The Effect of Velocity of the Melt Entering the Die Cavity on Fracture Toughness of AZ91 Magnesium Alloy

Magnesium alloys in liquid-phase condition form surface oxide films due to their high rate of oxidation. Oxide films enter the melt as a result of surface turbulence, and remain inside the workpiece after solidification. The incoherent nature of their interface causes decreases in mechanical properties and reliability of castings. The effect of critical velocity and greater velocities of entering the melt into the die cavity on fracture toughness has been studied in this research. Results indicated that an increase in the velocity decreases the fracture toughness, e.g. an increase of the velocity from 0.4 m/s (critical gate velocity of magnesium alloys) to 3.0 m/s decreases the fracture toughness for about 40%. Furthermore, increasing the velocity of melt entering the mold from 0.4 m/s to 1.0 and 3.0 m/s resulted in a decrease in the Weibull modulus from 13.34 to 7.28 and 4.93, respectively, and consequently, caused the results to be more scattered. SEM micrographs showed that increasing the velocity beyond the critical value increases significantly the quantity and size of the oxide films. Moreover, for the case of speeds greater than the critical value, double-layer films are observable due to considerable surface turbulence of the melt. Analysis of the fractured surfaces also showed that the failure mode of specimens produced using velocities greater than the critical value is of brittle type, whereas it was found to be ductile for the case of critical velocity.