3D printing of dual phase-strengthened microlattices for lightweight micro aerial vehicles

Xiao, Ran; Li, Xiang; Jia, Huaiyuan; Surjadi, James Utama; Li, Jingqi; Lin, Weitong; Gao,Libo; Chirarattananon, Pakpong; Lu, Yang

Published in:Materials and Design

Published: 01/08/2021

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Published version (DOI):10.1016/j.matdes.2021.109767

Publication details:Xiao, R., Li, X., Jia, H., Surjadi, J. U., Li, J., Lin, W., Gao, L., Chirarattananon, P., & Lu, Y. (2021). 3D printing ofdual phase-strengthened microlattices for lightweight micro aerial vehicles. Materials and Design, 206, [109767].https://doi.org/10.1016/j.matdes.2021.109767

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Materials & Design 206 (2021) 109767

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Materials & Design

journal homepage: www.elsevier .com/locate /matdes

3D printing of dual phase-strengthened microlattices for lightweightmicro aerial vehicles

https://doi.org/10.1016/j.matdes.2021.1097670264-1275/� 2021 The Author(s). Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⇑ Corresponding authors at: School of Mechano-Electronic Engineering, XidianUniversity, Xian 710071, China (L. Gao); Department of Biomedical Engineering,City University of Hong Kong, Kowloon, Hong Kong Special Administrative Region(P. Chirarattananon); Department of Mechanical Engineering, City University ofHong Kong, Kowloon, Hong Kong Special Administrative Region (Y. Lu).

E-mail addresses: lbgao@xidian.edu.cn (L. Gao), pakpong.c@cityu.edu.hk (P.Chirarattananon), yanglu@cityu.edu.hk (Y. Lu).

1 These authors contributed equally to this work

Ran Xiao a,1, Xiang Li b,c,1, Huaiyuan Jia d,1, James Utama Surjadi a, Jingqi Li c, Weitong Lin a, Libo Gao e,f,⇑,Pakpong Chirarattananon d,⇑, Yang Lu a,c,f,⇑aDepartment of Mechanical Engineering, City University of Hong Kong, Kowloon, Hong Kong Special Administrative Regionb School of Sciences, Nanjing University of Science and Technology, Nanjing 210094, ChinacNano-Manufacturing Laboratory (NML), Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, ChinadDepartment of Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong Special Administrative Regione School of Mechano-Electronic Engineering, Xidian University, Xian 710071, ChinafCityU-Xidian Joint Laboratory of Micro/Nano-Manufacturing, Shenzhen 518057, China

h i g h l i g h t s

� Dual-phase strengtheningmechanism-inspired microlatticemetamaterial was designed.

� The metamaterial’s strength, stiffnessand energy absorption ability wereenhanced.

� Large-scale metamaterial was 3Dprinted and applied to a micro aerialvehicle (MAV).

� The MAV shows greatly increasedflight duration with considerableweight reduction.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 December 2020Revised 21 April 2021Accepted 25 April 2021Available online 28 April 2021

Keywords:3D printingMechanical metamaterialDual-phase strengthening

a b s t r a c t

The rapid advancement in CAD and 3D printing technology have brought the rise of mechanical metama-terials which inspired from nature and have optimized microstructural features to exhibit superiormechanical properties over conventional materials for various structural applications. Here, by adoptingdual-phase strengthening mechanism in crystallography, we proposed a microlattice strengthening strat-egy which incorporates stretching-dominated octet-truss (OCT) units as the second phase particles intothe diagonal planes of the bending-dominated body-centered cubic (BCC) lattice matrix, to form an ani-sotropic OCT-BCC lattice. The OCT-BCC dual-phase microlattice possess superior specific compressivestrengths that are ~300% and 600% higher than BCC microlattices along its horizontal direction and lon-gitudinal direction, respectively, accompanied with a significant increase in stiffness and energy

