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Optics and Laser Technology

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Metallic integrated thermal protection structures inspired by the Norwayspruce stem: Design, numerical simulation and selective laser meltingfabrication

Kaijie Lin, Kaiming Hu, Dongdong Gu⁎

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, PR ChinaJiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics andAstronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, PR China

H I G H L I G H T S

• Integrated thermal protection structures, inspired by Norway spruce, were designed.

• Thermal behaviors of those structures were investigated by finite element method.

• The thermal conductivity of SLM-processed Ti6Al4V ITP structures was measured.

• The simulation results were consistent with the measured thermal conductivity.

A R T I C L E I N F O

Keywords:Integrated thermal protection structureBio-inspiredGradient structureSelective laser meltingAdditive manufacturingTi6Al4V

A B S T R A C T

With the development of hypersonic vehicles and reusable launch vehicles, the design of integrated thermalprotection (ITP) structures needs to meet the demanding requirement of thermal protection. More complexgeometries, such as lattice structures and bio-inspired structures, have been introduced into the design of ITPstructures, aiming to enhance the thermal protection performance. Additive manufacturing holds high flexibilityin processing and enables more complex designs, which is suitable for the fabrication of complex ITP structures.In this paper, inspired by the structures of Norway spruce stem, a series of ITP structures with different gradienthollow designs was proposed and manufactured by selective laser melting (SLM). The steady-state and transientthermal behavior of those bio-inspired ITP structures were investigated by finite element method (FEM). Toverify the accuracy of the FEM simulation results, Ti6Al4V components with different bio-inspired ITP structureswere fabricated by SLM and thermal conductivities of those SLM-processed components were experimentallymeasured. The FEM thermal simulation revealed that the gradient-structure with larger hollow tubes near thetop and bottom plates and smaller hollow tubes in the center possessed the lowest bottom surface temperature of262.8 °C, which was 21.4 °C lower than the structure with the highest bottom surface temperature. The thermalconductivity measurement revealed that the gradient-structure, with the lowest bottom surface temperatureobtained by FEM thermal simulation, had the lowest thermal conductivity of 2.321W/(m⋅K), which was about29% lower than that of the structure possessing the highest thermal conductivity.

1. Introduction

In the field of aerospace industries, extensive efforts have beendevoted to the development of hypersonic vehicles and reusable launchvehicles. These vehicles have to withstand severe aerodynamic heatingduring the high-speed cruise or the reentry stages. Therefore, a thermalprotection system (TPS) is of great importance to ensure the safety and

reliability of those vehicles. An effective TPS not only protects the in-ternal structure of vehicles from the high heat fluxes on the externalsurfaces but also provides sufficient load-bearing capacity. However,the traditional TPS consists of thermal protection structures and loadbearing structures, and these separate structures are assembled by glueor fasteners [1,2]. These concepts not only make the manufacturingprocess more complex and lead to structural robustness problem, but

https://doi.org/10.1016/j.optlastec.2019.02.003Received 7 November 2018; Received in revised form 30 December 2018; Accepted 2 February 2019

⁎ Corresponding author at: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016,Jiangsu Province, PR China.

E-mail address: dongdonggu@nuaa.edu.cn (D. Gu).

Optics and Laser Technology 115 (2019) 9–19

Available online 11 February 20190030-3992/ © 2019 Published by Elsevier Ltd.

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also significantly increase the structural weight. To overcome the lim-itation of the traditional TPS, a novel design concept, called integratedthermal protection (ITP) system, has been proposed [3,4]. An ITPsystem could simultaneously fulfill the functions of thermal protectionand load bearing [5]. The integration of thermal protection structuresand load bearing structures greatly reduces the structural weight andthe difficulty of maintenance, as well as enhances the structural ro-bustness.

