Browse Publications Technical Papers 2005-01-1687
2005-04-11

CT Inspection of Castings With Improved Calibration 2005-01-1687

Maintaining the high quality level of today's cars requires inspection by a variety of non-destructive testing methods. Specialized techniques are needed for highly stressed parts such as cast aluminum cylinder heads. Computerized Tomography (CT) is one of the best testing methods for checking complex areas such as combustion chambers, inlet/exhaust runners, and coolant passages. CT inspection simultaneously detects in-homogeneities, pores, shrinkages, and cracks while acquiring complete 3D dimension information on all internal and external surfaces. This adaptation of well-proven medical technology is now maturing to applications in the industrial environment and achieving harmonization with other production tools such as coordinate measuring machines and CAD software packages.
An important goal in today's fast paced design environment is rapid feedback to engineers from first article inspection and process control activities. CT provides accurate repeatable external and internal measurements or complete geometry capture without physical sectioning.
However, the reliability of CT systems' results must be verifiable against acceptable well defined standards and practices. Many factors in the use of industrial CT systems affect measurement accuracy without being well understood. One well-known phenomenon is the circular artifact problem caused by variations in the response of the system's detector array according to time, temperature, material, path length, or x-ray energy. A well-designed system mitigates all of these factors under control of the designer. The one variable that is always an unknown quantity by definition is the path length through the part being inspected.
In this paper, we present the results of a new calibration technique that pre-measures and calibrates the detector response for path length variations prior to inspection of all series and prototype parts (cylinder heads, manifolds) in the Technology Centre of Volkswagen Foundry at Hannover. The level of ring artifacts is significantly reduced in images produced with the new calibration technique. As a result, flaw detection, measurement accuracy and repeatability are improved.

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