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The effect of ultrasonic melt treatment (UST) on the microstructure and mechanical properties of Al-7Si-0.35Mg (A356) casting alloys was investigated. The particular aim of this study was to analyze the mechanism involved in the strengthening of the A356 alloys when fabricated by UST. The UST had little effect on the sizes of the α-Al grain and eutectic Si at a melt temperature of 750 C, and the yield strength of the A356 alloy was increased by UST by approximately 16%. After T6 heat treatment, however, alloys prepared with UST and without UST both had similar levels of yield strength. These results are possibly associated with a change in the type and the volume fraction of intermetallics due to UST. UST greatly reduced the volume fractions of the intermetallics which formed upon solidification, resulting in alloys with predominantly β-Al5FeSi instead of π-Al8FeMg3Si6. However, T6 heat treatment, especially a solid solution treatment at 530 °C for 8 hours, led to the dissolving of intermetallics such as Mg2Si and π-Al8FeMg3Si6 and as a result their volume fractions were further reduced to similar levels in both alloys with, and without UST. (Received May 4, 2016; Accepted October 5, 2016) keywords : hypoeutectic Al-Si alloy, casting, ultrasonic melt treatment, microstructure, mechanical properties 1. A356  Al-Si                       [1-3]. A356    α-Al   Al-Si , Mg2Si, π-Al8FeMg3Si6, β-Al5FeSi     ,       Si ,        [1,2].     Ti   [4], Sr, Na    Si [5],         [6].      Si         ,               [4,5]. ,   (ultrasonic melt treatment, UST)     [7-11],  Si [7-11],  [7,8]          .           ,        dendrite fragmentation  cavitation-induced heterogeneous nucleation    .          (cavitation threshold)       bubble   bubble      .      bubble  ‘--’         . Dendrite fragmentation       dendrite  bubble         acoustic streaming               .         dendrite    /           . Cavitation-induced heterogeneous nucleation          . , bubble     (pressure pulse)           [12,22]. , bubble      Clapeyron   (melting point)   ,              [23].       bubble        bubble           .    acoustic streaming             [12].  Al-Si                 . Puga           α-Al      Si,       [7]      α-Al            [13].    Al-Si      680 C  [17]         ,     /       [7-11,14-16].                     [7-11,14-16].      1   horn         horn              .                            [9,14,15,18].      Al-7Si-0.35Mg ( A356 )         . ,          A356          .  [7-11,14-18]        /       ,                   .

The Effect of Ultrasonic Melt Treatment on the Microstructure and Mechanical Properties of Al-7Si-0.35Mg Casting Alloys