Materials Science & Engineering A 773 (2020) 138857

Available online 24 December 20190921-5093/© 2019 Elsevier B.V. All rights reserved.

Laser additive manufacturing of bio-inspired lattice structure: Forming quality, microstructure and energy absorption behavior

Yuexin Du a,b, Dongdong Gu a,b,*, Lixia Xi a,b, Donghua Dai a,b, Tong Gao c,d, Jihong Zhu c,d, Chenglong Ma a,b

a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing, 210016, Jiangsu Province, PR China b Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing, 210016, Jiangsu Province, PR China c State IJR Center of Aerospace Design and Additive Manufacturing, MIIT Lab of Metal Additive Manufacturing and Innovative Design, Northwestern Polytechnical University, 710072, Xi’an, Shaanxi, PR China d NPU-QMUL Joint Research Institute of Advanced Materials and Structure, Northwestern Polytechnical University, Xi’an, 710072, PR China

A R T I C L E I N F O

Keywords: Additive manufacturing (AM) Selective laser melting (SLM) Bio-inspired Lattice structure Mechanical properties

A B S T R A C T

Development of additive manufacturing technology effectively extends the design and application of novel lattice structures. A novel lattice structure, inspired from the beetle’s front wing, was designed and fabricated by se-lective laser melting to investigate the effects of processing parameters on densification behavior, microstructure and mechanical properties. The lattice structure exhibited high densification (>97%) at the optimized processing parameters. As the laser power increased, the transformation from the cellular dendritic to the columnar den-dritic was observed, simultaneously accompanying the grain coarsening. The load-displacement curves of all lattice structures exhibited three characteristic deformation stages: (i) the elastic deformation; (ii) the inho-mogeneous plastic deformation; (iii) the failure/breaking. During the compressive process, the successive transfer of stress concentration from the intersections of the horizontal struts to the intersections of the diagonal struts occurred. At the optimized processing parameter (P ¼ 375 W, v ¼ 3500 mm/s), the lattice structure exhibited excellent energy absorption capability (3.45 J) with high bearing force (2.95 kN) and large displacement (1.18 mm).

1. Introduction

Ultra-light lattice structures are defined as a statically/statically indeterminate porous-ordered structure that simulates the atomic lattice configuration [1]. The lattice structures, as a porous structure, are a periodic combination of a certain unit cell through numerous identical elements. Lattice structures allow for design freedom beyond the capa-bility of solid materials and meanwhile offer unique functional charac-teristics including high specific strength and stiffness, enhanced absorption of mechanical energy and heat transfer control. As a typical lightweight structure, the lattice structures are widely used in many engineering fields such as airplanes, auto-mobile industries and vehicles [2,3]. Particularly in the aerospace, a lightweight design of parts can reduce fuel consumption and increase the carrying capacity. For instance, Airbus has designed a new aircraft cabin area based on growth

structures of organic cell and bone, leading to a weight decrease of the original structure by 45% [4]. Microlattice, a light and strong structure produced from Boeing, enabled to balance on top of a dandelion [5]. High specific strength of this structure offers great potentials for air-planes and vehicles.

Mechanical performances of lattice structures depend on various factors such as the cell topology, number of cells, geometric parameters (e.g. strut diameter and cell size), material and manufacturing process, as well as structural boundary and loading conditions [6]. In previous studies, Maskery et al. [7] investigated the equation of modulus, relative density, and relative cell volume of body-centered cubic (BCC) lattice structure, the effect of the number and size of cell on mechanical properties by experiments. It is found that modulus and strength sig-nificant decreased as unit cell size increased at the fixed relative density. Wallach et al. [8] studied the effects of unit cell size on the elastic

* Corresponding author. 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).

Contents lists available at ScienceDirect

Materials Science & Engineering A

journal homepage: http://www.elsevier.com/locate/msea

https://doi.org/10.1016/j.msea.2019.138857 Received 8 November 2019; Received in revised form 18 December 2019; Accepted 20 December 2019

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modulus, shear modulus and Poisson’s ratio of triangulated lattice structures. It is found that the elastic modulus and the shear modulus decreased as the cell size increased.