Because of the inherent ductility and forming flexibility, the me-tallic ITP system has attached considerable interest. During the lastdecade, the widely researched metallic ITP systems are honeycombplates [6–8] and corrugated core sandwich panels [9–11]. However,with the ever-growing requirement of the design speed of vehicles, theimproved properties of thermal protection and load-bearing, as well asthe further weight reduction of the ITP system are desperately desired.In order to fulfill these requirements, researches have been focused onthe structural optimization of ITP systems. Xie et al. [12] apply an ideacombing a transient heat transfer model and an efficient optimizationmodel to determine the optimal thickness of insulation layers in acorrugated core sandwich panel ITP system. Meng et al. [13] proposethree web configurations based on corrugated core sandwich panels,namely a web with cooling fins, a web with lightening holes, and a webwith both lightening holes and ribs, and establish a thermal–mechanicalanalysis model to investigate the thermal and mechanical capabilities ofdifferent structures. Fang et al. [14] design a new ITP system panel withspecial corrugated-core webs with cutouts for weight reduction andapply the finite element method (FEM) to obtain optimal dimensions. Ji[15] designs some lightening holes on the webs of a conventionalcorrugated-core sandwich panel and uses FEM to analysis coupledthermal and structural respond of the structure. From the above lit-eratures, it can be seen that the current researches mainly focus on thestructure re-design of the exciting ITP system structures, far too littleattention has been paid to the development of new structures. In ad-dition, due to the high structural complexity, those re-design structuresare hard to process by traditional processing methods. Therefore,thermal and mechanical performances could only be predicted by nu-merical simulations and the accuracy of the simulation is hard to verify.

Selective laser melting (SLM) is an additive manufacturing (AM)process, which can directly make complex three-dimensional metalparts according to a CAD data by selectively melting successive layers ofmetal powders [16–19]. Manufacturing of fully dense metal parts (evenover 99.9%) without the need of post-processes such as infiltration,sintering or hot isostatic pressing, and a high individuality and degreeof geometric freedom are considered to be its major advantages [2021]. The maturing of SLM technology gives rise to the possibility ofcreating new structural concepts inspired by biological structures,which would not be feasible with conventional means of manufacturing[22]. Therefore, parts of previously unattainable complexity can bemade [23], such as porous scaffolds [24–26], lattice structures [27],gradient structures [28] and multi-materials structures [29–33]. Thehigh fabrication flexibility of SLM process provides enormous freedomfor structure design and dramatically reduces the design cycle [34,35].The combination of design flexibility, excellent process capabilities andtime and cost-saving features makes SLM more attractive to aeronauticsand astronautics industries [36,37]. Many components in the field ofaerospace have been successfully fabricated by SLM [38], such asbrackets of satellites [39], connectors of space launchers [40] and le-vers of civil aircrafts [40]. SLM technology also finds its application inthermal related applications, such as heat sinks [41] and heat pipes[42]. However, to our best knowledge, SLM technology is rarely appliedto fabricate TPS components.

In nature, many species exhibit excellent thermal insulation per-formance, especially those live in Antarctic and Arctic area. Dawsonet al. [43] study the thermal behavior of Gentoo penguin feathers andclaim that the multilayer structure of interacting barbules effectivelyshields heat transfer. The unique hollow fur structure of polar bears

[44] provides good thermal insulation performance, which helps themto survive in the arctic cold at−50 °C. Besides animals, some plants canalso resist low temperature. Norway spruce is a species of spruce, whichcan live in the Arctic. The special hollow and gradient microstructure ofthe Norway spruce stem contributes to its cold-resistant performance.In addition, the gradient microstructure also contributes to weight re-duction as well as excellent mechanical properties [45–50]. However,natural structures usually possess highly complex geometries and greatdifficulty in processing, which hinder their application in the field ofthermal protection. In this paper, a series of ITP structures, inspired bythe Norway spruce stem, was designed. Finite element method wasemployed to investigate the static and transient thermal behaviors ofthose ITP structures. In order to verify the simulation results, the ITPstructures were fabricated by selective laser melting with Ti6Al4Vpowder and the thermal conductivity of different ITP structures weremeasured and compared with the simulation results. And finally, theinfluence of gradient structures on thermal insulation performance wasdiscussed.

