Investigation of high-cycle fatigue properties of 3D-printed steel parts

• Main Author: Neo, Benjamin Jun Wei
• Other Authors: Liu Erjia
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/78770
• Institution: Nanyang Technological University

Additive manufacturing (AM), also known as three-dimensional (3D) printing, is rapidly gaining popularity in many industries due to its ability to fabricate components with complex geometries. Due to technology advancements, more metallic parts with intricate details are required in the marine offshore industry. High Strength Low Alloy (HSLA) steels such as ASTM A131 EH36 and API 5L X65 are commonly used in the fabrication of their applications due to their excellent mechanical properties. Despite its advantages, there are possibilities of process-related defects being present in the components fabricated using additive manufacturing that could result in mechanical inferiorities of the components. Some of these defects include voids (porosities), inclusions, un-melted powders (lack of fusion), etc. Hence, the objective of this Final Year Project is to carry out an investigation of high-cycle fatigue properties of 3D-printed ASTM A131 EH36 and API 5L X65 steel specimens of different orientations which were fabricated using an additive manufacturing process, known as Laser Engineered Net Shaping (LENS). The four different materials and orientations tested in this Final Year Project are ASTM A131 EH36 XZ-45°, ASTM A131 EH36 XZ-90°, API 5L X65 XY-0° & API 5L X65 XY-45°. First, tensile tests were carried out to determine the mechanical properties of the specimens of different materials and orientations. Next, fatigue tests were carried out to determine their corresponding fatigue properties and to plot their respective stress-life (S-N) curves. Finally, fractographic analysis of the fractured surfaces for the fatigue specimens were conducted using a Scanning Electron Microscope (SEM). After conducting the tensile tests, the results were tabulated and the ranking order of the different materials and orientations in terms of Yield Strength (YS) and Ultimate Tensile Strength (UTS) is as follows: API 5L X65 XY-45° (Strongest), API 5L X65 XY-0°, ASTM A131 EH36 XZ-90°, ASTM A131 EH36 XZ-45°. The results of the fatigue tests were also tabulated and the ranking order of the different materials and orientations in terms of fatigue life when subjected to a stress level of 500 MPa is as follows: API 5L X65 XY-45° (Highest Fatigue Life), A131 EH36 XZ-45°, API 5L X65 XY-0°, A131 EH36 XZ-90°. Fractographic analysis of the fractured surfaces for the fatigue specimens were conducted and most of them displayed the three distinct stages that characterise a fatigue failure; crack initiation, crack propagation and final failure. The fractographic analyses also revealed the directions of crack propagation due to the characteristic markings such as beach marks and striations, which radiated from its corresponding crack initiation sites. Most of the crack initiation sites were formed due to the above-mentioned LENS process-related defects, which could have resulted in the reduced mechanical and fatigue properties of the specimens.