Salt-metal feedstocks for the creation of stochastic cellular structures with controlled relative density by powder bed fabrication

A novel type of metallic feedstock material for powder-bed additive manufacturing (AM) processes is proposed that enables the manufacture of cellular structures without the time consuming and computationally intensive step of digitally representing the internal geometry of a part. The feedstock is a blend of metal and salt particles and, following Selective Laser Melting (SLM) processing, the salt is dissolved to leave a metallic, cellular structure. The conditions for successfully processing the feedstock are first demonstrated, followed by an investigation into how the feedstock composition can be used to control the relative density of the cellular material. Mechanical testing reveals that the strength and stiffness of the cellular structures can be tuned through control of feedstock composition, and hence, relative density. This presents a significant enhancement to the state-of-the-art for materials preparation for AM since, for the first time, cellular structures can be created with specific properties without explicitly defining or analysing the unit cell geometry

Generation of graded porous structures by control of process parameters in the selective laser melting of a fixed ratio salt-metal feedstock

The demonstration of salt dissolution incorporated within laser powder-bed fusion fabrication processes has allowed the creation of complex porous structures without the need for sophisticated design algorithms. This serves to simplify the process, for porous structure creation in powder-bed fabrication techniques, creating a new opportunity for the realisation of optimised structures. A new methodology is presented here in which modulation of the energy density while using a single feedstock material enables three-dimensional control of porosity, ranging from 20 % to 49 %. Through structured experimentation, the response of the material to varying the process parameters in selective laser melting is evaluated and nested structures of distinct densities and morphologies are created. Correlation of the process parameters with modulus and ultimate compressive stress are established. A simple-assembly algorithm was used to generate complex parts consisting of locally assigned porosities having characteristic properties

The creep behaviour of nickel alloy 718 manufactured by laser powder bed fusion

Components manufactured by laser powder bed fusion (LPBF) are limited by their performance for use in critical applications. LPBF materials have microstructural defects, such as suboptimal grain size and morphology, and macroscale anomalies, such as lack of fusion. This results in LPBF components performing below their wrought counterparts for various mechanical properties, such as creep which has seldom been researched. To understand the creep behaviour of LPBF alloy 718, parts were fabricated using different scanning strategies and build orientations and creep tested at 650 °C under a 600 MPa load. Heat treatment increased the creep life by a factor of 5, confirming its necessity. The build orientation and stress state were shown to be determining factors in the creep failure mechanisms. The Meander scanning strategy resulted in a 58% increase in creep life compared to the Stripe strategy, due to the detrimental effects of the numerous laser overlapping regions in the Stripe strategy. For a given strategy, a 24% increase in creep life compared to wrought alloy 718 was observed, indicating that LPBF has the potential to surpass wrought material properties. As a result of this work, it is possible to propose build strategies for high temperature creep applications

Resonant transmission through an open quantum dot

We have measured the low-temperature transport properties of a quantum dot formed in a one-dimensional channel. In zero magnetic field this device shows quantized ballistic conductance plateaus with resonant tunneling peaks in each transition region between plateaus. Studies of this structure as a function of applied perpendicular magnetic field and source-drain bias indicate that resonant structure deriving from tightly bound states is split by Coulomb charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure

Chemical Bonding in Solids

This chapter discusses the various classes of hydride compounds, with a special focus on saline and metallic hydrides as well as oxyhydrides. It includes the following topics: thermodynamic stability, crystal chemistry, synthesis, and physical properties. The chapter also highlights recent progress in understanding hydride ion mobility in alkaline earth hydrides. It further deals with hydride compounds and in particular those containing alkali, alkaline earth, and transition and rare earth metals. The saline hydrides, that is, AH and AeH2 (with A=Li, Na, K, Rb, and Cs; Ae=Mg, Ca, Sr, and Ba) are proper ionic materials, in which hydrogen is present as hydride anions, H−. Saline hydrides show many similarities with their halide analogues, especially concerning crystal and electronic structures and, perhaps to a lesser extent, physical attributes such as brittleness, hardness, and optical properties

Quantification and characterisation of porosity in selectively laser melted Al–Si10–Mg using x-ray computed tomography

We used X-ray computed tomography (CT), microscopy and hardness measurements to study Al–Si10–Mg produced by selective laser melting (SLM). Specimens were subject to a series of heat treatments including annealing and precipitation hardening. The specimen interiors were imaged with X-ray CT, allowing the non-destructive quantification and characterisation of pores, including their spatial distribution. The specimens had porosities less than 0.1%, but included some pores with effective cross-sectional diameters up to 260 μm. The largest pores were highly anisotropic, being flat and lying in the plane normal to the build direction. Annealing cycles caused significant coarsening of the microstructure and a reduction of the hardness from (114 ± 3) HV, in the as-built state, to (45 ± 1) HV, while precipitation hardening increased this to a final hardness of (59 ± 1) HV. The pore size and shape distributions were unaffected by the heat treatments. We demonstrate the applicability of CT measurements and quantitative defect analysis for the purposes of SLM process monitoring and refinement

From the outside in: narratives of creative arts practitioners working in the criminal justice system

This is an accepted manuscript of an article published by Wiley-Blackwell in The Howard Journal of Crime and Justice on 31/12/2019, available online: https://doi.org/10.1111/hojo.12318 The accepted version of the publication may differ from the final published version.The penal voluntary sector is highly variegated in its roles, practices and functions, though research to date has largely excluded the experiences of front-line practitioners. We argue that engaging with the narratives of practitioners can provide fuller appreciation of the potential of the sector’s work. Though life story and narrative have been recognised as important in offender desistance (Maruna, 2001), the narrative identities of creative arts practitioners, who are important ‘change agents’ (Albertson, 2015), are typically absent. This is despite evidence to suggest that a practitioner’s life history can be a significant and positive influence in the rehabilitation of offenders (Harris, 2017). Using narratological analysis (Bal, 2009), this study examined the narratives of 19 creative practitioners in prisons in England and Wales. Of particular interest were the formative experiences of arts practitioners in their journey to prison work. The findings suggest that arts practitioners identify with an ‘outsider’ status and may be motivated by an ethic of mutual aid. In the current climate of third sector involvement in the delivery of criminal justice interventions, such a capacity may be both a strength and weakness for arts organisations working in this field