A global shift towards reducing carbon (CO2) emissions in the automotive industry while increasing fuel efficiency and range security has triggered the exploration of new processing routes and material alternatives for automotive components. To achieve such goals, manufacturing processes such as high vacuum high pressure die casting (HV-HPDC) have gained attention in recent years to fabricate cast Al alloys for structural automotive components. HV-HPDC allows for increased and more economical production as compared to other manufacturing methods due to the minimal steps involved in the process. Higher degrees of tolerance and precision can be upheld with HV-HPDC, ceasing the need for secondary operations to form the component into desired complex shapes. In this research, the effect of pre-solidified grains (PSG) and heightened metal residence time on the microstructure and mechanical properties were investigated in a new heat-treatable casting alloy, (Al-1.1wt%Fe-4.7wt%Zn-0.95wt%Mg)-0.07wt%Ti, also known as Nemalloy HE700 alloy, manufactured via HV-HPDC. Developed at McMaster University in conjunction with Nemak USA/CAN and CanmetMATERIALS, Nemalloy HE700 alloy is intended for structural automotive applications with its higher strength and increased light weighting capabilities. Nemalloy HE700 serves as a suitable candidate to replace existing Al-Si alloys such as Aural-5 (Al-8wt%Si-Mg-Mn), currently used in the market today. As-cast test plate castings adhering to two geometries: a 3-step plate geometry (nominal plate thicknesses of 3 mm, 2.5 mm, and 2.3 mm) and a singular plate (2.5 mm) with increasing shot delay intervals of 3 additional seconds to a total of 10 seconds from normal operating conditions (i.e., 1, 4, 7, and 10 seconds) were fabricated with the intention of increasing PSG content within the final cast components to study the underlying effects. Experimental efforts through metallography revealed that, much like traditional high pressure die cast (HPDC) components, PSG gravitated toward the centers of the castings in all operating conditions with heightened agglomerations and potential abnormal grain growth in higher delay samples. Moreover, distributions of PSG became more dispersed through the cross-sections as the delay time was increased. Size distributions of PSG adhered to a standard characteristic grain of 100 µm to sizes of 1000+ µm. Larger sizes of PSG grew substantially in equivalent circular diameter (ECD) and extent in higher delay interval samples. Affected area percentage as a result of an increase in PSG content uncovered higher degrees of porosity presenting themselves as shrinkage and gas porosities in the microstructure. A rise in gas porosity size and quantity was realized with higher delay intervals. Uniaxial mechanical testing of tensile specimens from both geometries indicated a directional relationship of PSG where samples were increasingly more brittle and demonstrated adverse mechanical properties when testing was performed parallel to the metal flow direction as opposed to when performed perpendicularly. Moreover, Nemalloy HE700 alloy exhibited a lower propensity of formation of PSG than Aural-5 in higher levels of shot delay times, primarily due to compositional and differing solidification behaviours of the two alloys. The research presented characterizes the nature of PSG formation in HV-HPDC Al alloys with increased metal residence time and the resultant adverse effects on performance. As efforts shift toward manufacturing structural Al components using HV-HPDC, a greater understanding of such effects will aid in alloy development, die mould design, and disseminate information on HV-HPDC to produce components of heightened quality. Additionally, the resultant findings aim to address gaps in current literature as automotive manufacturers transition from non-structural HPDC components to structural HV-HPDC products for commercial use. / Thesis / Master of Applied Science (MASc)