Thixotropic Strength and Thixotropic Criteria in Semi-Solid Processing

2008 ◽  
Vol 141-143 ◽  
pp. 319-323 ◽  
Author(s):  
W.C. Keung ◽  
Y.F. Lee ◽  
Wei Wei Shan ◽  
Shou Jing Luo
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Shape Factor ◽  
Shear Thickening ◽  
Solid Material ◽  
Solid Content ◽  
Solid Particles ◽  
Thixotropic Behavior ◽  
Semi Solid ◽  
Constant Shear Rate

Thixotropy is essential to semi-solid processing, and because of it the semi-solid material is characterized by ‘shear shinning’. Here, thixotropic strength and thixotropic criteria in semi-solid processing are put forward based on related theories and experiments, and thixotropic mechanism and its influencing factors are also investigated. The results are as follows: 1) the term of thixotropic strength means that with constant shear rate at semi-solid temperature, the semi-solid body begins to flow when the shear stress reach a certain value. This value of shear stress is defined as the thixotropic strength; 2) Thixotropic behavior happens with ‘shear thinning’ because of the deagglomeration of solid particles, while ‘shear thickening’ happens because of the agglomeration at the same time. With increasing shear time, the shear stress increases first and then decreases rapidly to reach a stable value. 3) There are three important factors that influence ‘thixotropic strength’: temperature (hence solid content), initial microstructure (including size, shape factor and uniformity of solid particles) and shear rate.

Thixotropic Behavior of Semi-Solid Magnesium Alloy

2006 ◽  
Vol 116-117 ◽  
pp. 648-651 ◽  
Author(s):  
Hsueh I Chen ◽  
Jyh Chen Chen
Keyword(s):  
Shear Stress ◽  
High Temperature ◽  
Magnesium Alloy ◽  
Shear Rate ◽  
Experiment Data ◽  
Az91d Magnesium Alloy ◽  
Thixotropic Behavior ◽  
Semi Solid ◽  
Rising Time ◽  
Shear Thinning Fluid

By a high-temperature Couette type viscometer, we studied the thixotropic behavior of the semi-solid AZ91D magnesium alloy slurry. According to different variable conditions, we could measure the change of the shear stress. The results showed that the shear stress of semi-solid AZ91D magnesium alloy slurry increased at starting shearing, and the shear stress fell down at the maintained shear rate shearing. In our experiment data, we found that the semi-solid AZ91D magnesium alloy slurry had the behavior of shear thinning fluid. As the maximum shear rate increased, the measured shear stress increased at a dissimilar level. When the rising time was shorter, the increasing range of shear stress was larger. As the shearing time of the maintained shear rate increased, the falling down degree of the shear stress increased and it can be presented as a function of shearing time, such as: e 0.0159t min 11331 τ = − . At the same shear rate, the area of hysteresis loop was bigger and the thixotropic behavior was more obvious.

Physical Aging and Thixotropy in Sludge Rheology

2008 ◽  
Vol 18 (1) ◽  
pp. 13495-1-13495-8 ◽  
Author(s):  
J.C. Baudez
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Physical Aging ◽  
Abrupt Change ◽  
Rheological Behaviour ◽  
Critical Value ◽  
Solid Content ◽  
Structural Parameter ◽  
Critical Shear Rate ◽  
A New Technique

Abstract Sewage sludge presents a dual rheological behaviour with an abrupt change between the two regimes. Using a new technique of reconstruction of the velocity profile, the behaviour can be modelled by a unique equation including liquid and solid components but also a structural parameter. It is also rigorously demonstrated that the only one rheological behaviour in steady state in the liquid regime is a truncated power-law which can be defined only for a shear rate and a shear stress higher than a critical value, γ̇c and τc. Moreover, the critical shear rate and shear stress increase with the solid content and depend on the fractal dimension of flocs which implies that thixotropic effects are all the more important as the sludge is thick and fresh.

