Effect of porosity on the Tensile and Fatigue Behaviour of Inconel 738 Manufactured Through Laser Powder Bed Fusion

This work provides a comprehensive investigation of LPBF Inconel 738, a material fabricated using laser powder bed fusion technique and enhanced through a novel crack-healing post-processing technique. The research explores the microstructural characteristics and mechanical performance of this advanced material, comparing it systematically with conventionally cast Inconel 738. Employing advanced characterisation methods, multi-scale mechanical testing, and finite element modelling, the study examines tensile performance and fatigue behaviour under varying stress levels. Techniques such as Digital Image Correlation, Scanning Electron Microscopy, and X-ray Computed Tomography were utilised to analyse material behaviour and microstructural evolution.
The findings reveal that LPBF Inconel 738, enhanced through the Liquid-Induced Healing post- process, exhibits superior mechanical properties compared to cast alloys, including higher ultimate tensile strength and elongation to failure. Microstructural analysis highlights the refined, equiaxed grain structure of LPBF materials, contrasting with the larger, columnar grains of cast specimens. Lower porosity levels in LPBF materials (below 0.04%) contribute to their enhanced performance, as observed in slower crack propagation and greater fatigue resistance under cyclic loading. Primary cracking mechanisms for both materials have been proved to be intergranular fracture, where propagation occurs along the grain boundaries. LPBF specimens displayed superior fatigue life, though variability in performance was noted due to process-induced defects. Finite element modelling demonstrated the impact of porosity on fatigue behaviour, with larger and more irregularly shaped pores significantly increasing stress concentrations. The results emphasise that achieving low porosity levels and promoting refined, equiaxed grain structures are crucial for enhancing the fatigue resistance and overall mechanical performance of LPBF Inconel 738 material.
This study represents the first detailed exploration of the tensile and fatigue properties of LPBF Inconel 738, addressing critical gaps in the literature. The research establishes a foundation for adopting LPBF Inconel 738 in high-performance applications and identifies pathways for optimising additive manufacturing processes to enhance material durability and reliability.