Conventional methods such as stamping [9], extrusion process [10], investment casting [11] that are used to produce periodic lattice struc-tures, consume much time and cost [12]. The structures fabricated by conventional methods possess relatively simple geometry and limited design freedom, restricting their widespread applications [13]. With the development of additive manufacturing (AM), selective laser melting (SLM) can offer fabrication possibility for complex geometries [14–16, 41]. SLM applies a high-energy laser beam as an energy source to penetrate into the powder-bed for the selective fusion and fabricates the geometrically complex lattice structures by user-defined computer aided design (CAD) data files without tools or molds. The as-built complex parts achieved precisely by SLM exhibit high dimensional precision, perfect surface quality and outstanding performance [17–19,42,43]. So far, some researchers have studied the performances of complex lattice structures fabricated by SLM. Qiu et al. [20] studied the influence of laser power and scanning speed on the diameter, shape and porosity of the diamond unit cell struts. The strut diameter deviated from the design value, monotonously increased with laser power. Diameters of struts and mechanical properties of diamond lattice structures could be well controlled of by variations of laser process parameters. Leary et al. [21] proposed five lattice structures and the corresponding specimens were successfully manufactured by SLM. The FBCCZ (Face and body centered cubic unit cell with vertical struts) structure were demonstrated to provide stable crushing behavior and excellent energy absorption characteristics. McKown et al. [22] fabricated a series of metallic lattice structures based on two kinds of unit cells including octahedral and pillar-octahedral topologies by SLM process. The change of the macro-scopic shape at different compressive strains was assessed and the development of shear bands was identified.

Organisms have evolved for millions of years to adapt to aggressive environments, which can provide good inspiration for novel structures. It is expected that the structures from organisms can develop the func-tional properties of the complex structures. Zhang et al. [23] were inspired from honeycombs and designed the regular hexagonal hierar-chical honeycombs with a smaller regular hexagon replacing each three-edge vertex of a base hexagonal. The out-of-plane crashworthiness of the new hexagonal layered honeycomb concept was studied. It was found that layered honeycombs showed more efficiently distribution on the network than traditional hexagonal cells. Chen et al. [24] studied a group of hierarchically architected metamaterials constructed by replacing cell walls with hexagonal, Kagome and triangular lattices and found that the layered design could effectively improve the heat resis-tance and load carrying capacity of conventional honeycomb structures. Wang et al. [25] designed a reticulated shell structure inspired from water spider. The effect of strut diameter on mechanical property and energy absorption capacity was studied. The load carrying capacity of the reticulated shell structure increased with the increase of the strut diameter. The structure with 1.25 mm diameter showed excellent en-ergy absorption capability.

In this work, a lattice structure inspired from the front wing of beetles was designed, which was fabricated by SLM technology of AlSi10Mg powder with different process parameters. The effects of process parameters on densification level, microstructures, mechanical properties of SLM-processed lattice structures were studied. Combining the finite element simulations, the fracture mechanism of SLM- processed lattice structures was elucidated.

2. Bionic design and experiment approaches

2.1. Bionic design

In nature, beetles with diverse species have evolved over a long period of time. The front wing has the dual function of protecting the

body and promoting flight ability. It is characterized by high degree of optimization, diverse design, lightweight and high strength. Therefore, bionic thin-walled structure based on the beetle’s front wing micro-structure may be an excellent energy absorber under the impact load [26,27].

Fig. 1a and (b) respectively show the adult Allomyrina dichotoma beetle and the corresponding microstructure of the cylindrical tube within the beetle’s front wing. According to the microstructure of the circular column tube in Fig. 1b, the shape of hyperboloid (Fig. 1c) can be obtained. From the perspective of lightweight structures, the rod is used instead of the surface to obtain a lattice structure (Fig. 1d). The height of the specimen was designed as 21 mm, and the diameter of each strut was 1 mm.

2.2. Experiment approaches

A series of bioinspired lattice structures were manufactured by the SLM-150 machine developed by Nanjing University of Aeronautics and Astronautics (NUAA). The SLM system consisted mainly of a YLR-500 ytterbium fiber laser with a maximum laser power of 500 W and the laser spot size of 70 μm (IPG Laser GmbH, Burbach, Germany), an inert argon gas protection system, automatic powder layering device, and a computer system for processing controlling. An aluminum substrate was initially fixed on the building platform and leveled. The building chamber was sealed and the argon gas was filled with. Afterwards, the spherical AlSi10Mg powder material was deposited on the substrate layer by layer. The laser beam was then controlled according to the CAD data to scan the powder bed selectively. Fig. 2a gives the SLM processing of the bioinspired lattice structures and the final as-fabricated specimens are exhibited in Fig. 2b, which had the same configuration parameters. For the laser processing parameters, the laser power varied between 350 W–450 W in an increment of 25 W, and the scanning speed (v) ranged from 2750 mm/s to 3500 mm/s in an increment of 250 mm/s. The thickness of layer was 30 μm and, AlSi10Mg alloy powder was used as the raw material in this study, in consideration of its low density, good cast ability, excellent corrosion resistance and wide application in aerospace fields.

