Influence of Grain Refinement on Slurry Formation and Surface Segregation in Semi-solid Al-7Si-0.3Mg Castings
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is commonly used in the aluminium casting industry.[1] Smaller and more globular crystals form in grain-refined aluminium alloys, which results in enhanced feeding, a decreased hot tearing tendency and more uniform dispersion of porosity and intermetallic phases.[2,3] Additionally, improved mechanical properties in the casting, fatigue resistance and increased uniformity of the anodized surface are obtained in grain-refined aluminium alloys.[4] There is divergence in the literature regarding the effects of stirring on the grain refiner effectiveness. Yang et al.[5] concluded that the grain refinement effect of Al-Ti-B additions was lost after electromagnetic stirring of an A356 aluminium alloy. Sharma[6] found a reduction in size and increased globularity of primary a-Al obtained by inoculation of the liquid prior to stirring in the Rapid Slurry Forming (RSF) process. In the New Rheocasting (NRC) process, a superheated liquid alloy is poured into
JORGE SANTOS, ANDERS E.W. JARFORS, and ARNE K. DAHLE are with the Department of Materials and Manufacturing, School of Engineering, Jo¨nko¨ping University, P.O. Box 1026, 55111 Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] LOTHAR H. KALLIEN is with the Department of Applied Sciences, Aalen University, Beethovenstrasse 1, 73430 Aalen, Germany. Manuscript submitted January 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a chill cup to generate copious nucleation of crystals. Easton et al.[7] reported that the addition of a grain refiner did not significantly change the microstructure at low pouring temperatures in the NRC process. A surface segregation layer is a typical feature in High Pressure Die Casting (HPDC),[8] direct-chill casting[9] and, particularly, Semi-Solid Metal (SSM) processing.[10] Surface segregation involves a solute-enriched region at the casting surface, with a distinct microstructure compared with the center of the casting and consequently strongly contributes to heterogeneous properties along the cross-section.[8] Gourlay et al.[11] studied defect bands and surface segregation layer formation in HPDC and suggested, in addition to the migration of the externally solidified crystals to the center, that the origin of the segregation layer is a combined effect of inverse segregation and exudation. In both segregation modes, a solute-enriched liquid flows toward the casting surface through the mushy zone to compensate for solidification shrinkage.[11] Exudation occurs when the partially solidified alloy next to the die wall shrinks because of solidification shrinkage and thermal contraction, and a gap is formed between the solid alloy and die wall. Consequently, a pressure differential is formed between the gap and the interior of the solidifying alloy, and the solute-enriched liquid is forced to flow through inter-crystal channels into the space between the solidifying alloy and die wall[11] and solidifies into an almost fully eutectic microstructure.[8]
In inverse segregation, the solute-enriched liquid that advanced toward the casting surface to compens
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