2. Materials and methods

2.1. Structure design

The ITP structures, proposed in this paper, were inspired by themicro-structure of Norway spruce stem [51,52] as shown in Fig. 1a. Inorder to investigate the influence of arrangements of gradient structureson thermal insulation performance, four different ITP structures weredesigned, namely a structure with no-gradient hollow tubes (as con-trolled group) and three structures with different arrangements ofhollow tubes. For the no-gradient (NG) structure (Fig. 1b), all tubes hadthe same dimension. The external diameter and the wall thickness oftubes was set as 4mm and 0.735mm, respectively. All of the threegradient structures consisted of two layers of big tubes and four layersof small tubes, the only difference among them was the arrangement ofsmall tubes and big tubes. The wall thickness of big tubes and smalltubes was 0.5mm, the external diameter of big tube and small tube was4mm and 2mm, respectively. For the gradient structure A (GA, shownin Fig. 1c), two layers of small tubes adjoined the top and bottom plate,respectively, and two layers of big tubes were placed in the center. Forthe gradient structure B (GB, as shown in Fig. 1d), one layer of big tubesadjoined the top and bottom plate, respectively, and four layers of smalltubes were arranged in the center. For the gradient structure C (GC, asshown in Fig. 1e), one layers of small tubes adjoined the top and bottomplate, respectively; the other two layers of small tubes were placed inthe center; one layer of big tubes was sandwiched between layers ofsmall tubes, respectively. To avoid the influence of structural weightand overall dimension on thermal behavior, all the four structures weredesigned to have the identical solid volume and overall dimensions. Theoverall dimensions of all ITP structures were20mm×20mm×17.2mm and the thickness of the top and bottomplate for all structures was 1mm. In order to ensure sound connectionamong tubes, an overlap of 0.3mm in Z axis and X axis between ad-jacent tubes was designed for all structures.

2.2. FEM thermal analysis model

2.2.1. Governing equationsConsidering that material thermal properties change with tem-

perature and pressure, the heat transfer problem turns into a non-lineartransient problem which is relatively complicated. According to theabove assumptions, the governing heat conduction equation can beexpressed as follows [55]:

∂∂

⎛⎝

∂∂

⎞⎠

= ∂∂X

k TX

ρc Tt (1)

where ρ, k, c, t, T is density, thermal conductivity, the specific heat of

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the insulation material, time and temperature, respectively. By dis-cretization of Eq. (1), the finite element equation in the heat transferanalysis is transformed into the general form:

+ =C T T K T T Q T[ ( )][ ] [ ( )][ ] [ ( )]c' (2)

where C, Kc, Q denotes the specific heat matrix, conductivity matrix andnodal heat flow vector, respectively. T and T′ is the nodal temperaturevector and the derivative of the nodal temperature versus time, re-spectively. Each nodal transient temperature can be obtained by usingthe Newmark method combined with the Newton-Raphson method tosolve Eq. (2). Eq. (2) is subjected to the initial and boundary conditionsas follows:

= =T x T K( , 0) 3070 (3)

=T t T t(0, ) ( )1 (4)

=T L t T t( , ) ( )2 (5)

where T0 is the initial temperature of the whole ITP structure, T1 is thetemperature load applied on the top surface of the structure, T2 is thetemperature of the bottom surface, and L is the thickness of the wholeITP structure.

A large portion of heat is radiated back to the ambient by the uppersurface of the top plate. The net thermal radiation is determined by thefollowing equation:

= −q ε σ T T( )r s s t a4 4 (6)

where qr, εs, σs denotes the radiant heat flux, surface emissivity, StefanBoltzmann constant, respectively. Tt is the top face sheet temperature,and Ta is the ambient temperature of the bottom face.

