Effect of ECAE on Microstructure and Tribological Properties of Cu–10%Al–4%Fe Alloy
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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.
KeywordsEqual channel angular extrusion Grain refinement Microstructure Friction Wear
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 . 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.
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.
Chemical composition of the GCr15 steel (wt.%)
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 :
Results and Discussion
Microstructure and mechanical properties after ECAE
Effect of ECAE on friction and wear properties
The friction coefficient is in direct proportion to real contact area A r, and the real contact area A r can be expressed as :
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.
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 . The effect of hardness on wear properties of an alloy can be expressed using Archard’s law:
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
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.
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.
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.
This research is financially supported by National Natural Science Foundation of China (Grant No. 50275093) and instrumental analysis center of Shanghai Jiao Tong University.
- 15.Yu, J., Xu, Z., Zhao, D.C., Liu, Z.L.: Tribology. Hunan Science and Technology Press, Changsha (1994)Google Scholar