Grain Refinement and Improvement of Solidification Defects in Direct-Chill Cast Billets of A4032 Alloy by Melt Condition
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irect chill (DC) casting of Al alloys, a grain-refined as-cast microstructure is generally desirable. McCartney has defined grain refinement as deliberate suppression of columnar grain growth in castings and formation of a fine equiaxed solidification structure throughout the material.[1] Effective grain refinement brings many direct and indirect benefits, including the possibility of faster production of DC castings. During DC casting, grain refinement directly affects the formation of porosity, the tendency of hot tearing, the pattern of macro-segregation, and the scale of micro-segregation. Grain refinement also influences the mechanical
HU-TIAN LI, JAYESH B. PATEL, and ZHONGYUN FAN are with BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, U.K. Contact e-mails: [email protected], [email protected] PIZHI ZHAO is with Chinalco Materials Application Research Institute Co., Ltd., Beijing 100082, P.R. China. RONGDONG YANG and XIANGFU CHEN are with the Southwest Aluminum (Group) Co., CHINALCO, Chongqing 401326, P.R. China. Manuscript submitted December 12, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
properties of solidified material, the kinetics of homogenization treatment, the formability and defect formation during downstream thermomechanical processing, and anodizing quality.[1–3] Al-Ti-B master alloys are commonly used as grain refiners during DC casting of wrought Al alloys in the industry. Nevertheless, less than 1 pct of the added TiB2 particles (nuclei particles in Al-Ti-B master alloys) are active for nucleation of the a-Al phase.[2] On the other hand, the presence of solute is not always beneficial to grain refinement; the solute can interact with TiB2 particles rendering them ineffective during the nucleation process. This is the so-called poisoning effect on grain refinement.[4–9] In Al-Si cast alloys, when the Si content exceeds ~2 wt pct, a poisoning effect is observed and it becomes worse with increased Si content. When Si content is greater than 3 wt pct, the grain refining efficiency of commercial Al-Ti-B master alloys is poor.[4–9] With the increase in billet size and resulting increase in solidification time, equiaxed grain settling becomes more important in determining the columnar to equiaxed transition (CET) and, therefore, grain refinement. This makes the achieving grain refinement even more challenging. The other issue is the nonuniform microstructure that forms across the cross section of the billet. The structure that forms in
different zones of the billet depends on their chemical composition and local cooling rate. The local cooling rate (dT/dt) varies from 0.4 to 1 K/s (C/s) in the center of the billet to around 10 to 20 K/s (C/s) in the surface zone.[10] In industrial practice, even with the grain refiner (Al-Ti-B master alloy) or modifier (Al-Sr master alloy) additions, direct-chill cast billets of near eutectic Al-Si alloys, such as A4032 alloy, display severe nonuniform microstructures consisting of a coarse dendritic primary a-Al phase and coarse blocks and acicular/flake-like par
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