Fig. 2 shows the microstructure of HPDC AM60B magnesium
alloy at different slow shot speeds. The microstructure comprised
ESCs, α-Mg grains, porosity (gas pore and shrinkage pore), defect
band and β-phase. At the surface of the specimen, the microstructure
was dense and few porosity presented, whereas in the
center, the microstructure was coarse and many large ESCs and
porosity existed. The defect band comprising clustering of porosity
was observed to exist at all cases, i.e. with different slow shot
speeds. In addition, a lower slow shot speed would lead to larger
and more dendritic ESCs, and the porosity in defect band and in
center became larger and more complex. This could be attributed
to the fact that a lower slow shot speed would lead to more time
for ESCs to grow, and consequently, the large and complex ESCs
existed, which led to the large and complex porosity