Hypereutectic Al-Si based alloys with a thixotropic microstructure produced by ultrasonic treatment

The present investigation attempts to evaluate the effect of an ultrasonic treatment on the microstructure of hypereutectic AI-SI based alloys. In conventional casting solidified at a moderate cooling rate, the primary silicon crystallizes in the form of hexagonal plates joined together at the centre into star-shaped particles, as they appear in cross-section. During the ultrasonic treatment most of the silicon plates were disconnected and broken, forming spheroidized crystals. The ultrasonic treatment results in an increase of the plasticity and strength of the alloys. To study the thixotropic behaviour, ultrasonically treated and non-treated specimens were upset in an electrohydraulic press under semi-solid conditions. The investigation confirms great advantages of ultrasonically treated ingots of Al-Si based alloys upon deformation in the semi-solid state. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.