High strength steels are a vital material for aerospace applications but are also prone to damage... more High strength steels are a vital material for aerospace applications but are also prone to damage from fatigue, corrosion, and wear. Additive manufacturing (AM) processes such as laser directed energy deposition (L-DED) offer a means for repairing both the geometry and structure of damaged steels; however, significant variation in tensile properties have been reported following repair. While previous studies have tried to improve performance through postdeposition heat treatment, such practices may not be possible for commercial parts due to risks of distortion and thermal damage to the substrate. Instead, this investigation analyses the role of the intrinsic heat treatment effect on as-deposited tensile properties through a detailed review of both AM and AM repair literature. By assessing a wide variety of high strength steels, the links between conventional heat treatment parameters and steel performance in AM are established, and the role of steel composition understood. This review is supported by additional AM and L-DED repaired samples, with consistent parameters used between steels to ensure similar thermal histories, and eliminate potential discrepancies seen between AM machines. The results demonstrate the effect of intrinsic heat treatment on martensitic and precipitation hardening steels, the role of residual heat and heat extraction through the substrate, and flag potential issues faced by steels at risk of temper embrittlement. Taken together, these findings provide a clear vision for the advancement of AM repair and the optimization of mechanical performance.
Lattice structures fabricated via Additive Manufacturing (AM) offer improved performance over tra... more Lattice structures fabricated via Additive Manufacturing (AM) offer improved performance over traditional manufacturing methods, however, predicting their mechanical behaviour both accurately and with acceptable computational efficiency remains a challenge. AM associated defects combined with multiple high aspect-ratio strut elements require fine 3D finite-element (FE) meshes; resulting in high computational complexity that limits the number of lattice unit cells that can be practically simulated. Alternatively, Euler-Bernoulli or Timoshenko beam elements can be specified to reduce computational complexity. However, these beam elements are typically based on idealised representations that exclude AM associated defects. This research proposes a novel method which combines data driven AM defect modelling, Markov Chains and Monte Carlo (MCS) simulation techniques to predict the stiffness of an AM lattice structure. Furthermore, this method accommodates stochastic distributions of AM associated defects within computationally effective beam models; thereby enabling the simulation of large-scale lattice structures at a relatively low computational cost. The proposed method is aimed at reliability analysis or a probabilistic approach to structural analysis of AM lattice structures. The combination of generating AM strut digital realisations and MCS, resulted in a variety of possible strut deformation shapes and effective diameters under axial compression. The propagation of effective diameter variability to the lattice-scale level displayed the possible variation in the mechanical response of AM lattice structure. Simulations are validated and insight into how a lattice structures unit cell topology affects simulation accuracy is discussed.
The International Journal of Advanced Manufacturing Technology, May 1, 2020
Metal additive manufacturing (MAM) enables the fabrication of structures with complexity and reso... more Metal additive manufacturing (MAM) enables the fabrication of structures with complexity and resolution that cannot be achieved by traditional manufacturing techniques, including lattice structures. However, MAM processes inherently induce local manufacturing defects, resulting in variation between the idealised and as-manufactured geometry and potentially introducing stress concentrations that are detrimental to structural performance. Quantification of these effects on mechanical performance enables the manipulation of intended lattice geometry to enhance structural performance. However, due to the geometric complexity and small scale of geometric defects, experimental testing and numerical simulation of lattice structures are technically difficult and time-consuming. To overcome this limitation, a novel methodology for quantifying the effect of manufacturing defects on the mechanical properties of MAM lattice structural elements is proposed. This method involves the automated analysis of microscope images of as-manufactured lattice structures to generate numerical models that automate the identification of plastic hinge behaviour in node elements based on custom MAM material properties. This method is applied to Ti-6Al-4V lattice structures fabricated by selective laser melting (SLM) with a range of strut and node diameters and cell sizes. This novel method is shown to predict the effect of local manufacturing defects on bulk lattice mechanical response and provides an efficient tool for the optimisation of as-manufactured MAM lattice structures.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jun 1, 2020
Abstract Selective Laser Melting (SLM) allows the fabrication of complex geometries with high res... more Abstract Selective Laser Melting (SLM) allows the fabrication of complex geometries with high resolution and robust mechanical properties. However, the manner of manufacture – melting of metallic powder with a laser power source – affects microstructure and results in mechanical anisotropy. While some studies have sought to characterise the microstructure and performance of SLM AlSi10Mg, the dynamic response, particularly with regard to anisotropic effects, remains relatively undefined. To overcome this deficit AlSi10Mg specimens were fabricated using SLM with three different build orientations, and quasi-static and dynamic split-Hopkinson tensile bar tests were performed to characterise the tensile properties of the material at strain rates ranging from 3.33 x 10-2 to 2.4 x 103 s-1. The microstructure of as-manufactured specimens and fracture surfaces of failed specimens were analysed. Quasi-static and dynamic results showed little difference between build orientations with regard to strength, but components loaded perpendicular to the build direction were found to be more ductile than other build orientations. Significant scatter was observed in dynamic results, suggesting no strain rate sensitivity of the material in the tested strain rate range. Build orientation was found to affect fracture surface morphology of dynamically tested specimens due to fracture paths following melt pool boundaries. These results assist in the characterisation of the anisotropic effects of build orientation on quasi-static and dynamic behaviours of SLM AlSi10Mg towards the further commercial adoption of the manufacturing technique and material.