Compressive test was conducted on a CMT5205 testing machine (MST Industrial Systems, China) at room temperature following the standard GB/T 7314-2017. To guarantee the accuracy and reproduc-ibility of experimental results, three parallel specimens with the same laser processing parameters were fabricated for the compressive test. During the compressive test, maximum displacement and cross-head velocity was set as 2 mm and 1 mm/min, respectively. Subsequently, the energy absorption values during the compressive process were calculated by the integral operation of force with respect to displace-ment according to curves, in order to assess the effects of laser pro-cessing parameters on the energy absorption behavior of SLM-fabricated lattice structures. The total energy absorbed from displacement 0 to hmax can be calculated from Ref. [40]:

WT ¼

Z hmax

0PðhÞdh (1)

where WT is the total energy absorption and P is the loading force. An Olympus PMG3 optical microscope (OM) was used to observe the pore distribution and molten pool morphology of the as-fabricated specimen. To reveal the molten pool configuration, the selected strut was cut, ground and polished following the standard metallographic procedures, and then etched by Keller’s reagent for 10–30 s. Microstructure of the strut was characterized using an S-4800 field emission SEM (FE-SEM) (Hitachi, Japan).

In order to reveal the fracture mechanism, predict stress concentra-tion and fracture locations and understand the fracture behavior of the lattice structure better, the nonlinear explicit finite element (FE) code ANSYS LS-DYNA was used to simulate the compressive behaviors of the

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lattice structure. A rigid plane with a constant velocity of 10 m/s was applied, and a fixed rigid support was set at the bottom of the lattice structure. Two types of contacts were used in the numerical analysis. A single-surface contact was used for the lattice structure itself, while an automatic surface-to-surface contact was applied between the lattice structure and the rigid body. The coefficient of friction and dynamic between all surfaces was set to 0.2. An elastic linear strain hardening model was used to characterize the true stress-strain relationship of the wall base material.

3. Results and discussion

3.1. Forming quality of SLM-fabricated bio-inspired lattice structure

3.1.1. Densification behavior Fig. 3 shows the influence of laser power P and scanning speed v on

the relative density of the SLM-fabricated lattice structures. It was found that high relative density of 99.93% was obtained when the laser power was 450 W and the scanning speed was 3500 mm/s, while the combined parameters of P (400 W) and v (2750 mm/s) resulted in the low relative density of 97.47%. It did not show a simply linear relationship between the densification and the laser processing parameters as the P or v was fixed. Nevertheless, the high-densification region was always associated with the combination of high P and v based on Fig. 3b. To clearly

Fig. 1. (a) The adult Allomyrina dichotoma beetle [27], (b) The microstructure of cylindrical tube in the beetle front wing [26], (c) Spatial hypersurface geometry, (d) CAD model of lattice structure inspired from the cylindrical tube.

Fig. 2. (a) Process of fabricating lattice structures by SLM; (b) Lattice structures fabricated by SLM technique.

Fig. 3. Effects of laser power and scanning speed on relative densities of lattice structures.

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illuminate the effects of laser processing parameters on the size and distribution of pores in SLM-fabricated lattice structures, three different laser powers of 375 W, 400 W and 450 W were chosen at a fixed v of 3500 mm/s. Fig. 4 gives the cross-sectional OM micrographs of diagonal struts of the SLM-fabricated lattice structures under the above three different laser powers. High-magnification OM images corresponding to four typical regions (A, B, C and D) along the building direction (BD) are further displayed (Fig. 4). Region B and region C were representative of the intersection positions of two different struts. When the P was 375 W, many large spherical pores with size ranging from 10-30 μm could be only found near the down-skin surface of the strut in region A in the top zone of the strut. In this case, the relative density of the SLM-fabricated lattice structure was 98.15%. As the P increased to 400 W, the number of spherical pores in region A decreased significantly (Fig. 4b), and the relative density reached 98.32%. When the applied P further increased to 450 W, the structure was closely full density with a densification degree of 99.93% and few tiny pores remained in the strut (Fig. 4c). Furthermore, in consideration of the great slope of as-designed struts, apparent slag at all laser processing parameters was invisible.

