Closed-cell aluminum foam was shot peened at different processing time (0 s, 5 s, 10 s, and 20 s), the intensity was the 0.12 mmA. The X-ray diffraction results showed that the reflections became weakened obviously with the shot peened time increased. Combined with Popa model and lognormal distribute model, the surface microstructure of closed-cell aluminum foam was inves-tigated by using the Rietveld whole pattern fitting analysis method. The results revealed that domain size and microstrain fluctuated along different reflection directions after shot peened, which attributed to the random and anisotropic deformation direction. With the shot peened processing time prolonged, a decrease in domain size and an increase in microstrain were also observed. Moreover, the corrosion behavior of closed-cell aluminum foam was studied by weight-loss test. The results indicated that corrosion properties of specimen subjected to shot peened processing was better than the unpeened specimens.
Aluminum foam, a kind of new function-structure materials, has a development application prospect in many aspects. According to the cell space structure, the aluminum foam can be classified as closed cell aluminum foam (CAF) and open aluminum foam. The particularity of foam metal structure endows these materials with many excellent properties, such as light-weight, thermal-insulation, energy-absorbing, permeation performance etc. Thus, the foam aluminum has got extensive application in different fields [
Recently, great progress has been made in the CAF corrosion resistance. Nima Movahedi et al. [
In this paper, the SP treatment was applied to CAF. The surface information of CAF at different time has been obtained by X-ray diffraction (XRD), and the evolution of microstructure induced by SP was further investigated by Rietveld whole pattern fitting analysis method. In addition, the corrosion behavior and mechanism of CAF was discussed.
CAF was fabricated by melt-foaming method at the Xinjiang University of China. The amount of thickening agent (high purity Ca about 3 wt% of the melt mass) was added to the molten aluminum (commercial purity Al), and foaming agent (high purity TiH2, 1.5 wt% of the melt mass) was added. Meanwhile, by the agitation of the foaming agent were dispersed and release gas, after a certain time of heat preservation, the molten aluminum foaming after cooling solidification in the mold was the CAF. The apparent densities of the foams were about 0.67 g∙cm−3, cell size was approximately 3 - 7 mm, porosity was 74%, and selecting the same pore structure of CAF specimens was cut into 18 × 11 × 4.5 mm by the electrical discharge machining. SP pellets used in this study were conditioned carbon steel cut wire shot (CCW14) balls with the diameter of ~0.30 mm and the average hardness of pellets was 610 Hv. The specimens were shot-peened on one side with the surface coverage of more than 100%. SP intensity was set at 0.12 mmA, and SP time was 0 s, 5 s, 10 s and 20 s, respectively. Specimens processing status and number were shown in
The corrosion resistance of the specimens was obtained by Weight-loss testing [
A = w 0 − w 1 w 0 × 100 % (1)
where w 0 was the initial mass of the specimens before the corrosion and w 1 was the finial mass of the specimen after the corrosion.
Prior to and after the weight-loss testing, the weight of specimen was measured accurately using the SARTORIUS CP225D analytic balance with the accuracy of 10−4 g. The surface structure analysis of specimens were performed by Bruker D8 Advanced X-ray diffractometer with Cu-Ka radiation (λ = 0.1542 nm), using step-scan ways with 2θ ranging from 30˚ - 120˚. The microstructure was further analyzed by the XRD linear analysis methods. The surface topography was characterized by scanning electron microscope (SEM), and the surface chemical composition of specimens was conducted by the energy dispersive X-ray spectroscopy (EDS).
time increasing. It can be seen that the diffraction peaks of Al (PDF#04-0787) was clearly, and there was no other new phase after SP. With the increasing of SP processing time, the intensity of the diffraction peak was significantly weakened and the breath of [
In order to further obtain the microstructure information of the CAF, the Rietveld whole pattern fitting method was applied with use Materials Analysis Using Diffraction (MAUD) software. The fitting result of the SP3 specimen was showed in
XRD linear analysis method was one of the most effective methods to characterize the microstructure of materials. X-ray diffraction pattern refers to the diffraction intensity with the change curve of the diffraction angle.