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

Unmanned aerial vehicle (UAV)Lightweight structures

absorption as well. Moreover, a large-scale OCT-BCC lattice metamaterial with dimensions up to 5.0 cm� 2.0 cm � 1.0 cm was successfully manufactured and integrated into a micro aerial vehicle (MAV). Themetamaterial-integrated MAV has an airframe that is ~65% lighter than its bulk counterpart, resulting in asignificant increase (~40%) in flight duration. This work not only provides an effective metamaterialenhancement design strategy, but also promotes the practical application of large-scale 3D printed meta-material in the field of micro unmanned aerial vehicle.� 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Mechanical metamaterials represent a new class of promisingadvanced materials which can customize its mechanical perfor-mance by changing their geometric and microstructural parame-ters [1–6]. Mechanical metamaterials such as microlattices havethe ability to exhibit numerous desirable mechanical propertiessuch as lightweight, high specific strength, and high energyabsorption, demonstrating their application potential in the areasof medical implants [7–12], sports equipment [13] and unmannedaerial vehicles (UAVs) [14,15], etc. Due to the precise geometricfeatures and the complexity of the hierarchical structures, fabrica-tion of microlattices is highly challenging for the traditionalmanufacturing methods. In the recent years, high resolution 3Dprinting technology has enabled the manufacture of large-scalemicrolattices with highly precise and intricate geometries[16–21]. Nevertheless, typical microlattices with periodic unit cellsoften exhibit insufficient mechanical properties, hindering theirbroad practical applications. By learning from nature, variousdesign strategies such as bio-inspired [22–25] andcrystallography-inspired [26–29] strategies have been proposed,bringing about potential strategies for the strengthening of meta-materials [20,30,31].

For crystallography-inspired designs, there are already classiclattices such as body-centered cubic (BCC), face-centered cubic(FCC), octet-truss (OCT) lattice materials [28,32–35], etc. whichhave been adopted for engineering applications. In contrast tothe classical single-phase lattices composed of a periodic arrange-ment of a single type of unit cell geometry, researchers haverecently proposed multi-phase lattices consisting of multiple unitcell designs [31,36–38]. The multi-phase metamaterials are ofteninspired by the strengthening mechanisms in crystallography. Forexample, inspired by the isotropy groups in crystals undergoing aphase change, Vangelatos et al. [31] designed a metamaterialthrough assembling three-compound octahedrons in differentplacement directions to tailor its buckling and stiffening. Addition-ally, inspired by the precipitation hardening in crystallography, Yinet al. [36] developed dual-phase mechanical metamaterial com-posites with improved strength and energy absorption capability.Zhao et al. [37] proposed a dual-layer lattice composed of a cubiclattice core and G7 lattice shell which can exhibit higher fatiguestrength and higher energy absorption than their single-phasecounterparts. Pham et al. [38] incorporated the combinational met-allurgical strengthening mechanisms in alloy design includinggrain-size effect, precipitation, and multiphase hardening todevelop multi-phase metamaterials with robust and damage-tolerant features. The superior mechanical properties demon-strated by the prior works on crystal-inspired metamaterialsimplies that it could be an effective strategy to improve themechanical properties through dual-phase strengthening.

Here, inspired by the dual-phase strengthening mechanism incrystallography [39], we proposed an enhancement strategy forBCC microlattice by incorporating OCT cells as the second phase‘‘precipitates” into the 45� diagonal planes of BCC lattice ‘‘matrix”[4,40–43]. Compared with the original BCC matrix, the

2

compressive specific strengths of the OCT-BCC dual-phase micro-lattice were increased by ~300% and ~600% along its horizontaldirection and longitudinal direction, respectively. The enhancedstrength is also accompanied by a significant improvement ofenergy absorption ability. Furthermore, to demonstrate the scala-bility of the manufacturing process [44] and the practical applica-tion potential of this advanced dual-phase OCT-BCC microlattice,we manufactured large scale OCT-BCC microlattice with dimen-sions up to 5.0 cm � 2.0 cm � 1.0 cm, and successfully integratedit into a micro aerial vehicle (MAV) which causes a 65% reductionin airframe weight, consequently increasing flight duration by~40%. This work not only proposes an effective metamaterialenhancement design method, but also demonstrates the practicalapplication potential of 3D printed metamaterials in the field ofUAVs.

