project maker
08-08-2014, 12:32 PM
Study on Mechanical Properties of nano SiCp reinforced AA6061
[attachment=66746]
Abstract
Metal matrix nano composites (MMNCs) have significance in aerospace, naval and other light weight applications. The ultrasonic cavitation based technique is found to be effective method to disperse nano particles uniformly and to produce a wide range of MMNCs. The uniform dispersion of nanoparticles in aluminum (AA6061) matrix resulted in enhanced mechanical properties. Various weight percentages of SiC nano particles are reinforced into the aluminum matrix. The microstructures and mechanical properties are compared and presented.
. The interaction between high-intensity ultrasonic waves and nano-sized particle clusters in aluminum melts.
Due to the application of high intensity ultrasonic waves the acoustic streaming traps the nano particles into the melt effectively. They generate some important non-linear effects in liquids, namely transient cavitation and acoustic streaming which are mostly responsible for benefits including refining microstructures, degassing of liquid metals for reduced porosity, and dispersive effects for homogenizing. Acoustic streaming, a liquid flow due to acoustic pressure gradient, is very effective for stirring. Acoustic cavitation involves the formation, growth, pulsating, and collapsing of tiny bubbles in liquid under cyclic high-intensity ultrasonic waves (thousands of micro bubbles will be formed, expanding during the negative pressure cycle and collapsing during the positive pressure cycle). By the end of one cavitation cycle, the tiny bubbles implosively collapse (in less than 10−6 s), producing transient (in the order of microseconds) micro “hot spots” that can reach temperatures of about 5000 0C, pressures of about 1000atms, and heating and cooling rates above 1010K/s. The strong impact coupling with local high temperatures can potentially break nanoparticles clusters and clean the particle surface. Since nanoparticles clusters are loosely packed together, air could be trapped inside the voids in the clusters, which serves as nuclei for cavitations[127]. The size of the clusters ranges from nano- to micro-meters due to the attraction force among nanoparticles and the poor wettability between nanoparticles and molten metal. Although these SiC clusters appear as micro clusters, most of the nanoparticles in the micro-clusters are still separated to single nanoparticle or sintered nano-clusters. The negative effects of some SiC micro-clusters are balanced by the positive effects of the grain refining effects and strengthening effects of the well dispersed SiC nanoparticles. The improvement of ductility in SiC nanocomposites can also be partly attributed to the decreasing of porosity and crack-like shrinkage areas. Because of the wide freezing temperature range, some porosity and crack-like shrinkage areas form at the end of solidification, which are detrimental to the ductility.
Nanoparticles reinforcements can significantly increase the mechanical strength of the matrix by more effectively promoting particle hardening mechanisms than micron size particles. A fine and uniform dispersion of nanoparticles provides a good balance between the strengthener (non-deforming particles, such as SiC nanoparticles) and inter-particle spacing that results in maximizing the yield strength and creep resistance. A combination of good distribution and dispersion of micro particles can be achieved by mechanical stirring. However, to produce aluminum matrix nanocomposites, it is extremely challenging for the conventional mechanical stirring method to distribute and disperse nanoparticles uniformly in molten metal because of the much higher specific surface areas in nanoparticles.
Hardness of the composites is measured at different locations on various heat treated samples samples. The hardness increased nearly linear with various weight percentages of the nano SiC particulate. The hardness of the aluminum AA6061 alloy is found to be Hv-106. The hardness values of the MMNCs are plotted. The various plates are solutionised at 560oC for one hour and ageing is done at 160oCfor12hrs.The Vickers’s hardness tests are conducted on solutionised and aged composites for different weight percentages of nano particulate and in the table.1.
Due to the application of high intensity ultrasonic waves, the acoustic streaming traps the nano particles into the melt effectively. Hardness of the composites is measured at different locations on the samples. The hardness increased nearly linear with various weight percentages of the nano SiC particulate.
. Microstructures
The properties of the metal matrix nano composites depend on the distribution of the reinforcing particles and interface bonding between metal matrix and the dispersed particles. Samples with ultrasonic processing were examined with the optical microscope and SEM. Typical microstructures are shown in Fig. Fig shows the aluminum matrix without addition of nanoparticles.
The nanoparticles were well dispersed in the AA6061matrix, although some microclusters remained in the matrix. The high-intensity ultrasonic waves generated strong cavitations and acoustic streaming effects. These transient cavitations can produce an implosive impact strong enough to break up the clustered particles and disperse them uniformly in the liquid.
