Microstructure and mechanical properties of aluminium alloy cellular lattice structures manufactured by direct metal laser sintering

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Abstract

This study thoroughly investigated the microstructure and mechanical properties of AlSi10Mg periodic cellular lattice structures with a wide range of volume fractions (5–20%) and unit cell sizes (3–7 mm) fabricated via direct metal laser sintering (DMLS). It was found that the arc-shaped melt pools are overlapping with each other and comprising near fully dense struts (relative densities≥99%) of the as-built lattice structures. The melt pools of the struts are characterized with very fine cellular-dendritic microstructure. Two distinctive zones in the melt pool can be distinguished: the boundary of melt pool possesses the coarse cellular/dendritic microstructure with the cell size or dendrite arm spacing ranging of 2–4 µm, while the interior of melt pool exhibits the much finer cellular microstructure consisting of the 400–700 nm cells mainly filled with the α-Al matrix and some embedded rod-type Si-phases, and the network boundaries predominantly generated by the aggregates of approximately 20 nm Si particles. Both compression strength and microhardness decrease with the increase in the unit cell size when the volume fraction is fixed. This is mainly because the thinner struts of the smaller unit cell size lattice structures were cooled faster by their surroundings and then exhibit a higher cooling rate, leading to finer microstructure. The compression strength increases with increasing the volume fraction, and an equation based on the Gibson–Ashby model is established to estimate the compression strength of DMLS-produced AlSi10Mg gyroid cellular lattice structures with the 3 mm unit cell size.

Introduction

The aluminium foams offer a wide range of high performance features including superior energy absorption characteristics, light weight, excellent thermal and acoustic insulation properties and high strength–weight ratio. They are increasingly demanded by aerospace and automotive industries due to the tighter requirements of the transport industry for lightweight and lower fuel consumption as well as higher payloads [1], [2]. Aluminium alloy foams can be classified to two types: stochastic foams and periodic cellular lattice structures. A stochastic foam has a random distribution of open or closed voids, whereas a periodic cellular lattice structure possesses a designed topology constructed by repeating a unit cell. Previous research revealed that periodic cellular lattice structures with properly designed unit cells exhibit property profiles superior to those demonstrated by their stochastic analogues at the same porosity [3], [4]. These periodic lattice structures, however, currently face higher manufacturing complexity and fabrication costs than the stochastic structures [5].

Nowadays, the manufacturing processes for closed-cell aluminium stochastic foams seem to abound. They can be fabricated by starting from melted metals or metal powders, where gas bubbles are injected directly or generated chemically by the decomposition of blowing and foaming agents [6], [7]. Comparatively, the techniques for manufacturing aluminium cellular lattice structures are quite limited. These lattice structures are normally produced by several conventional methods including investment casting and metal sheet folding. For instance, the manufacturing process of Duocel aluminium open-cell foams (supplied by ERG Materials and Aerospace Corporation) [8] is based on investment casting. Firstly, an open-cell cellular polymer pattern is formed and coated with a slurry of heat resistant materials. Then the polymer is removed and a molten aluminium alloy is casted into the resultant cavity. Finally, the mould material is mechanically removed to get the remaining aluminium cellular structure [9]. The procedure of this investment casting process is really long and complicated, leading to the difficulties in controlling part defects and accuracy, and high price of Duocel foams. Kooistra et al. (2004) [4] proposed a perforated sheet folding/brazing method to manufacture macro-scale lattice trusses, in which hexagonal perforated sheets were bent to create single layer tetrahedral truss lattices, and then the folded structures were bonded to each other by brazing to form lattice truss cores for sandwich structures. But, the lattice structures produced by metal sheet folding have relatively simple geometries and limited design freedoms, and consequently lack advanced functionality to meet the requirements of many applications.

