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1.
Experimental determination of mushy zone permeability in aluminum-copper alloys with equiaxed microstructures 总被引:1,自引:0,他引:1
Ø. Nielsen Sintef L. Arnberg A. Mo H. Thevik 《Metallurgical and Materials Transactions A》1999,30(9):2455-2462
An experimental apparatus for measuring the mushy zone permeability of aluminum-copper alloys with equiaxed microstructures
has been constructed. Permeabilities have been measured for high solid fractions (0.68 to 0.91) and different dendrite morphologies.
Microstructure characterizations on both the interdendritic and extradendritic length scales have been performed on the samples.
The results are in fairly good agreement with the predictions of the Kozeny-Carman relation and with more recent theory that
takes flow partitioning between interdendritic and extradendritic regions into account. 相似文献
2.
Jie Wanqi 《Metallurgical and Materials Transactions B》1994,25(5):731-739
After a review over former works about the solute redistribution during dendritic solidification, a new“local solute redistribution
equation ”is deduced based on Flemings's model, where lim-ited diffusion in solid during solidification is carefully treated.
Because a form parameter is also included, the equation can be used for the solidification processes with different shapes
of den-drites. By solving the equation at the condition of directional solidification, more completef
s
-C, functions for both needlelike and platelike dendritic solidifications with both linear and parabolic solidification rates
are obtained. As examples, the volume fractions of nonequilibrium phase in Al-4.5 pct Cu alloy is evaluated with differentf
s
-C
l
functions. On the thinking that the dendrites in actual solidification process is usually between needlelike and platelike
ones, the volume fraction of the nonequilibrium phase is suggested to be in the region between the one calculated by the model
for platelike dendrites and that for needlelike dendrites. The relationship between the region and local solidification time
is also presented by figures, which are compared with the data of former researchers. 相似文献
3.
4.
Measurements of liquid permeability in the mushy zones of Al-15.42 pct Cu and Al-8.68 pct Cu alloy samples were performed
isothermally just above the eutectic temperature, using eutectic liquid as the fluid. A modified method was developed to determine
the specific permeability as a function of time (K
s) during the test from the data collected on these alloys. Factors affecting permeability measurements are discussed. The
permeabilities are observed to vary throughout the experiment. This is attributed to microstructural coarsening and channeling
that occur in the sample during the experiment. Coarsening rates are determined for the isothermal coarsening tests without
fluid flow, and the results are observed to be less than the rates indicated from permeability tests where fluid flow is present.
Careful measurement of the volume fraction of liquid (g
L) shows that g
L decreases during the test. The permeability is then related to the microstructure of the sample using the Kozeny-Carman equation.
The correlation between the measured K
S, g
L, and specific solid surface area (S
V) improves markedly when compared to previous studies, when microstructural parameters at the initial stage of the test are
used. 相似文献
5.
J. N. DuPont 《Metallurgical and Materials Transactions A》2006,37(6):1937-1947
A simple mathematical framework is provided for calculating solidification paths of ternary alloys for equilibrium, paraequilibrium,
and nonequilibrium conditions. The results are useful for estimating the possible range of primary solidification paths, the
type of monovariant reaction expected to occur, and the relative amount of primary, monovariant eutectic and ternary eutectic
constituents that form during solidification. Example calculations show that, for solidification of a ternary eutectic alloy
under equilibrium conditions, the monovariant eutectic reaction will only occur when the nominal alloy concentration is above
the maximum solid solubility. This result is similar to the binary case. For paraequilibrium conditions, a monovariant eutectic
reaction is always expected to occur, but solidification can terminate before the invariant ternary eutectic reaction is reached.
Finally, for nonequilibrium conditions, both the monovariant and invariant ternary eutectic reactions are always expected
to occur, which is similar to the binary nonequilibrium case. The direction of liquid enrichment on the monovariant eutectic
line can also be determined from the results. 相似文献
6.
