Formation Of Solid Bridges Research Articles

The effect of surface chemistry on the caking behaviour of sucrose crystals

The caking behaviour of organic crystals presents a significant challenge in both the food and pharmaceutical industries. This study aimed to investigate how surface impurities influence the caking behaviour of sucrose (sugar) crystals. Sucrose crystals were selected as the model material due to their relevance in various industries such as food processing and pharmaceuticals. The impact of surface chemistry was assessed for sucrose crystals with varying levels of impurities, namely white sugar, light brown sugar, and dark brown sugar. These crystals were subjected to controlled cycles of humidity exposure, compaction, and drying to induce caking, simulating real-world scenarios. The caking propensity was evaluated using a compression test, while the surface chemistry was characterized through Inverse Gas Chromatography (IGC). Moisture sorption properties were evaluated using a Dynamic Vapor Sorption (DVS) technique, and the formation of solid bridges was studied using scanning electron microscopy (SEM) and X-ray computed tomography (XCT). The results indicate that impurities play a crucial role in enhancing the caking behaviour of sucrose crystals by influencing their hygroscopicity and facilitating solid bridge formation. This study provides valuable insights into the relationship between surface chemistry, moisture sorption properties, and the caking behaviour of sucrose crystals, offering strategies to mitigate caking issues.

Co-pyrolysis of plastic waste and eucalyptus waste wood for fuel pellets production: A study of fuel, mechanical, and combustion characteristics under varying binder proportion

The study explores the effect of varying molasses proportions as a binder on the characteristics of densified char obtained through the slow co-pyrolysis of plastic waste and Eucalyptus wood waste (Waste low-density polyethylene – Eucalyptus wood (WLDPE–EW) and Waste Polystyrene – Eucalyptus wood (WPS–EW)). Pyrolysis was conducted at 500 °C with a residence time of 120 min, employing plastic to wood waste ratios of 1:2 and 1:3 (w/w). The focus was on how varying the proportion of molasses (10–30 %), influences the physical and combustion properties of the resulting biofuel pellets. Our findings reveal that the calorific value of the pellets decreased from 28.94 to 27.44 MJ/Kg as the molasses content increased. However, this decrease in calorific value was compensated by an increase in pellet mass density, which led to a higher energy density overall. This phenomenon was attributed to the formation of solid bridges between particles, facilitated by molasses, effectively decreasing particle spacing. The structural integrity of the pellets, as measured by the impact resistance index, improved significantly (43–47 %) with the addition of molasses. However, a significant change in the combustion characteristics depicted by lower ignition and burnout temperatures were observed due to decrease in fixed carbon value and increase in volatile matter content, as the proportion of molasses increased. Despite these changes, the pellets demonstrated a stable combustion profile, suggesting that molasses are an effective binder for producing biofuel pellets through the densification of char derived from the co-pyrolysis of plastic and Eucalyptus wood waste. The optimized molasses concentration analyzed through multifactor regression analysis was 16.96 % with 28 % WLDPE proportion to produce WLDPE–EW char pellets. This study highlights the potential of using molasses as a sustainable binder to enhance the mechanical and combustion properties of biofuel pellets, offering a viable pathway for the valorization of waste materials.

Effect of process parameters on quality of alfalfa block

To address the imbalance in the supply of grass resources caused by seasonality and regional factors, it is crucial to efficiently store and transport alfalfa. Exploring suitable grass feed processing techniques contributes to the stable transportation of grass blocks and long-term storage of nutritional components. The Central Composite Design response surface design was used to design experiments, with moisture content and compressive force as the test factors. Based on the experimental results, it was found that lower moisture content and a certain compressive force were beneficial for the stability, high density, and protein storage of alfalfa blocks. The microscopic examination of alfalfa particles revealed that a certain moisture content (15%) facilitates the formation of solid bridges between particles, leading to more stable alfalfa blocks. The final optimized process parameters were moisture content of 14.3% and compressive force of 34.8 kN. Under these conditions, the density of the molded alfalfa block was 1001 kg/m3, with R-CP at 96.96%, R-EE at 67.23%, and R-CF at 114.13%.