Simplified Rheocasting of Al-Si Alloys Sheared by Mechanical Stirring

2015 ◽  
Vol 651-653 ◽  
pp. 1545-1550 ◽  
Author(s):  
Alfredo Hernández ◽  
J. Federico Chávez ◽  
Raquel E. Hernández
Keyword(s):  
Stainless Steel ◽  
Solid State ◽  
Shear Rate ◽  
Microstructural Characterization ◽  
Dendritic Morphology ◽  
Solid Alloy ◽  
Mechanical Stirring ◽  
Simplified Method ◽  
Thixotropic Behavior ◽  
Semi Solid

A rheocasting prototype for the semi-solid processing of Al-Si alloys was designed and assembled at laboratory scale, this is the first attempt for the development of S2P equipment in Mexico. This work describes the simplified method of mechanical stirring and the calculation of the theoretical parameters for the operation of the experimental device in order to provide shear rate into the slurry by a stainless steel impeller inside a crucible made of hot work steel. Moreover, a vertical pressure die-rheocasting system it is applied with the semi-solid alloy flowing counter the position of the injector during the displacement of the crucible in order to fill the mold. New design is applied in this rheocasting equipment.The semi-solid state with thixotropic behavior is produced in the stirred AlSi7Mg alloy; this condition allows the non-dendritic morphology because the intensively stirring at the beginning of the solidification produces the trimming of dendrites, growing so the primary solid (α) in globular shape surrounded by eutectic microstructure. Preliminary microstructural characterization was performed.

Plastic Deformation Kinetics of 95.5Sn4Cu0.5Ag Solder Joints

1995 ◽  
Vol 117 (2) ◽  
pp. 100-104 ◽  
Author(s):  
Z. Guo ◽  
Yi-Hsin Pao ◽  
H. Conrad
Keyword(s):  
Plastic Deformation ◽  
Shear Stress ◽  
Shear Rate ◽  
Solder Joints ◽  
Deformation Kinetics ◽  
Constant Shear ◽  
Controlling Mechanism ◽  
Deformation Equation ◽  
Kinetics Of ◽  
Constant Shear Rate

The plastic deformation kinetics of 95.5Sn4Cu0.5Ag solder joints were determined in monotonic loading shear over the temperature range of 25°–150°C using three types of tests: (a) constant shear rate, (b) constant shear stress (creep), and (c) differential tests (changes in shear rate or temperature during an otherwise isothermal constant shear rate test). The deformation kinetics were evaluated in terms of the Dorn high temperature plastic deformation equation γ˙p=A(μb/kT)D(b/d)P(τ/μ)n where γ˙p is the shear rate, μ the shear modulus, b the Burgers vector, D the appropriate diffusion coefficient, d the grain size and τ the shear stress. A, p, and n are constants whose values depend on the rate controlling mechanism. It was found that n increased with stress from ~4 at 2 MPa to ~20 at 25 MPa, relatively independent of temperature. The activation ΔH was determined to be 21.1 ± 2 kcal/mole. The constant A, however, decreased with temperature from a value of ~1018 at 25°C to ~1010 at 150°C. The values of n and ΔH suggest that dislocation glide and climb is the rate controlling mechanism and hence that p ≈ 0. It is speculated that the large decrease in A with temperature may be the result of an effect on the microstructure.

scholarly journals Aging, Rejuvenation and Thixotropy in Complex Fluids: Time-dependence of the Viscosity at Rest and under Constant Shear Rate or Shear Stress

2008 ◽  
Vol 18 (5) ◽  
pp. 53298-1-53298-13
Author(s):  
Daniel Quemada
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Volume Fraction ◽  
Complex Fluids ◽  
Transient Behaviour ◽  
Volume Fractions ◽  
Constant Shear ◽  
Constant Shear Stress ◽  
Constant Shear Rate ◽  
Viscosity Changes

Abstract Complex fluids exhibit time-dependent changes in viscosity that have been ascribed to both thixotropy and aging. However, there is no consensus for which phenomenon is the origin of which changes. A novel thixotropic model is defined that incorporates aging. Conditions under which viscosity changes are due to thixotropy and aging are unambiguously defined. Viscosity changes in a complex fluid during a period of rest after destructuring exhibit a bifurcation at a critical volume fraction ϕc2. For volume fractions less than ϕc2 the viscosity remains finite in the limit t →∞. For volume fractions above critical the viscosity grows without limit, so aging occurs at rest. At constant shear rate there is no bifurcation, whereas under constant shear stress the model predicts a new bifurcation in the viscosity at a critical stress σB, identical to the yield stress σy observed under steady conditions. The divergence of the viscosity for σ≤σB is best defined as aging. However, for σ > σB, where the viscosity remains finite, it seems preferable to use the concepts of restructuring and destructuring, rather than aging and rejuvenation. Nevertheless, when a stress σA(≤σB) is applied during aging, slower aging is predicted and discussed as true rejuvenation. Plastic behaviour is predicted under steady conditions when σ > σB. The Herschel-Bulkley model fits the flow curve for stresses close to σB, whereas the Bingham model gives a better fit for σ >> σB. Finally, the model’s predictions are shown to be consistent with experimental data from the literature for the transient behaviour of laponite gels.