2.2.2. Boundary and initial conditionsIn this paper, the commercial finite element software ANSYS was

employed to conduct the thermal simulations, including steady-statethermal analysis and transient thermal analysis. Thermal and physicalproperties of the Ti6Al4V applied in the simulation are temperature-dependent and presented in Table 1 [56]. To ensure simulation accu-racy of the finite element analysis, the hexahedral element (solid186)with the size of 0.5 mm was used in the simulation. The potential ap-plication of ITP structures is the skin of high-speed vehicles. According

to the literature [57 58], during the cruise of vehicles, the surfacetemperature of most of the upper vehicle fuselage and wing is around300 °C. Therefore, in the simulation, the applied thermal loading on thetop surface was set as 300 °C. To simplify and reduce the simulationprocessing time, all side faces of the structure were assumed to beperfectly insulated.

During the working condition of ITP structures, the ambient tem-perature and the convective heat transfer coefficient of the top andbottom surfaces are different. Based on these, the boundary conditionsof the finite element analysis were presented in Fig. 2 and listed asfollows:

• The initial temperature of the top surface was 34 °C, and the con-vective heat transfer coefficient was 215W/ (m2⋅K).

• The initial temperature of the bottom surface was 22 °C, and theconvective heat transfer coefficient was 10W/ (m2⋅K).

• The gaps among solid part inside the structure is small, and the airinside those gaps was set to be static. Therefore, the thermal con-vection inside structure can be ignored.

2.3. SLM processing and characterization

The gas atomized Ti6Al4V powder with spherical shape was used asraw materials in this study (Fig. 3a), and the particle size distributionwas shown in Fig. 3b. The composition of Ti6Al4V powder is shown inTable 2. The SLM apparatus used in this study was self-developed byNanjing University of Aeronautics and Astronautics (NUAA), and thedetails of this apparatus can be found in other literatures [59,60].Through a series of preliminary experiments [61], an optimized para-meter setting was employed to fabricate those ITP structures: scan

Fig. 1. (a) The design inspiration (Norway spruce [53] and its cross-section structure of stem [54]) of the ITP structures, (b) no gradient structure (NG), (c) gradientstructure A (GA), (d) gradient structure B (GB), (e) gradient structure C (GC).

Table 1Thermo-physical properties of Ti6Al4V alloy [56].

Temperature (°C) Density (kg/m3)

Thermal conductivity(W/(m⋅K))

Specific heatcapacity (J/ (kg⋅K))

25 4420 7.0 546100 4406 7.45 562200 4395 8.75 584300 4381 10.15 606

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speed 800mm/s, laser power 250W, laser spot 70 µm, powder layerthickness 50 µm and hatch spacing 80 µm. In order to reduce thermalstresses, the substrate was preheated to 200 °C. Island scanning strategywas employed to fabricated all components. High-purity argon wasused as the protective gas during SLM processing.

The relative density of SLM-processed parts was measured using theArchimedes' principle. In order to ensure the accuracy of measure-ments, ethanol was used as the liquid medium [62]. The surface mor-phology of SLM-processed components was observed by a scanningelectron microscope (SEM) (Jeol, JSM-6360LV). Thermal conductivitymeasurements were conducted using a hot disk thermal constant ana-lyzer (Hot Disk, TPS-2500S) and the schematic diagram of the testingsetup was shown in Fig. 4. Two identical samples are required for themeasurement. A sensor was sandwiched between the samples. In thisstudy, the thermal loading was set at 300 °C, which was applied by thesensor.

3. Results

3.1. Steady-state thermal simulation

The steady-state thermal analysis was conducted by the means of

finite element method (FEM). The results of the steady-state thermalanalysis were presented in Fig. 5. From the temperature field dis-tribution maps, it can be qualitatively seeming that different structuredesigns did affect the temperature distribution.

In order to quantitatively compare the thermal protection perfor-mance, the temperature values of five different faces along the thick-ness direction were extracted and charted in Fig. 6. Temperatureloading of 300 °C was applied on the top face, therefore the temperatureof the top face stayed at 300 °C during the steady state. With the in-crease of distance from the top face, the temperature gradually de-creased. Expect the top face temperature, the temperature values atother faces varied among different ITP structures. It is obvious that theGB structure showed the lowest temperature at all other faces exceptthe top face, followed by the NG structure and the GC structure. The GAstructure showed the highest temperature values at all other faces

Fig. 2. The heat source loading and boundary conditions of thermal simulation of gradient structures.

