Numerical Investigation of Air Entrapment Dynamics for High-Speed Thermal Spraying

Abstract

For thermal spraying, bubble entrapments are highly undesired, as this would lead to pores in the final coating and lower its adhesion quality. This understanding warrants an investigation of the process behind their formation. Nevertheless, the air entrapment process is difficult to study via experimental methods since molten droplets are always opaque and hard to visualize. Most numerical models are focused on air entrapment at the moment of impact, which could only explain the pores observed around the center of the splat. Here, in this paper, the air entrapment of a micron-sized molten nickel droplet impacting on a stainless-steel substrate is numerically studied. The results show that, besides the air entrapped during the high-speed impacting (impacting air bubbles/IM bubbles), bubbles may also be entrapped due to the fallback of the pointed-out finger on the edge during the spreading process (spreading air bubbles/SP bubbles). The number and size of the entrapped SP bubbles are related to the solidification rate and spreading rate. Therefore, both low (50 m/s) and high (200 m/s) impacting speeds could achieve an entrapped bubble ratio that is about 10% lower than that of a medium one (100 m/s). However, the formed coating is thick for low impacting speeds, and the low entrapped bubble ratio is obtained due to the cut-off of the peripherical fingers, which is actually unwanted.