Aluminum brazing rods should be heated to a working temperature of 720 to 750 degrees Fahrenheit. This temperature range allows for the rods to be easily used for brazing aluminum joints without distortion, discoloration, or loss of strength. It is important to note that the vacuum levels should be maintained in the range of 10-5 mbar (10-5 Torr) or better during the brazing process.
The brazing of aluminum components typically requires heating the parts to a temperature of 575-590°C (1070-1100°F), depending on the alloy used. Temperature uniformity is critical, with a tolerance of ±5.5°C (±10°F) or better. Multiple-zone temperature-controlled furnaces are commonly used to achieve this uniformity. The cycle times for brazing aluminum can vary depending on factors such as furnace type, part configuration, and part fixturing. Larger parts and dense loads may require longer cycle times.
During aluminum brazing, a soldering alloy with a lower melting point than the base material is used to join components made of the base alloy. The filler metal, in the form of a strip or roll bonded to the base material, is inserted between the elements to be brazed. The filler metal melts at temperatures ranging from 580-620°C (1076-1148°F), while the base metal remains solid. The melted filler metal spreads and fills the cracks between the elements, forming a brazed joint. The joint solidifies during the cooling period.
There are some considerations to keep in mind when brazing aluminum. The solidus temperature of the aluminum alloy should be higher than the minimum brazing temperature of the filler metal used, typically above 600°C (1112°F). Cast aluminum with a solidus temperature around 570°C (1058°F) may not be suitable for brazing. The magnesium content in the alloy is also important, as a magnesium content of over 2% can lead to stable oxide formation on the surface, making brazing difficult or impossible.
Certain aluminum alloys can be brazed, including non-hardenable alloys such as series 1xxx (99% Al), series 3xxx (Al-Mn), and series 5xxx (Al-Mg) with a low level of magnesium.
When brazing aluminum, it is recommended to use the lowest possible brazing temperature within the recommended range. For many braze alloys, the minimum brazing temperature should be at least 25ºC (50ºF) above the liquidus temperature. This is particularly important when using free-flowing braze alloys, brazing large gaps, or brazing thin materials. The time at brazing temperature should be sufficient to ensure that all sections of the part and all parts within the load reach the desired temperature, typically ranging between 5 and 10 minutes.
After the brazing process, it is recommended to cool the load to a temperature at least 25ºC (50ºF) below the solidus temperature of the braze alloy before gas quenching. This allows the molten braze alloy to re-solidify and prevents it from being blown away from the joint during quenching.
The atmosphere within the furnace should be slightly positive when brazing with hydrogen, and the optimal temperature for brazing copper is usually between 1,100 and 1,500℉.
Overall, the temperature and brazing process for aluminum depend on the specific alloy being used and the desired results. It is important to follow the recommendations and guidelines provided by the manufacturer of the brazing rods and the specific alloy being brazed.
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