By properly proportioned SiC particles with different sizes and using squeeze infiltration process, SiCp/Al composites with high volume fraction of SiC content (Vp = 60.0%, 61.2%, 63.5%, 67.4%, and 68.0%) were achieved for optical application. The flexural strength of the prepared
The aluminum matrix composites reinforced by SiCp particles have been extensively applied in industry for their low density and high specific strength. In recent years, these materials with high fraction of
The bonding strength of SiCp/Al composite-glass components plays an important role in the reliability. The bonding property at glass/metal interface is deemed as the key benchmark for assessing the quality of the space mirror, because it will be posited in the harsh serving condition. Mantel [
As far as Al matrix composite, it is well known that the thickness of alumina film under natural condition is only several nanometers, which is difficult to form a stable metal/glass bonding interface. After the anodizing process on the
So far, there have been few reports about anodized SiCp/Al composite, even less high volume fraction SiCp/Al composite. Because the local melt structure around SiC particle affects the microstructures of the composites and the properties of the interface [
The chemical compositions of the Al matrix are listed in Table
Chemical composition of 6061-Al.
Specification | Composition (wt %) | |||||||
---|---|---|---|---|---|---|---|---|
Cu | Mg | Fe | Si | Zn | Mn | Pb | Al | |
Aluminum alloy | 0.258 | 1.08 | 0.255 | 0.962 | 0.24 | 0.168 | 0.09 | 96.947 |
The properties of the prepared SiCp/Al composites.
Gradation composition ( | | | | | | Measured/Wu (%) | Measured/H-S (%) | ||
---|---|---|---|---|---|---|---|---|---|
Measured data | Wu model | H-S model | |||||||
45 : 8 : 2 = 1000 : 100 : 400 | 34.63 | 68.0 | 483 | 0.28 | 206.1 | 245.1 | 271.7 | 84 | 76 |
45 : 8 : 2 = 1000 : 200 : 300 | 36.69 | 67.4 | 544 | 0.33 | 195.8 | 243.3 | 269.8 | 80 | 73 |
45 : 8 : 2 = 1000 : 250 : 250 | 37.38 | 63.5 | 548 | 0.35 | 195.7 | 231.9 | 257.7 | 84 | 76 |
45 : 8 : 2 = 1000 : 300 : 200 | 39.74 | 61.2 | 554 | 0.35 | 180.8 | 225.6 | 251.0 | 80 | 72 |
45 : 8 : 2 = 1000 : 400 : 100 | 44.87 | 60.0 | 569 | 0.36 | 174.2 | 222.5 | 247.5 | 78 | 70 |
A hydraulic machine was carried out for the preparation of SiCp preform. In this process the mixed SiCp powder was compacted in a cylindrical graphite mold under a pressure of 25 MPa. Thereafter, the preform was sintered at 1600°C for 2 h and cooled down to room temperature in the atmosphere.
The SiCp/Al composite was manufactured by using the squeeze casting machine. The prepared preform was preheated to 700°C in a steel mold. The molten Al (superheated to 820°C) was infiltrated to the steel mold by applying the hydraulic pressure from 8 to 90 MPa. The steel mold was cooled down to the room temperature when the infiltration process was completed.
The formed SiCp/Al composites were machined into the standard tensile and flexural specimens according to GB/T 228.1-2010 and GB/T 232-2010, respectively.
Flexural samples were also fabricated according to the standard of GB/T 232-2010. The dimension for the flexural sample was 65 mm × 7 mm × 7 mm. The parallelepiped samples was machined with the size of 8 mm × 8 mm × 8 mm. The parallelepiped samples were used for anodizing process. Before anodizing operation, the samples was etched in KOH solution (50 g/L) at room temperature for 2 min. Thereafter, it was rinsed in distilled water. Chemical pickling process was carried out in terms of HNO3 solution (1 mol/L) at room temperature for 3 min. Again, the sample was cleaned by distilled water and dried in a drying oven. The anodizing electrolyte was sulfuric acid. The sulfuric acid concentration was 180 g/L; the polar distance was 3 cm; the anodic current density was 1.6 A. An aluminum alloy plate was used as the cathode materials. Sulfuric and nitric acids were analytical grade chemicals. Different anodic oxidation times were employed in order to obtain different thickness of the oxidation film. The time durations for the anodizing process were independently set at 5 min, for 10 min, for 15 min, for 20 min, for 25 min, and for 30 min. The parallelepiped samples were mechanically ground P 1500 grade paper and then polished.
In order to obtain a better interface bonding strength, high volume fraction SiCp/Al was anodized prior to vacuum hot-pressing. Two sets of strain gauges that scatter in orthogonal planes were stuck on the tensile specimens. The gauge length was marked on the surface of the tensile specimens (Figure
The schematic of tensile and flexural specimen size and the photo of test sample.
