Chemical heterogeneity as a factor of improving the strength of steels manufactured by selective laser melting technology

Abstract

The aim of this paper was to establish the causes of the heterogeneity of the chemical composition of the metal obtained by the LC technology. The powdered raw material was made from a monolithic alloy, which was fused by the SLM, the initial raw material was a laboratory melting metal of a low-carbon chromium-manganese-nickel composition based on iron. To determine the distribution pattern of alloying chemical elements in the resulting powder, electron-microscopic images of thin sections were combined with X-ray analysis data on the cross-sections of the powder particles. As a result, it was found that transition (Mn, Ni) and heavy (Mo) metals are uniformly distributed over the powder particle cross-sections, and the mass fraction of silicon (Si) is uneven: in the center of the particles, it is several times larger in some cases. The revealed feature in the distribution of silicon is supposedly due to the formation of various forms of SiO4 upon the cooling of the formed particles. The internal structure of the manufactured powder is represented by the martensitic structure of stack morphology. After laser fusion, etched thin sections revealed traces of segregation heterogeneity in the form of a grid with cells of ~ 200 μm. The boundaries of segregations and the fine-grained structure were the dominant mechanisms of steel hardening in the SLM process. Under compression of the obtained samples, the yield strength was 720 MPa, and the greatest value of the deformation resistance reached 1050 MPa, which exceeds the performance of a monolithic material of the similar chemical composition. In the diagrams σ (ε) in the parabolic hardening section, randomly located extrema of local hardening are recorded, followed by a sharp load decrease. The appearance of the diagrams indicates an inhomogeneous internal structure of the samples.