Shear Flow and Large Amplitude Oscillation Shear Study of Solutions of Aggregating Micellar Casein Particles

2008 ◽  
Vol 18 (2) ◽  
pp. 23050-1-23050-7 ◽  
Author(s):  
Anne Pitkowski ◽  
Taco Nicolai ◽  
Dominique Durand
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Shear Flow ◽  
Heating Temperature ◽  
Constant Shear ◽  
Micellar Casein ◽  
Large Amplitude Oscillation ◽  
Increasing Temperature ◽  
Small Shear ◽  
Constant Shear Rate

Abstract Small micellar casein particles were formed in aqueous solutions of native casein after addition of polyphosphate. These so-called submicelles aggregated and gelled with a rate that increased with increasing temperature. The evolution of the viscosity during this process was determined at constant shear rate or shear stress. When applying a small shear stress the viscosity increased strongly until the shear rate became immeasurably slow, but when the applied shear stress exceeded a critical value (σc) the aggregates broke up and the viscosity reached a maximum. At longer times the viscosity decreased rapidly at first, followed by a very slow decrease. σc was independent of the shear rate and heating temperature, but increased strongly with increasing casein concentration. At constant shear rate the stress remained close to σc, but fluctuated irregularly. After cessation of shear flow, gels were formed rapidly. Oscillation shear measurements for σ > σc showed a strongly non-linear response at the time of maximum viscosity.

ELECTRORHEOLOGY OF DISPERSIONS OF BaxSr(1-x)TiO3 IN SILICONE OIL UNDER AC OR DC ELECTRIC FIELD

2012 ◽  
Vol 26 (14) ◽  
pp. 1250081 ◽  
Author(s):  
GLAUBER M. S. LUZ ◽  
ANTONIO J. F. BOMBARD ◽  
SILVIO L. M. BRITO ◽  
DOUGLAS GOUVÊA ◽  
SHEILA L. VIEIRA
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Electric Field ◽  
Silicone Oil ◽  
Ferroelectric Materials ◽  
Dc Electric Field ◽  
Ac Fields ◽  
Er Fluid ◽  
Constant Shear Rate

Electrorheology (ER) of ferroelectric materials such as nanometric BaTiO 3 is still not fully understood. In this paper, nanoparticles of Ba x Sr (1-x) TiO 3 (where x = 0.8, 0.9 or 1.0) were synthesized using the method of Pechini, calcinated at 950°C, and after, lixiviated under pH 1 or pH 5. A controlled stress rheometer (MCR-301) was used to make the ER characterization of dispersions made of Ba x Ti 1-x O 3 in silicone oil (30% w/w), where (a) shear stress as a function of DC electric field (under constant shear rate) or (b) shear stress as a function of shear rate (under constant AC or DC electric field) were measured. We observed that electrophoresis occurred under electric field DC, creating a concentration gradient which induced phase separation in ER fluid. On the other hand, under AC fields above 1 kV/mm, the ER effect is stronger than for DC field, and almost without electrophoresis. Furthermore, there is an AC frequency, dependent on the disperse phase, where the ER effect has a maximum.

Cold start analysis of an engine coolant-MWCNT nanofluid: Synthesis and viscosity behavior under shear stress

2021 ◽  
pp. 095440702110192
Author(s):  
Luiz U R Sica ◽  
Edwin M C Contreras ◽  
Enio P Bandarra Filho ◽  
José A R Parise
Keyword(s):  
Thermal Conductivity ◽  
Shear Stress ◽  
Shear Rate ◽  
Internal Combustion Engines ◽  
Shear Thickening ◽  
Cold Start ◽  
Shear Thinning ◽  
Pollutant Emissions ◽  
Flow Index ◽  
Engine Coolant

