Effect Of Stirring Research Articles

Friction Stir Spot Weldability of AA7075-T6 Sheets with a Pinless Tool Providing Enhanced Stirring Effect

Friction Stir Spot Weldability of AA7075-T6 Sheets with a Pinless Tool Providing Enhanced Stirring Effect

TiC reinforced Ti6Al4V fabricated by circular oscillating laser directed energy deposition: Microstructure and wear resistance

Gaussian laser is a common laser source in the field of laser directed energy deposition (L-DED). However, Gaussian laser sources have certain limitations, as they produce elevated temperature differentials and internal stresses during L-DED, which can result in significant distortion and fracturing. To address these restrictions, this research proposes using a circular oscillating laser as the energy source for L-DED. TC4 deposition samples and 5 wt% TiC/TC4 composite deposition samples were prepared using both Gaussian laser directed energy deposition (GL-DED) and circular oscillating laser directed energy deposition (COL-DED), and the deposition samples prepared under the two laser modes were compared and studied. The results indicate that, in the process of COL-DED, the laser oscillates periodically at a constant frequency in the high-temperature molten pool, which produces a more consistent temperature gradient and improved stirring effect in the molten pool. This inhibits grain growth and effectively refines the grains. Compared to the TiC/TC4 deposition samples prepared with Gaussian laser, the average grain size of COL TiC/TC4 deposition samples decreased by approximately 54.3 %. Due to the effects of the high-power laser beam, TiC particles partially melted and decomposed into Ti and C. As the laser advanced, the molten pool cooled swiftly, ultimately precipitating uniformly distributed gray-white TiC hard phases. Compared to the TC4 alloy prepared with the Gaussian laser, the TiC/TC4 composite material manufactured with a circular oscillating laser exhibited a 24.5 % increase in microhardness (430.76 HV0.2) and a reduction in wear rate of approximately 42.15 % (to 1.4 × 10−6 g/mm). Circular oscillating laser directed energy deposition can, to a certain extent, improve the hardness and wear resistance of materials compared to Gaussian laser directed energy deposition.

Effect of Electromagnetic Stirring on Solidification Structure and Central Porosity of Large‐Sized Round Bloom in Continuous Casting

The effect of strand (S‐) and final (F‐) electromagnetic stirring (EMS) on solidification structure characteristic of a φ690 mm continuously cast round bloom has been investigated industrially and theoretically. The newly designed S1‐EMS equipped just below the foot zone takes great effect on both equiaxed ratio and distribution alignment, as well as central porosity. Larger current S1‐EMS leads to higher equiaxed ratio, more symmetrical equiaxed zone, and smaller diameter of central porosity, and the effect is much more obvious than conventional S2‐EMS and F‐EMS. It is also noticed that there should be a saturation effect of S1‐EMS on the promotion of equiaxed dendrite nucleation. The S2‐EMS is beneficial for the expanding of equiaxed zone and improving of central porosity. When S1‐EMS is weak, strong S2‐EMS results in more asymmetrical equiaxed zone. However, when S‐EMS is strong, strong S2‐EMS results in more symmetrical distribution of equiaxed zone. The F‐EMS takes a little impact on the expanding of equiaxed zone in the large‐sized round bloom, and almost no effect on the alignment. The central porosity, however, is improved by F‐EMS with the help of decrease in temperature gradient and compensation of solidification contraction.

