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1.
S. Otarawanna C. M. Gourlay H. I. Laukli A. K. Dahle 《Transactions of the Indian Institute of Metals》2009,62(4-5):499-503
In order to optimise the high pressure die casting (HPDC) process, more understanding of microstructure and defect formation is essential. This article gives an overview of the key microstructural features and the mechanisms of microstructure formation in this process. The incavity solidified grain size in HPDC can be as low as 10 μm, but externally solidified crystals (ESCs) as large as 200 μm are often also present. The eutectic microstructure is very fine with an interlamellar spacing around 500 nm. Bands of positive macrosegregation and sometimes with cracks/porosity, so-called defect bands, are also often observed. Intensification pressure (IP) is one of the major factors governing the porosity level in the casting. At high IP, defect bands form in the gate region and appear to be assisting the feeding during the intensification stage. 相似文献
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Christopher M. Gourlay Arne K. Dahle Hans I. Laukli 《Metallurgical and Materials Transactions A》2004,35(9):2881-2891
Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears,
and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic
nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters
on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys
were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect
of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference
to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering
flow past a solidifying immobile wall layer. 相似文献
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Macro-microscopic models have been developed to describe the macrosegregation behavior associated with the peritectic reaction
of low carbon steel. The macrosegregation model has been established on the basis of previously published work and experimental
data. A microscopic model of a three-phase reaction L+δ→γ has been modeled by using Fredriksson’s approach. Four horizontal and unidirectional solidified experimental groups simulating
continuous casting have been performed with a low carbon steel containing 0.13 wt pct carbon. The extent of macrosegregation
of carbon was determined by wet chemical analysis of millings. It is confirmed, by comparing calculated results with experimental
results, that this model successfully predicts the occurrence of macrosegregation. The results indicate that a peritectic
reaction which is associated with a high cooling rate generates high thermal contraction and a high tensile strain rate at
the peritectic temperature. Therefore, the macrosegregation, particularly at the ingot surface, is very sensitive to the cooling
rate, where extremely high positive segregation was observed in the case of a high cooling rate. However, in the case of slow
cooling rate, negative segregation was noted. The mechanism of macrosegregation with peritectic reaction is discussed in detail. 相似文献
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An approximate method for calculating the macrosegregation in a multicomponent aluminum alloy is proposed. This method is
based on the use of a predefined solidification path (i.e., relation between the solute concentration in the liquid phase and the solid fraction) instead of addressing the fully coupled
micro-macrosegregation problem. In determining the solidification path, it is assumed that the total solute concentration
is constant, and that the solidification history is the same everywhere in the casting. In this manner it becomes quite easy
to take into account how the macrosegregation development is affected by the solute diffusion in the dendrites and the precipitation
of secondary phases, provided that such effects are accounted for in the model used for determining the solidification path.
In order to demonstrate the approximate method, the inverse segregation formation at a chill surface of an Al-4 pct Mg-0.2
pct Fe-0.15 pct Si-0.3 pct Mn (AA5182) alloy is calculated. In this case study, the solidification path is determined prior
to the macrosegregation computation by a microsegregation model discussed elsewhere, and the solid and liquid densities are
related to the concentrations of the different alloying elements by a simple mixture law without distinguishing between the
different solid phases that are formed. The accuracy of the approximate method is discussed by considering a binary alloy.
It turns out that the macrosegregation formation at a chill surface of an Al-4 pct Mg alloy is fairly close to that resulting
from a modeling in which the variation of the total solute concentration is taken into account. Furthermore, the mixture law
is compared to a more elaborate treatment of the densities involving both primary and eutectic solid phases. This comparison
is carried out for an Al-4.5 pct Cu alloy for which literature data exist. The mixture law is found to give a reasonable accuracy
in the calculated macrosegregation. 相似文献
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M. El-Bealy 《Metallurgical and Materials Transactions B》2000,31(2):331-343
In the first part of this two-part article, mathematical models have been developed to characterize temperature, interdendritic
stain, and segregation distributions during dendritic solidification. This aims to predict the effect of interdendric strain
associated with sudden changes in the cooling conditions on the macrosegregation distributions, i.e., the combined effect of interdendric strain and macrosegregation on the dendritic structure. These theoretical models were
verified on a laboratory scale. Four laboratory ingots of 0.53 and 0.9 wt pct C steels were cast horizontally and unidirectionally
in a static mold under cooling conditions designed to approximate those in the continuous-casting process. Thermocouples recorded
temperatures in the ingot at different locations from copper chill. The ingots were examined for macro-microstructure, and
the extent of carbon macrosegregation was determined by wet chemical analysis. The experimental results indicate that static
mold with sudden changes in the cooling conditions on the copper chill provides an approximately similar structure and macrosegregation
