Page 80 - MSAM-3-2

Materials Science in Additive Manufacturing Mechanical properties of NiTi TPMS

in elastic strain, causing permanent deformation of the presented in Figure 18. On heating the samples above the A f
structure. temperature, the formation of the martensite phase, induced
The recoverable and unrecoverable strains of RGCS by residual stress, underwent re-transformation into the
and SGCS under various compressive strains are depicted austenite phase, leading to partial recovery of residual
in Figure 15. The recoverable strain of RGCS remained deformation. The irreparable deformation primarily
relatively consistent under 2% compressive strain. stemmed from internal structural fractures under stress.
However, as the compressive strain reached 4% and 6%, For RGSC-A0, the shape recovery ratio was measured at
an expected increase in recoverable strain was observed, 36.93%, while for SGCS-A0, it stood at 40.7%. The recovery
ratio of SGCS and RGCS structures gradually increased
with a slight augmentation noted with prolonged aging with aging time. Specifically, the shape recovery ratio of
time. Among these rod-shaped structures, RGCS-A10 RGCS-A6 and SGCS-A6 increased to 97.63% and 97.62%,
exhibited the largest recoverable strain, measuring at respectively. Notably, between 4 h and 6 h of aging time,
1.77%, 3.68%, and 5.36%, following compression strains the enhancement of SMEs was more pronounced, with the
of 2%, 4%, and 6%, respectively. Conversely, within the recovery ratio reaching its peak. Sun et al. measured the
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sheet-shaped structures, the SGCS-A4 sample exhibited recovery strain of the NiTi gyroid structures compressed by
the highest recoverable strain, measuring 1.84%, 3.82%, 4%, 8%, and 12% strain following immersion in a silicone
and 5.75% after undergoing 2%, 4%, and 6% compression oil bath at 150°C. The recorded recovery strains were
strain, respectively. The aging heat treatment exerted a 1.65%, 4.19%, and 5.91%, respectively. Yang et al. reported
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positive effect on enhancing the superelasticity of the NiTi- an overall shape recovery ratio of NiTi gyroid structures
TPMS structures. Moreover, maintaining a constant aging with varying volume fractions and unit sizes, ranging from
temperature while appropriately increasing the aging time 96.5% to 98.8% after heating under compressive strain.
significantly enhanced superelasticity. Therefore, NiTi alloy samples fabricated through LPBF can

3.4. Effect of aging heat treatment on shape undergo aging heat treatment to enhance the recovery ratio
memory and improve the SME.
The SME is an important functional characteristic of SMA, 4. Conclusion
denoting its capability to restore its original shape after
deformation induced by applied stress. This phenomenon This study investigates the effects of various aging times
on the microstructure, phase transformation behavior,
hinges on the reversible transformation between the mechanical properties, and functional properties of
martensite phase and the austenite phase. The results
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obtained from DSC and XRD analyses indicated that NiTi-TPMS structures after LPBF remelting. The main
the samples predominantly existed in the B2 phase at conclusions drawn from this study are as follows:
room temperature. The recovery of deformation after (i) The microstructure of the samples manifested as
compression primarily stemmed from martensitic coarse columnar crystals. On aging heat treatment,
transformation induced by stress excitation. The cyclic precipitation of the B19’ phase and NiTi phase
2
compression curves of RGCS and SGCS samples subjected occurred. With increasing aging heat treatment
to aging heat treatments are illustrated in Figures 16 and 17. time, M initially decreased and then increased,
f
It was observed that the stress hysteresis of both RGCS M increased from 22.5°C to 27.9°C, and A rose
s
s
from 32.7°C to 54.9°C. Notably, from sample A0
and SGCS peaked during the initial cycle. Subsequent to sample A10, the microhardness value increased
cycles witnessed a decline in stress hysteresis, suggesting from 278.2 HV to 477.8 HV, representing a 71.74%
a reduction in energy dissipation during the deformation increase.
process. The recoverable strain during the first cycle was (ii) The plasticity of RGCS was enhanced after aging
measured at 4.96% for RGCS and 4.46% for SGCS. Over heat treatment, while the strength remained almost
15 cycles, the total irrecoverable strain decreased to 4.3% unchanged. Due to the aging hardening effect, the
for RGCS and 3.6% for SGCS. With increasing aging time, strength of SGCS increased while the plasticity and
the overall recoverable strain of both RGCS and SGCS toughness declined. When aging time reached 2 h, the
gradually decreased, while the total unrecoverable strain minimum elastic modulus of RGCS-A2 was 361.52
increased. By the time the aging time reached 10 h, the MPa, and the maximum elastic modulus of SGCS-A2
recoverable strain decreased to 2.67% for RGCS and 2.88% was 1262.82 MPa. The maximum compressive
for SGCS. strength of SGCS-A4 reached 69.48 MPa, which
The shape memory recovery ratios of the samples was 61.39% higher than the compressive strength of
after cyclic compression and heating in a water bath are SGCS-A0. Compared with RGCS-A0, the maximum

Volume 3 Issue 2 (2024) 17 doi: 10.36922/msam.3137