrigerant between them. The heat exchanger 10 further includes a fluid tube 16 for directing the fluid into the heat exchanger 10 formed in one manifold 12 and a fluid tube (not shown) for directing the fluid out of the heat exchanger 10 formed in the other manifold. The heat exchanger 10 also includes a plurality of convoluted or serpentine fins 18 disposed between the tubes 14 and 4 attached to an exterior of each of the tubes 14. The fins 18 serve as a means for conducting heat away from the tubes 14 while providing additional surface area for convective heat transfer by air flowing over the heat exchanger 10. It should be appreciated that the heat exchanger 10 could be used as a heat exchanger in other applications besides motor vehicles.

Referring to FIG. 2, a tube-to-manifold assembly, generally indicated at 20, for the heat exchanger 10 is shown. The tube-to-manifold assembly 20 is a sub-component affixed to the heat exchanger 10. The tube-to-manifold assembly 20 includes a first component being the fluid tube 16 and a second component being the manifold 12. The fluid tube 16 and manifold 12 are made of an aluminium based l=- material. The term "aluminium based" as used herein is meant that the aluminium based composition comprises mostly aluminium, but may be alloyed with other metals like silicon, copper, magnesium, zinc and so forth. The aluminium based material is preferably selected from the Aluminium Association 1XXX, 3XXX, 5XXX and 6XXX series aluminium alloys, preferably being 6XXX aluminium alloy. The aluminium based material may and desirably does include magnesium.

Preferably, the aluminium based material includes magnesium in an amount greater than 0.25 wt.%, more preferably between about 0.4 and 2.5 wt.%. It should be appreciated that the heat exchanger 10 may include other sub-components besides the tube-to-manifold assembly 20.

As illustrated in FIG. 2, the tube-to-manifold assembly includes the manifold 12 disposed adjacent the fluid tube 16, which is to be joined by CAB brazing to the fluid tube 16. For example, the manifold 12 has an aperture 21 for accepting the fluid tube 16. The fluid tube 16 has an internal surface 22 and an external surface 24. The internal surface 22 and external surface 24 each have a composition cladding thereon. Filler material 26 is applied to a portion of the external surface 24 of the fluid tube 16, the filler material 26 being made of Aluminium Association 4XXX series aluminium alloy. Preferably, the filler material 26 is a 4047 filler metal. During the CAB brazing process, the filler material 26 melts and flows into the joint between the external surface 24 of the fluid tube 16 and the aperture 21 of the manifold 12 to seal the joint.

To form the precursor tube-to-manifold assembly 20, a flux is applied to the joint between the fluid tube 16 and the manifold 12. The flux can be applied onto the joint area by suitable means such as brushing, dipping, and water based spray or electrostatic spray, the latter being preferred because it provides a more uniform application..

The flux is a mixture of caesium fluoride (CsF), lithium fluoride (LiF) and potassium aluminium fluoride (KalF). The flux contains lithium fluoride (LiF) within a range from is about 0.5 wtA to about 15 wtA, caesium fluoride (CsF) within a range from about 0.1 wtA to about 14 wtA, balance potassium aluminium fluoride (KalF). Preferably, the flux contains lithium fluoride (LiF) about 1 wt.%, caesium fluoride (CsF) about 10 wt.%, balance potassium aluminium fluoride (KalF).

For assembly of the tube-to-manifold assembly 20 for the heat exchanger 10, the manifold 12 is joined to the fluid tube 16 using a CAB furnace brazing process. The precursor tube-to-manifold assembly 20 is applied with the flux in the areas of the joint to be formed. During the CAB process, the flux melts or liquefies at or about 5500C and flows through a porous aluminium oxide (A1203) layer on the external surface 24 to wet the external surface 24. This wetting provides the medium to continue the dispersement of the oxide layer and allows the filler material 26, which has become molten to wet and flow into the joint and create a braze. The brazed tube to-manifold assembly 20 can then be removed from the furnace, cooled, and applied for its intended use. It should be appreciated that the CAB furnace brazing process is conventional and known in the art.

Several different compositions of flux and melting point of the fluxes were obtained from a thermal analyser (not 6 shown). The composition and melting point of the fluxes are as follows:

FLUX COMPOSITION MELTING POINT (OC) 1%LiF 9.3%CsF 558 1'-.LiF 14%CsF 554 10-0.LiF 565 3.3%LiF 2.3%CsF 569 10.3%LiF 4.7%CsF 561 15-0.LiF 587 1% LiF 572 3.3%LiF 9.3%CsF 558 5.7%LiF 567 10.3%LiF 2.3%CsF 562 is 5.7% LiF 4.7%CsF 562 1%LiF 4.7%CsF 566 5.7%LiF 9.3%CsF 559 5.7%LiF 4.7%CsF 562 Nocolok' 554 It should be appreciated that the above flux compositions that include LiF and/or CsF are in wt.% and the balance is KAlF.