Soil colloids as binding agents in the formation of soil microaggregates in wet-dry cycles: A case study for arable Luvisols under different management

In the hierarchical model of soil aggregates, small soil microaggregates (small SMA; <20 μm) are often considered to be fundamental building units at the micron scale. Below which, soil colloids (<1 µm) have recently been proposed as binding agents of (micro)aggregates. However, the way in which soil colloids contribute to the formation and stability of soil micro- and macroaggregates remains largely unknown. For clarification, we evaluated potential impacts of the colloidal content, particularly the <450 nm colloids, on the aggregation of small SMA. Free water stable small SMA and <450 nm colloids were isolated from Ap-horizons of Stagnic Luvisols under different management (cropped and bare fallow). The size-resolved elemental composition of the <450 nm colloids was analyzed by asymmetric flow field-flow fractionation in combination with an inductively coupled plasma mass spectrometer and an organic carbon detector. To vary the colloidal content in small SMA, (1) suspensions containing different amounts of <450 nm colloids were added in small SMA, or (2) <1 µm colloids were removed from small SMA by centrifugation. In the maximum colloidal addition treatment, the mass ratios of added colloids to small SMA were 3.0 and 5.1 wt% for the cropped and bare fallow soil samples, respectively. Aggregation of small SMA with different colloidal amounts was performed in three successive wet-dry cycles. Afterwards, the size distribution of the resulting aggregates was measured by laser diffraction. Our results indicated that, in wet-dry cycles, colloids were important binding agents for the formation of SMA. Their presence, especially those <450 nm, was likely to support the formation of solid bridges during drying at particle contacts of 1–10 µm small SMA, favoring hereby SMA build-up in a relatively small size range of 1–40 µm. In contrast, the absence of <1 μm colloids in small SMA led to a preferential generation of relatively large aggregates in wet-dry cycles, i.e., typically with sizes >40 μm up to 1700 μm in maximum. Our study on aggregation in wet-dry cycles revealed that the colloidal content has a controlling effect on the size distribution of resulting aggregates by acting as a binding agent and provides hereby new insights into the evolvement of aggregate hierarchy in soils.

Optimization and Advantages of Molded Tablets Using Trehalose as a Binder.

Molded tablets are manufactured by molding wet powder at low pressure and drying. Typically, water-soluble polymers are used as a binder; however, the ratio to achieve both tablet strength and rapid disintegration is limited, and designing an optimal formulation according to the active ingredients can be challenging. In addition, production may be temporarily interrupted owing to the adherence of wet powder to the inside of the mortar, which can hamper stable production. Therefore, optimization was performed by design of experiments to utilize the disaccharide trehalose as a binder for molded tablets. We formulated placebo tablets with high tablet strength and rapid disintegration. On examining the tablet interior, we confirmed the formation of solid bridges between particles and high porosity, suggesting that trehalose can be used as a binder for molded tablets. The viscosity of the trehalose saturated solution was lower than that of the polyvinyl alcohol (PVA) solution (3.8 wt%). Moreover, the trehalose formulation exhibited a significantly lower wet powder adhesion rate to the upper punch than the PVA formulation. This study provided valuable results for the future formulation design of molded tablets.

The formation of solid bridges during agglomeration in a fluidized bed: Investigation by Raman spectroscopy and image analyses

Solid bridge formation in the agglomeration process is still not well-understood. This work has aimed to analyze particle class evolution, Raman spectra, and microscopy to comprehend granule formation in a fluidized bed throughout the process. Microcrystalline cellulose was agglomerated by maltodextrin solutions. The experimental statuses were classified in wet and dry conditions according to the combination of operating parameters. The effect of solution concentration (5.0, 20.0, and 35.0% w/w), solution flow rate (1.0 and 4.0 mL/min), and air temperature (30 and 90 °C) on particle size was checked. Afterward, the solution flow rate was increased from 1.0 to 4.0 mL/min while maintaining constant solution concentration and air temperature. Wet conditions led to more effective particle formation since the amount of liquid binder on the particle surface favored the formation of a sticky layer. The examination of data and images made it possible to identify the solid bridge formation and stability.