Fig. 3. (a) SEM image of Ti6Al4V powder, (b) particle size distribution of the Ti6Al4V powder.

Table 2Chemical composition of Ti6Al4V powder.

Element Al V C Ni O H Fe Y Other Ti

wt % 6.75 4.5 0.08 0.05 0.2 0.0125 0.3 0.005 0.1 Bal.

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except the top face. It is interesting to find that the introduction ofgradient structures did not always positively affect the thermal pro-tection performance, only the GB structure exhibited lower steady-statetemperature than that of the NG structure. The bottom face sheet (BFS)temperature is a crucial performance for ITP system. The order of BFStemperature values of the ITP structures was ranked as follow: GBstructure (262.8 °C) < NG structure (269.7 °C) < GC structure(276.4 °C) < GA structure (284.2 °C).

3.2. Transient thermal simulation

A transient thermal analysis was implemented on different ITPstructures under the same incident heat flux, and the temperature fieldmaps of GB structure at different times, namely 10 s, 20 s, 50 s, 100 s,150 s and 500 s, were presented as representatives and shown in Fig. 7.At the beginning of heat loading (from 10 s to 50 s), the temperaturefield changed greatly. With the increase of time, the temperature field

Fig. 4. The schematic diagram of the testing setup of thermal conductivitymeasurements.

Fig. 5. The steady-state temperature field map of the bio-inspired ITP structures: (a) NG, (b) GA, (c) GB, (d) GC (The scale bars are different among differentstructures).

Fig. 6. The steady-state temperature distribution at different faces of the bio-inspired ITP structures.

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gradually stabilized, and the BFS temperature approached to thesteady-state value (262.8 °C).

The temperature evolutions against time of different faces, in-cluding 1/4 face, 1/2 face, 3/4 face and bottom face, were presented inFig. 8. Because the curves of different ITP structures were close to eachother after 200 s, the enlarged views from 200 s to 500 s were presentedas insets. From the comparison between different faces, the heating ratedecreased with the increase of distance from the top face. For the 1/4face (Fig. 8a), the heating rate of four ITP structures was significantlydifferent. Especially in the early stage (from 0 s to 20 s), the heating rateof NG, GA, GB, and GC structure was 6.38 °C/s, 7.98 °C/s, 5.40 °C/s and7.04 °C/s, respectively. In addition, the temperature difference betweendifferent ITP structures became larger with the increase of distancefrom the top face. In the four different faces, the temperature values ofthe GB structure were the lowest during the whole process, followed bythe NG structure, the GC structure and the GA structure.

3.3. SLM-processed ITP structures

In order to verify the accuracy of simulation results, the ITP struc-tures were fabricated by the means of SLM process. The images of SLM-processed Ti6Al4V components with different ITP structures werepresented in Fig. 9a–b. In order to avoid over-hanging structures duringSLM processing, the building direction (BD) of all components wasparallel to the length direction of tubes. All the structures were suc-cessfully fabricated without any forming defects, such as cracks anddeformation. The dimensional accuracy of SLM-processed componentswas measured and presented in Fig. 9c. All components possessed ac-ceptable forming accuracy (≤±0.21mm). The weight and relativedensity values of all the SLM-processed components were shown inFig. 9d. The weight of all the SLM-processed components was close toeach other, ranging from 17.1 g to 17.6 g. The relative density values ofall components were above 98%. Specifically, the NG component

Fig. 7. The transient temperature distribution of GB structure at different times: (a) 10 s, (b) 20 s, (c) 50 s, (d) 100 s, (e) 150 s, (f) 500 s (The scale bars are differentamong different times).

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exhibited the highest relative density (98.9%), and the gradient struc-tures exhibited close relative densities, ranging from 98.4% (GA),98.3% (GB) to 98.1% (GC).

