Page 349 - IJB-10-1

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

and the depth of corrosion pits . To enhance the corrosion workpiece with a length of 100 mm and a diameter of ~26
48
resistance of AM-prepared 316L steel, one hypothesized mm (circular cross-section) was manufactured using a
solution is to combine AM and post-processing, which is a Meander printing strategy; the principle of which lies in
thermomechanical treatment 15-17 . cladding individual lines of the powder next to each other
The main focus of the present study was to correlate along the entire length of the designed workpiece 16,53-56 .
the microstructures and corrosion behaviors of 316L The meander printing strategy is favorable as it represents
steel workpieces, prepared by a combination of the SLM a favorable trade-off between high deposition rate and
methods and thermomechanical treatment performed with low-temperature gradients. After the SLM, we applied a
rotary swaging. The swaging method is highly favorable heat treatment at 900°C for 30 min, primarily to impart
for the processing of powder-based and pre-sintered (partial) homogenization and relaxation of the residual
materials, mainly because it features predominantly stress. On the other hand, a relatively short dwell time was
compressive stress state during processing, which supports selected to avoid the risk of (negatively) affecting the utility
57,58
consolidation of the material and the elimination of properties, including the corrosion ones .
possible printing defects and voids, and incrementally Having finished the preparation of the workpiece,
imparts shear strain, which supports shear mixing and thermomechanical post-processing via rotary swaging 59-61
gradual grain refinement 16,49 . In the current study, an at the temperature of 900°C was applied. The swaging was
original method was utilized to prepare the material, and performed in several consecutive passes to acquire swaged
electrochemical corrosion tests were performed in an rods with diameters of 20 mm, 17 mm, and 15 mm. The
ionized simulated body fluid (i-SBF) 50-52 (as documented applied swaging ratio in each consecutive pass can be
above, if the corrosion behavior of AM-prepared 316L steel calculated using Equation (I):
for prospective bio-application has been examined, it has
mostly been in NaCl-based solutions). The results of the
electrochemical corrosion tests were correlated with the ϕ = S n (I)
results of microscopic investigations of microstructure and S 0
substructure, as well as with the results of residual stress
analyses performed by means of neutron diffraction. where S and S are the cross-sectional areas of the
0
n
workpiece at the input and output from the swaging dies,
2. Materials and methods respectively. The swaging ratios for the individual samples
2.1. Materials were as follows: 0.5 for sample 20, 0.8 for sample 17, and
16,17
The original workpiece of the AISI 316L stainless steel 1.1 for sample 15. In the above mentioned works ,
was prepared using SLM from a steel powder delivered the authors documented that it is necessary to perform
by Renishaw, using a Renishaw AM400 machine (both a few initial swaging passes to sufficiently increase the
by Renishaw company, Wotton-under-Edge, UK). The density and reduce the residual porosity of AM-prepared
3D printing was performed under an inert atmosphere workpieces. Toward this end, we examined the samples
(Ar of 99.998% purity). The powder featured the particle subjected to the three last swaging passes.
size distribution of 15–45 µm and the exact chemical 2.2. Experiment
composition as presented in Table 1. As for the building
direction, we selected a vertical laser cladding strategy 2.2.1. Neutron diffraction
(based on the results of studies by Kunčická et al. 16,17 , In order to examine the positive effects of rotary swaging
who investigated the internal structures and intrinsic on the residual stress, its distribution within the workpiece
properties of AM-prepared AISI 316L workpieces laser- swaged to the diameter of 20 mm was investigated by means
cladded in the vertical and horizontal directions). The of neutron diffraction-strain scanning technique, which

Table 1. Declared and experimentally measured chemical compositions of AISI 316L steel powder

Element Cr Ni Mo Mn Si O N C S Fe
Composition by
manufacturer 16–18 10–14 2–3 <2 <1 <0.1 <0.1 <0.03 <0.03 bal.
(wt.%)
Measured
composition 18.3 ± 0.5 11.2 ±1 .2 2.2 ± 0.1 1.9 ± 0.2 1.1 ± 0.2 0.05 ± 0.01 0.03 ± 0.01 0.02 ± 0.01 0.004 ± 0.001 bal.
(wt.%)

Volume 10 Issue 1 (2024) 341 https://doi.org/10.36922/ijb.1416