Tribology Letters

, Volume 27, Issue 2, pp 221–225 | Cite as

Effect of ECAE on Microstructure and Tribological Properties of Cu–10%Al–4%Fe Alloy

Original Paper

Abstract

Equal channel angular extrusion (ECAE) process was carried out for a commercial aluminum bronze alloy (Cu–10%Al–4%Fe) produced by hotrolling at high temperature. The effect of ECAE on microstructure, mechanical, and tribological properties of the alloy was investigated. Experimental results showed that the grain size of the alloy decreased with the increase of the pass number of ECAE. After applying ECAE with six passes, the hardness and yield strength of the alloy increased from 118 kgf/mm2 and 356 MPa to 165 kgf/mm2 and 588 MPa, respectively. The friction coefficient and wear rate of the aluminum bronze alloy were largely reduced due to the improvement of mechanical properties after ECAE. The adhesive wear was the primary wear mechanism for the specimen without ECAE, while abrasive wear was dominant for the specimen with ECAE after six passes.

Keywords

Equal channel angular extrusion Grain refinement Microstructure Friction Wear 

Introduction

Aluminum bronzes are a special class of engineering tribo-materials due to their high strength and excellent wear resistance. They are usually used where parts of high hardness and wear resistance are required, such as engineering tools and dies, bushings and guide plates. However, little research on the wear behavior of alloys and their improvement has been reported. In practice, many modern aluminum bronze components require high strength, low friction, and wear, therefore, it is necessary to improve the mechanical properties and wear resistance of the alloys to meet such requirements.

Severe plastic deformation is an important way to obtain high strength and fine grains of an alloy. Many plastic deformation processes have been developed to enhance mechanical properties of an alloy using simple shear. One technique that has been the focus of numerous investigations in recent years is ECAE. It was reported that significant grain refinement occurred after applying ECAE, which increased the strength of an alloy [1]. ECAE has been successfully used for various metals such as copper [2, 3, 4], Al alloys [5, 6, 7, 8] and Ti alloys [9, 10]. However, these studies mainly focus on the effect of ECAE on microstructure and mechanical properties of alloys. The effect of ECAE on friction and wear characteristics of alloys is still not fully explored. In this work, ECAE was applied to Cu–10%Al–4%Fe aluminum bronze alloy to improve its mechanical and tribological properties. The changes of the microstructure and mechanical properties of the alloy before and after applying ECAE were studied, and the effect of ECAE on friction and wear behavior of the alloy under dry sliding condition was investigated.

Experimental Procedures

A commercial aluminum bronze alloy rod, Cu–10%Al–4%Fe (the concentration of alloy elements was given in wt.%) was used as experimental material for ECAE. The billets (9.6 mm × 9.6 mm in cross-section and 100 mm in length) were cut from the rod in the as-rolling condition. The die used for ECAE consisted of two rectangular channels of cross-section area 10 × 10 mm2 intersecting at an angle of 90°. The billets were coated with a lubricant containing graphite to reduce the friction between the die and the billets during ECAE. The ECAE processes were carried out at start temperature of 650°C.

Optical microscope (OM, Model: MEF4M, Leica Inc.) was used to observe the microstructural evolution of the alloy. The specimens for microstructure observation were cut along the extrusion direction, and were grounded mechanically using abrasive papers. After that, the surfaces of the specimens were etched by immersing in a solution of 8% HF, 22% HNO3, and 70% H2O for about 15 s. Buehler 6406 microhardness tester was used to determine the Vickers microhardness of specimens under a load of 50 g for 13 s. The specimens for tensile tests were machined from the as-received and the extruded billets with gauge length of 25 mm and cross-section of 2 × 2 mm2. The tensile tests were conducted on an Instron 1185 machine with strain rate of 10−3 s−1 at room temperature.

