Page 65 - MSAM-3-2

Materials Science in Additive Manufacturing Mechanical properties of NiTi TPMS

domains such as biomedicine, aerospace, automotive structures. LPBF uses laser spots to selectively melt metal
engineering, shock absorption systems, and propulsion powder on a powder bed to form metal parts with complex
devices, capitalizing on the characteristics of NiTi alloy. For shapes. However, the LPBF process applied to NiTi SMA
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example, porous biomaterials enable adjustments of elastic can result in residual thermal stress and the formation
modulus, while the biocompatibility of NiTi alloy enhances of Ni-rich precipitates in the sample, due to the high
implant integration with host bone tissue and improves thermal cycling, substantial thermal gradient, and rapid
bone tissue regeneration. In addition, the porous structure cooling rate involved. Heat treatment serves as a means
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increases surface area, facilitating the functionalization of to alleviate residual stress, dissolve precipitates, adjust Ni
biomaterials. 11,12 Due to their lightweight nature, extensive content, control precipitate existence and distribution, and
contact area, and excellent transmission characteristics, consequently affect microstructure, phase transformation
minimal surface structures offer great advantages in characteristics, and mechanical properties. Given the
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aerospace thermal management technologies. 13,14 apparent over-solid solubility of the B2 phase relative
The triply periodic minimal surface (TPMS) structure to Ni, Ni-rich NiTi alloy lends itself to solid solution
represents a form of porous architecture characterized by heat treatment to dissolve the second phase, resulting in
smooth surfaces devoid of sharp edges, thus mitigating uniform microstructure, high plasticity, elimination of
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stress concentration. Fundamental properties of TPMS, residual stress, and reduction of dislocation density.
such as the type, size, and porosity of monomer cells, are However, the solution temperature generally tends to
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controllable through adjustment of functional parameters, be relatively high, which can promote grain growth.
with significant implications for mechanical properties. Subsequent aging heat treatment is often necessary
Stress in the NiTi sheet gyroid structure primarily localizes after the solution heat treatment. Aging heat treatment
at the joint of inclined surfaces under compressive loading. facilitates the precipitation of the Ni Ti phase with
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As the volume fraction increases, mechanical properties uniform size and dispersed distribution, thereby altering
such as compression modulus and ultimate yield strength phase transformation behavior and enabling controlled
improve. Research by Shi et al. highlights that gyroid, adjustments of mechanical properties, shape memory
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diamond, and I-graph-wrapped package (IWP) structures effect (SME), and superelasticity. 35
are prone to shear failure, coupled with flexural and The mechanical properties of NiTi alloy are influenced
torsional coupling deformation. Primitive deformation by varying aging temperatures and durations. For
predominantly involves stretching, manifesting in layer-by- example, the residual strain in samples aged at 350°C
layer collapse. Gyroid and IWP structures exhibit heightened was significantly lower compared to those aged at 450°C.
energy absorption among the four TPMS lattices. Axial This disparity arises from a slight increase in dislocation
loading results in a notable increase in effective stress and formation during the loading process in samples aged at
martensite volume fraction with rising relative density of 350°C, resulting in a significantly lower content of retained
TPMS cells, with the diamond structure exhibiting superior stable martensite. After aging heat treatment at 600°C,
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mechanical properties. In cyclic compression experiments, the tensile strength of NiTi alloy could reach 729 MPa,
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the residual strain of the gyroid structure escalates with the with a strain recovery ratio of 92.85%. This enhancement
number of cycles. A rise in maximum strain from 4% to is attributed to the gradual formation of Ni Ti and NiTi
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8% corresponds to an increase in residual strain from 1% precipitates, which affects the adjustments of properties
to 4%, indicative of favorable superelasticity. Jin et al. and microstructure. As the aging temperature increases
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similarly affirm the superior superelastic properties of to 700°C, particle aggregation, dislocation formation, and
Ni-Ti gyroid TPMS lattice structure. In addition, the gyroid strain-induced boundary migration occur. Yan et al.
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structures exhibit superior static mechanical properties concluded that aging at 750°C for 5 h could improve the
and fatigue resistance compared to traditional octahedral mechanical strength of NiTi alloy, with the effect of aging
minimum trabecular lattice structures at equivalent volume temperature on the hardness of NiTi alloy surpassing that of
fractions. In addition to mechanical research, TPMS aging time. These studies underscore the significant impact
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structures find applications in sound absorption, 21-23 heat of aging temperature on microstructure characteristics and
dissipation, 24,25 catalysis, 26,27 and other fields. However, thermal properties such as phase transformation process,
traditional machining methods encounter challenges in temperature, and rate. Moreover, increasing aging time
forming TPMS structures due to the high work hardening leads to uneven precipitation of Ni-rich intermetallic
and strength characteristics of NiTi SMA, 28,29 resulting in compounds such as Ni Ti and Ni Ti from the matrix. In
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poor processing ability. addition, the type, volume fraction, size, and distribution
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Laser powder bed fusion (LPBF) technology offers of precipitated phases within the matrix after LPBF can
a viable approach for manufacturing metallic porous affect phase transformation behavior and mechanical
Volume 3 Issue 2 (2024) 2 doi: 10.36922/msam.3137