Grain refinement of AZ91D magnesium alloy by Al–Ti–B master alloy and its effect on mechanical properties

Abstract

The effects of grain-refining parameters on grain size of AZ91D alloy have been investigated using an Al–Ti–B master alloy as refiner and an adequate refining technique has been developed. Simultaneously, the ultimate tensile strength (UTS) and hardness of the as-cast, solutionized and solution-aged alloys have also been examined at room temperature. The results indicate that the Al–Ti–B master alloy is an effective grain refiner for AZ91D alloy. Increasing pouring temperature or cooling rate from holding temperature to pouring temperature is beneficial for obtaining small grains. But for addition amount, holding temperature or holding time, there is an optimal value. The grain size can be decreased from 422 μm to 79 μm after being refined according to the obtained technique. Accordingly, both the UTS and hardness are obviously enhanced. Solution treatment can further improve the UTS but decrease the hardness while solution-age treatment leads these two properties to increase. Compared with the un-refined as-cast alloy, the UTS and hardness of the solution-aged alloy are increased by 71% and 53% respectively. Correspondingly, the fracture mode during tensile testing changes with the treatments. These changes in mechanical properties and fracture mode are attributed to the different contributions of dissolution or precipitation of β phase to grain bonding strength, solution strengthening and precipitation strengthening. In addition, the microstructure sensitivity of the alloy refined by this refiner to casting thickness is quite low compared with that of the alloy refined by SiC particles.