The friction and wear behavior of the aluminum bronze alloy block sliding against a GCr15 steel ring (Young’s modulus, 208 GPa; Poisson ratio, 0.33; HRC, 60) was evaluated on an M-2000 model ring-on-block test rig. The block has a size of 20 × 8 × 8 mm, and the GCr15 ring is 40 mm in diameter and 10 mm in length. The chemical compositions of the GCr15 steel are given in Table 1.
Table 1

Chemical composition of the GCr15 steel (wt.%)

C

Mn

P

S

Si

Cr

0.75–0.85

0.20–0.40

≤0.027

≤0.020

0.15–0.35

1.30–1.65

The tests were carried out with a linear velocity of 0.42 m/s, ambient temperature around 20°C and relative humidity of 50%. Before each test, the GCr15 steel ring and the block were abraded with No. 900 water-abrasive paper to reach Ra of 0.1 and 0.1–0.2 μm, respectively. During the tests, the load was gradually increased until the maximum possible load could be applied or until seizure took place. The friction force can be recorded by the tester and all the values were averages of at least five measurements.

A characteristic value, which describes the wear performance under the chosen conditions for a tribosystem, is the specific wear rate [11]:

$$ K = \frac{{\Delta V}} {{F_{N} \cdot \,L}} $$
(1)
$$ \Delta V = {\left[ {R^{2} \, \cdot \arcsin \,\frac{b} {{2R}} - \frac{{b{\sqrt {R^{2} - {\left( {b \mathord{\left/ {\vphantom {b 2}} \right. \kern-\nulldelimiterspace} 2} \right)}} }^{2} }} {2}} \right]}\, \cdot B $$
(2)
Where L is the sliding distance (m), \( \Delta V \) the worn volume loss (m3), b the width of the wear track (m), B the width of specimen (m). The wear track width was measured using a vernier. The morphologies of worn surfaces were examined using a scanning electron microscope (SEM, Model: S-520, Hitachi Inc.).

Results and Discussion

Microstructure and mechanical properties after ECAE

Figure 1 shows the optical images of the specimens without ECAE and with ECAE after six passes. It is clear that α phase and the second phase are elongated along rolling direction (vertical direction) and α phase parallels the second phase with few intersections before ECAE, as shown in Fig. 1(a). After six passes of ECAE, the grain size is refined remarkably, and the distribution of the second phase particles is more homogeneous, as shown in Fig. 1(b). The grain boundaries are no longer straight, but more curved than the boundaries without ECAE.
Fig. 1

Microstructure of aluminum bronze specimens: (a) without ECAE, (b) with ECAE after six passes

The effects of ECAE on grain size and mechanical properties of the alloy are shown in Figs. 2 and 3, respectively. It is seen that, with the increase of pass number, the grain size of the alloy decreases while the mechanical properties increase. ECAE can refine grains and improve dislocation density of an alloy [12]. With the decrease of grain size, grain refinement strengthening occurs according to Hall-Petch equation [13]. Dislocation strengthening also presents due to the dislocation networks and tangles within grains and near grains make dislocation glide more difficult [14]. Therefore, the cooperative interaction of grain refinement and the improvement of dislocation density lead to the significant increase in mechanical properties.
Fig. 2

Effect of ECAE on grain size of aluminum bronze specimens

Fig. 3

Effect of ECAE on mechanical properties of aluminum bronze specimens

Effect of ECAE on friction and wear properties

Figure 4 shows the friction coefficients of the specimens with different pass number of ECAE under a load of 100 N and a sliding speed of 0.42 m/s. It is noticeable that the friction coefficient of the alloy decreases considerably with increasing pass number, and the specimen with ECAE after six passes has the lowest-friction coefficient.
Fig. 4

Friction coefficient of specimens versus pass number at 100 N normal load and sliding speeds of 0.42 m/s

The friction coefficient is in direct proportion to real contact area A r, and the real contact area A r can be expressed as [15]:

$$ A_{r} = \frac{W} {H} $$
(3)

Where W is the normal load and H is the hardness of the alloy. With the increase of hardness, the real contact area decreases, accordingly the friction coefficient decreases. It is shown in Fig. 3 that the hardness of the alloy increases with the increase of pass number after ECAE. Thus, the low-friction coefficient of the ECAE-treated specimens is due to the increase of the hardness.

