Electronic Theses and Dissertations

Date

2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Committee Chair

Amir Hadadzadeh

Committee Member

Gladius GL Lewis

Committee Member

Reza RM Molaei

Committee Member

Sanjay SM Mishra

Committee Member

Gary GB Bowlin

Committee Member

Yue YG Guan

Abstract

The current work is focused on Laser-powder bed fusion (L-PBF) of Ti-6Al-2Sn-4Zr-2Mo-0.08Si (Ti-6242) which is a near-α titanium alloy used for high-temperature applications. The fabrication of Ti-6242 is common using conventional techniques such as casting, forging, and powder metallurgy whereas the L-PBF of Ti-6242 is comparatively new. This offers the opportunity to develop the process-microstructure-property relationship of Ti-6242 fabricated through L-PBF. This includes tasks like 3D printing of Ti-6242 followed by the study of mechanical properties and the governing microstructures of as-built (AB) and after post-processing (heat treatment in this case). In the current study, the 3D printing of Ti-6242 was done by varying the volumetric energy density (E_v) from 41.67 to 66.67 J/mm3 by changing the scanning speeds. The variation in E_v results in variation in mechanical properties. Both the microstructure and defect analysis are performed. The defect characteristics are changed from keyhole to lack of fusion by varying the E_v from high to low whereas the microstructural characteristics are almost consistent. This concluded that the changes in mechanical properties of laser-powder bed fused Ti-6242 (L-PBF-Ti-6242) are defect-driven rather than microstructure-driven in the AB condition. The sample fabricated with process parameters offering the best combination of strength and ductility is selected for further studies. The systematic study of L-PBF-Ti-6242 is further proceeded by investigating the kinetics of α to β phase transformation using differential scanning calorimetry (DSC). Both non-isothermal and isothermal kinetics models are developed using the DSC results and employing the Johnson-Mehl-Avrami-Kolmogorov equation. As the AB microstructure of L-PBF-Ti-6242 consists of α’ martensite which is a brittle constituent it shows high strength and low ductility. To achieve the strength-ductility synergy, the modeled α to β phase transformation kinetics is used to design and develop two-step (solutionizing and aging) post-process heat treatment with an emphasis on preserving the unique, ultrafine, and hierarchical microstructure. The solutionized microstructure, 900°C for 10 minutes (S), has α/α’ and β phases with different dislocation densities. This condition results in the lowest strength and highest ductility, governed by the presence of the bcc phase (β), and high mean effective slip length in the α/α’ phase. The aging process, 300°C for 48 hours following the solutionizing step (STA) results in changes in dislocation substructure in the α/α’ phase which leads to an increase in strength, controlled by a reduction in mean effective slip length. The formulated two-step heat treatment leads to the best strength-ductility synergy in this study. The high-temperature mechanical property of L-PBF-Ti-6242 was investigated for AB, S, and STA conditions. In the AB condition, the LPBF-Ti-6242 shows superior strength at high temperatures in comparison with Ti-6Al-4V. In the case of L-PBF-Ti-6242, although the STA shows superior strength at room temperature, both S and STA exhibit almost similar strength at high temperatures. It is proposed that the developed heat treatment recipe attained a better combination of strength and ductility at room as well as high temperature. This justified the importance of the study of phase transformation kinetics to develop the heat treatment recipe for titanium alloys newly fabricated through L-PBF.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest.

Notes

Open Access

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