Preparation of metallic semisolid slurries using the cooling slope method is increasingly becomin... more Preparation of metallic semisolid slurries using the cooling slope method is increasingly becoming popular because of the simplicity of design and control of the process. Microstruc-tural features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast micro-structure.
An optical semisolid simulating device was developed and used to study the morphological evolutio... more An optical semisolid simulating device was developed and used to study the morphological evolution of 99% pure succinonitrile alloy under forced fluid flow. The real time microscopic observations and flow analyses at different locations in the simulating device help to clarify some conflicting claims about the relative importance of the shear rate and turbulent flow on the formation of globular microstructure. It was revealed that the presence of the turbulent flow rather than shear rate is the critical factor for the formation of globular particles during semisolid solidification. This new finding can be practically significant in the development of new and more effective rheocasting methods.
Growth morphology of succinonitrile crystals under different cooling and stirring conditions was
... more Growth morphology of succinonitrile crystals under different cooling and stirring conditions was experimentally investigated by real time microscopic observation of the solidifying melt. It was found that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions the growth of solid particles followed a spherical mode with a high growth velocity from the initial stages of solidification. When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy. The new findings can be used for more effective refinement of rheocast microstructures.
The purpose of this study was to develop a new method for formation of non-dendritic
microstructu... more The purpose of this study was to develop a new method for formation of non-dendritic microstructure during sand casting of aluminum alloys without any previous special processing. For this purpose, sand molds with three different types of running systems were designed. The first design included a traditional reverse tapered vertical sprue. The second design was similar to the first one except for a conical sand core placed inside the lower half portion of the sprue creating a narrow gap for the flow of the melt into the mold. In the third design, the core was fitted with metal chills. Molten A356 alloy was then cast from fully liquid state under gravity and centrifugal casting conditions in the molds and the microstructures of the castings were studied. The results showed that non-dendritic microstructure could be achieved by using the last design under low superheat temperature and centrifugal casting conditions. The findings can pave the way for small quantity production of semisolid castings in expendable molds.
A theoretical model has been developed to investigate the effects of fluid flow on the dendrite
c... more A theoretical model has been developed to investigate the effects of fluid flow on the dendrite coarsening. The accelerated convective solute transport due to convection is considered by incorporating the new apparent diffusivity parameter into the classical coarsening model for stagnant melts. The model predicted that the dendrite arm coarsening followed the d~c1 :c 0:12t0:37 1 and d~c2 :c 0:26t0:42 1 relationships under laminar and turbulent fluid flow conditions respectively. The model predictions are found to be in good quantitative agreement with experimental data in literature.
Morphological evolution of a transparent model succinonitrile (SCN) material during
solidificatio... more Morphological evolution of a transparent model succinonitrile (SCN) material during solidification was investigated in an apparatus resembling a shearing-disc viscometer. The in situ microscopic observations showed that fragmentation decreased the average particles size, but did not result in transition of dendritic to spherical morphology. At low shear rates, the degenerated dendrites and at high shear rates, the pseudo-cluster morphology was observed. It was revealed that coarsening has the most important effect on the final morphology of solid particles. The quantitative influences of shearing rate and intensity on the size and morphology of solid crystals were also discussed based on the measurements on the microstructures.
A new criterion for prediction of hot tearing susceptibility
of cast alloys is suggested which ta... more A new criterion for prediction of hot tearing susceptibility of cast alloys is suggested which takes into account the effects of both important mechanical and metallurgical factors and is believed to be less sensitive to the presence of volume defects such as bifilms and inclusions. The criterion was validated by studying the hot tearing tendency of Al-Cu alloy. In conformity with the experimental results, the new criterion predicted reduction of hot tearing tendency with increasing the copper content.
