Abstract: Molecular dynamics simulations are used to study the dynamics of a single Al nanosphere (singlet) colliding with an aggregate of two Al nanospheres (doublet) with initial I-shaped configuration. Depending on the initial impact velocity, there are four collision outcomes, namely bounce, adhesion, aggregation and melting. At a very low velocity, the repulsive force between the nanospheres will cause the nanospheres to rebound without contact, and the critical velocity of bounce decreases with the increase of the diameter of the nanosphere. As the velocity increases, the nanospheres are sintered together due to adhesion between them and the formation of new bonds. The phase transformation and atomic diffusion during singlet-doublet collisions are quantitatively characterized by Common Neighbor Analysis, Dislocation Analysis and mean square displacement to explore the underlying sintering mechanism. The critical impact velocity of singlet melting is obtained by monitoring the temperature of singlet with different diameters.