The macro-photographs and SEM images of the SLM-processed GBcomponent were presented in Fig. 10. It can be seen from the SEMimage (Fig. 10b) that the small tubes were well fabricated without anyblock of powders, and tubes were soundly fused together without anycracks. From the higher magnification SEM image (Fig. 10c), theaverage wall thickness (t ave) of tubes was around 507.4 μm, which wasquite close to the design value (500 μm). On the other hand, due to thenature of SLM processing, some adhered particles can be observed onthe side surfaces of tubes, which was commonly found in SLM-pro-cessed components [63]. The powder adhesion is inevitable during SLMprocessing and negatively affects the forming accuracy [64] and surfaceroughness [65] of SLM-processed components. The residual heat fromthe molten pool partially melt powders near the boundary of built solidpart and led to the adhesion of particles.

3.4. Experimental results of thermal conductivity

The measured thermal conductivity values of SLM-processed com-ponents with different ITP structures under 300 °C were charted inFig. 11. From the comparison of thermal conductivities, the GB com-ponent possessed the lowest thermal conductivity (2.321W/(m⋅K)),followed by the NG component (2.593W/(m⋅K)), the GC component(2.983W/(m⋅K)) and the GA component showed the highest thermal

conductivity (3.277W/(m⋅K)). Under the same thermal loading on thetop surface, a lower thermal conductivity leads to a relatively lowertemperature on the bottom surface. The order of thermal conductivitywas matched well with the order of simulated steady-state bottomsurface temperature of the ITP structures as shown in Fig. 6. In addi-tion, from the results of the transient thermal analysis, the GB structureshowed the slowest heating rate, followed by the NG structure, GCstructure and the GA structure. Generally speaking, a low thermalconductivity causes a slow heating rate. Therefore, the results of thetransient simulation were also consistent with the results of measuredthermal conductivities.

4. Discussions

As mentioned in the Section 2.1, the solid volume and overall di-mensions of the designed ITP structures were identical. From the resultspresented in Section 3.3, the actual weight and size of the SLM-pro-cessed components were close to each other. Hence, the influence ofweight and size of structures on the thermal behavior can be ignored. Inaddition, under the identical SLM parameter setting, the relative den-sity of different components was close to each other (Fig. 9d), as well asthe surface roughness of components. It can be concluded that thedifferent thermal behavior among structures was caused by the dif-ferent structures, more specifically, the arrangement of tubes withdifferent diameter. For those ITP structures proposed in this paper, thecross-section along the axial direction of tubes was constant, hence, 3D

Fig. 8. The temperature evolutions at different faces of ITP structures: (a) 1/4 face, (b) 1/2 face, (c) 3/4 face and (d) bottom face.

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structures could simplify to 2D structures (Fig. 12). The gaps amongsolid part inside the structure were small, and the air inside those gapswas set to be static. Therefore, the thermal convection inside the ITPstructures can be ignored. The relation between absolute thermal re-sistance (Rθ) and structure parameters, namely length and cross-sectionarea of heat transfer path, can be described by the Fourier’s Law forheat conduction:

=×

R xA kθ (7)

where Rθ is the absolute thermal resistance, x is the length of heat flowpath, A is the cross-sectional area (perpendicular to the path of heatflow), k is the thermal conductivity of the material. The top quarter partof each ITP structure was selected as representative to investigate. Theheat transfer path in top quarter part of different ITP structures wasschematically shown in Fig. 12. For NG and GB structures, the topquarter part only consisted of one big tube layer, and the structure canbe equivalent to parallel thermal resistance (Fig. 12a & c). While for GAand GC structures, the top quarter consisted of two layers of tubes/halftube, and the structure was equivalent to two group of parallel thermalresistances in series (Fig. 12b & d).

