Research Information System
Thesis Type: Doctorate
Institution Of The Thesis: Gazi University, Fen Bilimleri Enstitüsü, Turkey
Approval Date: 2022
Thesis Language: Turkish
Student: Mehmet MOLLAMAHMUTOĞLU
Supervisor: Oğuzhan Yılmaz
Abstract:
In this study, the thermal behavior of powder bed fusion processes and the effect of this thermal behavior on the microstructure are discussed. For this purpose, the nature of the two principal powder bed processes, selective laser melting and electron beam melting, was investigated. Then, the processes were mathematically modeled, and some improvements were made. A new volumetric heat source model and an empirical absorptivity correlation have been developed for the Ti-6Al-4V alloy in the selective laser melting process. In the electron beam melting process, the evaporation phenomenon is emphasized. For this purpose, evaporative energy loss and the effects of evaporation on the melt pool profile were investigated. A mathematical expression has been derived for the Anisimov method to define the evaporation heat loss and the recoil pressure on the melt pool. Experimental work was carried out for both processes, and various Ti-6Al-4V samples were manufactured. The microstructures formed by the selective laser melting process were examined and correlated with the thermal outputs obtained by numerical analysis. Thus, four cooling rate regimes were determined as below 20 K/s, 20 to 410 K/s, 410 to 1900 K/s, and above 1900 K/s. The possible recoil pressure and evaporative heat loss occurring in the vacuum environment in the electron beam melting process were investigated via the solidified melt pool profiles obtained from the experiment and numerical analysis. In the scans with moderate process parameters, it was observed that the heat loss by evaporation was around 10%. It has been concluded that in a vacuum environment, various speed regimes may occur under various conditions and process parameters, and evaporation heat loss may increase under these regimes.
Key Words
: Additive manufacturing, powder bed fusion processes,
thermal model, microstructure