Page 101 - MSAM-4-3

Materials Science in Additive Manufacturing Heterostructures of A131 steel by DED

Figure 6C displays the Kernel average misorientation tests were performed along ND and TD, respectively.
(KAM) mappings, revealing the misorientation angle From the stress-strain curves (Figure 8A), AB A131 steel
of the grains related to the lattice distortion. It can be exhibited outstanding tensile strength compared to the
observed that the misorientation was related to the grain other groups, while elongation significantly decreased
size, with smaller grains exhibiting larger misorientation compared to that of commercial steel along TD (HR-TD).
angles. A large misorientation angle corresponds to higher In addition, AB A131 steel exhibited strong anisotropy in
stress within the crystal, most likely due to the martensite tensile strength, with both tensile strength and elongation
phase and rapid cooling rates. of AB A131 steel along ND (AB-ND) surpassing those
After tempering, the IPF mappings (Figure 7A) along TD (AB-TD). After the HT process, the performance
display equiaxed grains in HT A131 steel with of HT A131 steel tested along ND (HT-ND) also exceeded
uniformly distributed colors, suggesting reduced that along TD (HT-TD), suggesting that the HT process
crystal misorientation compared to AB A131 steel. The played a limited role in reducing anisotropy in mechanical
corresponding pole figures (Figures S1 and S2) further performance. Moreover, an interesting Portevin-Le
confirmed that the HT process contributed to reducing Chandelier (PLC) phenomenon 40,41 was observed in
the maximum orientation density of the grain (from 2.01° the curves of both AB-TD and HT-TD, leading to stress
to 1.59°). In addition, the heterostructure disappeared, serrations in the curves. This observation is likely due to
and both grain size and grain profiles became identical the negative strain rate sensitivity induced by dynamic
(average grain size: 7.93 µm), similar to that of the coarse strain aging effects, 42,43 where crystal misorientation of the
grain region. This indicated that the HT process led to grains led to increased dislocation interactions along TD
recrystallization, resulting in different orientations and the under the given strain rate.
formation of equiaxed grains. Compared to AB A131 steel, Figure 8B displays true stress-strain curves and
the KAM mappings (Figure 7B) of HT A131 steel displayed corresponding work-hardening behavior based on
smaller misorientation angles within the grains, likely due Figure 8A, indicating that the work-hardening curve of
to reduced martensite content and grain growth, which HR-TD gradually decreases after the yield strength (σ )
helped reduce lattice distortion. YS
point, with the rate of work-hardening declining gradually
3.2. Mechanical properties as elongation increases. The work-hardening rates of AB
A131 steel decreased dramatically against the increasing
To further investigate the effects of A131 steel microstructure rates of strains, especially for the AB-TD curve, likely
on mechanical performance, uniaxial tensile and hardness
due to limited elongation. Moreover, significant stress
A B serrations were observed in the work-hardening curves
along TD for both AB and HT A131 steel, attributed to
the PLC effects, leading to notable stress fluctuations. This
suggested that work hardening was limited in resisting
deformation during the tensile test along TD at the given
strain rate.
The corresponding σ , ultimate tensile strength
YS
(σ UTS ), and elongation are displayed in Figure 8C, and the
corresponding data are summarized in Table S3. Compared
to the σ (346.5 MPa) and σ UTS (545.0 MPa) of HR-TD, the
YS
σ and σ UTS of AB-ND dramatically improved by 168.3%
YS
and 78.0%, respectively, with the strain of 24.6% meets the
definition of the EH A131 standard (19~26% in strain).
The performance of AB A131 steel decreased slightly in
TD but reported enhanced σ and σ UTS of 132.2% and
YS
46.6%, respectively, with a 65.5% reduction in elongation.
This was attributed to the predominant acicular martensite
and fine equiaxed grain (Figures 4-6), resulting in notable
Figure 7. HT A131 steel heterostructure along TD: (A) IPF image; and improvements in strength. After the HT process, HT
(B) KAM mappings. (A, inset) a bar chart of the grainsize distribution. A131 steel reported reduced strength, with enhancements
Scale bars: 100 µm
Abbreviations: HT: Heat treatment; IPF: Inverse pole figures; of 70.0% and 24.1% in σ and σ UTS , respectively, for
YS
KAM: Kernel average misorientation; TD: Transverse direction HT-ND, while HT-ND exhibited a 4.6% decrease in σ UTS .

Volume 4 Issue 3 (2025) 8 doi: 10.36922/MSAM025220038