Page 428 - IJB-9-6

International Journal of Bioprinting Versatile pomelo peel-inspired structures

Figure 5. (A) Results of mesh sensitivity analysis. (B) The meshed numerical model.

w (
Table 3. Thermal properties’ parameters of the heat sink T − )
T
applied in this simulation R = Q f (XII)
th
Property BPPS Solid base w
Al-Mg-Sc-Zr Aluminum (pure)
Density (kg/m ) 2770 2700 3. Results and discussion
3
Thermal conductivity (W/mK) 165.2 188.1 3.1. LPBF formability analysis
Thermal diffusivity (mm /s) 5.533 7.426 The surface morphologies and dimensional accuracy of
2
the as-fabricated BPPS are shown in Figure 6. The BPPS
Nusselt number (Nu) is a dimensionless number were successfully processed by LPBF technology without
evaluating convective heat transfer data. macro defects, such as pores, cracks, or inadequate fusion
in the surface, exhibiting excellent forming quality and
hD geometric fidelity (Figure 6A). However, a small number
Nu = h (VII) of un-melted particles and spatter particles on the surface
λ
f of the strut and cavity were observed (Figure 6B), causing
where λ is the thermal conductivity of the fluid.
f a terrible impact on the geometric fidelity and surface
The dimensionless friction factor, f, defined in Equation roughness. The optical microscope (OM) image of the
VIII, is derived from the pressure drop. etched longitudinal section showed a homogenous layer-
wise molten pool with a mean depth of 40 μm (Figure 6C),
2Δ P D
f = ⋅ h (VIII) indicating great metallurgical bonding between adjacent
ρ aU m 2 l scanning tracks and layers. Figure 6D shows the 3D side
where ΔP is the pressure drop, and l is the distance between surface morphology of the BPPS, where the red region
the pressure points. represents a relatively rough area. The un-melted powder
particles, which caused the rough side surface of the BPPS,
In order to consider both heat transfer enhancement could be seen in the corresponding position as shown in
and pumping power, the heat transfer efficiency index Figure 6E. The SEM surface morphologies of the BPPS
(η), proposed by Gee and Webb , was introduced as the are displayed in Figure 6E–G, showing obvious irregular
[43]
thermal performance evaluation criteria. outlines and dross defects. These irregular outlines on
Nu the surface, which were induced by the adhesion of un-
Nu melted powder particles, and dross defects reduced the
η= 0 1 (IX) dimensional accuracy and increased the surface roughness
f 3 of the structure. The size of the as-fabricated structure
f 0 was slightly larger than the design value, with a maximum
where the empirical values of Nu and f were calculated deviation of no more than 10%. It could be observed that
the volume of dross in Figure 6F was meaningfully smaller
0
0
using the Dittus–Boelter correlation (X) and Blasius than that in Figure 6G.
correlation (XI), respectively.
The formation of the spatters was attributed to the
08.
Nu = 0 023. Re Pr 03. (X) localized laser heating due to the high-energy laser beam
0
f = 0 316. Re − 025. (XI) impinging on the powder bed, which led to surface boiling
and generated a strong vapor jet. The recoil pressure
0
Thermal resistance (R ) is a numerical representation of the created by the vapor jet pushed the melt surface downward,
th
degree of difficulty in heat transfer, which could evaluate contributing to a vapor depression. The high-speed upward
the heat dissipation performance of the structure . vapor flow of the vapor jet ejected powders and liquid
[44]
Volume 9 Issue 6 (2023) 420 https://doi.org/10.36922/ijb.1011