Investigation on microstructure and build defects for 3D printed steel parts

• Main Author: Seah, Christabel
• Other Authors: Liu Erjia
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/74835
• Institution: Nanyang Technological University

In order to determine the feasibility of using Laser engineered net shaping (LENS) 3D-printing for offshore construction, observation and analysis of built defects and the microstructure of LENS 3d-printed A131 EH36 grade steel was conducted. Two block samples were cut and mounted to study their horizontal and vertical cross-sections. After polishing, an optical microscope (OM) was used in conjunction with Image J software to measure porosity and observe other built defects. To observe the built defects of the samples after testing, penetrant testing was performed on samples that had undergone tensile, fatigue and Charpy testing. After chemical etching with nitric acid, OM and scanning electron microscope (SEM) was used to observe and analyse the sample microstructure. It was observed that there were two types of built defects: porosity and inclusions. Calculated porosity levels were 1% for the horizontal cross-section and 1.3% for the vertical cross-section. Porosity distributions for both samples were observed to be nonuniform. Due to excessively damp powder or trapped gases from the heated powder, gas bubbles are enclosed during the printing process and become pores. Energydispersive X-ray spectroscopy (EDS) was performed using SEM to analyse the chemical composition of the inclusions, and it was found that the weighted percentage of iron was 82.39%. It is possible that fall back from the gas shield or contamination of the powder source caused these inclusions. The microstructure was found to vary along the built direction, starting with very fine grains that are equiaxed in morphology nearer to the substrate, with the development of pearlite, ferrite and a little acicular ferrite and Widmanstätten ferrite phases at the middle. At the top of the sample dendritic Widmanstätten ferrite, austenite and acicular ferrite were observed, indicating an increase in temperatures and a decrease in cooling rates as the sample was built. In conclusion, although the presence of acicular ferrite indicates high strength and toughness, the uneven distribution of pores and the presence of inclusions may result in points of weakness within the material.