Gas Entrapment Research Articles

Enhancement of Mechanical Properties of Pure Aluminium through Contactless Melt Sonicating Treatment.

A new contactless ultrasonic sonotrode method was previously designed to provide cavitation conditions inside liquid metal. The oscillation of entrapped gas bubbles followed by their final collapse causes extreme pressure changes leading to de-agglomeration and the dispersion of oxide films. The forced wetting of particle surfaces and degassing are other mechanisms that are considered to be involved. Previous publications showed a significant decrease in grain size using this technique. In this paper, the authors extend this research to strength measurements and demonstrate an improvement in cast quality. Degassing effects are also interpreted to illustrate the main mechanisms involved in alloy strengthening. The mean values and Weibull analysis are presented where appropriate to complete the data. The test results on cast Al demonstrated a maximum of 48% grain refinement, a 28% increase in elongation compared to 16% for untreated material and up to 17% increase in ultimate tensile strength (UTS). Under conditions promoting degassing, the hydrogen content was reduced by 0.1 cm3/100 g.

The numerical modeling and study of gas entrapment phenomenon in non-isothermal polymer filling process

• The problem of gas entrapment in the non-isothermal polymer filling process is simulated. • The finite element method coupled with implicit discontinuous Galerkin method is developed for the simulation. • The influence of injection velocity and gate size on the gas entrapment phenomenon have been analyzed. In this paper, the intricate gas entrapment phenomenon in the non-isothermal polymer filling process is investigated via a combined finite element and discontinuous Galerkin numerical algorithm. The evolving melt interface is captured via level set method and the eXtended Pom-Pom (XPP) constitutive model is employed to describe the viscoelastic fluid. The governing equations of flow field are solved via a coupled continuous and discontinuous Galerkin method. The implicit discontinuous Galerkin method is utilized for the solution of the XPP constitutive equation, level set and its re-initialization equations. The good agreement of experimental and simulation results illustrates the validity of this combined computational algorithm. The influence of injection velocity and the gate size on the gas entrapment phenomenon are both studied in the irregular cavity with complex insert. We have found that the entrapped gas is easy to appear for the case of higher injection velocity and smaller gate size. The distribution of temperature field in different cases are also shown.

Site-specific microstructure, porosity and mechanical properties of LENS™ processed Ti–6Al–4V alloy

In the present study, pre-alloyed Ti–6Al–4V powder is deposited on CP-Titanium substrate by laser engineered net shaping (LENS™) process using parameters optimized for best adhesion and densification. The optical montages from the three surfaces (front, side and top) show columnar β grains growing along the building direction due to conductive heat transfer through the substrate. The as-deposited microstructure contains thin lamellar (α+β)-colonies besides prior β grain boundaries and basket-weave (α+β)-structure inside prior β grains. Narrow band-like structure forms between consecutive layers due to re-melting of previously deposited layers, thereby creating additional interfaces in the microstructure. In addition, tiny isolated pores appears in negligible fraction throughout the LENSTM-processed specimen due to gas entrapment, shrinkage during cooling and un-melted or partially melted powder particles. Both the number and volume of the pores increase along the building direction. Hardness on different surfaces (front, side and top) differs considerably due to the presence of different heating/cooling zones, residual stresses and variations in the thermal cycles and consequent change in the α'-martensite phase fraction. Larger variation in the hardness between these surfaces is observed in nano-indentation technique signifying for inhomogeneity in nano-scale structure. These microstructural variations also resulted in measurable changes in the coefficient in friction (COF) during scratch testing from the substrate along the building direction due to presence of different heating/cooling zones (diffused vs. reheating/re-melting zones). The variation in hardness and COF along different directions can ultimately lessen the in-service performance of the as-deposited parts.

