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1.
Nickel aluminide intermetallics (e.g., Ni3Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net
shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes
directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic
compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation
observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were
obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus
cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be
minimized or eliminated by the use of a N2-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated
from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the
substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient
microstructure were also produced by the in-situ reactive processing. 相似文献
2.
A study of laser energy transfer efficiency, melting efficiency, and deposition efficiency has been conducted for the laser-engineered
net-shaping process (LENS) for H-13 tool steel and copper powder deposits on H-13 tool steel substrates. This study focused
on the effects of laser deposition processing parameters (laser power, travel speed, and powder mass flow rate) on laser beam
absorption by the substrate material. Measurements revealed that laser energy transfer efficiency ranged from 30 to 50 pct.
Laser beam coupling was found to be relatively insensitive to the range of processing parameters tested. Melting efficiency
was found to increase with increasing laser input power, travel speed, and powder mass flow rate. A dimensionless parameter
model that has been used to predict melting efficiency for laser beam welding processing was investigated for the LENS process.
From these results, a semiempirical model was developed specifically for the LENS processing window. Deposition efficiency
was also investigated and results show that under optimum processing conditions, the maximum deposition efficiency was approximately
14 pct. A semiempirical relation was developed to estimate deposition efficiency as a function of process efficiencies and
LENS processing parameters. Knowledge of LENS process efficiencies measured in this study is useful to develop accurate heat
flow and solidification models for the LENS process. 相似文献
3.
TiAl-based titanium aluminide alloys and their composites reinforced with ceramic particles are considered to be important
candidate materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a layered manufacturing
process, which involves laser processing fine powders into three-dimensional components directly from a computer-aided design
(CAD) model. In this work, the LENS process was employed to fabricate carbide-particle-reinforced titanium aluminide-matrix
composites using TiC and gas-atomized Ti-48Al-2Cr-2Nb powders as the feedstock materials. The composites deposited by the
LENS process were susceptible to solid-state cracking due to high thermal stresses. The microstructures of the laser-deposited
monolithic and composite titanium aluminide materials were characterized using light optical microscopy (LOM), scanning electron
microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) analysis, electron-probe microanalysis (EPMA), and X-ray diffraction
(XRD) techniques. Effects of the LENS processing parameters on the cracking susceptibility and microstructure were studied.
Crack-free deposits were fabricated by preheating the substrate to 450 °C to 500 °C during LENS processing. The fabricated
composite deposits exhibit a hardness of more than twice the value of the Ti-6Al-4V alloy. 相似文献
4.
TiAl-based titanium aluminide alloys and their composites reinforced with ceramic particles are considered to be important
candidate materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a layered manufacturing
process, which involves laser processing fine powders into three-dimensional components directly from a computer-aided design
(CAD) model. In this work, the LENS process was employed to fabricate carbide-particle-reinforced titanium aluminide-matrix
composites using Tic and gas-atomized Ti−48Al−2Cr−2Nb powders as the feedstock materials. The composites deposited by the
LENS process were susceptible to solid-state cracking due to high thermal stresses. The microstructures of the laser-deposited
monolithic and composite titanium aluminide materials were characterized using light optical microscopy (LOM), scanning electron
microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) analysis, electron-probe microanalysis (EPMA), and X-ray diffraction
(XRD) techniques. Effects of the LENS processing parameters on the cracking susceptibility and microstructure were studied.
Crack-free deposits were fabricated by preheating the substrate to 450 °C to 500 °C during LENS processing. The fabricated
composite deposits exhibit a hardness of more than twice the value of the Ti−6Al−4V alloy. 相似文献
5.
Laser-engineered net shaping (LENS) is a solid freeform fabrication process that has the capability of producing functionally
graded material (FGM) components by selectively depositing different powder materials in the melt pool at specific locations
in the structure during part buildup. The composition in each layer of an FGM is dependent upon the degree of dilution between
the substrate (or previous layer) and powder material. A study on the effects of LENS processing parameters (laser power,
travel speed, and powder mass flow rate) on dilution was conducted for deposits of H-13 tool steel and copper powder on H-13
tool steel substrates. When varying a single processing parameter while holding all others constant, the dilution was found
to increase with increasing laser input power and travel speed and decrease with increasing powder mass additions into the
melt pool. A method for estimating dilution in LENS deposits was developed from knowledge of LENS process efficiencies and
material thermophysical properties. A reasonable correlation was shown to exist between the experimentally measured dilution
and the dilution calculated from the model. 相似文献
6.
