title: Additive manufacturing of Ti64 alloy in-situ modified with additions of boron or oxygen

authors: Jackson, Benjamin Ivan

subjects: Additive manufacturing

abstract: Selective laser melting (SLM) is a metal additive manufacturing process that enables the production of complex, near net shape, parts with fully functional mechanical properties. The use of titanium alloys in SLM is a good fit due to a high material utilisation and net shape production with this expensive material. While titanium alloys have attractive properties of high specific strength and excellent corrosion resistance, these and other attributes may be modified through various methods for further improvements of the material. Strengthening mechanisms of interstitial hardening and particulate reinforcement are viable approaches for enhancing titanium alloys, however these often increase processing difficulty. Near net shape processes, like SLM, have the potential to produce functional parts with these modified properties.
This study explored the use of SLM for the fabrication of titanium parts, modified in-situ during processing with small amounts of boron or oxygen to develop enhanced properties. The in-situ incorporation of these different elements was performed during SLM using blended powders. Simple mixing of feedstock Ti64 alloy powder with small amounts of either amorphous boron or TiO₂, and subsequent SLM consolidation, has the potential to develop interstitial (oxygen) and particulate (boron) strengthened materials. 
This work analytically compares the properties of samples created using different process parameters, related through energy density. By examining variation in porosity and hardness with respect to these process parameters, it was possible to establish optimum processing conditions for manufacturing the in-situ modified material. Novel samples were developed for SLM that simplify the multi-level, multi-factor investigation of these process parameters into a single specimen for easy preparation and testing. Further investigation of how processing may influence the distribution and homogeneity of the modified material was studied through the influence of stress relief and sub-beta transus annealing heat treatments. Characterisation of these materials and treatment conditions were performed to determine the effect the additives have on the mechanical and wear properties. 
Most significantly this work demonstrated that small amounts (approx. 0.2 wt%) of boron or oxygen could be effectively combined with Ti64 via SLM processing. Optimised parameters (resulting in energy density of 54.4 J/mm³) were able to produce high density parts with uniform incorporation. Typical microstructures were developed for SLM produced Ti64, consisting of columnar colonies of fine lamellar martensite that extend in the build direction. Additional oxygen changed this very little, however boron had a significant effect, refining the grain structure and preventing the formation of columnar colonies. Enhancements in compressive strength and hardness resulted from boron or oxygen addition with compressive yield strength increased by up to 10% and 16% respectively. Improvements in these properties were retained in the different heat treatment conditions when compared to unmodified Ti64. While strengthening of Ti64 was achieved by inclusion of the additives, little improvement in wear performance was found.
This work identifies a viable method of modifying properties of titanium through in-situ processes during SLM. The findings of this study will have implications for the production of Ti64 alloys during the SLM processes and may guide future work on developing materials specific for additive manufacturing.

text: Additive manufacturing of Ti64 alloy in-situ modified with additions of boron or oxygen Selective laser melting (SLM) is a metal additive manufacturing process that enables the production of complex, near net shape, parts with fully functional mechanical properties. The use of titanium alloys in SLM is a good fit due to a high material utilisation and net shape production with this expensive material. While titanium alloys have attractive properties of high specific strength and excellent corrosion resistance, these and other attributes may be modified through various methods for further improvements of the material. Strengthening mechanisms of interstitial hardening and particulate reinforcement are viable approaches for enhancing titanium alloys, however these often increase processing difficulty. Near net shape processes, like SLM, have the potential to produce functional parts with these modified properties.
This study explored the use of SLM for the fabrication of titanium parts, modified in-situ during processing with small amounts of boron or oxygen to develop enhanced properties. The in-situ incorporation of these different elements was performed during SLM using blended powders. Simple mixing of feedstock Ti64 alloy powder with small amounts of either amorphous boron or TiO₂, and subsequent SLM consolidation, has the potential to develop interstitial (oxygen) and particulate (boron) strengthened materials. 
This work analytically compares the properties of samples created using different process parameters, related through energy density. By examining variation in porosity and hardness with respect to these process parameters, it was possible to establish optimum processing conditions for manufacturing the in-situ modified material. Novel samples were developed for SLM that simplify the multi-level, multi-factor investigation of these process parameters into a single specimen for easy preparation and testing. Further investigation of how processing may influence the distribution and homogeneity of the modified material was studied through the influence of stress relief and sub-beta transus annealing heat treatments. Characterisation of these materials and treatment conditions were performed to determine the effect the additives have on the mechanical and wear properties. 
Most significantly this work demonstrated that small amounts (approx. 0.2 wt%) of boron or oxygen could be effectively combined with Ti64 via SLM processing. Optimised parameters (resulting in energy density of 54.4 J/mm³) were able to produce high density parts with uniform incorporation. Typical microstructures were developed for SLM produced Ti64, consisting of columnar colonies of fine lamellar martensite that extend in the build direction. Additional oxygen changed this very little, however boron had a significant effect, refining the grain structure and preventing the formation of columnar colonies. Enhancements in compressive strength and hardness resulted from boron or oxygen addition with compressive yield strength increased by up to 10% and 16% respectively. Improvements in these properties were retained in the different heat treatment conditions when compared to unmodified Ti64. While strengthening of Ti64 was achieved by inclusion of the additives, little improvement in wear performance was found.
This work identifies a viable method of modifying properties of titanium through in-situ processes during SLM. The findings of this study will have implications for the production of Ti64 alloys during the SLM processes and may guide future work on developing materials specific for additive manufacturing.

