Grain size prediction and investigation of 7055 aluminum alloy inoculated by Al–5Ti–1B master alloy

The effect of Al–5Ti–1B master alloy addition level on the grain size of 7055 aluminum alloy was studied by the standard T-golf method and the free growth model. The features of the commercial Al–5Ti–1B master alloy were analyzed by X-ray diffraction (XRD), optical microscope (OM) and scanning electron microscope (SEM). The size distribution of TiB2 particles in the Al–5Ti–1B master alloy is well fitted by a log-normal function. The cooling curve of 7055 aluminum alloy is measured by a thermocouple. As the amount of Al–5Ti–1B master alloy increases, the grain size of the 7055 aluminum alloy decreases rapidly first and then becomes slowly. When the refiner content exceeds 0.5 wt%, the grain size tends to be stable, and the refinement limit is about 40 μm. The grain sizes were calculated by the free growth model with input of cooling rate, solute concentration, addition level of Al–5Ti–1B master alloy and size distribution of TiB2 particles. The prediction results are basically consistent with the experimental results. The fractions of active TiB2 particles are less than 5% at all addition levels and the efficiency progressively decreases with addition level. The grain size distribution is progressively uniform with the increase of Al–5Ti–1B master alloy addition. As the grain size of the 7055 aluminum alloy decreases, the second phases are diffusely distributed on the grain boundaries. So the area fraction of coarse second phases is reduced, and the defects such as porosities are also reduced.