Mingjun Li Takehiko Ishikawa Shinichi Yoda Kosuke Nagashio Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》2005,36(11):3254-3257
Employing an electrostatic levitator (ESL) equipped with a CO2 laser heating setup, we solidified Ni99B1 bulk crystals through containerless processing at high undercoolings and observed grain-refined microstructures. The electron
backscatter diffraction pattern (EBSP) and analysis of the twin directions were accomplished, from which the primary growth
traces with a cellular-like structure were revealed on a macro-millimeter scale. In comparison with the strong mechanical
electromagnetic stirring in a sample processed on an electromagnetic levitator, the ESL provides a quite quiescent state for
the melt, which enables identification of the primary growth traces after solidification. The present observation supplied
experimental evidence that the refined microstructure in the Ni99B1 alloys at the high undercooling regime was due to fragmentation of the primary growth crystal, rather than dynamic nucleation. 相似文献
7.
The metastable L12-Al3Zr phase has been obtained as a solidification product on melt-spinning ternary Al-X-Zr (X = Cu, Ni) alloys. Different non-equilibrium
effects of the metastable Al3Zr phase (L12) have been observed in the as-solidified and heat-treated alloys. The solidification sequence begins with the formation of
the L12-Al3Zr (cubic) phase as a primary phase, followed by heterogeneous nucleation of α-Al. Morphological changes in the primary phase result in a shape transformation from a faceted cube to one with concave interfaces
and protrusions along the corners, having a preferential growth along the 〈111〉 direction. This is brought about by a kinetic
effect taking place during the growth of the L12-Al3Zr phase into the surrounding liquid, as the alloy is quenched. In another instance, the primary L12-Al3Zr phase nucleates as solid-state precipitates of the same L12-Al3Zr phase on annealing, by dissolution and reprecipitation of solute, under the influence of moving grain boundaries. A third
case shows the metastable L12 phase nucleating on the equilibrium DO23-Al3Zr phase, upon solidification. This is attributed to the sluggish growth kinetics of the latter. 相似文献
8.
In this article, the effect of CaS formation on the evolution of Al2O3-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature
confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate
with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The
shape change was found to be due to the reaction between the Al2O3-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions.
The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell
was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated,
the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary
depression. 相似文献
9.
Abdulhaqq A. Hamid S. C. Jain P. K. Ghosh Subrata Ray 《Metallurgical and Materials Transactions A》2005,36(8):2211-2223
Cast particulate composites, containing in-situ generated reinforcing particles of alumina, have been developed by solidification of slurry obtained by dispersion of externally
added manganese dioxide particles (MnO2) in molten aluminum, and alumina is formed by reaction of manganese dioxide with molten aluminum. The chemical reaction also
releases manganese into molten aluminum. Magnesium is added to the melt in order to help wetting of alumina particles by molten
aluminum and to retain the particles inside the melt. The present work aims to understand the influence of key parameters
such as processing temperature, time, and the amount of MnO2 particles added on the microstructure and mechanical properties of the resulting cast in-situ composites. The sequence of addition of MnO2 particles and magnesium has significant influence on the microstructure and mechanical properties. Increasing processing
temperature and time increases the extent of reduction of MnO2 particles, generating more alumina particles as well as releasing more manganese to the matrix alloy. Alumina helps to nucleate
finer and sometimes blocky MnAl6 in the matrix of the composite and thereby results in relatively higher ductility and increased strength in the composite
as compared to the base alloy of similar composition. Even in the presence of relatively higher porosity of 8 to 9 vol pct,
one observes a percent elongation not below 7 to 8 pct, which is considerably higher than those observed in cast Al(Mg)-Al2O3 composite synthesized by externally added alumina particles. 相似文献
10.
