- Email: [email protected]
Microstructure and dry sliding wear behavior of Cu–10%Al–4%Fe alloy produced by equal channel angular extrusion
Wear 265 (2008) 986–991
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Microstructure and dry sliding wear behavior of Cu–10%Al–4%Fe alloy produced by equal channel angular extrusion L.L. Gao a , X.H. Cheng a,b,∗ a b
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China The State Key Laboratory of Tribology, Tsinghua University, Pekin 100084, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2006 Received in revised form 13 January 2008 Accepted 18 February 2008 Available online 18 April 2008 Keywords: ECAE Aluminum–bronze alloy Friction Wear
a b s t r a c t A commercial aluminum bronze alloy (Cu–10%Al–4%Fe) produced by hot-rolling was subjected to equal channel angular extrusion (ECAE) process at high temperature. The effect of ECAE on microstructure, mechanical and tribological properties of the alloy was investigated. Experimental results showed that grain size decreased and the second phase was rearranged after ECAE. The hardness and strength of the alloy with ECAE were higher than that of the alloy without ECAE and increased with the increase pass number. The wear resistance of the alloy after ECAE was significantly improved. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aluminum bronzes have been used as high strength, corrosion and wear resistance for many decades. Its high strength and wearresisting properties make them suitable for hardness and wear resistance applications, as engineering tools and dies, bushings, guide plates, etc. In practice, many modern engineering applications require a rather sophisticated combination of mechanical and tribological properties. At present, the main method to improve the mechanical properties of the alloys is to change the composition of the material. However, this method has its own limitation and cannot improve the tribological properties of the alloys significantly. It is necessary to find a new method to improve the mechanical properties and wear resistance of the alloys. It has been intensively studied for producing bulk ultrafinegrained materials by using severe plastic deformation techniques [1]. Recently, equal channel angular extrusion (ECAE) has been proven to be available for fabricating ultrafine-grained bulk materials. During ECAE, significant grain refinement occurs, resulting in the spectacular enhancement of the strength of a working material [2]. Active research efforts have been made recently, and successful applications have been reported for various materials such as copper [3,4], Al alloys [5] and Ti alloys [6], etc. How-
∗ Corresponding author at: School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China. Tel.: +86 21 62932404; fax: +86 21 34205880. E-mail address: [email protected] (X.H. Cheng). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.014
ever, previous ECAE studies are concerned mostly with producing an ultrafine-grained microstructure from single-phase alloys with high ductility at room temperature. For the two-phase alloy, they are generally difficulty-to-deformation materials and an increase of the extrusion temperature is required to increase their ductility [7]. Considering the industrial importance of two-phase materials, it is essential to establish optimum processing conditions of such difficult-to-deformation materials to study the microstructural changes and the effects of ECAE on mechanical and tribological properties of two-phase alloys. In this research, ECAE was carried out on a two-phase aluminum bronze alloy, Cu–10%Al–4%Fe. The effects of ECAE on the microstructure, mechanical and tribological properties of the alloy were investigated. 2. Experimental procedures A commercial aluminum bronze alloy rod, Cu–10%Al–4%Fe (the concentration of alloying elements was given in wt.%) obtained in the as-rolling condition was used as experimental material for ECAE. The billets with 9.6 mm ×9.6 mm in cross-section and 100 mm in length were cut from the alloy rod. The die used for ECAE consisted of two rectangular channels with the cross-section area 10 mm ×10 mm intersecting at angle of 90◦ . The extrusion velocity was 5 mm/s and an effective strain of about 1 was yielded by a single pass. 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 temperature of 600 ◦ C.
