DETERMINATION OF DENDRITIC COHERENCY IN SOLIDIFYING MELTS BY THEOLOGICAL MEASUREMENTS

The instant, during equiaxed solidification, when the dendrites start to impinge and form a network is called the dendritic coherency point. The paper presents a method for measuring the dendritic coherency point. The method utilizes the fact that the shear strength of the solidifying material increases sharply at the coherency point. A probe is slowly rotated in the melt during controlled solidification and the torque and temperature are measured. The coherency point can be deduced from the derivative of the torque/temperature curve. Coherency measurements of solidifying aluminium alloys have shown that the coherency point for a given alloy is reproducible for given cooling conditions, but varies greatly with alloying composition, grain refinement and cooling rate. It has been found that the fraction solid at which coherency occurs varies between less than 0.1 for poorly grain refined Al foundry alloys and more than 0.5 for fine grained Al alloys.