Specifications for Aerospace Aluminum Brazing (AMS 2750E and Nadcap)

AMS 2750E is a pyrometry specification that focuses on continual improvement in the automotive and aerospace industries by employing procedures, calibration data, timelines, record archiving, thermocouple guidelines and applications, System Accuracy Testing (SAT) and Temperature Uniformity Surveys (TUS).

Nadcap provides a systematic approach of checkpoints to ensure the achievement of required control and reproducibility of a specific process for which approval is being sought. A Nadcap-accredited organization generally meets the requirements of AMS 2750E, AWS C3.7M/C3.2005 and AMS 2769B.

Nadcap Accreditation for Heat Treatment Processes

Most heat-treating companies will look for approval pertaining to their actual processes for aerospace applications. Customers often seek Nadcap approval on:

  • AC7102/1 for brazing
  • AC7102/2 for aluminum

With possible additional compliance to:

  • An American National Standard
  • AWS C3.7M/C3.7:2005
  • AMS 2759
  • PRI Introduction to Pyrometry
  • AMS 2769A
  • AWS C3.7M/C3.2005

However, specifications for aerospace primes are often more rigorous than those needed for AMS 2750E and Nadcap certification. Having adequate knowledge about AMS 2750E and Nadcap is of high importance for heat-treating companies. When attempting to comply with AMS 2750E, one must consider their process, the configuration of their equipment and the classification of their equipment. These guidelines are outlined in the AMS 2750E standard to ensure validation, calibration and corroboration of the thermal processing instrument's ability to demonstrate process reproducibility.

Process and Equipment Configuration and Classification

The first step in complying with the AMS 2750E standard is gaining knowledge about the process, the components being processed and the specifications that have to be met for that specific process.

The next step in the compliance process is selecting an appropriate vacuum heat treatment system that meet's the required specifications of the components being processed.

Furnace classification is another factor affecting compliance. An internal view of an aluminum vacuum brazing furnace is depicted in Figure 1. Class and instrumentation type are the two categories required by the AMS 2750E standard for furnaces. Furnace class is determined by the temperature uniformity range; whereas, furnace instrumentation type is determined by the number, function and position of the sensors in the furnace.

Vacuum aluminum brazing furnace (Ipsen R&D)

Figure 1. Vacuum aluminum brazing furnace (Ipsen R&D)

Furnaces are classified into six types. Class One is the highest class rating and Class Six is the lowest class rating. The following factors need to be considered in the selection of furnaces for aerospace aluminum brazing:

  • Materials, geometry and weight of the components to be processed
  • Brazing furnace adjusted for core and/or flat plate brazing processes
  • Double-ended opening door systems (ease of cleaning)
  • Cold-wall temperature regulation (water vapor)
  • Temperature zone offset abilities
  • Temperature uniformity regulation and zones of control
  • Furnace vacuum levels during braze
  • Furnace pumping systems and time needed to evacuate
  • Control of retained water vapor from stamping to brazing
  • Slow pumping bypass event to negate core shifting
  • Optional part (in furnace) cooling systems
  • Geographical location of end user’s facility

AMS 2750E Vacuum Specifications

According to many field inspectors, if the process uses nitrogen or argon gas for partial pressure in excess of one micron, it is necessary to have the vacuum gauge (Figure 2) as part of the calibration and accreditation of the vacuum furnace systems.

 Vacuum gauge

Figure 2. Vacuum gauge

SAT and TUS

Many heat-treating companies are not able to clear Nadcap audits due to a lack of knowledge and understanding of the basic requirements for the AMS 2750E standard. They need to maintain the historical records and have work instructions in place, according to the guidelines. They must also prove process repeatability and the availability of trained operators before approaching Nadcap accreditation.

Figures 3, 4, 5 and 6 depict TUS fixtures. A SAT is a test carried out to validate that an accurate temperature reading is being given by the thermocouples (Figure 7) and instruments.

TUS fixture (Ipsen R&D)

Figure 3. TUS fixture (Ipsen R&D)

TUS fixture in furnace (Ipsen R&D)

Figure 4. TUS fixture in furnace (Ipsen R&D)

Vacuum aluminum brazing furnace (Ipsen R&D)

Figure 5. Vacuum aluminum brazing furnace (Ipsen R&D)

Vacuum aluminum brazing furnace (Ipsen R&D)

Figure 6. Vacuum aluminum brazing furnace (Ipsen R&D)

Types of thermocouples

Figure 7. Types of thermocouples

A TUS is performed using a fixture to precisely place the thermocouples (TC) for validation of the temperature uniformity across the heating zones. When choosing survey TCs, their maximum correction factor needs to be below the required specification for the vacuum furnace.

Cleaning Process for Vacuum Aluminum Brazing Furnace

Mechanical scraping and a combination of air and vacuum cleanup cycles are used to clean vacuum aluminum brazing furnace. In Figure 8, the front and back doors of the aluminum brazing furnaces from Ipsen are open for cleaning. Different levels of magnesium buildup within the aluminum brazing furnace are shown in Figures 9, 10 and 11. Customers can gain knowledge on how to maintain the proper balance of magnesium within the furnace through experience and time.

Vacuum aluminum brazing furnace (Ipsen R&D)

Figure 8. Vacuum aluminum brazing furnace (Ipsen R&D)

New aluminum brazing furnace (Ipsen R&D)

Figure 9. New aluminum brazing furnace (Ipsen R&D)

New aluminum brazing furnace (Ipsen R&D)

Figure 10. New aluminum brazing furnace (Ipsen R&D)

Four-year-old aluminum brazing furnace – Normal (Ipsen R&D)

Figure 11. Four-year-old aluminum brazing furnace – Normal (Ipsen R&D)

Conclusion

Aerospace aluminum brazing continues to evolve and fractionate into various aerospace niches. Moreover, there is an increasing demand for system throughput improvements, reduced brazing costs and enhanced furnace capabilities to process intricate component geometries. This necessitates compliance with more stringent guidelines and procedures. In that regard, gaining knowledge about the specifications and prerequisites outlined by AMS 2750E and Nadcap as it pertains to the processes of end users will facilitate the accreditation process.

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This information has been sourced, reviewed and adapted from materials provided by Ipsen.

For more information on this source, please visit Ipsen.

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