2. Method

2.1. Design and manufacturing

The lattice models were established by SolidWorksTM. Accord-ing to the structures of the BCC cells and OCT cells, the positionsof the vertices of the structure were determined through 3-dimentional coordinates based on mechanical consideration andthen connected to form the complete 3D printing model. A high-resolution PlSL 3D printer Nano ArchTM S140 (BMF Material Tech-nology Inc.) and a photosensitive polymer (mainly consist of acyc-lic acid) were applied to print the mechanical samples. Duringprinting, layer thickness was set as 10 lm, and the exposure inten-sity was set at 40. The exposure time of the first 5 layers whichstick to the printing plane was set as 15 s to provide better supportfor the sample, from layer 6th on, the exposure time was set as10 s. The printed samples were quickly cleaned by ethanol toremove the remaining resin and were placed in a vacuum dryingbox with silica gel desiccant until dry. The average weight of theBCC microlattice samples was 0.0058 � 10-3 kg, with an averageoverall size of 4.54 mm � 4.54 mm � 2.12 mm. The average weightof the OCT-BCC microlattice samples was 0.0954 � 10-3 kg, with anaverage overall size of 10.12 mm � 10.12 mm � 4.51 mm. Theweight and dimensions were used to calculate the compressivespecific stresses. Moreover, an extra BCC lattice and an extraOCT-BCC lattice were coated with conductive Ag thin film by sput-tering machine for morphology observation by a field emissionscanning electron microscope (SEM, FEITM Quanta FEG 450).

2.2. Mechanical testing

Compression experiments on the original BCC microlattice andthe optimized OCT-BCC microlattice were conducted by a microtesting system (GatanTM Microtest) with a strain rate of8 � 10�4 s�1, shown in Fig. 2b. BCC microlattice sample was puton the side of the indenter in horizontal direction (with a sectionsize of 4.54 mm � 2.12 mm), which in contact with the indenter.The anisotropic OCT-BCC microlattices were placed in both

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

horizontal direction (direction 1, abbreviated as d1 in Figs. 2c and3a, with a section size of 10.12 mm � 4.51 mm) and longitudinaldirection (direction 2, abbreviated as d2 in Figs. 2c and 3a, with asection size of 10.12 mm � 10.12 mm). Each experiment wasrepeated at least three times.

2.3. Finite element analysis

Finite element analysis (FEA) was conducted by using ABAQUS/Explicit commercial package to study the strengthening mecha-nism and the deformation process of the original BCC microlatticeand the optimized OCT-BCC microlattice under applied pressure. A2-node linear beam in space B31 was used, and at least 5 elementswere set on each rib to guarantee good convergence of results.Poisson’s ratio of the base material was assumed to be 0.3. An idealelastic-plastic constitutive relationship of the base material wasadopted in the calculations and proportional loads were adopted.

2.4. OCT-BCC lattice application and flight experiments

A large-scale OCT-BCC microlattice metamaterial with dimen-sions of 5.0 cm � 2.0 cm � 1.0 cm was fabricated as shown inFig. 1b. Solid MAV airframe parts and lightweight OCT-BCC micro-lattices MAV airframe parts were designed, manufactured andinstalled as shown in Fig. 4. And the weight of the 3D printed air-frame parts, the overall MAV weights, the hovering power con-sumptions, and the flight durations of both MAVs installed withfour-part solid airframe and OCT-BCC microlattice airframe weremeasured and compared.

3. Results and discussions

3.1. Design and manufacturing of the dual-phase OCT-BCC microlatticemetamaterial

Dual-phase strengthening mechanism refers to the strengthen-ing caused by the dispersion of strong and hard particles hinderingthe movement of dislocations in a softer matrix [45–49]. To obtaindesired reinforcement effect, the reinforce phase should be typi-cally stronger than the matrix phase. According to Deshpandeet al., stretching-dominated lattices possess superior strength

BCC cell

OCT cell original BCC unit designed OCT-Besigned OCT-B

a

inspired

dual-phase strength45 slip of BCC lattice

500nm

Fig. 1. Design, manufacturing, and application of the dual-phase OCT-BCC microlattice mmetamaterial with dimensions up to 5.0 cm � 2.0 cm � 1.0 cm; c) The MAV constructe

3

and stiffness over bending-dominated lattices [32]. Thus, in thiswork, stretching-dominated OCT structure was chosen as thestronger particles to reinforce the bending-dominated BCC matrix.Fig. 1a shows the overall design strategy of the dual-phase OCT-BCC microlattice unit. As shown in Fig. 1a, 5 � 5 � 5 BCC cells (5cells arrayed in each dimension) with strut diameter of 61 lmwere arrayed to make a BCC unit with overall dimensions of 2.8mm � 2.8 mm � 2.8 mm, and OCT cells with the same strut diam-eter and cell size were introduced into the diagonal planes of theBCC matrix as the second-phase particles to form an OCT-BCCmicrolattice. As the 45� diagonal planes are usually the slip planein a BCC unit under compression, introducing the stronger OCTcells as second-phase particles into the 45� diagonal planes ofthe BCC matrix should effectively prevent/retard the slip and catas-trophic fracture of matrix BCC lattice along its 45�directions, whichwould result in a strengthening effect. Additionally, to demon-strate the scalability of the manufacturing process and practicalapplication potential, a large-scale OCT-BCC microlattice metama-terial with dimensions of 5.0 cm � 2.0 cm � 1.0 cm was manufac-tured (Fig. 1b) and incorporated into the airframe of a MAV(Fig. 1e).