Purpose Fused deposition modelling (FDM) is increasingly being explored as a commercial fabricati... more Purpose Fused deposition modelling (FDM) is increasingly being explored as a commercial fabrication method due to its ability to produce net or near-net shape parts directly from a computer-aided design model. Other benefits of technology compared to conventional manufacturing include lower cost for short runs, reduced product lead times and rapid product design. High-performance polymers such as polyetherimide, have the potential for FDM fabrication and their high-temperature capabilities provide the potential of expanding the applications of FDM parts in automotive and aerospace industries. However, their relatively high glass transition temperature (215 °C) causes challenges during manufacturing due to the requirement of high-temperature build chambers and controlled cooling rates. The purpose of this study is to investigate the mechanical properties of ULTEM 1010, an unfilled polyetherimide grade. Design/methodology/approach In this research, mechanical properties were evaluated through tensile and flexural tests. Analysis of variance was used to determine the significance of process parameters to the mechanical properties of the specimens, their main effects and interactions. The fractured surfaces were analysed by scanning electron microscopy and optical microscopy and porosity was assessed by X-ray microcomputed tomography. Findings A range of mean tensile and flexural strengths, 60–94 MPa and 62–151 MPa, respectively, were obtained highlighting the dependence of performance on process parameters and their interactions. The specimens were found to fracture in a brittle manner. The porosity of tensile samples was measured between 0.18% and 1.09% and that of flexural samples between 0.14% and 1.24% depending on the process parameters. The percentage porosity was found to not directly correlate with mechanical performance, rather the location of those pores in the sample. Originality/value This analysis quantifies the significance of the effect of each of the examined process parameters has on the mechanical performance of FDM-fabricated specimens. Further, it provides a better understanding of the effect process parameters and their interactions have on the mechanical properties and porosity of FDM-fabricated polyetherimide specimens. Additionally, the fracture surface of the tested specimens is qualitatively assessed.
High strength steels are a vital material for aerospace applications but are also prone to damage... more High strength steels are a vital material for aerospace applications but are also prone to damage from fatigue, corrosion, and wear. Additive manufacturing (AM) processes such as laser directed energy deposition (L-DED) offer a means for repairing both the geometry and structure of damaged steels; however, significant variation in tensile properties have been reported following repair. While previous studies have tried to improve performance through postdeposition heat treatment, such practices may not be possible for commercial parts due to risks of distortion and thermal damage to the substrate. Instead, this investigation analyses the role of the intrinsic heat treatment effect on as-deposited tensile properties through a detailed review of both AM and AM repair literature. By assessing a wide variety of high strength steels, the links between conventional heat treatment parameters and steel performance in AM are established, and the role of steel composition understood. This review is supported by additional AM and L-DED repaired samples, with consistent parameters used between steels to ensure similar thermal histories, and eliminate potential discrepancies seen between AM machines. The results demonstrate the effect of intrinsic heat treatment on martensitic and precipitation hardening steels, the role of residual heat and heat extraction through the substrate, and flag potential issues faced by steels at risk of temper embrittlement. Taken together, these findings provide a clear vision for the advancement of AM repair and the optimization of mechanical performance.
Lattice structures fabricated via Additive Manufacturing (AM) offer improved performance over tra... more Lattice structures fabricated via Additive Manufacturing (AM) offer improved performance over traditional manufacturing methods, however, predicting their mechanical behaviour both accurately and with acceptable computational efficiency remains a challenge. AM associated defects combined with multiple high aspect-ratio strut elements require fine 3D finite-element (FE) meshes; resulting in high computational complexity that limits the number of lattice unit cells that can be practically simulated. Alternatively, Euler-Bernoulli or Timoshenko beam elements can be specified to reduce computational complexity. However, these beam elements are typically based on idealised representations that exclude AM associated defects. This research proposes a novel method which combines data driven AM defect modelling, Markov Chains and Monte Carlo (MCS) simulation techniques to predict the stiffness of an AM lattice structure. Furthermore, this method accommodates stochastic distributions of AM associated defects within computationally effective beam models; thereby enabling the simulation of large-scale lattice structures at a relatively low computational cost. The proposed method is aimed at reliability analysis or a probabilistic approach to structural analysis of AM lattice structures. The combination of generating AM strut digital realisations and MCS, resulted in a variety of possible strut deformation shapes and effective diameters under axial compression. The propagation of effective diameter variability to the lattice-scale level displayed the possible variation in the mechanical response of AM lattice structure. Simulations are validated and insight into how a lattice structures unit cell topology affects simulation accuracy is discussed.
