Hot-Zone Design Meets Tight Brazing Temperature Requirements
Furnaces used for vacuum aluminum brazing operate at temperatures in the 1200 to 1300 F (650 to 705 C) range and at a vacuum level of 10-5 torr. In addition, because aluminum brazing is very sensitive to temperature uniformity, these furnaces are required to have a very tight temperature uniformity within +/-5 F (+/-3 C) or better. The heat-treating process requires uniform heating to ensure that portions of the workload are not overheated or underheated. In the case of brazing, the term "temperature window" describes the upper and lower allowable temperatures for the process to be accomplished without distortion from overheating and not melting the filler metal because of underheating. The temperature window in certain heat-treating processes can be as narrow as 20 to 30 F (10 to 15 C) throughout the workload. Therefore, controlling the furnace trim zones within a few degrees of the required temperature in that work location ensures performance of the process without rejects. PV/T achieves excellent uniformity control using multiple temperature trim zones (typically 6 to 12 zones), with each zone having its own control system and power transformer.
Various PV/T customers also were looking for furnaces capable of operating at higher temperatures in the range of 2500 to 2600 F (1370 to 1430 C) to vacuum braze stainless steel, nickel, titanium and other alloys. In addition, they wanted multiple trim zones for tight temperature uniformity control and wanted to be able to achieve a clean high vacuum in the range of 10-6 torr to handle their critical applications.
These demands led PV/T to design state-of-the-art high-temperature vacuum furnaces that could provide the required workload heating uniformity, reduce the amount of inert gas used, as well as reduce maintenance costs.
The approach used to meet the customer's demands started with a concept of tuning the heat zones automatically to match the thermal characteristics of the workload, such that the bottom, corners, sides, top and ends could receive different amounts of heat. For example, a rectangular workload resting on a support grid may require more heat at the bottom than at the top and less heat in the corners because the area-to-mass ratio is higher in the corners. Similarly, the ends at the bottom may require more heat input than the ends at the top.
Band heating elements radiate the same amount of heat around the hot zone circumference and do not permit variations from bottom to top or in the corners. Circumferential band heating elements emit the same amount of heat around the workload, requiring the heating cycle time to be extended to allow the bottom to heat up without overheating the corners or top. If the heat input is variable around the circumference, more heat can be delivered in the bottom and less in the top and in the corners, allowing a shorter cycle to be completed without overheating. In other cases, the heat input pattern may be unknown requiring the controls to automatically adjust the heating pattern. To accomplish the desired performance, the new patented proprietary hot zone design is configured with longitudinal pairs of heating elements as illustrated in Fig. 1.
Each opposing horizontal pair of heating elements forms a separate zone, which is automatically controlled to produce the desired temperature in that zone. Heavy thermal mass sections of the workload receive more radiant heat than low thermal mass sections. For example, when heating a large rectangular workload, Z2 and Z5 run at a reduced heat input to prevent the corners from overheating, as sensed by the thermocouples in those zones. Zone Z6 may run hotter to compensate for the work supports and fixtures, which typically are heavier at the bottom. This arrangement including the end zones enables the temperature profile to be adjusted vertically and horizontally for better temperature uniformity and faster heating by matching the workload thermal characteristics.
Another objective of the new hot-zone design is to minimize distortion of the heating elements and radiant heat shields. The shape of the elements and shields shown (Fig. 2) was selected with reduction of maintenance costs as a primary consideration.
Cylindrical heat shields are inherently more stable than flat heat shields at high temperatures. PV/T's "bar" heating-element configuration has more stiffness than a flat element because the bar shape has a higher section modulus. In addition, considering maintenance access and safety, the bar heating element is easy to slide in and out without entering the hot zone. The element power bars are disconnected with access through the rear chamber door and the heating element is slid out through the front door. Should it be necessary to clean the chamber interior, the hot zone is designed to easily roll in or out of the chamber (Fig. 3).
A programmable temperature-modulated, dual-channel flow arrangement is used to achieve controlled cooling to prevent distortion of the workload when using an external inert-gas forced-convection system. The cold inert gas is first circulated around the hot zone exterior to extract a large percentage of the heat stored while the work inside the hot zone cools gradually. The dual-channel cooling system modulates the flow of inert gas as the workload and hot-zone temperature is lowered in a controlled manner to a desirable crossover temperature, gradually reducing the bypass flow and increasing flow directly through the hot zone interior, which eventually becomes 100%. This accelerates cool down of the workload after the critical temperature for distortion is passed. In the case of brazing, the critical temperature is below the filler metal solidification range.
Conserving inert gas
Another advantage of the cooling system design is that it allows the external cooling system to be isolated from the chamber using vacuum-sealed valves (Fig. 4). This arrangement retains the inert gas contained inside the external cooling system when the chamber is opened, saving approximately 20 to 30% of the total inert gas compared with a non-isolated system. During the vacuum cycle, outgassing from the heat exchanger and pressure blower does not occur, because at that time in the cycle, the external cooling system is isolated and vacuum levels in the 10-6 to 10-7 torr range are achieved.
Recent vacuum brazing furnace installations are based on the new design for higher temperature vacuum brazing. Included are two installations, each with a round hot zone furnace having bar heating elements for brazing stainless steel, and a second furnace with rectangular hot zone for aluminum brazing (Fig. 5). The applications are for heat exchanger cores used in the European Airbus and other aircraft. Also shown is a recent rectangular hot zone vacuum brazing furnace with the bar heating elements used in the production of aluminum charge-air coolers for diesel engines (Fig. 6).
After approximately two years of production service, the new, patented hot zones show no signs of distortion and have not required replacement of parts or maintenance (Fig.7). Durability of this hot zone is due to the design and also the selection of materials of construction. For example, molybdenum-lanthanum (ML) oxide-stabilized alloy is used for the heating elements and key heat shields, and TZM alloy (titanium- and zirconium-alloyed molybdenum) is used for the work supports.
Observed temperature uniformity during acceptance testing demonstrated that at an equilibrium temperature of 2460 F (1350 C), the zone-to-zone temperature uniformity was within +/-2 F (+/-1 C), which is the limit of the digital controllers. Vacuum levels in the 10-6 to 10-7 torr range are routinely achievable. The first production workload brazed in each of the four furnaces was acceptable for shipment.