profiles to those in a continuous-casting process. It is concluded that these cooling conditions have a significant effect
on the fluctuated macrosegregation phenomenon. The sudden drop in the heat flux on the chill causes a positive segregation,
whereas a sudden increase in heat flux results in a negative segregation. Also, the metallographic examination shows that
there is high inelastic deformation of the dendrites due to the sudden drop in heat flux on the chill. 相似文献
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The squeeze casting of an Al-7 wt pct Si alloy was carried out in order to investigate the conditions for the formation and
the prevention of macrosegregation. The effects of process parameters such as applied pressure, die temperature, pouring temperature,
delay time, degassing, and inoculation on the formation of macrosegregation were investigated, in correlation with the evolution
of macrostructure and shrinkage defects. Three critical applied pressures were defined, based on the experimental results
for the squeeze-cast Al-7 wt pct Si. The first is the critical applied pressure under which shrinkage defects form (P
SC). The second is the critical applied pressure above which macrosegregates form (P
MS). The third is the critical applied pressure above which and under which minor segregation forms. (P
m and P
MS, respectively). With the concept of these three critical pressures, an experimental diagram describing the optimum process
conditions was proposed for obtaining sound squeeze castings. It was concluded that sound castings without macrosegregation
and shrinkage defects can only be obtained when the applied pressure is in the range where P
SC < P<P
m (<P
MS). Both degassing and inoculation treatments greatly enhanced the soundness of the castings. It was also found that the pouring
temperature and the delay time should not exceed T
D-critical and t
D-critical, respectively, in order to achieve sound castings. 相似文献
11.
Detailed investigations of the salient microstructural features and casting defects of the high-pressure die-cast (HPDC) AlSi9Cu3(Fe) alloy are reported. These characteristics are addressed to the mechanical properties and reliability of separate HPDC tensile bars. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes throughout the tensile specimen. The results indicate that the die-cast microstructure consists of several microstructural heterogeneities such as positive eutectic segregation bands, externally solidified crystals (ESCs), cold flakes, primary Fe-rich intermetallics (sludge), and porosities. In addition, it results that sludge particles, gas porosity, as well as ESCs, and cold flakes are concentrated toward the casting center while low porosity and fine-grained structure is observed on the surface layer of the castings bars. The local variation of the hardness along the cross section as well as the change of tensile test results as a function of gage diameter of the tensile bars seem to be ascribed to the change of porosity content, eutectic fraction, and amount of sludge. Further, this behavior reflects upon the reliability of the die-cast alloy, as evidenced by the Weibull statistics. 相似文献
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Haijie Zhang Menghuai Wu Yongjian Zheng Andreas Ludwig Abdellah Kharicha 《Transactions of the Indian Institute of Metals》2018,71(11):2639-2643
A unidirectional solidification experiment of hypoeutectic Al-7.0 wt% Si alloy against gravity direction in a cylindrical mold with cross-sectional change was made, and the macrosegregation in different parts of the as-solidified sample was investigated (Ghods et al. in J Cryst Growth 441:107–116, 2016; J Cryst Growth 449:134–147, 2016). The current study is to use a two-phase columnar solidification model to analyze the segregation mechanisms as used in this experiment. Following flow phenomena and their contributions to the formation of macrosegregation are simulated and compared: (1) solidification shrinkage-induced feeding flow; (2) thermo-solutal convection; and (3) combined thermo-solutal convection and shrinkage-induced feeding flow. The shrinkage-induced feeding flow leads to an inverse (positive) segregation in the bottom part, and a severe negative segregation in the part below cross-sectional change. Thermo-solutal buoyancy leads to a so-called steepling convection in the main part of the sample (away from the bottom and cross-sectional change), and this kind of flow leads to a positive macrosegregation near the sample surface. The calculations have successfully explained the experimental result of macrosegregation. 相似文献
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Sindo Kou David R. Poirier Merton C. Flemings 《Metallurgical and Materials Transactions B》1978,9(4):711-719
A computer model is presented for predicting macrosegregation in rotated electroslag or vacuum arc remelted ingots. Sample calculations of segregation are carried out for ingots of the model alloy Sn-12 pet Pb in which the liquid density increases during solidification and for two hypothetical alloys; in one, the liquid density decreases during solidification, and in the other, liquid density first increases and then decreases during solidification. In alloys such as Sn-Pb in which liquid density increases during solidification, segregation is positive at the ingot centerline and if solidification is sufficiently slow, “freckles” form near the centerline. Positive segregation and freckles are found at the outer periphery of the ingot when liquid density decreases during solidification. Positive segregation and freckles are found at midradius when liquid density first increases and then decreases during solidification, and when the solidus isotherm changes shape abruptly at midradius (with density increasing during solidification). Ingot rotation, by introducing a radial component to the force field, alters interdendritic flow behavior and therefore macrosegregation. Modest rotation speeds eliminate “freckles” and reduce macrosegregation in all modeling studies conducted. Greater rotational speeds can accentuate the segregation. Experiments were conducted on simulated remelted ingots of Sn-Pb alloy. The ingots were 8 cm diam, rotated at speeds up to 119 rpm and solidified at rates from 5.3 × 10?3 to 1.36 × 10?2 cm/s. Segregation behavior obtained agrees qualitatively and quantitatively with theory. 相似文献
14.
