Effect of external ultrasonic treatment on hypereutectic cast aluminium–silicon alloy N. U ¨ nal, H. E. C ¸ amurlu*, S. Koc ¸ak and G. Du ¨ztepe The effect of ultrasonic treatment during casting of a hypereutectic Al–Si alloy (Etial 195) containing 18%Si on the microstructure and some mechanical properties has been investigated. For this purpose, a steel mould was placed inside an ultrasonic bath, and molten aluminium alloy at 740uC was poured into the mould. Ultrasound was applied to the casts from the external side of the mould during solidification. Cooling curves were obtained by measuring and recording the temperature of the cast. The samples were subjected to X-ray diffraction analysis and hardness, wear and impact tests. The microstructure of the samples was examined by optical and scanning electron microscopies and investigated statistically with image analysis software. Hardness values increased from 49?8–52?3 to 57?7–61?6 HRB upon ultrasonic treatment during solidifica- tion. The application of ultrasonic vibration to the cast from the external side of the mould resulted in modification of the microstructure and improvement in mechanical properties. Keywords: Aluminium cast alloy, Hypereutectic alloy, External vibration, Ultrasound, Wear Introduction Al–Si cast alloys are widely used in the aerospace and automotive industries because of their excellent cast- ability, thermal conductivity and corrosion resistance. The most important effect of Si alloying on aluminium is the improvement in castability. The fluidity and mechanical properties of aluminium increase upon alloying with silicon, whereas the density and thermal expansion coefficient decrease. Furthermore, silicon increases the wear resistance considerably. 1–3 Al–Si alloys are widely used in the production of automotive engine parts due to their typical properties, like high strength and lightness. There is a growing interest in hypereutectic Al–Si alloys. A high amount of Si brings about superior mechanical properties; there- fore, these hypereutectic alloys are expected to replace the hypoeutectic alloys that are commercially being used in the production of high quality engine parts. 4 The amount of silicon and the size distribution and morphology of Si particles play a crucial role in the mechanical properties of the cast parts. For example, the tensile strength increases with the reduction in the size of Si particles. The cooling rate in cast Al–Si alloys provides grain refinement of Si particles since Si forms as primary crystals and eutectic product. However, sufficient cooling rate can be achieved to a certain degree, depending on the size of the cast part. 5 Microstructure refinement, minimisation of segrega- tion, modification of formation and distribution of secondary phases have been investigated in various studies by subjecting the molten alloys to ultrasound. In these studies, ultrasonic sources of 0?5–1?5 kW power were utilised at 18–25 kHz frequencies for applying ultrasonic waves directly to the melt in the mould via a water cooled probe. 6–9 It has been demonstrated that the application of ultrasonic vibrations to the solidifying alloys prevents columnar grain formation, refines the equiaxed grains and in some circumstances leads to the formation of non-dendritic, globular grains. 6,7 Alloys having the mentioned microstructure are re- ported to possess both increased ductility and in- creased strength. 8–10 Some mechanisms were proposed for the formation of non-dendritic grains and grain refinement as a result of ultrasonic vibrations. These effects are suggested to be related with the temperature and pressure fluctuations created by cavitations, which form as a result of ultrasonic vibrations. Temperature and pressure fluctua- tions are believed to originate heterogeneous nucleation in the melt and also cause the dendrites to break up. 5 According to these mechanisms, cavitation formed in the melts subjected to ultrasonic treatment creates the suitable conditions for the intensification of the physicochemical activities that take place before solidi- fication. Acoustic cavitation enhances the processes requiring heat and mass transfer, such as diffusion, wetting, solution and dispersion. These processes com- prise the bases for many technologies used for degas- sing, cleaning, purification and solidification of alloys. 8 Campbell has reported that sonic treatment during casting improves the mechanical properties and also the corrosion resistance. 11 On the other hand, some undesired effects, such as the formation of porosity when vibrating in cavitation mode, coarsening of dendritic structures and second phases, etc., have been encountered. 11 Mechanical Engineering Department, Akdeniz University, Antalya 07058, Turkey *Corresponding author, email erdemcamurlu@gmail.com 246 ß 2012 W. S. Maney & Son Ltd. Received 23 January 2011; accepted 2 February 2012 DOI 10.1179/1743133612Y.0000000011 International Journal of Cast Metals Research 2012 VOL 25 NO 4