Fundamental mechanisms of microstructural evolution during spray forming.

摘要

Spray forming consists of two physical stages: atomization and deposition. In principle, the spray forming process inherently avoids the extreme thermal excursions, extensive macrosegregation and concomitant degradation in mechanical properties, normally associated with ingot metallurgy. This synthesis approach also eliminates the need to handle fine reactive metallic powders as is necessary with powder metallurgy. In addition, spray forming can be readily used to synthesize particulate reinforced metal matrix composites through different approaches. Presently, spray forming is used for the production of a wide variety of alloys and materials such as Al, Mg, Fe and Ni-based alloys and metal matrix composites, as well as intermetallics. Significant technical progress over the past three decades has helped spray forming to mature into a manufacturing technique. However, review of the published literature reveals that, despite numerous commercial success stories, the fundamentals of spray forming, in many cases, remain to be established. The present dissertation is organized as follows. First, computational fluid dynamic techniques are implemented to analyze the gas flow behavior in a typical atomization configuration with nitrogen as the modeled gas. Second, a numerical approach is implemented to analyze the heat transfer, nucleation and growth of individual droplets during both flight and deposition, and the calculated results, along with the microstructural observation, are used to interpret the formation of the heterogeneous grain morphology in the initially deposited material and the generation of the equiaxed grain morphology beyond a critical deposited thickness. Third, a numerical model is established to describe the thermal environment of a collection of individual droplets thereby predicting the formation of a mushy layer under different processing parameters. Fourth, the aforementioned model is used to investigate the cooling processes of individual droplets and their deposited material, and to identify the factors controlling the microstructure and properties of the deposited material. Fifth, three different-spacing layered MMCs are synthesized by co-injection to investigate the relationship between the tensile properties and fracture toughness of the MMCs and the layered geometrical structure, and to gain insight into optimizing the layered geometrical arrangement for further improved mechanical properties.