Successive deposition of uniform metal droplet is a new kind of 3D printing and rapid prototyping (RP) technology. This paper presents a systematic numerical investigation of the transient transport phenomenon during the overlap of successive aluminum droplets impinging onto a substrate surface. The physical mechanisms of the overlap process, including the bulk liquid, capillarity effects at the liquid-solid interface, heat transfer, and solidification, are identified and quantified numerically. The 3D models based on a volume of fluid (VOF) method were developed to investigate the successive deposition of molten metal droplets on a horizontally fixed aluminum substrate surface. The numerical models are validated with experiments. The comparison between numerical simulations and experimental findings shows a good agreement. The effects of various parameters such as impact velocity, substrate temperature, droplet diameters on the maximum spread factor during impacting and spreading with solidification of a molten droplet onto an aluminum surface under different parameters was studied. Based on the above research, a semi-quantitative relationship between external morphology and internal microstructure was proposed, which was further certified by investigating the piled overlap of successive droplets. This investigation is essential to implement effective process control in metal micro-droplet deposition manufacture.