Ultrasonic testing (UT) is an indispensable component of quality inspection for aluminium extrusion billet (logs) and forging stock. Depending on the application, fully automatic linear or helical testing stations are available. They are usually integrated in the production flow, and occasionally installed as stand alone units.

Testing cast billet for centre cracks and cavities has been standard practice in aluminium casthouses for many years. However, portable hand-held UT equipment is commonly used, which means only random checks can be made on a sample of output.

For the production of highly stressed components, particularly for the aircraft and automotive industries, examination of all the billets used is required.

Ultrasonic testing should be perfomed directly after casting to avoid costly further processing of faulty material.

ASTM B594 standards class-A (1,2 mm flat bottom hole reflection) and class-B (2,0 mm flat bottom hole reflection) sensitivity can be typically achieved with the as-cast surface.

 
Ultrasonic Testing

In a linear testing station two test probes, 90° apart, are elastically pressed against the surface of the log, while the log is moved lengthwise through the testing station.The acoustic probe-to-specimen contact is maintained by a steady laminar flow of water, forming a water „cushion“ between probe head and log.

During the linear travel of the log through the testing station, 100% of the center region is examined for cracks and inclusions, but not the surface-near zone of logs. However, as centre cracks and centre cavities are by far the most common material flaws in logs, this method affords a good standard of quality verification.

Plant description: Linear Testing

 

With helical testing, the log is rotated while one or several probes are moved along its length. The log is thus inspected along a helical path. The pitch during testing is adjustable between 4 and 15 mm per revolution, depending on parameters such as probe geometry, received signal to noise ratio, billet surface condition, internal grain structure, desired sensitivity class, etc.

This method allows testing of the entire log except an outermost skin some 5 mm deep. To examine this skin layer requires further probes positioned at an angle to the billet surface (angle beam testing).

The helical testing method requires substantial capital investment and rather longer testing times per billet than linear. It is therefore applied only for particularly meticulous quality testing requirements.

Plant description: Helical Testing


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