Heat exchange – With coated – roughened or polished surface
Reexamination Certificate
1999-12-14
2001-05-22
Flanigan, Allen (Department: 3743)
Heat exchange
With coated, roughened or polished surface
C228S183000, C428S654000
Reexamination Certificate
active
06234243
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an improved heat exchanger for automotive vehicles manufactured by controlled atmosphere brazing (“CAB”).
BACKGROUND ART
Many automotive vehicles include heat exchangers such as condensers, evaporators, heater cores and coolers generally made of aluminum or aluminum alloys. These heat exchangers are alternating rows of tubes or plates. The heat exchangers often include convoluted fins brazed to the external surfaces of the tubes and turbulators disposed within the tubes and brazed to their inner surfaces. One way of brazing the fins and turbulators to the tube surfaces is by a vacuum furnace. Also, a process known as “controlled atmosphere brazing” (CAB) has been employed.
CAB furnace brazing typically is preferred over vacuum furnace brazing due to improved production yields, lower furnace maintenance requirements and greater braze process robustness. When aluminum components are exposed to air, the surface layer oxidizes and forms aluminum oxide. Although heat exchangers are pre-cleaned using alkaline cleaning agents which reduce the native aluminum oxide layer, the surface of the heat exchanger will re-oxidize in the CAB furnace due to the presence of the oxygen and water vapor in the nitrogen gas used in the furnace. In order to braze aluminum components together, a flux is provided at a joint between the tube and any component to be joined thereto in order to disrupt any aluminum oxide which might interfere with the formation of a sound joint. A flux commonly used in CAB furnace brazing is NOCOLOK™ (potassium fluoaluminate represented often as “KALF”).
Magnesium is commonly included in aluminum based tubing or core materials to improve their strength and corrosion resistance. Magnesium is also generally included in the aluminum alloy cladding generally disposed on the core materials. U.S. Pat. No. 5,422,191, issued Jun. 6, 1995, to Childree discloses aluminum cladding materials which include lithium in addition to magnesium to increase the post braze strength of the brazed joint. Childree teaches that for CAB processing, NOCOLOK™ flux can be used.
U.S. Pat. No. 5,771,962, issued Jun. 30, 1998, to Evans et al. which is incorporated herein by reference discloses that the use of a standard KALF flux works less than desirable with core and clad materials which contain desirably high levels of magnesium. Evans et al. teaches a modified aluminum brazing flux including lithium fluoride, cesium fluoride or their mixture into an aluminum flux like NOCOLOK™. Evans teaches that because the lithium and cesium in the flux have relatively low melting temperatures compared to magnesium, the lithium and cesium will melt first and flow into the joint area before the magnesium forming a sound braze joint.
As disclosed in Evans et al., the magnesium melts during processing and flows into the joint area. At high processing temperatures, magnesium readily forms magnesium oxides which are not broken down by conventional aluminum fluxes such as KALF and hence this oxide and the aluminum oxides present on the aluminum surfaces interfere with the integrity of the brazed joint. Such interferences occur by reducing the “wetability” of the molten clad layer and its ability to form an effective braze joint. Additionally, because a conventional KALF flux is not effective in CAB brazing for disrupting the complex MgO and Al
2
O
3
surface oxide, if and when wetting does occur, the braze joint is discontinuous and does not represent a sound braze joint. The end result of using a conventional KALF flux is a heat exchanger with porous and weak braze joints.
The heat exchanger assembly disclosed in Evans et al. has an aluminum based alloy cladding disposed on the core materials. The cladding may include a weight percentage of lithium (Li) along with other elements. The inclusion of the lithium in the clad material acts to lessen the magnesium from migrating out of the core material, creating a barrier to lessen the formation of undesirable magnesium oxides which interfere with the formation of a sound braze joint.
Although the cladding disclosed in Evans et al. provides a magnesium barrier, it is desirable to reduce the amount of lithium used. The less lithium needed to establish a sufficient magnesium barrier, the less aluminum is required, thereby reducing the weight of the cladding and, in turn, reducing costs.
Thus, what is needed is a more efficient magnesium barrier within a heat exchanger assembly.
Although the core material used in Evans et al. is strong, it is desirable to use the strongest core material as is economically feasible. The stronger the core material used, the lighter the heat exchange assembly will be, thereby reducing manufacturing cost and increasing fuel efficiency.
Thus, what is needed is a stronger core material of a heat exchanger assembly to lessen the weight thereof.
DISCLOSURE OF THE INVENTION
Accordingly, it is an object of the present invention to provide a more efficient magnesium barrier within an improved heat exchanger assembly.
It is a further object of the present invention to provide a stronger tubing or core material of a heat exchanger assembly to lessen the weight thereof.
A more specific object of this invention is an improved heat exchanger assembly. The improved heat exchanger assembly includes at least one aluminum based tube including magnesium adapted to be exposed to an ambient atmosphere and having an internal surface and an external surface, an aluminum based cladding disposed adjacent one of the internal and external surfaces, and at least one aluminum based component disposed adjacent the cladding. A brazing flux is applicable between the aluminum based cladding and the aluminum based component to facilitate brazing in the ambient atmosphere. An aluminum based lithium enriched layer is disposed between the one of the internal and external surfaces and the aluminum based cladding. The aluminum based lithium enriched layer is adapted to scavenge sufficient oxygen from the ambient atmosphere to form a sufficient barrier against the diffusion of magnesium from the aluminum based tube that the formation of undesirable magnesium oxide is resisted.
Another specific object of this invention is a method of manufacturing an improved heat exchanger assembly for an automotive vehicle. The method includes providing at least one aluminum based tube including magnesium adapted to be exposed to an ambient atmosphere and having an internal surface and an external surface, applying an aluminum based lithium enriched layer to at least one of the internal and external surfaces, and applying an aluminum based cladding to the aluminum based lithium enriched layer. Then, the method further includes disposing at least one aluminum based component adjacent the cladding, and applying a brazing flux to a joint between the cladding and the at least one aluminum based component to facilitate brazing in the ambient atmosphere. Finally, the method includes joining the at least one tube and the at least one aluminum based component together in the ambient atmosphere using a controlled atmosphere brazing process.
REFERENCES:
patent: 3891400 (1975-06-01), Robinson
patent: 4209059 (1980-06-01), Anthony et al.
patent: 5069980 (1991-12-01), Namba et al.
patent: 5316863 (1994-05-01), Johnson et al.
patent: 5422191 (1995-06-01), Childree
patent: 5771962 (1998-06-01), Evans et al.
patent: 6120848 (1998-11-01), Van Evans et al.
Evans Timothy Van
Murching Nagendra Narayana
Flanigan Allen
Shelton Larry I.
Visteon Global Technologies Inc.
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