Effects of Porosity and Microstructure on the Fatigue Fracture Properties of Ti6Al4V Alloy Produced through Selective Laser Melting

Selective laser melting (SLM) was employed to fabricate Ti6Al4V samples, focusing on laser energy density ranges suitable for industrial applications. The process involved nine levels of laser energy density (66.66–113.09 J/mm³) to achieve full density in the SLM-fabricated products. Experimental results demonstrated that the Archimedes relative density of the specimens increased to 99.86%. At this density level, characterized by minimal porosity, microstructural analysis revealed the presence of pores both on the surface and within the internal regions. The various laser energy density levels, including lower, higher, and optimal values, promoted the αˈ-phase microstructure, thereby affecting the microhardness. X-ray diffractometer (XRD) analysis confirmed the presence of acicular αˈ-phases in the SLM Ti6Al4V components. Fatigue tests, including high-cycle fatigue (HCF) and low-cycle fatigue (LCF), were conducted on the samples exhibiting minimal porosity and refined microstructure, yielding fatigue limits of 140 MPa and lifetimes up to 10 ⁷ cycles. The results demonstrated that fatigue cracks in the Ti6Al4V alloy predominantly originated from surface-connected pores. The relationships between microstructure, porosity, and fractography revealed that the rapid cooling in the SLM process limited crystal growth, leading to smaller crystal sizes and influencing fatigue performance. The findings highlight the practical significance of optimizing laser energy density in improving the mechanical and fatigue properties of SLM-fabricated Ti6Al4V components by reducing porosity and promoting the formation of strong metallurgical bonds in the microstructure.