The difference in densification behavior could be attributed to the complicated thermodynamic characteristics of the non-equilibrium molten pool induced by different laser processing parameters. At a low v (<2850 mm/s), as the P gradually increased, the laser energy input enhanced remarkably, thus leading to the vapor depression instability, penetration and fluctuation of molten pool [28]. In this case, the melting mode transformed from the conduction to the keyhole mode, producing excessive porosity during the subsequent solidification process. On the other hand, large laser energy density tended to intensify the evapora-tion of Mg constituent, which also contributed to a decrease of the densification degree. For the relatively high v of 3500 mm/s, the densification behavior of SLM-fabricated parts might mainly depend on the motion of gas pores in the molten pool. According to Weingarten’s work [28], spherical pores in SLM-fabricated Al-based alloys mainly resulted from the moisture existing within the powder. Due to the re-action between Al and H2O, a great mass of hydrogen was formed in the melt, further inducing the nucleation and growth of hydrogen pores in the melt pool. These pores were prone to emerge near the down-skin surface of the strut (region A in Fig. 4b) at ultra-high cooling rate of SLM. Additionally, the re-melting of the previous layer further caused

the upward migration of the residual pores under the convection induced by the buoyancy and surface tension of the molten pool, which accounted for the fact that more pores were observed in region A. Herein, formation of some large pores might derive from an integration of adjacent hydrogen pores during the escaping process. As a combi-nation of high P and high v (e.g. 450 W and 3500 mm/s) was applied, strong Marangoni flow was formed, thus accelerating the escape of hydrogen pores and contributing to high densification level.

3.1.2. Dimensional accuracy Fig. 5 shows the effect of laser power on the strut diameters of the

lattice structures at a fixed v of 2750 mm/s. The designed strut diameter was 1 mm. The result indicated that the strut diameter increased continuously with increasing the laser power. As the laser power increased from 350 W to 400 W, the actual strut diameters deviated significantly from the designed value, increased by 3.8%, 7.8% and

Fig. 4. Optical micrographs of cross-section of a diagonal strut from SLM-fabricated lattice structures at different laser processing parameters: (a) P ¼ 375 W, v ¼3500 mm/s; (b) P ¼ 400 W, v ¼ 3500 mm/s; (c) P ¼ 450 W, v ¼ 3500 mm/s.

Fig. 5. Variation in strut diameters with laser powers for lattices produced at a fixed scanning speed of 2750 mm/s.

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13.2%. As the laser power further increased to 450 W, the strut diameter increased slowly. The actual strut diameters changed from 1.132 mm to 1.142 mm, increased by 14.2%. This is caused by wider laser scanning track as high laser power leads to much wider melt pools than low power. This observation is consistent with Qiu et al.’s work [20], where strut diameters of diamond unit cell struts increased with laser power.

3.2. Microstructural evolution of bio-inspired lattice structure

Fig. 6 shows the optical and SEM images of cross-sectional micro-structures of unit cell lattice structures produced by SLM at different laser powers. In consideration of the microstructure difference at various positions of the strut of the SLM-fabricated structure, the same position corresponding to the region A for these three samples in Fig. 4 were chosen. From the optical images displaying the morphology of the molten pool (Fig. 6a, 6d, and 6g), it was found that the homogeneity of the molten pool size got gradually enhanced with the laser power increasing from 375 W to 400 W, and then got worse when the laser power further increased to 450 W. In the melt pool, two typical different zones could be distinguished, as the white box indicated in Fig. 6a, namely the center of the molten pool and the boundary of the molten pool. Similar to the microstructure of SLM-fabricated bulk sample, cellular-dendritic structures could be observed for all samples, consist-ing of aluminum-rich phases, and residual Si precipitated at the inter-cellular boundary. And it should be pointed out that the structure inside the molten pool (Fig. 6b) was finer than that at the boundary of the molten pool (Fig. 6c). As the applied laser power increased from 375 W to 400 W, an apparent coarsening in the dendrite spacing could be found. The average dendrite spacing in the molten pool increased from 0.093 μm to 0.156 μm, and the average dendrite spacing at the molten pool boundary increased from 0.142 μm to 0.241 μm (Fig. 6e and 6f). When the laser energy input further increased, the columnar-dendritic

structure features became coarse in the whole molten pool (Fig. 6h and 6i). The average dendrite spacing in and at the boundary of molten pool increased to 0.195 μm and 0.255 μm, respectively.

