Microstructure and damping behavior of SiCp/Gr/2024Al metal matrix composites by squeeze casting technology
LENG Jin-feng(冷金凤), WU Gao-hui(武高辉)
Center for Metal Matrix Composites Engineering Technology, Harbin Institute of Technology,
Harbin 150001, China
Received 28 July 2006; accepted 15 September 2006
Abstract: SiCp/Gr/2024Al metal matrix composites were processed by squeeze casting technology. The microstructure of composites was observed by SEM and TEM, and the effects of graphite particulates and SiC particulates on the damping behaviors of composites were also investigated. The results show that the microstructure of composites was dense and homogeneous, without any interfacial reactivity among reinforcement/matrix interfaces. Compared with the damping capacity of 2024Al, the damping capacity of composites was enhanced significantly by addition of SiC or graphite particulates. The main damping mechanisms of SiCp/Al composites were ascribed to the dislocation damping, and those of SiCp/Gr/2024Al were attributed to the intrinsic damping and interface damping.
Key words: metal matrix composites; SiCp/Gr/2024 composite; graphite; squeeze casting; damping capacity
1 Introduction
In a structure, the application of high-damping material may allow undesirable noise and vibration to be passively attenuated. Therefore，the use of high-damping materials may eliminate the need for special energy absorbers or dampers which sometimes are quite massive. Particulates reinforced MMCs have shown promising improvements in damping and mechanical properties [1-9], and SiC, Al2O3, and graphite particulates are most regularly used in MMCs. The SiC and Al2O3 particulates are hard particulates which improve the strength and stiffness of the aluminum. While graphite particulates, the soft particles, are found to have negative influence to the mechanical properties but exhibit a relatively high damping capacity when measured in its bulk form.
Various fabrication processes were investigated to produce particulate-reinforced composites. ZHANG et al[1,10] used spray atomization and deposition to process 6061Al/SiC/graphite hybrid MMCs and reported that the damping capacity of 6061Al was significantly improved. ROHATGI et al[2,11] studied the damping behaviour of SiC/Al and Gr/Al composites processed by gravity cast- ing. Their experimental results show that the damping capacity of the MMCs is proportional to the volume fraction of graphite and the addition of SiC in aluminum alloy has no evidence to improve in damping capacity. However, the MMCs made by gravity casting have their own disadvantages, such as nonuniform and low volume fraction, which result in low mechanic properties of MMCs and couldn’t satisfy the need of structure.
Avoiding these shortages, squeeze casting processing is believed to be a promising technology. The MMCs fabricated by squeeze casting technology has several advantages, such as the volume fraction of reinforcement to 50%, uniform distribution of reinforcement in metal matrix and the minimization of deleterious interfacial reactions which can satisfy the requirement of industrial manufacture.
The present paper was undertaken with the objectivity to study the effects of SiC and graphite particulates on the damping capacity of MMCs. On the basis of the analysis of microstructure and damping measurements, the operative damping mechanisms in the particle reinforced MMCs were discussed.
2 Experimental
The matrix alloy in the paper was 2024Al with chemical composition (in mass fraction) 4.79%Cu, 1.49%Mg, 0.611%Mn, 0.245%Fe, 0.168%Si, 0.068%Zn, 0.046%Ti, 0.013%Ni, 0.049%Cr and balance in Al. The 40% volume fraction SiC reinforcing particulates used were of 3 μm in average diameter. Flake graphite was chosen for the purpose to improve the damping capacity, which has an average diameter of 70 μm and two volume fractions of 4% and 3%.
The SiCp/Gr/2024Al composites were fabricated by squeeze casting technology. Firstly, the SiC particulates and graphite particulates were mixed with different mass ratio by mechanical stirring for 30 min. Then they were filled and pressed into a mold to produce a SiC/Gr preform and then pre-heated. At the same time, the aluminum alloy was melt, degassed and cleaned in a graphite crucible and heated to 800 ℃. Subsequently, the molten aluminum was poured into the tool steel die and a vertical pressure up to 100 MPa was applied to force molten aluminum to infiltrate into SiC/Gr preform entirely. The pressure was maintained for about 5min until the solidification was complete.
