Page 96 - MSAM-4-2

Materials Science in Additive Manufacturing Additively manufactured high carbon steel

A B

C D

Figure 1. Materials selection and gas atomized alloy powders. (A) Time-temperature-transformation (TTT) diagram of the present alloy with varying
carbon concentrations calculated using JMatPro (version 7). Red arrows indicate the decreasing martensite start (M ) temperature as a function of
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s
increasing carbon concentration. (B) Phase fraction calculated using JMatPro (version 7) based on the nominal composition. (C and D) Particle size
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distribution (C) and secondary electron micrograph (D) of gas-atomized powders
thermal gradients, can impact phase transformations austenitic as-built alloy, which can then be subsequently
and the resulting microstructure. LPBF-fabricated parts heat treated to develop complex, hierarchical micro-
commonly exhibit cellular dendritic microsegregation of constituents.
alloying elements, 23,27,28 leading to spatial variations in local Pre-alloyed charges were inductively melted and gas-
composition at the sub-micron scale, which can deviate atomized using an-house double-induction gas atomizer.
from the nominal alloy chemistry. Metal charges were melted under ultra-high purity argon
These compositional inhomogeneities can significantly (99.99%), shrouded in an alumina crucible, and atomized
influence phase stability and transformation behavior, using ultra-high purity argon pre-set to 3 MPa. Gas
making it difficult to accurately predict or control the final atomized powders were mechanically sieved using an
microstructure based solely on nominal compositions. industry-standard No. 200 (75 μm) sieve compatible with
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To address this, calculating the M temperature across a a Ro-Tap mechanical shaker. Particle size was verified
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range of carbon concentrations offers valuable insight into using a laser diffraction particle size analyzer (Beckman
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phase stability during LPBF. As displayed in Figure 1A, the Coulter LS 13 320; Beckman Coulter, United States
M temperature decreases substantially with increasing to obtain a particle size distribution suitable for LPBF
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carbon content – dropping to approximately, 116°C at outlined in Figure 1C. The powder morphology is featured
0.85 wt.% C, and further to ~74°C at 1 wt.% C. Coupled in Figure 1D, and the composition of the powders reported
with rapid solidification inherent to LPBF and a relatively in Table 1 was measured using X-ray energy-dispersive
low substrate temperature of 100°C, it becomes feasible spectroscopy (XEDS).
to suppress martensitic transformation below room X-ray diffraction (XRD) patterns were acquired
temperature at higher carbon levels. This would enable using a PANalytical Empyrean diffractometer (Malvern
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the retention of a predominantly austenitic matrix in the Panalytical, the Netherlands) equipped with a Cu K
α
as-printed state. Accordingly, the primary focus of this radiation source operating at 45 kV and 40 mA. The XRD
study is to exploit the rapid cooling rates in LPBF and patterns were obtained using a step size of 0.03°, dwell time
the elevated carbon content to produce a predominantly of 60 s, and 2θ range between 30° and 85°. The direct

Volume 4 Issue 2 (2025) 3 doi: 10.36922/MSAM025100011