d ∗ = 2 sin θ / λ (2)
The observed diffraction linear specimens h ( x ) can be expressed as instrument linear g ( y ) and physical linear f ( x − y ) convolution of specimens. The reason for instrument line broadening were that: 1) The wavelength of the incident X-ray was not completely monochromatic; 2) X-ray light source, specimen, receiving slit was not completely meet the focused condition; 3) The incident
X-ray was not completely parallel etc. The mainly factors affected the physical linear microstructure included: domain size, strain and crystal structural defects responsible for line etc., these factors responsible for line broadening effects was independently.
h ( x ) = ∫ ∞ ∞ g ( y ) f ( x − y ) d y (3)
f ( x − y ) function was related to grain size and strain. Direction-dependent crystallite size and strain models were put forward by Popa [
In this model, the normalized peak profile was:
V H ( z ) = ∫ d ( Δ z ′ ) G H d ( Δ z ′ ) L H ( z + Δ z ′ ) (4)
The Gaussian component GH represents the strain effect, and the Lorentzian component LH represents the size effect. In terms of the integral breadths β G H , β L H these functions were:
G H ( z ) = β G H − 1 exp ( − π z 2 β G H 2 ) − 1 (5)
L H ( z ) = β L H − 1 ( 1 + π 2 z 2 β L H 2 ) − 1 (6)
For the constant-wavelength diffraction method, z = 2 θ and the quantities β G H β L H were shown as follows.
β G H = 2 tan θ H ( 2 π 〈 ε h k l 2 〉 ) 1 / 2 (7)
β L H = 2 λ ( 3 〈 R h 〉 cos θ H ) (8)
According to the obtained diffraction line parameters and based on the follow principles, direction-dependent domain size and microstrain of the CAF has been obtained from the broadening of diffraction peaks. For face-centered cubic (f, c, c) crystal, The space group was Fm3m, domain size and microstrain expression shown as below:
< D > = D 0 + D 1 K 4 1 ( x , φ ) + D 2 K 6 1 ( x , φ ) + D 3 K 6 1 ( x , φ ) + … (9)
< ε 2 > E H 4 = E 1 ( h 4 + k 4 + l 4 ) + 2 E 2 ( h 2 k 2 + h 2 l 2 + k 2 l 2 ) (10)
The results obtained via XRD linear analysis methods were showed Figures 3-6. It can be seen from
The variation trend of microstrain was shown in
the deformation of cell structure and shear collapse of CAF was random, and the magnitude of the stress was not affected.
The average domain size along with SP time was characterized by the lognormal distribution model, and results were shown in
The effect of SP treatment on corrosion resistance of CAF was also discussed. It can be seen from
The surface morphologies were observed by SEM from Figures 8(a)-(d).
compressive residual stress field still remain dominate, which were much better than disadvantage brought by the surface damage of the CAF.
The chemical composition of specimen’s surface was analyzed by X-ray spectroscopy (EDS). The element contents were given in
more nucleus to form denser nature oxide film than ordinary grain [
In this paper, with the following important conclusion being the list: The XRD results showed that the reflection became broaden obviously with the SP time increased. And the nanocrystalline grain of SP specimens was also obtained. The Rietveld whole pattern fitting analysis method results revealed that domain size and microstrain fluctuated along different reflection directions after SP, which attributed to the random and anisotropic deformation direction. Weight-loss test indicated that corrosion properties of CAF subjected to SP processing was better than the unpeened specimens. The Rietveld whole pattern fitting analysis method and experimental results showed that 5 seconds was the optimal processing time.
Financial support from the High-tech development project (201515106) of Xinjiang Uyghur Autonomous Region and Natural Science Foundation of China under Project No. 11264037 was acknowledged.
Projects (11264037) supported by the National Natural Science Foundation of China; Project (201515106) supported by high-tech development project of Xinjiang Uyghur Autonomous Region.
Zhang, C.W., He, Y.D., Chen, Y.H., Mu, Y.L., Zhao, F.J. and Li, X.C. (2017) The Effect of Shot-Peening Treatment on Microstructure and Corrosion Behavior of Closed-Cell Aluminum Foam. World Journal of Engineering and Technology, 5, 89-98. https://doi.org/10.4236/wjet.2017.54B010