The microstructure of as-fabricated SiCp/Gr/2024Al was examined using S-570 scanning electron microscope. A CM12 transmission electron microscope was used to observe the interface of composite material and dislocation in the metal matrix. The slices of the TEM samples were mechanically thinned to 40 μm and final thinning on argon ion plasma bombardment with a liquid nitride platform.
Commonly used measures to report damping capacity include loss angle(f), loss tangent (tanf), loss factor η, inverse quality factor (Q-1). In this study, the measurement of the damping capacity is inverse quality factor, Q-1. The inverse quality factor Q-1 was measured using acoustic frequency internal friction appearance under its own resonance frequency. The specimens used in damping measurements by T6 heat treatment was 2 mm×4 mm×70 mm in size. The major technical parameters were as follows: working temperature of 298 K, strain amplitudes of 2×10-5 and the maximum resolution of 1×10-4. Resonance frequencies of 2024Al, 40%SiC/2024Al, 40%SiC/3%Gr/2024Al and 40%SiC/ 4%Gr/2024Al composites are 1 136, 1 123, 1 575 and 1 430 Hz, respectively.
3 Results and discussion
3.1 Microstructure characterization
Fig.1 shows the distribution of SiC and graphite reinforcements in the SiC/Gr/Al MMCs. Inspection of Fig.1 confirms that three MMCs are all dense and macroscopically homogeneous, and seldom particulates cluster. Flake graphite particulates distribute uniformly and lack of micrometer sized pores in the micro- structure.
Fig.1 SEM images of SiC/Gr/Al composite: (a) 40%SiC/4%Gr/ Al; (b) 40%SiC/Al.
To further analyze the interface state, TEM images of the SiC/Gr/Al MMCs are shown in Fig.2. It can be seen that a lower density of dislocation is found in the matrix of SiC/Gr/Al composites compared with that of SiC/Al composites. These dislocations may be caused by the large thermal misfit between particulates and matrix during the processes of composites preparation and consequent heat treatment. These dislocations are located primarily near the reinforcement-matrix interface and decrease with increasing distance from the interface. In SiC/Al composites, due to higher volume fraction of 40% SiC particulates in the matrix, the dislocation density increased in the matrix. However, in SiC/Gr/Al composites, the graphite particulates can be considered ‘soft’ and the bind force between flakes of graphite is weak, so it can relax the thermal mismatch stress in the process of plastic deformation and reduce the dislocation density in the matrix.
Fig.2 TEM images of SiC/Al and SiC/4%Gr/Al MMCs: (a), (b) SiCp/Al; (c), (d) SiCp/Gr/Al
The addition of graphite particulates to molten aluminum alloy often leads to severe reactivity between graphite and Al under the thermodynamic conditions that are normally present in common casting fabrication techniques. However, no interfacial reactivity was observed on interfaces between SiC particles and Al matrix as well as graphite particulates and Al matrix by squeeze casting method (Fig.2). Molten aluminum alloy was filtered into SiC/Gr preform, in which SiC contacted with molten aluminum in short time accompanied by the effect of cooling from preform and mold to accelerate the solidification rate of molten aluminum alloy. The short time of contacting between molten aluminum alloy and SiC particulates or graphite particulates avoids the possibility of interfacial reactions. These feactures of microstructure may affect the damping capacity of composites.
3.2 Damping capacity
The results of damping capacity measurement on SiC/Gr/Al and aluminum alloy are shown in Fig.3. The Q-1 of SiC/Al composites is 0.013 1, and it reveals that the addition of 40% SiC particulates (in volume fraction) increases the damping capacity of 2024Al by 2.8 times. Data in Fig.3 also indicate that the dispersion of 4% graphite particulates (in volume fraction) in the SiC/Al composites results in the further increase of damping capacity which reaches 3.8 times in Q-1 as that of 2024Al. In this paper, the damping capacity is directly related to the volume fraction of graphite particulates.