Additionally, a method, according to the present invention, of brazing the tube-to-manifold assembly 20 for the heat exchanger 10 is disclosed. The method includes the steps of providing at least one component being the fluid tube 16 having an internal surface 22 and an external surface 24 and applying a flux to the external surface 24. The method may include disposing a second component being the manifold 12 adjacent the external surface 24 and joining the fluid tube 16 and manifold 12 together using a controlled atmosphere brazing (CAB) process.

In the CAB process, the precursor tube-to-manifold assembly 20 for the heat exchanger 10 is placed on a braze holding furnace fixture (not shown) and preheated, for example, to a temperature in a range from about 4250 to about 7 475OF (2240-246OC). The heat exchanger 10 and braze holding furnace fixture are transferred to a prebraze chamber where it is soaked for about 3-15 minutes at about 750OF (3990C).

Subsequently, the hot tube-to-manifold assembly 20 and braze holding furnace fixture are transferred to a conveyor and moved through a CAB furnace. In the CAB furnace, the heat exchanger 10 is kept for 2-3 minutes at about 10950-1130OF (5910-610OC). The brazed tube-to-manifold assembly 20 is then cooled, removed and applied for its intended use.

- 8

Claims

A flux for CAB brazing an aluminium heat exchanger comprising:

caesium fluoride, lithium fluoride and potassium aluminium fluoride; and wherein said caesium fluoride is present in a range of 0.1 wt.-o. to 14 wt.% and said lithium fluoride is present in a range of 0.5 wtA to 15 wt.% and said potassium aluminium fluoride being a balance thereof.
2. A flux as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 9.
3 wt.% and said lithium fluoride is present in an amount about 1 wt.%.

is 3. A flux as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 4.7 wtA and said lithium fluoride is present in an amount about 1 wt.%.
4. A flux as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 14 wtA and said lithium fluoride is present in an amount about 1 wt.%.
5. A flux as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 9.3 wtA and said lithium fluoride is present in an amount about 3.3 wt.%.
6. A flux as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 9.3 wtA and said lithium fluoride is present in an amount about 5.7 wt.%.
7. A heat exchanger comprising:

a first component and a second component, wherein at least one of said first component and said second component is made of an aluminium based material containing magnesium greater than 0.25 wt.%; 9 a flux applied to at least one of said first component and said second component, wherein said flux comprises a mixture of caesium fluoride, lithium fluoride and potassium aluminium fluoride; and wherein said caesium fluoride is present in a range of about 0.1 wt.% to about 14 wt.% and said lithium fluoride is present in a range of about 0.5 wt.% to about 15 wt.% and said potassium aluminium fluoride being a balance thereof.
8. A heat exchanger as claimed in claim 7 including a filler material disposed in a joint between said first component and said second component.
9. A heat exchanger as claimed in claim 8, wherein said filler material is an Aluminium Association 4XXX series aluminium alloy.
10. A heat exchanger as claimed in claim 7, wherein said caesium fluoride is present in an amount of about 9.3 wt.% and said lithium fluoride is present in an amount about 1 wt.%.
11. A heat exchanger as claimed in claim 7, wherein said caesium fluoride is present in an amount of about 14 wtA and said lithium fluoride is present in an amount 1 wt.%.
12. A heat exchanger as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 9.3 wt.% and said lithium fluoride is present in an amount about 3.3 wt.%.
13. A heat exchanger as claimed in claim 1, wherein said caesium fluoride is present in an amount of about 9.3 wt.% and said lithium fluoride is present in an amount about 5.7 wt.-oo.
14. A method of brazing an aluminium heat exchanger, said method comprising the steps of:

providing a first component and a second component, wherein at least one of the first component and the second component is made of an aluminium based material containing magnesium greater than 0.25 wtA7 applying a flux to at least one of the first component and the second component, wherein the flux comprises a mixture of caesium fluoride, lithium fluoride and potassium aluminium fluoride, wherein the caesium fluoride is present in a range of 0.1 wtA to 14 wt.% and the lithium fluoride is present in a range of 0.5 wtA to 15 wtA and the potassium aluminium fluoride being a balance thereof; disposing the first component adjacent the second component; and joining the first component and second component together using a controlled atmosphere brazing (CAB) process.
15. A method as claimed in claim 14 including the step of disposing a filler material in a joint between the first component and the second component.
16. A method as claimed in claim 15, wherein said filler material is 4047 filler metal.
17. A method as claimed in claim 14, wherein said caesium fluoride is present in an amount of about 9.3 wtA and said lithium fluoride is present in an amount about 1 wt.%.
18. A method as claimed in claim 14, wherein said caesium fluoride is present in an amount of about 14 wtA and said lithium fluoride is present in an amount about 1 wt.%.
19. A method as claimed in claim 14, wherein said caesium fluoride is present in an amount of about 9.3 wtA and said lithium fluoride is present in an amount about 3.3 wt.%.
20. A method as claimed in claim 14, wherein said caesium fluoride is present in an amount of about 9.3 wtA and said lithium fluoride is present in an amount about 5.7 wt.%.
21. A flux for CA.B brazing an aluminium heat exchanger lo substantially as hereinbefore described with reference to the accompanying drawings.
22. A method of brazing an aluminium heat exchanger substantially as hereinbefore described with reference to is the accompanying drawings.