During cold start of internal combustion engines, coolant temperature, and thermal conductivity are key parameters in the heat transfer processes that ultimately affect pollutant emissions and engine performance. Hereupon the use of coolants with suspended nanoparticles, to enhance thermal conductivity, emerged as a promising technology. However, for Newtonian materials, viscosity also increases with nanoparticle concentration. To overcome increased pumping power, the use of non-Newtonian nanofluids makes such application potentially feasible, specifically for shear-thinning materials, in which a higher shear rate leads to reducing shear viscosity due to higher shear stress. Accordingly, a nanofluid, suitable for engine cooling (0.2 wt.% MWCNT-engine coolant/distilled water 30/70 v/v%), was here fabricated and mapped. Shear rate and temperature were varied, with focus on cold start investigation. Shear thinning and shear thickening regions were mapped according to the shear rate levels, for each temperature considered. The nanofluid behaved as shear-thinning material for the entire range of temperatures (−10°C–25°C). Above shear rates of 500 s−1 and flow curves with temperatures below −5°C, a prominent shear thickening behavior was observed. Additionally, the relative apparent viscosity data were compared with four classical models. Regarding the curve fitting parameters of a modified Herschel-Bulkley equation, above 0°C, the apparent yield stress, [Formula: see text], was invariant with temperature. Besides, for the temperature range from 0°C to 20°C, the flow index remained approximately constant. For temperatures above −5°C, infinite-shear-rate viscosity and consistency index presented a linear decrease and a third-degree polynomial-like behavior, respectively.

scholarly journals Rheological and Technological Aspects in Designing the Properties of Shear Thickening Fluids

Materials ◽  
2021 ◽  
Vol 14 (21) ◽  
pp. 6585
Author(s):  
Radosław Żurowski ◽  
Paweł Falkowski ◽  
Justyna Zygmuntowicz ◽  
Mikołaj Szafran
Keyword(s):  
Kinetic Energy ◽  
Functional Properties ◽  
Raw Materials ◽  
Shear Thickening ◽  
Dissipation Factor ◽  
Solid Content ◽  
Solid Particles ◽  
Raw Material ◽  
Protective Properties ◽  
Shear Thickening Fluids

This work focuses on shear thickening fluids (STFs) as ceramic–polymer composites with outstanding protective properties. The investigation aims to determine the influence of raw material parameters on the functional properties of STFs. The following analyses were used to characterize both the raw materials and the STFs: scanning electron microscopy, dynamic light scattering, matrix-assisted laser desorption/ionization time-of-flight, chemical sorption analysis, rheological analysis, and kinetic energy dissipation tests. It was confirmed that the morphology of the solid particles plays a key role in designing the rheological and protective properties of STFs. In the case of irregular silica, shear thickening properties can be obtained from a solid content of 12.5 vol.%. For spherical silica, the limit for achieving shear thickening behavior is 40 vol.%. The viscosity curve analysis allowed for the introduction of a new parameter defining the functional properties of STFs: the technological critical shear rate. The ability of STFs to dissipate kinetic energy was determined using a unique device that allows pure fluids to be tested without prior encapsulation. Because of this, it was possible to observe even slight differences in the protective properties between different STFs, which has not been possible so far. During tests with an energy of 50 J, the dissipation factor was over 96%.

Microstructure Evolution of Mg-6Zn-6Al Magnesium Alloy during Semi-Solid Partial Remelting

2011 ◽  
Vol 306-307 ◽  
pp. 608-612
Author(s):  
Xiao Feng Huang ◽  
K. Feng ◽  
Y. Ma ◽  
F.Y Yan ◽  
Ti Jun Chen
Keyword(s):  
Solid State ◽  
Magnesium Alloy ◽  
Shape Factor ◽  
Solid Particles ◽  
Dendritic Microstructure ◽  
Partial Remelting ◽  
Semi Solid ◽  
Average Size ◽  
Microstructure Of The Material ◽  
Holding Temperature

A new magnesium alloy, named as Mg-6Zn-6Al(ZA66), using for thixoforming production has been developed. The microstructure of the material during partial remelting holding in the semi-solid state was characterized. The results indicate that non-dendrite microstructure in ZA66 magnesium alloy billets can be obtained, but the proper partial remelting temperature and holding time should be select. After being treated at 575°Cfor 20 min, the ZA66 alloys can obtain a non-dendritic microstructure with finer unmelted primary solid particles (37 μm) and shape factor about 0.6. With the increasing holding temperature from 575°C to 590°C,the average size of unmelted primary solid particles increases and globular tendency becomes more obvious.

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