A Comparison of the Use of Ultrasonic Melt Treatment and Mechanical Vibration in the Manufacture of Al5Si5Zn Alloy Feeds
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TION
THE efficiency of semisolid material (SSM) processing depends on adequate control of the solid–liquid transition and the semisolid slurry microstructure.[1,2] In fact, the ‘‘sensitivity of the liquid fraction, dfL/dT, at the desired liquid fraction, fL, exclusively for the primary phase must be as low as possible’’.[3–6] Solid particles in the primary phase in the eutectic liquid must be refined (maximum grain size 150 lm), homogeneously distributed and highly globular (i.e., as round as possible)
C.T.W. PRONI, G.L. BROLLO, and E.J. ZOQUI are with the Materials and Manufacturing Department, School of Mechanical Engineering, University of Campinas UNICAMP, R. Mendelev, n. 200, Campinas, SP, 13083-860, Brazil. Contact e-mail: [email protected] Manuscript submitted April 17, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
and contain a limited amount of melted eutectic entrapped in their interior.[7,8] When these conditions are met, the semisolid slurry will exhibit the thixotropic rheological behavior required for SSM processing (rheocasting or thixoforming).[1–4] Control of the solid–liquid transition depends almost exclusively upon the chemical composition of the material but is also affected by its morphology, which can be improved by chemical or physical refinement. Many industries use chemical agents for grain refinement and to improve spheroidization. However, only a fraction of the grain refiner truly acts as nucleation sites, and the other fraction remains as impurities in the alloy (mainly TiB2), acting as potential sources of crack initiation under cyclical stress and leading to inferior mechanical performance. To overcome this limitation, the use of physical agents is suggested for grain refinement. Examples include cooling of the ingot mold during casting,[9] mechanical stirring of the ingot during casting[10,11] and application of ultrasonic cavitation in the molten metal before casting.[12,13]
566.6 to 615.2 C, within which the liquid fraction varies from 45 to 100 pct and the liquid-fraction sensitivity is always lower than the recommended value of 0.03 C1.[17] Refining the microstructure of this alloy is, therefore, key to producing suitable feedstock for thixoforming. To evaluate the effects of different physical refinement techniques that could be used to this end, three processing conditions were tested: simple casting in a cooled copper mold (AC), casting under mechanical vibration (MV) and casting after ultrasound melt treatment (UST).
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EXPERIMENTAL PROCEDURE
The alloy was melted in a conventional furnace and poured at 70 C above the liquidus into a cooled copper mold (30 mm D 9 220 mm L). The flow rate of the cooling water was 15 L/min. This procedure corresponds to the AC condition referred to in the previous section and was also used for the other conditions. For the MV condition, the alloy was mechanically vibrated as it solidified in the cooled mold using an eccentric shaft (mechanical hammer) with an amplitude of 0.7 mm, frequency of approximately 800 Hz and acceleration ranging fr
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