Microstructure and mechanical properties of high strength, low alloy steel via additive manufacturing

• Main Author: Low, Zuhao
• Other Authors: Liu Erjia
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/77862
• Institution: Nanyang Technological University

Additive Manufacturing has been made very popular in recent year due to many economical and efficiency advantages over more conventional subtractive manufacturing techniques. Due to a larger range of printable materials such as various metals, many industries have been looking in to AM, specifically Direct Energy Deposition (DED). In the maritime industry, high strength, low alloy (HSLA) X65 grade steel is very commonly used in maritime structures due to its ideal mechanical properties. DED allows fabrication of complex parts with the optimisation of materials. However, this process may cause defects in the microstructure such as voids and pores, which weakens the mechanical properties of the X65 grade steel. Therefore, the aim of this project is to investigate the tensile, fatigue and toughness behaviour of DED-fabricated X65 grade steel specimens. The X65 grade steel specimens were fabricated in different orientations (XY-0° and XY-45°), which were then subjected to tensile, fatigue and toughness tests. In the tensile testing, most of the specimens passed the ASTM standards. The tensile results were plotted on the stress-strain curve. Fractographic analysis was also conducted using Scanning Electron Microscope (SEM). Generally, tensile failure was mainly caused by defects introduced during the DED process such as pores and lack of fusion. Most specimens showed obvious ductile failure such as necking. In the toughness testing, the specimens were subjected to Charpy V-notch (CVN) impact test. The CVN results were recorded to be above ASTM standards. Fractographic analysis was also conducted using the SEM. The impact failure was mainly caused by defects introduced during the DED process such as pores and lack of fusion. Most specimens showed three obvious stages of crack initiation, propagation, and final failure.