M. Srinivas B. Majumdar G. Phanikumar D. Akhtar 《Metallurgical and Materials Transactions B》2011,42(2):370-379
The effect of planar flow melt spinning (PFMS) parameters on the continuity, surface quality, and structure of 10-mm-wide
Fe68.5Si18.5B9Nb3Cu1 ribbons has been investigated. The change in shape and stability of the melt puddle as a function of the processing parameter
was studied using a high-speed imaging system and was correlated to ribbon formation. A window of process parameters for obtaining
continuous ribbons with good surface quality has been evaluated. It has been observed that thinner ribbons are found to be
more continuous because of higher ductility. The higher melt temperature leads to the formation of crystalline phase in as-spun
ribbons, and this deteriorates the soft magnetic properties on annealing. The experimental results are corroborated with the
numerical estimates, which suggest that the critical thickness for amorphous phase formation decreases with increasing initial
melt temperature. 相似文献
11.
Xiaoming Wang Jie Song Wei Vian Haibin Ma Qingyou Han 《Metallurgical and Materials Transactions B》2016,47(6):3285-3290
In the grain refinement of aluminum, Al3Ti and TiB2 particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB2 particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al3Ti particles in the master alloys. The titanium in solution either forms Al3Ti layers on the surface of TiB2 particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB2/Al3Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al3Ti on the surface of TiB2 particles, a sign of the formation of an Al3Ti layer on the surface of TiB2 particles in grain refinement practice. The Al3Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al3Ti and TiB2 particles leading to the loss of efficiency in grain refinement. 相似文献
12.
Louis FerrantiJr. Naresh N. Thadhani 《Metallurgical and Materials Transactions A》2003,34(11):2671-2678
Two-phase TiB2+Al2O3 ceramics with an interconnected or dispersed TiB2 (minor)-phase microstructure can be produced by variations in processing parameters. A standard method of quantitative characterization
of the microstructural bias, i.e., the degree of TiB2 phase connectivity relative to its dispersion, is necessary to comprehend the mechanism(s) controlling the evolution of microstructure
during processing. In this work, techniques derived from stereology were used to quantitatively characterize the microstructural
bias on the basis of the connectivity and dispersion of the minor phase (TiB2), in addition to the size of the TiB2- and Al2O3-phase regions. The mean integral curvature calculated using the area particle-count and area tangent-count methods was determined to quantitatively describe the connectivity of the TiB2 minor phase around the Al2O3 major phase. The results illustrate that, in spite of partial and mixed bias, integral curvature measurements (particularly
those based on the area tangent-count method) provide a reliable and reproducible means for quantitative characterization
of the two-phase biased microstructure. 相似文献
13.
P. Coursol A. D. Pelton P. Chartrand M. Zamalloa 《Metallurgical and Materials Transactions B》2005,36(6):825-836
The phase diagram of the CaSO4-Na2SO4-Ca3(AsO4)2-Na3(AsO4) system was measured by differential thermal analysis and by an equilibration and quenching technique. Thermodynamic models
were developed giving the Gibbs energies of all phases as functions of temperature and composition. Optimized model parameters
were obtained by assessment of all available thermodynamic and phase equilibrium data. The models, which reproduce all the
data within experimental error limits, were used to calculate the liquidus surface of the system. The modified quasi-chemical
model in the quadruplet approximation was used for the liquid solution. For the various solid solution phases, the modified
quasi-chemical model, which accounts simultaneously for short-range-ordering among first-nearest-neighbor (FNN) and second-nearest-neighbor
(SNN) pairs, was used for the first time within the framework of the compound energy formalism. The distinction between true
model parameters and formalism parameters is made. Implications of the work for the potential use of sulfate fluxes for copper
refining are discussed. 相似文献
14.
