Microstructure and fatigue behaviour of 3D printed EH36 high strength low carbon steel

• Main Author: Tay, Justin Jia Wei
• Other Authors: Liu Erjia
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/150290
• Institution: Nanyang Technological University

Additive manufacturing (AM) has been on a rising trend in the recent years due to the many advantages it has to offer over traditional manufacturing methods. It is widely utilized in many major industries such as medical, aerospace, automotive and marine offshore. As the marine offshore industry advances to meet different needs, more complex components are required for its various applications. Structural steels such as ASTM A131 EH36 are commonly used for ship building due to its exceptional mechanical properties, however, the AM methods employed are not yet fully optimized to prevent process induced defects such as porosities, lack of fusion, inclusions and unmelted feedstock from occurring. In this Final Year Project, the microstructure and fatigue behaviour of 3D printed EH36 wire high strength low carbon steel by Laser Engineered Net Shaping (LENS) method will be investigated, in particular the XZ45° and XZ90° build directions. Firstly, tensile test were conducted to determine its mechanical properties such as ductility, yield and tensile strengths and compared with the minimum requirements by Keppel. Secondly, fatigue test were conducted with subjected stresses based on its respective yield strengths. The fatigue properties were then consolidated into a table and plotted on a S-N curve for comparison across different specimens. Next, fractographic analysis using Scanning Electron Microscope (SEM) will be performed on the various fractured surfaces to identify defects causing the failure of the specimens and discuss the impact on its fatigue life. All the specimens were subjected to ductile fracture and the three stages of fatigue failure were clearly observed. Finally, microstructure analysis using Optical Microscope (OM) was carried out on etched specimens before and after testing to relate the mechanical properties to the microstructure exhibited by the specimens. The fatigue specimens subjected to lower cyclic stresses were not always inversely related to their fatigue life. The study concluded that the mechanical properties of a material were not only dictated by its microstructure, but also significantly influenced by process induced defects. Hence, there is a need for further optimization of printing parameters to reduce and even eliminate such defects to improve the mechanical properties of the materials.