Quantitative distribution characterization of gradient composition of additive-manufactured stainless steel using micro-beam X-ray fluorescence

The fabrication of the compositionally graded metals provides a new frontier in additive manufacturing and the characterization of the elemental distribution of graded materials is strongly desired for the quality control of powder mixing and improvement of additive manufacturing process parameters. In this study, Micro-beam X-ray fluorescence (μ-XRF) was used to characterize the elemental distribution of additive-manufactured compositionally graded stainless steel on a large area. A poly-capillary X-ray optics unit was used to obtain a high spatial resolution with the spot size of 20-μm. The quantitative distribution characterization of Ni, Mo, Cr, Mn, Si and Cu elements in the whole deposition direction have been realized through the improvement of μ-XRF quantitative calibration method by using certified reference materials with composition and microstructure similar to the graded sample. The composition distribution variation in different cladded layer was also investigated with the pixel distance decreased to 20 μm. Moreover, laser-induced breakdown spectroscopy was used to analyze the composition distribution of the composition-graded steels to verify the reliability of quantitative distribution characterization results by μ-XRF method. The elemental distribution results from two methods have good agreement, but the results determined by μ-XRF can give more details about the composition distribution in different cladded layers because of its higher spatial resolution, which is closely related to additive manufacturing process. The microstructure content varied linearly with the change of composition. The austenite volume fraction increased with an increase in Ni content and the evolution rule of the structure agreed with the calculated thermodynamics model results. Cracks appeared in the martensite and austenite transformation region with a large volume change when the Ni content ranged between 6.5% and 8.5% for the gradient stainless steels.