Hot cracking during additive manufacturing of D2 tool steels

Abstract

In this research, the mechanism of hot cracking during direct laser deposition of D2 tool steels was studied with the perspective of the liquation cracking due to the difference in the melting point of microstructural phases and thermal history. Thin-wall specimens were deposited under different laser powers and the length of the cracks varied with the laser power and the position in the specimen. Microstructures of primary austenite dendrites with the inter-dendritic regions which comprised eutectic austenite and carbides were developed, and the cracks were formed in the inter-dendritic regions. It is confirmed that the melting point of the eutectics is lower than the primary dendrites by atom probe tomography analysis and thermodynamic calculations. The distribution of the cracks was varied with the mis-orientation between the dendrites and it is explained by the decrease of the dendrite coalescence temperature. The thermal history of the building specimen was measured by pyrometry and the size of the cracks increased with the increasing number of reheating events above the solidus temperature. Based on these results, it is concluded that the re-heating behavior during the deposition significantly affects the formation and growth of the hot cracking.