The Study of Spray-Freeze-Drying Technique for Development of Novel Combination pMDIs, Part II: In Vitro and In Vivo Evaluations

AbstractThe mometasone furoate (MF) and formoterol fumarate dihydrate (FF) inhalable microparticles prepared by different methods, such as micronized active pharmaceutical ingredients (APIs), microparticles of APIs prepared by spray-freeze drying technique (SFD APIs), and phospholipid microparticles of APIs prepared by SFD (SFD Lip-APIs), showed different inhaled drug delivery characteristics. Study on the physicochemical characteristics of those microparticles and the effect of matrix excipients on pharmacokinetic (PK) behaviors of inhalable microparticles is helpful for the development of new methods for inhalable microparticles with excellent performance of inhalation characteristics. In this study, the crystal state of the microparticles was investigated by powder X-ray diffraction and differential scanning calorimetry. The density was investigated by a bulk density method. The suspension and dispersion characteristics were determined by observing its state in hydrofluoroalkane (HFA). Meanwhile, the PK behaviors of SFD Lip-APIs in beagle dogs were also investigated by airway administration to evaluate the effect of phospholipids on drug release. The results indicated that the presence of phospholipids prevents the formation of solid bridges bonding to each other during SFD of pure drug solutions. In comparison to the conventional micronized microparticles, inhalable drug–phospholipid microparticles were easily dispersed and suspended in HFA. The embedded drugs were in a crystal state that endowed a better physical stability, and most interestingly, have similar PK behavior to the control (a mixed solution of MF/FF), suggesting that the phospholipids, as matrix excipients, had no effect on absorption. Given above, our designed SFD phospholipid microparticles may represent an efficient carrier for pulmonary delivery of MF and FF for further clinical treatment.

Formation of liquid and solid bridges under the interactions between droplet-polyethylene particle pairs

The formation mechanism of liquid and solid bridges between particles and their effects in a gas–solid fluidized bed with liquid injection on particle aggregation is essential for the operational controls and improve operations. We used high-speed imaging, infrared thermography, and scanning electron microscope (SEM) to study the liquid film behaviors during the collision between droplets and double polyethylene particles and the formation of solid bridges. Three modes of solid bridge formation were found: the liquid film stimulated slow growth, the liquid bridge stimulated intermittent growth, and the liquid bridge stimulated rapid growth. The different growth positions and growth rate of the solid bridge of the three forms show that the influence of particle wetting degree is stronger than the liquid holding capacity on the solid bridge growth characteristic.

Inhalable Hydroxychloroquine Powders for Potential Treatment of COVID-19.

Background: Hydroxychloroquine (HCQ) is one of the repurposed drugs proposed for the treatment of coronavirus disease 2019 (COVID-19). However, all the published clinical trials involve oral administration of the drug, although the disease is primarily a respiratory one. Direct inhaled delivery could reduce the side effects associated with oral use and ensure a high concentration of the drug in the lungs. In this study, inhalable HCQ powders were prepared and characterized for potential COVID-19 therapy. Methods: Hydroxychloroquine sulfate (HCQ-sul) was jet milled (JM) followed by conditioning by storage at different relative humidities (43%, 53%, 58%, and 75% RHs) for 7 days. The solid-state properties, including particle morphology and size distribution, crystallinity, and vapor moisture profiles of HCQ-sul samples, were characterized by scanning electron microscopy, laser diffraction, X-ray powder diffraction, differential scanning calorimetry, thermogravimetric analysis, and dynamic water vapor sorption. The aerosol performance of the HCQ-sul powders was assessed using a medium-high resistance Osmohaler coupling to a next-generation impactor (NGI) at a flow rate of 60 L/min. Results: The jet-milled powder showed a volume median diameter of 1.7 μm (span 1.5) and retained the same crystalline form as the raw HCQ-sul. A small amount of amorphous materials was present in the jet-milled HCQ-sul, which was convertible to the stable, crystalline state after conditioning at 53%, 58%, and 75% RH. The recovered fine particle fraction (FPF)recovered and the emitted fine particle fraction (FPFemitted) of the HCQ-sul sample immediately after jet milling and the samples after conditioning at 43%, 53%, and 58% RH were similar at ∼43% and 61%, respectively. In contrast, the sample having conditioned at 75%RH showed lower corresponding values at 33% and 26% respectively, due to the formation of solid bridges caused by excessive moisture. Conclusion: Inhalable crystalline powders of HCQ-sul were successfully prepared, which can be used for clinical testing as a potential inhaled COVID-19 treatment.

