Additively manufactured metals are increasingly being considered for hydrogen-related applications, including pressure vessels, pipelines, and fuel cell components. However, their susceptibility to hydrogen embrittlement remains a critical concern. Unlike conventionally manufactured metals, additively produced materials exhibit unique microstructural features, porosity, and residual stresses that can exacerbate hydrogen-induced degradation. Understanding these factors and their relationship with printing parameters is essential for developing hydrogen-resistant components.

Porosity is a common feature in additively manufactured metals, arising from incomplete fusion, gas entrapment, or keyhole instability during the printing process. These voids act as stress concentrators and provide nucleation sites for hydrogen-induced cracking. The size, distribution, and morphology of pores influence hydrogen diffusion and trapping. Larger, irregularly shaped pores tend to accumulate higher hydrogen concentrations, accelerating crack initiation. Printing parameters such as laser power, scan speed, and hatch spacing directly affect porosity levels. Higher energy densities generally reduce porosity but may introduce other microstructural defects if not optimized. For instance, excessive laser power can lead to keyhole porosity, while insufficient power results in lack-of-fusion voids. Post-processing techniques like hot isostatic pressing can mitigate porosity but may alter grain structure in ways that affect hydrogen diffusion.

Residual stresses in additively manufactured metals are another critical factor in hydrogen embrittlement. The rapid heating and cooling cycles inherent in processes like laser powder bed fusion generate significant thermal gradients, leading to tensile residual stresses. These stresses can combine with applied loads to lower the threshold for hydrogen-assisted cracking. Regions with high residual stress, such as near-surface layers or support structure interfaces, are particularly vulnerable. Printing strategies like optimized scan patterns and preheating the build plate can reduce residual stresses. However, layer-by-layer deposition inherently creates anisotropic stress distributions, complicating stress management. Post-build heat treatments are often employed to relieve residual stresses, but the temperature and duration must be carefully controlled to avoid excessive grain growth, which can also influence hydrogen susceptibility.

The unique microstructures of additively manufactured metals further differentiate their hydrogen embrittlement behavior from that of wrought or cast materials. Rapid solidification results in fine, often non-equilibrium microstructures with high dislocation densities and metastable phases. These features increase hydrogen trapping, often leading to higher apparent hydrogen solubility but also potentially enhancing embrittlement. The cellular substructures commonly observed in additively manufactured alloys provide additional trapping sites at cell boundaries, influencing hydrogen diffusion kinetics. Printing parameters such as scan strategy and cooling rates determine the extent of these microstructural features. For example, bidirectional scanning can produce more uniform microstructures compared to unidirectional patterns, while slower cooling rates may reduce dislocation density but promote phase transformations that affect hydrogen interactions.

Grain morphology and texture in additively manufactured metals also play a role in hydrogen embrittlement. The epitaxial grain growth typical of many additive processes results in elongated grains aligned with the build direction. This anisotropy affects both mechanical properties and hydrogen diffusion pathways. Grain boundaries act as hydrogen traps, but their effectiveness depends on boundary character and energy. High-angle grain boundaries are more effective traps but may also serve as preferential crack propagation paths under hydrogen exposure. Printing parameters like layer thickness and laser focus can influence grain size and texture, offering potential avenues for microstructure optimization to mitigate embrittlement.

The interaction between hydrogen and the various microstructural features in additively manufactured metals is complex and depends on multiple factors. Hydrogen trapping at dislocations, grain boundaries, and interfaces can initially reduce the amount of diffusible hydrogen available for embrittlement. However, under mechanical loading, these trapped hydrogen atoms may be released, contributing to crack growth. The relative contributions of different trapping sites depend on their binding energies and densities, which are influenced by printing parameters and post-processing. For example, solution annealing can reduce dislocation density but may increase grain boundary availability for hydrogen trapping.

Mitigating hydrogen embrittlement in additively manufactured metals requires a holistic approach that considers the interplay between porosity, residual stresses, and microstructure. Process optimization to minimize defects must be balanced with the need to achieve desired mechanical properties. Post-processing treatments should be tailored to the specific alloy and intended service environment. Advanced characterization techniques, including in-situ hydrogen charging and mechanical testing, are essential for understanding failure mechanisms and validating mitigation strategies. As additive manufacturing continues to advance, developing hydrogen-compatible materials and processes will be crucial for enabling their use in critical hydrogen infrastructure.

The relationship between printing parameters and hydrogen embrittlement susceptibility provides opportunities for targeted material design. By controlling energy input, cooling rates, and build orientation, it may be possible to engineer microstructures that resist hydrogen-assisted failure while maintaining other performance requirements. Continued research is needed to establish quantitative relationships between process variables and hydrogen embrittlement resistance across different alloy systems. This knowledge will support the development of robust additive manufacturing protocols for hydrogen service applications.