This study introduces a new apparatus for quantitative assessment of hot tearing tendency in cast... more This study introduces a new apparatus for quantitative assessment of hot tearing tendency in casting alloys. In this method, molten metal is cast in a T-shaped mold cavity. Each arm of the T-shaped casting is hooked onto a rigid flask which restrains its free contraction during solidification. One of the hooks connects to a load cell which enables the real-time measurement of the contraction load during the solidification process as well as the load-time curve of each experiment. Temperature-time curves are also plotted using K-type thermocouples placed in the casting hot spot and connected to a data-acquisition system. Through the use of this set up, it is possible to estimate the solid fraction at which hot tearing occurs. Experiments were conducted with Al-9 %Si alloy to investigate the accuracy of the apparatus and modify its operation. The influence of the mold thermal gradient on the load-time curve and the hot tearing severity were investigated. Microscopic study of the observed hot tear regions was also performed to characterize the hot tearing characteristics.
Melt squeezing process in semisolid state was used for the first time for production of open-cell... more Melt squeezing process in semisolid state was used for the first time for production of open-cell Al–Si foams with improved microstructural and mechanical characteristics. First a given amount of preheated NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing temperature, melt to salt ratio and piston pressure for production of uniform and high integrity foams were obtained to be 3400 lm, 605 C, 1.5:1 and 10 MPa, respectively. The results showed that the semisolid processing employed could refine and modify the microstructure of the cell ligaments in the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness were also improved by the semisolid processing.
Preparation of metallic semisolid slurries using the cooling slope method is increasingly becomin... more Preparation of metallic semisolid slurries using the cooling slope method is increasingly becoming popular because of the simplicity of design and control of the process. Microstructural features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast microstructure.
To investigate the effects of curing condition on the surface characteristics of polyurethane coa... more To investigate the effects of curing condition on the surface characteristics of polyurethane coatings, the prepared coatings had undergone different curing conditions in which the rate of solvents evaporation differed. Differential scanning calorimeter, Fourier transform infrared spectroscope, atomic force microscope, and contact angle measurement system were used to characterize the coating solution as well as the cured coatings. The results showed that the curing condition substantially influenced the samples’ surface morphology and can be used as a parameter for precise engineering of surface pattern. The highest contact angle of 103◦ was obtained for the ambient cured sample.
A356 aluminum alloys reinforced with carbon nano-tubes (CNTs) were produced by stir casting and c... more A356 aluminum alloys reinforced with carbon nano-tubes (CNTs) were produced by stir casting and compocasting routes and their microstructural characteristics and hardness were examined. In order to alleviate the problems associated with poor wettability, agglomeration and gravity segregation of CNTs in the melt, CNTs were introduced into the melts by injection of CNT deposited aluminum particles instead of raw CNTs. Aluminum particles with mean diameters of less than 100 μm were first deposited by CNTs using Ni-P electroless plating technique and then injected into the melt agitated by a mechanical stirrer. The slurry was subsequently cast at temperatures corresponding to full liquid as well as 0.15 and 0.30 solid fractions. The results show that addition of CNTs to A356 matrix can significantly refine both full liquid and semi-solid cast microstructures. Hardness of the samples is also significantly increased by addition of CNTs and A356-CNT composite cast at 0.3 solid fraction produces the highest hardness.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2008
In the present study, in-situ Al matrix composites containing alumina particles were synthesized ... more In the present study, in-situ Al matrix composites containing alumina particles were synthesized by a new method that involved injection of an activated powder mixture into the molten aluminum, which was subsequently squeeze cast to minimize its porosity. The activated powder mixture was prepared by milling of powders of commercially pure aluminum and zinc oxide for a predetermined time. Milling for 60 minutes led to optimum activation of the reactant powders. Such activation reduced the reaction temperature from above 1000 °C to about 640 °C. Microstructural characterization studies revealed the formation of submicron size alumina particles with near equiaxed morphology and good particle/matrix interface. The composite specimen synthesized using the activated powder injection (API) method exhibited higher hardness, greater yield and tensile strengths, and lower elongation when compared to the monolithic specimen of similar composition. This article places particular emphasis on explaining the uniqueness of the processing methodology used in this study for synthesis of in-situ Al/Al2O3 composites.
Preparation of metallic semisolid slurries using the cooling slope method is increasingly becomin... more Preparation of metallic semisolid slurries using the cooling slope method is increasingly becoming popular because of the simplicity of design and control of the process. Microstruc-tural features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast micro-structure.