According to the above simplification, the thermal resistances of the

top quarter part of different ITP structures were calculated according toEq. (7) and presented in Fig. 13. The GB structure exhibited the highestthermal resistance (5.71 K/W), followed by NG structure (4.11 K/W),GC structure (4.09 K/W) and GA structure (2.46 K/W). The relationbetween thermal resistance and temperature difference can be ex-pressed as followed:

=R TP

Δθ (8)

where Rθ is the absolute thermal resistance, ΔT is the temperaturedifferent and P is the heat flow. Under the same thermal loading, higherthermal resistance causes a larger temperature difference. This resultwas in good agreement with the simulation results of steady-statetemperature. According to the simulation results of steady-stste tem-perature (Fig. 6), the temperature difference between the top face andthe 1/4 face of GB structure was the highest (11.06 °C), followed by NGstructure (7.3 °C), GC structure (5.31 °C) and GA structure (2.83 °C).

From the thermal resistance point of view, internal structure designis crucial for ITP system. Under a given overall dimension, longerthermal transfer path and smaller cross-section area contribute tohigher thermal resistance, and thus better thermal insulation perfor-mance. Design of complex internal structure of ITP system puts higher

Fig. 9. Images of SLM-processed Ti6Al4V components with different ITP structures: (a) as-fabricated, (b) removed from the substrate plate; (c) dimension error ofSLM-processed Ti6Al4V components with different ITP structures; (d) relative density and weight of SLM-processed Ti6Al4V components.

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requirements for processing technologies. Thanks to the bottom-upfabrication of SLM, complex internal and external geometries can befabricated directly from digital CAD files. With the help of SLM tech-nology, fine internal structures, such as pins and fins, can be built toincrease surface area [66]. Some key locations, which act as bottlenecks for the heat flow (such as connection nodes), can be carefullydesigned. In addition, active thermal protection systems with complexgeometries of internal channel can be customizablely fabricated to fa-cilitate medium flow, thus enhancing heat removal [67]. Besides thoseadvantages offered by SLM, some drawbacks of SLM also benefit thethermal behavior. For example, the adhesion of particles is consideredto be harmful for most applications. However, the significantly enlarged

surface area caused by particle adhesion will definitely benefit the heatexchange in thermal applications. Therefore, besides the ITP applica-tion, SLM technology also has strong potential in other applications inthermal management field, such as heat exchangers and heat sinks.

5. Conclusions

In the present study, bio-inspired gradient hollow structures weredesigned for the application of integrated thermal protection. Thesteady-state and transient thermal behaviors of those bio-inspired in-tegrated thermal protection structures were investigated by the meansof finite element method. And the thermal conductivity of SLM-pro-cessed components with those integrated thermal protection structureswas measured to verify the simulation results. The following conclu-sions can be drawn from this study:

• The results of steady-state and transient thermal simulations showedthat the gradient structure B (GB), with larger hollow tubes near thetop and bottom plates and smaller hollow tubes in the center, ex-hibited the lowest temperature value at every critical faces along thethickness direction, as well as the slowest heating rate among thoseITP structures. However, other gradient structures (GA and GC)showed even worse thermal protection performance than that of theno gradient structure (NG). This finding suggested that the in-troduction of gradient structure did not guarantee the improvementof thermal protection property, hence the gradient structure neededto be carefully designed.

• Dense and crack-free Ti6Al4V components with the bio-inspired ITPstructures were successfully fabricated by the method of SLM. Thethermal conductivity of those SLM-processed components was ex-perimentally measured, and the GB component exhibited the lowestthermal conductivity, followed by NG component, GC component

Fig. 10. (a) The macro-photograph and (b) (c) SEM images of the SLM-processed GB component.

Fig. 11. The measured thermal conductivity of SLM-processed Ti6Al4V com-ponents with different ITP structures.

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and GA component. This order was in good agreement with the si-mulation results, indicating the simulation model proposed in thisstudy was valid and the results were accurate.

• The geometries of internal structure directly governed the thermalresistance and consequently influenced the thermal protection per-formance. Therefore, thanks to the high capacity to fabricate com-plex internal structures, SLM technology holds great potential inthermal management field.

Acknowledgments

The authors gratefully acknowledge the financial support from theNational Natural Science Foundation of China (No. 51735005).

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