Macrosegregation

Macrosegregation is a macroscopic compositional inhomogeneity that develops in cast or welded metallic components during solidification and subsequent cooling. It refers to variations in alloying element concentration that span lengths from millimeters to centimeters, and is distinct from microsegregation, which occurs within individual grains on a much smaller scale. Macrosegregation can persist after solidification and, if left untreated, may affect performance.

The primary mechanisms involve solute partitioning at the solid–liquid interface, the existence of a wide freezing range, and convection in the mushy zone. Density differences between liquid and solid, together with temperature gradients, can drive thermo-solutal flows that move liquid rich or poor in solute along channels or across cross-sections. Channel segregation occurs when interdendritic or liquid-filled paths transport solute during solidification, often producing elongated, high- or low-concentration regions. Inadequate feeding of liquid metal to compensate solidification shrinkage can also promote macrosegregation by redistributing solute as the cast cools.

Macrosegregation is commonly observed in large ingots, plates, and welded joints in alloys such as steel, aluminum, copper, and nickel-based systems. It can lead to significant property variation, liquation cracking during welding, reduced corrosion resistance, hot tearing, and difficulties in further processing.

Mitigation strategies include controlling solidification rate and pouring temperature to reduce convection, alloy design to minimize partitioning effects, and thorough homogenization heat treatment to even out solute distribution. Other approaches involve directional solidification or improved feeding efficiency to limit channel formation and long-range diffusion during cooling.

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