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
987
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
Optical electron microscope was used to study the microstructural evolution. The specimens for microstructure observation were cut along the extrusion direction. The specimens were ground mechanically using abrasive papers and alumina powders, and their surfaces were etched by immersing in a solution of 8% HF, 22% HNO3 and 70% H2 O for about 15 s. Vickers microhardness measurements were taken with loads of 50 g applied for 13 s. The specimens for tensile testing were machined from the as-received and the extruded billets with gauge length of 10 mm and cross-section of 2 mm × 2 mm. The specimen axis was aligned with the extrusion direction. The tensile tests were conducted at room temperature with strain rate of 10−3 s−1 . The friction and wear behavior of the aluminum bronze alloy blocks sliding against a GCr15 steel ring was evaluated on an M2000 model ring-on-block test ring. The contact schematic diagram of the frictional pair is shown in Fig. 1. The blocks in a size of 20 mm × 8 mm × 8 mm were made of the alloy, and the rings of ˚ 40 mm × 10 mm were made of GCr15 steel. The chemical compositions of the GCr15 steel are given in Table 1. Before each test, the GCr15 steel ring and the block were abraded with No.900 water-abrasive paper to reach a surface roughness Ra of 0.1 and 0.1–0.2 m, respectively. The steel ring and the blocks were thoroughly degreased by acetone and dried before the commencement of each wear test. The sliding velocity is 0.42 m/s.
Fig. 1. Contact schematic diagram for the frictional pair (unit: mm).
During the tests, the load was increased gradually until the maximum possible load could be applied or until seizure took place. The friction coefficient, , is calculated by the expression =
T FN R
(1)
where T is the friction torque recorded by the tester (N m), FN the normal force (N), R the radius of the ring (m). A characteristic value, which describes the wear performance under the chosen conditions for a tribosystem, is the specific wear rate K=
V FN L
Fig. 2. Microstructure of aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
(2)
988
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
V =
R2 arcsin
b b − 2R
R2 − (b/2) 2
2
B
(3)
where L is the sliding distance (m), V the worn volume loss (m3 ), b the width of the wear track (m), B the width of specimen (m). The morphologies of worn surfaces were examined by using a scanning electron microscope (SEM, Model: S-520, Hitachi Inc.). 3. Results and discussion 3.1. Microstructural evolution Fig. 2 shows the optical images of the specimens without ECAE, after ECAE with one pass and two passes. Before ECAE, it is apparent that ␣ phase and the second phase are elongated along rolling direction and ␣ phase parallels the second phase, as shown in Fig. 2(a). After ECAE with one pass, severe plastic deformation is imposed during ECAE. The microstructure of the alloy mainly consists of low-angle boundaries and the grain size is decreased, as shown in Fig. 2(b). After ECAE with two passes, the grain size of the alloy is further decreased and some equiaxed grains occur in some areas, as shown in Fig. 2(c). The specimens undergo essentially simple shear during ECAE [8]. The low-angle boundaries are truncated by shear planes and form a microstructure with large angle boundaries. As a result, the grain size reduces progressively with the increase of pass number. It is also seen from Fig. 2(b) and (c) that the volume fraction of second phase increases remarkably after the first pass. This is mainly attributed to the use of a pressing temperature of 600 ◦ C which leads to a increasing of the second phase during ECAE process. 3.2. Mechanical properties Vickers hardness and yield strength of specimens tested under the as-received condition and after ECAE with one and two passes are shown in Fig. 3. It is seen that the hardness and the yield strength of specimens after ECAE are higher than that of specimen without ECAE and both of them increase with the increase of pass number. It also can be seen that the increase of hardness and yield strength after one pass is larger than that of the specimen after two passes of ECAE. The increase of hardness and strength is attributed to grain refinement produced by ECAE. The grain refinement strengthen-
Fig. 3. Vickers microhardness and yield strength of the aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
Fig. 4. Wear volume of aluminum bronze specimens with sliding distance under a load of 50 N.
