Additive manufacturing of high Si aluminium alloys

• Main Author: Lee, Boon Yao
• Other Authors: Zhou Wei
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/149067
• Institution: Nanyang Technological University

The effects of laser power variation and post additive manufactured (AM) heat treatment on the microstructure of primary silicon phase and mechanical properties have been studied in this paper. This observation was done through optical microscope and Vickers hardness testing. It was discovered that primary silicon microstructure improves with increasing laser power. As the higher laser power provides more nucleating sites which resulted in smaller primary silicon phase particles. Beyond 800 W laser setting, the columnar primary silicon particles transform into a blocky morphology that reduces in effective diameter with increasing laser power. By increasing laser power from 800 W to 1900 W, the primary silicon diameter decreases by 84%, which shows significant microstructure improvement at higher power setting. However, the eutectic silicon phase at various laser power exhibits no significant microstructural change. At 800 W and 1350 W lower power settings it was observed that defects such as porosity and cracking appeared more frequently and is predominantly occurring near the melt pool surface. It is discovered that at 1900 W power setting, the deposit exhibit homogenous hardness at 180 HV, While the 1350 W and 800 W displayed decreasing hardness value towards the melt pool surface. To reduce the occurrence of defects, heat treatment at 300 ℃ to 500 ℃ proved to be successful in preventing defect occurrence by thermal stress relieving during the heat treatment process. From 400 ℃ heat treatment temperature onwards, spheroidization of eutectic silicon was observed. At 400 ℃ heat treatment onwards, there is a transformation of dendritic eutectic silicon into a composite like silicon reinforced aluminium matrix that surrounds the primary silicon phase. It is discovered that at all heat treatment temperature, the deposit exhibit homogenous hardness across the height of the melt pool. It is determined that 300 ℃ heat treatment temperature produces the best hardness result at ~155 HV.