Automatic Conductivity Scanning of Rolled Aluminum Plates for Aerospace Applications

The use of eddy current techniques to measure electrical conductivity has long been a major QA requirement for rolled aluminum plates used in aerospace applications. The main reason for conductivity testing is to detect “soft spots” that may occur as a result of improper quenching after heat treatment or macro-segregation. Most specifications require spot measurements, on both sides of the plate, in a specific grid e.g. 100 by 100 mm. Typically a hand held device can be used to record the results in a data sheet. This technique can be practical for a small production environment but it is labor intensive and limiting for manufacturing mills where larger plates are produced. The idea of automating conductivity measurements on aluminum plates to produce a conductivity map (C-scan) has been entertained by a few rolling mills in the past two decades as a process control tool. However, this has resulted in only limited application due to extreme sensitivity of eddy currents to liftoff (distance between the sensor and the plate surface) which become a major factor with a plate scanning system. The authors describe a unique measurement system in which conductivity values are dynamically corrected for liftoff while both sides of the plate are being scanned simultaneously. In addition to performing required testing procedures to meet aerospace specifications, the system can also produce more detailed conductivity maps making it a useful tool to analyze trends of conductivity variation related to heat treatment e.g. quenching nozzles, as well as casting issues such as micro segregation

References
1. Blitz J. “Electrical, Magnetic, and Visual Methods of Testing Materials” Vol. 2 “Eddy Current Methods” Butterworth, 1969. 2. ASTM E1004-09 “Standard Test Method for Determining Electrical Conductivity Using the Electromagnetic (Eddy Current) Method” 3. MIL-STD-1537C “Test method Standard for Electrical Conductivity test for Verification of Heat Treatment of Aluminum Alloys Eddy Current Method”, June 25 2002
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