ing is generally described by a Hall–Petch equation [9]. The grain of the aluminum bronze alloy is refined and the grain size decrease with the increase of pass number, as shown in Fig. 2. Thus, the hardness and yield strength of the alloy increase after ECAE. In addition, the volume fraction of the second phase increases and the second phase is more homogeneously distributed in the alloy after ECAE. It is well known that hardness and strength are directly proportional to the volume fraction of the second phase and the dispersed distribution of the second phase can result in an increase of hardness and strength [10]. From Fig. 2(b) and (c), it can be seen that the volume fraction of the second phase increases after the first pass and does not increase after the second pass. As a result, the increase in hardness and yield strength after the first pass is larger than that of the specimen after two passes of ECAE. 3.3. Friction and wear properties Fig. 4 shows the wear volume of specimens without and after ECAE with one and two passes as a function of sliding distance. It can be seen that wear volume of specimen without ECAE is higher than that of the specimens with ECAE. The wear volume is the lowest for the specimen after two passes of ECAE, which is correlated to the lowest grain size of the specimens with and without ECAE. Fig. 5 shows the variations of wear rate of specimens without and with ECAE at various loads. It can be seen that the wear rate of all specimens increases with the increase of load. Under the same load, the wear rate of specimens with ECAE-treated is lower than that of specimen without ECAE-treated. The wear rate decreases with the increase of pass number, which indicates that grain refinement can decrease wear rate of the alloy. It also can be seen that the wear rate has a dramatically increase when seizure happens. The wear resistance is improved and the resistance to seizure increases after ECAE. Thus, it appears that grain refinement can improve the load bearing capacity of the aluminum bronze alloy during wear. Fig. 6 shows the variations of friction coefficient of specimens without ECAE and with ECAE under the load of 50 N after 30 min. It is observed that the friction coefficient of specimens with ECAE is lower than that of specimen without ECAE. And the friction coefficient decreases with the decrease of grain size. The decrease of friction coefficient and wear rate of the alloy after ECAE is mainly due to the improvement of mechanical prop-
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
Fig. 5. Variations of wear rate with load for the aluminum bronze specimens with and without ECAE.
erties caused by grain refinement. The friction coefficient is related to adhesive wear and abrasive wear. It can be expressed as following [11]: f =
b 2 + ctg s
(4)
989
Fig. 6. Friction coefficient of the aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
where b is the shear strength and s is the yield strength of the alloy. The first part of Eq. (4) is mainly caused by adhesive wear and the second part of Eq. (4) is decided by abrasive wear. After ECAE, the yield strength of the alloy increase with the increase of pass number, as shown in Fig. 3. The grain was refined after ECAE, which decreases the size of abrasive particles detached from the
Fig. 7. SEM morphologies of the worn surface of aluminum bronze specimens with 100 N load: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
990
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
aluminum bronze alloy. From Eq. (4), it can be concluded that the friction coefficient decrease after ECAE. The wear volume is inverse proportion to hardness according to Archard’s law [12]. With the decrease of grain size after ECAE, the hardness of the aluminum bronze alloy also increases, accordingly the wear volume decreases. This is due to the hardness of ECAE can improve the deformation resistance of the alloy. Furthermore, the volume fraction of second phase increases and the distribution of second phase rearranges after ECAE, which strengthen the surface of the alloy. As a result, the wear resistance of the alloy improves remarkably after ECAE. 3.4. SEM and EDX analysis of worn surfaces Fig. 7 shows the scanning electron micrographs of the worn surfaces of specimens with and without ECAE under dry sliding condition. For the specimen without ECAE, adhesive wear is the main mechanism and severely plastic deformation occurs during wear, which results in the larger extent of delaminating, as shown in Fig. 7(a). It is seen in Fig. 7(b) that adhesive wear decrease a lot and light plastic deformation remains when the grain size of the specimen decreases after one pass. The worn surface of the specimen after ECAE with two passes is characterized by deep wear