Fig.3 Damping capacity of SiC/Gr/2024Al composites and 2024Al
The damping mechanisms of MMCs at low temperature were attributed to the intrinsic damping of metal matrix and reinforcement particulate, dislocation damping and interface damping. 2024Al and SiC are low damping materials with loss factors in the range from 0.001 6 to 0.005[10], and their contributions to MMCs damping depend on their modification to the microstructure of composites.
There are no interface reaction and well-bonded interface between SiC particulates and metal matrix as shown in Fig.2. LEDERMAN[12] considered that the damping stems only from the strain-independent damping capacity under this conditions. In other words, the well-bonded interface could not contribute much to the damping characteristics of material since interfacial slip didn’t occur. In this circumstance, the contribution of SiC particulates to damping capacity of MMCs may directly be related to the increase in dislocation density in metal matrix. The relative movement of dislocation in matrix metal under cyclic stress promoted frictional losses or energy dissipation and thereby contributed to the damping of materials. The dislocation mechanism affecting the damping behavior is generally considered as the Granato-Lucke mechanism[13-14]. This theory assumes that the dislocations break away from weak pinning points under cycle loading and result in energy loss, which is dissipated by breakaway and the sweeping motion is proportion to the area traversed. According to the G-L dislocation theory, the damping capacity of materials is proportion to the dislocation density, which is expressed by
(1)
where Q-1 is the damping capacity from dislocation, C1 and C2 are materials constants; C1 is proportional to dislocation density in the matrix.
As described the above, the addition of graphite particulates in SiC/Al composites further improved the damping capacity of MMCs. The higher damping capacity of SiC/Gr/Al composites compared with that of SiC/Al composites was contributed to the intrinsic damping of graphite particulates and the weak bonds between graphite particulates and metal matrix. Graphite, a layered soft material, is capable of dissipating large amounts of energy by microplastic deformation within the particulates. The other primary source of damping is due to the friction energy loss caused by sliding on the interface between the graphite particulate and metal matrix. Interfacial sliding stems from the development of high shear stresses at the particulate-matrix. Under cycle loads, the shear stresses increase until it is sufficient to overcome frictional loads and the energy is dissipated by friction as the matrix slides over the particulates. Fig.2 shows that the density of dislocation in the matrix of SiC/Gr/Al composites reduces compared with that of SiC/Al composites, and results in low dislocation damping. However, the improvement of intrinsic damping capacity and interface damping capacity of graphite is greater than the decrease caused by dislocation damping and thus leads to the improvement of the overall damping capacity. Increasing the volume percentage of graphite particulate, the damping capacity of SiC/Gr/Al composites is further increased, as the same reported by ROHATGI et al [11].
4 Conclusions
1) The squeeze casted SiC/Al and SiC/Gr/2024Al composites are macroscopically dense and homogeneous; a high density of dislocation is found in the matrix of SiC/Al composites. No interfacial reactivity is observed on SiC/Al interface and graphite/Al interface.
2) The addition of 40% SiC particulates (in volume fraction) increases the damping capacity of 2024Al by 2.8 times. The dispersion of 4% (volume fraction) graphite particulates in the SiC/Al composites results in further increase of damping capacity which reaches 3.8 times as that of 2024Al, and the damping capacity depends on the volume fraction of graphite.
3) The main damping mechanisms of SiC/Al composites are ascribed to dislocation damping, while those of SiC/Gr/Al are attributed to the intrinsic damping and interface damping.
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(Edited by HE Xue-feng)
Corresponding author: LENG Jin-feng, Tel/Fax: +86-451-86412164; E-mail: jfleng@126.com
