2.4 Metallographic specimen preparation and microstructure analysis

All LPBF-printed samples were first cut of the substrate plate using a manually operated abrasive cutter with water cooling. Specimens were then sectioned using a precision cutting machine with the plane of sectioning being parallel to the building and gas flow direction and at the approximate middle of the specimen. Melted specimens of the boron- or yttrium-containing alloys were cut parallel to the expected macroscopic solidification direction in the as-cast state. The CP titanium rod material, used for comparison during the microstructure analysis and tensile tests (see Section 2.6), was studied for its cross section. All specimens were subsequently warm embedded in EpoMet™ G (Buehler) and BAKELIT (ATM Qness) mounting compound (the latter used as filler material) and then ground, polished, and etched according to standard metallographic specimen procedures. Etching was performed for approximately 10 s using a mixture of 86 vol% H₂O, 4.5 vol% HNO₃, 12 vol% H₂O₂, and 5 vol% HF. Regarding porosity measurements, grinding was performed by using SiC grinding papers of sizes P180, P240, P320, P400, P600, P800, P1200, and P2500 on an automatic grinding machine (ATM SAPHIR 550). The P1200 and P2500 grinding papers were thereby pretreated with wax. Polishing was conducted using MasterPrep™ (Buehler) polishing suspension with a particle size of 0.05 µm together with oxalic acid for approximately 10 min followed by 1 min rinsing with tap water.

The porosity of the samples was determined via optical analysis on polished and cleaned (soap water and ethanol) samples. A reflected light microscope (ZEISS Axio Imager.M2m) was used in the bright field mode for image capturing, and the Fiji software (Schindelin et al., 2012) was used for porosity measurements. For each specimen, nine images were obtained with ×100 magnification in a pre-defined 3 × 3 grid, excluding borders and support structures. During the image analysis, the upper threshold value was individually set for each image and determined by sight. This and some residual ethanol spots or artifacts from the metallographic specimen preparation left on the specimens' surface lead to a slight uncertainty or error on the measured porosity. This uncertainty is hardly quantifiable, since the error in measurement due to choice of the upper threshold value will be a function of the individual porosity within the analyzed picture and the pores' contour sharpness, which is the outcome of the specimen preparation procedure. This contour sharpness and the amount of residual polishing artifacts can be specimen dependent. Regarding the residual ethanol spots, the relative error could possibly be comparably high if the total porosity is low. Accordingly, the measured porosities or relative densities should be seen as approximate values. Nevertheless, two decimals are used for measured porosities, since otherwise fully dense specimens would be partly indicated by the results.

For microstructural investigations, a reflected light microscope (Olympus BX51M) was used in order to obtain panorama images of the samples, which consist of several individual images taken with ×50 or ×100 magnification, which have been captured and stitched together automatically using the Stream Motion software (Olympus Corporation, 2011). In addition, a tabletop SEM microscope (Hitachi TM3000) operated with an acceleration voltage of 15 kV (with and without increased beam current) and equipped with a BSE detector and an EDS system (QUANTAX 70, Bruker Nano) was used to obtain high-magnification images and perform chemical analyses.