An optical semisolid simulating device was developed and used to study the morphological evolutio... more An optical semisolid simulating device was developed and used to study the morphological evolution of 99% pure succinonitrile alloy under forced fluid flow. The real time microscopic observations and flow analyses at different locations in the simulating device help to clarify some conflicting claims about the relative importance of the shear rate and turbulent flow on the formation of globular microstructure. It was revealed that the presence of the turbulent flow rather than shear rate is the critical factor for the formation of globular particles during semisolid solidification. This new finding can be practically significant in the development of new and more effective rheocasting methods.
Growth morphology of succinonitrile crystals under different cooling and stirring conditions was
... more Growth morphology of succinonitrile crystals under different cooling and stirring conditions was experimentally investigated by real time microscopic observation of the solidifying melt. It was found that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions the growth of solid particles followed a spherical mode with a high growth velocity from the initial stages of solidification. When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy. The new findings can be used for more effective refinement of rheocast microstructures.
The purpose of this study was to develop a new method for formation of non-dendritic
microstructu... more The purpose of this study was to develop a new method for formation of non-dendritic microstructure during sand casting of aluminum alloys without any previous special processing. For this purpose, sand molds with three different types of running systems were designed. The first design included a traditional reverse tapered vertical sprue. The second design was similar to the first one except for a conical sand core placed inside the lower half portion of the sprue creating a narrow gap for the flow of the melt into the mold. In the third design, the core was fitted with metal chills. Molten A356 alloy was then cast from fully liquid state under gravity and centrifugal casting conditions in the molds and the microstructures of the castings were studied. The results showed that non-dendritic microstructure could be achieved by using the last design under low superheat temperature and centrifugal casting conditions. The findings can pave the way for small quantity production of semisolid castings in expendable molds.
A theoretical model has been developed to investigate the effects of fluid flow on the dendrite
c... more A theoretical model has been developed to investigate the effects of fluid flow on the dendrite coarsening. The accelerated convective solute transport due to convection is considered by incorporating the new apparent diffusivity parameter into the classical coarsening model for stagnant melts. The model predicted that the dendrite arm coarsening followed the d~c1 :c 0:12t0:37 1 and d~c2 :c 0:26t0:42 1 relationships under laminar and turbulent fluid flow conditions respectively. The model predictions are found to be in good quantitative agreement with experimental data in literature.
Morphological evolution of a transparent model succinonitrile (SCN) material during
solidificatio... more Morphological evolution of a transparent model succinonitrile (SCN) material during solidification was investigated in an apparatus resembling a shearing-disc viscometer. The in situ microscopic observations showed that fragmentation decreased the average particles size, but did not result in transition of dendritic to spherical morphology. At low shear rates, the degenerated dendrites and at high shear rates, the pseudo-cluster morphology was observed. It was revealed that coarsening has the most important effect on the final morphology of solid particles. The quantitative influences of shearing rate and intensity on the size and morphology of solid crystals were also discussed based on the measurements on the microstructures.
A new criterion for prediction of hot tearing susceptibility
of cast alloys is suggested which ta... more A new criterion for prediction of hot tearing susceptibility of cast alloys is suggested which takes into account the effects of both important mechanical and metallurgical factors and is believed to be less sensitive to the presence of volume defects such as bifilms and inclusions. The criterion was validated by studying the hot tearing tendency of Al-Cu alloy. In conformity with the experimental results, the new criterion predicted reduction of hot tearing tendency with increasing the copper content.
This study introduces a new apparatus for quantitative assessment of hot tearing tendency in cast... more This study introduces a new apparatus for quantitative assessment of hot tearing tendency in casting alloys. In this method, molten metal is cast in a T-shaped mold cavity. Each arm of the T-shaped casting is hooked onto a rigid flask which restrains its free contraction during solidification. One of the hooks connects to a load cell which enables the real-time measurement of the contraction load during the solidification process as well as the load-time curve of each experiment. Temperature-time curves are also plotted using K-type thermocouples placed in the casting hot spot and connected to a data-acquisition system. Through the use of this set up, it is possible to estimate the solid fraction at which hot tearing occurs. Experiments were conducted with Al-9 %Si alloy to investigate the accuracy of the apparatus and modify its operation. The influence of the mold thermal gradient on the load-time curve and the hot tearing severity were investigated. Microscopic study of the observed hot tear regions was also performed to characterize the hot tearing characteristics.
