Page 16 - MSAM-2-1

Materials Science in Additive Manufacturing AM-produced CoCrFeMnNi properties

Figure 9. Average composition of selective laser melting-produced
CoCrFeMnNi at 450 and 750 mm/s scanning speeds before and after heat
treatment.

is far less than the melting temperature of Mn (1246°C).
Mn evaporation is believed to contribute to the formation
of voids in SLM-produced samples with the higher Figure 10. Open circuit potential with respect to normal hydrogen
energy density as discussed above. In the literature, electrode for selective laser melting-produced CoCrFeMnNi at 450 and
similar phenomenon was observed in SLM-produced 750 mm/s scanning speeds, with and without heat treatment.
CoCrFeMnNi . Together with the presence of the
[36]
aforementioned micro-cracks, this slight depletion in Mn decrease is more pronounced as E of 750 HT case
corr
might be another indication of the need for further process is almost 100 mV <450 HT case. Gradual decrease in
parameter optimization. On the other hand, the heat corrosion resistance with increase of scan speed was
[61]
[39]
treatment hardly affects the composition of SLM-produced also reported for SLMed Ti-6Al-4V , 316L steel and
CoCrFeMnNi. CoRrW .
[62]
3.3. Electrochemical corrosion behavior It appears that heat treatment also reduces corrosion
resistance of the SLMed CoCrFeMnNi. In some cases, its
Open circuit potential (OCP) is a straightforward method effect is stronger compared to scan speed. As shown in
to study the corrosion behavior. Figure 10 shows OCP Figure 12, corrosion potential decreased from −0.200 to
results for four conditions. Potentials of both AB conditions −0.216 V versus NHE after heat treatment of 450 sample
gradually shift toward the anodic direction or in positive while it decreases from −0.225 to −0.291 V versus NHE
(more noble) potential values. This is an indication that a after heat treatment of 750 sample. Corrosion resistance
passive film was formed on the surface of AB samples [36,60] . decline was also reported for SLMed AlCoCrMnNi HEA
The OCP of 750 AB sample has dipped toward the end of and was attributed to phases formed after heat treatment,
the experiment, which is an indication of a breakdown and which result in less protective passive film . Zhu et al.
[35]
[63]
re-passivation. The opposite phenomenon was observed reported decrease in corrosion resistance of CoCrFeMnNi
with OCP of HT samples. OCPs of both 450 HT and with annealing time longer than 6 h. Decrease in the anti-
750 HT samples shift towards the cathodic direction or corrosion property was attributed to σ precipitates and
in the negative (less noble) potential values and after 1 h, Mn-rich inclusions, which increase the susceptibility to
and the potential stabilizes at about −0.1 V versus normal pitting corrosion.
hydrogen electrode (NHE). In this case, the implication is 3.4. Mechanical properties
that passive layers did not form.
Tafel potentiodynamic polarization curves of 450 3.4.1 Tensile properties
and 750 samples at AB and HT conditions are shown in The engineering stress-strain curves, ultimate tensile
Figure 11. Potential at the dip of such curves represents strength, and elongation of the tensile samples are presented
corrosion potential (E ). This parameter reflects the in Figure 13. It can be seen that heat treatment causes only
corr
stability of the system: the higher the E and the smaller insignificant changes in ultimate tensile strength (<0.5%
corr
the corrosion tendency [37,39] . Figure 12 summarizes the for the 450 mm/s cases, and up to 4% for the 750 mm/s
corrosion potentials for all four cases obtained in this cases), while it considerably improves the ductility (59.3%
experiment. As can be seen from the two figures, higher and 23.2% increases for the 450 and 750 mm/s cases,
scanning speed results in slight decrease in corrosion respectively). Similar results were observed in spark
resistance in the AB case. After heat treatment, such plasma-sintered CoCrFeMnNi . Decrease in tensile
[3]

Volume 2 Issue 1 (2023) 10 https://doi.org/10.36922/msam.42