This paper presents a systematic numerical investigation of the transient transport phenomenon during the pileup of molten metal droplets on the substrate. The physical mechanisms of the pileup process, including the bulk liquid, capillarity effects at the liquid-solid interface, heat transfer, and solidification, are identified and quantified numerically. The droplet diameter is 100 μm, and the impact velocities are 1–3 m/s. These conditions correspond to Re = O(100), We = O(1). The initial substrate temperature is 350 K. The initial droplet temperature of aluminum alloy molten droplets is 960 K. The numerical models are validated with experiments. The comparison between numerical simulations and experimental findings shows a good agreement. The effects of impacting velocity and relative distances between two successive molten droplets on the end-shapes of impact regime are examined. This investigation is essential to implement effective process control in metal microdroplet deposition manufacture.