In situ monitoring of the melt pools in laser powder bed fusion (LPBF) has enabled the elucidation of process phenomena. There has been an increasing interest in also using melt pool monitoring to identify process anomalies and control the quality of the manufactured parts. However, a better understanding of the variability of melt pools and the relation to the incidence of internal flaws are necessary to achieve this goal. This study aims to link distributions of melt pool dimensions to internal flaws and signal characteristics obtained from melt pool monitoring. A process mapping approach is employed in the manufacturing of Hastelloy X, comprising a vast portion of the process space. Ex situ measurements of melt pool dimensions and analysis of internal flaws are correlated to the signal obtained through in situ melt pool monitoring in the visible and near-infrared spectra. It is found that the variability in melt pool dimensions is related to the presence of internal flaws, but scatter in melt pool dimensions is not detectable by the monitoring system employed in this study. The signal intensities are proportional to melt pool dimensions, and the signal is increasingly dynamic following process conditions that increase the generation of spatter.

Titanium-based alloys are susceptible to hydrogen embrittlement (HE), a phenomenon that deteriorates fatigue properties. Ti-6Al-4V is the most widely used titanium alloy and the effect of hydrogen embrittlement on fatigue crack growth (FCG) was investigated by carrying out crack propagation tests in air and high-pressure H₂ environment. The FCG test in hydrogen environment resulted in a drastic increase in crack growth rate at a certain $\Delta$ K, with crack propagation rates up to 13 times higher than those observed in air. Possible reasons for such behavior were discussed in this paper. The relationship between FCG results in high-pressure H₂ environment and microstructure was investigated by comparison with already published results of cast and forged Ti-6Al-4V. Coarser microstructure was found to be more sensitive to HE. Moreover, the electron beam melting (EBM) materials experienced a crack growth acceleration in-between that of cast and wrought Ti-6Al-4V.

Additive manufacturing, particularly laser powder bed fusion (LPBF), has received much attention in recent years because of its multiple benefits over traditional manufacturing. One of the key factors affecting the repeatability and performance of the materials is the existence of defects. Defects can be driven by process by‐products called spatters, which consist of particles covered by an oxide layer formed during their travel time in the process atmosphere. As a standard process atmosphere consists of argon containing a residual oxygen level of around 0.1%, one possible way of addressing spatter‐driven defect formation is by reducing the oxygen level, thereby reducing the oxidation of the spatter particles. In this study, Hastelloy X powder was processed by means of LPBF in an argon atmosphere containing 1000‐ppm O ₂ or 50‐ppm O ₂ . Spatter particles were collected in a controlled manner, allowing sampling particles of different sizes, which were analyzed in terms of their surface chemical composition by means of X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). By combining these two tools, a comprehensive assessment of the surface chemical composition was conducted, taking advantage of XPS for the evaluation of the overall surface chemistry and of Auger nanoprobe analysis for high lateral analytical resolution combined with depth profiling. It is shown that tighter oxygen control will both limit the overall oxidation and affect the surface chemical composition. At regular O ₂ level in the process atmosphere (1000 ppm), spatter particles are covered by a 70‐nm‐thick oxide layer, on average. The thickness is substantially greater than that measured in spatter particles collected from the process atmosphere containing 50‐ppm oxygen, which averages 6 nm and is comparable with that of the virgin powder, thus revealing a potential for defect mitigation through control of the process atmosphere. Nonetheless, substantial differences in the surface chemical composition were identified between spatters and virgin powder, notably with the appearance of Al‐ and Ti‐oxides on spatter particles, revealing the influence of the manufacturing process on surface characteristics.