The variation of wear rate with load for specimens with and without ECAE is shown in Fig. 5. It can be seen that the wear rate of all specimens increases with the increase of load. Under the loads from 20 N to 260 N, specimen with six passes of ECAE shows the lowest-wear rate, while specimen without ECAE presents the highest. It also can be seen from Fig. 5 that the seizure load for specimens increases from 160 N before ECAE to 253 N after ECAE with six passes. These indicate that the wear resistance of the aluminum bronze alloy is obviously improved by ECAE.
Fig. 5

Variations of wear rate with load for the aluminum bronze specimens with different pass number

Low-wear resistance indicates the increase in the ease of removal of the surface, which happens when the friction force on per unit area exceeds the shear strength of the sliding material. It is well known that the shear strength of an alloy very much depends on its hardness [16]. The effect of hardness on wear properties of an alloy can be expressed using Archard’s law:

$$ Q = K\frac{{LN}} {H} $$
(4)
Where Q is the volumetric wear loss, N the applied load, L the total sliding distance, K the friction coefficient and H is the hardness of the wear surface [17]. Figure 6 shows the wear volume of the specimens with different pass number under the load of 100 N and the sliding speed of 0.42 m/s. It can be seen from Fig. 6 that the wear volume of the alloy follows almost a linear trend with the sliding distance. This is in accordance with the Archard’s law.
Fig. 6

The wear volume of the specimens with different pass number under the load of 100 N and the sliding speeds of 0.42 m/s

As shown by Archard’s law, the increased hardness and decreased friction coefficient will lead to a low-wear loss of the aluminum bronze alloy. Therefore, the improvement in wear resistance of the alloy is also due to the increase of hardness after ECAE.

SEM studies on worn surface

Figure 7 shows the scanning electron micrographs of the worn surfaces of specimens with and without ECAE under dry sliding condition. It can be seen that the three specimens have very different wear characteristics. For the specimen without ECAE, the wear track shows large extent of adhesion wear and severely plastic deformation in the sliding direction, which results in large extent of delaminating of the alloy, as shown in Fig. 7(a). Severe plastic deformation and adhesion increases the formation of asperity junctions in friction, therefore, the specimen has very high friction coefficient and wear rate without ECAE. After three passes of ECAE, as shown in Fig. 7(b), the plastic deformation and adhesion wear of the alloy decreases a lot, the worn surface is smoother, accordingly the friction coefficient and wear rate of the alloy decrease. With the increase of the pass number of ECAE, the observed plastic deformation on the worn surface of the alloy becomes less intensive, and the features of delamination and adhesion wear are replaced by abrasive wear, as shown in Fig. 7(c). This indicates that the improved hardness of the alloy after six passes of ECAE reduces the adhesion between the alloy and the counterpart, which decreases the friction coefficient and wear rate of the alloy.
Fig. 7

SEM morphologies of the worn surface of aluminum bronze specimens with 100 N load: (a) without ECAE, (b) with ECAE after three passes, (c) with ECAE after six passes

Conclusions

  1. (1).

    The grain sizes of the Cu–10%Al–4%Fe alloy decreases gradually with the increase of pass number. The hardness and the strength of the alloy increase considerably as a result of grain refinement strengthening and dislocation strengthening after ECAE.

     
  2. (2).

    The friction coefficient and wear rate of the alloy obviously decrease after ECAE. The improvement in tribological properties of the alloy is due to the increase of hardness after ECAE.

     
  3. (3).

    The adhesive wear is the primary wear mechanism for the specimen without ECAE, while abrasive wear is dominant for the specimen with ECAE after six passes.

     

Notes

Acknowledgments

This research is financially supported by National Natural Science Foundation of China (Grant No. 50275093) and instrumental analysis center of Shanghai Jiao Tong University.

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Copyright information

© Springer Science+Business Media, LLC 2007

Authors and Affiliations

  1. 1.School of Mechanical EngineeringShanghai Jiao Tong UniversityShanghaiP.R. China
  2. 2.National Power Traction Laboratory of Southwest Jiaotong UniversityChengduP.R. China

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