Page 22 - MSAM-1-1

Materials Science in Additive Manufacturing Spheroidization of 304L SS powder for LPBF process

manufacturing method, including forming, machining, in the spheroidization process to produce spherical tungsten
welding, and assembling, into a single-step process [3-6] . particles since the gas atomization method was not found
Advanced AM processes, such as laser foil printing capable of generating the required high temperature to meet
(LFP) and fused deposition modeling (FDM), have been tungsten’s high melting point. In another study, an increase
developed to fabricate complex-geometry metal parts in ferrite volume fraction, generation of nano-sized particles,
with high mechanical properties [7-11] . The widespread use and an increase in powder-bed density due to finer particles
of AM has primarily benefited the aerospace industry by filling the inter-particle voids were observed by Ji et al.
[29]
fabricating 3-dimensional (3D) parts using metals [12,13] . through investigating the spheroidization impacts on AISI
As the powder-based AM technology has matured, the stainless steel 316L powder. Depending on the type of raw
criticality of the powder used has become increasingly material, which may be the powder of a pure metal or a multi-
apparent, especially in the laser powder bed fusion (LPBF) element alloy, the evaporation rate per element during the
process with its special thermal history conditions [14-16] . spheroidization process would be different due to the different
LPBF uses a wiper to spread a layer of powder with a melting points and vapor pressures of constitutive elements.
typical thickness ranging 30 – 100 µm on the substrate The different reaction of constitutive elements to high
to be scanned by the laser according to the computer- temperatures changes the chemical composition of yielded
aided design (CAD) data provided by the part geometry spheroidized powder compared to its as-received condition.
in a layer-by-layer fashion . The powder’s ability to In a study conducted by Park et al. , the best set of process
[30]
[17]
properly spread on the substrate when being pushed by parameters for the spheroidization of AISI stainless steel
the wiper significantly affects the powder bed density, 316 powder was analyzed. A slight decrease in the particle
which consequently affects the porosity and mechanical size and a considerable increase in powder flowability were
properties of the fabricated parts ; in addition, these observed. Although these previous studies present valuable
[18]
might impact the tensile strengths and fracture toughness knowledge on the spheroidization process for improving
of fabricated parts [19,20] . powder properties, a substantial knowledge gap still exists
The powder flowability strongly depends on the on the relationship between powder’s chemical composition
geometry of powder particles . The irregular particle before the LPBF process and part properties after the LPBF
[21]
geometries tend to engage, agglomerate, and interlock process. Such lack of knowledge creates ambiguity in terms of
together, hindering the powder flow. In contrast, spherical the morphological, chemical, and microstructural properties
particles show better flowability, mainly due to the absence of the powder feedstock. The work presented in this paper
of inter-particle friction forces . Therefore, to have a covers these gaps with a significant focus on tailoring the
[22]
better powder flowability, the delivered powder feedstock powder properties with plasma spheroidization for use in the
to the LPBF machine should be in spherical geometries . LPBF process.
[23]
The commonly used powder feedstock in the LPBF In this study, the gas-atomized AISI 304L stainless
process is currently produced using inert gas atomization. steel powder, composed of Fe, Ni, Cr, Mn, and Si, was
The molten material, generated through induction spheroidized by an induction plasma spheroidization
melting, is atomized by an inert gas, such as argon or process to investigate the impact of this process on powder
nitrogen, when falling under gravity in the atomization characteristics and part properties. The spheroidized
chamber [24,25] . Although most of the produced gas- powder was characterized in morphology, chemistry,
atomized powder particles are in acceptable spherical and microstructure, which were compared with those of
geometries, the presence of a few irregular particles is still the as-received powder. Some parts were fabricated with
easily observable. Therefore, improving the properties of the spheroidized powder using the LPBF process. Their
gas-atomized powder by creating even more spherical mechanical properties were compared with those of the
particles without the appearance of irregular geometries is parts fabricated with the as-received powder to assess the
crucial. effects of the spheroidization process on the mechanical
properties of LPBF parts.
Plasma spheroidization changes the geometries of powder
particles by creating surface tensions when subjecting the 2. Material and methods
particles to plasma around 10,000 K, which is high enough to 2.1. Material
melt and decompose most materials . Due to this process’
[26]
considerable benefits, such as improving particle geometries The gas-atomized (in argon media) AISI 304L stainless
and reducing chemical impurities, several researchers have steel powder was provided by LPW Technology (Carpenter
recently studied the plasma spheroidization process, a Technology Corp., USA). The manufacturer reported the
comprehensive review of which has been conducted by Sehhat particle size distribution of as-received powder as 13, 20,
et al. . Wang et al. took advantage of the high temperature and 30 µm for D , D , and D ,respectively.
[27]
[28]
10 50 90
Volume 1 Issue 1 (2022) 2 http://doi.org/10.18063/msam.v1i1.1