An analytical heat transfer model is presented that describes the temperature distribution and the positions of solidus and liquidus isotherms in the unidirectional solidification of binary alloys. The proposed technique employs the mathematical expedient of replacing the interfacial thermal resistance by equivalent layers of material. The application of the model is demonstrated by comparison with experimental data and with a finite difference method. 相似文献
7.
Aage Stangeland Asbjørn Mo Dmitry Eskin 《Metallurgical and Materials Transactions A》2006,37(7):2219-2229
The parameters in a recently developed constitutive equation for macroscopic thermal strain in the mushy zone have been determined
for the commercial alloys A356, AA2024, AA6061, and AA7075 in addition to an Al-4 wt pct Cu alloy. The constitutive equation
for macroscopic thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone
at low solid fractions and that the thermal strain in the mushy zone approaches thermal strain in the fully solid material
as the solid fraction increases toward 1. The development of thermal strain in the mushy zone is determined by combining experimentally
measured contraction of a cast sample with thermomechanical stimulations. Experiments were performed at cooling rates in the
range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract,g
s
th, are in the range from 0.63 to 0.94. Grain refinement increasesg
s
th for all the tested alloys. For most of the tested alloys the thermal strain in the mushy zone increases rapidly to the same
level as thermal strain in fully solid material once the solid fraction becomes higher thang
s
th. 相似文献
8.
Conclusions In the stage of growth of Al-Cu compacts during liquid-phase sintering diffusion zones of solid solutions form on the surfaces of the aluminum particles, while a liquid phase appears at the grain boundaries. At a copper concentration in the mixture within the limits of solid-phase solubility the structure of the compacts does not change during holding at the sintering temperature. In the shrinkage stage, when the liquid phase is retained during the whole period of sintering, a polyhedral structure is formed, whose grain increases in size with increasing duration of the process. The principal mechanism of formation of the polyhedral cast structure is apparently crystallization of the melt on the undissolved particles during cooling. The subsequent coarsening of the structure with time during isothermalsintering is due to recrystallization of the solid phase through the melt.Translated from Poroshkovaya Metallurgiya, No. 3(279), pp. 19–23, March, 1986. 相似文献
9.
C. J. Paradies R. N. Smith M. E. Glicksman 《Metallurgical and Materials Transactions A》1997,28(3):875-883
Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying
in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt
was made to relate the results to the controllable parameters of the system. A microscope-video system recorded the mushy
zone-melt interface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis
of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates,
and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all
runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface.
Fragmentation rates appear to correlate well with the magnitude of particle velocities near the interface, with increasing
fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation
is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation
are discussed. Although it appears that either constitutional remelting or capillary pinching are likely of importance, hydrodynamic
shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence
suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The
results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone
as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings. 相似文献
10.
C. J. Paradies R. N. Smith M. E. Glicksman 《Metallurgical and Materials Transactions A》1997,28(13):875-883
Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying
in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt
was made to relate the results to the controllable parameters of the system. A microscope-video system recorded the mushy
zone-melt interface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis
of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates,
and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all
runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface.
Fragmentation rates appear to correlate well with the magnitude of particle velocities near the interface, with increasing
fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation
is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation
are discussed. Although it appears that either constitutional remelting or capillary pinching are likely of importance, hydrodynamic
shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence
suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The
results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone
as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings.
C. J. PARADIES, formerly Graduate Research Assistant, Rensselaer Polytechnic Institute 相似文献
11.
12.
13.
