Toward more realistic viscosity measurements of tyre rubber–bitumen blends

The measurement of rheological properties of the tyre rubber bitumen blends is often challenging due to presence of suspended tyre rubber’s crumbs. Furthermore, the phase separation during the course of measurements makes the viscosity of these non-homogeneous blends difficult to ascertain. In this study, a new dual helical impeller was designed and manufactured to be used with a rotational viscometer in order to have a real-time control of the viscosity while performing a laboratory mixing of the blends. Layer based manufacturing techniques showed to be a convenient method to produce complex shaped impeller prototypes before manufacturing the more expensive stainless steel assembly. Impeller geometry was optimised to create a convective like flow within the sample and so minimise phase separation. Shear rate constant is geometry dependent and a calibration exercise was carried out to ascertain this. Results of both calibration and validation phases showed that the new impeller provides reliable viscosity measurements of homogenous fluids such as neat bitumen. With regards to complex fluids the new impeller showed a more stable and realistic trend than that obtained by using a standard spindle. In fact, it was demonstrated that the new impeller significantly decreases phase separation within the blend and in turns provides a more realistic measurement of the viscosity. This system represents a feasible and improved solution for optimising the laboratory modification process of tyre rubber bitumen blends by adapting the rotational viscometer as a low-shear mixer

Energy distribution modulation by mechanical design for electrochemical jet processing techniques

The increasing demand for optimised component surfaces with enhanced chemical and geometric complexity is a key driver in the manufacturing technology required for advanced surface production. Current methodologies cannot create complex surfaces in an efficient and scalable manner in robust engineering materials. Hence, there is a need for advanced manufacturing technologies which overcome this. Current technologies are limited by resolution, geometric flexibility and mode of energy delivery. By addressing the fundamental limitations of electrochemical jetting techniques through modulation of the current density distribution by mechanical design, significant improvements to the electrochemical jet process methods are presented. A simplified 2D stochastic model was developed with the ability to vary current density distribution to assess the effects of nozzle-tip shape changes. The simulation demonstrated that the resultant profile was found to be variable from that of a standard nozzle. These nozzle-tip modifications were then experimentally tested finding a high degree of variance was possible in the machined profile. Improvements such as an increase in side-wall steepness of 162% are achieved over a standard profile, flat bases to the cut profile and a reduction of profile to surface inter-section radius enable the process to be analogous to traditional milling profiles. Since electrode design can be rapidly modified EJP is shown to be a flexible process capable of varied and complex meso-scale profile creation. Innovations presented here in the modulation of resistance in-jet have enabled electrochemical jet processes to become a viable, top-down, single-step method for applying complex surfaces geometries unachievable by other means

The importance of microstructure in electrochemical jet processing

© 2018 Electrochemical jet processing (EJP) is an athermal technique facilitating precision micromachining and surface preparation, without recast layer generation. The role of the microstructure in determining machining characteristics has been largely overlooked. In this study, we show that in order to optimise EJP for a given material, fundamental material factors must be considered to ensure the desired near-surface response in terms of metallurgy, topography and dimensional accuracy. In this work, specimens have been prepared from the same feedstock material (brass, Cu39Zn2Pb), to appraise the role of microstructure in the determination of key removal characteristics, such as resultant topography, removal efficiency and form. Topography is shown to be highly dependent upon microstructure across large current density ranges, whereby the phase ratio is generally the dominant amplitude-defining material property, where preconditions with divergent ratios result in lower amplitudes. The microstructure, specifically the phase ratio, significantly changes the form, where predominantly single-phase conditions result in deeper and narrower features (up to 15% deeper compared with as-received condition). In addition, removal efficiency is greater (by 6%) at low current density for small grained dual-phase conditions, than for predominantly single-phase, due to erosion complementing anodic dissolution. Mechanisms are discussed for these removal phenomena and used to inform industrial practice

Defect evolution in laser powder bed fusion additive manufactured components during thermo-mechanical testing

The mechanical performance of additively manufactured (AM) components remains an issue, limiting the implementation of AM technologies. In this work, a new method is presented, to examine the evolution of defects in an Inconel 718 two-bar test specimen, manufactured by laser powder bed fusion AM, during thermo-mechanical testing. The test was interrupted at specific extensions of the specimen, and X-ray computed tomography measurements performed. This methodology has allowed, for the first time, the evolution of the defects in an AM specimen to be studied during a thermo-mechanical test. The number and size of the defects were found to increase with time as a result of the thermo-mechanical test conditions, and the location and evolution of these defects have been tracked. Defect tracking potentially allows for accurate prediction of failure positions, at the earliest possible stage of a thermo-mechanical test. Ultimately, when the ability to locate defects in this manner is coupled with manipulation of build parameters, laser powder bed fusion practitioners will be able to further optimise the manufacturing procedure in order to produce components of a higher structural integrity