Comparative study of microstructural evolution and mechanical properties in friction stir processed, reinforced friction stir processed, and heat-treated reinforced friction stir processed Al composites

The development of friction stir welding (FSW) created the basic foundation for friction stir processing (FSP). This process entails refining the local microstructure and achieving the localised plastic deformation due to the stirring effect of the tool, which may involve adding additional filler material as required. The rotating tool involved in the process has a shoulder and pin responsible for heat generation for plastic deformation and localised mixing. The rotating tool is inserted and traversed on the desired surface, tailoring the material properties. The tool’s exposure to the surface causes localised plastic deformation and thermal gradient, and microstructural changes occur locally. The present investigation uses Friction Stir Processing (FSP) to fabricate an aluminium metal matrix composite. Recently, the use of aluminium alloy has been replaced by composites because of their increased mechanical and physical properties (specific properties). The experiment leads to manufacturing an aluminium metal matrix composite using aluminium oxide, boron carbide and commercially pure copper dust. Later, the effect of heat treatment on the respective metal matrix composite is studied. A comparative study among three different types of FSP techniques viz. FSP, Reinforced FSP, and Heat-treated Reinforced FSP have been performed. The tool revolution speed (RPM) and tool traverse speed of 400 RPM and 10 mm/min had been implemented, respectively. The T4 Heat-Treatment cycle was used during the current investigation. This investigative study was performed to analyse the changes in grain sizes and hardness values at the stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and on the base materials (BM) of the samples. The reinforced FSP metal matrix composite (MMC) showed an 81.26% increase in the micro-hardness value (HV) compared to the base metal. Further on analysing the microstructures, the reinforced FSP composite showed a 65.20% reduction in grain size. Moreover, the Heat-Treated sample showed an increase in the micro-hardness value by 99.8%.

Frequency Stirring Effects on Reference Antenna Efficiency Measurements in Reverberation Chambers

Frequency Stirring Effects on Reference Antenna Efficiency Measurements in Reverberation Chambers

Effect of beam oscillating amplitude on forming quality, microstructure, and mechanical performance of Al-Mg-Sc alloy fabricated by laser-arc hybrid additive manufacturing

The oscillating laser-arc hybrid additive manufacturing was applied to fabricate Al-Mg-Sc alloy and the effect of beam oscillating amplitude on macro/micro-morphology and the mechanical properties were investigated in this study. With the oscillating amplitude increased from 0 to 6 mm, the stirring effect of laser beam on molten pool was firstly enhanced due to the expanded active region and then weakened because the laser spot located at the edge of pool. As a result, the improved forming quality, the reduced porosity and the refined microstructures were achieved when the amplitude reached 4 mm, and then negative effect occurred when the amplitude was 6 mm. Eventually, the optimal mechanical performance was achieved at the amplitude of 4 mm and the as-deposited tensile properties reached 353 MPa and 24 % for UTS, and EL in vertical direction, respectively, which were 13.5 % and 50 % higher than that of 0 mm.

Effect of electromagnetic stirring on interfacial microstructure and mechanical properties of Al/Mg bimetal produced by lost foam compound casting

The Al/Mg bimetal was fabricated using a novel spiral magnetic field-assisted lost foam compound casting (LFCC) process, and the effects of electromagnetic stirring (EMS) on the interfacial microstructure and mechanical properties of the Al/Mg bimetal were investigated. The results indicated that the interface of the Al/Mg bimetal without EMS consisted of Al–Mg intermetallic compounds area (IMCs area) and Al–Mg eutectic area (E area). The Mg2Si particles exhibited a reticulated agglomeration in the IMCs area, and a continuous oxide film was present at the junction of the IMCs area and E area. During shear strength measurement, interface cracks propagated along the oxide film until interface fracture occurred, leading to lower bonding strength at the interface. With EMS applied, there was no change in the phase composition of the Al/Mg interface. The EMS facilitated the diffusion of Si elements in the interface, resulting in the refinement and dispersion of the Mg2Si particles within the IMCs area. Moreover, the Mg2Si content around the oxide film significantly increased, the oxide film remained but became discontinuous. During shear strength measurement, the interface cracks didn’t propagate along the oxide film but extended from the IMCs area to the E area, as a result, the shear strength increased by 29.1% compared to the samples without EMS. The study results demonstrated that applying EMS during the lost foam casting process is a feasible method for improving the microstructure and properties of the Al/Mg bimetals.