W.J. Boettinger D.E. Newbury K. Wang L.A. Bendersky C. Chiu U.R. Kattner K. Young B. Chao 《Metallurgical and Materials Transactions A》2010,41(8):2033-2047
The solidification microstructures of three nine-element Zr-Ni–based AB2 type C14/C15 Laves hydrogen storage alloys are determined. The selected compositions represent a class of alloys being examined
for usage as an MH electrode in nickel metal-hydride batteries that often have their best properties in the cast state. Solidification
is accomplished by dendritic growth of hexagonal C14 Laves phase, peritectic solidification of cubic C15 Laves phase, and
formation of cubic B2 phase in the interdendritic regions. The B2 phase decomposes in the solid state into a complex multivariate
platelike structure containing Zr-Ni–rich intermetallics. The observed sequence C14/C15 upon solidification agrees with predictions
using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. Experimentally,
the closeness of the compositions of the C14 and C15 phases required the use of compositional mapping with an energy dispersive
detector capable of processing a very high X-ray flux to locate regions in the microstructure for quantitative composition
measurement and transmission electron microscope examination. 相似文献
15.
Zongye Ding Qiaodan Hu Wenquan Lu Siyu Sun Mingxu Xia Jianguo Li 《Metallurgical and Materials Transactions A》2018,49(5):1486-1491
The morphology and growth behavior of Al3Ni in the liquid Al/solid Ni interface were observed through synchrotron radiation. The formation time and mechanism of Al3Ni are connected with saturation of the molten layer. In unsaturated conditions, the growth of columnar Al3Ni formed during solidification governed by the melting of small grains accompanied by the growth of adjacent large grains and coalescence of grains near the tips. Conversely, the scallop-type Al3Ni formed in holding showed annexation of adjacent small grains with its morphology changing from scallop to hemisphere. 相似文献
16.
17.
T. D. Anderson J. N. DuPont T. DebRoy 《Metallurgical and Materials Transactions A》2010,41(1):181-193
Autogenous welds on the single-crystal (SX) alloy CMSX-4 were prepared over a wide range of welding parameters and processes
to investigate the formation and behavior of stray grains (SGs). The quantity and location of SGs in the welds were analyzed
by orientation imaging microscopy (OIM). Heat- and fluid-flow modeling was conducted to understand the influence of welding
parameters on the local solidification conditions and resultant SG formation tendency. The results indicate that constitutional
supercooling and SG formation are generally reduced in low-power, high-travel-speed welds. Because of the complex effect of
travel speed on temperature gradient and solidification velocity, the worst conditions for SG formation in alloy CMSX-4 for
the conditions examined here occur at intermediate travel speeds of ~6 mm/s. These findings were corroborated with heat-transfer/fluid-flow
modeling simulations that were coupled with SG predictions. These calculations also indicate that SG formation will be greatest
where different regions of dendrite growth intersect, due to the so-called off-axis heat flow. For a given set of welding
conditions, the amount of SGs will also vary with substrate orientation. This effect is attributed to differences in the number
and location of dendrite growth intersection regions within the melt pool that occur with changes in substrate orientation. 相似文献
18.
Seiji Miura Miki Aoki Yasuhiko Saeki Kenji Ohkubo Tetsuo Mohri Yoshinao Mishima 《Metallurgical and Materials Transactions A》2005,36(3):489-496
The effect of Zr on the formation of Nb/Nb5Si3 lamellar microstructure by eutectoid decomposition reaction of Nb3Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb3Si phase into Nb and Nb5Si3 phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation
(TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition
of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role
of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs)
and Zr distribution in the parent Nb3Si phase during solidification.
This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place
March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects
Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory
Metals Committee. 相似文献
19.
20.
The crystallization kinetics of CaO-SiO2-Al2O3-MgO (CSAM) slags was studied with the aid of single hot thermocouple technique (SHTT). Kinetic parameters such as the Avrami exponent (n), rate coefficient (K), and effective activation energy of crystallization (E A ) were obtained by kinetic analysis of data obtained from in situ observation of glassy to crystalline transformation and image analysis. Also, the dependence of nucleation and growth rates of crystalline phases were quantified as a function of time, temperature, and slag basicity. Together with the observations of crystallization front, they facilitated establishing the dominant mechanisms of crystallization. In an attempt to predict crystallization rate under non-isothermal conditions, a mathematical model was developed that employs the rate data of isothermal transformation. The model was validated by reproducing an experimental continuous cooling transformation diagram purely from isothermal data. 相似文献