grooves, as shown in Fig. 7(c). These deep grooves on the wear surfaces are produced by hard particles which abrade the specimen surfaces on their entrapment. The hard particles may stem from the detachment of the second phase, work-hardened particles formed by shearing and rolling of the ␣ phase. Fig. 8 is the EDX spectrum of the worn counterfaces for the alloy without ECAE and after ECAE with two passes. It can be seen that large amount of Cu and Al are determined on the worn counterpace for specimen without ECAE. However, very little Al is determined on the worn counterface for specimen after ECAE with two passes. It is suggested that the Cu and Al on the worn counterface for the specimen after ECAE with two passes mainly comes from the aluminum bronze alloy in friction. It can be concluded that adhesive wear is the main mechanism of the specimen without ECAE, while abrasive wear is the main mechanism of the specimens after ECAE with two passes. This indicates that grain refinement of the alloy can increase the alloy’s resistance to adhesion and reduce the adhesive wear of the alloy. This is due to the grain refinement produced by ECAE can vary the degree and severity of plastic deformation. The ␣ phase grain refinement is an important factor in promoting the wear resistance of the aluminum bronzes and the morphology and amount of the second phase affect the mechanical, friction and wear behaviors of the alloy greatly [13]. Grain refinement increases the hardness significantly and thereby reduces the plastic deformation of the specimens with ECAE. Therefore, the features of delamination and adhesion wear of specimen without ECAE are replaced by numerous wear debris formed on the wear surface specimen with ECAE after two passes. 4. Conclusions (1) The grains are refined and grain size decreases considerably after ECAE. The hardness and strength of the alloy increase after ECAE. (2) The friction coefficient of specimen without ECAE is higher than that of specimens with ECAE and decreases with the decrease of grain size. (3) The adhesive wear is primary wear mechanisms for the specimen without ECAE under dry sliding, while abrasive wear is the primary wear mechanism of the specimen after two passes of ECAE. (4) The grain refinement can improve wear resistance and the load bearing capacity of the alloy during dry sliding. A high load bearing capacity of 220 N is observed for the alloy after two passes of ECAE. 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. References
Fig. 8. EDX spectrum of the worn counterfaces for aluminum bronze alloy: (a) without ECAE and (b) after ECAE with two passes.
[1] S. Ferrasse, V.M. Segal, F. Alford, Effect of additional processing on texture evolution of Al0.5Cu alloy processed by equal channel angular extrusion (ECAE), Mater. Sci. Eng. A 372 (2004) 44–55. [2] K.-T. Park, S.-H. Myung, D.H. Shin, C.S. Lee, Size and distribution of particles and voids pre-existing in equal channel angular pressed 5083 Al alloy: their effect on cavitation during low-temperature superplastic deformation, Mater. Sci. Eng. A 371 (2004) 178–186. [3] E. Rabkin, I. Gutman, M. Kazakevich, E. Buchman, D. Gorni, Correlation between the nanomechanical properties and microstructure of ultrafine-grained copper produced by equal channel angular pressing, Mater. Sci. Eng. A 396 (2005) 11–21. [4] W.H. Huang, L. Chang, P.W. Kao, C.P. Chang, Effect of die angle on the deformation texture of copper processed by equal channel angular extrusion, Mater. Sci. Eng. A 307 (2001) 113–118.
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991 [5] Y.C. Chen, Y.Y. Huang, C.P. Chang, P.W. Kao, The effect of extrusion temperature on the development of deformation microstructures in 5052 aluminium alloy processed by equal channel angular extrusion, Acta Mater. 51 (2003) 2005–2015. [6] Z.H. Li, G.Q. Xiang, X.H. Cheng, Effects of ECAE process on microstructure and transformation behavior of TiNi shape memory alloy, Mater. Des. 27 (2006) 324–328. [7] G.G. Yapici, I. Karaman, Z.P. Luo, H. Rack, Microstructure and mechanical properties of severely deformed powder processed Ti–6Al–4V using equal channel angular extrusion, Scripta Mater. 49 (2003) 1021–1027. [8] J.Y. Suh, J.H. Han, K.H. Oh, J.C. Lee, Effect of deformation histories on texture evolution during equal- and dissimilar-channel angular pressing, Scripta Mater. 49 (2003) 185–190.