Melt squeezing process in semisolid state was used for the first time for production of open-cell... more Melt squeezing process in semisolid state was used for the first time for production of open-cell Al–Si foams with improved microstructural and mechanical characteristics. First a given amount of preheated NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing temperature, melt to salt ratio and piston pressure for production of uniform and high integrity foams were obtained to be 3400 lm, 605 C, 1.5:1 and 10 MPa, respectively. The results showed that the semisolid processing employed could refine and modify the microstructure of the cell ligaments in the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness were also improved by the semisolid processing.
Preparation of metallic semisolid slurries using the cooling slope method is increasingly becomin... more Preparation of metallic semisolid slurries using the cooling slope method is increasingly becoming popular because of the simplicity of design and control of the process. Microstructural features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast microstructure.
To investigate the effects of curing condition on the surface characteristics of polyurethane coa... more To investigate the effects of curing condition on the surface characteristics of polyurethane coatings, the prepared coatings had undergone different curing conditions in which the rate of solvents evaporation differed. Differential scanning calorimeter, Fourier transform infrared spectroscope, atomic force microscope, and contact angle measurement system were used to characterize the coating solution as well as the cured coatings. The results showed that the curing condition substantially influenced the samples’ surface morphology and can be used as a parameter for precise engineering of surface pattern. The highest contact angle of 103◦ was obtained for the ambient cured sample.
A356 aluminum alloys reinforced with carbon nano-tubes (CNTs) were produced by stir casting and c... more A356 aluminum alloys reinforced with carbon nano-tubes (CNTs) were produced by stir casting and compocasting routes and their microstructural characteristics and hardness were examined. In order to alleviate the problems associated with poor wettability, agglomeration and gravity segregation of CNTs in the melt, CNTs were introduced into the melts by injection of CNT deposited aluminum particles instead of raw CNTs. Aluminum particles with mean diameters of less than 100 μm were first deposited by CNTs using Ni-P electroless plating technique and then injected into the melt agitated by a mechanical stirrer. The slurry was subsequently cast at temperatures corresponding to full liquid as well as 0.15 and 0.30 solid fractions. The results show that addition of CNTs to A356 matrix can significantly refine both full liquid and semi-solid cast microstructures. Hardness of the samples is also significantly increased by addition of CNTs and A356-CNT composite cast at 0.3 solid fraction produces the highest hardness.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2008
In the present study, in-situ Al matrix composites containing alumina particles were synthesized ... more In the present study, in-situ Al matrix composites containing alumina particles were synthesized by a new method that involved injection of an activated powder mixture into the molten aluminum, which was subsequently squeeze cast to minimize its porosity. The activated powder mixture was prepared by milling of powders of commercially pure aluminum and zinc oxide for a predetermined time. Milling for 60 minutes led to optimum activation of the reactant powders. Such activation reduced the reaction temperature from above 1000 °C to about 640 °C. Microstructural characterization studies revealed the formation of submicron size alumina particles with near equiaxed morphology and good particle/matrix interface. The composite specimen synthesized using the activated powder injection (API) method exhibited higher hardness, greater yield and tensile strengths, and lower elongation when compared to the monolithic specimen of similar composition. This article places particular emphasis on explaining the uniqueness of the processing methodology used in this study for synthesis of in-situ Al/Al2O3 composites.
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Papers by behzad niroumand
pure succinonitrile alloy under forced fluid flow. The real time microscopic observations and flow analyses at
different locations in the simulating device help to clarify some conflicting claims about the relative importance
of the shear rate and turbulent flow on the formation of globular microstructure. It was revealed that
the presence of the turbulent flow rather than shear rate is the critical factor for the formation of globular
particles during semisolid solidification. This new finding can be practically significant in the development
of new and more effective rheocasting methods.
experimentally investigated by real time microscopic observation of the solidifying melt. It was found
that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology
immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions
the growth of solid particles followed a spherical mode with a high growth velocity from the initial
stages of solidification. When the stirring was combined with rapid heat extraction from the rotor,
a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of
dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical
particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy.