Generally, the solidification parameter G/R at the solid-liquid interface determines the solidification morphology, where G and R are the thermal gradient and the solidification rate, respectively. The so-lidification rate R depends on the speed of the moving source v and the angle θ between the direction of the moving source and the growth di-rection of the solidified material. The stability of the solidification front and hence the resulting solidification mode is determined by the G/R. Planar dendrites, cellular dendrites, columnar dendrites, and equiaxed dendrites are formed in decreasing order of G/R. As the laser power increases, R increases as well, which results in the decrease of the value of G/R. The transformation from the cellular-dendritic structure to the columnar-dendritic one was observed [29]. The relationship between the dendrite spacing λ, temperature gradient G and the solidification rate R was shown below [30]:

λ¼ 4:3�

ΔT0⋅Dl⋅Γk

�0:25

⋅ G� 0:5⋅R� 0:25 (2)

where Dl is the diffusion coefficient of solute atoms in liquid phase, and ΔT0 is equilibrium solidification interval. k and Г represent solute dis-tribution and Gibbs-Thomson coefficient, respectively. From equation (2), it could be found that the dendrite spacing λ is inversely related to G and R, and G exerts more effects on λ than R [30]. The temperature gradient G and the solidification rate R vary in the molten pool with the movement of heat source. Due to the difference types of the temperature gradient, cooling rate and local solidification time during the SLM process, the dendrites showed various morphologies. The cooling rate in the melt pool decreases with an increase of laser power at a fixed scanning speed [31]. Therefore, a fine structure was obtained at a low

Fig. 6. Typical cross-sectional microstructure features of SLM-fabricated lattice structures at various laser processing parameters: (a)–(c) P ¼ 375 W, v ¼ 3500 mm/s; (d)–(f) P ¼ 400 W, v ¼ 3500 mm/s; (g)–(i) P ¼ 450 W, v ¼ 3500 mm/s (b), (e), and (h) correspond to the region I marked in (a), (d) and (g), while (c), (f) and (i) correspond to the region II marked in (a), (d), and (g), respectively.

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laser power.

3.3. Mechanical property and energy absorption mechanism

To further characterize the mechanical properties of the SLM- fabricated unit cell lattice structures, the corresponding compressive tests were performed. The compressive force-displacement curves of the SLM-fabricated lattice structures under various processing conditions are displayed in Fig. 7. During the loading process, the force exerted on the lattice structure experienced successive increase as the displacement increased. When the force increased to a certain value, a certain strut of the structure started to break, leading to a sharp drop of the force and then continued to increase until the next fracture occurred. Based on the typical features of load, three stages could be divided into: (I) elastic deformation; (II) inhomogeneous plastic deformation; (III) failure. The stage I exhibited a nearly linear relationship between the force and the displacement. Among the three samples, the slope of the one fabricated by SLM at the laser power of 400 W was maximum, while the others showed the similar value. The slope of the curve could reflect the magnitude of the structural rigidity. It indicated that the sample pro-cessed at 400 W had the maximum rigidity. It is found that the inho-mogeneous plastic deformation started to dominate and the maximum compressive bearing capacity Fmax could be obtained in the stage II. The highest Fmax (2.95 kN) with the largest displacement value (1.18 mm) was achieved in the sample at 375 W. As the laser power further increased to 400 W or 450 W, the accumulated displacement drastically decreased. Note that the sample at 450 W present the similar Fmax as that of the sample at 375 W, while the sample at 400 W exhibited the min-imum Fmax (2.62 kN). At the end of stage II, force dropped sharply accompanying the failure of a certain strut occurred. The structure at 400 W firstly broke, followed by the structures at 450 W and 375 W. For the stage III, as the displacement increased, the struts of the lattice structure broke successively. Specially, a maximum critical displace-ment for each breaking of the strut was obtained for the lattice structure fabricated by SLM at 375 W.

Fig. 8 (a) shows the energy absorption-displacement curves of lattice structures under different processing conditions. Before the first strut broke, the value of energy absorption grew rapidly and then the growth rate of the value slowed down. At the beginning of the compression test, the values of energy absorption of lattice structures under various pro-cessing conditions had no distinct difference. When the displacement was larger than 0.8 mm, the value of energy absorption at 350 W was

larger than that of other processing conditions. At a laser power of 350 W, energy absorption reached a maximum value due to the structure subjected to a large force, while its energy absorption decreased to a minimum value at 400 W due to a relatively low force. The values of maximum energy absorption of the lattice structures are shown in Fig. 8b. The maximum energy absorption of the lattice structure at 350 W was 37.45% higher than that of the lattice structure at 400 W.

