Geometric defects and mechanical response of additively manufactured lattice structures
Creators: name: ALGHAMDI, Ahmad Saeed S
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401499
subject: Manufacturing engineering not elsewhere classified

description: Additively Manufactured (AM) lattice structures provide an emerging opportunity for innovation in the design of inexpensive, high-performance engineered structures. These high-value lattice systems enable important commercial outcomes, including for medical and aerospace applications. Despite these commercial opportunities, there remains relatively little research data and associated design methodologies for understanding the influence of design decisions on as-manufactured lattice geometry and associated mechanical performance. This limitation incurs high development costs and lead time for AM lattice structure deployment, and thereby imposes limits on the commercial application of AM lattice systems. This research responds to this identified deficiency and provides a commercially focussed research investigation into AM lattice geometric defects, and the influence of these geometric defects upon observed mechanical performance. 

The thesis contributions are presented within four interconnected investigations. The first and second investigations are associated with the characterisation of as-manufactured geometric defects of AM lattice structures; these analyses are provided in terms of the influence of digital data resolution on geometry and the influence of design decisions on local roughness. These investigations are highly relevant as much of the available predictive techniques focus on idealised response, resulting in under-design or excessive safety factors. The third and fourth investigations are concerned with the characterisation of mechanical response. These analyses, which are demonstrated in this context for the Selective Laser Melting (SLM) AM technology classification, are nonetheless developed from a fundamental basis that is independent of the underlying AM technology and can therefore be utilised for broad-focus AM manufacturability research. Multiple materials are assessed to provide insight into material-specific observations for the SLM manufacturing method.
 
The first geometric investigation characterises the effect of design data input resolution on the circularity of the as-manufactured AM strut element. Digital design input data is represented by the polygon order used to represent the geometric design input of the lattice strut element. The isoperimetric quotient is proposed to characterise the as-manufactured circularity in comparison to a perfect circle; a well-formed cylinder having an isoperimetric quotient near unity. This research found that strut elements with higher polygon order display a higher isoperimetric quotient; and that a fundamental threshold exists, below which increased design resolution does not improve part quality. This observed threshold is found to be dependent upon the material of manufacture. These findings allow increased confidence in the necessary computational cost for robust representation of AM lattice. 

The second investigation quantifies the effect of geometric design and material on lattice strut surface roughness. An experimental design is implemented to quantify influence of strut length, strut diameter, node diameter and the inclination angle on arithmetic mean profile roughness of upward facing and downward facing surfaces. This investigation was enabled by novel methods to efficiently extract surface roughness directly from high-resolution digital microscopy images. The inclination angle was found to be the dominant geometric influence on surface roughness for all materials investigated, with downward facing surface showing higher roughness. The effect of fabrication temperature on surface roughness was characterised with custom numerical simulations to approximate the strut temperature field during SLM manufacture. These simulations confirm that the highest fabrication temperatures occurred in the more acutely inclined struts, and that base supports influence fabrication temperature. Regression analysis confirms that inclination angle and simulated temperature field can be used to predict surface roughness with similar accuracy to the geometric design factors alone. These a priori predictive tools provide generalisable outcomes on the effect of design decisions on as-manufactured surface roughness and are valuable for applications that require confidence on the as-manufactured surface roughness prior to production.

The third investigation explores the buckling of lattice strut elements based on experimental, analytic and numerical simulation. Experimentally fabricated lattice strut elements were imaged using microscopy and micro-computed tomography (¿CT) and the associated critical loads acquired by mechanical testing. Data from microscope images and ¿CT were used to estimate the critical buckling load according to Euler and Secant methods. Experimental data confirms the effectiveness of the proposed ¿effective eccentricity¿ to quantify the effect of as-manufactured geometric defects on critical buckling loads. Linear eigenvalue and non-linear numerical buckling solutions were generated and compared to experimental results. The non-linear model shows strong correlations, while the eigenvalue method is accurate only for struts with large slenderness ratio. These outcomes provide a mechanism to accurately predict lattice strut element buckling from ¿CT scan data and provides a means for non-destructive quality assurance of buckling limited lattice structural elements.
 
The fourth investigation focusses on the influence of geometric defects on the mechanical response of as-manufactured lattice node elements. Aluminium lattice structures were fabricated with a range of geometric design inputs and then imaged by high-resolution optical microscopy. Custom scripts were specified to convert the high-resolution images into a plane stress finite element models of individual nodes and their adjacent struts. Plastic hinges were found to predominantly form in the struts adjacent to the node, implying that node strength is dependent on strut diameter and cell size, but largely independent of the nominal node size. Node stiffness was found to be dependent on strut diameter and cell size, with only the largest strut diameters showing a dependence upon the node diameter. A reduced stress concentration effect was observed when struts are loaded in the non-build direction.
 
These novel methods and results provide a robust contribution to the understanding of the geometric and structural behaviour of AM lattice structures and enable insight into appropriate methods to characterise the effect of these responses in engineered lattice structures. In addition to providing previously unavailable insights and design data, this research provides Design for Additive Manufacturing (DFAM) tools that enable further research based on extension of the underlying design methodologies. The DFAM tools generated in this research are explicitly defined within the thesis structure by both illustrative examples and custom implementation code; this data will benefit future researchers and engineers that intend to further understand and optimise the geometric characteristics and associated mechanical response of engineered AM lattice structures.