Enhanced fuel characteristics and physical chemistry of microwave hydrochar for sustainable fuel pellet production via co-densification

Microwave assisted hydrothermal treatment (MHTC) was compared with torrefaction in terms of carbonization efficiency and physicochemical characteristics of char products. The utilization of produced char was optimized for composite solid biofuel production. The results show that MHTC significantly improved the binding capability of the microwave hydrochar (MHC) particles during co-densification with unprocessed biomass and coal. One possible contributor to the improved binding is the pseudo lignin formed during the MHTC, which led to a better interlocking of the feedstock particles and promoted the solid bridge formation. Composite pellet prepared with 80 wt% of torrefaction char (TC-120), 10 wt% of microwave hydrochar (MHC-30), and 10 wt% of Coal-04 showed a higher heating value of 24.54 MJ/kg and energy density of 26.43 GJ/m3, which is significantly higher than that of the raw cotton stalk pellet (16.77 MJ/kg and 18.76 GJ/m3, respectively), showing great promise as a solid biofuel. The moisture resistance and oxidation reactivity are also significantly improved. The results demonstrate that MHCs provides dual functionalities in acting as binder and fuel promoter in the production of composite biofuel. This study can provide new insight into the unique functions of MHC during fuel application, which demonstrates the great potential of applying MHTC in energy recovery from lignocellulosic biomass.

Phenytoin-loaded lipid-core nanocapsules improve the technological properties and in vivo performance of fluidised bed granules.

Lipid-core nanocapsules (LNCs) were recently reported by our group as a suitable binder system to produce fluidised bed granules. However, there is still a lack of knowledge about the influence of using these nanocarriers loaded with a drug on the properties of the granules and their in vivo performance. Therefore, this study was designed to produce innovative fluidised bed granules containing phenytoin-loaded LNCs (LNCPHT) as a strategy to evaluate the influence of the presence of the drug-loaded nanocarriers on their in vitro and in vivo properties. Granules were produced using a mixture of maltodextrin and phenytoin (1:0.004 w/w) as substrate. They were prepared by fluid bed granulation using water or LNCPHT as the liquid binder, affording good yields (73-82%) of granules with low moisture content (<5%). Granules prepared with LNCPHT had larger mean size (122μm) compared to maltodextrin primary particles (50μm) due to the formation of solid bridges. Moreover, the use of LNCPHT as the liquid binder improved their powder flow properties. The nanocarriers were recovered after aqueous dispersion (3.00mg.mL-1 of PHT) with a redispersibility close to 90%. After reconstitution in water, granules containing LNCPHT showed an improved dissolution behaviour compared to those prepared without them. In addition, they showed a higher mucoadhesive effect due to a combined effect of the LNCPHT and maltodextrin in the interactions with porcine intestinal mucosa. Regarding the in vivo studies, granules containing the combination of non-encapsulated PHT and PHT-loaded lipid-core nanocapsules increased the latency to seizures compared to placebo granules, showing effective anticonvulsant effect in mice. In conclusion, the use of drug-loaded nanocapsules as binder is an encouraging approach to produce fluidised bed mucoadhesive granules with improved technological properties and in vivo performance.