Complex functional surface design for additive manufacturing

This paper presents a new methodology for the creation of advanced surfaces which can be produced by Additive Manufacturing (AM) methods. Since there is no cost for enhanced complexity, AM allows for new capabilities in surface design. Micro-scale surface features with varying size, shape and pitch can be manufactured by Two-Photon Polymerisation (2PP). Computer-Aided Design (CAD) tools allowing for this variation to be incorporated into the surface design are only just emerging. With the presented methodology, surfaces are created from a single feature design. Variation is applied to the surface features through algorithmic design tools, allowing for arrays of hundreds of unique features can be created by non-CAD experts. The translation of these algorithmic expressions from CAD to a physical surface is investigated. Using the proposed methodology, 2PP is used to create quasi stochastic surfaces for the purpose of enhancing the biointegration of medical implants against current state-of-the-art

Loose powder detection and surface characterization in selective laser sintering via optical coherence tomography

Defects produced during selective laser sintering (SLS) are difficult to non-destructively detect after build completion without the use of X-ray-based methods. Overcoming this issue by assessing integrity on a layer-by-layer basis has become an area of significant interest for users of SLS apparatus. Optical coherence tomography (OCT) is used in this study to detect surface texture and sub-surface powder, which is un-melted/insufficiently sintered, is known to be a common cause of poor part integrity and would prevent the use of SLS where applications dictate assurance of defect-free parts. To demonstrate the capability of the instrument and associated data-processing algorithms, samples were built with graduated porosities which were embedded in fully dense regions in order to simulate defective regions. Simulated in situ measurements were then correlated with the process parameters used to generate variable density regions. Using this method, it is possible to detect loose powder and differentiate between densities of ±5% at a sub-surface depth of approximately 300 μm. In order to demonstrate the value of OCT as a surface-profiling technique, surface texture datasets are compared with focus variation microscopy. Comparable results are achieved after a spatial bandwidth- matching procedure

Spatially resolved acoustic spectroscopy for selective laser melting

Additive manufacturing (AM) is a manufacturing technique that typically builds parts layer by layer, for example, in the case of selective laser melted (SLM) material by fusing layers of metal powder. This allows the construction of complex geometry parts, which, in some cases cannot be made by traditional manufacturing routes. Complex parts can be difficult to inspect for material conformity and defects which are limiting widespread adoption especially in high performance arenas. Spatially resolved acoustic spectroscopy (SRAS) is a technique for material characterisation based on robustly measuring the surface acoustic wave velocity. Here the SRAS technique is applied to prepare additively manufactured material to measure the material properties and identify defects. Results are presented tracking the increase in the measured velocity with the build power of the selective laser melting machine. Surface and subsurface defect measurements (to a depth of ∼24 μm) are compared to electron microscopy and X-ray computed tomography. It has been found that pore size remains the same for 140 W to 190 W melting power (mean: 115–119 μm optical and 134–137 μm velocity) but the number of pores increase significantly (70–126 optical, 95–182 velocity) with lower melting power, reducing overall material density

In-situ synthesis of titanium aluminides by direct metal deposition

This study explores the capabilities of methods for in-situ synthesis of titanium aluminides using the Direct Metal Deposition process. This allows for the functional grading of components which will be required for next generation aerospace components. The feasibility of three techniques are explored here; firstly, a new process of powder preparation for Additive Manufacturing, satelliting, in which a larger parent powder is coated with a smaller powder fraction. Here, Al parent particles are satellited with fine TiO2 to produce an intermetallic matrix composite with Al2O3 particulates. The satelliting procedure is shown to increase capability and mixing of in situ synthesis. Secondly, combined wire and single powder feeding is explored through the use of Ti wire and Al powder to create Ti-50Al (at%). Finally, a combination of wire and loose mixed powders is explored to produce the commercially deployed Ti-48Al-2Cr-2Nb (at%) alloy. The simultaneous wire and powder delivery is designed to overcome issues encountered when processing with single powder or wire feedstocks, whilst allowing for on-the-fly changes in elemental composition required for functional grading. Characterisation of the deposits produced, through OM, SEM, and EDX, reveal the influence of key processing parameters and provides a meaningful basis for comparison between the techniques. Results show that it is possible to produce α2+γ twophase microstructures consistent with previous studies which have relied upon more expensive and harder to obtain pre-alloyed feedstocks. This represents a move forward in manufacturability for an emergent process type

Modelling of single spark interactions during electrical discharge coating

Electrical discharge coating (EDC) methods may be used to enhance the surface functionality of electrical discharge machined components. However, industrial uptake of EDC has been restricted due to limited understanding of the fundamental interactions between energy source and workpiece material. The fraction of energy transferred to the workpiece, Fv, as a consequence of sparking, is an important parameter which affects directly crater geometry and the microstructural development of the near surface modified layer. In this paper, a 2D transient heat transfer model is presented using finite difference methods, validated against experimental observations, to estimate effective values for Fv as a function of processing conditions. Through this method we can predict coating layer thicknesses and microstructures through appropriate consideration of heat flow into the system. Estimates for crater depths compared well with experimentally determined values for coating layer thicknesses, which increased with the increasing fraction of energy transfer to the workpiece. Predictions for heat transfer and cooling of melt pools, arising from single spark events, compared well with experimental observations for the developed cermet microstructures. In particular, intermediate processing conditions were associated with the development of complex, banded, fine-grained microstructures, reflecting differences in localised cooling rates and the competing pathways for heat conduction into the substrate and convection within the dielectric fluid. Increased pulse-on times were associated with a propensity towards increasing grain size and columnar growth, reflecting the higher energies imparted into the coatings and slower cooling rates