Variation in powder size distribution for electron beam melting of Ti48Al2Cr2Nb - density and surface roughness effects
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Date
2024-04-25
Authors
Oluwajuyigbe, Tomisin
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
Additive manufacturing (AM), also known as 3D printing, is an innovative method for producing complex geometries with minimal material waste. Aerospace, medical, and battery fields are a few sectors that employ AM as a means to further create innovation in materials design, structural design and structural/mechanical property development. The aerospace industry is on of such sectors continuously seeking ways to reduce energy/fuel consumption by shifting towards lightweight and high-temperature resistant materials. Ti-48Al-2Cr-2Nb (γ-TiAl) is a popular choice due to its desirable weight-to-density ratio and superior high-temperature mechanical properties. However, traditional manufacturing methods for the Ti-48Al-2Cr-2Nb alloy are expensive. Additive manufacturing (AM), specifically electron beam melting (EBM), is a cost-effective alternative that offers the potential for optimal ductility and fracture toughness for γ-TiAl. To promote economic sustainability, this study explores varying particle size distribution (PSD) in the EBM process.
The research focuses on Ti-48Al-2Cr-2Nb alloy for PSD 45-150 µm, 38-150 µm, and 38-180 µm, specifically with the potential of PSD 38-180 µm as a more cost-effective reliable material to promote economic sustainability. By examining density and porosity morphology, as well as surface roughness, the study investigates how to maintain good density and surface qualities, irrespective of PSD, as the mechanical properties of the printed materials are dependent on the density, pore formation and surface quality obtained during the AM process. To mitigate pore defects and increase surface quality, process parameters involved in AM must be tuned to ensure the target density, porosity type and surface roughness of the final product. In this work, gamma titanium aluminide (Ti-48Al-2Cr-2Nb) was deployed in an electron beam melting (EBM) powder bed fusion process. The focus was to analyze and classify the printed part quality in terms of surface roughness (SA, STR, and SPC), relative density, lack of fusion defects and gas pore defects to establish trends across the parameter space.
This thesis follows various sets of experiments that detail process parameter combinations that aim to provide a wide range or part quality results, therefore gathering enough unique data to conduct a full analysis on the behavior of the EBM-printed parts. The experimental study involves data analysis and data visualization utilized to identify the relationship between the process inputs (parameters and engineered features such as normalized enthalpy and volumetric energy density), the resulting density and pore type (gas pores and lack of fusion pores) and the surface roughness parameters (spatial, feature, and height parameters).
The EBM process can face challenges in achieving low density levels and poor surface quality due to the applied energy and powder characteristics. More specifically, this thesis examines the relationship between PSD and porosity by identifying and investigating lack of fusion (LOF) and gas porosity types within threshold levels of 'Excellent,' 'Good,' 'Poor,' and 'Failed', with pre-defined ranges for the three classes. Additionally, the relationship between the PSD and surface roughness (SA, STR, and SPC) is also confirmed but utilizing a full factorial design of experiment (DOE) to create three sub-experimental hypothesis that explores the various prominent surface quality influencer (layer thickness and higher energy electron beam). Based on this experimental study, it was found that bulk density ranges spanned 88% - 99.99%, with gas porosity spanning 0.01% - 0.3%, and lack of fusion porosity spanning 0.001% - 12%. Additionally, for surface roughness, SA reports a range spanning (39.41 µm – 109.74 µm), STR reports (0.0984 – 0.8126), and SPC reports (2344.2 1/µm – 4084.31 1/µm) across the reference PSD 45-150 µm and the widest PSD in this study 38-180 µm. Based on resulting outcomes and literature, data distributions and statistical analysis were employed to classify and quantitatively defined the results obtained from the experimental study. The results demonstrate the relationship between the process parameters, density, and porosity type as well as the surface roughness. The study contributes to a comprehensive understanding of the interplay between PSD and porosity as crucial factors in the EBM AM process.
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Keywords
Additive Manufacturing, Electron beam melting, Titanium aluminide, Ti-48Al-2Cr-2Nb, Pore analysis, Density, Powder size distribution