Physical and Combustion Properties of Binder-Assisted Hydrochar Pellets from Hydrothermal Carbonization of Tobacco Stem

Hydrothermal carbonization (HTC) combined with pelletization is an efficient upgrading process to convert biomass into energy-dense pellets. Binders usually serve an important role in the quality of biofuel pellets. Here, hydrochar was produced from tobacco stem through HTC, and was pelletized in the presence of various binders. The effect of four kinds of binders (K2CO3, CaCO3, xanthan gum and carboxymethyl cellulose) on the basic fuel properties, mechanical strength, moisture adsorption and combustion characteristics was investigated. The results showed that K2CO3 assisted pellet had decreased compressive strength, increased hygroscopicity and high combustion residues. The compressive strength of pellets increased with increasing amount CaCO3 binder probably due to the hardening effect of enhanced adhesion attraction forces between adjacent particles. Meanwhile, the CaCO3 assisted pellet exhibited the lowest moisture uptake behavior. In addition, the introduction of organic binders led to improved mechanical strength, especially for xanthan binder, which could be ascribed to the formation of solid bridges and enhancement of adhesion forces between particles. TGA results suggested that the binder-assisted pellets exhibited desirable combustion characteristics such as decreased ignition temperature and increased comprehensive combustibility index. Kinetic parameters of the combustion process of binder-assisted pellets were also determined by employing the model free isoconversional Kissinger–Akahira–Sunose method. This study demonstrated that HTC of tobacco stem combined with pelletization in the presence of appropriate binders provide an alternative for solid biofuel pellets production. Hydrothermal carbonization (HTC) of tobacco stem combined with pelletization in the presence of appropriate binders provide an alternative for solid biofuel pellets production.

Investigation of Electrostatic Behavior of Dry Powder-Inhaled Model Formulations

The accumulation of electrostatic charge on drug particles and excipient powders arising from interparticulate collisions or contacts with other surfaces can lead to agglomeration and adhesion problems during the manufacturing process, filling, and delivery of dry powder inhaler (DPI) formulations. The objective of the study was to investigate the role of triboelectrification to better understand the influence of electrostatic charge on the performance of DPIs with 2 capsule-based dimensionally similar devices constructed with different materials. In addition, strategies to reduce electrostatic charge build up during the manufacturing process, and the processes involved in this phenomenon were investigated. Electrostatic charge measurements showed that there was a significant difference in electrostatic charge generated between tested formulations and devices. This affects particle detachment from carrier and thus significantly impacts aerosol performance. Conditioning fluticasone DPI capsules at defined temperature and humidity conditions reduced electrostatic charges acquired during manufacturing. Conditioning salmeterol DPI capsules at same conditions seemed disadvantageous for their aerosol performance because of increasing capillary forces and solid bridge formation caused by water absorption. Knowledge and understanding of the role of electrostatic forces in influencing DPI formulation performance was increased by these studies.

Co-hydrothermal carbonization of food waste-woody biomass blend towards biofuel pellets production

Co-hydrothermal carbonization of food waste-woody biomass blend was conducted to enhance the pelletization and hydrochar-fuel properties. The hydrochar was characterized by proximate, elemental analysis and HHVs, whilst energy consumption of pelletization, tensile strength, and combustion characteristics of hydrochar pellets were evaluated. Results showed that food waste (FW) blended with 0–50% mainly decreased H/C of hydrochar, while blend ratio from 75% to 100% mainly decreased O/C. When FW blended from 0% to 75%, the energy consumption for hydrochar palletization decreased about 12–17 J, whereas tensile strength of pellets increased about 2.4–5.5 MPa by formation of solid bridge when woody biomass (WS) ratio was increased. The hydrochar pellets from high ratio FW had decreased ignition temperature and maximum weight loss rate with wider temperature range, indicating the increased flammability and moderate combustion. These findings demonstrate that HTC of food waste-woody biomass blend was suitable for pelletization towards solid biofuel production.

Influence of an Added Fraction of Hygroscopic Salt Particles on the Operating Behavior of Surface Filters for Dust Separation

AbstractAn innovative approach to manipulating the operating behavior of surface filters for dust separation by raw gas conditioning using hygroscopic salt particles is described. The basis for a targeted manipulation of the operating behavior of surface filters is the fact that a favorable performance adjusts itself at long cycle times. These are obtained by a slow increase in pressure drop during the filtration phase and by a low residual pressure drop after the regeneration. Technically, such states can be realized by so‐called raw gas conditioning. The deliquescence and efflorescence properties of hygroscopic salt particles can be considered as possibilities for the formation of solid bridges between dust particles in the dust cake. With this concept, a flexible and targeted strengthening of the cohesion in the dust cake without an accompanying modification of the adhesion between the cake layer and the filter medium is theoretically possible.

