The porosity that is commonly associated with discrete droplet processes, such as plasma spraying and spray deposition, effectively degrades the quality of the sprayed material. In the present study, solidification and micropore formation during the deformation and interaction of molten droplets impinging into a flat substrate in plasma spray processes have been investigated numerically. The numerical simulation has been accomplished on the basis of the full Navier-Stokes equations and the volume of fluid (VOF) function by using a two-domain method for the thermal field and solidification problem and a two-phase flow continuum model for the flow problem with a growing solid layer. A new phenomenon during the impingement of droplets is found to be responsible for the formation of micropores in the solidified layer. On the basis of this mechanism, some fundamental trends and effects of important processing parameters on microporosity may be reasonably explained, and optimal processing conditions for reducing microporosity may be determined.