991
[9] W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Mechanical properties and microstructures of an AZ61 Mg Alloy produced by equal channel angular pressing, Scripta Mater. 47 (2002) 39–44. [10] V.V. Stolyarov, R. Lapovok, I.G. Brodova, P.F. Thomson, Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure, Mater. Sci. Eng. A 357 (2003) 159–167. [11] S.Z. Wen, Tribology, Tinghua University Press, Beijing (1990) 384. [12] Z.N. Farhat, Y. Ding, D.O. Northwood, A.T. Alpas, Effect of grain size on friction and wear of nanocrystalline aluminum, Mater. Sci. Eng. A 206 (1996) 302– 313. [13] Y.Y. Li, T.L. Ngai, W. Xia, Mechanical, friction and wear behaviors of a novel high-strength wear-resisting aluminum bronze, Wear 194 (1996) 130–136.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Microstructure and dry sliding wear behavior of Cu–10%Al–4%Fe alloy produced by equal channel angular extrusion L.L. Gao a , X.H. Cheng a,b,∗ a b
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China The State Key Laboratory of Tribology, Tsinghua University, Pekin 100084, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2006 Received in revised form 13 January 2008 Accepted 18 February 2008 Available online 18 April 2008 Keywords: ECAE Aluminum–bronze alloy Friction Wear
a b s t r a c t A commercial aluminum bronze alloy (Cu–10%Al–4%Fe) produced by hot-rolling was subjected to equal channel angular extrusion (ECAE) process at high temperature. The effect of ECAE on microstructure, mechanical and tribological properties of the alloy was investigated. Experimental results showed that grain size decreased and the second phase was rearranged after ECAE. The hardness and strength of the alloy with ECAE were higher than that of the alloy without ECAE and increased with the increase pass number. The wear resistance of the alloy after ECAE was significantly improved. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aluminum bronzes have been used as high strength, corrosion and wear resistance for many decades. Its high strength and wearresisting properties make them suitable for hardness and wear resistance applications, as engineering tools and dies, bushings, guide plates, etc. In practice, many modern engineering applications require a rather sophisticated combination of mechanical and tribological properties. At present, the main method to improve the mechanical properties of the alloys is to change the composition of the material. However, this method has its own limitation and cannot improve the tribological properties of the alloys significantly. It is necessary to find a new method to improve the mechanical properties and wear resistance of the alloys. It has been intensively studied for producing bulk ultrafinegrained materials by using severe plastic deformation techniques [1]. Recently, equal channel angular extrusion (ECAE) has been proven to be available for fabricating ultrafine-grained bulk materials. During ECAE, significant grain refinement occurs, resulting in the spectacular enhancement of the strength of a working material [2]. Active research efforts have been made recently, and successful applications have been reported for various materials such as copper [3,4], Al alloys [5] and Ti alloys [6], etc. How-
∗ Corresponding author at: School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China. Tel.: +86 21 62932404; fax: +86 21 34205880. E-mail address: [email protected] (X.H. Cheng). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.014
ever, previous ECAE studies are concerned mostly with producing an ultrafine-grained microstructure from single-phase alloys with high ductility at room temperature. For the two-phase alloy, they are generally difficulty-to-deformation materials and an increase of the extrusion temperature is required to increase their ductility [7]. Considering the industrial importance of two-phase materials, it is essential to establish optimum processing conditions of such difficult-to-deformation materials to study the microstructural changes and the effects of ECAE on mechanical and tribological properties of two-phase alloys. In this research, ECAE was carried out on a two-phase aluminum bronze alloy, Cu–10%Al–4%Fe. The effects of ECAE on the microstructure, mechanical and tribological properties of the alloy were investigated. 2. Experimental procedures A commercial aluminum bronze alloy rod, Cu–10%Al–4%Fe (the concentration of alloying elements was given in wt.%) obtained in the as-rolling condition was used as experimental material for ECAE. The billets with 9.6 mm ×9.6 mm in cross-section and 100 mm in length were cut from the alloy rod. The die used for ECAE consisted of two rectangular channels with the cross-section area 10 mm ×10 mm intersecting at angle of 90◦ . The extrusion velocity was 5 mm/s and an effective strain of about 1 was yielded by a single pass. 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 temperature of 600 ◦ C.
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
987
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
Optical electron microscope was used to study the microstructural evolution. The specimens for microstructure observation were cut along the extrusion direction. The specimens were ground mechanically using abrasive papers and alumina powders, and their surfaces were etched by immersing in a solution of 8% HF, 22% HNO3 and 70% H2 O for about 15 s. Vickers microhardness measurements were taken with loads of 50 g applied for 13 s. The specimens for tensile testing were machined from the as-received and the extruded billets with gauge length of 10 mm and cross-section of 2 mm × 2 mm. The specimen axis was aligned with the extrusion direction. The tensile tests were conducted at room temperature with strain rate of 10−3 s−1 . The friction and wear behavior of the aluminum bronze alloy blocks sliding against a GCr15 steel ring was evaluated on an M2000 model ring-on-block test ring. The contact schematic diagram of the frictional pair is shown in Fig. 1. The blocks in a size of 20 mm × 8 mm × 8 mm were made of the alloy, and the rings of ˚ 40 mm × 10 mm were made of GCr15 steel. The chemical compositions of the GCr15 steel are given in Table 1. Before each test, the GCr15 steel ring and the block were abraded with No.900 water-abrasive paper to reach a surface roughness Ra of 0.1 and 0.1–0.2 m, respectively. The steel ring and the blocks were thoroughly degreased by acetone and dried before the commencement of each wear test. The sliding velocity is 0.42 m/s.