According to the above force-displacement curves, it obviously indicated that the energy absorption behavior of the lattice structure significantly relied on the corresponding deformation features during the compressive process. To illustrate more insight into the deformation behavior and the failure mechanism of the lattice structure, computa-tional modeling was conducted to disclose the stress distribution and the failure position of the lattice structure during the loading process. As shown in Fig. 9a, the stress concentration tended to appear at the intersection of horizontal struts (namely the side of the hexagonal frame), as the displacement increased to 0.6 mm corresponding to the stage I. In this condition, the total stress level within all diagonal struts was lower by comparison with that of the horizontal struts. It should be pointed out that the predicted maximum stress σmax had reached ~586.5 MPa, much higher than the measured experimental data of SLM- fabricated AlSi10Mg alloy, leading to the occurrence of strut breaking at the intersection of the horizontal struts. As the displacement further increased from 0.6 mm to 1.0 mm (Fig. 9b), the strut breaking existed, in which case the previous accumulated stress of the whole lattice structure released significantly. Subsequently, the gap between the adjacent broken struts became enlarged when the displacement continued to increase. As a result, the upper regions of the diagonal struts became curved, thus resulting in the formation of new stress concentration po-sitions, as marked by the red arrow in Fig. 9c. Once the stress reaching the compress strength of the Al-based material, the attendant breaking emerging on the diagonal strut accompanied with a remarkable decrease of the total stress level. The real-time images of the lattice structure captured by the camera at various compressive stages are given in Fig. 10, which demonstrates high consistence with the simulated results. It suggested that the successive transfer of the stress concentration accounted for stress drop and strut breaking. At a compressive displacement of 1.0 mm, the first breaking occurred at the six in-tersections of the horizontal struts by simulation rather than at one of six intersections in actual. This was because only structural parameters were considered in the simulation instead of different densification and microstructures.

The morphology of the first fracture (Stage II) of the structure pro-duced by SLM at three different parameters is shown in Fig. 11. It could be found that a number of dimples were visible on the top of the fracture surface of AlSi10Mg lattice structure produced by SLM using laser power of 375 W and scanning speed 3500 mm/s, exhibiting a ductile type of fracture. On the middle and bottom of the fracture surface, the fracture surface was smooth with tongue patterns, exhibiting a brittle type of fracture. Therefore, the fracture model was dominated by brittle fracture with weak ductile fracture (Fig. 11a) at 375 W and 3500 mm/s. As the power increased to 400 W, there was a certain angle between the frac-ture surface and the horizontal plane (Fig. 11b). A large number of dimples appeared on the fracture surface. In this case, the fracture model was mainly ductile fracture. As shown in Fig. 11c, the fracture surface was unevenness. There were river patterns at the bottom, exhibiting a brittle type of fracture. While there were dimples at the top, exhibiting a ductile type of fracture. The fracture model was a mixture of brittle fracture and ductile fracture when the laser power was 450 W.

According to the above results, as the applied laser power increases, the strength and ductility of SLM-fabricated lattice structures fluctuated significantly. For the parts processed at 375 W, 425 W and 450 W, the compressive of the lattice structure decrease as the laser power increase. The evolution of their compressive strength can be explained by their microstructure, which fine microstructure leads to high strength. High ductility obtained from the part at 375 W can be attributed to the

Fig. 7. Force-displacement (F-D) curves of lattice structures under various processing conditions. Three different stages were divided based on the F-D curve of lattice structure fabricated by SLM at the combined parameters of P ¼375 W and v ¼ 3500 mm/s.

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residual pores existing in the down-skin surface of the strut. According to the simulated results (Fig. 9), the down-skin surface of the strut suffers from the compressive stress during the loading. Hence, the residual pores can effectively enhance the ductility. For the part fabricated at 350 W, a relatively high porosity, especially with some irregular pores induced by insufficient laser energy input, as well as the heterogenous distribution of Si particles might result in a remarkable decrease of compressive strength and ductility. The lowest compressive property is obtained in the part at 400 W, which can be attributed to an increase of residual pores emerging in up-skin surface of the strut, where tensile stress is induced during the loading.