The optical micrograph and SEM images of MMNCs reinforced with 1.5wt% of nano SiCp are presented in figures below. The optical images of the base metal in fig-3 show the grain boundaries clearly. With the optical microscopy no pores are observed. The grains in fig 3(a) are larger in size and that of MMNC’s are smaller as seen in fig-3(b). The fig-4 shows images at higher magnification the grain boundaries are seen clearly. The SEM images reveal that, there is uniform distribution of the nano SiC particles in the matrix (fig-5,6) and also small agglomerates of the powders and voids are seen at higher magnification fig-5. The overall microscopic analysis shows that there is good bonding between the matrix and the ceramic particulate, indicating uniform distribution of particles due to ultrasonic cavitation and grain boundaries are seen distinctly in fig-6 at lower magnification.
conclusions
Nano-sized SiC particles reinforced magnesium composites
were fabricated by casting with the help of high-intensity
290 J. Lan et al. / Materials Science and Engineering A 386 (2004) 284–290
ultrasonic cavitation technique. From the study of microstructural
characterization and hardness determination, the results
suggest:
High-intensity ultrasonicwaves are capable of distributing
and dispersing nanoparticles in Mg matrix with non-linear
effects in liquids, especially transient cavitation. From the
high-resolution SEM observation, SiC nanoparticles are almost
uniformly distributed in the matrix, although some small
clusters (less than 300 nm) still exist in matrix. EDS analysis
indicates that the SiC nanoparticels are partly oxidized.
Compared to pure cast AZ91D, cast AZ91D/5SiC yields
Mg2Si compounds. The Mg2Si compound in the composites
might be resulted from chemical reactions taking place during
the ultrasonic processing of composites. Si could be introduced
into the magnesium matrix by reactions between Mg
and the SiO2 layer that covers the surfaces of SiC nanoparticles.
The XPS analysis also indicates the existence of SiO2.
The microhardness of nanoparticle reinforced magnesium
composites improved with the increasing fraction of SiC
nanoparticles. The microharness of AZ91D/5SiC increased
by 75% compared to that of AZ91D.
Bulk Al-based nanocomposites with nanosized SiC were fabricated
by an ultrasonic based nanomanufacturing process. The microstructure
study shows that high-power ultrasonic is effective to
disperse nanosize SiC particles in aluminum alloy A356 and enhances
the wettability between the particles and Al matrix. However,
it is typical that a small amount of microclusters remained in
the matrix. The EDS spectrum shows that the process is well
protected from oxidation. The superior nanoparticle dispersion resulted
in significantly improved mechanical properties.
Adding nanoparticles into the melts was successful by manual
handling and mechanical delivery. But this process was not very
efficient, demanding long ultrasonic processing time. Two other
feeding techniques, the master powder carrying method and compressed
inert gas spraying, were experimented with but the results
were not very promising. Better methods to improve the efficiency
of nanoparticle feeding into melts during nanomanufacturing are
needed.
[attachment=66746]
Abstract
Metal matrix nano composites (MMNCs) have significance in aerospace, naval and other light weight applications. The ultrasonic cavitation based technique is found to be effective method to disperse nano particles uniformly and to produce a wide range of MMNCs. The uniform dispersion of nanoparticles in aluminum (AA6061) matrix resulted in enhanced mechanical properties. Various weight percentages of SiC nano particles are reinforced into the aluminum matrix. The microstructures and mechanical properties are compared and presented.
. The interaction between high-intensity ultrasonic waves and nano-sized particle clusters in aluminum melts.
Due to the application of high intensity ultrasonic waves the acoustic streaming traps the nano particles into the melt effectively. They generate some important non-linear effects in liquids, namely transient cavitation and acoustic streaming which are mostly responsible for benefits including refining microstructures, degassing of liquid metals for reduced porosity, and dispersive effects for homogenizing. Acoustic streaming, a liquid flow due to acoustic pressure gradient, is very effective for stirring. Acoustic cavitation involves the formation, growth, pulsating, and collapsing of tiny bubbles in liquid under cyclic high-intensity ultrasonic waves (thousands of micro bubbles will be formed, expanding during the negative pressure cycle and collapsing during the positive pressure cycle). By the end of one cavitation cycle, the tiny bubbles implosively collapse (in less than 10−6 s), producing transient (in the order of microseconds) micro “hot spots” that can reach temperatures of about 5000 0C, pressures of about 1000atms, and heating and cooling rates above 1010K/s. The strong impact coupling with local high temperatures can potentially break nanoparticles clusters and clean the particle surface. Since nanoparticles clusters are loosely packed together, air could be trapped inside the voids in the clusters, which serves as nuclei for cavitations[127]. The size of the clusters ranges from nano- to micro-meters due to the attraction force among nanoparticles and the poor wettability between nanoparticles and molten metal. Although these SiC clusters appear as micro clusters, most of the nanoparticles in the micro-clusters are still separated to single nanoparticle or sintered nano-clusters. The negative effects of some SiC micro-clusters are balanced by the positive effects of the grain refining effects and strengthening effects of the well dispersed SiC nanoparticles. The improvement of ductility in SiC nanocomposites can also be partly attributed to the decreasing of porosity and crack-like shrinkage areas. Because of the wide freezing temperature range, some porosity and crack-like shrinkage areas form at the end of solidification, which are detrimental to the ductility.
