The dynamic behaviour of AlSi10Mg lattice structures fabricated via selective laser melting
Creators: name: Maconachie, Tobias
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400202
subject: Automotive engineering materials
classificationCode: 401499
subject: Manufacturing engineering not elsewhere classified

description: Additive manufacturing (AM) techniques such as Selective Laser Melting (SLM) enable the fabrication of lattice structures, which provide opportunities for automotive weight reduction through the replacement of solid sections with lattice structures to significantly reducing weight while maintaining the required stiffness and strength. However, while many previous studies have sought to characterise the quasi-static behaviour of lattice structures, automotive applications of these structures inherently depend on their dynamic behaviour, which remains relatively unexplored. The performance and behaviour of lattice structures are primarily dictated by three factors: the properties of the structure’s parent material; the topology of the unit cell, which refers to the arrangement and connectivity of the unit cell’s structural elements; and, the geometry of those structural elements. This dissertation discusses these three factors specifically with reference to their effects on the dynamic behaviour of AM lattice structures.

To understand the dynamic structural response of AM lattice structures, the dynamic material properties must first be understood. AlSi10Mg is casting alloy with many favourable properties, that has become the most studied aluminium for SLM manufacture and is the selected material for this project. The orientation of components is an important design concern for AM and has implications for the manufacture of lattice structures due to the various orientation of lattice struts within their structure. To investigate the dynamic behaviour of SLM AlSi10Mg and the effect of build orientation, tensile specimens with three different build orientations were manufactured and tested under quasi-static and dynamic loadings. Although there was limited effect of build orientation on the strength of specimens, a consistent relationship between build orientation angle and ductility was identified, with horizontally built specimens having greater ductility than vertically built specimens. However, no statistically significant relationship between tensile properties and strain was identified, suggesting the material can be considered non rate sensitive. 

Unit cell topology significantly affects the qualitative behaviour of lattice structures, as the arrangement of lattice struts dictates whether a structure will behave in a bending-dominated or stretch-dominated manner. To investigate the effect of topology on the dynamic behaviour of lattice structures, specimens with five different topologies and all other parameters kept constant were tested under quasi-static and dynamic loading conditions. Topologies with struts orientated in the loading direction were stiffer and stronger than those without, and specimens failed by the emergence of diagonal shear planes, the orientation of which was dependent on topology. However, no rate sensitivity was observed in the structures in the tested range of strain rates (0.001 – 133.33 s-1) for any of the tested topologies.

The performance of lattice structures can be predicted based on the relative density of the unit cells, which itself depends on the unit cell topology and is a function of the geometric parameters of the unit cell – cell size and strut diameter. To identify the effects of unit cell geometry on relative density and the resulting mechanical performance, lattice specimens with two topologies, one stretch-dominated (BCCZ) and one bending-dominated (BCC), were designed with different arrangements of cell size and strut diameter to achieve two target relative densities (10% and 20%). Predictive formulae were generated to relate geometric parameters to the resulting relative density, and for a given target relative density, BCC specimens were found to be more consistent than BCCZ. Again, no rate sensitivity was identified for any specimens, suggesting rate sensitivity is not affected by unit cell geometry in the range of strain rates tested.

Previous studies have found that the addition of cellular materials to thin-walled tubes is an efficient means of improving the energy absorption performance, yet no previous studies have explored the use of lattice structures for this application, despite the superiority of lattice structures over alternative cellular materials. To overcome this deficit, thin-walled tubes filled with lattice structures, referred to as “crash cans”, were manufactured with different wall thicknesses and relative densities of the lattice structures. The addition of lattice structures was found to significantly increase the energy absorption performance of the tubes and improve the consistency of the response. Furthermore, and upper limit of performance improvement was identified when the crushing load of the lattice structure significantly exceeded that of the tube.

The findings of this research contribute to the comprehensive characterisation of SLM lattice structures towards their further commercial adoption.