Numerical investigation of arc characteristics and metal properties in ELAMF-assisted WAAM with wire retraction

In this study, a multi-physics integrated model of wire arc additive manufacturing (WAAM) with wire retraction and external longitudinal alternating magnetic field (ELAMF) assistance is proposed and validated, and the effect of ELAMF on the arc plasma characteristics and metal properties are systematically and quantitatively investigated. The model comprehensively takes the arc plasma heating, wire dynamic retraction, droplet transfer, the formation and diffusion of metal vapor, the melting and solidification of molten metal, and the stirring effect of ELAMF into account. The simulated results show that ELAMF has compressed the arc morphology, and decreased the temperature, potential, current density and static pressure, which are most pronounced when the wire is at an elevated position and the transient current is at its peak phase. Meanwhile, the formation of low-temperature and low-velocity cavities in the center of arc column are promoted by ELAMF, consequently diminishing the energy transferred from the arc plasma to molten pool through thermal conduction and mitigating the squeezing of the arc plasma on the surface of molten pool. The results also demonstrate that the ELAMF has reduced the temperature gradient of liquid metal inside the molten pool, and intensified the forced convection for the occurrence of multiple circulations.

Gas–Liquid Mixability Study in a Jet-Stirred Tank for Mineral Flotation

Micro- and nano-bubble jet stirring, as an emerging technology, shows great potential in complex mineral sorting. Flow field characteristics and structural parameters of the gas–liquid two-phase system can lead to uneven bubble distribution inside the reaction vessel. Gas–liquid mixing uniformity is crucial for evaluating stirring effects, as increasing the contact area enhances reaction efficiency. To improve flotation process efficiency and resource recovery, further investigation into flow field characteristics and structural optimization is necessary. The internal flow field of the jet-stirred tank was analyzed using computational fluid dynamics (CFDs) with the Eulerian multiphase flow model and the Renormalization Group (RNG) k − ε turbulence model. Various operating (feeding and aerating volumes) and structural parameters (nozzle direction, height, inner diameter, and radius ratio) were simulated. Dimensionless variance is a statistical metric used to assess gas–liquid mixing uniformity. The results indicated bubbles accumulated in the middle of the vessel, leading to uneven mixing. Lower velocities resulted in low gas volume fractions, while excessively high velocities increased differences between the center and near-wall regions. Optimal mixing uniformity was achieved with a circumferential nozzle direction, 80 mm height, 5.0 mm inner diameter, and 0.55 radius ratio.

Comparison of the Effects of Stirring and Standing on Chemical Reactions

AbstractFor hundreds of years, it seems that people have needed stirring to conduct chemical experiments. This operation can be seen everywhere in chemical, pharmaceutical, and materials laboratories and factories. People generally believe that stirring helps with processes such as material dispersion, dissolution, and collision, thereby enabling more-efficient reactions. However, why do chemical reactions that occur in Nature not require stirring? What are the facts? For this purpose, we investigated a total of 329 organic chemical reactions in eight categories and 25 types, including 26 chemical reactions magnified to gram or even kilogram levels. Under the same conditions of temperature, humidity, pressure, and reaction time, we compared the reaction yields under stirring and standing conditions. More than 600 results showed that stirring or not stirring had almost no effect on the efficiency of chemical reactions in solution. If most chemists performing reactions turned off the agitator, it would not be difficult to imagine how much electricity could be saved!

Dissolution of EAF slag minerals in aqueous media: Effects of sonication on brownmillerite and gehlenite