X-ray diffraction (XRD) analysis of the parallelepiped samples was carried out on a SIEMENS D8 ADVANCE diffractometer using Cu radiation. Electron backscattered diffraction (EBSD) was used to evaluate the oxide layers of the samples. The oxygen content in the anodized samples was analyzed by Energy Dispersive Spectrometer (EDS). The parallelepiped samples were sputtered with platinum for 70 seconds to characterization. SEM was employed to observe the microstructure of the anodic film. The differential Scanning Calorimetry (DSC) measured the endothermic peak of the composites. The heating rate was 5°C/min. The DSC scanning was initiated at 30°C and completed at 750°C.
Testing results in Table
The properties of SiC particles and aluminum alloy at room temperature.
Specification | | | |
---|---|---|---|
(MPa) | (GPa) | ||
SiC | 550 | 410 | 0.14 |
Aluminum alloy | 398 | 130.1 | 0.40 |
The effect of SiC content to the flexural strength of composites is presented in Figure
(a) Effect of SiC vol.% on flexural strength, (b) effect of SiC vol.% on elastic modulus, (c) effect of mean grain size on flexural strength, (d) effect of mean grain size on measured elasticity modulus, (e) effect of SiC vol.% on Poisson’s ratio, and (f) effect of mean grain size on Poisson’s ratio.
(a) SEM images of the mixture SiC particle (gradation composition and proportion 45
Testing data in Table
The measured elastic modulus of composites was reported in Table
Poisson’s ratios of composites are 0.36, 0.35, 0.35, 0.33, and 0.28 for the 60.0%, 61.2%, 63.5%, 67.4%, and 68.0% composites, respectively. The effect of SiC volume fraction on Poisson’s ratio of composites is shown in Figure
Figure
Fracture morphology of
Figure
XRD patterns of the
Figure
Figure
Micrograph of the Al 60.0 vol%
Figure
Figure
The thickness of oxidation layers under different anodizing time is presented in Table
The thickness of anodized film after oxidizing for different time.
| Thickness of anodized film for different oxidizing time ( | |||||
---|---|---|---|---|---|---|
(%) | 5 min | 10 min | 15 min | 20 min | 25 min | 30 min |
68.0 | 12.5 | 14.5 | 15.5 | 17.9 | 18.3 | 19.6 |
67.4 | 12.9 | 15.9 | 17.3 | 20.2 | 22.4 | 24.4 |
63.5 | 13.5 | 16.3 | 19.2 | 21.2 | 23.4 | 26.4 |
61.2 | 16.9 | 18.7 | 20.8 | 23.0 | 24.8 | 27.3 |
60.0 | 17.5 | 19.6 | 22.4 | 24.0 | 26.2 | 28.4 |
The result is in agreement with the reports about the growth of oxide film [
The relationship (a) between the anodic film thickness and SiC volume fraction and (b) between the anodic film thickness and anodizing time. (c) The schematic of mass transfer channel.
SEM micrograph of 68.0% SiCp/Al composite after being anodized with different time.
The DSC thermograms of nonoxidizable and anodized composites obtained are shown in Figures
DSC curves of the composite containing 60.0 vol% SiCp, (a) before and (b) after anodization.
At present, a number of scholars have applied micromechanics theory to study the quantitative relationship between the properties of composites and the properties of its constituents, and established the mathematical model, such as the Hashin-Shtrikman model [
Hashin and Shtrikman consider the strain, interface bonding, and stress transfer between reinforcement and matrix. They assume that the strain cross section of the composite under uniaxial loading is uniform, the interface bonding between particle-matrix is perfect, and stress transfer between reinforcement and matrix is effective. They have grouped many analysis methods, such as direct methods, variation methods, and approximation methods. Finally, they derived a model for the effective elastic modulus of two-phase composite materials. The model is expressed as follows:
The predicted results of the H-S model are presented in Table
In consideration of the difficulties for characterizing the microstructures of two-phase composites, Wu introduces an unknown parameter, which can be determined from experiment. He derives an equation for calculating the effective elastic modulus of composites. Wu’s equation can be written as follows:
The calculated data from the Wu model are listed in Table
In the present study, the mechanical and anodized surface properties of high volume fraction
Si and Mg are added in smaller amounts to the aluminum alloy, effectively suppressing the formation of Al4C3. With an increase in SiC volume fraction, the flexural strength and Poisson’s ratio decrease while the elastic modulus increases. With the mean grain size increasing, the flexural strength increases. From the fracture feature, it is found that the dominant fracture mechanism of composites is a cleavage fracture with occurrence of ductile fracture in the local of composites.
Through anodic oxidation treatment, an oxidation film with porous structure can be prepared on the surface of the composites. The anodic film is uniformly distributed. The oxide growth rate of composites linearly increases with anodizing time and decreases with SiC content increasing. SiC content and anodic films on the surface of composites have a negligent effect on the endothermic peaks of the composites.
The measured elastic modulus is in good agreement with predicted values based on Wu’s model.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was funded by the National Major Project of China (JCKY2016208A002). All investigations were performed in the Advanced Materials Processing Laboratory at South China University of Technology.