Fig. 1. Contact schematic diagram for the frictional pair (unit: mm).
During the tests, the load was increased gradually until the maximum possible load could be applied or until seizure took place. The friction coefficient, , is calculated by the expression =
T FN R
(1)
where T is the friction torque recorded by the tester (N m), FN the normal force (N), R the radius of the ring (m). A characteristic value, which describes the wear performance under the chosen conditions for a tribosystem, is the specific wear rate K=
V FN L
Fig. 2. Microstructure of aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
(2)
988
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
V =
R2 arcsin
b b − 2R
R2 − (b/2) 2
2
B
(3)
where L is the sliding distance (m), V the worn volume loss (m3 ), b the width of the wear track (m), B the width of specimen (m). The morphologies of worn surfaces were examined by using a scanning electron microscope (SEM, Model: S-520, Hitachi Inc.). 3. Results and discussion 3.1. Microstructural evolution Fig. 2 shows the optical images of the specimens without ECAE, after ECAE with one pass and two passes. Before ECAE, it is apparent that ␣ phase and the second phase are elongated along rolling direction and ␣ phase parallels the second phase, as shown in Fig. 2(a). After ECAE with one pass, severe plastic deformation is imposed during ECAE. The microstructure of the alloy mainly consists of low-angle boundaries and the grain size is decreased, as shown in Fig. 2(b). After ECAE with two passes, the grain size of the alloy is further decreased and some equiaxed grains occur in some areas, as shown in Fig. 2(c). The specimens undergo essentially simple shear during ECAE [8]. The low-angle boundaries are truncated by shear planes and form a microstructure with large angle boundaries. As a result, the grain size reduces progressively with the increase of pass number. It is also seen from Fig. 2(b) and (c) that the volume fraction of second phase increases remarkably after the first pass. This is mainly attributed to the use of a pressing temperature of 600 ◦ C which leads to a increasing of the second phase during ECAE process. 3.2. Mechanical properties Vickers hardness and yield strength of specimens tested under the as-received condition and after ECAE with one and two passes are shown in Fig. 3. It is seen that the hardness and the yield strength of specimens after ECAE are higher than that of specimen without ECAE and both of them increase with the increase of pass number. It also can be seen that the increase of hardness and yield strength after one pass is larger than that of the specimen after two passes of ECAE. The increase of hardness and strength is attributed to grain refinement produced by ECAE. The grain refinement strengthen-
Fig. 3. Vickers microhardness and yield strength of the aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
Fig. 4. Wear volume of aluminum bronze specimens with sliding distance under a load of 50 N.
