heat treating of brazed assemblies

Heat treating is always a part of aluminum brazing process. At the braze temperature the raw material loses all temper and is brought to the “O” condition. This is the Solution heat treating stage and called such due to the alloying elements of the material being dispersed throughout the material in a solid solution. With 6061 alloy this occurs above 529C (985F). If allowed to cool very slowly it will remain in this soft state. With the heat treatable alloys of 6061, 6063, and 6951 a rapid cooling once removed from the braze bath accomplishes the first stage of the heat treat process called quenching. At this point the assembly is in an unstable condition designated “AQ” or “as quenched”. Over time at room temperature the constituent elements of the alloy will precipitate to the grain boundaries. In the case of 6061 alloy the magnesium and silicon in the alloy form magnesium silicide and this is the primary element creating the strength. Just after this quenching the assembly is soft and can be mechanically straightened to correct for distortions occurring during the quenching. After 96 hours at room temperature the assembly will achieve a T4 condition which is called natural aging. In place of natural aging a low temperature process called artificial aging can be employed to obtain T6 properties. This artificial aging, also called precipitation hardening, is one of two profiles: 8 hours at 177C (350F), or 18 hours at 160C (320F). Both profiles accomplish the same end result and the choice will largely depend on how the customer defines the heat treating requirements.

Heat treating of complex brazed chassis, enclosures, and heat exchangers requiring full T6 mechanical properties, and specifying tight dimensional requirements can be a more difficult challenge than the brazing. The distortions caused by a water immersion quench are almost always greater than can be mechanically straightened without destroying the assembly. Our experience covers a broad range of assembly types and heat treating requirements and we can help you design the optimum quench chamber using a combinations of air, fine water spray, and/or spray fog to achieve the highest mechanical properties with the least distortion and meet the requirements of AMS 2770.

There are times when the customer specifies a combination of mechanical properties, final dimensional requirements, and part configuration that are just not achievable even with the best quenching, straightening, and final machining practices. This is when it is necessary to negotiate with the customer engineering for relief in some or all areas. Success in this effort requires a great depth of experience to help the customer understand from an engineering and process stand point why what they are asking is not possible. We have that experience to support you through this process.