Additive manufacturing (AM) is able to make three-dimensional objects with complex shapes from computer-aided design (CAD) models [10]. Direct metal laser sintering (DMLS), also termed as selective laser sintering (SLM), is a powder bed fusion (PBF) process of AM technologies and capable of fabricating near fully dense metal components with complex geometries by selectively melting successive layers of metal powders [11]. It shows great potential to make metallic cellular lattice structures beyond current limitations. Recent studies have employed DMLS to build lattice structures using different metal materials including stainless steel [12], [13], pure titanium [14], titanium alloy [11], [15], [16] and copper [17]. For example, McKown et al. (2008) [12] manufactured 316L stainless steel lattice structures based on two types of unit cells by SLM, and then studied their compressive and blast loading behaviours. Mullen et al. (2009) [14] produced titanium cellular structures through SLM for bone in-growth applications. These cellular structures can be given design porosity of 10–95% and compressive strength of 0.5–350 MPa to be comparable to the typical property range of natural bones.

However, there is yet little research investigating the DMLS manufacturing of aluminium alloy cellular lattice structures to achieve low volume fractions and lightweight functionality. This may be because aluminium and its alloys are much more difficult to be processed by DMLS compared to stainless steels and commercially pure titanium and its alloys due to their poor flowability, high laser reflectivity and thermal conductivity and oxidation [18], [19]. Only more recently, Ameli et al. (2013) [20] used DMLS to produce the aluminium heat pipes with porous wick structures designed by repeating an octahedral unit cell. This work concentrated on the functional properties of heat pipes such as permeability, but did not evaluate the microstructure and mechanical properties of the as-made metal lattices. Our previous research [21] revealed the manufacturability and sufficient mechanical properties of the DMLS-made AlSi10Mg cellular lattice structures, which were designed by repeating a unit cell called “Schwartz Diamond”. But, it did not carry out an in-depth study on the microstructure of the struts of the lattice structures and the influence of unit cell size on the strength and microhardness.

The objective of this study is to fully characterize the microstructure and mechanical properties of AlSi10Mg aluminium alloy cellular lattice structures made by DMLS. The periodic cellular lattice structures were designed by repeating a unit cell type called “Schoen Gyroid”, which was proved to possess a “self-supporting” feature that makes it more suitable for DMLS manufacturing [13]. AlSi10Mg gyroid cellular lattice structures with a wide range of cell sizes (3–7 mm) and volume fractions (5–20%) has been built for the evaluation of their strut microstructure, density and microhardness, and mechanical properties.

Section snippets

Materials

The cellular lattice structures were built with an AlSi10Mg alloy powder, which was bought from Electro Optical System (EOS) GmbH, Germany. The chemical compositions of the AlSi10Mg alloy include 9.0–11.0 wt% Si, which is near the eutectic composition (12.5 wt% Si) according to Al–Si equilibrium phase diagram, and 0.2–0.45 wt% Mg that allows hardenability by natural or artificial ageing [22]. Fig. 1 shows the SEM image of the as-received AlSi10Mg alloy powder. Most of the powder particles exhibit

Optical microscope observation

The optical microscope images of the cross sections of the struts were taken along the build direction (z axis) and parallel to the powder deposition plane (xy-plane), as shown in Fig. 3(a) and (b), respectively. In Fig. 3(a), the cross-sections of the melt pools are visible in the optical microscope image along the build direction. The melt pools exhibit an arc-shaped cross section along the build direction. Based on the pixel calculation, the depth of the melt pools is 140±9 µm, bigger than

Conclusions

This study thoroughly investigated the microstructure and mechanical properties of light-weight AlSi10Mg periodic cellular lattice structures with a wide range of volume fraction (5–20%) and unit cell size (3–7 mm) manufactured by direct metal laser sintering (DMLS). The major findings of this research are:

  • (1)

    The arc-shaped melt pools are overlapping with each other and comprising the near fully dense struts of the as-built lattice structures with the relative densities≥99%, and show the very fine

Acknowledgement

This work is supported by the UK Technology Strategy Board (TSB) Funded Project (TP14/BA036D) entitled “SAVING – Sustainable product development via design optimisation and AdditiVe manufacturing” and National Natural Science Foundations of China (Grant no. 51375188 and 51375189). The authors would like to thank Dr Simon Lawrence Bubb in 3T RPD Ltd. for manufacturing the samples and Dr Hong Chang, Dr Yat-Tarng Shyng and Dr Wear Lesley in University of Exeter for the assistance with the

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