Air conditioning systems used in automobiles and other vehicles conventionally include an evaporator unit into which a cooled liquid refrigerant is sprayed so as to revaporize the refrigerant. The heat of vaporization required for vaporizing the refrigerant is drawn from the incoming outside air, which is blown around the evaporator. So as to maximize the amount of surface area available to the incoming air and, correspondingly, to more efficiently cool and dry that air, the design of the evaporator unit is typically a tubeplate-and-air center type heat exchanger containing flat ribbed tubes, similar to the tube-and-fin type designs.
The evaporator is assembled by stacking and brazing together matching clad aluminum sheet components, referred to as tubeplates. Each tubeplate is formed from an aluminum brazing stock material which has been stamped so as to define an internal passage through the evaporator when properly mated with another tubeplate. The evaporator unit is then formed by stacking and brazing a number of tubeplates together.
Due to the numerous brazements which must be formed, it is most efficient to braze the tubeplates during a single brazing operation. Generally, this is accomplished by forming the tubeplates from an aluminum alloy brazing stock material. A typical brazing stock material consists of, for example, an appropriate aluminum alloy core which has been clad on both sides with an aluminum-based brazing alloy. Typically, the cladding layers are an aluminum-silicon eutectic brazing alloy characterized by a melting point lower than the core aluminum alloy. Therefore, the clad layers of brazing alloy melt during the vacuum brazing operation and flow toward the desired joint regions and, upon cooling, solidify to form the brazements. The core aluminum alloy does not melt during the brazing operation and thereby constitutes the structural part of the tubeplate-and-center type evaporator.
In the past, the aluminum alloy brazing stock material routinely used to form these types of plate-type evaporators has consisted of a core layer of aluminum alloy AA 3005, as designated by the Aluminum Association (AA), which has been clad on both sides by an aluminum-silicon brazing alloy, such as aluminum alloy AA 4047. Generally speaking, this particular aluminum alloy brazing stock material has performed satisfactorily over the years. In particular, the material is easily stamped for formation of the tubeplates, there are no inherent brazing difficulties associated with its use, and the structural integrity of the material is considered to be sufficient.
However, the corrosion resistance of evaporator units formed from this specific material is less than desired. Corrosion is particularly problematic if the tubeplates of the evaporator unit are exposed to long periods of wetness due to the use of a climate control switch within the passenger compartment of the automobile which automatically regulates the temperature of the incoming air. The corrosion problem associated with the AA 3005 alloy appears to be due not only to the presence of the condensate which is particularly aggressive to aluminum, but also the presence of sulfur and chloride compounds within the incoming air.
In the past, the corrosion resistance of an air conditioning evaporator formed from this conventional aluminum brazing stock material would typically be enhanced by coating the evaporator with a protective chromate layer. However, due to increased concerns about the environment, the industry is working to eliminate the use of this protective chromate coating because of the toxic byproducts produced by the process. Therefore, it would be preferable to eliminate the use of the chromate conversion process entirely.
U.S. Pat. No. 5,176,205 to Anthony, assigned to the assignee of this invention, significantly overcomes the corrosion problems associated with prior art aluminum alloy brazing stock materials, such as those which employ the AA 3005 alloy. Specifically, Anthony teaches an improved aluminum alloy brazing stock which contains an aluminum alloy core material characterized by a higher copper concentration and a lower manganese concentration as compared to the conventional aluminum alloy AA 3005. The higher concentration of copper within the aluminum alloy core material reduces the electrode potential differential between the core alloy and the aluminum-silicon brazing alloy, which is clad to the core alloy and which forms the multitude of brazements within the evaporator unit during the brazing operation. As a result, the electrochemical potential difference between the core alloy and the brazing alloy is minimized, such that the galvanic corrosion driving force between the two alloys is reduced.
A particularly advantageous feature of the alloy developed by Anthony is that an air conditioning evaporator unit formed from the alloy is characterized by enhanced corrosion resistance, as compared to the conventional AA 3005 aluminum alloy brazing stock material which has been treated with a protective chromate coating. Further, the strength of the alloy is sufficient to contain a high pressure refrigerant even over extended thermal cycling which is characteristic of an automotive environment.
However, further improvements in corrosion resistance are continuously being sought in the relevant industries to further enhance service life and durability. Such an improvement is disclosed in U.S. Ser. No. 08/052,975 to Rungta et al., filed Apr. 27, 1993, and also assigned to the assignee of this invention. This improvement involves rapidly quenching the evaporator unit immediately after brazing so as to substantially prevent the formation of copper-aluminum precipitates in the grain boundaries of the aluminum alloy core material. In accordance with the teachings of Ser. No. 08/052,975, it was discovered that unexpected and substantial improvements in the resistance of a 3000 series aluminum alloy to intergranular corrosion can be attained by appropriately processing such alloys to keep the relatively low level of copper in these alloys in solid solution, so as to inhibit the precipitation of CuAl.sub.2 particles at the grain boundaries. Greater consistency in corrosion resistance was also attained by this method, so as to minimize the occurrence of premature failures of evaporator units made from these alloys. Finally, it was also determined that evaporator units formed by such processing methods exhibited enhanced mechanical properties, enabling the evaporator units to more readily withstand repeated thermal cycling within a typical automotive environment.
However, the implementation of such a quenching method within a manufacturing process cannot always be readily adopted, in that additional facilities are required to suitably quench the evaporator units as they leave the brazing furnace. Therefore, it would be desirable to provide a method for further enhancing the corrosion resistance of an evaporator unit, so as to survive the aggressive automotive environment, without the requirement to protect the evaporator unit with an additional protective chromate coating and without the requirement for quenching the evaporator unit immediately after the brazing process.