Towards in-situ texture analysis during LPBF: optical microscopy and texture control

• Main Author: Wang, Xiaogang
• Other Authors: Matteo Seita
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Material testing and characterization; Engineering::Materials::Metallic materials
• Online Access: https://hdl.handle.net/10356/165550
• Institution: Nanyang Technological University

Laser-powder bed fusion (L-PBF) is one of the most popular metal additive manufacturing technologies owing to its high production rate and high dimensional accuracy. The possibility of optimizing parts design to minimize weight while retaining high mechanical performance is a key advantage of L-PBF over traditional manufacturing methods. However, due to the complexity of the L-PBF process— which relies on the orchestrated interplay of hundreds of different process parameters—parts may exhibit large property scatter or poor performance. This uncertainty around L-PBF products hinders the adoption of the technology by the industry, especially in applications with stringent safety factors. The root cause of this material variability stems from the uncontrolled incorporation of defects in the build—which may lead to unexpected catastrophic failure of parts—or the formation of heterogeneous microstructures during part production—which bestow markedly different properties across nominally identical products. Examples of this microstructure heterogeneity include the occurrence of spatially-varying textures— namely regions in the material comprising grains with preferred crystallographic orientation—and grain structures, which directly affect the elastic and plastic behavior of alloys, as well as their resistance to failure. If it was possible to assess these microstructural features in situ, during L-PBF, the manufacturing process could be modified on the fly to control their occurrence, produce builds with more uniform microstructure, and thus improve the consistency of parts. In this thesis, I explore the possibility of achieving this ambitious goal using different characterization methods. First, I devise different optical techniques to acquire microstructural information from metals in a similar manner to common X-ray or electron-based diffraction techniques (described in Chapters 2). I opted for optical based technologies owing to their high measurement throughput, low price, and adaptability to different environments, which make them suitable for in situ monitoring of L-PBF processes. Secondly, I explore a wide range of L-PBF process parameters and their effects on the microstructure of stainless steel 316L (Chapter 3). The rationale for this investigation is to identify significantly different microstructures to test the optical methods on. Finally, I present my attempts atix enabling optical microstructural analysis, in-line, during L-PBF processes (Chapter 4). To this end, I integrated special surface processing techniques as well as the optical characterization methods described in the previous chapters inside a custom-made L-PBF system. Although the experiments are not successful, I discuss alternative possibilities to achieve this capability.