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The purpose of this paper is to assess the hydrogen embrittlement sensitivity of carbon gradient heterostructure materials and to verify the reliability of the scratch method.

The surface-modified layer of 18CrNiMo7-6 alloy steel was delaminated to study its hydrogen embrittlement characteristics via hydrogen permeation, electrochemical hydrogen charging and scratch experiments.

The results showed that the diffusion coefficients of hydrogen in the surface and matrix layers are 3.28 × 10⁻⁷ and 16.67 × 10⁻⁷ cm²/s, respectively. The diffusible-hydrogen concentration of the material increases with increasing hydrogen-charging current density. For a given hydrogen-charging current density, the diffusible-hydrogen concentration gradually decreases with increasing depth in the surface-modified layer. Fracture toughness decreases with increasing diffusible-hydrogen concentration, so the susceptibility to hydrogen embrittlement decreases with increasing depth in the surface-modified layer.

The reliability of the scratch method in evaluating the fracture toughness of the surface-modified layer material is verified. An empirical formula is given for fracture toughness as a function of diffused-hydrogen concentration.

This paper aims to investigate the relationship between in situ monitoring characteristics and surface defects in laser-based directed energydeposited Ti-6Al-4V.

In situ monitoring was conducted to extract and quantify the monitoring characteristics of each frame. A two-dimensional contour map was generated using the quantified characteristics to determine the defect formation locations. Computational thermal-fluid dynamics software was used to determine which surface tension terms or shielding gas had a significant effect on the depression of the molten pool.

This study has made a significant contribution by revealing the direct correlation between the molten pool size and brightness with defect formation in laser-based DED of Ti-6Al-4V. It was found that in regions of reduced height, the molten pool exhibited increased size and brightness, leading to surface depressions due to vapor recoil pressure flattening the molten pool. Moreover, the results highlighted that the enhanced Marangoni forces, caused by a high-temperature gradient, hindered the proper accumulation of molten metal, exacerbating height reductions. This insight provides a deeper understanding of how molten pool dynamics directly influence surface quality, which is a critical factor in DED processes.

This study contributes to understanding of the relationship between in situ monitoring characteristics and surface defects in laser-based directed energy-deposited Ti-6Al-4V. Additionally, by using in situ monitoring and computational analysis, significant insights were gained into the factors influencing molten pool behavior and subsequent surface defects.