ing is generally described by a Hall–Petch equation [9]. The grain of the aluminum bronze alloy is refined and the grain size decrease with the increase of pass number, as shown in Fig. 2. Thus, the hardness and yield strength of the alloy increase after ECAE. In addition, the volume fraction of the second phase increases and the second phase is more homogeneously distributed in the alloy after ECAE. It is well known that hardness and strength are directly proportional to the volume fraction of the second phase and the dispersed distribution of the second phase can result in an increase of hardness and strength [10]. From Fig. 2(b) and (c), it can be seen that the volume fraction of the second phase increases after the first pass and does not increase after the second pass. As a result, the increase in hardness and yield strength after the first pass is larger than that of the specimen after two passes of ECAE. 3.3. Friction and wear properties Fig. 4 shows the wear volume of specimens without and after ECAE with one and two passes as a function of sliding distance. It can be seen that wear volume of specimen without ECAE is higher than that of the specimens with ECAE. The wear volume is the lowest for the specimen after two passes of ECAE, which is correlated to the lowest grain size of the specimens with and without ECAE. Fig. 5 shows the variations of wear rate of specimens without and with ECAE at various loads. It can be seen that the wear rate of all specimens increases with the increase of load. Under the same load, the wear rate of specimens with ECAE-treated is lower than that of specimen without ECAE-treated. The wear rate decreases with the increase of pass number, which indicates that grain refinement can decrease wear rate of the alloy. It also can be seen that the wear rate has a dramatically increase when seizure happens. The wear resistance is improved and the resistance to seizure increases after ECAE. Thus, it appears that grain refinement can improve the load bearing capacity of the aluminum bronze alloy during wear. Fig. 6 shows the variations of friction coefficient of specimens without ECAE and with ECAE under the load of 50 N after 30 min. It is observed that the friction coefficient of specimens with ECAE is lower than that of specimen without ECAE. And the friction coefficient decreases with the decrease of grain size. The decrease of friction coefficient and wear rate of the alloy after ECAE is mainly due to the improvement of mechanical prop-
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
Fig. 5. Variations of wear rate with load for the aluminum bronze specimens with and without ECAE.
erties caused by grain refinement. The friction coefficient is related to adhesive wear and abrasive wear. It can be expressed as following [11]: f =
b 2 + ctg s
(4)
989
Fig. 6. Friction coefficient of the aluminum bronze specimens: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
where b is the shear strength and s is the yield strength of the alloy. The first part of Eq. (4) is mainly caused by adhesive wear and the second part of Eq. (4) is decided by abrasive wear. After ECAE, the yield strength of the alloy increase with the increase of pass number, as shown in Fig. 3. The grain was refined after ECAE, which decreases the size of abrasive particles detached from the
Fig. 7. SEM morphologies of the worn surface of aluminum bronze specimens with 100 N load: (a) without ECAE, (b) after ECAE with one pass and (c) after ECAE with two passes.
990
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991
aluminum bronze alloy. From Eq. (4), it can be concluded that the friction coefficient decrease after ECAE. The wear volume is inverse proportion to hardness according to Archard’s law [12]. With the decrease of grain size after ECAE, the hardness of the aluminum bronze alloy also increases, accordingly the wear volume decreases. This is due to the hardness of ECAE can improve the deformation resistance of the alloy. Furthermore, the volume fraction of second phase increases and the distribution of second phase rearranges after ECAE, which strengthen the surface of the alloy. As a result, the wear resistance of the alloy improves remarkably after ECAE. 3.4. SEM and EDX analysis of worn surfaces Fig. 7 shows the scanning electron micrographs of the worn surfaces of specimens with and without ECAE under dry sliding condition. For the specimen without ECAE, adhesive wear is the main mechanism and severely plastic deformation occurs during wear, which results in the larger extent of delaminating, as shown in Fig. 7(a). It is seen in Fig. 7(b) that adhesive wear decrease a lot and light plastic deformation remains when the grain size of the specimen decreases after one pass. The worn surface of the specimen after ECAE with two passes is characterized by deep wear
grooves, as shown in Fig. 7(c). These deep grooves on the wear surfaces are produced by hard particles which abrade the specimen surfaces on their entrapment. The hard particles may stem from the detachment of the second phase, work-hardened particles formed by shearing and rolling of the ␣ phase. Fig. 8 is the EDX spectrum of the worn counterfaces for the alloy without ECAE and after ECAE with two passes. It can be seen that large amount of Cu and Al are determined on the worn counterpace for specimen without ECAE. However, very little Al is determined on the worn counterface for specimen after ECAE with two passes. It is suggested that the Cu and Al on the worn counterface for the specimen after ECAE with two passes mainly comes from the aluminum bronze alloy in friction. It can be concluded that adhesive wear is the main mechanism of the specimen without ECAE, while abrasive wear is the main mechanism of the specimens after ECAE with two passes. This indicates that grain refinement of the alloy can increase the alloy’s resistance to adhesion and reduce the adhesive wear of the alloy. This is due to the grain refinement produced by ECAE can vary the degree and severity of plastic deformation. The ␣ phase grain refinement is an important factor in promoting the wear resistance of the aluminum bronzes and the morphology and amount of the second phase affect the mechanical, friction and wear behaviors of the alloy greatly [13]. Grain refinement increases the hardness significantly and thereby reduces the plastic deformation of the specimens with ECAE. Therefore, the features of delamination and adhesion wear of specimen without ECAE are replaced by numerous wear debris formed on the wear surface specimen with ECAE after two passes. 4. Conclusions (1) The grains are refined and grain size decreases considerably after ECAE. The hardness and strength of the alloy increase after ECAE. (2) The friction coefficient of specimen without ECAE is higher than that of specimens with ECAE and decreases with the decrease of grain size. (3) The adhesive wear is primary wear mechanisms for the specimen without ECAE under dry sliding, while abrasive wear is the primary wear mechanism of the specimen after two passes of ECAE. (4) The grain refinement can improve wear resistance and the load bearing capacity of the alloy during dry sliding. A high load bearing capacity of 220 N is observed for the alloy after two passes of ECAE. 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. References
Fig. 8. EDX spectrum of the worn counterfaces for aluminum bronze alloy: (a) without ECAE and (b) after ECAE with two passes.