Microbial-Induced Desaturation in Stratified Soil Conditions

A feasibility study was performed to assess the potential of microbially induced desaturation and precipitation (MIDP) through denitrification to reduce the risk of earthquake-induced liquefaction and improve the resilience of embankments along the lower Fraser River in Richmond, British Columbia, Canada. The denitrification process produces nitrogen gas, which gradually desaturates the soil, and dissolved inorganic carbon, which in presence of dissolved calcium results in precipitation of calcium carbonate minerals, cementing the soil particles. The trapped gas bubbles dampens pore pressure build-up during cyclic loading. Consequently, microbially induced desaturation (MID) the first phase of the MIDP process, has great potential as soil improvement technique, especially for liquefaction mitigation, independently from the precipitation phase of the process. Numerous studies have demonstrated the effect of biogenic gas formation on the mechanical response for treated sand samples at lab scale. However, there is still limited knowledge on the impact of partial desaturation on the hydraulic properties of the treated sediments and the durability of the entrapped gas phase, particularly in naturally stratified and heterogenous formations. This is important because changes in porosity and hydraulic conductivity may affect the injection flow patterns and distribution of substrates and products in the subsurface. The objective of this study was to develop a test procedure to evaluate the applicability and performance of the MID technique in stratified subsurface conditions. The study involved a two-dimensional model (2D) tank set-up, in which substrates were laterally injected and extracted. The results demonstrated the effectiveness of treatment and showed how gas formation, migration and entrapment and resulting degree of desaturation and hydraulic conductivity are affected by stratifications in a natural soil.

Characteristics and Genetic Mechanisms of Normal-Pressure Fractured Shale Reservoirs: A Case Study From the Wufeng–Longmaxi Formation in Southeastern Chongqing, China

Compared with the overpressure conditions of shale reservoirs in the Jiaoshiba area, exploring the controlling factors of changes in shale reservoir physical properties under normal-pressure accumulation is of great significance to shale gas exploration. To achieve this, X-ray powder diffraction, nitrogen adsorption, and scanning electron microscopy were carried out to determine mineral content, permeability, porosity, and pore structure for well core and outcrop samples of the Wufeng–Longmaxi Formation in southeastern Chongqing, China. Field observations and drilling cores revealed abundant structural fractures in southeastern Chongqing, offering potential main storage space for shale gas. Findings also determined that high-quality source rocks are located in the lower part of the Wufeng–Longmaxi Formation, which features low porosity and low permeability, yet existing structural fractures could potentially improve permeability to a certain extent. The main factors controlling the shale reservoir with the above characteristics include the depositional environment, mineral components, total organic carbon content, and tectonic movement. The deep-water anoxic depositional environment was conducive to the complete preservation of organic matter and sedimentation of the biogenic siliceous minerals, which had a strong effect on improving the porosity of organic matter and brittleness of the rocks. The high content of quartz was found to improve rock compression resistance and brittleness, and increasing clay proportion was found to enhance the interlayer pores of clay minerals. Total organic carbon content, specific surface area, pore volume, and fracture development were all found to have direct contributions to shale gas entrapment. Overall, the most important factor resulting in normal-pressure accumulation in southeastern Chongqing was the intense tectonic movements since the Late Jurassic period which destroyed the original overpressure accumulation conditions.

A Cavity-Tailored Metal-Organic Cage Entraps Gases Selectively in Solution and the Amorphous Solid State.

Here we report the subcomponent self‐assembly of a truxene‐faced Zn4L4 tetrahedron, which is capable of binding the smallest hydrocarbons in solution. By deliberately incorporating inward‐facing ethyl groups on the truxene faces, the resulting partially‐filled cage cavity was tailored to encapsulate methane, ethane, and ethene via van der Waals interactions at atmospheric pressure in acetonitrile, and also in the amorphous solid state. Interestingly, gas capture showed divergent selectivities in solution and the amorphous solid state. The selective binding may prove useful in designing new processes for the purification of methane and ethane as feedstocks for chemical synthesis.

Research and Evaluation of the Influence of the Construction of the Gate and the Influence of the Piston Velocity on the Distribution of Gases into the Volume of the Casting

Distribution of gasses to the cast volume and volume of pores can be maintained within the acceptable limits by means of correct setting of technological parameters of casting and by selection of suitable structure and gating system arrangement. The main idea of this paper solves the issue of suitability of die casting adjustment—i.e., change of technological parameters or change of structural solution of the gating system—with regards to inner soundness of casts produced in die casting process. Parameters which were compared included height of a gate and velocity of a piston. The melt velocity in the gate was used as a correlating factor between the gate height and piston velocity. The evaluated parameter was gas entrapment in the cast at the end of the filling phase of die casting cycle and at the same time percentage of porosity in the samples taken from the main runner. On the basis of the performed experiments it was proved that the change of technological parameters, particularly of pressing velocity of the piston, directly influences distribution of gasses to the cast volume.

