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<p>This study investigates the influence of sodium additions on the size, morphology, radial segregation, and hardness profile in centrifugally cast Al–15 wt% Mg<sub>2</sub>Si cylindrical composites. Under centrifugal acceleration, particle migration follows Stokes’ law in fluid mechanics, yet its interplay with chemical modification during solidification remains poorly understood. In the unmodified (Na-free) alloy, coarse hopper-shaped Mg<sub>2</sub>Si particles (~ 50 µm) migrated centripetally, forming a particle-enriched eutectic band near the inner wall, whereas the outer radial zone was depleted of primary particles and exhibited a marked hardness decline from ~ 95 to ~ 62 HB. To regulate this segregation, sodium was introduced as NaF flux at 0.1 and 0.4 wt% in composition of the melt. At 0.1 wt% Na, the primary Mg<sub>2</sub>Si particles were refined to ~ 27 µm with a transition toward well-defined polyhedral morphologies, accompanied by a fine rod-to-fibrous eutectic matrix. The segregation band broadened to ~ 10 mm (vs. ~ 8 mm in the base alloy), yielding a smoother hardness gradient (~ 105 HB in inner wall and ~73 HB in outer one). Conversely, 0.4 wt% Na caused partial coarsening (~ 37 µm cubic Mg<sub>2</sub>Si), deteriorated eutectic refinement, and slightly reduced outer hardness to about 70 HB. ICP–OES revealed nearly uniform Na distribution across radial layers, supporting the facet-poisoning mechanism of Mg<sub>2</sub>Si modification rather than heterogeneous nucleation. Overall, an addition of 0.1 wt% Na provides the optimum balance between Mg<sub>2</sub>Si refinement and segregation control, producing a functionally graded Al–Mg<sub>2</sub>Si composite with a gradual distribution of Mg<sub>2</sub>Si particles and mechanical transition along the radial direction.</p>

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Effect of Sodium Additions on the Morphology and Functionally Graded Microstructure of Centrifugally Cast Al–15 wt%Mg2Si Composites

  • 期刊论文

  • 发表日期：

    2026年5月11日

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  • Ebrahim Aghazadeh,
  • Ahad Samadi

摘要

This study investigates the influence of sodium additions on the size, morphology, radial segregation, and hardness profile in centrifugally cast Al–15 wt% Mg2Si cylindrical composites. Under centrifugal acceleration, particle migration follows Stokes’ law in fluid mechanics, yet its interplay with chemical modification during solidification remains poorly understood. In the unmodified (Na-free) alloy, coarse hopper-shaped Mg2Si particles (~ 50 µm) migrated centripetally, forming a particle-enriched eutectic band near the inner wall, whereas the outer radial zone was depleted of primary particles and exhibited a marked hardness decline from ~ 95 to ~ 62 HB. To regulate this segregation, sodium was introduced as NaF flux at 0.1 and 0.4 wt% in composition of the melt. At 0.1 wt% Na, the primary Mg2Si particles were refined to ~ 27 µm with a transition toward well-defined polyhedral morphologies, accompanied by a fine rod-to-fibrous eutectic matrix. The segregation band broadened to ~ 10 mm (vs. ~ 8 mm in the base alloy), yielding a smoother hardness gradient (~ 105 HB in inner wall and ~73 HB in outer one). Conversely, 0.4 wt% Na caused partial coarsening (~ 37 µm cubic Mg2Si), deteriorated eutectic refinement, and slightly reduced outer hardness to about 70 HB. ICP–OES revealed nearly uniform Na distribution across radial layers, supporting the facet-poisoning mechanism of Mg2Si modification rather than heterogeneous nucleation. Overall, an addition of 0.1 wt% Na provides the optimum balance between Mg2Si refinement and segregation control, producing a functionally graded Al–Mg2Si composite with a gradual distribution of Mg2Si particles and mechanical transition along the radial direction.