Improvement of mechanical property of M2 high-speed steel with hetero-microstructure tailored via electron beam melting

The aim of this study is to enhance the plasticity and strength of M2 high-speed steel through electron beam melting (EBM) technology. Therefore, by controlling different melting times to change the energy input, a gradient distribution of grain size and uniform distribution of carbides can be achieved. During the deformation process, the gradient structure causes geometrically necessary dislocations (GND) to accumulate near the boundary of regions in the soft zone, resulting in compressive stress in the soft zone and tensile stress in the hard zone, ultimately achieving heterogeneity-induced (HDI) work hardening. This study is to EBM technology for the design and fabrication of three types of M2 high-speed steel samples, denoted as SS-1, SS-2, and SS-3, utilizing three distinct scanning strategies: single melting, double melting, and a combination thereof. The microstructure and phase evolution of these samples were comprehensively characterized using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques. The findings demonstrate that when sample SS-3 was subjected to the combined scanning strategy involving both single melting and double melting approaches, its crystal structure exhibited a gradient distribution along with the presence of back stress during tensile testing. Sample SS-3 achieved an average tensile strength of 1479 MPa with an average elongation rate of 1.8%. This represents an enhancement in average tensile strength by approximately 8.6% compared to sample SS-1 (-1362 MPa) and a significant increase in average elongation rate by around 20% compared to SS-1 (-1.44%). The gradient grain structure materials show the microstructure transformation from fine grain to coarse-grain in the same material, thus reducing the stress concentration caused by the change of mechanical properties in each region. The SS-3 sample can maintain significant ductility while improving tensile strength. Consequently, this represents a pivotal direction for the advancement of high-performance structural materials. The conducted experimental studies have demonstrated that geometrically necessary dislocations (GNDs) and their associated strain hardening play a crucial role in enhancing the strength of grain structure materials. Additionally, these are essential to achieve an exceptional synergy between strength and toughness in these materials.