The manufacture of aluminum heat exchangers such as air conditioner evaporator cores and radiators is a sophisticated technology in that it requires the simultaneous formation of many brazed joints between the aluminum alloy tubes, fins and headers or tanks. In order to supply molten braze alloy at the intended joints, at least some of such components, for example, the tubes, are formed of two or more aluminum alloy layers in a sandwich-type sheet construction. In the sandwich, the principal layer or base layer is an aluminum alloy in sheet form selected for its strength and corrosion resistance requirements. Roll bonded to one or both surfaces of such base layer is a suitable aluminum brazing alloy composition. The aluminum braze alloy composition comprises aluminum with an appreciable amount of silicon such that the composition will melt and flow on the base layer at a brazing temperature below the melting point of the base layer.
In the production of such aluminum heat exchangers, aluminum alloy sheet materials are shaped and assembled together to form the desired part. The assembly must then be heated in a suitable environment to melt the braze alloy layers, cause them to flow and wet underlying and adjacent surfaces and form brazing fillets at many joints at the same time. In some applications, such brazing operations have been carried out in vacuum furnaces. In this practice, magnesium in suitable quantities is incorporated into either the base aluminum alloy composition, the braze alloy composition or both. When the assembled aluminum sheet components are heated in vacuum to a braze temperature, the magnesium vaporizes or sublimes to getter the environment in the vicinity of the molten alloy to facilitate wetting of the aluminum surfaces to be joined. This practice is disclosed, for example, in U.S. Pat. Nos. 3,321,828; 3,322,517; 3,373,482 and 3,373,483.
Where large numbers of aluminum workpieces are to be brazed in a continuous process, it is generally preferable to employ a multi-chamber vacuum brazing furnace. In the operation of such a continuously operating multi-chamber furnace, a plurality of workpieces are sequentially advanced into the inlet of the furnace through the respective isolatable operating chambers and the brazed products removed from the furnace outlet.
A suitable furnace might comprise, for example, three stages or chambers. The furnace may be aligned with the chambers in a straight flow path. The inlet and outlet for each chamber can be closed off with suitable doors so that the temperature, pressure and other environmental parameters therein may be controlled. In a typical flow-through vacuum brazing furnace, a group of assembled heat exchangers is first introduced into a preheat chamber where the absolute pressure is reduced to about 5.times.10.sup.-3 torr and the workpieces are heated by use of radiant heating elements to a temperature of about 800.degree. F. The preheated workpieces are then transported to the next chamber where the braze operation takes place. The pressure is reduced to a level of 10.sup.-5 to 10.sup.-6 torr. The temperature is increased to about 1100.degree. F. such that the braze alloy cladding on surfaces of at least some of the workpiece components melts and flows under capillary action to the joints to be formed. At the high temperatures and low pressures of the braze chamber, magnesium vapor evolves from the alloy to react with residual oxidizing constituents in the chamber that could inhibit wetting of the workpieces by the molten braze alloy and the formation of a braze joint. The still-hot workpieces are then moved into the third chamber which serves as an exiting vestibule. The exit chamber is initially evacuated to a low pressure suitably 5.times.10.sup.-3 torr. As soon as the workpieces have entered the chamber and it has been isolated from the braze chamber, dry air is admitted to effect some cooling of the workpieces and to raise the pressure in the chamber to that of the atmosphere. The outlet door of this chamber is then opened and the workpieces are removed from the furnaces into ambient air for further cooling.
In the prolonged operation of continuous multi-chamber vacuum brazing furnaces of the type described, problems have arisen that were not predictable from batch-type or other multi-chamber brazing operations. Successful continuous operation of these flow-through furnaces depends upon repeatedly obtaining the desired operating conditions in each chamber. Some flow-through furnaces are relatively large, having chambers, for example, that may be 6 feet wide by 10 feet high by 20 feet long. More than 100 automotive air conditioner evaporators may be treated at the same time in each of the three chambers. Magnesium that is evolved from the workpieces in the braze and exit chambers tends to collect as magnesium or magnesium oxide on the furnace wall surfaces. The furnaces must be cleaned from time to time to remove this oxide. In some such brazing lines, continuous operations have been interrupted by the occurrence of excessively high pressures in the exit and braze sections as the workpieces are shuttled into the exit chamber. The high pressures in the braze section overwhelm the vacuum pumps and cause shutdown of the braze line.
It is an object of the present invention to provide an improved practice for the operation of such multi-chamber aluminum vacuum brazing furnaces to prevent such high pressure events in the braze chamber and avoid interruptions of the desired continuous brazing operations. It is another object of the present invention to provide an improvement in the design and construction of the exit chamber of such vacuum brazing furnaces for more efficient removal of accumulated magnesium oxide deposits and for avoiding high pressure impulses that upset operations in the braze chamber of the furnace.