Page 8 - MSAM-2-1

Materials Science in Additive Manufacturing AM-produced CoCrFeMnNi properties

metallurgy knowledge suggests certain correlation between product surface finish, has become a major AM technique
the number elements in a multicomponent system and the for making complex metal components. Note that SLM
number of phases and intermetallic compounds formed belongs to the general powder bed fusion (PBF) group of
in such system. In 2004, Yeh et al. showed that single AM processes according to the ISO/ASTM classification.
[2]
or double solid solutions can become stabilized when In SLM, localized laser heating and high laser scanning
multiple elements are mixed in an appropriate ratio. This speed result in extreme short duration of laser-material
[11]
can happen because configurational entropy of mixing interaction . The resultant rapid cyclic heating and cooling
such elements is high enough to overcome the enthalpy process is the root cause for the unique microstructure and
of compound formation . This new class of materials mechanical properties of SLMed materials. SLM process
[1]
was named as high-entropy alloys (HEAs). HEAs can may also produce favorable properties for HEAs. For
be defined with respect to composition or entropy. For instance, it was discovered that CoCrFeNi HEA obtained
composition-based definition, HEAs are usually the alloys from SLM possess much higher yield strength compared
[13]
containing at least four dominant elements with atomic with that produced by arc melting . Li et al. investigated
[12]
percentage between 5% and 35%, in which additional SLM-produced CoCrFeMnNi and observed that elongated
minor elements are allowed if their atomic percentage is columnar grains grew epitaxially with a <001> orientation
<5%. For entropy-based definition, entropy of mixing is parallel to the build direction. The elemental distribution
n was homogeneous except Mn, which was present in
calculated for alloys : S =− R ∑ xlnx , where R is the
[1]
mix i i higher concentration in the boundary of melting pool.
i=1 Chen et al. studied the feasibility of in situ alloying of
[14]
gas constant and x is the molar fraction of the i-th element
i
in the mixture, and the alloys with entropy higher than elemental Mn with pre-alloyed CoCrFeNi in SLM. Only
1.5R are considered HEAs . a single FCC phase was found in the resultant materials.
[4]
The microstructure was characterized to be coarse
A major type of HEAs has been developed based on columnar growing through more than 10 layers in the
the parent alloy of CoCrFeNi, which consist of a single build direction, and a strong <001> texture was detected.
face-centered cubic unit cell (FCC) solid solution without Guo et al. investigated machinability of SLM-produced
[15]
[5]
segregation . The addition of more elements to this HEA CoCrFeMnNi. It was found that some machining operations
can reduce the diffusion, which enhances microstructure led to compressive stress in machined surface in the cases of
and mechanical properties such as creep resistance. For milling and grinding, or an increase in tensile stress in the
instance, CoCrFeMnNi, which also consists of a single- case of wire electrical discharge machining (EDM). Savinov
phase FCC, was first studied by Cantor et al. , and et al. evaluated CoCrFeMnNi HEAs obtained from two
[6]
[16]
now it is known as “Cantor alloy.” This alloy is one of major metal AM methods, that is, SLM and laser directed
the most thoroughly investigated HEAs, and it exhibits energy deposition (DED) processes. It was shown that both
some attractive mechanical properties, such as unusual AM methods led to a single-phase FCC material, but the
combination of high yield strength, high ultimate tensile average grain size of DED-produced materials was twice
strength, high ductility and fracture toughness at cryogenic that of SLMed materials.
temperatures . The damage-tolerance can be attributed to Tensile properties of CoCrFeMnNi alloy were
[7]
the low stacking fault energies (SFEs) ranging from 18.3 investigated by Li et al. , which showed the increase of
[13]
to 27.3 mJ/m ∙m at room temperature . The Cantor ultimate tensile strength with the increase of volumetric
2
[8]
−2
alloy was also shown to exhibit good radiation resistance. energy density until 123 J/mm , followed by a flattened
3
Damage-tolerance also rises from high lattice friction stress pattern beyond that. Zhang et al. observed little effect
[17]
which results from the apparently random distribution of of layer rotation strategy on yield and ultimate tensile
the solutes. This forms a true solid solution down to atomic strength, which were about 550 and 650 MPa, respectively.
scale which leads to high resistance to dislocation motion.
Chew et al. studied CoCrFeMnNi manufactured by
[18]
Unfortunately, the applications of HEAs are often laser DED. Its tensile strength was found to be higher than
plagued by the availability of manufacturing methods. the counterpart fabricated by SLM (660 MPa). Besides
Traditional manufacturing approaches are either expensive tensile properties, fracture toughness is also an important
or inefficient in dealing with complex shapes, while additive material property, which measures how good a given
manufacturing (AM) technology is well positioned to material hinders crack propagation at high-rate loading
overcome such challenge thanks to its nature of layer-wise and is related to the service life and safety of many load-
fabrication and significantly simplified process steps [8- bearing components . Together with high ductility,
[19]
10] . Compared with other AM techniques, selective laser toughness is especially important when the SLM-produced
melting (SLM), featured with high geometrical accuracy and HEAs are used as structural materials . Low toughness
[20]
Volume 2 Issue 1 (2023) 2 https://doi.org/10.36922/msam.42