title: Producing near-net shape titanium alloy parts by metal injection moulding

authors: Gnanavinthan, Thavanayagam

subjects: Titanium

abstract: Metal injection moulding (MIM), is a metalworking process for producing complex parts from metal powders. It is often more cost effective to use MIM to make small, precise componentry from expensive materials such as titanium than traditional metal processes such as casting, forming, extrusion, investment casting and machining. The four processing steps in MIM are: formulating homogenous feedstock of metal power and binders, injection moulding, removing the binder (debinding) and sintering. The binder affects feedstock rheology, allowing the required shape to be moulded. It then needs to be removed so the object can be sintered to consolidate into a near-net shaped part. As MIM has many advantages, including low wastage of material and the ability to produce intricate shapes, it is used in sophisticated applications such as biomedical instruments, in the aerospace industry, and in niche applications that use expensive materials such as titanium. 

The objective of this research was to identify binders and processing methods that produced easily moulded titanium-based feedstock and final products with minimal defects. This involves understanding how factors such as binder composition, powder loading, and processing method affect feedstock rheology, homogeneity, debinding and sintering, and to develop a model for investigating feedstock rheology.

Feedstocks were manufactured by mixing different amounts of hydride-dehydride titanium alloy powder (HDH Ti-6Al-4V) with a three-component binder containing different amounts of polyethylene glycol (PEG), polyvinyl butyral (PVB) and stearic acid (SA). Most research on titanium powder metallurgy uses spherical, fine powders with mean diameters (d50) between 0.3 and 19 μm. This research used HDH Ti-6Al-4V, an irregular, coarse powder with a d50 of 52 μm and a d90 of 117 μm. The effect of using different types of mixers for various mixing times was evaluated by measuring mixing torque and thermal analysis. Feedstock homogeneity was evaluated by density measurements and DTA/TGA analysis, and mouldability was evaluated by rheological properties. After mixing and extrusion, binders were removed from the formed (green) part in a two-stage process involving aqueous debinding followed by thermal debinding. The part was then sintered to remove any remaining binder and to create the final product. Morphological characteristics of feedstocks, moulded, debound and sintered parts were assessed using optical and scanning electron microscopes.

Polyethylene glycol, PVB and SA were used as binder components because they are inexpensive and readily available. PEG is environmentally safe, easily extractable in water and reduces feedstock viscosity. The PVB is water-insoluble and provides strength to the formed (green) parts and debound (brown or grey) parts so used as a backbone component. The SA acts as a surfactant and helps wet the powder particles. Thermal characteristics were determined with DTA/TGA and rheological analyses with a capillary rheometer.

The four-stage process developed to manufacture homogeneous material suitable for titanium-MIM (Ti-MIM) involved dry-mixing irregular HDH Ti-6Al-4V powder and binder components in a planetary mixer for 30 min at room temperature, followed by 16 h further mixing at room temperature in a roller mixer rotating at 250 rpm, then melt-mixing at 125 ˚C for 45 min in a roller compounder. The final step involved extruding the processed feedstock in a twin-screw extruder operating with a feed-to-nozzle temperature profile of 125, 140, 135, 135, 140 ˚C.

Viscosity, density and mixing torque data for the PEG-PVB-SA binders indicated that the maximum (so-called critical) powder loading was 65 vol. %. Feedstocks made with 75:20:5 vol. % PEG:PVB:SA and 80:15:5 vol. % PEG:PVB:SA binders and 60 vol. % powder loading, were optimal for Ti64-MIM. This powder loading is higher than reported for spherical, fine powders. The flow behaviour index of the feedstock could be used to identify optimal powder loading and temperatures for injection moulding. A feedstock with low viscosity (<80 Pa-s), low flow activation energy (<30 kJ/mol), high mouldability index (15 – 22), high melt flow rate (up to 860 g/10 min), and high stability was selected for Ti-MIM.
 
Solvent debinding green parts made of 60 vol. % Ti64 powders and PEG-PVB-SA binders at 35 °C was affected by binder composition. Between 4.67 – 5.20 h, PEG removal changed from dissolution-controlled to diffusion-controlled. Increasing debinding temperature to 55 °C decreased the time at which the transition occurred. Debinding time affected PEG removal, porosity and reaction depth to a similar extent, indicating these parameters are inter-related. The empirical relationship describing solvent debinding kinetics of Ti-MIM parts showed PEG removal (weight loss %) was linearly related to the nth root of debinding time (n√t). This model can be used to predict optimum solvent debinding temperature and time. 

The maximum relative density, tensile strength and the oxygen content of as-sintered parts were 95%, 455 MPa and 1.58 wt. % respectively. Fracture surfaces exhibited transgranular fracture. The poor mechanical properties are mainly due to porosity and high oxygen content.

