<p>The present study investigates the effect of various semi-solid processing routes on the structure–property relationship of 339 aluminum alloy. Semi-solid metal processing was performed through cooling slope experiments utilizing a custom designed in-house setup. Four distinct processing methods were employed: (i) baseline cooling slope setup, (ii) cooling slope with mold vibration applied, (iii) vibrating cooling slope, and (iv) vibrating cooling slope with additional mold vibration. The molten A339 alloy was poured at 585 °C on to a 500-mm long inclined cooling slope channel (vibrating and non-vibrating) set at 45° angle and collected on the mold with and without the application of 30 Hz vibration. The microstructure analysis revealed that the conventional cooling slope method produces rosette morphology of the primary aluminum with an elongated eutectic phase. Incorporating mold vibration altered the structure of the primary aluminum phase, resulting in a smaller grain size rosette with equiaxed morphology. The vibrating cooling slope technique results in the formation of equaixed grains, effectively eliminating the rosette structure and producing a mixture of fine and nearly globular grains. Meanwhile, the experiments conducted with vibrating cooling slope with mold vibration casting demonstrate that the primary aluminum takes on a nearly globular shape, which is more uniformly distributed in the matrix compared to the other three processing methods. The optimal mechanical properties and lower porosity, including a tensile strength of 220 MPa, elongation of 6.10%, and hardness of 98 BHN and 1.9%, are found in the vibrating cooling slope with a mold vibration casting system, surpassing all other processing techniques. These enhancements are attributed to the refined and uniformly distributed microstructure the investigation is further supported by explanations of fracture micro-mechanisms.</p>