Entrapped Gas and Process Parameter-Induced Porosity Formation in Additively Manufactured 17-4 PH Stainless Steel

Porosity is one of the most significant obstacles to widescale adoption of additive manufacturing since it acts as a stress concentrator that negatively impacts mechanical properties. Suboptimal process parameters and entrapped gas content in powder feedstock are two factors that can induce porosity in additively manufactured parts. This study utilizes laser diffraction, optical and scanning electron microscopy, and micro-x-ray computed tomography to study how four powders with varied entrapped gas concentrations and gas atomization environments create parts with different densities. Process maps of the four 17-4 PH stainless steel powders used in the EOS M 290 laser powder bed fusion machine, were developed. Furthermore, this paper also discusses how process parameters interact with the powder-entrapped gas porosity to influence the in-part porosity and the Charpy toughness of the as-built components.

Evaluation of Leachate Recirculation Effect on the Acceleration of Waste Mineralization Process by Using a Coupled Numerical Model

Accelerating the waste mineralization is of great significance to control the settlement of transportation facilities nearby landfills. Mineralized waste can also be used as road construction materials to recycle waste resources and reduce the construction cost of transportation facilities. A biochem-hydro-mechanical-solute migration-coupled model for describing complex interactions in landfills with high kitchen waste content has been developed. The proposed model can consider large leachate production and landfill gas entrapment due to the fast degradation of kitchen waste. The quantitative effects of three leachate recirculation conditions are investigated in this article via a typical landfill cell. According to the simulation results, introducing methanogen into landfills with leachate recirculation can relieve acidification caused by fast hydrolysis of kitchen waste and speed up the mineralization process of landfills with high kitchen waste content significantly. Furthermore, landfill gas generation potential loss and fast degradation compression should be considered in the implementation of leachate recirculation in landfills with high kitchen waste content, which helps to maintain the operation of transportation facilities nearby landfills and improve the economic and environmental benefits of leachate treatment.

Macro-, meso- and microstructural characterization of metallic lattice structures manufactured by additive manufacturing assisted investment casting

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications. Periodic lattices have tunable properties and may be manufactured by metallic additive manufacturing (AM) techniques. However, AM can lead to issues with un-melted powder, macro/micro porosity, dimensional control and heterogeneous microstructures. This study overcomes these problems through a novel technique, combining additive manufacturing and investment casting to produce detailed investment cast lattice structures. Fused filament fabrication is used to fabricate a pattern used as the mold for the investment casting of aluminium A356 alloy into high-conformity thin-ribbed (~ 0.6 mm thickness) scaffolds. X-ray micro-computed tomography (CT) is used to characterize macro- and meso-scale defects. Optical and scanning electron (SEM) microscopies are used to characterize the microstructure of the cast structures. Slight dimensional (macroscale) variations originate from the 3D printing of the pattern. At the mesoscale, the casting process introduces very fine (~ 3 µm) porosity, along with small numbers of (~ 25 µm) gas entrapment defects in the horizontal struts. At a microstructural level, both the (~ 70 μm) globular/dendritic grains and secondary phases show no significant variations across the lattices. This method is a promising alternative means for producing highly detailed non-stochastic metallic cellular lattices and offers scope for further improvement through refinement of filament fabrication.

On the Effect of the Gap of End Seals on Force Coefficients of a Test Integral Squeeze Film Damper: Experiments and Predictions

