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Mechanical and tribological properties of Al–40Zn–Cu alloys
ARTICLE IN PRESS Tribology International 42 (2009) 176– 182
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Mechanical and tribological properties of Al– 40Zn– Cu alloys Yasin Alemdag˘, Temel Savas- kan Mechanical Engineering Department, Karadeniz Technical University, Kanuni Campus, 61080 Trabzon, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 9 August 2007 Received in revised form 15 April 2008 Accepted 18 April 2008 Available online 6 June 2008
One binary Al–40Zn and five ternary Al–40Zn–Cu alloys with different copper contents were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated in ascast state. Friction and wear properties of the ternary alloys were studied using a conforming block-ondisc type tester. The results obtained were compared with those of SAE 65 bearing bronze. The microstructure of Al–40Zn–Cu alloys consisted of aluminium-rich a dendrites surrounded by eutectoid a+Z phases and y (CuAl2) particles. Hardness of the ternary alloys increased continuously with increasing copper content, but their tensile strength decreased above 3% Cu. Friction coefficient and temperature of the Al–40Zn–Cu alloys and bronze increased in the initial period of run. This was followed by a reduction in the properties and attainment of constant levels afterwards. However, volume loss of the alloys increased rapidly at the beginning of the test run and reached almost constant levels after a sliding distance of approximately 400 km. The Al–40Zn–Cu alloys were found to be much superior to the SAE 65 bronze, as far as their wear resistance is concerned. Among the alloys tested, highest strength and wear resistance were obtained with the Al–40Zn–3Cu alloy. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Al–40Zn–Cu alloys Effects of Cu content Mechanical and tribological properties
1. Introduction As a result of extensive research work carried out over the past, a number of zinc-based commercial alloys having eutectic (Zn–5Al), eutectoid (Zn–22Al) and monotectoid (Zn–40Al) compositions have been developed. The alloys showed better performance than either bronze or cast iron used in different engineering and tribological applications [1–14]. Zinc-based monotectoid alloys containing copper and silicon have been found to be superior to the alloys based on either eutectic or eutectoid compositions, as far as their mechanical and tribological properties are concerned [9,10,13,14]. Among the zinc-based ternary alloys, best mechanical properties and wear performance were obtained with the Zn–40Al–2Cu alloy [9,10,12]. However, only a few studies have been carried out on the evaluation of mechanical and tribological properties of aluminium-based ternary alloys containing zinc and small amounts of copper [15,16]. According to the results of these investigations, hardness, tensile strength and wear resistance of the Al-based ternary alloys increased with decreasing zinc and increasing copper contents [15,16]. Among the copper containing Al–Zn alloys, highest hardness and tensile strength were obtained with Al–40Zn–Cu alloys [15]. This indicates that the binary Al–40Zn alloy can be taken as the basis for preparing and investigating ternary
Corresponding author. Tel.: +90 462 377 2986; fax: +90 462 325 5526.
E-mail address: [email protected] (T. Savas- kan). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.04.008
Al–Zn–Cu alloys. However, the effects of copper content on the microstructure and properties of Al–40Zn–Cu alloys have not been fully understood. Especially, no work has been carried out pertaining to the lubricated friction and wear properties of these alloys. Therefore, the purpose of this work was to investigate the microstructure, mechanical properties and lubricated friction and wear behaviour of Al–40Zn–Cu casting alloys and to determine their most suitable chemical composition for tribological applications.
2. Experimental studies 2.1. Preparation of alloys, chemical composition and microstructure One binary Al–40Zn and five ternary Al–40Zn–Cu alloys were prepared by permanent mould casting. Commercially pure aluminium (99.7%), high-purity zinc (99.99%) and electrolytic copper (99.99%) were used for the preparation of the alloys. The alloys were melted in an electric furnace and poured at a temperature of approximately 650 1C into a permanent mould at room temperature. The mould had a conical shape with length 196 mm, internal diameter at the bottom 60 mm, and internal diameter at the top 72 mm. For comparing the wear behaviour of the Al–40Zn–Cu alloys with that of a conventional bearing material, a cylindrical ingot of centrifugally cast SAE 65 bronze (CuSn12) with a diameter of 60 mm was obtained from a commercial source.
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Fig. 1. Schematic diagram of the conforming block-on-disc test machine.
