Thermo-fluid mechanisms controlling droplet based materials processes

Recently, the successful application of thermal spray deposition techniques to the manufacture of bulk materials and thin film coatings with improved structural properties has illuminated the need for a fundamental understanding of the physical phenomena that control the deposition process. However, the spray parameters have a complex statistical interdependence that renders their analysis difficult. Discrete droplet deposition processes provide phenomenologically simpler systems than thermal spray deposition processes while still retaining their salient features. Furthermore, discrete droplet deposition processes have found important manufacturing applications from soldering to rapid prototyping. Consequently, this review focuses on recent research in the area of droplet based materials processes. The emphasis is on fluid dynamics and thermal aspects as they relate to materials science issues. The design and operation of a number of representative droplet deposition approaches is also discussed in detail, paying special attention to applications in near net shape manufacturing and digital microfabrication. Finally, outstanding questions are identified

Solidification in spray forming

Solidification in spray forming takes place in two distinct steps: typically half of the alloy latent heat is removed rapidly from the droplet spray created by gas atomization; the droplets are then constituted into a billet at deposition where the remaining liquid fraction solidifies relatively slowly. However, within the droplet spray, individual droplets have different thermal and solidification histories and depositing droplets may be solid, mushy, or liquid. Despite many studies of solidification behavior in spray forming, uncertainties and some misconceptions remain on how the solidification conditions in the spray and billet interact to give rise to the characteristic spray-formed microstructure comprising refined, polygonal/equiaxed primary grains with low levels of microsegregation. This article presents a simple numerical model for the spray-formed grain size arising from the deposition of the various droplets in the spray and combines insights provided by the model with previous investigations of the phenomena occurring during and immediately after deposition to propose a comprehensive description of the important solidification behavior during spray forming. Remelting, grain multiplication, thermal and elemental equilibration, and microstructural coarsening are proposed to play a critical role in the evolution of the spray-formed microstructure

Formation of solid splats during thermal spray deposition

International audienceThis paper reviews the findings of recent research on the formation of solid splats by the impact of thermal spray particles on solid substrates. It discusses methods of describing the substrate, by characterizing both chemical (oxide layers) and physical (surface topography, adsorbed and condensed contaminants) aspects. Recent experiments done to observe impact of thermal spray particle are surveyed and techniques used to photograph particle impact and measure cooling rates described. The use of numerical modeling to simulate impact and deformation of impacting particles is appraised. Two different break-up mechanisms are identified: solidification around the edges of splats; and perforations in the interior of thin liquid films created by droplet spreading without solidification. These two modes can be reproduced in numerical models by varying the value of thermal contact resistance between the splat and substrate. A simple criterion to predict the final splat shape is presented