Laser additive manufacturing of cellular structure with enhanced compressive performance inspired by Al–Si crystalline microstructure

Abstract

Additive manufacturing (AM), also known as 3D printing, has greatly promoted the development of lattice structures with complex configurations. However, these lattice structures usually consist of periodically arranged nodes and struts. Here, inspired by the three-dimensional crystalline microstructure of selective laser melted (SLM) Al–Si alloy, a type of novel cellular structure with irregular nodes and struts was designed and fabricated by the SLM process with Al–Si alloy powder. The as-fabricated cellular structures were multi-scale materials from nano- to macro-scale. Electron backscatter diffraction (EBSD) analysis revealed that compared with the edge region, the central region of the struts had larger grain size, dominant (001) grain orientation, and worse toughness. Most importantly, compared with the regular lattice structures, the novel cellular structures brought about maximum 32.8% and 38.3% improvement in volumetric energy absorption W_v and specific energy absorption W_s, respectively. Furthermore, the finite element simulation was employed to reveal the stress distribution and energy absorption mechanism of cellular components during compression. Finally, the different fracture modes between the edge and central regions of the struts were investigated. The Al–Si crystalline microstructure inspired cellular structures have potential applications in biomaterials, vibration and thermal insulation.

Introduction

Cellular structures have been applied in many engineering fields for their benefits such as lightweight, low thermal conductivity, high specific stiffness, and high impact energy absorption [1], [2]. Cellular structures were firstly derived from the biological structures in nature [3], [4], [5], [6], [7], [8], [9], and then the bioinspired cellular structures were formed through artificial manufacturing. Sullivan et al. [3] reviewed the cellular structural features of feathers and bones of birds, and they found that the nano- to macro-scale hierarchy of feathers and bones enabled the high bending/torsional stiffness and strength with light weight. Porcupine quill [4], a type of natural foam-filled tube, has a thin cortex and closed-cell foam. Compared with the structure with only the thin cortex, the existence of filled foam improves the mechanical properties of the tube and reduces the compression deformation. Ma et al. [5] fabricated NiTi cellular structures inspired by the microstructural feature of cancer pagurus’ claw, and they found that the bionic structures enhanced the toughness of NiTi parts. In addition, turtle shells [6], sea sponges [7], and pomelo peel [8] are other specific examples of high-strength-low-weight cellular structures. At the early research stage of mimicking natural cellular structures, foam materials were fabricated. However, the pores of foam materials are usually irregular and difficult to control, which results in the properties, such as density, Poisson’s ratio, strength, and stiffness, are difficult to achieve the expected values. Additive manufacturing technologies [10], [11], [12], such as fused deposition modeling (FDM), stereolithography (SLA), two-photon polymerization, laser melting deposition (LMD), and selective laser melting (SLM), have been widely used in manufacturing complicated structures because of their capacity to fabricate nearly arbitrary complexity structures directly from computer-aided-design (CAD) models. The additive manufacturing technology paves the way of fabricating lattice structures, which is a type of cellular structures with periodic arrangements of nodes and struts [13]. The mechanical properties of lattice structures manufactured by additive manufacturing are potentially highly controllable. Several authors have established various types of lattice structure models and fabricated by additive manufacturing, and investigated their deformation and energy absorption behaviors using experimental and finite element analysis (FEA) methods. Qiu et al. [14] designed lattice structures with diamond unit cells and fabricated the lattice structures by SLM process. They found that a series of shear bands at around 45° to the compression axis led to the sharp falling of compressive stress. Li et al. [15] investigated the deformation mode and failure mechanism of multi-layer lattice panels with body-centered cubic and vertical struct (BCCZ) cells through the compressive tests and FEA method. The weak boundary of cells closed to the edge resulted in the decreasing of modulus and strength and the main failure mode of the multi-layer panel was layer-by-layer progressive damage. To solve the rapid collapse of compressive strength, Pham et al. [16] applied metallurgical hardening principles, such as multiphase hardening, grain refinement, precipitation, to the design of lattice structures and offered a new opportunity to design damage-tolerant lattice materials with high strength and stiffness. For biomedical applications, the cellular structure should copy natural geometry at all levels for the properties of biocompatibility [17]. However, the lattice structures are not very similar to natural structures, such as bone, which has a hierarchical porous network of trabeculae [18]. Foaming processed foam materials and selective laser melted lattice structures are two types of cellular structures. The properties of foam materials are difficult to control and the lattice structure is not very similar to natural structure. Thus, further research of cellular structures with controllable structural parameters and mechanical properties is needed.

The selective laser melting (SLM) process can obtain components with fine microstructure due to the high cooling rate (∼105 K/s) [19], [20], [21]. In recent years, Al–Si alloy has been widely used in the field of aerospace and automobile industries for its relatively high specific strength, low density, good corrosion resistance, and good manufacturability [22], [23]. Simultaneously, considerable efforts have been made on fabricating Al–Si alloy components by the SLM process and the SLM parts show great mechanical properties. From the previous literature [24], [25], the microstructure of SLMed Al–Si alloy often shows a network of fine cellular structure, which is pseudoeutectic fibrous Si because of the high cooling rate. Similar to mimick biological structures in nature to establish bio-inspired structures, we designed a cellular structure that consisted of randomly distributed pores by mimicking the crystalline microstructure of SLM processed Al–Si alloy. In return, the SLMed cellular structure with Al–Si alloy will be the multiscale materials (from micro to macro) because both macroscopic and microscopic structures are cellular structures.

In this study, we demonstrated a type of cellular structure inspired by the 3D crystalline microstructure of SLM processed Al–Si alloy. The cellular structures with different relative densities were fabricated using the SLM process with Al–Si alloy powder. Using the EBSD characterization, we studied the microstructure and toughness differences between the edge region and the central region of the strut. The mechanical properties were investigated through compression tests and the finite element simulations were conducted to reveal the stress distribution and energy absorption mechanism during compression. We further studied the fracture modes of cellular components through fracture morphologies.