Magnesium alloy AE44-2, with composition Mg-4Al-4RE-0.2Mn in wt% where RE = mixed La + Ce
rare earth metals, is an attractive high pressure die casting (HPDC) alloy for automotive applications. However, microstructure formation during HPDC of this alloy is less well understood than the AM- and AZ-series magnesium alloys. This thesis focuses on the factors affecting the morphology and lengthscale of α-Mg grains, the α-Mg + Al11RE3 eutectic, and the large (Al,Si)2RE, Al2RE, Al10Mn7RE2 and Al8Mn4RE intermetallic phases during the HPDC of AE44. This thesis first investigates shot sleeve pre-solidification and its influence on the microstructure and tensile properties of HPDC. The pre-solidification
is always a critical factor in high pressure die casting (HPDC), which directly influences
the formation of defects and the final mechanical properties. This thesis developed the understanding of the pre-solidification as a step in developing methods to control the formation of pre-solidification, in order to reduce the defect formation and improve the mechanical properties of HPDC Mg alloys. Next,
geometric models are built to explain the morphology of the in-cavity solidified α-Mg. Finally, the influence of growth twinning and orientation relationships (ORs) on the morphology of the intermetallic compounds is explored. Whilst the identities of the blocky intermetallic phases in HPDC AE44 are fairly
well-established, much less is known about their growth crystallography during casting and how this affects the intermetallic particle morphology. Since the size and shape of large intermetallics in castings is known to determine their effects on mechanical performance. This thesis developed the understanding of the growth crystallography of the intermetallic as a step in developing methods to control their size
and shape.