[1] S. Ferrasse, V.M. Segal, F. Alford, Effect of additional processing on texture evolution of Al0.5Cu alloy processed by equal channel angular extrusion (ECAE), Mater. Sci. Eng. A 372 (2004) 44–55. [2] K.-T. Park, S.-H. Myung, D.H. Shin, C.S. Lee, Size and distribution of particles and voids pre-existing in equal channel angular pressed 5083 Al alloy: their effect on cavitation during low-temperature superplastic deformation, Mater. Sci. Eng. A 371 (2004) 178–186. [3] E. Rabkin, I. Gutman, M. Kazakevich, E. Buchman, D. Gorni, Correlation between the nanomechanical properties and microstructure of ultrafine-grained copper produced by equal channel angular pressing, Mater. Sci. Eng. A 396 (2005) 11–21. [4] W.H. Huang, L. Chang, P.W. Kao, C.P. Chang, Effect of die angle on the deformation texture of copper processed by equal channel angular extrusion, Mater. Sci. Eng. A 307 (2001) 113–118.
L.L. Gao, X.H. Cheng / Wear 265 (2008) 986–991 [5] Y.C. Chen, Y.Y. Huang, C.P. Chang, P.W. Kao, The effect of extrusion temperature on the development of deformation microstructures in 5052 aluminium alloy processed by equal channel angular extrusion, Acta Mater. 51 (2003) 2005–2015. [6] Z.H. Li, G.Q. Xiang, X.H. Cheng, Effects of ECAE process on microstructure and transformation behavior of TiNi shape memory alloy, Mater. Des. 27 (2006) 324–328. [7] G.G. Yapici, I. Karaman, Z.P. Luo, H. Rack, Microstructure and mechanical properties of severely deformed powder processed Ti–6Al–4V using equal channel angular extrusion, Scripta Mater. 49 (2003) 1021–1027. [8] J.Y. Suh, J.H. Han, K.H. Oh, J.C. Lee, Effect of deformation histories on texture evolution during equal- and dissimilar-channel angular pressing, Scripta Mater. 49 (2003) 185–190.
991
[9] W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Mechanical properties and microstructures of an AZ61 Mg Alloy produced by equal channel angular pressing, Scripta Mater. 47 (2002) 39–44. [10] V.V. Stolyarov, R. Lapovok, I.G. Brodova, P.F. Thomson, Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure, Mater. Sci. Eng. A 357 (2003) 159–167. [11] S.Z. Wen, Tribology, Tinghua University Press, Beijing (1990) 384. [12] Z.N. Farhat, Y. Ding, D.O. Northwood, A.T. Alpas, Effect of grain size on friction and wear of nanocrystalline aluminum, Mater. Sci. Eng. A 206 (1996) 302– 313. [13] Y.Y. Li, T.L. Ngai, W. Xia, Mechanical, friction and wear behaviors of a novel high-strength wear-resisting aluminum bronze, Wear 194 (1996) 130–136.