On the mechanical anisotropy of direct laser deposited 316L stainless steel workpieces

The purpose of this work is to evaluate the mechanical anisotropy of structures additively manufactured from heat-resistant steels under conditions where the dimensions of the workpiece do not allow cutting samples in three mutually perpendicular directions. The material for this research was cylindrical specimens from SS 316L alloy plates prepared through direct laser deposition with process parameters, providing mechanical properties of large-scale structures. The quasi-static isothermal tension tests up to fracture under room temperature and elevated temperatures carried out on specimens extracted along two possible orientations (namely: the build direction and the print direction) shown the similarity of the flow curves in these directions, which might be associated with the small degree of mechanical anisotropy of the workpiece. In contrast, the cross-section of the round bar specimen after tension becomes elliptical — both in the necking part of the specimen and in the part of uniform elongation, which demonstrates the persistence of anisotropy in the workpieces related with the difference of mechanical properties in the third, unstudied direction. On the basis of numerical simulation of the specimen’s deformation via finite element method in the ANSYS package a mathematical description of the material was constructed, providing coincidence of the simulated behavior with the experimental one — both in the load-elongation curve and in the cross-section ellipticity of the specimen. Typical for additive manufacturing method thermally stressed structure was numerically simulated to estimate the influence of the material anisotropy on structural strength. It is shown that taking into account anisotropy reduces the assessed cyclic durability by approximately two times. The performed work gives a reason for a more complete study of anisotropy, despite the methodological difficulties, such as manufacturing of sufficiently large 3D-printed workpieces, and additional amount of experimental work.

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