Abstract An integral squeeze film damper (ISFD) offers the advantages of a lower number of parts, a shorter axial span, a lighter weight, a split manufacturing, and high precision on its film clearance construction. An ISFD does not only add damping to reduce shaft vibration amplitudes and to enhance the stability of a rotor-bearing system but also can be used to tune a rotor-bearing system natural frequency, and thus increasing the operational safety margin between the running shaft speed and the system critical speed. In spite of the numerous commercial applications, the archival literature is scant as per the experimental quantification of force coefficients for ISFDs. This paper details the results of an experimental and analytical endeavor to quantify and to predict the dynamic force coefficients of an ISFD, hence bridging the gap between theory and practice. With an axial length of 76 mm, the test damper element has four arcuate film lands, 73 deg in arc extent at a diameter of 157 mm, and each with a clearance (c) equaling to 0.353 mm. As is customary, the damper has its axial ends sealed with end plates produced by a set of installed shims giving an axial gap (d) equal to 1.5c, 1.21c, and 0.8c. A baseline configuration, namely, open ends, is also tested without the end seals in place. In the test rig, the ISFD and its housing are flexibly mounted while the rotor is rigid and stationary (no spinning). The lubricant is an ISO VG46 oil supplied at a low pressure, 1 to 2 bar(g) and ∼47 °C inlet temperature, typical of compressor applications. The test procedure applies static loads on the ISFD and records the bearing static offset or eccentricity to verify the structure stiffness, and meanwhile, individual hydraulic shakers deliver dynamic loads along two orthogonal directions to produce motions over a set frequency range, 10 Hz to 160 Hz. The ISFD produces direct damping and inertia that increase with the journal static eccentricity albeit at a lower rate than predictions from a computational squeeze film flow model that includes lubricant compressibility. The end seals are effective in significantly raising the damping coefficient while reducing the oil through flow rate. The damper with the tightest sealed ends (d = 0.8c) shows nearly 20 times more damping that the open ends ISFD albeit also revealing a significant stiffness hardening (negative virtual mass) as the excitation frequency increases. On the contrary, the open ends ISFD and the sealed-ends configurations with gaps d = 1.21c and 1.5c produce a (positive) virtual mass that exceeds the test element physical mass and thus softens the test element direct dynamic stiffness. For the configurations with loose end seals (d = 1.21c or larger to open ends), the model predicts well the damping coefficients but under predicts the added masses by 50% or more. Note this virtual mass coefficient, largely ignored in practice, can make the test element either extremely stiff as with the sealed damper configuration with the smallest gap d = 0.8c, or very soft as with the ISFD with end seals gap = 1.21c or 1.5c. Hence, designers are cautioned not to pursue overly tight end sealed dampers as the mineral lubricant, nearly incompressible though always having a small amount of entrapped gas, may behave distinctly when confined to a squeezed film volume and having no adequate routes to escape or flow through.

Effects of entrapped gas on the surface of a plastic ball induced by ultrasonic irradiation on the enhancement of crystallization of acetaminophen form II

We performed crystallization of the acetaminophen form II by ultrasonic irradiation with a polyethylene (PE) ball or a teflon (PTFE) ball using acetonitrile as a solvent. A PTFE ball enhanced the nucleation probability of form II by ultrasonic irradiation, though the same effect was not observed with a PE ball. The enhancement effect by a PTFE ball was reduced if supersaturated solution with a PTFE ball was evacuated and gas was removed prior to the crystallization. Gas entrapped on the surface of a PTFE ball effectively increased the generation of cavitation bubbles under ultrasonic irradiation, and the cavitation bubbles induced the nucleation of the form II. Therefore, the amount of gases entrapped on the surface of a plastic ball is concluded to be a key parameter for effective crystallization using ultrasonic irradiation. Wettability of a solvent on solid surface dictates the amount of entrapped gases. This study indicated that efficient polymorphic crystallization can be realized by ultrasonic irradiation technique with the help of a plastic ball which features low wettability with solvent.

Study of Powder Gas Entrapment and Its Effects on Porosity in 17-4 PH Stainless Steel Parts Fabricated in Laser Powder Bed Fusion

Powder-entrapped gas, which can occur naturally in gas-atomized powder, can induce porosity in parts fabricated with powder-based metal additive manufacturing processes. This study utilized synchrotron-based x-ray computed tomography and an in situ high-speed imaging technique, dynamic x-ray radiography (DXR), to investigate the formation of powder-induced porosity using 17-4 PH stainless steel powders with a controlled size distribution and intentionally varied entrapped gas contents. While powder with a low entrapped gas content showed no net part porosity increase, the results showed a strong correlation between the porosity in the powder and the porosity in the builds made from powder with a high entrapped gas content relative to typical gas-atomized powder. A threshold value was developed to classify porosity induced by powder-entrapped gas based on pore morphology measured using computed tomography. Transfer and coalescence of pores during laser melting was observed directly with DXR.