Fig. 12 shows the compressive properties of different lightweight structures according to the previous reported investigations. The different data presented in different areas are taken from the literatures. For the same densification, the strength of bio-inspired lattice structure

significantly outperforms that of the conventional lattice structures. The bio-inspired lattice structure showed lightweight and high strength compared with the lattice structures prepared by the aluminum alloy in the literature. The bio-inspired lattice structure has excellent combina-tion of light weight and high strength [2,11,32–39].

4. Conclusions

In this paper, bio-inspired lattice structures have been designed and fabricated by SLM technology with AlSi10Mg powder at different pro-cess parameters. Forming quality, microstructure and energy absorption behavior of the SLM-processed lattice structures were investigated. The crush simulation of lattice structures was carried out to reveal the fracture mechanism. The following conclusions can be drawn from:

Fig. 8. Energy absorption curves of lattice structures under various processing conditions.

Fig. 9. Stress distribution and deformation feature of the designed lattice structure at different displacements: (a) 0.6 mm: stage I; (b) 1.0 mm: stage II; (c) 1.2 mm: stage III; (d) 1.8 mm: stage III; (e) The force-displacement curve obtained from the simulation.

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1) Near fully dense lattice structures (relative density>97%) were successfully fabricated by SLM of AlSi10Mg powder. At the opti-mized laser processing parameter (P ¼ 450 W, v ¼ 3500 mm/s), the SLM-processed lattice structures exhibited high densification of 99.93%.

2) As the laser power increased, the transformation from the cellular- dendritic to the columnar-dendritic was observed, simultaneously accompanying the coarsening of grains.

3) The load-displacement curves of lattice structures exhibited three characteristic deformation stages, namely the elastic deformation, the inhomogeneous plastic deformation and the failure. During the compressive process, the strut was fractured accompanying a sig-nificant stress drop, which could be explained by the successive transfer of the stress concentration from the intersections of the horizontal struts to the intersections of the diagonal struts.

Fig. 10. Compression images captured at different displacement during the compression test of lattice structure fabricated by SLM with the power of 375 W and the scanning speed of 3500 mm/s: (a) 0.3 mm; (b) 0.8 mm; (c) 1.2 mm; (d) 1.5 mm; (e) 1.7 mm; (f) 2.0 mm.

Fig. 11. Fracture morphologies of SLM-fabricated lattice structures at different laser processing parameters: (a) P ¼ 375 W, v ¼ 3500 mm/s; (b) P ¼ 400 W, v ¼ 3500 mm/s; (c) P ¼ 450 W, v ¼ 3500 mm/s.

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4) The lattice structure at 375 W exhibited excellent energy absorption of 3.45 J. At the same time, the value of bearing force and displacement was 2.95 kN and 1.18 mm, respectively.

Author contribution section

Yuexin Du conducted the experiments and numerical simulation work, as well as wrote this manuscript.

Dongdong Gu proposed the main idea of this work and provided the crucial advices and a lot of constructive discussions.

Lixia Xi and Donghua Dai designed the experiments and went over the whole manuscript to polish the English language.

Tong Gao and Jihong Zhu designed the lattice structure and analyzed the deformation behavior of this structure;

Chenglong Ma analyzed a part of experimental results and simula-tion results.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51735005); the National Key Research and Development Program “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100101); Na-tional Natural Science Foundation of China for Creative Research Groups (Grant No. 51921003); National Natural Science Foundation of China (Grant No. 51905269); Natural Science Foundation of Jiangsu for Youths (No. BK20170787); The 15th Batch of “Six Talents Peaks” Innovative Talents Team Program “Laser Precise Additive

Manufacturing of Structure-Performance Integrated Lightweight Alloy Components” (No. TD-GDZB-001) (Jiangsu Provincial Department of Human Resources and Social Security of China); 2017 Excellent Scien-tific and Technological Innovation Teams of Universities in Jiangsu “Laser Additive Manufacturing Technologies for Metallic Components” (Jiangsu Provincial Department of Education of China).

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Fig. 12. A summary of compressive strength for different lightweight structures, including our works, aluminum alloy (AlSi10Mg, 6061, 3003), stainless steel (316L stainless steel, 304L stainless steel), titanium alloy (Ti6Al4V). For the same density, the bio-inspired lattice structure outperforms the conventional lattice structures significantly for peak strength.

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