One of the first documented cases of tamping or hammering in Cold Spray coatings was by Li who showed that impacting particles cause densification of the upper layers of the coating and that there is distinguishable regions within the coating: the porous upper layer, and the densified lower layer. The effect of cold spray parameters (in particular deposition efficiency) on the prominence has now started to be investigated by researchers. There have been a number of studies which have shown that tamping can increase coating density and hardness; however, these studies have also not investigated in detail the effect of tamping with respect to cold spray parameters, and the detail of mechanical properties. Tamping was also briefly discussed in the work of Hao et al. where in one case the authors found that increasing gas heating temperature from 300 to 400 °C caused the porosity of an Al alloy coating to increase from 2 to 12%. It was hypothesised that the increase in DE caused a reduction in the tamping effect although a thorough explanation was not provided.

Conventional understanding of the relation between particle velocity and deposit strength in cold spray is that higher particle velocity leads to higher deposit strength. However, in this work, we found that the density and tensile strength of cold sprayed Al deposits were significantly improved under low particle velocity before and after heat treatment. The reason for this phenomenon is the enhanced plastic deformation and interparticle bond caused by the hammer effect of rebound particles under low velocity and deposition efficiency. This result for the first time demonstrated that lower particle velocity may benefit to the deposit properties.