Mössbauer spectroscopy for additive manufacturing be selective laser melting

Various experimental techniques (X-ray diffraction - XRD, scanning electron microscopy - SEM, energy dispersive spectroscopy - EDS) were used to improve the SLM process and to inspect the properties (surface morphology, microstructure, phase and elemental composition) of components produced by SLM. Mössbauer spectroscopy (MS) is a suitable method for examining iron-based components. Transmission 57Fe MS (TMS) can be used for investigation of metal powders. Backscattering 57Fe MS with registered conversion electrons (CEMS) and secondary X-rays (CXMS) allows surface layers to be studied selectively. When applying CEMS, we receive information from the surface layer with a depth of about 0.3 μm. The CXMS contains information from depths of tens of micrometers. The phase composition of stainless steel metal powders (CL20ES) and maraging steel powders (CL50WS) during annealing was studied by TMS in the temperature range of 500 - 1100 °C in air and in an inert atmosphere. Different iron oxides (α-Fe2O3, Fe3O4 and wüstite) were identified after thermal treatment in air at different temperatures. Depending on the annealing temperature in a nitrogen atmosphere, transformations of austenite ↔ martensite were observed. CXMS and CEMS showed changes in the phase composition of the parts depending on the annealing temperature and atmosphere. At the same time, some changes occur only in the surface layer with a thickness of approximately 0.3 μm. These are changes in the ratio of the austenitic and ferritic phases. Various iron oxides also occur during surface oxidation. In accordance with XRD and EDS, it has been shown that some phase transformations occur as a result of the diffusion of alloying elements in the surface layers. SEM showed an inhomogeneity of the emerging surface layer, which unfortunately results in difficulties in the registration of CEMS spectra.