Page 358 - IJB-10-1

International Journal of Bioprinting Corrosion behavior of SLM-prepared 316L steel

Table 2. Parameters of selective laser melting
Laser speed Scan speed Hatch spacing Layer thickness Energy density Build orientation
1400 mm/s 650 mm/s 60 µm 50 µm 1,025,650 J·mm3 Vertical

distributions of residual stress of a predominantly tensile dominant shear fiber texture of relatively high maximum
character 16,76 . The combination of AM and rotary swaging, intensity), and resulted in accumulation of dislocations
a deformation method which features a predominantly (sample 17 featured high density of dislocations and
compressive stress state 60,77,78 , favorably affected the stress occurrence of numerous dislocation tangles and cells). The
state within the workpiece and gradually transformed the development of substructure within this sample was also
tensile stress state toward a compressive one in the axial confirmed by the increased occurrence of misorientations
direction . At the beginning of the swaging process, the (see Figure 5B). Finally, swaging to the diameter of 15
compressive stress primarily occurred at the periphery of mm had no significant effect on further grain refinement,
the workpiece (tensile stress was still evident at the vicinity although it contributed to grain size homogenization. On
of the workpiece axis for sample 20). This positive effect the other hand, this swaging pass primarily influenced
was supposed to increase with increasing swaging ratio. the development of substructure—the increased imposed
In other words, continuing swaging further imparted strain induced the development of numerous subgrains.
homogenization of the residual stress, as regards its The occurrence of misorientations was comparable to that
compressive character . The presence of compressive stress observed within sample 17 (compare Figure 5B and C);
59
is advantageous from the viewpoint of corrosion behavior; however, a slight difference between these two structure
Hardes et al. reported that the occurrence of compressive states was observed. For sample 17, the misorientations
48
residual stress within AM-prepared 316L steel workpieces were primarily present in the vicinities of the boundaries
has a highly positive influence on the aggravation of micro- of the larger grains, as dislocations tended to accumulate at
cracks developed during corrosion. Moreover, the mutual grain boundaries as defects (irregularities) . Nevertheless,
79
effect of compressive stress and high imposed shear strain as the development of substructure progressed and
favorably contributed to fragmentation of grains during subgrains for sample 15 was formed (see also Figure 7C–
the swaging. F), misorientations occurred in a greater extent also in the
interiors of the larger grains—these were primarily related
Considering the results of grain size and KAM
analyses, the following hypothesis can be introduced. to the occurrence of subgrains.
Swaging of the original AM-prepared workpiece to the Not only the (residual) stress state, but also the
diameter of 20 mm primarily resulted in closing of the microstructure changes imparted by the swaging process
internal voids, elimination of residual porosity (no pores (according to the applied swaging ratio) substantially
were observed within the microstructure of sample 20), affected the behavior of the swaged samples during
and partial fragmentation of the grains. As the applied the corrosion testing. Kong et al. reported that a
80
swaging ratio was still relatively low, the grains within decreased dislocations density is related to a decrease
sample 20 were refined, but the grain size distribution was in the corrosion potential, i.e., the E parameter. The
corr
still inhomogeneous. Also, no substantial accumulation comparison of the dislocation densities for the swaged
of dislocations or significant development of substructure samples (based on TEM substructure observations and
was observed within this sample, which corresponded to KAM maps) shows that the dislocation density of sample
the relatively scarce occurrence of misorientations within 20 was incomparably lower than those of samples 17 and
the microstructure (Figure 5A). Continuing swaging to the 15, which corresponds to the lower acquired E value
corr
diameter of 17 mm caused further refinement of the grains for sample 20 (Table 3). The results also showed that the
and homogenization of their average size, but primarily CR decreased significantly after swaging to 17 mm, and
affected the grains’ orientations (sample 17 featured then increased very slightly after swaging to 15 mm (but

Table 3. Characteristic parameters of electrochemical corrosion by Tafel extrapolation
Sample Ecorr (mV) Icorr (µA/cm2) CR (mm/year) Rp (kΩ cm2) Epit (mV)
20 -179.7 ± 2.2 1.39 ± 0.18 0.0158 ± 0.0021 43.6 ± 0.5 973 ± 5
17 -166.9 ± 1.2 0.96 ± 0.15 0.0110 ± 0.0017 31.7 ± 2.2 1133 ± 20
15 -165.6 ± 1.6 1.13 ± 0.11 0.0129 ± 0.0013 45.1 ± 0.7 551 ± 5
Abbreviations: E , corrosion potential; I , corrosion current density; CR, corrosion rate; R , polarization resistance; E , critical pitting potential.
corr corr p pit
Volume 10 Issue 1 (2024) 350 https://doi.org/10.36922/ijb.1416