Singapore: Researchers Test Potential of High Entropy Alloys in EBM Metal 3D Printing

Taking the Art to the Cart

Singapore: Researchers Test Potential of High Entropy Alloys in EBM Metal 3D Printing

Metal 3D printing is becoming invaluable for many manufacturers today
worldwide, and the research regarding processes and materials continues
as researchers from both Singapore Institute of Manufacturing
Technology and Nanyang Technological University explore metal powders
being used in electron beam melting (EBM) technology today in
‘Additively manufactured CoCrFeNiMn high-entropy alloy via pre-alloyed
powder.’

Most studies regarding serious manufacturing practices
and their interest in 3D printing with metal center around the best ways
to produce strong, complex geometries. Here, the authors review whether
CoCrFeNiMn high entropy alloy (HEA) parts produced through EBM—very
similar to the popular selective laser sintering (SLS) process—is a
realistic improvement over conventional casting techniques.

CoCrFeNiMn
is known as an equiatomic alloy powder made through vacuum induction
via atomization with argon gas. As a single face-centered-cubic (FCC)
crystal structure, CoCrFeNiMn has been the focus of a wide variety of
research throughout the years due to:Strong mechanical propertiesCorrosion resistanceWear resistanceExcellent ductility

The
scientists point out that while HEAS like CoCrFeNiMn perform well in
cryogenic temperatures, melting, casting, and mechanical alloying are
the ‘dominant preparation methods,’ often leading to issues with both
voids and porosity. Powder bed fusion additive manufacturing (PBFAM)
offers potential for fabricated HEAs due to the following features:Short processing timeGeometrical accuracyReduced wasteCustomization possibilities

EBM
relies on high energy preheating up to 1100 °C, offers reduced stress
on reactive parts, and has been known to be successful in production of
HEA parts previously. Along with evaluating CoCrFeNiMn in terms of its
microstructure and mechanical properties, researchers were able to
produce it through gas atomization for this study, producing further
analysis in powder flow, particle size, density, defect, printability,
and more. For better ease in 3D printing, the atomized powder was
separated into four different categories: ≤25 μm, 25–45 μm, 45–105 μm
and 105–300 μm. Respectively, this allows for spark plasma
sintering/injection molding, selective laser melting (SLM), EBM, and
laser-aided AM.

Flowability is one of the important features in
PBFAM, and can be determined in different ways, but for this study was
evaluated through the Hall flowmeter funnel and pronounced excellent.
Particle size was evaluated, with good printability proven, and parts
were inspected for defects based on Archimedes principle and OM
observation. Further, microhardness was evaluated as follows:

“The
microhardness was examined on the polished specimen by using Matsuzawa
MMT-X3 Vickers hardness tester at 1 kg for 15 s. Dog-bone specimens with
a cross-section of 1 × 3 mm2 and a gauge length of 5 mm were cut from
the cuboid sample. An Instron 5982 universal tensile testing machine
with a 10 kN load cell was used for the tensile test with an initial
strain rate of 3.3 × 10−4 s−1 at room temperature. A video extensometer
was applied to record the strain. Three specimens were examined to
obtain the yield strength (YS), ultimate tensile strength (UTS), and
elongation to failure.”

Chemical composition analysis revealed
‘spherical morphology’ and only a few irregular particles, with all
particles overall created as solidifying droplets collided in the
‘turbulent flow’ of atomization.

“In addition to the satellites,
the spherical pores corresponding to entrapped gas during the
atomization process was revealed by cross-sectioned observation. In
contrast to the occasional appearance in the fine powder, these
spherical pores prevailed in the coarse powder. These entrapped gas
pores not only influence the true density but also cause defects in the
AM parts.”

“A relative narrow ranged powder was obtained after the
sieving process. The average particle size is 10.3, 36.2, 63.3 and
129.8 μm for the P1, P2, P3, and P4, respectively. The size distribution
overlap is caused by non-perfect sieving process, such as plugged mesh
by spherical powder, which can be improved by modifying the sieving
process.”

SEM images showing typical HEA powder morphology with
different magnifications. (a) and (b) for powder size ≤25 μm (P1); (c)
and (d) for powder size ranged from 45 to 105 μm (P3).

The
scientists agree that while previous processes may have led to obstacles
in using HEA powder, gas optimization makes such materials a definite
consideration for mass production, stating that the powders offered all
the following:Desirable and apparent densityTapped densityFlowabilityParticle size distribution

The
researchers do point out, however, that there could be safety issues
due to the ‘high density of satellites,’ although it does not seem to
affect the EBM printing process. Porosity is a concern however, and the
researchers tentatively suggest the hot isostatic press process for
elimination of such issues in additive manufacturing, but it is costly
and can be limiting for most applications.

“It is suggested that
powder with low porosity, for example, produced by plasma rotated
electrodes process, would be an ideal choice for critical industrial
parts that needs to be exposed in high operating temperature,” said the
researchers.

Ultimately, the team concluded that all important
features of the process studied here deem it suitable for PBFAM
technology and new materials, further stating:

“… the EBM-built
CoCrFeNiMn HEA parts had comparable mechanical properties (microhardness
and tensile properties) to their conventional cast counterparts.”

As
3D printing with metal in a variety of different methods begins to
infiltrate industries focused on intense manufacturing processes, the
study of the materials and powders that accompany this technology
continues to grow, as exemplified in the automobile industry, aerospace,
military and ammunitions endeavors—and far more. Find out more about
the PBFAM process and the use of new materials in 3D printing here.

What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

(a) Image of EBM-built part. (b) SEM of the rough surface from the side
view. (c) Typical top surface appearance of the cuboid samples with
different extent of swelling and lack of fusion. The yellow and red
arrows reveal the swelling and lack of fusion, respectively. (For
interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)

 

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