3.2. Mechanical analysis

3.2.1. Mechanical behaviors of the dual-phase OCT-BCC microlatticeFig. 2 shows the 3D-printed microlattices and their mechanical

testing results. Fig. 2a shows the BCC lattice sample and the OCT-BCC lattice sample printed by the high-resolution Projectionmicro-stereolithography (PlSL) 3D printer Nano ArchTM S140(BMF Material Technology Inc.). The uniform beam width and theclear geometry features of these samples indicate good printingquality. Fig. 2b shows the compression test on a 3D printed OCT-BCC microlattice conducted by the micro testing system (GatanTM

Microtest) along its direction 1 (horizontal direction, with sectionsize of 10.12 mm � 4.51 mm, abbreviated as d1 in Fig. 2c).Fig. 2c shows the compressive specific stress (defined as the ratioof compressive stress to the lattice apparent density) - straincurves of the original BCC microlattice and the OCT-BCC microlat-tice along its direction 1 and direction 2 (longitudinal direction,with section size of 10.12 mm � 10.12 mm, abbreviated as d2 inFig. 2c). According to Fig. 2b, the compressive specific

CC unitCC un

MAV with lightweight airframe

b

dual-phase OCT-BCC metamaterial

c

ening

etamaterial: a) Design strategy; b) The large-scale dual-phase OCT-BCC microlatticed with a four-part lightweight dual-phase OCT-BCC microlattice airframe.

ba

BCC

OCT-BCC d2

Strain

OCT-BCC d1

1mm2mm

500 m

Spe

cific

stre

ss (k

Pa·

m3 ·

kg-1

)

c

100 m

Fig. 2. Sample morphology and mechanical testing. a) 3D printed OCT-BCC microlattice and BCC microlattice; b) Exhibition of the compression test on a 3D printed OCT-BCCmicrolattice along its direction 1; c) Experimental compressive specific stress-strain curves of the 3D printed BCC microlattice and dual-phase OCT-BCC microlattice along itstwo directions.

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

stress-strain curve of the BCC microlattice exhibits no obviousyield stage, directly entering a stress plateau after the elastic defor-mation stage and the plastic deformation stages. On the otherhand, the compressive specific stress-strain curves of OCT-BCCmicrolattice on both directions show apparent yield phenomenon.

It is well accepted that the improvement of compressivestrength and stiffness is one of the characteristics of the dual-phase strengthening [46–49]. According to Fig. 2c, compared withthe compressive specific strength of the original BCC lattice of1 kPa�m3�kg�1, the compressive specific strength of OCT-BCC latticealong direction 1 and direction 2 are 4 kPa�m3�kg�1 and 6.8 kPa�m3-�kg�1, respectively. This represents an increase by ~300% and~600% along direction 1 and direction 2, indicating significantenhancement effect of the adopted design strategy we proposed.Additionally, the compressive stiffnesses of the dual-phase OCT-BCC lattice along direction 1 and direction 2 increased by ~250%and 300%, respectively, compared with that of the original BCC lat-tice. Moreover, based on the area under the specific stress-straincurve up to the densification strain, we can confirm that the speci-fic energy absorption of the OCT-BCC lattice in both compressiondirections is much higher than that of the original BCC lattice. Onthe one hand, it is because of the stronger OCT cells were originallyof higher energy absorption ability than the BCC cells [32,40]. Andone the other hand, the introducing of the OCT cells constrainedthe deformation of the BCC matrix, which leaded to additionalenergy absorption [36].

