Page 66 - MSAM-3-2

Materials Science in Additive Manufacturing Mechanical properties of NiTi TPMS

properties, thus complicating the effect of aging on the equipment (Guangzhou Laseradd Additive Technology
mechanical properties of NiTi alloy. Co., LTD., China). The powder employed was Ni 50.67Ti 49.33
In this study, sheet-shaped gyroid cellular structure (Shenzhen MINATECH CO., LTD., China), with a particle
(SGCS) and rod-shaped gyroid cellular structure (RGCS) size distribution range of 15 – 53 µm (D 10=17.8 µm,
were designed and manufactured using LPBF with NiTi D 50=33.6 µm, D 90=55.8 µm). Given the known benefits
alloy powder. Subsequently, the effects of aging heat of laser remelting in effectively increasing sample
treatments on the microstructure and phase transformation density, refining surface roughness, reducing defects, and
behavior of NiTi cellular structures were investigated optimizing microstructure through multiple laser scans
through metallographic microscopy and X-ray diffraction applied to each layer of slices, 41,42 the remelting process
(XRD) analyses. Furthermore, the influence of aging heat was deemed suitable for this study. The pertinent forming
treatments on the mechanical properties, superelasticity, process parameters are detailed in Table 1. Subsequently,
and SME of NiTi cellular structures was analyzed using based on these process parameters, the RGCS and SGCS
an electronic universal testing machine and digital were prepared using LPBF technology, as illustrated in
image correlation (DIC) technique. Finally, the fracture Figure 1B.
mechanism of NiTi cellular structures was elucidated
through detailed examination employing scanning 2.3. Aging heat treatment
electron microscopy (SEM). Before aging heat treatment, the manufactured samples
underwent solution heat treatment. They were placed
2. Materials and methods into quartz tubes and sealed with argon of 99.9% purity,
2.1. Design of gyroid surface structure then kept in a furnace at 1000°C for 2 h, followed by water
quenching. Different aging times were designed, as outlined
All models in this study were designed using MATLAB in Table 2. The heating rate for both heat treatments was
software (MathWorks Inc, America), employing the implicit set at 10°C/min. The designation “A0” denoted no heat
function of the gyroid structure expressed in Equation I:
treatment, while samples subjected to 2 h of aging heat
F Gyroid = sin (2πx/a) cos (2πy/a) + sin (2πy/a) cos (2πz/a) + treatment were labeled by SGCS-A2 and RGCS-A2.
sin (2πz/a) cos (2πx/a)−t(x,y,z) (I)
2.4. Characterization of microstructure
where a denotes the size of the cell body in millimeters
(mm), and t(x,y,z) represents the relative density variation The aged samples were cold-set and coarsely ground
parameter controlling the structure of minimal surfaces. using sandpaper ranging from 120 grit to 2000 grit.
Utilizing this function, samples of SGCS and RGCS with Subsequently, mechanical polishing was conducted using
a volume fraction of 15% were designed. The dimensions a 50 nm diamond abrasive paste. The polished surface was
of the cell body were set at 3 mm × 3 mm × 3 mm, and the then etched using an etching solution (HF: HNO :H O in
2
3
overall size of the structure was 15 mm × 15 mm × 15 mm, a volume ratio of 1:2:5) for 120 s. The samples were then
as illustrated in Figure 1A. For SGCS, the surface thickness thoroughly washed with water and alcohol. The phase
was set as 0.15 mm, while the minimum rod diameter for structure was observed using a metallographic microscope,
RGCS was set as 0.64 mm. and photographic documentation was performed.
Analysis of the crystalline phases presented in the samples
2.2. Manufacturing before and after the aging process was conducted using
The RGCS and SGCS samples were printed using the XRD patterns with Cu-Kα radiation, with a scanning
Dimetal-100H laser selective melting manufacturing angle ranging 20 – 90° and a scanning speed of 10°/min.

A B

Figure 1. NiTi alloy porous structures. (A) Illustrations of sheet-shaped gyroid cellular structure (SGCS) and rod-shaped gyroid cellular structure (RGCS)
structures, and (B) the structures fabricated through laser powder bed fusion.

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