The new findings can be used for more effective refinement of rheocast microstructures.
microstructure during sand casting of aluminum alloys without any previous special processing. For
this purpose, sand molds with three different types of running systems were designed. The first
design included a traditional reverse tapered vertical sprue. The second design was similar to the
first one except for a conical sand core placed inside the lower half portion of the sprue creating a
narrow gap for the flow of the melt into the mold. In the third design, the core was fitted with metal
chills. Molten A356 alloy was then cast from fully liquid state under gravity and centrifugal casting
conditions in the molds and the microstructures of the castings were studied. The results showed
that non-dendritic microstructure could be achieved by using the last design under low superheat
temperature and centrifugal casting conditions. The findings can pave the way for small quantity
production of semisolid castings in expendable molds.
coarsening. The accelerated convective solute transport due to convection is considered by
incorporating the new apparent diffusivity parameter into the classical coarsening model for
stagnant melts. The model predicted that the dendrite arm coarsening followed the d~c1
:c
0:12t0:37
1
and d~c2
:c
0:26t0:42
1 relationships under laminar and turbulent fluid flow conditions respectively. The
model predictions are found to be in good quantitative agreement with experimental data in
literature.
solidification was investigated in an apparatus resembling a shearing-disc viscometer. The in situ
microscopic observations showed that fragmentation decreased the average particles size, but did
not result in transition of dendritic to spherical morphology. At low shear rates, the degenerated
dendrites and at high shear rates, the pseudo-cluster morphology was observed. It was revealed that
coarsening has the most important effect on the final morphology of solid particles. The quantitative
influences of shearing rate and intensity on the size and morphology of solid crystals were also
discussed based on the measurements on the microstructures.
materials researchersduringlastdecades.Useofelectriccurrentduringsolidification hasbeenrecently
proposed asasuitabletoolforrefinement andmodification ofdendriticmicrostructures.Mechanismsby
which anelectric field couldaffectadendriticmorphologyarenotentirelyclearasitisdifficult tostudy
the solidification phenomenaduringcooling.Thispaperreportsthedesign,fabricationandtestingofan
instrument forin-situstudyofthedendriticgrowthoftransparentmodelmaterialsinthepresenceofan
applied electric field. Thesystemconsistsofanisothermalbath,agrowthchamberandanopticaland
imaging system.Thestructureofthegrowthchamberisdesignedtoallowthegrowthofonlyasingle
dendrite undertheappliedelectric field. Incontrasttosomepreviouslymadeinstrumentswhichuse
two orthogonallyplacedcamerastotakecareofdendritetiltingorrotationerrors,inthisresearcha
mathematical approachforcalculatingtheactualdimensionsandgrowthvelocityofthedendriteusing
only onecamerawassuggested.Thisgreatlysimplifies thestructureoftheinstrument.Solidification ofa
transparentmodelmaterial,i.e.asuccinonitrilealloy,wasinvestigatedasacasestudyunderdifferent
applied electric fields usingtheinstrument.Theresultsofthestudyshowedareductioninthedendrite
tip velocityinthepresenceoftheelectric field.
of cast alloys is suggested which takes into account
the effects of both important mechanical and metallurgical
factors and is believed to be less sensitive to the
presence of volume defects such as bifilms and inclusions.
The criterion was validated by studying the hot tearing tendency of Al-Cu alloy. In conformity with the experimental results, the new criterion predicted reduction of hot tearing tendency with increasing the copper content.
In this method, molten metal is cast in a T-shaped mold cavity. Each arm of the T-shaped casting is hooked
onto a rigid flask which restrains its free contraction during solidification. One of the hooks connects to
a load cell which enables the real-time measurement of the contraction load during the solidification process
as well as the load-time curve of each experiment. Temperature-time curves are also plotted using K-type
thermocouples placed in the casting hot spot and connected to a data-acquisition system. Through the use
of this set up, it is possible to estimate the solid fraction at which hot tearing occurs. Experiments were
conducted with Al-9 %Si alloy to investigate the accuracy of the apparatus and modify its operation. The
influence of the mold thermal gradient on the load-time curve and the hot tearing severity were investigated.