Nanoparticles reinforcements can significantly increase the mechanical strength of the matrix by more effectively promoting particle hardening mechanisms than micron size particles. A fine and uniform dispersion of nanoparticles provides a good balance between the strengthener (non-deforming particles, such as SiC nanoparticles) and inter-particle spacing that results in maximizing the yield strength and creep resistance. A combination of good distribution and dispersion of micro particles can be achieved by mechanical stirring. However, to produce aluminum matrix nanocomposites, it is extremely challenging for the conventional mechanical stirring method to distribute and disperse nanoparticles uniformly in molten metal because of the much higher specific surface areas in nanoparticles.
Hardness of the composites is measured at different locations on various heat treated samples samples. The hardness increased nearly linear with various weight percentages of the nano SiC particulate. The hardness of the aluminum AA6061 alloy is found to be Hv-106. The hardness values of the MMNCs are plotted. The various plates are solutionised at 560oC for one hour and ageing is done at 160oCfor12hrs.The Vickers’s hardness tests are conducted on solutionised and aged composites for different weight percentages of nano particulate and in the table.1.
Due to the application of high intensity ultrasonic waves, the acoustic streaming traps the nano particles into the melt effectively. Hardness of the composites is measured at different locations on the samples. The hardness increased nearly linear with various weight percentages of the nano SiC particulate.
. Microstructures
The properties of the metal matrix nano composites depend on the distribution of the reinforcing particles and interface bonding between metal matrix and the dispersed particles. Samples with ultrasonic processing were examined with the optical microscope and SEM. Typical microstructures are shown in Fig. Fig shows the aluminum matrix without addition of nanoparticles.
The nanoparticles were well dispersed in the AA6061matrix, although some microclusters remained in the matrix. The high-intensity ultrasonic waves generated strong cavitations and acoustic streaming effects. These transient cavitations can produce an implosive impact strong enough to break up the clustered particles and disperse them uniformly in the liquid.
The optical micrograph and SEM images of MMNCs reinforced with 1.5wt% of nano SiCp are presented in figures below. The optical images of the base metal in fig-3 show the grain boundaries clearly. With the optical microscopy no pores are observed. The grains in fig 3(a) are larger in size and that of MMNC’s are smaller as seen in fig-3(b). The fig-4 shows images at higher magnification the grain boundaries are seen clearly. The SEM images reveal that, there is uniform distribution of the nano SiC particles in the matrix (fig-5,6) and also small agglomerates of the powders and voids are seen at higher magnification fig-5. The overall microscopic analysis shows that there is good bonding between the matrix and the ceramic particulate, indicating uniform distribution of particles due to ultrasonic cavitation and grain boundaries are seen distinctly in fig-6 at lower magnification.
conclusions
Nano-sized SiC particles reinforced magnesium composites
were fabricated by casting with the help of high-intensity
290 J. Lan et al. / Materials Science and Engineering A 386 (2004) 284–290
ultrasonic cavitation technique. From the study of microstructural
characterization and hardness determination, the results
suggest:
High-intensity ultrasonicwaves are capable of distributing
and dispersing nanoparticles in Mg matrix with non-linear
effects in liquids, especially transient cavitation. From the
high-resolution SEM observation, SiC nanoparticles are almost
uniformly distributed in the matrix, although some small
clusters (less than 300 nm) still exist in matrix. EDS analysis
indicates that the SiC nanoparticels are partly oxidized.
Compared to pure cast AZ91D, cast AZ91D/5SiC yields
Mg2Si compounds. The Mg2Si compound in the composites
might be resulted from chemical reactions taking place during
the ultrasonic processing of composites. Si could be introduced
into the magnesium matrix by reactions between Mg
and the SiO2 layer that covers the surfaces of SiC nanoparticles.
The XPS analysis also indicates the existence of SiO2.
The microhardness of nanoparticle reinforced magnesium
composites improved with the increasing fraction of SiC
nanoparticles. The microharness of AZ91D/5SiC increased
by 75% compared to that of AZ91D.
Bulk Al-based nanocomposites with nanosized SiC were fabricated
by an ultrasonic based nanomanufacturing process. The microstructure
study shows that high-power ultrasonic is effective to
disperse nanosize SiC particles in aluminum alloy A356 and enhances
the wettability between the particles and Al matrix. However,
it is typical that a small amount of microclusters remained in
the matrix. The EDS spectrum shows that the process is well
protected from oxidation. The superior nanoparticle dispersion resulted
in significantly improved mechanical properties.
Adding nanoparticles into the melts was successful by manual
handling and mechanical delivery. But this process was not very
efficient, demanding long ultrasonic processing time. Two other
feeding techniques, the master powder carrying method and compressed
inert gas spraying, were experimented with but the results
were not very promising. Better methods to improve the efficiency
of nanoparticle feeding into melts during nanomanufacturing are
needed.