Development of Novel Semisolid Powder Processing for Micromanufacturing

Wu, Yufeng
Major Professor
Gap-yong Kim
Committee Member
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Mechanical Engineering
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Mechanical Engineering

Semisolid powder processing (SPP) is a promising approach for near net–shape forming of features in micro/meso–scale. By combining the concept of forming in the semisolid state and conventional powder metallurgy, SPP provides a novel solution to various processing and materials engineering challenges faced in micromanufacturing. Replacing bulk materials with powdered materials adds a new dimension to the traditional semisolid technique by allowing tailoring of material properties. In this dissertation, experimental study to understand flow characteristics of metallic powders in the semisolid state is performed, and its potential application to the fabrication of a functionally graded structure (FGS) is demonstrated.

The viscosity and phase segregation behavior of Al–Si powders in the semisolid state were first studied with back extrusion experiments. Effects of process parameters including shear rate, extrusion ratio, heating time and pre–compaction pressure were analyzed using the design of experiments method. The results showed that the effects of shear rate, extrusion ratio and heating time were statistically significant factors influencing the viscosity. The semisolid state powders showed a shear thinning behavior. Moreover, microstructure analysis of extruded parts indicated severe phase segregation during the forming process. As the extrusion opening became small (about 400 μm), the phase segregation increased.

A two–layer FGS with one layer reinforced by SiC particles was fabricated with SPP. The results indicate that SPP is capable of fabricating graded structures with promising microstructures and mechanical properties. When the SiC particles are larger than the matrix powder, dense and strong parts were formed. Smaller SiC particles can isolate the metal powders and result in porous and weak structures. The roughness of the SiC particle surface affects interface bonding between SiC particles and Al–Si–Cu matrix phase.

In summary, SPP has the potential to become a viable micromanufacturing method that can be used to make graded structures with low cost, good microstructure and promising properties.