STUDY OF DENDRITE COHERENCY IN AL-SI ALLOYS DURING EQUIAXED DENDRITICSOLIDIFICATION

Dendrite coherency (or impingement) is important for the formation of solidification structures. The effects of silicon concentration and mo dification of eutectic silicon crystals on dendrite coherency in Al-Si alloys have been studied by continuous torque measurement in solidify ing samples. The fraction solid at the dendritic coherency point in th e alloys decreases with increasing silicon concentration, and varies f rom 0.09 to 0.39 when the silicon concentration in the alloy is in the range 0.5 to 11 mass%. Modification of eutectic silicon crystals may increase the coherency fraction solid somewhat, but not much. When a g rain refiner and a modifier are added together the coherency fraction solid will increase greatly. A theoretical approach is proposed to des cribe how these factors affect the dendrite coherency in the alloy.