Particle agglomeration of chitosan–magnesium aluminum silicate nanocomposites for direct compression tablets

Exfoliated nanocomposites of chitosan-magnesium aluminum silicate (CS-MAS) particles are characterized by good compressibility but poor flowability. Thus, the aims of this study were to investigate agglomerates of CS-MAS nanocomposites prepared using the agglomerating agents water, ethanol, or polyvinylpyrrolidone (PVP) for flowability enhancement and to evaluate the agglomerates obtained as direct compression fillers for tablets. The results showed that the addition of agglomerating agents did not affect crystallinity, but slightly influenced thermal behavior of the CS-MAS nanocomposites. The agglomerates prepared using water were larger than those prepared using 95% ethanol because high swelling of the layer of chitosonium acetate occurred, allowing formation of solid bridges and capillary force between particles, leading to higher flowability and particle strength. Incorporation of PVP resulted in larger agglomerates with good flowability and high strength due to the binder hardening mechanism. The tablets prepared from agglomerates using water showed lower hardness, shorter disintegration times and faster drug release than those using 95% ethanol. In contrast, greater hardness and more prolonged drug release were obtained from the tablets prepared from agglomerates using PVP. Additionally, the agglomerates of CS-MAS nanocomposites showed good carrying capacity and provided desirable characteristics of direct compression tablets.

Mechanism by Which Magnesium Oxide Suppresses Tablet Hardness Reduction during Storage.

This study investigated how the inclusion of magnesium oxide (MgO) maintained tablet hardness during storage in an unpackaged state. Tablets were prepared with a range of MgO levels and stored at 40°C with 75% relative humidity for up to 14 d. The hardness of tablets prepared without MgO decreased over time. The amount of added MgO was positively associated with tablet hardness and mass from an early stage during storage. Investigation of the water sorption properties of the tablet components showed that carmellose water sorption correlated positively with the relative humidity, while MgO absorbed and retained moisture, even when the relative humidity was reduced. In tablets prepared using only MgO, a petal- or plate-like material was observed during storage. Fourier transform infrared spectrophotometry showed that this material was hydromagnesite, produced when MgO reacts with water and CO2. The estimated level of hydromagnesite at each time-point showed a significant negative correlation with tablet porosity. These results suggested that MgO suppressed storage-associated softening by absorbing moisture from the environment. The conversion of MgO to hydromagnesite results in solid bridge formation between the powder particles comprising the tablets, suppressing the storage-related increase in volume and increasing tablet hardness.

Anomalous size evolution of partially amorphized pharmaceutical particles during post-milling storage