To verify the reliability of the enhancement effect obtainedfrom the experiments, we also obtained the compressive specificstress-strain curves through FEA as shown in Fig. 3a. In simulation,the original BCCmicrolattice shows a compressive specific strengthof 1.4 kPa�m3�kg�1, while the specific compressive strengths of theOCT-BCC microlattice along both direction 1 and direction 2 are3.2 kPa�m3�kg�1 and 6.2 kPa�m3�kg�1, respectively. Therefore, thesimulated compressive specific strengths as well as their enhance-ment efficiencies is in good agreement with the experimentalresults. These consistencies, especially the consistency of theenhancement efficiency of the OCT-BCC lattice in the two direc-tions, demonstrated the reliability of the enhancement effect ofthe OCT-BCC lattice obtained from experiments. We also noticed

4

that the simulated stress plateaus of the specific stress-straincurves, especially for that of the OCT-BCC lattice compressed alongdirection 1, exhibited more fluctuations than the experimentalresults. This difference may be caused by the difference betweenthe actual printing material properties and the ideal material prop-erties set by the simulation.

3.2.2. Reinforcement mechanism analysisHere, we will explain the reasons for the observed significant

reinforcement of the OCT-BCC lattice along its two directions fromthe following two aspects: On one hand, as mechanical propertiesare highly related with deformation modes, we can explain thisdual-phase lattice enhancement mechanism by comparing thedeformation modes of the OCT-BCC lattice and the BCC lattice dur-ing compression. Fig. 3b shows the compressive deformationbehavior of the original BCC microlattice, from both simulationand experiment. Fig. 3c and 3d shows the compressive deforma-tion behaviors of the OCT-BCC lattice along direction 1 and direc-tion 2, from both simulation and experiment. As shown inFig. 3b-3d, there is no obvious brittle failure demonstrated by boththe BCC lattice and OCT-BCC lattice (direction 1 and direction 2). Aswe mentioned in 3.1, the stretching-dominated OCT structure wasintroduced into the diagonal of the bending-dominated BCC matrixto reinforce the weaker BCC matrix. Due to the difference in defor-mation behavior between OCT units and BCC units, the existence ofthe OCT units would effectively change and disrupt the originalload transfer route and the deformation pattern of the BCC matrix.From Fig. 3b, we can clearly see that the compressive deformationof the BCC lattice was concentrated at its diagonal planes. Andaccording to Fig. 3c, we can see that the squeeze and collapse firstoccurred on the BCC cells, then the OCT cells touched together witheach other and line up neatly, due to the collapse of BCC cells. Afterthat, the OCT cells in contact with each other began to be com-pressed and deformed. Compared with the BCC microlattice whichhas a deformation concentrate on its diagonal as shown in Fig. 3b,due to the obstruction caused by the stronger OCT cells, the defor-mation along diagonal planes were not observed in the OCT-BCClattice, while occurs a more gradual, layer-by-layer like deforma-tion behavior (Fig. 3c and d). Due to the constrained deformation

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

of the BCC lattice and presence of strong OCT units along its stressconcentrated regions, the OCT-BCC lattice exhibits both higherspecific stiffness, strength, and energy absorption.

As can be seen from the top view of the compression direction 2(Fig. 3e), the OCT-BCC lattice is essentially an array of OCT colum-nar pillars surrounded by BCC pillars. Since the OCT column andthe BCC column have the same unit height and overall height,the stronger and stiffer OCT cells will mainly be responsible forresisting the compression force. It is also important to note thatwhen the OCT column was subjected to a certain compressivestrain, the BCC column would undergo the same compressivestrain. Unlike in direction 1 where the BCC unit cells tend to col-lapse first before the OCT cells and pull each other, the equal strainin the OCT and BCC unit columns produces an even smootherdeformation behavior in direction 2, thereby saved energy con-sumption and resulted in higher specific stiffness, strength, andenergy absorption. This deformation behavior is further confirmedby the simulation results.

On the other hand, we can utilize the Gibson–Ashby model toexplain the dual-phase lattice enhancement mechanism. Accordingto the theoretic Gibson–Ashby model, the effective Young’s modu-lus and the relative density have the relation [28]:

40%

40%

b

d

e f

OCT-BCC d1

OCT-BCC d2

BCC

0.05 0.1

0.001

0.01

E/E

s

/ s

Ed1/Es 0.14 ( / s)

1.53

Strain

Spec

ific

stre

ss (k

Pa·m

3 ·kg

-1)a

BCC pillar

OC

T pi

llar

p

Fig. 3. The FEA results and the deformation processes of the BCC microlattice and the duacurves of the 3D printed BCC microlattice and dual-phase OCT-BCC microlattice; b) Expedual-phase OCT-BCC microlattice along its direction 1; d) and direction 2; e) Top view ofresults of the effective young’s modulus of the dual-phase OCT-BCC microlattice along i

