Refinement of the grain structure of additive manufactured titanium alloys via epitaxial recrystallization enabled by rapid heat treatment

The coarse prior-β grain structure in titanium alloys produced by additive manufacturing is associated to mechanical anisotropy and limited fatigue life. Here we report a novel methodology to refine such structure by rapid heat treatment of Ti-6Al-4V produced by laser powder-bed fusion. The refinement was analysed using high-temperature EBSD that showed, for the first time, how high angle boundary β grains nucleated and grew with quasi-equiaxed morphology by epitaxial recrystallization. These findings show the potential for such heating regime to be applied to control grain size, morphology and distribution in a wider category of structural alloys produced by additive manufacturing

Understanding selectivity in radio frequency and microwave sorting of porphyry copper ores

Continuous high-throughput microwave treatment followed by infrared thermal imaging (MW-IR) has previously been shown to provide attractive separations for a number of porphyry copper ores, leading to rejection of a large proportion of barren fragments from ore-grade material or concentration of copper values from waste-grade material. However, the efficacy of the sorting process is reduced by the presence of hydrated clays and pyrite. Literature measurements have shown differences in the conductivity of pyrite and copper sulphides such as chalcopyrite at radio frequencies. In this work the potential of using radio frequency (RF) heating to exploit these differences and achieve improved selectivity between copper and iron sulphides, is investigated. For the first time a novel bulk materials handling and presentation method that facilitates even heating of angular ore fragments in parallel plate RF systems is discussed. The fragment-by-fragment thermal response of five ore samples under equivalent pilot MW-IR and RF-IR processing conditions is evaluated, showing that there is an increase in selectivity in the heating of hydrated clay minerals in RF compared to microwave. It is suggested, again for the first time, that selectivity in the microwave processing of ores containing semi-conducting minerals is due predominantly to magnetic absorption (induction heating) caused by eddy currents associated with the magnetic field component of electromagnetic energy. In radio frequency processing, where electric field is the dominant component, heating of semi-conducting minerals is limited by the electric field screening effect. This effect is demonstrated using synthetic fragments. Thermal response profiles of synthetic fragments show that approximately 2.5 times the mass of sulphide minerals to hydrated clay minerals would result in an equal temperature increase for microwave heated fragments in which the microwave-heating minerals are evenly disseminated throughout the matrix. This understanding provides the foundations for development of models incorporating different thermal responses for individual heated phases, alongside other textural and treatment variables, that can be used to predict how close to intrinsic sortability ores will perform in MW-IR and RF-IR without the need for extensive processing trials

A tool for predicting heating uniformity in industrial radio frequency processing

Radio frequency energy is utilised for heating in a wide range of applications, particularly in the food industry. A major challenge of RF processing is non-uniform heating in loads of variable and angular geometry, leading to reduced quality and product damage. In the study, the specific effects of geometry on the heating profiles of a range of geometrically variable loads in an industrial scale RF system are analysed, and the understanding used to derive a general tool to predict heating uniformity. Potato was selected as a test material for experimental work; dielectric properties were measured using a 44mm coaxial probe. Analysis of simulated and experimental surface temperature profiles and simulated power uniformity indices indicates that the presence of vertices and edges on angular particles, and their proximity to faces perpendicular to the RF electrodes increases localised heating; faces parallel to the electrodes heated less than those faces perpendicular to them. Comparison of the same geometrical shape in different orientations indicates that overall power absorption uniformity can be better even when localised heating of edges is greater. It is suggested, for the first time, that the rotation of angular shapes within a parallel plate electric field can improve heating uniformity, and that this can be achieved through the design of bespoke electrode systems. A Euler characteristic based shape factor is proposed, again for the first time, that can predict heating uniformity for solid, dielectrically homogenous shapes. This provides industry with a tool to quickly determine the feasibility for uniform RF heating of different three dimensional shapes based on geometry alone. This provides a screening method for food technologists developing new products, allowing rapid assessment of potential heating uniformity and reducing the need for early stage specialist computational modelling