Microscopic study of the observed hot tear regions was also performed to characterize the hot tearing characteristics.
foams with improved microstructural and mechanical characteristics. First a given amount of preheated
NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry
until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated
piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by
leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing
temperature, melt to salt ratio and piston pressure for production of uniform and high integrity
foams were obtained to be 3400 lm, 605 C, 1.5:1 and 10 MPa, respectively. The results showed that
the semisolid processing employed could refine and modify the microstructure of the cell ligaments in
the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness
were also improved by the semisolid processing.
features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope
surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast microstructure.
cured coatings. The results showed that the curing condition substantially influenced the samples’ surface morphology and can be used as a parameter for precise engineering of surface pattern. The highest contact angle of 103◦ was obtained for the ambient cured sample.
and their microstructural characteristics and hardness were examined. In order to alleviate the problems associated with poor wettability, agglomeration and gravity segregation of CNTs in the melt, CNTs were introduced into the melts by injection of CNT deposited aluminum particles instead of raw CNTs. Aluminum particles with mean diameters of less than 100 μm were first deposited by CNTs using Ni-P electroless plating technique and then injected into the melt agitated by a mechanical stirrer. The slurry was subsequently cast at temperatures corresponding to full liquid as well as 0.15 and 0.30 solid fractions. The results show that addition
of CNTs to A356 matrix can significantly refine both full liquid and semi-solid cast microstructures. Hardness of the samples is also significantly increased by addition of CNTs and A356-CNT composite cast at 0.3 solid fraction produces the highest hardness.
pure succinonitrile alloy under forced fluid flow. The real time microscopic observations and flow analyses at
different locations in the simulating device help to clarify some conflicting claims about the relative importance
of the shear rate and turbulent flow on the formation of globular microstructure. It was revealed that
the presence of the turbulent flow rather than shear rate is the critical factor for the formation of globular
particles during semisolid solidification. This new finding can be practically significant in the development
of new and more effective rheocasting methods.
experimentally investigated by real time microscopic observation of the solidifying melt. It was found
that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology
immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions
the growth of solid particles followed a spherical mode with a high growth velocity from the initial
stages of solidification. When the stirring was combined with rapid heat extraction from the rotor,
a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of
dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical
particles in the microstructure. Formation of the layer was experimentally confirmed for an AleSi alloy.
The new findings can be used for more effective refinement of rheocast microstructures.
microstructure during sand casting of aluminum alloys without any previous special processing. For
this purpose, sand molds with three different types of running systems were designed. The first
design included a traditional reverse tapered vertical sprue. The second design was similar to the
first one except for a conical sand core placed inside the lower half portion of the sprue creating a
narrow gap for the flow of the melt into the mold. In the third design, the core was fitted with metal
chills. Molten A356 alloy was then cast from fully liquid state under gravity and centrifugal casting
conditions in the molds and the microstructures of the castings were studied. The results showed
that non-dendritic microstructure could be achieved by using the last design under low superheat
temperature and centrifugal casting conditions. The findings can pave the way for small quantity
production of semisolid castings in expendable molds.
coarsening. The accelerated convective solute transport due to convection is considered by
incorporating the new apparent diffusivity parameter into the classical coarsening model for
stagnant melts. The model predicted that the dendrite arm coarsening followed the d~c1
:c
0:12t0:37
1
and d~c2
:c
0:26t0:42
1 relationships under laminar and turbulent fluid flow conditions respectively. The
model predictions are found to be in good quantitative agreement with experimental data in
literature.
solidification was investigated in an apparatus resembling a shearing-disc viscometer. The in situ
microscopic observations showed that fragmentation decreased the average particles size, but did
not result in transition of dendritic to spherical morphology. At low shear rates, the degenerated
dendrites and at high shear rates, the pseudo-cluster morphology was observed. It was revealed that
coarsening has the most important effect on the final morphology of solid particles. The quantitative
influences of shearing rate and intensity on the size and morphology of solid crystals were also
discussed based on the measurements on the microstructures.
