Microstructure evolution during laser-directed energy deposition of tool steel by in situ synchrotron x-ray diffraction

Abstract

Additive manufacturing (AM) processing of tool steels has a high level of complexity due to the martensitic transformation and consequently tempering during the component manufacturing. The thermal history generated over the part is very complex and challenging to evaluate after the AM process. By using high energy synchrotron X-ray diffraction (HEXRD), this work performs a series of in situ laser-directed energy deposition (L-DED) experiments to reveal the phase transformation and lattice defect evolution at different parts of a printed thin wall made of X40CrMoV5–1 (AISI H13) steel. To complement and correlate the findings obtained during the AM process, ex situ characterizations of the wall are done by means of HEXRD scans, electron backscattering diffraction (EBSD), micro hardness, and optical microscopy. The results show that the intrinsic heat treatment (IHT) at any position of the wall can be represented by a combination and repetition of five different types of phase transformation cycles. Overtime, the heat accumulation on the wall causes a decrease in cooling rates, modifying the transformation kinetics of these phase transition. The martensite carbon content evolution shows that over the cycles, less carbon remains in solution due to stronger self-tempering effects and formation of precipitates. While dislocation density evolution in martensite also shows a decrease over time due to self-tempering effects, the density evolution in austenite shows an increase, indicating an accumulation of defects over time. The ex situ characterization of the manufactured wall confirm the microstructure heterogeneities seen during the in situ L-DED experiments. The IHT created high hardness quenched martensite at the very top while regions in the middle and bottom were subjected to different severities of tempering. Additionally, the chemical maps shows the expected microsegregation of C, Mo, Cr and V over the manufactured sample and the presence of oxides caused by contamination during the L-DED process.