Pore characteristics governing the dynamic failure of laser metal deposition-fabricated ultra-high-strength steel

Laser Metal Deposition (LMD) is a promising additive manufacturing technology for ultra-high-strength steel but inevitably introduces gas pores that may impair dynamic mechanical performance. However, the quantitative effects of pore size and spatial distribution under high-strain-rate loading remain unclear. In this study, the dynamic behavior of LMD-fabricated ultra-high-strength steel with different porosity levels was investigated using Split Hopkinson Pressure Bar (SHPB) tests. Industrial computed tomography (CT) was employed to characterize the internal pore structures, and Continuum-Discontinuum Element Method (CDEM) models reconstructed from the CT data were developed to simulate deformation and fracture under impact loading. The results show that the pores are predominantly spherical gas pores, with the largest exceeding 600 μm in diameter. Pronounced strain concentration develops around pores during impact, and the maximum strain adjacent to a 600 μm pore is 28% higher than that in pore-free regions, decreasing rapidly with increasing distance from the pore. Numerical simulations reveal that pores located on the dominant shear plane play a role in degrading the dynamic mechanical properties. Compared with the pore-free numerical model, the model containing the complete pore distribution exhibits a 7.3% reduction in peak compressive stress, whereas removing the pore on the dominant shear plane reduces the reduction in peak compressive stress to only 1.4%. Under the present loading conditions, pores located on the dominant shear plane have the greatest influence on the dynamic failure behavior, and the transition from a limited to a pronounced pore-size effect occurs between 200 and 400 μm.