Research Information System
Thesis Type: Doctorate
Institution Of The Thesis: Gazi University, Fen Bilimleri Enstitüsü, Turkey
Approval Date: 2022
Thesis Language: Turkish
Student: AYŞE KÜBRA YILDIZ
Supervisor: Oğuzhan Yılmaz
Abstract:
In Selective Laser Melting (SLM), due to the high temperatures and rapid cooling rates, martensite α' microstructures with poor ductility are formed in Ti-6Al-4V alloy. However, the higher strength and ductility can be obtained by α+β microstructure. Via in-situ martensite decomposition method, the martensite α' microstructure can be turned into lamellar (α+β) microstructures. However, special conditions are required for decomposition to occur, and these conditions are difficult to determine. In this thesis, an efficient multi-tracks and multi-layer model has been developed to evaluate the everchanging thermal behavior of the laser scanning process selectively on the laid powder layer. The model includes empirical approaches to determine conductivity enhancement factor in the z-direction (λz) and absorptivity for Ti-6Al-4V. A phenomenological approach has been employed, which allowed to directly relate the local time dependent temperature gradients to the microstructure transformation evolution. These transformations in relation to process parameters and the required stages of martensite decomposition have been explained. The consistency of the experimental studies with the model was checked. In addition, parameters were estimated for a powder layer thickness of 30 μm and a laser with a beam diameter of 85 μm, where decomposition would be difficult, and a sample geometry was fabricated on a laser powder bed fusion additive manufacturing machine and its microstructure was analyzed. In addition, the effects of different support structures, build orientations, scanning strategies and reheating processes on martensite decomposition were investigated by thermal analysis. New parameter sets were determined that allow the decomposition of martensite by improving the powder laying times at high substrate temperatures. While positive effects of high-void support structures and expanding cross-section structures on martensite decomposition were detected, it was observed that the scanning strategy and reheating process did not have a significant effect. Substrate temperature and powder laying times were determined to be important parameters for decomposition.
Key Words : Selective Laser Melting, Additive Manufacturing, Ti-6Al-4V