New rheological models to predict feedstock flow properties, relative viscosity and critical powder loading were developed and validated. An improved model for predicting relative viscosity and critical powder loading from powder loading, surface area, shear rate, and flow behaviour index is proposed.

The Einstein relationship for relative viscosity of dilute suspensions was modified and used for MIM feedstocks, which have a much higher powder loading.
 
Recommendations for further work are given.

text: Producing near-net shape titanium alloy parts by metal injection moulding Metal injection moulding (MIM), is a metalworking process for producing complex parts from metal powders. It is often more cost effective to use MIM to make small, precise componentry from expensive materials such as titanium than traditional metal processes such as casting, forming, extrusion, investment casting and machining. The four processing steps in MIM are: formulating homogenous feedstock of metal power and binders, injection moulding, removing the binder (debinding) and sintering. The binder affects feedstock rheology, allowing the required shape to be moulded. It then needs to be removed so the object can be sintered to consolidate into a near-net shaped part. As MIM has many advantages, including low wastage of material and the ability to produce intricate shapes, it is used in sophisticated applications such as biomedical instruments, in the aerospace industry, and in niche applications that use expensive materials such as titanium. 

The objective of this research was to identify binders and processing methods that produced easily moulded titanium-based feedstock and final products with minimal defects. This involves understanding how factors such as binder composition, powder loading, and processing method affect feedstock rheology, homogeneity, debinding and sintering, and to develop a model for investigating feedstock rheology.

Feedstocks were manufactured by mixing different amounts of hydride-dehydride titanium alloy powder (HDH Ti-6Al-4V) with a three-component binder containing different amounts of polyethylene glycol (PEG), polyvinyl butyral (PVB) and stearic acid (SA). Most research on titanium powder metallurgy uses spherical, fine powders with mean diameters (d50) between 0.3 and 19 μm. This research used HDH Ti-6Al-4V, an irregular, coarse powder with a d50 of 52 μm and a d90 of 117 μm. The effect of using different types of mixers for various mixing times was evaluated by measuring mixing torque and thermal analysis. Feedstock homogeneity was evaluated by density measurements and DTA/TGA analysis, and mouldability was evaluated by rheological properties. After mixing and extrusion, binders were removed from the formed (green) part in a two-stage process involving aqueous debinding followed by thermal debinding. The part was then sintered to remove any remaining binder and to create the final product. Morphological characteristics of feedstocks, moulded, debound and sintered parts were assessed using optical and scanning electron microscopes.

Polyethylene glycol, PVB and SA were used as binder components because they are inexpensive and readily available. PEG is environmentally safe, easily extractable in water and reduces feedstock viscosity. The PVB is water-insoluble and provides strength to the formed (green) parts and debound (brown or grey) parts so used as a backbone component. The SA acts as a surfactant and helps wet the powder particles. Thermal characteristics were determined with DTA/TGA and rheological analyses with a capillary rheometer.

The four-stage process developed to manufacture homogeneous material suitable for titanium-MIM (Ti-MIM) involved dry-mixing irregular HDH Ti-6Al-4V powder and binder components in a planetary mixer for 30 min at room temperature, followed by 16 h further mixing at room temperature in a roller mixer rotating at 250 rpm, then melt-mixing at 125 ˚C for 45 min in a roller compounder. The final step involved extruding the processed feedstock in a twin-screw extruder operating with a feed-to-nozzle temperature profile of 125, 140, 135, 135, 140 ˚C.

Viscosity, density and mixing torque data for the PEG-PVB-SA binders indicated that the maximum (so-called critical) powder loading was 65 vol. %. Feedstocks made with 75:20:5 vol. % PEG:PVB:SA and 80:15:5 vol. % PEG:PVB:SA binders and 60 vol. % powder loading, were optimal for Ti64-MIM. This powder loading is higher than reported for spherical, fine powders. The flow behaviour index of the feedstock could be used to identify optimal powder loading and temperatures for injection moulding. A feedstock with low viscosity (<80 Pa-s), low flow activation energy (<30 kJ/mol), high mouldability index (15 – 22), high melt flow rate (up to 860 g/10 min), and high stability was selected for Ti-MIM.
 
Solvent debinding green parts made of 60 vol. % Ti64 powders and PEG-PVB-SA binders at 35 °C was affected by binder composition. Between 4.67 – 5.20 h, PEG removal changed from dissolution-controlled to diffusion-controlled. Increasing debinding temperature to 55 °C decreased the time at which the transition occurred. Debinding time affected PEG removal, porosity and reaction depth to a similar extent, indicating these parameters are inter-related. The empirical relationship describing solvent debinding kinetics of Ti-MIM parts showed PEG removal (weight loss %) was linearly related to the nth root of debinding time (n√t). This model can be used to predict optimum solvent debinding temperature and time. 

