Synthesis and characterisation of advanced ball-milled Al-Al2O3 nanocomposites for selective laser melting

Selective laser melting (SLM) offers significant potential for the manufacture of the advanced complex-shaped aluminium matrix composites (AMCs) used in the aerospace and automotive domains. Previous studies have indicated that advanced composite powders suitable for SLM include spherical powders with homogeneous reinforcement distribution, a particle size of < 100 μm and good flowability (Carr index < 15%); however, the production of such composite powders continues to be a challenge. Due to the intensive impacts of grinding balls, the high-energy ball-milling (HEBM) process has been employed to refine Al particles and disperse the nano Al2O3 reinforcements in the Al matrix to improve their mechanical properties. Notwithstanding, the specific characteristics of ball-milled powders for SLM and the effect of milling and pause duration on the fabrication of composite powders have not previously been investigated. The aim of this study was to synthesise Al-4 vol.% Al2O3 nano-composite powders using HEBM with two different types of milling and pause combinations. The characteristics of the powders subjected to up to 20 h of milling were investigated. The short milling (10 min) and long pause (15 min) combination provided a higher yield (66%) and narrower particle size distribution range than long milling (15 min) and a short pause (5 min). The nano Al2O3 reinforcements were observed to be dispersed uniformly after 20 h of milling, and the measured Carr index of 13.2% indicated that the ball-milled powder offered good flowability. Vickers micro-hardness tests indicated that HEBM significantly improved the mechanical properties of the ball-milled powders

Characterisation and milling time optimisation of nanocrystalline aluminium powder for selective laser melting

The aim of this study is to investigate the properties of high-energy ball-milled nanocrystalline aluminium powders and to determine the optimum milling time required to produce an advanced aluminium powder for selective laser melting (SLM). Previous research has indicated that powders suitable for SLM include milled nanocrystalline aluminium powders with an average grain size of 60 nm and good flowability (Carr index less than 15 %). This study employs advanced nanometrology methods and analytical techniques to investigate the powder morphology, phase identification, average grain size and flowability of ball-milled powders. Stearic acid is used to prevent excessive cold welding of the ball-milled powder and to reduce abrasion of the grinding bowl and balls. The results indicate that, whilst the average particle size achieves a steady state after 14 h of milling, the grain size continues to decrease as the milling time progressed (e.g. the transmission electron microscopy measured average grain size is 56 nm after 20 h of milling compared to 75 nm for 14 h of milling). The aluminium powders milled for 16 and 20 h exhibit good flow behaviour, achieving a Carr index of 13.5 and 15.8 %, respectively. This study shows that advanced nanocrystalline aluminium powders suitable for SLM require ball milling for between 16 and 20 h, with 18 h being the optimum milling time

Comparing different data collection and analysis techniques for quantifying healthy knee joint function during stair ascent and descent

There is currently no standard data collection or analysis method for the assessment of stair gait using motion analysis. This makes the comparison of results from different studies difficult. It is important to gain an appreciation of the discrepancies in kinematic and kinetic information generated by employing different computational approaches, as these differences may be critical in cases where methodologies were to change over a long-term study. This study explores the effect of using different methodologies for the assessment of non-pathological knee function of ten subjects during stair ascent and descent. Two methods of computing knee kinematics were compared: (a) using in-house software and a pointer method of anatomical calibration and (b) using commercial software, Visual3D (C-motion, Inc.) and skin-mounted markers. Significant differences were found between the two methods when calculating a frontal plane range of motion (p<0.05). Three methods of computing knee moments were compared. Knee moments computed using the inverse dynamic analysis (IDA) approach of Visual3D (C-motion, Inc.) were significantly different (p<0.05) to those calculated using in-house IDA software that ignores the foot and ankle and to those computed using a vector cross-product approach. This study highlights the implications of comparing data generated from different collection and analysis methods

Detection of cracking in mild steel fatigue specimens using acoustic emission and digital image correlation

The aim of this investigation was to identify sources of AE in mild steel fatigue specimens and rel1ate them to damage mechanisms. Digital Image Correlation (DIC), a full-field strain measurement technique, was used to validate the findings. This paper describes in detail the results of a ‘dog bone’ style specimen undergoing uni-axial fatigue loading. This test forms part of a much larger programme designed to develop an AE monitoring system to identify damage initiation and growth from background noise in fatigue testing of automotive steels subjected to corrosion. Crack growth was monitored in the test using two AE sensors and, to allow a comparison with the detected and located signals, DIC images were captured periodically at peak loads. As part of the initial analysis located signals were compared with areas of high deformation and crack growth as identified by the DIC system. Results demonstrated that it is possible to distinguish the different AE signals originating from various possible failure mechanisms such as Plastic deformation, delamination of DIC paint and crack initiation and propagation. This might be utilized for an effective and powerful approach to monitor multiple failure mechanisms; this has significant applications in automotive chassis testing

Feasibility of detecting orthopaedic screw overtightening using acoustic emission

A preliminary study of acoustic emission during orthopaedic screw fixation was performed using polyurethane foam as the bone-simulating material. Three sets of screws, a dynamic hip screw, a small fragment screw and a large fragment screw, were investigated, monitoring acoustic-emission activity during the screw tightening. In some specimens, screws were deliberately overtightened in order to investigate the feasibility of detecting the stripping torque in advance. One set of data was supported by load cell measurements to directly measure the axial load through the screw. Data showed that acoustic emission can give good indications of impending screw stripping; such indications are not available to the surgeon at the current state of the art using traditional torque measuring devices, and current practice relies on the surgeon’s experience alone. The results suggest that acoustic emission may have the potential to prevent screw overtightening and bone tissue damage, eliminating one of the commonest sources of human error in such scenarios