Investigation of microstructure of 3D printed Ti-6Al-4V alloy under various heat treatment conditions
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Journal Title
Journal ISSN
Volume Title
Insinööritieteiden korkeakoulu |
Master's thesis
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Author
Date
2023-08-21
Department
Major/Subject
Additive Manufacturing for Full Flexibility
Mcode
Degree programme
Master’s Programme in Manufacturing
Language
en
Pages
104
Series
Abstract
As additive manufacturing (AM) continues to show great potential in building complex designs and optimizing geometries without the constraints of traditional manufacturing methods, the use of Ti-6Al-4V (Ti64) alloy has become prominent for its exceptional strength and excellent biocompatibility and reduced stress shielding. However, AM-built parts that face issues such as high porosity, surface roughness, microstructure variations, and residual stress, particularly exhibit lower mechanical properties compared to wrought Ti64 alloy. This Thesis, aimed to examine the microstructure of materials printed using different technologies, namely Electron Beam Melting (EBM) and Laser Powder Bed Fusion (LPBF). They conducted a series of post-heat treatments on the samples to explore their effects on microstructure, phase transformations, and residual stresses. This investigation aimed to address the limitations associated with the LPBF and EBM processes. The experiments involved the use of X-ray Diffraction (XRD) as the primary method to analyse 10 samples manufactured under different conditions. Relevant calculations and parameter plotting were conducted to understand the microstructure, phase transformations, and residual stresses. A comparative analysis was performed, utilizing hardness values from prior research project for result validation. The key findings are as follows: EBM Analysis: In the analysis of the Electron Beam Melting (EBM) samples, the heat treatment performed in Production 1 (M6) showed significantly better performance in relieving strains compared to the post-heat treated sample in Production 2 (M11). The heat-treated sample from Production 2 displayed surface discolouration, indicating potential oxidation during heat treatment and faster cooling. Additionally, this sample exhibited higher Vickers hardness and a significant level of microstrain compared to the heat-treated sample from Production 1. These findings highlight the importance of heat treatment parameters and their effects on the microstructure and mechanical properties of the printed materials. LPBF samples manufactured in Production 1: In the analysis of the LPBF samples manufactured in Production 1, the heat treatment (JR001T) resulted in lower lattice strain compared to the heat-treated sample in Production 2 (JR0027). The post-heat treatment transformations, including α'→ α+β peak splitting and an overall peak shift towards lower angles, aligned with existing literature. Notably, the heat-treated sample from Production 1 (JR001T) exhibited a more prominent β peak compared to the heat-treated sample from Production 2 (JR0027), suggesting differences in heating rates or temperatures in the heat treatment recipes. These observations provide insights into the effects of heat treatment on the lattice strain and phase transformations of the printed materials. LPBF samples manufactured in Production 2: In the study of the LPBF samples, preheating the bed in the as-built condition (3) resulted in an increased c/a ratio and decreased micro-strain compared to the non-preheated condition (4). However, post-heat treating the preheated condition (3) (2) did not have a significant impact on the lattice strains, and an erratic spike in the c/a ratio was observed, which could be attributed to the heat treatment parameters and cooling process. Furthermore, the lattice strains exhibited by the post-heat-treated samples without preheating and with preheating (1, 2) closely resembled the strains observed in the preheated as-built sample (3). These findings shed light on the influence of preheating and post-heat treatment on the c/a ratio and lattice strains of the LPBF-printed materials.Description
Supervisor
Salmi, MikaThesis advisor
Celikin, MertKeywords
additive manufacturing,, laser powder bed fusion., electron beam machining,, post heat treatment,, Ti-6Al-4V