Milling is simple and inexpensive method for particle size reduction. The micronization process such as fluid-energy impact mills (e.g. spiral jet mills and fluidized bed jet mills) is commonly used in the pharmaceutical industries to attain optimal/desire particle size distribution for pharmaceutical powders. However, this process may also cause several undesirable effects on active pharmaceutical ingredients (APIs) such as aggregation of fine particles, induction of electrostatic charges, mechanochemical transformation, defects on the surface of drug particles (amorphization) and changes in other physicochemical properties which may significantly affect the physical stability of API powders.In this study, the effect of milling-induced amorphization on the properties of APIs jet-milled at different energy-inputs (high (I), medium (II) and optimal (III)) provided from industry was investigated. Dynamic vapor sorption (DVS) technique has been established to quantify the amorphous content of industry milled APIs (I, II and III). A time-course study has been conducted to evaluate the physical stability of X-ray amorphous (generated by in-house cryo-milling as control) and three industrial samples I, II and III under accelerated stability test conditions (A=40°C/75%RH and B=25°C/55%RH) and any accompanying changes in key physicochemical properties such as crystallinity, agglomerate strength, particle size distribution (PSD) and morphology. In descending order, the amorphous content (%) of industrial API samples (milled different energy-inputs (high (I), medium (II) and optimal (III))) were ranked from I (50.9%), II (31.6%) to III (14.6%).Re-crystallization of X-ray amorphous and industrial samples was observed under both conditions A and B during storage of 13weeks. It was observed that there were changes in the particle size of APIs during post-micronization storage. There were reductions in particle size of all industrial samples under conditions A and B after 13weeks of storage. A possible explanation on the observation of anomalous particle size shift during post-milling storage was attributed to two different phenomena occurring at the same time and competing with each other. These two phenomena are called “surface re-crystallization” and “stress relaxation”. Re-crystallization of amorphous surface (observed from SEM) may lead to enlargement of particle size due to formation of solid bridges during storage. Microcompressor tester (MCT) analysis also suggested that the agglomerate strength increases during storage due to the surface re-crystallization. In case of particle size reduction, PSD analysis suggested that the milled agglomerates exhibited breakage pattern termed as localized disintegration whereby clusters of primary particles broke off from the outer surface of milled agglomerates. This was likely attributed to the energy-intensive micronization process that induces residual stresses, which propagate over time and lead to the breakage of particle agglomerates. Overall, the stress relaxation is more dominant than the re-crystallization and results in overall particle size reduction after storage.

Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting

During solidification of metallic alloys, coalescence leads to the formation of solid bridges between grains or grain clusters when both solid and liquid phases are percolated. As such, it represents a key transition with respect to the mechanical behavior of solidifying alloys and to the prediction of solidification cracking. Coalescence starts at the coherency point when the grains begin to touch each other, but are unable to sustain any tensile loads. It ends up at mechanical coherency when the solid phase is sufficiently coalesced to transmit macroscopic tensile strains and stresses. Temperature at mechanical coherency is a major input parameter in numerical modeling of solidification processes as it defines the point at which thermally induced deformations start to generate internal stresses in a casting. This temperature has been determined for Al-Zn alloys using in situ X-ray diffraction during casting in a dog-bone-shaped mold. This setup allows the sample to build up internal stress naturally as its contraction is prevented. The cooling on both extremities of the mold induces a hot spot at the middle of the sample which is irradiated by X-ray. Diffraction patterns were recorded every 0.5 seconds using a detector covering a 426 × 426 mm2 area. The change of diffraction angles allowed measuring the general decrease of the lattice parameter of the fcc aluminum phase. At high solid volume fraction, a succession of strain/stress build up and release is explained by the formation of hot tears. Mechanical coherency temperatures, 829 K to 866 K (556 °C to 593 °C), and solid volume fractions, ca. 98 pct, are shown to depend on solidification time for grain refined Al-6.2 wt pct Zn alloys.

&lt;i&gt;In Situ&lt;/i&gt; X-Ray Diffraction during Casting: Study of Hot Tearing in Al-Zn Alloys

During solidification of metallic alloys, coalescence corresponds to the formation of solid bridges between grains when both solid and liquid phases are percolated. As such, it represents a key transition with respect to the mechanical behaviour of solidifying alloys and to the prediction of solidification cracking. Coalescence starts at the coherency point when the grains begin to touch each other, but are unable to sustain any tensile loads. It ends up at the rigidity temperature when the solid phase is sufficiently coalesced to transmit macroscopic tensile strains and stresses. This temperature, also called mechanical or tensile coherency temperature, is a major input parameter in numerical modelling of solidification processes as it defines the point at which thermally induced deformations start to generate internal stresses in a casting. The rigidity temperature has been determined in Al Zn alloys using in situ X-ray diffraction (XRD) during casting in a dog bone shaped mould. This set-up allows the sample to build up internal stress naturally as its contraction is prevented. The cooling on both extremities of the mould induces a hot spot at the middle of the sample which is irradiated by X-rays. Diffraction patterns were recorded every 0.5 s using a detector covering a 426 x 426 mm2area. The change of diffraction angles allowed us to observe agglomeration/decohesion of growing grain clusters and to determine a solid volume fraction at rigidity around 98 % depending on solidification time for grain refined Al 6.2 wt% Zn alloys.