BRAZING & SOLDERING TODAY Fig. 10 — Wide band heating element. Fig. 11 — New vacuum aluminum brazing furnace. Vacuum Aluminum Brazing Furnace Characteristics Vacuum Pumping System As noted previously, one of the key process parameters in vacuum aluminum brazing is a deep vacuum level and adequate pumping throughput to keep up with the significant outgassing that takes place during the heating cycle due to magnesium vaporization. Typical vacuum aluminum brazing furnaces have large diffusion pumps and backing pumps to accomplish these requirements. Figure 9 shows typical pumping arrangements for these furnaces. The pumping capacity required for a given aluminum brazing furnace is dependent on the load, specifically the load surface area being brazed. The larger the load surface area, the larger the pumping capacity required. Due to the fact that most of the magnesium vaporization occurs in the 10–4 to 10–5 torr range, it is the diffusion pump(s) that must handle the gas load during the mag burst with adequate backing pumps. To facilitate the vacuum pumping in the furnace, the cooling jacket around the vacuum vessel is run at higher than ambient temperatures. This warm wall design helps prevent condensation of water vapor inside the vessel when the 54 FEBRUARY 2014 door is open for loading/unloading. Water vapor is the enemy of aluminum brazing. It both slows pumping speed and breaks down, releasing oxygen into the furnace. The warm wall design has also proven to lessen the bonding strength of the magnesium oxide that forms during the brazing process and ultimately condenses on the inner wall of the chamber, making it easier to mechanically clean. A vacuum aluminum brazing furnace must maintain a low leakup rate to avoid outside atmosphere from entering the furnace Fig. 12 — Four-year-old vacuum aluminum brazing during the brazing cycle. The quality of the vacuum contains a low PPM of oxygen throughout the brazing cycle. Good design practices for vacuum chambers that have a low leak-up rate typically include minimal use of pipe thread joints, use of a 63 microfinish or better on sealing surfaces for O-rings, and use of the correct O-ring material for the temperature of the sealing area. Heating Elements Other important process parameters are precise temperature control and temperature uniformity. Placing the sensing junctions of the thermocouples near the heating elements results in faster and more accurate control of the process parameters. furnace (normal). Exceptional temperature uniformity ± 5°F (3°C) is accomplished by utilizing many heating control zones arranged within the hot zone. The wideband design provides a substantial radiating surface to the process parts, which facilitates faster heating and better temperature uniformities. Batch-type vacuum aluminum brazing furnaces that contain 10 to 20 individually controlled heating zones are common. The heating element surface area as a percentage to load surface area is important. The larger the surface area of the heating elements, the lower the watt density is on that surface, resulting in element temperatures that are only slightly above the load temperature at steady-state soaking conditions. This is important to ensure the outside surface
Welding Journal | February 2014
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