Study of pore defect and mechanical properties in selective laser melted Ti6Al4V alloy based on X-ray computed tomography

Abstract

The selective laser melting (SLM) technology is widely used for the manufacturing of Ti6Al4V alloy components in the biomedical and aerospace industries, and the residual pores are one of the significant defects obstructing its further application. X-ray computed tomography (XCT) technique was applied for the examination and analysis of the spatial and morphological features of pore defects, which will influence the tensile properties of the SLM-manufactured Ti6Al4V samples. Hence, the effects of processing parameters on pore defects and their formation mechanism are discussed thoroughly. The results showed that two major types of pore defects are observed: the lack-of-fusion caused by low volumetric energy density (VED), and the keyhole mode resulting from excessive energy densities. The highest relative density of 99.995% was obtained at a VED value of 58.8 J/mm3. Besides, the quantitative information of the porosity defects, such as size distribution, morphology and formation direction, was evaluated using XCT to reveal their formation mechanisms. Furthermore, the tensile properties of SLM-manufactured Ti6Al4V alloy under different VED values were evaluated to correlate with the porosity variation. Overall, the tensile strength and ductility were notably degraded with the increase of porosity. The fractography study based on XCT and SEM showed that lack-of-fusion and porosities are responsible for the early fracture. In conclusion, the XCT technique can provide direct and precise inspection of the process of laser-based additive manufacturing to control the overall quality.