2.3. Characterization of the Powder and Printed Parts

Differential Scanning Calorimetry analyses (DSC) were performed in a Netzsch DSC 200 F3, to determine the melting temperature and the melting enthalpy of the powder along subsequent processing cycles. The samples heating was carried out from 20 °C to 220 °C, at a rate of 10 °C/min and maintained at 220 °C during 1 min. Then, the samples were cooled down until 20 °C at a rate of 10 °C/min. Nitrogen, purged at 50 mL/min, was used to ensure an inert environment.

Scanning Electron Microscopy (SEM) using FEG-SEM FEI Nova 200 field emission gun scanning electron microscope, from FEI Company, Hillsboro, OR, USA, was used to characterize the powder morphology. High voltage of 10 kV and secondary electrons (SE) mode were selected for the analysis.

For characterization purposes, two distinct sample geometries were produced for each printing cycle:

Parts printed in each (re)processing cycle: (a) tensile test sample and (b) CT sample for porosity characterization (dimensions in mm).

The coordinates of the samples in the build space were kept fixed in all the printing cycles (corresponding to the first layers of the center of the build space).

A Shimadzu AG-X tester was used to conduct the tensile tests until break. The deformation was monitored through a video extensometer. The tests were performed at a 10 mm/min speed, at room temperature, and a load cell of 1 kN was used.

The printed samples illustrated in Figure 3b were analyzed in a computed tomography equipment, XT H 225 S, from Nikon Metrology, using a tungsten filament. The 360° scans were performed with beam energy of 180 kV, power beam current of 11 µA, 20 W of power, and exposure of 4 fps. Two frames were taken for each projection. All images were analyzed in the Visual Graphics Studio software. For each sample, three longitudinal cuts (see Figure 4a) performed at 1.0, 2.5, and 4.0 mm from the sample wall marked with a C symbol, were made, and four areas were considered in each section (see Figure 4b). Thus, a total of twelve values of porosity were determined for each sample. Additionally, three transversal cuts (see Figure 4c) were made to check the shape of the samples. The porosity results are presented as the average values from three independent samples. Statistical analysis was evaluated by one-way ANOVA followed by Tukey’s test for multiple comparisons, considering p < 0.05 as significant.

Computed tomography analysis: (a) 3D view; (b) one of the three longitudinal cuts made in each sample, showing the four sections where porosity values were taken; and (c) one of the three transversal cuts made for sample shape checking.

During the successive printing cycles, a visual inspection of the parts was carried out to qualitatively evaluate their aesthetic quality. This is a relevant issue since SLS is often used to produce parts for applications where good surface finishing is required.