Philip N. Adler Robert L. Schulte Harold Margolin 《Metallurgical and Materials Transactions A》1990,21(7):2003-2007
Deuterium surface segregation has been investigated in α, α + β, and β-phase titanium that were deuterium charged over the
range of 2 to approximately 300 wppm. Surface segregation was observed in samples that were essentially α-phase materials,i.e., high-purity commercial α-Ti, Ti-6A1, and Ti-3A1-2.5V, whereas Ti-6A1-4V had slight enrichment and β-Ti-13Mn had no detectable
segregation. Nuclear reaction analysis (NRA) techniques were used to measure the near-surface deuterium concentration, and
the segregation has been localized to within 50 nm of the surface. The time-dependent increase of deuterium at the surface
is consistent with deuterium diffusion from the bulk to the surface and a room-temperature diffusivity of approximately 3
× 10-9 cm2/s. Surface enrichment in excess of 30 times the bulk concentration was observed in charged samples and in excess of 60 times
for samples that had been charged and then vacuum annealed. Polishing was found to be of importance in causing segregation.
The presence of deuterides or a surface defect state is suggested to explain the deuterium surface enrichment. 相似文献
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S. Otarawanna H.I. Laukli C.M. Gourlay A.K. Dahle 《Metallurgical and Materials Transactions A》2010,41(7):1836-1846
This work focuses on understanding the feeding behavior during high-pressure die casting (HPDC). The effects of intensification
pressure (IP) and gate thickness on the transport of material through the gate during the latter stages of HPDC were investigated
using an AlSi3MgMn alloy. Microstructural characterization of the gate region indicated a marked change in feeding mechanism
with increasing IP and gate size. Castings produced with a high IP or thick gate contained a relatively low fraction of total
porosity, and shear band-like features existed through the gate, suggesting that semisolid strain localization in the gate
is involved in feeding during the pressure intensification stage. When a low IP is combined with a thin gate, no shear band
is observed in the gate and feeding is less effective, resulting in a higher level of porosity in the HPDC component. Although
shear banding through the gate was found to reduce porosity in HPDC parts, if gates are not properly designed, deformation
of the mushy zone through the gate can cause severe macrosegregation, large pores, and large cracks, which could severely
reduce the performance of the component. 相似文献
16.
P.A. Nikrityuk K. Eckert R. Grundmann 《Metallurgical and Materials Transactions B》2009,40(3):317-327
This article considers the nondirectional solidification of a binary-metal alloy in a cylindrical cavity, which is cooled
along its outer vertical wall and the bottom. To study the influence of convection within the liquid phase on the final segregation,
three cases are examined: the purely buoyancy-driven convection (case 0), the impact of an external steady axial magnetic
field on the melt flow during solidification (case 1), and the effect of the combination of an external magnetic field with
a steady electrical current (DC) applied directly to the melt (case 2). The results show that convection in the form of multivortices
caused by the thermosolutal buoyancy leads to macrosegregations in the form of V-channels. The application of an external
axial magnetic field alone suppresses the multivortex structure and, thus, the macrosegregation. By contrast, the parallel
use of an additional voltage of 10−3 V leads to an increase in final macrosegregation.
This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC
2007), which occurred in September 2007 in Nancy, France. 相似文献
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Deepika Sachdeva Shashank Tiwari Suresh Sundarraj Alan A. Luo 《Metallurgical and Materials Transactions B》2010,41(6):1375-1383
Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles.
This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures
between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype
control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC
AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control
arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development
by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural
parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys
that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior. 相似文献
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Macrosegregation has been observed in a series of Pb-Sn alloys isothermally treated in an α + L (or β + L) two-phase region. Depending on the alloy composition, the Pb-rich α (or the Sn-rich β) solid phase quickly settles (or floats) to the bottom (or top) of the crucible as the alloy is heated into the solid-plus-liquid
two-phase region. This settling or floating persists until such time as a skeletal solid structure is formed at the crucible
bottom or top. The skeleton (the “mushy” zone) resembles liquid-phase-sintered structures; that is, it consists of interconnected
solid and liquid phases. The initial settling can give rise to macrosegregation. Elimination of macrosegregation requires
substantial time and is accompanied by a reduction in length of the mushy zone, giving rise to apparent sedimentation of this
zone. We argue that this sedimentation results from the compositional readjustments accompanying macrosegregation elimination.
On this basis, it appears that many observations of sedimentation in liquid-phase-sintered structures are due to these readjustments. 相似文献