5

EEs

¼ Cqqs

� �n

where E and Es are the effective Young’s modulus of the lattice andthe Young’s modulus of the base material; q and qs are the densitiesof the lattice and the base material, respectively. For the BCC lattice,C ¼ 0:1697, n ¼ 1:9311 [40]. While, for the OCT lattice, C ¼ 0:2,n ¼ 1 [32]. The relative density of the OCT-BCC is measured usingSolidworksTM. From Fig. 3f, we can see that the effective Young’smodulus of the OCT-BCC in direction 1 and the relative densityhas the relation:

Ed1

Es¼ 0:1433

qqs

� �1:526

We can see that the effective Young’s modulus of the OCT-BCCin direction 1 is much higher than that of the original BCC lattice.While seen from Fig. 3g, the effective Young’s modulus of theOCT-BCC in direction 2 is larger than its direction 1, and has therelation:

Ed2

Es¼ 0:1946

qqs

� �1:336

=40%

=40%

15%32%

15%

c

32%

0.05 0.1 0.15 0.2 0.25

0.01

Ed2/Es 0.19 ( / s)

1.34

/ s

E/ E

s

0.15 0.2 0.25

g

l-phase OCT-BCC microlattice. a) FEA results of the compressive specific stress-strainrimental and simulation deformation processes of the BCC microlattice; c) and thethe compression direction 2 of the dual-phase OCT-BCC microlattice; f) Simulationts direction 1; g) and direction 2.

a

e

d

f g

flight duration: 189s flight duration: 266s

c

1cm

solid airframe lattice airframe

123

b

Fig. 4. Dual-phase OCT-BCC microlattice metamaterial applied on a MAV. a) The weight of only one solid MAV airframe part; b) The weight of three dual-phase OCT-BCCmicrolattice MAV airframe parts; c) Magnified dual-phase OCT-BCC microlattice MAV airframe part; d) Lightweight MAV installed with four-part dual-phase OCT-BCCmicrolattice airframe; e) Top view of the dual-phase OCT-BCC cylinder in c); f) The hovering performance and the flight duration of the MAVs installed with 3D printed solidairframe; g) and 3D printed dual-phase OCT-BCC microlattice airframe.

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

The above two equations illustrate the mechanical propertyenhancement of the OCT-BCC analytically.

3.3. Application of the dual-phase OCT-BCC microlattice metamaterialon a micro aerial vehicle

Unmanned aerial vehicles (UAVs) have broad application poten-tial in civil, military, reconnaissance, rescue and entertainment[50–52]. However, the insufficient flight duration of the UAVs,especially the small-sized type with current battery limitation,micro aerial vehicles (MAVs), greatly limits their practical applica-tion scenarios such as long-distance flight and task/operation timeduration. Reducing the weight of a MAV is thus an effectivemethod for enhancing its flight duration as the required thrust ofthe MAV during flight would decrease, consequently reducingpower consumption which prolong the battery life [53–56]. Theapplication of 3D printed microlattices which are lightweight,robust, and with higher structural efficiency, is a promising wayfor MAV weight reduction [51,57–59]. Furthermore, microlatticesare also beneficial for MAV heat dissipation, and their outstandingenergy absorption capability could help MAVs to resist impact andcollisions [60,61]. As our dual-phase OCT-BCC microlattice meta-material is not only lightweight and robust, but also has goodenergy absorption capacity which could help UAVs to resist impactand collisions during flight, it is a suitable material for the MAVs.

As shown in Fig. 1b, to demonstrate the large-scale formabilityand the practical application potential of this advanced dual-phaseOCT-BCC metamaterial, we manufactured a large-scale OCT-BCCmicrolattice metamaterial with dimensions up to 5.0 cm � 2.0 cm� 1.0 cm, and applied it into a four-part airframe of a MAV, as illus-trated in Fig. 4. It can be found in the figure that the weight of threeOCT-BCC lattice MAV parts was 3.44 g (Fig. 4b), which is a ~65%weight reduction compared with the weight of only one solidMAV part (3.31 g, Fig. 4a). After the airframes (each airframe con-sists of four parts) were installed (the two MAVs are identicalexcept for the airframes), the weight of the entire MAV constructedwith the solid airframe was 49.4 g, whereas the weight of the MAVwith the OCT-BCC microlattice airframe was 40.6 g, showing a~18% overall weight reduction.

