| Summary: | 碩士 === 中華大學 === 機械與航太工程研究所 === 92 === ABSTRACT
A significant problem with superplastic aluminum alloys is the internal cavity formation during superplastic deformation. The cavities nucleate at second phase particles or grain boundaries, their subsequent growth and coalescence could cause premature failure during superplastic forming, and the presence of these cavities in the superplastically formed parts would have a deleterious effect on any post-form applications.
The purpose of this work is to explore the deformation characteristics of a superplastic 5083 Al alloy during multaxial deformation. A series of interrupted experiments were performed to bulge the sheet into a die with a cylindrical cavity. Deformation rate and lubrication were included in the experimental forming processes to investigate the influence of the forming variables on the thickness distribution, deformation state, and variation of strain rate of the formed parts. Effects of the deformation variables on the cavitation were also analyzed in the present study.
Experimental results showed cavitation characteristics could be separated into two stages during forming a cylindrical cup . In stage I; the sheet deformed freely as part of a hemi-cylindrical shape, cavity volume increased exponentially with deformation. The evolution of cavity volume was due to both nucleation and growth of cavities. In the second stage; the surface friction would restrict thinning of the sheet, the die surface condition would influence the flow patterns of metal flow during forming. The thickness distribution of the formed parts with lubrication was found to be more uniform than that formed without lubrication, and cavity levels were also less in the parts formed with lubrication. The cavity volume first increased and then decreased with forming time for all test strain rates in the second stage of forming. Decrease in cavity volume in the later stage could be related to the cavity shrinkage rose from sintering effect.
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