Electromagnetic heating for industrial kilning of malt: a feasibility study

Industrial malting operations use ~800kWh/t of energy to produce the heat required to kiln malt. Electromagnetic heating technologies are suggested as a way to potentially improve the energy efficiency of the kilning processing. In this work, the potential for using electromagnetic heating to dry malt to commercially acceptable moisture levels, whilst preserving the activity of enzymes critical for downstream brewing processes is investigated. The 2450 MHz bulk dielectric properties of malt at moisture contents consistent with those occurring at different points in the kilning process are evaluated; 12% is shown to be a critical moisture level below which drying becomes more energy intensive. Calculated penetration depths of electromagnetic energy in malt at radio frequency are 100 fold higher than at microwave frequencies, showing a significant advantage for commercial scale batch processing. The moisture contents and alpha and beta amylase activity of malt subjected to RF heating at different temperatures, treatment times and RF energy inputs in the intermediate and bound water drying regions were determined. It is shown for the first time that whilst significantly reduced process times are attainable, significant energy efficiency improvements compared to conventional kilning can only be achieved at higher product temperatures and thus at the expense of enzyme survival. It is suggested that RF heating may be feasible where higher bulk temperatures are not critical for downstream use of the material or when used in hybrid systems

Microstructure and tensile properties of additive manufactured Ti-6Al-4V with refined prior-β grain structure obtained by rapid heat treatment

Heat treatments tailored for additive manufactured Ti-6Al-4V can be used to attain desirable microstructures associated with superior mechanical properties. Rapid heat treatments, for example, can be used to recrystallise the microstructure of laser powder bed fusion (L-PBF) Ti-6Al-4V to obtain refined and quasi-equiaxed prior-β grains. In this study, we discuss the interplay between refined microstructures and the tensile properties of Ti-6Al-4V specimens. The influence of individual microstructural constituents on the tensile properties are examined in detail and directly compared to those found in specimens subject to conventional annealing treatments. It was found that rapid heat treatments into the β phase field can significantly refine the size of the prior-β grains found in as-built L-PBF Ti-6Al-4V, as well as alter the size and morphological arrangement of the resulting α laths. The proposed new heat treatments are clearly shown to have strengthened the alloy with no apparent detriment to the ductility of the material

Combining continuous flow oscillatory baffled reactors and microwave heating: Process intensification and accelerated synthesis of metal-organic frameworks

We have constructed a continuous flow oscillatory baffled reactor (CF-OBR) equipped with a homogeneous and controllable microwave applicator in an entirely novel design. This affords a new route to chemical production incorporating many of the principles of process intensification and allows, for the first time, investigation of the synergistic benefits of microwave heating and CF-OBRs such as; faster and continuous processing; improved product properties and purity; improved control over the processing parameters; and reduced energy consumption. The process is demonstrated by the production of a metal-organic framework (MOF), HKUST-1, a highly porous crystalline material with potential applications in gas storage and separation, catalysis, and sensing. Our reactor enabled the production of HKUST-1 at the 97.42 g/h scale, with a space time yield (STY) of 6.32 × 105 kg/m3/day and surface area production rate (SAPR) of 1.12 × 1012 m2/m3/day. This represents the highest reported STY and fastest reported synthesis (2.2 seconds) for any MOF produced via any method to-date and is an improvement on the current SAPR for HKUST-1 by two orders of magnitude owing to the superior porosity exhibited by HKUST-1 produced using our rig (Langmuir surface area of 1772 compared to 600 m2/g)

Towards large scale microwave treatment of ores: Part 1 – Basis of design, construction and commissioning

