An investigation of droplet behavior during spray-based manufacturing processes.

摘要

Spray forming is a state-of-the-art, droplet-based technology currently being used to manufacture advanced materials that exhibit improved physical behavior. It provides a unique approach for achieving near-net-shape control of products with a combination of superior properties that are otherwise unachievable with equilibrium materials. The present investigation was carried out to study droplet-related phenomena in spray forming using advanced diagnostic methods (phase Doppler and light diffraction techniques) and conventional methods (mechanical sieving). The phase Doppler interferometry (PDI) can provide an in-depth understanding of fluid flow phenomena of metal sprays, in-situ, including spatial and temporal droplet size and distribution, droplet velocity and distribution which are of interest in spray forming. Accordingly, an in-depth understanding of such phenomena allows us to compare experimental observations by PDI with predictions generated by modeling simulations thereby leading to model verification. In the present study, PDI was first applied to evaluate the currently available linear nozzles using water as a model media. Subsequently, PDI was applied to evaluate the currently available linear and circular nozzles for achieving optimum performance and further enhancing nozzle design concepts. Single point, as well as line measurements of the droplet size and velocity were conducted in the present study. Comparing the velocity profiles along the X direction (parallel to the nozzle slit) and Y direction (Perpendicular to the nozzle slit) indicated that the droplet velocity along the X direction has a more uniform distribution. It is worth noting that the droplet velocity shows a symmetric distribution along both directions around the centerline. However, the results of the droplet size distribution demonstrate that along the X direction the droplet diameter decreases from the centerline toward the edges of the spray, whereas along the Y direction, the droplet size shows a minimal value in the center of the spray. Comparison of the Sauter mean diameter obtained by PDI, laser diffraction and mechanical sieving showed a reasonable agreement.; PDI was also used for a preliminary characterization of the performance of a circular atomizer. The point measurements of droplet size and velocity at different axial locations and gas pressures showed that the droplet velocity exhibits the maximum around a droplet diameter of 45 $m$m, and that the droplet velocity at low atomizing gas pressure decays faster with axial distance, compared to high gas pressure.; Co-injection of fine SiC particles for the synthesis of MMCs was used in the present work to illustrate the importance of droplet behavior during MMC processing. The reinforcing particle distribution and microstructural characteristics of SiC_p reinforced Al based MMCs were analyzed in the current study using TEM, SEM and optical microscopy, aiming at exploring the potential superplastic forming behavior.; In the present study, influence of powder particle size, as well as cooling rate, which primarily depends on the relative velocity between atomizing gas and droplet, on microstructural features of the atomized Al-Fe powders were also investigated. Microstructures were characterized using SEM, XRD and optical microscopy to understand the relationship between solidification behavior, microstructure and operating parameters.