稀有金属(英文版) 2015,34(08),560-563

收稿日期：26 August 2013

基金：financially supported by the National Natural Science Foundation of China (No. 51361010);the Natural Science Foundation of Jiangxi Province (No. 20114BAB216015);the Scientific Research Fund of Jiangxi Provincial Education Department (GJJ12320);the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University (No. SKLSP201321);

Stirred casting Al–Pb monotectic alloys with high damping capacity

Di-Qing Wan

Key Laboratory of Ministry of Education for Conveyance and Equipment, East China Jiaotong University

Abstract：

In this study, the stirred casting with various processing parameters, such as stirring temperature and stirring speeds, was carried out on the Al–Pb monotectic alloys in order to make Pb particles distribute much more uniformly. More importantly, their damping capacities were systematically studied. The results show tha mechanical stirring can not only make Pb in the aluminum matrix uniformly distribute but also dynamically influence the damping capacity of this alloy system. The Al–Pb alloy was prepared under a slow speed at solid–liquid temperature region, wherein high volume fraction of Pb in alloy could be obtained. The high volume fraction of Pb gives high overall damping capacity. The dislocation damping and interface damping theories are mainly dominated to the alloys.

Keyword：

Al–Pb alloy; Mechanical stirring; Damping capacity;

Author： Di-Qing Wan,e-mail: padwan@tom.com;

Received： 26 August 2013

1 Introduction

Al–Pb bearing material has advantages of cheap, good wear resistance, good corrosion resistance, becoming as wearing materials of this century [1–6]. However, owing to the big difference in specific gravity and wide immiscibility gap, the manufacturing of Al–Pb alloys is difficult [7, 8]. Hence, conventional methods of melting and casting are inadequate for producing an Al–Pb system. Therefore, in most of these studies, the main focus was on manufacturability with homogeneous microstructure. In view of the above, various researchers developed different processing techniques for Al–Pb alloys [9–15]. As compared, stirred casting method is the most popular traditional method to prepare high volume factional Pb particle reinforced aluminum matrix composites.

Generally, the previous literatures are focused on the wear and mechanical properties of Al–Pb alloy systems. However, it is not paid much attention to their damping capacity. To our best knowledge, Fan et al. [16] revealed that Pb particles on the interface between Al₁₈B₄O₃₃and Al resulted in a change in damping properties of the Al₁₈B₄O₃₃/Al composites. Furthermore, the internal friction of Al–Pb alloy was studied by Pang [17]. However, with respect to the Al–Pb alloy, it is important to know the effect of Pb particle distribution to its damping performance. Hence, in this study, it was systematically studied the damping properties of the stirred Al–Pb monotectic alloys with the difference Pb particle distributions.

2 Experimental

According to Al–Pb phase diagram [18], the monotectic point for Al–Pb is of 1.52 % Pb. In order to get monotectic alloy, Pb content was selected as 1.52 % in this study. Smelting process of the alloy was as follows: the crucible was preheated to 400 °C, subsequently adding aluminum, and the temperature was set at 700 °C. At that temperature, aluminum was completely melted, and then the lead was added; the melt was cooled at the temperatures as shown in Table 1, incubated for 5 min. Then the melt was taken for stirring processing. In this study, the stirring speed was 900 and 540 r min^-1, respectively. The melt was immediately poured into an air-cooled mold with pipe diameter of U8 mm after stirring. The sample was for microstructure observation etched by 1 wt% HF solution within 5 min.

Table 1 Parameters of stirred casting Al–Pb  下载原图

Table 1 Parameters of stirred casting Al–Pb

In this study, the measure of damping capacity utilized is loss tangent (tanφ), which was measured using a precision damping performance measurement system (TPA-8). A sample with 30 mm 9 5 mm 9 1 mm installed in the testing head was constrained at each end by a clamping bar arrangement with one end fixed to a rigid frame, and the other end driven by an electromagnetic vibrator via a composite drive shaft. The resulting sinusoidal force and deflection data were recorded and analyzed by modulus. The calculation of the loss tangent was based on the following forced vibration equation [19].

where E'is the loss modulus, and E'' is the storage modulus.

The vibration frequency was held at 1 Hz at room temperature during the strain amplitude varies. Furthermore, the influence of frequencies on damping capacity was examined with frequencies ranging from 0.5 to 3.0 Hz with holding strain amplitude of 3 ×10^-6, which was the minimum strain amplitude of the testing system achieved.

3 Results and discussion

3.1 Microstructure characterization

According to the Refs. [20–23], the stirring temperature and stirring speed are most important processing parameters to the mechanical stirred Al–Pb alloys. Figure 1 shows the microstructures for distribution of lead particles at different stirring temperatures. The volume fraction of Pb in Al–Pb alloy was calculated by a quantitative image analysis software (image-pro plus) [24]. When the stirring temperature is 650 °C, the maximum size to the particles lead is reaching 10.6 lm, and the volume fraction of lead is 6.2 %. However, as the stirring temperature decreases to 580 °C, the size of lead particles approaches 12.6 lm, and the volume fraction increases to 9.2 %. According to the phase diagram [18], when stirring at 650 °C, the alloy is almost for whole liquid, so that there are more opportunities to cause the lead particles coagulation and even sedimentation. However, with temperature decreasing to 580 °C, the alloy starts to solidify, which will cause the viscosity to the melt increase sharply in the stirring process, much more energy will be dissipated in the melt during the stirring process. Hence, the volume fraction of lead particle increases.

