• Increasing mold wall thickness from 2 mm to 5 mm reduces filling speed from 25.41 m/s to 11.07 m/s, leading to wider and less distinct defect bands.
• The ratio of defect band width to grain size (w/d) increases significantly to 24.47 for 5 mm thickness, compared to 7-18 for 2-4 mm, indicating a critical thickness effect.
• Externally solidified crystals (ESCs) diffuse from center to defect band as wall thickness increases, maintaining high shear strength during filling and influencing defect band morphology.
• The 5 mm-thick casting exhibits high porosity (5.25%), highlighting the detrimental effect of thick sections on casting quality.