To quantify the impact of the application of OCT-BCC microlat-tice metamaterial on the flight performance of the MAV, we per-formed flight experiments and compared the hovering power

6

consumption and flight duration between two MAVs composedof solid airframe and OCT-BCC lattice airframe, as illustrated inFig. 4f to g. During hovering, we observed that the root meansquare of errors (RMSEs) of the hovering position during controlledflights of both MAVs were within 2 cm, demonstrating that bothMAVs possessed satisfactory hovering performance. During hover-ing, the measured power consumption of the lightweight MAVmade with the OCT-BCC lattice was ~9.45 W, while the power con-sumption of the MAV installed with solid airframe was ~10.92 W,which represents a ~14% reduction. Furthermore, the flight dura-tion of the MAV installed with the OCT-BCC microlattice airframewas ~266 s (Supplementary Video S1, replayed at 10 times speed),~40% longer compared with the flight duration of the MAVinstalled with the solid airframe (~189 s). The difference betweenthe power consumption reduction ratio and flight enduranceenhancement ratio is mainly caused by the fact that the practicalbattery capacity is dependent on the discharge rate. The larger pro-pelling thrust and associated increased current required by theMAV installed with the solid airframe further reduced the flightduration [59–64]. These results show that the introduction of theOCT-BCC microlattice metamaterial to MAVs would highly benefitthe weight reduction and flight duration increment of the MAVs,which is of great significance for promoting the practical applica-tions of MAVs.

It’s also worth mentioning that many existing works of apply-ing 3D printing lattices to UAVs are limited to the design andprinting capability for advanced lattice components and has lim-ited enhancement as demonstrated in field tests [51,57–59]. More-over, these existing works cannot be applied to these emergingMAVs due to the microlattice manufacturing challenge and insuf-ficient printing resolution. Our work demonstrated the high-resolution micro manufacturing by PuSL, as well as the significanteffect on weight reduction and power saving, thus should be ofsignificance for promoting the practical application of high-resolution 3D printed microlattice metamaterial in the field ofmini/micro UAVs.

4. Conclusions

This work adopted the dual-phase strengthening mechanism incrystallography to guide the design and manufacturing of robustlightweight microlattice metamaterials. By introducing the OCT

R. Xiao, X. Li, H. Jia et al. Materials & Design 206 (2021) 109767

cells as the second-phase particles into the BCC matrix, weobtained OCT-BCC microlattice which shows considerablyincreased compressive specific strengths along its horizontal direc-tion and longitudinal direction (~300% and ~600%, respectively), aswell as highly improved stiffness and energy absorption ability.Moreover, a large-scale OCT-BCC microlattice metamaterial withdimensions up to 5.0 cm � 2.0 cm � 1.0 cm was successfully man-ufactured and applied into a MAV. The MAV incorporated with thedual-phase OCT-BCC microlattice metamaterial shows a ~65%reduction in airframe weight, resulting in a ~40% increase in flightduration. Though there is still room for further optimizing andimprovement, we anticipate the strategy can promote the applica-tions of the high-resolution 3D printed microlattice metamaterialsin MAV and other advanced applications. This work also demon-strates an effective strategy for the design of high-performancemechanical metamaterials based on crystallography-inspiredmicrolattices for future aerospace and vehicle engineering.

Author contributions

Ran Xiao, Xiang Li and Huaiyuan Jia contributed equally to thiswork. Ran Xiao performed the experiments and wrote the manu-script. Xiang Li contributed to the modeling and simulation parts.Huaiyuan Jia conducted the flight experiments and Pakpong Chi-rarattananon supervised the flight experiments. James Utama Sur-jadi helped with the writing modification, Jingqi Li helped with theairframe modeling and Weitong Lin helped with the sketch map.Libo Gao and Yang Lu proposed the idea and supervised theresearch. All authors contributed to the revision of the manuscriptand have approved the final version of the paper.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgements

These authors acknowledge the financial support fromShenzhen Science and Technology Innovation Committee underthe Grant (No. JCYJ20170818103206501), City University of HongKong (No.7005070 and 9667153), National Natural Science Foun-dation of China (No.61904141), Natural Science Foundation ofShaanxi Province (No.2020JQ-295) and the Key Research andDevelopment Program of Shaanxi (No.2020GY-252) and ChinaPostdoctoral Science Foundation (No. 2020 M673340).

Data availability

The raw data required to reproduce these findings are availableupon request to the authors.

Appendix A. Supplementary material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.matdes.2021.109767.

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