The accumulation of electric arc furnace slag (EAFS) in landfills has been causing severe environmental problems. This study examines the dissolution properties of EAFS minerals, including brownmillerite and gehlenite, essential for their possible use in resource recovery. An investigation was conducted to compare the effects of sonication and stirring on mineral dissolution while also assessing the usage of citrate as a complexing agent for gehlenite. Synthetic brownmillerite and gehlenite minerals were dissolved in aqueous solutions at room temperature using a 1:100 g/ml ratio. The dissolved elements were measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), while zeta potential and X-ray Photoelectron Spectroscopy (XPS) were used to assess changes in surface chemistry. Brownmillerite had significant dissolution extents, with Al and Ca dissolving up to 16 % and 8 %, respectively, in contrast to gehlenite, which dissolved less than 2 % under similar conditions. Sonication significantly increased the dissolution of brownmillerite by up to 100 %, although its impact on gehlenite dissolution varied depending on the duration of time. Besides, adding citrate enhanced the leaching of Al and Ca from gehlenite by facilitating complexation. XPS data demonstrated differences in elemental ratios on brownmillerite and gehlenite surfaces affected by the method used and the presence of citrate. Lastly, the dissolution extents of Al and Ca from EAFS were up to 12 %, depending on time and mixing method, with a preference for sonication over stirring. In conclusion, this study showed that minerals in EAFS have distinct dissolution characteristics, and sonication and citrate can considerably enhance dissolution.

Effect of fast frequency double pulse current on microstructural characteristics and mechanical properties of wire arc additively manufactured Ti-6Al-4V alloy

This study introduces an innovative technique known as fast-frequency double pulse wire arc metal additive manufacturing (FFDP-WAAM). This method employs periodic fluctuations in arc plasma and force to enhance the stirring effect within the molten pool, resulting in the fragmentation of β grains and the formation of finer prior-β grains. The microstructure primarily comprises α', attributed to the high cooling rates and minimal heat accumulation. Compared to the conventional gas tungsten arc welding-based WAAM (CGT-WAAM) process, FFDP-WAAM significantly reduces α-variant selection, thereby achieving a more uniform α phase orientation distribution. Additionally, ultrasonic vibration in the FFDP-WAAM process facilitates recrystallization and mitigates residual strain. The tensile strength and elongation of the FFDP-WAAM specimens reached 939.2 MPa and 9.0 %, respectively, whereas the CGT-WAAM specimens showed a lower tensile strength of 830 MPa and elongation of 9.2 %. The enhanced strength, fatigue life and microhardness of Ti-6Al-4V produced by FFDP-WAAM is ascribed to the refined grain-size distribution. Furthermore, the low Schmid factor (SF) distribution and the stable triangular structure of Category I α-clusters are anticipated to contribute to the increased strength of the FFDP-WAAM-fabricated wall.

Monomer Transport by Collisions in (Mini) Emulsion Polymerization, a Personal Perspective

AbstractTransport of monomer from droplets to growing latex particles in emulsion polymerization in general is assumed to proceed via diffusion through the aqueous phase. Especially in miniemulsion polymerizations the direct transfer of very hydrophobic species from droplet to droplet is assumed to also proceed via collisions. Amongst the hydrophobic species where this is shown to play a role are monomers, initiators, inhibitors and (catalytic) chain transfer agents. It is well known that the reactor geometry and the stirring speed can have a profound effect on emulsion polymerizations. The 1972 paper of Nomura on the effect of stirring on emulsion polymerization is cited more than 100 times and until today keeps scientists intrigued. Diffusion limitations of monomer going from the droplet into the aqueous phase can occur for very hydrophobic monomers. The alternative route of transport via collisions is often not considered. In this perspective, paper will discuss the evidence for collision based transfer in miniemulsion polymerization and also consider whether collision based monomer transport can play a role in regular emulsion polymerizations.