The maximum relative density, tensile strength and the oxygen content of as-sintered parts were 95%, 455 MPa and 1.58 wt. % respectively. Fracture surfaces exhibited transgranular fracture. The poor mechanical properties are mainly due to porosity and high oxygen content.

New rheological models to predict feedstock flow properties, relative viscosity and critical powder loading were developed and validated. An improved model for predicting relative viscosity and critical powder loading from powder loading, surface area, shear rate, and flow behaviour index is proposed.

The Einstein relationship for relative viscosity of dilute suspensions was modified and used for MIM feedstocks, which have a much higher powder loading.
 
Recommendations for further work are given.

year: 2016

title: The development of summative assessment literacy: An exploration of the experiences of beginner secondary science teachers in New Zealand

authors: Edwards, Frances

subjects: assessment literacy

abstract: Summative assessment has been identified as an essential element of effective pedagogy. In the New Zealand context, secondary teachers are responsible for summative assessment within their own classrooms and for summative assessment that contributes to standards-based high stakes school leaving qualifications for students.  For this they need high levels of summative assessment literacy. However, there is a lack of evidence on the nature of summative assessment literacy and its development, especially in the New Zealand context.
A qualitative approach underpinned by an interpretivist research paradigm was used to investigate summative assessment literacy development in eight beginner secondary science teachers. The study tracked the teachers from their entrance to a graduate one-year initial teacher education programme until they had completed six months teaching in their first schools. Each teacher was interviewed at five points through this approximately 18 month period. At each interview they were asked to present artefacts that they felt provided evidence of their understanding and use of summative assessment. These interviews helped establish the beginner teachers’ understandings of summative assessment and allowed them to reflect on what and how they were learning. A questionnaire was used three times during the study (at the beginning and at the completion of the teacher education programme and after six months of teaching). The researcher also observed the students within their university classes. The data was analysed initially using a thematic analysis. Subsequently, a rubric (the SALRubric) was developed to allow further analysis of the development of specific dimensions of summative assessment literacy over time. 
In the study, summative assessment literacy was found to be an amalgam of knowledges. Teacher commentary indicated that personal and contextual factors acted as amplifiers and/or filters for what teachers chose to implement in practice. Personal factors included teacher prior knowledge, experience, and conceptions about assessment; beliefs about ethical matters; and emotional responses to assessment consequences. Contextual factors included beginner teachers’ interactions and experiences with influential others as well as wider influences such as school policy and practice, and national policy. Beginner teachers attributed their summative assessment development to particular people, learning activities and teaching experiences. The profile of development of their summative assessment literacy was largely idiosyncratic with all beginner teachers demonstrating shifts over time. Development was more evident following practicum experiences.
This thesis provides a comprehensive and nuanced view of summative assessment literacy and its development. It highlights the need for policy makers as well as those working with beginner teachers in initial teacher education and schools to take a broad perspective on the nature of summative assessment literacy. A shared understanding regarding its development would allow beginner teachers and those working to support them to better understand the complexity of the process and lead to programmes that better prepare teachers for the challenge of this most complex and important task. The SALRubric developed from this study is a useful tool for tracking summative assessment literacy that could be used by and for beginner teachers to track their development and help focus learning.

text: The development of summative assessment literacy: An exploration of the experiences of beginner secondary science teachers in New Zealand Summative assessment has been identified as an essential element of effective pedagogy. In the New Zealand context, secondary teachers are responsible for summative assessment within their own classrooms and for summative assessment that contributes to standards-based high stakes school leaving qualifications for students.  For this they need high levels of summative assessment literacy. However, there is a lack of evidence on the nature of summative assessment literacy and its development, especially in the New Zealand context.
A qualitative approach underpinned by an interpretivist research paradigm was used to investigate summative assessment literacy development in eight beginner secondary science teachers. The study tracked the teachers from their entrance to a graduate one-year initial teacher education programme until they had completed six months teaching in their first schools. Each teacher was interviewed at five points through this approximately 18 month period. At each interview they were asked to present artefacts that they felt provided evidence of their understanding and use of summative assessment. These interviews helped establish the beginner teachers’ understandings of summative assessment and allowed them to reflect on what and how they were learning. A questionnaire was used three times during the study (at the beginning and at the completion of the teacher education programme and after six months of teaching). The researcher also observed the students within their university classes. The data was analysed initially using a thematic analysis. Subsequently, a rubric (the SALRubric) was developed to allow further analysis of the development of specific dimensions of summative assessment literacy over time. 
In the study, summative assessment literacy was found to be an amalgam of knowledges. Teacher commentary indicated that personal and contextual factors acted as amplifiers and/or filters for what teachers chose to implement in practice. Personal factors included teacher prior knowledge, experience, and conceptions about assessment; beliefs about ethical matters; and emotional responses to assessment consequences. Contextual factors included beginner teachers’ interactions and experiences with influential others as well as wider influences such as school policy and practice, and national policy. Beginner teachers attributed their summative assessment development to particular people, learning activities and teaching experiences. The profile of development of their summative assessment literacy was largely idiosyncratic with all beginner teachers demonstrating shifts over time. Development was more evident following practicum experiences.
This thesis provides a comprehensive and nuanced view of summative assessment literacy and its development. It highlights the need for policy makers as well as those working with beginner teachers in initial teacher education and schools to take a broad perspective on the nature of summative assessment literacy. A shared understanding regarding its development would allow beginner teachers and those working to support them to better understand the complexity of the process and lead to programmes that better prepare teachers for the challenge of this most complex and important task. The SALRubric developed from this study is a useful tool for tracking summative assessment literacy that could be used by and for beginner teachers to track their development and help focus learning.

