Hall-Petch relationship in selective laser melting additively manufactured metals: using grain or cell size?

The mechanical properties of many materials prepared by additive manufacturing technology have been greatly improved. High strength is attributed to grain refinement, formation of high density dislocation and existence of cellular structures with nanoscale during manufacturing. In addition, the super-saturated solid solution of elements in the matrix and the solid solution segregation along the wall of the cellular structures also promote the improvement of strength by enhancing dislocation pinning. Hence, the existence of cellular structure in grains leads to differences in the prediction of material strength by Hall-Petch relationship, and there is no unified calculation method to determine the d value as grain size or cell size. In this work, representative materials including austenite 316L SS were printed by selective laser melting (SLM), and the strength was predicted. The values of cell size and grain size were substituted into Hall-Petch formula, and the results showed that the calculation error for 316L is increased from 4.1% to 11.9%. Therefore, it is concluded that the strength predicted by grain size is more accurate than that predicted by cell size in additive manufacturing materials. When calculating the yield strength of laser additive manufacturing metal materials through the Hall-Petch formula, the grain size should be used as the basis for calculation.