year: 2022

title: How topography-dependent are topographic effects? Complementary numerical modeling of centrifuge experiments

authors: Jeong, Seokho

subjects: Science & Technology

abstract: Topographic effects, the modification of seismic shaking by irregular topographies compared to flat ground, have been extensively studied. Very few studies, however, have investigated the effects of the stratigraphy and nonlinear response of the underlying geology on topographic amplification. Furthermore, most experimental studies have been performed in the field, where it is often difficult to establish an ideal flat-ground reference station, as well as to characterize the soil properties and their spatial variability in sufficient detail. Dafni [1] recently tested the seismic response of step-like slopes in a series of centrifuge experiments, where the incident motion, reference station and material properties were characterized in detail. In this study, we investigated the influence of the container boundary on topographic effects observed in the centrifuge experiments by performing numerical simulations with and without the container boundary. Our analysis suggested that the rigid-body rocking motion of the centrifuge container likely increased the experimental topographic spectral ratios, contributing to the discrepancy between the simulated and observed spectral ratios. We also found that although the laminar box lateral boundaries caused spurious reflections, they didn’t qualitatively affect the ground surface amplification pattern compared to numerical predictions of the same configuration without boundaries. At the same time, and most importantly, however, we found that the baseplate –by trapping waves scattered and distracted by the slope– amplified the ground motion at the crest up to one order of magnitude compared to numerical predictions of the response in absence of the baseplate. Our results show that topographic effects can be significantly affected by the underlying soil stratigraphy; and allude to the potentially significant role of this phenomenon in elevating seismic risk in regions with strong topographic relief. The findings of this study also suggest that future studies will benefit from clear understanding and careful considerations of capabilities and limitations of different investigation methods and that the numerical modelling and the lab testing (or the field testing) methods should complement each other.

text: How topography-dependent are topographic effects? Complementary numerical modeling of centrifuge experiments Topographic effects, the modification of seismic shaking by irregular topographies compared to flat ground, have been extensively studied. Very few studies, however, have investigated the effects of the stratigraphy and nonlinear response of the underlying geology on topographic amplification. Furthermore, most experimental studies have been performed in the field, where it is often difficult to establish an ideal flat-ground reference station, as well as to characterize the soil properties and their spatial variability in sufficient detail. Dafni [1] recently tested the seismic response of step-like slopes in a series of centrifuge experiments, where the incident motion, reference station and material properties were characterized in detail. In this study, we investigated the influence of the container boundary on topographic effects observed in the centrifuge experiments by performing numerical simulations with and without the container boundary. Our analysis suggested that the rigid-body rocking motion of the centrifuge container likely increased the experimental topographic spectral ratios, contributing to the discrepancy between the simulated and observed spectral ratios. We also found that although the laminar box lateral boundaries caused spurious reflections, they didn’t qualitatively affect the ground surface amplification pattern compared to numerical predictions of the same configuration without boundaries. At the same time, and most importantly, however, we found that the baseplate –by trapping waves scattered and distracted by the slope– amplified the ground motion at the crest up to one order of magnitude compared to numerical predictions of the response in absence of the baseplate. Our results show that topographic effects can be significantly affected by the underlying soil stratigraphy; and allude to the potentially significant role of this phenomenon in elevating seismic risk in regions with strong topographic relief. The findings of this study also suggest that future studies will benefit from clear understanding and careful considerations of capabilities and limitations of different investigation methods and that the numerical modelling and the lab testing (or the field testing) methods should complement each other.

year: 2019

title: Weaving kaupapa Māori and e-Learning

authors: Neal, Terry

subjects: New Zealand

abstract: This paper describes the creation and design of an online professional development course that aims to help educators in New Zealand to weave kaupapa Māori and e-Learning approaches together. The team sought to model weaving of kaupapa Māori and e-Learning approaches in how they worked together and the educational design for the material. The authors propose that educators with these skills are crucial if we are to effectively meet the needs of Māori learners, more than that, of all New Zealand learners, in the 21st century.

text: Weaving kaupapa Māori and e-Learning This paper describes the creation and design of an online professional development course that aims to help educators in New Zealand to weave kaupapa Māori and e-Learning approaches together. The team sought to model weaving of kaupapa Māori and e-Learning approaches in how they worked together and the educational design for the material. The authors propose that educators with these skills are crucial if we are to effectively meet the needs of Māori learners, more than that, of all New Zealand learners, in the 21st century.

year: 2006-09