分析提出了连铸流动与凝固耦合数值模拟中, 钢液在两相区流动时的糊状区系数(Amush)与渗透率的关系; 通过建立大方坯连铸结晶器三维耦合数值模型, 揭示了不同糊状区系数对钢液流动、传热与凝固进程的影响, 以及早期相关研究结果差异的源头.结果表明: 糊状区系数越大, 钢液在糊状区内的流动阻力越强, 凝固时钢液流动速度降低越快.采用较大的糊状区系数时, 糊状区呈较窄的"带状"分布在固液相之间; 当糊状区系数较小时, 糊状区范围变大, 钢液在结晶器内温降过快, 自由液面处出现过冷现象, 凝固坯壳局部发生重熔.结合实验数据验证与模型分析, 认为糊状区系数取值1×108~5×108 kg·m-3·s-1可以较可靠地揭示连铸结晶器内的实际凝固现象. 相似文献
14.
S. D. Felicelli D. R. Poirier J. C. Heinrich 《Metallurgical and Materials Transactions B》1998,29(4):847-855
The formation of macrosegregation defects known as “freckles” was simulated using a three-dimensional finite element model
that calculates the thermosolutal convection and macrosegregation during the dendritic solidification of multicomponent alloys.
A recently introduced algorithm was used to calculate the complicated solidification path of alloys of many components, which
can accommodate liquidus temperatures that are general functions of liquid concentrations. The calculations are started from
an all-liquid state, and the growth of the mushy zone is followed in time. Simulations of a Ni-Al-Ta-W alloy were performed
on a rectangular cylinder until complete solidification. The results reveal details of the formation of freckles not previously
observed in two-dimensional simulations. Liquid plumes in the form of chimney convection emanate from channels within the
mushy zone, with similar qualitative features previously observed in transparent systems. Associated with the formation of
channels, there is a complex three-dimensional flow produced by the interaction of the different solutal buoyancies of the
alloy solutes. Regions of enhanced solid growth develop around the channel mouths, which are visualized as volcanoes on top
of the mushy zone. The prediction of volcanoes differs from our previous calculations with multicomponent alloys in two dimensions,
in which the volcanoes were not nearly as apparent. These and other features of freckle formation phenomena are illustrated. 相似文献
15.
The formation of macrosegregation in a rectangular ingot with reduced cross section from the riser to the casting, chilled
from the bottom, has been studied numerically. In addition to positive inverse segregation occurring near the chilled surface,
very severe negative segregation around the under-riser region and moderate positive segregation near the top corners of the
casting were found. Although large circulating vortexes are created by natural convection in the under-riser region during
the early stage of solidification, the fluid flow in the mushy zone is dominated by solidification shrinkage. As a result,
the final solute distribution in the casting is determined by the flow of solute-rich liquid in the mushy zone owing to the
combined effects of solidification shrinkage and change of cross section from casting to riser. Detailed explanations regarding
the effect of different flow phenomena on the formation of the segregations are provided. The effects of riser size and cooling
condition at the bottom of the ingot on the formation of macrosegregation also were studied. The predicted negative and positive
macrosegregations in the casting compared very well with the available experimental data. 相似文献
16.
An analytical model for solute redistribution during solidification of planar,columnar, or equiaxed morphology 总被引:8,自引:0,他引:8
Existing models for solute redistribution (microsegregation) during solidification were reviewed. There are no analytical
models that take into account limited diffusion in both the liquid and the solid phases. A new analytical mathematical model
for solute redistribution was developed. Diffusion in liquid and in solid was considered. This model does not require a prescribed
movement of the interface. It can be used for one-dimensional (1-D) (plate), two-dimensional (cylinder), or three-dimensional
(3-D) (sphere) calculations. Thus, it is possible to calculate microsegregation at the level of primary or secondary arm spacing
for columnar dendrites or for equiaxed dendrites. The solution was compared with calculations based on existing models, as
well as with some available experimental data for the segregation of base elements in as cast Al-4. 9 wt pct Cu, INCONEL 718,
625, and plain carbon (0. 13 wt pct C) steel. 相似文献
17.