materials researchersduringlastdecades.Useofelectriccurrentduringsolidification hasbeenrecently
proposed asasuitabletoolforrefinement andmodification ofdendriticmicrostructures.Mechanismsby
which anelectric field couldaffectadendriticmorphologyarenotentirelyclearasitisdifficult tostudy
the solidification phenomenaduringcooling.Thispaperreportsthedesign,fabricationandtestingofan
instrument forin-situstudyofthedendriticgrowthoftransparentmodelmaterialsinthepresenceofan
applied electric field. Thesystemconsistsofanisothermalbath,agrowthchamberandanopticaland
imaging system.Thestructureofthegrowthchamberisdesignedtoallowthegrowthofonlyasingle
dendrite undertheappliedelectric field. Incontrasttosomepreviouslymadeinstrumentswhichuse
two orthogonallyplacedcamerastotakecareofdendritetiltingorrotationerrors,inthisresearcha
mathematical approachforcalculatingtheactualdimensionsandgrowthvelocityofthedendriteusing
only onecamerawassuggested.Thisgreatlysimplifies thestructureoftheinstrument.Solidification ofa
transparentmodelmaterial,i.e.asuccinonitrilealloy,wasinvestigatedasacasestudyunderdifferent
applied electric fields usingtheinstrument.Theresultsofthestudyshowedareductioninthedendrite
tip velocityinthepresenceoftheelectric field.
of cast alloys is suggested which takes into account
the effects of both important mechanical and metallurgical
factors and is believed to be less sensitive to the
presence of volume defects such as bifilms and inclusions.
The criterion was validated by studying the hot tearing tendency of Al-Cu alloy. In conformity with the experimental results, the new criterion predicted reduction of hot tearing tendency with increasing the copper content.
In this method, molten metal is cast in a T-shaped mold cavity. Each arm of the T-shaped casting is hooked
onto a rigid flask which restrains its free contraction during solidification. One of the hooks connects to
a load cell which enables the real-time measurement of the contraction load during the solidification process
as well as the load-time curve of each experiment. Temperature-time curves are also plotted using K-type
thermocouples placed in the casting hot spot and connected to a data-acquisition system. Through the use
of this set up, it is possible to estimate the solid fraction at which hot tearing occurs. Experiments were
conducted with Al-9 %Si alloy to investigate the accuracy of the apparatus and modify its operation. The
influence of the mold thermal gradient on the load-time curve and the hot tearing severity were investigated.
Microscopic study of the observed hot tear regions was also performed to characterize the hot tearing characteristics.
foams with improved microstructural and mechanical characteristics. First a given amount of preheated
NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry
until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated
piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by
leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing
temperature, melt to salt ratio and piston pressure for production of uniform and high integrity
foams were obtained to be 3400 lm, 605 C, 1.5:1 and 10 MPa, respectively. The results showed that
the semisolid processing employed could refine and modify the microstructure of the cell ligaments in
the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness
were also improved by the semisolid processing.
features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope
surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast microstructure.
cured coatings. The results showed that the curing condition substantially influenced the samples’ surface morphology and can be used as a parameter for precise engineering of surface pattern. The highest contact angle of 103◦ was obtained for the ambient cured sample.
and their microstructural characteristics and hardness were examined. In order to alleviate the problems associated with poor wettability, agglomeration and gravity segregation of CNTs in the melt, CNTs were introduced into the melts by injection of CNT deposited aluminum particles instead of raw CNTs. Aluminum particles with mean diameters of less than 100 μm were first deposited by CNTs using Ni-P electroless plating technique and then injected into the melt agitated by a mechanical stirrer. The slurry was subsequently cast at temperatures corresponding to full liquid as well as 0.15 and 0.30 solid fractions. The results show that addition
of CNTs to A356 matrix can significantly refine both full liquid and semi-solid cast microstructures. Hardness of the samples is also significantly increased by addition of CNTs and A356-CNT composite cast at 0.3 solid fraction produces the highest hardness.