| Summary: | 碩士 === 國立成功大學 === 機械工程學系 === 87 === In this study, the correlation of working parameters and characteristics as well as microstructure evolutions of Ti-6Al-4V by forging process was investigated. There are two main concerns: (1) The evolution and mechanism of a-case formation on the forging surface. The thickness and hardness distribution of a-case was measured. (2) The correlation of microstructure and hardness of specimens relates to the working parameters.
Study result on a-case formation shows that oxygen concentration is highest on forging surface and decreases with the increasing distance from the forging surface. This proves the formation of a-case is controlled by the diffusion mechanism. Furthermore, the three-forging-pass-finished specimens have stronger diffusion effect than the five-forging-pass-finished specimens. As a result, a thicker a-case and higher hardness depth were formed by there-forging-pass-finished specimens as compared to the five-pass-forging-finished ones.
Summarized results from the relationships of working parameters and overall hardness showed that if forging is performed on temperature 40℃ above Tb in addition to high strain, whatever forging pass was set, temperature difference will not result significant hardness change. However, if forging temperature is much higher than Tb , hardness will decrease rapidly. The results of hardness test demonstrate that the region with high strain has better hardness stability. This means that severe plastic flow in material will promote the homogeneity of Ti-6Al-4V. Moreover, basing on our literatures survey and microstructure observations, three different microstructure evolution mechanisms were classified according to forging temperature: (1) Forging began and completed in single b phase region. (2) Forging began and completed in a＋b region. (3) Forging Began in b region and completed in a＋b region. Typical microstructures and properties are thus developed. Basing on this relationship, the working parameters were further modified according to the developed microstructure.
As a whole, basic forging characteristics of Ti-6Al-4V were summarized in this thesis. Through microstructure observation and measurement results, primary process-microstructure-property correlation of Ti-6Al-4V was constructed.
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