Lilia C. Nicolli Asbjørn Mo Mohammed M’Hamdi 《Metallurgical and Materials Transactions A》2005,36(2):433-442
A two-phase volume-averaged continuum model is presented that quantifies macrosegregation formation during solidification
of metallic alloys caused by deformation of the dendritic network and associated melt flow in the coherent part of the mushy
zone. Also, the macrosegregation formation associated with the solidification shrinkage (inverse segregation) is taken into
account. Based on experimental evidence established elsewhere, volumetric viscoplastic deformation (densification/dilatation)
of the coherent dendritic network is included in the model. While the thermomechanical model previously outlined (M. M’Hamdi,
A. Mo, and C.L. Martin: Metall. Mater. Trans. A, 2002, vol. 33A, pp. 2081–93) has been used to calculate the temperature and velocity fields associated with the thermally
induced deformations and shrinkage driven melt flow, the solute conservation equation including both the liquid and a solid
volume-averaged velocity is solved in the present study. In modeling examples, the macrosegregation formation caused by mechanically
imposed as well as by thermally induced deformations has been calculated. The modeling results for an Al-4 wt pct Cu alloy
indicate that even quite small volumetric strains (≈2 pct), which can be associated with thermally induced deformations, can
lead to a macroscopic composition variation in the final casting comparable to that resulting from the solidification shrinkage
induced melt flow. These results can be explained by the relatively large volumetric viscoplastic deformation in the coherent mush resulting from the applied constitutive model, as well as the relatively large
difference in composition for the studied Al-Cu alloy in the solid and liquid phases at high solid fractions at which the
deformation takes place. 相似文献
18.
19.
Strength development in the mushy zone during solidification of three aluminum alloys (Al-4 wt pct Cu, Al-7 wt pct Si-1 wt
pct Cu, and Al-7 wt pct Si-4 wt pct Cu) has been measured with two different techniques—horizontal tensile testing and direct
shear cell testing. The strength results from the two methods correspond with one another to a much higher degree than suggested
by the results presented in the literature. Tensile strength starts to develop at the maximum packing solid fraction, confirmed
by the shear strength measurements. The maximum packing fraction represents the point where the internal network structure
of the mushy zone is established and ligaments of the network must be broken to rearrange the dendrites. The data indicate
a converging trend of the shear and tensile strength at high solid fractions, therefore indicating that the deformation mechanisms
are also becoming similar. The results presented in this article suggest that it is possible to develop constitutive equations
for the mechanical properties of the mushy zone over the entire solid fraction regime, i.e., from coherency to complete solidification. These equations could be used for the prediction of stress development as well
as defect formation. 相似文献
20.
Menghuai Wu Dr.-Ing. Andreas Ludwig Lorenz Ratke 《Metallurgical and Materials Transactions A》2003,34(12):3009-3019
A two-phase volume averaging approach to model Marangoni-induced droplet motion of the minority liquid phase and the convection
in the parent melt during solidification of the hypermonotectic alloys is presented. The minority liquid phase decomposed
from the parent melt as droplets in the miscibility gap was treated as the second-phase L
2. The parent melt including the solidified monotectic matrix was treated as the first phase L
1. Both phases were considered as different and spatially interpenetrating continua. The conservation equations of mass, momentum,
solute, and enthalpy for both phases, and an additional transport equation for the droplet density, were solved. Nucleation
of the L
2 droplets, diffusion-controlled growth, interphase interactions such as Marangoni force at the L
1-L
2 interface, Stokes force, solute partitioning, and heat release of decomposition were taken into account by corresponding
source and exchange terms in the conservation equations. The monotectic reaction was modeled by adding the latent heat on
the L
1 phase during monotectic reaction, and applying an enlarged viscosity to the solidified monotectic matrix. A two-dimensional
(2-D) square casting with hypermonotectic composition (Al-10 wt pct Bi) was simulated. This paper focused on Marangoni motion,
hence gravity was not included. Results with nucleation, droplet evolution, Marangoni-induced droplet motion, solute transport,
and macrosegregation formation were obtained and discussed. 相似文献