PSI - Issue 15

Enzoh Langi et al. / Procedia Structural Integrity 15 (2019) 41–45 Author name / Structural Integrity Procedia 00 (2019) 000–000

43

3

2.2. Characterisation of microstructure Standard metallography techniques were used to prepare the specimens for electron backscatter diffraction (EBSD) and chemical composition (Energy Dispersive X-ray spectroscopy, EDX) analyses. All specimens went through sectioning, mounting, grinding and polishing (down to 0.5 µm). The VibroMet 2 was combined with MasterMet 2 Colloidal Silica to chemo-mechanically polish the samples to a surface finish suitable for EBSD. The samples were analysed in a field emission scanning electron microscope (SEM JEOL 7100F) equipped with an Oxford Nordlys II S EBSD detector. The microscope was operating at a 20-kV acceleration voltage. The step size of 0.5 µm was selected for all the samples. 3. Results and Discussion The scanning electron image for the 316L SLMed tube is shown in Figure 2, with no observed internal porosity. This result indicates that AM can be used to fabricate tubes for producing stents. In contrast, other studies (Brooks et al., 2015; Khairallah et al., 2017) showed that additive manufacturing via selective laser melting can lead to formation of pores within manufactured parts, affected by a lack of fusion, entrapment of gases during manufacturing, un-melted or partially melted particles of powders and delamination between deposited layers (Frazier, 2014). The SEM/EDS analyses revealed similar chemical composition for the SLMed tube and the commercial stent, with Fe, Cr, Ni and C as the main alloying elements and Mo, Mn and Si as the secondary elements (Table 1). Some differences were seen in weight fraction of alloy elements between the two samples, which could affect the material properties and the residual stresses introduced during stent manufacturing.

Fig. 2. SEM image for radial cross-section of the 316L stainless steel SLMed tube showing no pores

Table 1. Chemical composition (wt%) for 316L SLMed tube and commercial stent Specimen Fe Cr Ni C Mo

Mn 0.9

Si

316L Stainless steel SLMed tube 316L Stainless steel commercial stent

65.3

16.4

10.5

4.2

2.2

0.6

60.8

17.9

13.4

4.6

2.7

0.6

The inverse pole figure (IPF) maps are given in Figs. 3 (a) and (c) for the 316L stainless steel SLMed tube and 316L commercial stainless steel, where each colour represents a certain crystallographic orientation as presented in the colour-coded triangle. It can be seen (Figs. 3(b) and (d)) that most of the grains have high angle grain boundaries. For SLMed stainless steel tube, the sample consists of fine grains towards the wall edges and coarse grains at the middle of the tube wall. The coarse grains are elongated in the longitudinal direction, with a length of hundreds of microns but a width of only tens of microns (see Fig. 3 (a)). While the commercial stent has equiaxed and fine grains,