On the role of laser in situ re-melting into pore elimination of Ti–6Al–4V components fabricated by selective laser melting

Laser re-melting is a potential technique performed during the selective laser melting (SLM) process to improve the quality of Ti–6Al–4V alloy, especially the density. In this study, a transient mesoscale model with a packed powder-bed is proposed by a simulation to reveal the pore elimination mechanism during the laser re-melting process intuitively. The effect of the laser re-melting on the temperature distribution, molten pool dynamics and resultant surface morphology is investigated. The results show that the re-melting scanning speed played an essential role in controlling the elimination effect on pores. A higher re-melting scanning speed does not eliminate most of the pores due to the limited energy input. In contrast, a lower re-melting scanning speed introduces new types of pores caused by the unstable molten pool. At an appropriate re-melting scanning speed, most of the pores are eliminated attributing to the sufficient spreading of the molten material due to the drag force of melt flow and the escape of the entrapped gas driven by thermocapillary force. The density and surface quality are determined experimentally. The relative density after laser re-melting at a scanning speed of 300 mm/s is improved 99.2% and the surface roughness reduced to Ra 1.9 μm. The simulation results can serve to provide guidance for parameter optimization and improve the density in high effectiveness and efficiency finally.

Dynamic compressive behaviour of auxetic and non-auxetic hexagonal honeycombs with entrapped gas

This work aims to study the uniaxial dynamic compression response of hexagonal honeycombs with different cell morphologies in the presence of an entrapped gas. A theoretical model is proposed to estimate the dynamic crushing strength of non-auxetic honeycombs while including the effect of the entrapped gas on the crushing process. The theoretical predictions are shown to agree well with the finite element (FE) simulations. From the numerical simulations, Hugoniot relations between the shock velocity and the impact velocity are obtained for various honeycomb geometries. It has been observed that shock velocity varies almost linearly with impact velocity. Using this fundamental relation, we derive the stress-impact velocity Hugoniot from the conservation law of momentum. The dynamic stress-strain states of a regular hexagonal honeycomb obtained from the FE simulations show a good agreement with the Hugoniot predictions. It is shown that the dynamic stress-strain states for various impact velocities lie on a unique curve, which is different from the quasi-static stress-strain response. The local strains behind the shock front are significantly lowered in the presence of an entrapped gas, and the stresses behind the shock front are higher as compared to the case where there is no entrapped gas. The variation of the plateau stresses with the cell morphology has been explained, and correlated to the energy absorption capacity of the honeycombs. A new method to characterize the energy absorption capacity of honeycombs is proposed, and the performance of various honeycombs has been compared through the dissipation performance parameter.

Characterization of Energy Absorption and Strain Rate Sensitivity of a Novel Elastomeric Polyurea Foam

Elastomeric polymer foams are widely used in sports and other protective padding applications due to their unique properties, such as excellent cushioning and relatively high‐energy absorption to weight ratio. This work investigates the mechanical and energy absorption performance of an elastomeric hybrid structure polyurea foam in response to low‐velocity impact. The examined polyurea foams are synthesized using a novel self‐foaming process that leads to the development of a semi‐closed cellular structure. The quasi‐static response of the foam is first characterized by measuring the global stress–strain and energy absorption characteristics. The evolution of the foam's Poisson's ratio is also characterized by in situ digital image correlation (DIC) measurements. The same properties are also studied in dynamic loading conditions by subjecting the foam samples to controlled impact tests. A strain‐dependent rate sensitivity parameter is used to quantify differences between the quasi‐static and dynamic strength and energy absorption responses of the foam. The examined foam shows significant enhancement in strength at increased strain rates while retaining its excellent energy absorption capacity. This unique characteristic of the examined foam is discussed in terms of the concurrent effects of entrapped gas and the rate sensitivity of the parent polymer.