The speed of stirring could also effectively affect size and morphology of the secondary phase particles. Figure 2 shows the microstructures at different stirring rates, namely the stirring speed 900 and 540 r min^-1, respectively. In fact, it is explored that the different stirring rates could significantly affect the morphology of Pb particles. Under a low stirring speed, the morphology of Pb phase is near spherical, as shown in Fig. 2a. Meanwhile, it is found that under a higher stirring speed of 900 r min^-1, the morphology of Pb particle transforms into flat-like, which is marked by the arrow in Fig. 2b.

As we know, when temperature is below the melting point of the alloy, the solidification will occur, thus generating dendrite. Under the effect of mechanical stirring, the slender dendrite is constantly interrupted with stirring proceeding, and then interrupted dendrites are gradually transformed into spherical or ellipsoidal particles in the solid. While stirring at high rate, under a large shear force, the second phase will be plastically deformed dynamically. Hence, it can be obtained flat-like Pb particles of irregular shape, which is clearly shown in Fig. 2b.

Fig.1 SEM images of Al–Pb alloys under different stirring temperatures with stirring speed of 540 r min-1: a 650 °C and b 580 °C

Fig.2 SEM images of Al–Pb alloys with different stirring speeds at 580 °C: a 540 r min-1and b 900 r min-1

3.2 Damping properties of stirred Al–Pb alloys

Damping capacity of the alloy is due to energy dissipation on defects of the solid. Thus, damping measurements can sensitively reflect the characteristics of the microstructure evolution of alloys. Figure 3 shows the damping performance of Al–Pb alloys with the strain amplitude increasing at the room temperature with vibration frequency of 1 Hz. As seen from Fig. 3, the damping capacity of the alloys performs differently. Alloy C has the highest damping capacity, which is significantly higher than other alloys. Recently, it is considered that when damping value tan/ [ 0.01, the material exhibits high damping capacity. Accordingly, when the strain amplitude is higher than 3 9 10^-6, the damping value of Alloy C is greater than 0.01, which belongs to the high damping materials. However, in terms of other Al–Pb alloys, the damping value tan/ basically maintains less than 0.01 even at high strain amplitude. Moreover, it is necessary to note that the strain amplitude-dependent damping performance of alloys is different from each other, which is mainly due to its different microstructures. Figure 4 shows the damping capacity of Al–Pb alloy with frequency variation from 0.1 to 3.0 Hz at room temperature, in which the strain amplitude is held at 3 9 10^-6, which is the minimum strain amplitude of the testing system achieved. It is clearly shown that the damping capacity of casting Al–Pb alloy does exponentially vary with frequency. Malhotra and Van [25] also investigated the frequency dependence of the damping capacity for the alloys. It is found that the damping capacity decreases exponentially with frequency increasing from 0.1 to 3.0 Hz. It is suggested that the high mobile dislocation is dominated to the damping mechanism.

Generally, the well-accepted theory on the dislocation damping mechanism of Al–Pb alloys is G–L theory [26, 27]. The G–L theory describes the pinned or unpinned dislocations to explain the anelastic energy loss, which can be applied to describe the strain amplitude-dependent damping properties of the alloys. The damping performances of all the alloys are increasing with the strain amplitude increasing which confirms that the G–L dislocation damping theory is dominated to the alloys.

With respect to the Al–Pb monotonic alloys, other damping mechanism could be arisen due to the secondary phase. As an appearance of the secondary phase in the alloy, it brings the presence of the interface damping. The interface damping can be attributed to the energy loss of interfacial slip between the particles and the matrix under the cyclic loading. Hence, for the case of weakly bonded interface of Al–Pb alloys, when the interface shear stress is large enough to overcome the frictional resistance, the interface slip can occur. Therefore, the Pb particle/matrix interfacial slip energy loss will become a major source. Lederman [28] successfully established a model to analyze the weak binding interface damping, which is listed as below:

Fig.3 Strain amplitude-dependent damping of Al–Pb alloys

Fig.4 Frequency versus damping capacity of Al–Pb stirring casting alloys

where, l is friction coefficient between for the particles and the metal the substrate, V_pis the particle volume fraction, k is the radial stress concentration factor at the interface, and C is a constant. Thus, the volume fraction of Pb and its morphology is direct factors impacting on the interface damping mechanism. As referred previously, Alloy C was prepared under a slow speed at solid–liquid temperature region, wherein the Pb is largely broken, and it distributes uniformly on the matrix. Moreover, the volume fraction of Pb in Alloy C is significantly higher than that of other alloys. Therefore, according to the Lederman’s model, the high volume fraction of Pb gives high overall damping.

4 Conclusion

The stirring temperature can influence Al–Pb alloys. When the stirring temperature is 650 °C, the maximum size of the particles of lead reaches 10.6 lm, and the volume fraction of lead is 6.2 %. As the temperature decreases to 580 °C, the size of lead particles approaches 12.6 lm, the volume fraction approaches 9.2 %.

Mechanical stirring speed can influence Al–Pb alloys. The alloy was prepared under slow speed, wherein the Pb is largely broken, and the morphology of Pb is almost spherical. While under a higher ration speed, it is flat-like.

A high damping Al–Pb alloys can be accomplished by applying the rotation of stirrer at speed of 900 r min^-1and stirring at 580 °C. The Al–Pb alloy was prepared under a slow speed at solid–liquid temperature region, wherein high volume fraction of Pb in alloy is obtained. The high volume fraction of Pb gives high overall damping. The dislocation damping and interface damping theories are mainly dominated to the alloys.