Structure Modification upon Ultrasonic Processing of an AA4032 Piston Alloy: Comparison of Permanent Mold and Direct-Chi
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idely used in engine parts in automotive industry because of their properties, i.e., light weight, thermal stability, good corrosion, and wear resistance. These alloys are usually produced into billet
SUWAREE CHANKITMUNKONG is with the Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-Utid Rd., Bangmod, Tungkhru, Bangkok, 10140 Thailand and also with Brunel University London, BCAST, Uxbridge, Middlesex UB8 3PH, United Kingdom. Contact e-mail: [email protected] DMITRY G. ESKIN is with Brunel University London and also with Tomsk State University, Tomsk 634050, Russian Federation. CHAOWALIT LIMMANEEVICHITR is with the Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi. Manuscript submitted August 26, 2019. Article published online December 9, 2019 818—VOLUME 51A, FEBRUARY 2020
by direct-chill (DC) casting for subsequent processing such as extrusion or forging. The mechanical and service properties of these alloys depend on the presence of hard and thermally stable phase particles such as Si, c-Al7Cu4Ni, d-Al3CuNi, b-phase (Al9Fe2Si2), and Q-Al5Cu2Mg8Si6 phases[1,2] as well as on the phases that form precipitates upon annealing such as Al2Cu and Mg2Si. Higher Cu and Ni concentrations are shown to be beneficial for high-temperature properties.[3,4] On the other hand, most of the particles of solidification origin, primary or eutectic, are rather brittle and, in the case of large size and/or elongated morphology, may act as stress concentrators in the structure, causing premature fracture or overall decrease in ductility and toughness.[5,6] The morphology and size can be controlled by additions of some alloying elements (e.g., Mn for AlFeSi phases)[7] or application of physical treatment (e.g., ultrasonic melt processing (USP))[8] as well as modification of eutectics (e.g., by alkali-earth metals),[9] which positively affects mechanical properties such as ductility and strength at a high temperature. The overall
METALLURGICAL AND MATERIALS TRANSACTIONS A
structure, i.e., primary aluminum grains, can be refined by an addition of AlTiB master alloys,[10] which is not working very well in high-Si alloys, or by USP through a variety of mechanisms reviewed elsewhere.[11–13] It is important that these mechanisms are working at the different stages of solidification, e.g., the activation of heterogenous nuclei—in the liquid state, while the fragmentation of dendrites—below the liquidus.[12] Acoustic streaming flow is another interesting phenomenon that accompanies USP. Acoustic streaming flow can be potentially beneficial for dispersing the solid phase particles and redistributing the melt in the liquid and semi-liquid parts of the billet, affecting the structure uniformity and macrosegregation.[11] USP has significant effects on the primary Si and AlFeSi phases, especially in hypereutectic Al-Si alloys with high Fe concentration. The suggested mechanisms included the acceleration of peritectic r
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