Despite over thirty years of work, microwave pre-treatment processes for beneficiation of ores have not progressed much further than laboratory testing. In this paper we present a scaleable pilot-scale system for the microwave treatment of ores capable of operating at throughputs of up to 150tph. This has been achieved by confining the electric field produced from two 100kW generators operating at 896MHz in a gravity fed vertical flow system using circular choking structures yielding power densities of at least 6x108 W/m3 in the heated mineral phases. Measured S11 scattering parameters for a quartzite ore (-3.69±0.4dB) in the as-built applicator correlated well with the simulation (-3.25dB), thereby validating our design approach. We then show that by fully integrating the applicator with a materials handling system based on the concept of mass flow, we achieve a reliable, continuous process. The system was used to treat a range of porphyry copper ores

Microwave digestion of gibbsite and bauxite in sodium hydroxide

It was hypothesized that bauxite digestion may be improved by using microwave heating as it has been shown in literature that some material processes have improved efficiency. To test this hypothesis, a set of digestion experiments were conducted using gibbsite, one of the major minerals in bauxite. Gibbsite was digested at various temperatures (50, 75, and 95 °C) in either 1 M or 6 M sodium hydroxide solutions for 30 min using either a convection oven or a 2.45 GHz microwave applicator. Results show that microwave heating provided an increase of 5–7% in the digestion after 30 min and required around 1/10th the time to heat the solutions compared to conventional heating. Electromagnetic simulations show that preferential heating occurs at the solution surface creating a temperature gradient within the solution. Although vigorous stirring of the solution was used to minimize the temperature gradient, it could still be responsible for the observed difference in digestion. Digestion of bauxite itself yielded similar results to the gibbsite

Utilization of dielectric properties assessment to evaluate the catalytic activity and rate of deactivation of heterogeneous catalysts

The use of dielectric property assessment to gauge the catalytic activity and rate of deactivation of heterogeneous catalysts is reported. Four supported catalysts containing a combination of Fe and Ni active sites and γ-Al₂O₃, ZSM-5, MCM-41, and SBA-15 supports were synthesized, characterized, and utilized to catalyze a Fischer–Tropsch process over a temperature range of 250–400 °C that was specifically directed toward the production of lower olefins. While the highest conversion was obtained from ZSM-5 and MCM-41 supports containing Fe and Ni as active sites at 350 °C, all these catalysts were observed to be deactivated by the formation of carbon on their surface. The dielectric properties of the fresh, used catalysts and supports were evaluated and correlated with their catalytic activity and structural/textural properties. It was clearly shown that the dielectric property measurement could demonstrate both the presence and magnitude of carbon deposits on the catalyst via the differences in the values of fresh and used catalysts. Furthermore, the ability to differentiate between the levels of the carbon deposition observed was shown to be independent of the morphology exhibited by the carbon deposit demonstrating that this is a method that can be generally applied

Improving the design of industrial microwave processing systems through prediction of the dielectric properties of complex multi-layered materials

Rigorous design of industrial microwave processing systems requires in-depth knowledge of the dielectric properties of the materials to be processed. These values are not easy to measure, particularly when a material is multi-layered containing multiple phases, when one phase has a much higher loss than the other and the application is based on selective heating. This paper demonstrates the ability of the Clausius-Mossotti (CM) model to predict the dielectric constant of multi-layered materials. Furthermore, mixing rules and graphical extrapolation techniques were used to further evidence our conclusions and to estimate the loss factor. The material used for this study was vermiculite, a layered alumina-silicate mineral containing up to 10 % of an interlayer hydrated phase. It was measured at different bulk densities at two distinct microwave frequencies, namely 934 and 2143 MHz. The CM model, based on the ionic polarisability of the bulk material, gives only a prediction of the dielectric constant for experimental data with a deviation of less than 5 % at microwave frequencies. The complex refractive index model (CRIM), Landau, Lifshitz and Loyenga (LLL), Goldschmidt, Böttcher and Bruggeman-Hanai model equations are then shown to give a strong estimation of both dielectric constant and loss factor of the solid material compared to that of the measured powder with a deviation of less than 1 %. Results obtained from this work provide a basis for the design of further electromagnetic processing systems for multi-layered materials consisting of both high loss and low loss components