Dynamic mechanical properties of the selective laser melting NiCrFeCoMo0.2 high entropy alloy and the microstructure of molten pool

Abstract

To improve the dynamic mechanical properties of the selective laser melting (SLM)-NiCrFeCoMo0.2 high entropy alloy (HEA), dynamic impact experiments were carried out on this alloy with different characteristic molten pools. Melt pool structures of different special sizes can be obtained by adjusting the laser power and scanning speed during the SLM process. In this work, the split-Hopkinson pressure bar (SHPB) is used to conduct dynamic impact on the SLM-NiCrFeCoMo0.2 HEA samples with different molten pool size. The X-ray diffractometer (XRD), the scanning electron microscope (SEM), the electron probes micro-analyzer (EPMA) and the transmission electron microscope (TEM) were used to characterize the microstructure and deformation characteristics of the of the molten pool. The SLM-NiCrFeCoMo0.2 HEA shows high impact energy absorption density and compressive strength at high strain rate. Volume energy density (VED) refers to the power delivered by the laser per unit volume of material during a SLM process, which can affect the size of the molten pool formed. Mo-rich σ phase precipitations are distributed near the molten pool boundary. In the cooling stage of the SLM process, the center of the molten pool has a faster cooling rate than the molten pool boundary, so area near the boundary stays in the temperature range where the precipitation occurs for a longer time, which is the reason why the precipitates are concentrated near the boundary. The precipitation strengthening in the SLM-NiCrFeCoMo0.2 HEA is considered by measuring the relationship between the molten pool structure and the precipitation distribution, considering the material as a composite of the precipitation dense region (PDR) and the precipitation free region (PFR), and the strength contribution of precipitation is estimated by mixing rules.