year: 2017

title: A Relationships Framework for Organisational Responsiveness to Te Tiriti/Treaty of Waitangi in the Work of Groups and Organisations in the Community and the Public Sector of Aotearoa New Zealand

authors: Spelman, Anthony Cyril

subjects: Organisation Development

abstract: This thesis is focussed on the development and implementation of a Relationships Framework based on Te Tiriti/Treaty of Waitangi.  The Framework has been designed to enable working together between Tangata Whenua and Tangata Tiriti in the community and Public Sector in Aotearoa New Zealand.  Its development is explored and its implementation critiqued.  Within that Framework, an original organisation development tool based on a Tiriti/Treaty two-worldview approach is examined in terms of its potential to facilitate change in the operation and management of public life in Aotearoa New Zealand.
My aim is to establish an effective approach for working with worldview difference in the context of Tiriti/Treaty of Waitangi relationships and their implications.  I have a belief that Tangata Whenua are currently excluded from participating in processes that govern public life due to the dominance of Tangata Tiriti worldviews.  Therefore I am committed to making a contribution to a change in this dynamic so that Tangata Whenua views are not only heard but form an integral part of how the infrastructure of our public life is developed and managed. 
The Tiriti/Treaty of Waitangi Relationships Framework [The Framework] presented in the thesis is critiqued in two ways: i) through an examination of the concepts that inform it, and ii) by exploring six examples of its operation and further development in the community and public sector in Aotearoa New Zealand.  The Framework and the Tiriti/Treaty Two-worldview approach used to inform it has worked effectively in a variety of settings.  Through my research I show that organisation development of this type is complex.  It appears that there need be no fixed starting point in the development process within an organisation.  However all key elements of that process must eventually be addressed if effective relationships are to be achieved and the resulting organisational change is to be sustained.  Commitment to organisation development of this type is therefore long-term and requires a commitment to leading and supporting change in behaviour, systems and processes, and structures.

text: A Relationships Framework for Organisational Responsiveness to Te Tiriti/Treaty of Waitangi in the Work of Groups and Organisations in the Community and the Public Sector of Aotearoa New Zealand This thesis is focussed on the development and implementation of a Relationships Framework based on Te Tiriti/Treaty of Waitangi.  The Framework has been designed to enable working together between Tangata Whenua and Tangata Tiriti in the community and Public Sector in Aotearoa New Zealand.  Its development is explored and its implementation critiqued.  Within that Framework, an original organisation development tool based on a Tiriti/Treaty two-worldview approach is examined in terms of its potential to facilitate change in the operation and management of public life in Aotearoa New Zealand.
My aim is to establish an effective approach for working with worldview difference in the context of Tiriti/Treaty of Waitangi relationships and their implications.  I have a belief that Tangata Whenua are currently excluded from participating in processes that govern public life due to the dominance of Tangata Tiriti worldviews.  Therefore I am committed to making a contribution to a change in this dynamic so that Tangata Whenua views are not only heard but form an integral part of how the infrastructure of our public life is developed and managed. 
The Tiriti/Treaty of Waitangi Relationships Framework [The Framework] presented in the thesis is critiqued in two ways: i) through an examination of the concepts that inform it, and ii) by exploring six examples of its operation and further development in the community and public sector in Aotearoa New Zealand.  The Framework and the Tiriti/Treaty Two-worldview approach used to inform it has worked effectively in a variety of settings.  Through my research I show that organisation development of this type is complex.  It appears that there need be no fixed starting point in the development process within an organisation.  However all key elements of that process must eventually be addressed if effective relationships are to be achieved and the resulting organisational change is to be sustained.  Commitment to organisation development of this type is therefore long-term and requires a commitment to leading and supporting change in behaviour, systems and processes, and structures.

year: 2013