On the Significance of Ageostrophic Meridional Eddy-Induced Heat Flux in the Surface Ocean of the Antarctic Circumpolar Current

Abstract Eddy-induced heat flux (EHF) convergence plays an important role in balancing the cooling of mean flows in the heat budget of the Southern Ocean. This study investigates the EHF in the Southern Ocean and the surface ocean heat budget over the Antarctic Circumpolar Current (ACC) estimated through a high-resolution ocean assimilation product. In contrast to previous studies in which the estimation of the EHF in the Southern Ocean was based on the assumption that mesoscale eddies are quasigeostrophic turbulence, we find that more than one-third of the total meridional EHF in the surface layer is attributed to ageostrophic currents of eddies and that the ageostrophic component of the EHF convergence is as important as its geostrophic component for the surface ocean heat budget over the ACC. In particular, the ageostrophic meridional EHF convergence accounts for 22% of the warming needed to balance the cooling from the mean flows during winter, equivalent to warming the surface ocean of the ACC by 0.14°C. The ageostrophic meridional EHF is likely caused by the stirring effect of ageostrophic secondary circulations in mesoscale eddies, which are induced by the turbulent thermal wind balance to restore the vertical shear of the upper layer in mesoscale eddies destructed by intense winter winds.

Morphology, mechanical property, and molten pool dynamics by narrow gap AMF-GTAW with swing filler wire

The present study employed swing filler wire to enhance the microstructure and properties of thick joints in Ti–6Al–4V titanium alloy by narrow gap alternating-magnetic-field-assisted tungsten gas welding (AMF-GTAW). Results show that the addition of swing filler wire resulted in the refinement of prior β grains and α laths. Consequently, the tensile strength exhibited an increase from 877 ± 13 MPa to 913 ± 24 MPa (4.1% increase), while the fracture elongation showed a significant improvement from 6.3% ± 1.1% to 8.7% ± 0.9% (38.1% increase). The introduction of swing filler wire alters the flow state of the molten pool and exerts a stirring effect on its entirety. This stirring effect promotes nucleation and dendrite fragmentation within the molten pool.

Mechanism and kinetics of ultrasound-enhanced CaCO3 precipitation for indium enrichment in zinc oxide dust leaching solution

In this study, ultrasound-enhanced calcium carbonate precipitation was used to enrich indium in zinc oxide dust leachate, and the effects of precipitation endpoint pH and ultrasound power on the indium precipitation behaviour were investigated, and the optimal conditions of ultrasound-enhanced precipitation were obtained to be the precipitation endpoint pH of 4.0 and the ultrasound power of 200 W. The precipitation rate of indium under these conditions was 99.79 %. At the same time, the effects of ultrasonication and conventional stirring on the indium precipitation kinetics were compared, which proved that ultrasound can shorten the time for precipitation to reach equilibrium and reduce the amount of calcium carbonate used, and the theory of ultrasonication activation energy was put forward. The activation energy of ultrasonication was Eu-a = 2.63 KJ/mol, and that of conventional precipitation was 9.78KJ/mol, which proved that ultrasonication could reduce the activation energy of the precipitation reaction, and promote the rapid precipitation reaction. The kinetic model of ultrasound-enhanced indium precipitation is lnC0-lnCt = exp(0.11339–318.54/W).t + A. In addition, the mechanism of ultrasound-enhanced calcium carbonate precipitation of indium was revealed by XRD, SEM-EDS, XPS and TEM analyses of the precipitated residue, it was demonstrated that ultrasound can inhibit the precipitation of zinc, and the ZnCO3 phase was found in the ultrasonically precipitated residue. This study provides a new idea for indium enrichment, and the future focus will be on the scale-up of the ultrasound-enhanced precipitation device.

Study on molten pool flow and hot cracking of narrow gap laser welding of 316LN stainless steel for fusion reactor

In this paper, narrow-gap laser welding (NGLW) of 316LN stainless steel was studied. The results indicated that continuous laser welding with filler wire (CLWFW) and pulsed laser welding with filler wire (PLWFW) can obtain good weld formation under the transition of the wire bridge, and when the pulsed laser was a trapezoidal wave, the hot cracks of the filling layer can be greatly inhibited. The intermittent output of the pulsed laser reduced the temperature gradient of the molten pool and enhanced the morphology of the caudal region of the molten pool. Meanwhile, timely liquid backfilling effectively relieved the stress concentration of the weld, thus inhibiting the generation of hot cracks. Besides, the intermittent pulse energy input had a stirring effect on the molten pool, which disturbed the growth direction of the dendrite grain and refined the grain; consequently, the impurity elements were fully diffused, and the distribution of elements in the weld center area was more uniform.

