Publications

Abstract

Critically engineered stiffness and strength of a scaffold are crucial for managing maladapted stress concentration
and reducing stress shielding. At the same time, suitable porosity and permeability are key to facilitate biological
activities associated with bone growth and nutrient delivery. A systematic balance of all these parameters are
required for the development of an effective bone scaffold. Traditionally, the approach has been to study each of
these parameters in isolation without considering their interdependence to achieve specific properties at a
certain porosity. The purpose of this study is to undertake a holistic investigation considering the stiffness,
strength, permeability, and stress concentration of six scaffold architectures featuring a 68.46–90.98% porosity.
With an initial target of a tibial host segment, the permeability was characterised using Computational Fluid
Dynamics (CFD) in conjunction with Darcy’s law. Following this, Ashby’s criterion, experimental tests, and Finite
Element Method (FEM) were employed to study the mechanical behaviour and their interdependencies under
uniaxial compression. The FE model was validated and further extended to study the influence of stress concentration
on both the stiffness and strength of the scaffolds. The results showed that the pore shape can influence
permeability, stiffness, strength, and the stress concentration factor of Ti6Al4V bone scaffolds.
Furthermore, the numerical results demonstrate the effect to which structural performance of highly porous
scaffolds deviate, as a result of the Selective Laser Melting (SLM) process. In addition, the study demonstrates
that stiffness and strength of bone scaffold at a targeted porosity is linked to the pore shape and the associated
stress concentration allowing to exploit the design freedom associated with SLM.

Abstract

Additive manufacture (AM) of metals and alloys using powder-bed fusion (PBF) often employs a 400W
(1060–1100 nm wavelength) fibre laser as the primary energy source for Selective Laser Melting (SLM). Highly
reflective and thermally conductive materials such as pure silver (Ag) offer significant challenges for SLM due to
insufficient laser energy absorption at the powder bed. Accordingly, this work pioneers the processing, analysis,
and fabrication of 99.9% (pure) atomised Ag using PBF AM featuring a 400W fibre laser system. The atomised
pure silver powder is characterised for its morphology, size, shape, distribution and compared to current AM
sterling silver. Laser-powder interaction is then investigated through single track fabrication to assess the feasibility
of laser melting pure Ag. Varied process parameter single laser pass and single-track fabrication on both
copper and steel build substrates are conducted and analysed with optical and scanning electron microscopy
(SEM) techniques. The resulting SLM process parameters are then used to create pure Ag 3D structures and the
effects of laser power, scan speed, hatch distance and layer thickness on material density is evaluated.
Furthermore, SEM analysis of the 3D structures was conducted to identify optimum laser power, scan speed,
hatch distance and layer thickness required to create dense pure Ag structures. The results of this study show that
SLM processing of pure Ag utilising PBF AM is feasible. The optimum process parameters required for the
generation of controlled track formation and 3D fabrication of pure Ag at a 97% density is reported.

Abstract

Literature on the mechanical performance of additively manufactured (AM) negative Poisson’s ratio (
υ) structures
has been primarily focused on beam‐based re‐entrant structures with chevron crosslinks. The walled variants
of this architecture have been shown to exhibit lateral instability. This is where a layered framework can
be advantageous as they provide increased lateral stability. Much less is known regarding the behaviour of such
architecture, let alone their thin/thick‐walled variants. This study explores the influence of design parameters
namely wall thickness (t) and angle (θ) on the mechanical performance of thin and thick‐walled inherently
stable
υ lattices. The design is achieved through conceiving linearly arranged AlSi10Mg re‐entrant unitcells
while discarding the traditional chevron crosslinks. The printed prototypes were experimentally tested
and response surface (RS) models were generated to study the parametric influence on the elastic modulus
(E), compressive strength (σc), failure strain (ɛf ),
υ and relative density (ρr ). The results demonstrate that both
thin‐ and thick‐walled structures exhibit υ of−0.108 to−0.257 despite the interaction effects between t and θ.
The elastic modulus can be increased by either increasing t or θ without considering the interaction effects at
0:3≤t ≤1 mm and 45° ≤ θ ≤ 85°. This study presents a new understanding regarding the fabrication and performance
of re‐entrant structures by AM.