The International Journal of Advanced Manufacturing Technology, May 1, 2020
Metal additive manufacturing (MAM) enables the fabrication of structures with complexity and reso... more Metal additive manufacturing (MAM) enables the fabrication of structures with complexity and resolution that cannot be achieved by traditional manufacturing techniques, including lattice structures. However, MAM processes inherently induce local manufacturing defects, resulting in variation between the idealised and as-manufactured geometry and potentially introducing stress concentrations that are detrimental to structural performance. Quantification of these effects on mechanical performance enables the manipulation of intended lattice geometry to enhance structural performance. However, due to the geometric complexity and small scale of geometric defects, experimental testing and numerical simulation of lattice structures are technically difficult and time-consuming. To overcome this limitation, a novel methodology for quantifying the effect of manufacturing defects on the mechanical properties of MAM lattice structural elements is proposed. This method involves the automated analysis of microscope images of as-manufactured lattice structures to generate numerical models that automate the identification of plastic hinge behaviour in node elements based on custom MAM material properties. This method is applied to Ti-6Al-4V lattice structures fabricated by selective laser melting (SLM) with a range of strut and node diameters and cell sizes. This novel method is shown to predict the effect of local manufacturing defects on bulk lattice mechanical response and provides an efficient tool for the optimisation of as-manufactured MAM lattice structures.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jun 1, 2020
Abstract Selective Laser Melting (SLM) allows the fabrication of complex geometries with high res... more Abstract Selective Laser Melting (SLM) allows the fabrication of complex geometries with high resolution and robust mechanical properties. However, the manner of manufacture – melting of metallic powder with a laser power source – affects microstructure and results in mechanical anisotropy. While some studies have sought to characterise the microstructure and performance of SLM AlSi10Mg, the dynamic response, particularly with regard to anisotropic effects, remains relatively undefined. To overcome this deficit AlSi10Mg specimens were fabricated using SLM with three different build orientations, and quasi-static and dynamic split-Hopkinson tensile bar tests were performed to characterise the tensile properties of the material at strain rates ranging from 3.33 x 10-2 to 2.4 x 103 s-1. The microstructure of as-manufactured specimens and fracture surfaces of failed specimens were analysed. Quasi-static and dynamic results showed little difference between build orientations with regard to strength, but components loaded perpendicular to the build direction were found to be more ductile than other build orientations. Significant scatter was observed in dynamic results, suggesting no strain rate sensitivity of the material in the tested strain rate range. Build orientation was found to affect fracture surface morphology of dynamically tested specimens due to fracture paths following melt pool boundaries. These results assist in the characterisation of the anisotropic effects of build orientation on quasi-static and dynamic behaviours of SLM AlSi10Mg towards the further commercial adoption of the manufacturing technique and material.
Purpose Fused deposition modelling (FDM) is increasingly being explored as a commercial fabricati... more Purpose Fused deposition modelling (FDM) is increasingly being explored as a commercial fabrication method due to its ability to produce net or near-net shape parts directly from a computer-aided design model. Other benefits of technology compared to conventional manufacturing include lower cost for short runs, reduced product lead times and rapid product design. High-performance polymers such as polyetherimide, have the potential for FDM fabrication and their high-temperature capabilities provide the potential of expanding the applications of FDM parts in automotive and aerospace industries. However, their relatively high glass transition temperature (215 °C) causes challenges during manufacturing due to the requirement of high-temperature build chambers and controlled cooling rates. The purpose of this study is to investigate the mechanical properties of ULTEM 1010, an unfilled polyetherimide grade. Design/methodology/approach In this research, mechanical properties were evaluated through tensile and flexural tests. Analysis of variance was used to determine the significance of process parameters to the mechanical properties of the specimens, their main effects and interactions. The fractured surfaces were analysed by scanning electron microscopy and optical microscopy and porosity was assessed by X-ray microcomputed tomography. Findings A range of mean tensile and flexural strengths, 60–94 MPa and 62–151 MPa, respectively, were obtained highlighting the dependence of performance on process parameters and their interactions. The specimens were found to fracture in a brittle manner. The porosity of tensile samples was measured between 0.18% and 1.09% and that of flexural samples between 0.14% and 1.24% depending on the process parameters. The percentage porosity was found to not directly correlate with mechanical performance, rather the location of those pores in the sample. Originality/value This analysis quantifies the significance of the effect of each of the examined process parameters has on the mechanical performance of FDM-fabricated specimens. Further, it provides a better understanding of the effect process parameters and their interactions have on the mechanical properties and porosity of FDM-fabricated polyetherimide specimens. Additionally, the fracture surface of the tested specimens is qualitatively assessed.
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