Microstructure and bending resistance performance of sprayed NiCoCrAlY sealing coating anchored by circular oscillating laser

A novel pinning-process using circular oscillating laser is utilized to improve the performance of NiCoCrAlY sealing coatings prepared by atmospheric plasma spraying. the stirring effect of the oscillating laser and Marangoni convection affect the outline morphology of pinned area, promote the recrystallization of the sprayed coating, eliminate the initial defects, transform the bonding behavior, and improve the mechanical property. The microstructure of the circular oscillating laser pinning area becomes compact, and the grain size is significantly refined, with an average size of 4.50 μm. The combination between the coating and the substrate changes from mechanical bonding to metallurgical bonding. The enhanced uniformity of the microstructure in the laser pinning region resulted in a decrease of the microhardness by 5.6 %. Furthermore, the pinning process increased the toughness of the coatings by 29 % in yield strength from 470 MPa to 600 MPa, and by 20 % in maximum failure strength from 1160.91 MPa to 1392.97 MPa.

Heat and mass transfer behavior in CMT plus pulse arc manufacturing

The metal transfer mode, temperature distribution, flow behavior, and microstructure characteristic in the process of cold metal transfer plus pulse (CMT+P) arc manufacturing were investigated by the in-situ observation and numerical simulation. A novel coupling model for droplet-pool interaction under the CMT+P arc was established, along with a new heat source model that considered Marangoni forces on droplets. These innovations provided significant insights into the heat and mass transfer mechanisms in the CMT+P hybrid arc manufacturing process. The results show that the droplets generated by the CMT+P hybrid arc exhibited a mixed transfer mode: short-circuit transfer in the CMT period and projected transfer of one droplet per pulse during the pulse period. The short-circuit transfer was more stable than the projected transfer, attributed to the smooth transfer of droplets to the molten pool through the liquid metal bridge, dominated by surface tension and Marangoni forces. The main energy of the molten pool was provided by droplets, which had a dominant ability to melt the substrate. However, the liquid metal bridge in the CMT period slowed down the heat transfer to the molten pool and had significant oscillating and stirring effects. During the pulse period, the droplet was accelerated and projected transfer by gravity and electromagnetic force. After solidification, the microstructure exhibited an alternating distribution of columnar and equiaxed grains from the bottom-up in the monolayer deposited layer, with a deflected fusion interface towards the scanning direction. At the top of the deposited layer, coarse columnar grains were distributed obliquely or laterally, inhibiting the growth of equiaxed grains and forming a distinct boundary with the equiaxed grains below.

'Accelerated' Deactivation of Carbon Nitride Photocatalyst for Solar Hydrogen Evolution.

Carbon nitride photocatalysts are among the most studied candidates for efficient solar hydrogen (H2) production due to their abundance of precursors, suitable bandgap, and visible light utilization. However, the polymeric nature of carbon nitride materials raises concerns regarding the self-decomposition during photocatalytic redox processes. Yet, the operational stability of carbon nitride photocatalysts for solar H2 production remains under-explored. Here we evaluate the photostability of carbon nitride photocatalysts with platinum (Pt) as the co-catalyst for solar H2 evolution and significant deactivation of this photocatalyst is observed under'accelerated' testing conditions. It is demonstrated that the detachment of the Pt co-catalyst on the surface of carbon nitride is the major reason for this deactivation, which can be attributed to a synergistic effect of photo-corrosion and mechanical stirring. The photo-corrosion weakens the interfacial bonding between carbon nitride and Pt co-catalyst, while continuous collisions from the mechanical stirring promote the detachment of co-catalysts from the surface of carbon nitride. These understandings provide insights into the rational design of photocatalysts and photocatalytic systems for improved operational stability.