An investigation into the formations of the internal microstructures of solid dispersions prepared by hot melt extrusion

Hot melt extrusion (HME) is a widely used manufacturing process for pharmaceutical solid dispersions. The complexity of the HME formulations and the number of excipients used in the process are increasing with the advancement of the relevant knowledge. However, one of the areas that is still significantly lacking understanding is the control of internal microstructure of extrudates. Internal microstructure, consisting of voids, in hot melt extruded amorphous solid dispersions is often observed without the causes having been systemically investigated in the literature. In this study, we investigated a range of factors that demonstrated their impacts on the formation of the voids. These include the effect of the types of the materials (i.e. drug, polymer and additive) used in the formulation, the quantity of the drug and the additives used, the key extrusion processing parameters, the type of extruder, and the drying of the raw materials prior to extrusion. The results indicate that the appropriate viscosity and the presence of phase-separated particulates are essential for the formation of the voids. The particulates act as nuclei for the entrapped gas bubbles and the viscosity of the mixture during extrusion governs the collapse/escape of the bubbles. To minimise void formation, the results of this study indicate that slow screw speed, low moisture content of the raw materials, fewer particulates and the addition of lubricants, such as low melting lipid excipients, could be beneficial. This study systematically examines the mechanism of void formation in HME extrudates and generates new strategies that can be used to manage such void formations.

Study on effect of laser keyhole weld termination regimes and material composition on weld overlap start-stop defects

Abstract The effects of two different laser termination regimes; laser power ramp-down and laser defocusing, on the weld overlap start-stop region, have been studied on three different compositions of S355J2 + N grade steel plates. The plates have varying amounts of deoxidisers, such as manganese and silicon, to allow for the effect of dissolved oxygen on melt pool dynamics and defect formation to be studied. Observations show that gas entrapment, manifesting as porosity, is the main issue in both the laser termination regimes studied, due to melt pool instabilities related to keyhole termination. However, it is more severe during laser defocusing due to the formation of porosity clusters below a specific power density and melt pool size. The percentage of dissolved oxygen and deoxidisers was found to be vital and could be correlated to the response of the melt pool during laser processing. Steel, with a higher percentage of deoxidisers, such as manganese and silicon, would combine with the dissolved oxygen in the weld pool and thereby affect the surface tension and melt pool dynamics. The effect of lower dissolved oxygen would lead to a reduction in penetration depth, but more importantly, a reduction in porosity and porosity cluster formation, as observed for both laser termination regimes. Successful laser termination with defect-free weld overlaps at the start-stop region could be achieved using both termination regimes for the steel with a higher proportion of deoxidising elements.

What We Should Consider for Full Densification when Sintering.

To fully densify a powder compact, we should avoid two things: (i) entrapment of insoluble gases within pores and (ii) entrapment of isolated pores within grains. This paper describes general directions for promoting full densification in view of the above two points. Emphasis is placed on ways to potentially prevent pore entrapment in terms of grain growth control. Currently available techniques that can enhance densification while suppressing grain growth are briefly described, and their major mechanisms are discussed.

Application of Vacuum Techniques in Shell Moulds Produced by Additive Manufacturing

This research shows the feasibility of the additive manufacturing technique (AM), Binder Jetting (BJ), for the production of shell moulds, which are filled by vacuum suction in the field of aluminium parts production. In addition, this study compares the gravity pouring technique and highlights the advantages of using vacuum techniques in AM moulds. A numerical simulation was carried out to study the behaviour of the liquid metal inside the moulds and the cooling rate of parts was analysed. The results show that in the gravity-pouring mould, the velocity in the gate causes moderate turbulence with small waves. However, vacuum suction keeps the velocity constant by eliminating waves and the filling process is homogeneous. Regarding dimensional accuracy, the staircase effect on the surface of the 3D moulds was the most critical aspect. The vacuum provides very homogeneous values of roughness across the entire surface of the part. Similarly, 3D scanning of castings revealed more accurate dimensions thanks to the help of vacuum forces. Finally, the microstructure of the cross section of the moulded parts shows that the porosity decreases with the vacuum filled. In both cases, the origin of the pores corresponds to gas entrapment and shrinkage during the filling process, the binder vaporization and nucleation points creation, leading to pores by shrinkage, gas entrapment or a mixture of both. This is the first study that uses vacuum filling techniques in moulds created by BJ, demonstrating the feasibility and advantages of AM using vacuum techniques, as an alternative to traditional casting.