Chemical compositions of the alloys were determined by atomic absorption analysis. Metallographic samples of the aluminium-based alloys and bronze were prepared using standard techniques and etched in sodium hydroxide and ferric chloride solutions, respectively. The microstructure of the alloys was examined using both optical and scanning electron microscopy (SEM). 2.2. Physical and mechanical tests Density of the alloys was determined by measuring their volume and mass. Brinell hardness was measured using a 2.5 mm diameter ball indenter at a load of 62.5 kgf. Vickers microhardness of different phases of the alloys was also measured at a load of 50 gf. Tensile properties of the alloys were studied using specimens with 8 mm gauge diameter and 40 mm gauge length at a strain rate of 6.25 10 3 s 1. The hardness and tensile test results were determined by taking the average of at least three readings. 2.3. Friction and wear tests The friction and wear tests were carried out using a conforming block-on-disc test machine shown in Fig. 1. The machine consists of a disc, a block (specimen) and its mounting system, a speed control unit, a loading system and friction force and temperature measuring systems. The disc having a diameter of 15070.01 mm was fabricated with SAE 4140 steel (0.41% C, 0.90% Cu, 0.14% Mo, 0.90% Mn, 0.20% Si and balance Fe) and hardened to 5571 HRC. Samples 10 mm 15 mm 35 mm were prepared from the alloy castings and one end was machined with a 149.670.05 mm diameter cutter so that the resulting curved surface conformed exactly to the edge of the disc. The contact surface area was calculated to be 150.2 mm2. Friction and wear tests were performed at a constant pressure of 6 MPa and a sliding speed of 1.5 m s 1. Lubrication was provided by dropping SAE 20W/50 oil onto the revolving disc at a rate of 1.5 cm3 h 1. The friction force was measured using a load-cell and the friction coefficient was calculated by dividing the frictional force by normal load. Friction test for each sample was carried out for 12 h corresponding to a sliding distance of 64.8 km. The temperature of the samples was monitored by inserting a copper–nickel thermocouple in a hole at a distance of 1.5 mm
Table 1 Chemical composition of the alloys Alloy
Al–40Zn Al–40Zn–1Cu Al–40Zn–2Cu Al–40Zn–3Cu Al–40Zn–4Cu Al–40Zn–5Cu
Chemical composition (wt%) Al
Zn
Cu
61.2 59.1 57.3 56.9 56.6 55.3
38.8 39.7 40.5 40.0 39.3 39.5
– 1.2 2.2 3.1 4.1 5.2
from the rubbing surface. Each wear sample was ultrasonically cleaned and weighed before the wear test using a balance with the accuracy of 0.01 mg. The disc was cleaned with solvents to remove oil traces or other surface contaminants before each wear test. The sample was removed during the tests at an interval of 24 h corresponding to a sliding distance of 129.6 km, cleaned in solvents and reweighed to determine the weight loss. However, the first two readings were taken at an interval of 12 h to determine the initial wear behaviour of the alloys more accurately. The mass loss of the samples was converted into volume or wear loss using the measured density values of the alloys. Each wear test was carried out for 168 h corresponding to a sliding distance 907 km. The wear surface of the samples was examined with SEM.
3. Results 3.1. Chemical composition and microstructure The chemical composition of alloys is shown in Table 1. The microstructure of the binary alloy consisted of aluminium-rich a dendrites surrounded by eutectoid a+Z phase, Fig. 2a. In addition to these phases, y (CuAl2) particles formed in the interdendritic regions of the ternary Al–40Zn–Cu alloys (Fig. 2b–d). The y particles coarsened with increasing copper content of the alloy system. The microstructure of the SAE 65 bronze was observed to consist of copper-rich a dendrites and eutectoid a+d phases (Fig. 2e). The phases in the alloys and the bronze were identified
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Fig. 2. Microstructures of (a) Al–40Zn, (b) Al–40Zn–1Cu, (c) Al–40Zn–3Cu, (d) Al–40Zn–5Cu alloys and (e) SAE 65 bronze (CuSn12).
by optical and electron microscopy, energy dispersive analysis and X-ray diffraction work. 3.2. Physical and mechanical test results The change of density, hardness, tensile strength and elongation of the aluminium-based alloys and microhardness of a phase as a function of copper content are shown in Fig. 3. The density and hardness of the ternary alloys increased continuously with increasing copper content up to 3%. However, when the copper content exceeded 3%, the tensile strength of the alloys decreased. The microhardness of the a phase also increased with increasing copper content up to 3% and became constant above this level. However, the percentage elongation of the alloys decreased continuously with increasing copper content. 3.3. Friction and wear test results The variation of friction coefficient and temperature of the ternary Al–40Zn–Cu alloys and SAE 65 bronze as a function of sliding distance shown in Figs. 4 and 5. The friction coefficient of the alloys attained constant levels following an initial decrease.
However, their temperature increased initially, followed by a decrease and a steady-state value (Fig. 5). In addition, friction coefficient and temperature of the SAE 65 bronze were considerably higher than those of the Al–40Zn–Cu alloys. Volume or wear loss of the alloys is plotted as a function of sliding distance in Fig. 6. The volume loss rapidly increased in the beginning of the tests and reached constant levels after a sliding distance of approximately 400 km. Among the alloys, lowest volume loss was obtained for the Al–40Zn–3Cu alloy, while the SAE 65 bronze showed the highest. The volume loss of the ternary Al–40Zn–Cu alloys as a function of copper content is shown in Fig. 7. The volume loss of the alloys decreased with increasing copper content up to 3%, but the trend reversed above this level. Wear surfaces of three representative Al–40Zn–Cu alloys and the SAE 65 bronze are shown in Fig. 8a–d, respectively. The wear surfaces of Al–40Zn–Cu alloys are characterized by fine scratches and extensive smearing (Fig. 8a–c), while deep scratches and limited smearing were observed to be the main features of the wear surfaces of SAE 65 bronze (Fig. 8d). These observations indicate that smearing was the most effective wear mechanism for the Al–40Zn–Cu alloys, whereas scratches in addition to smearing were responsible for the wear of the SAE 65 bronze.
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Fig. 3. The change of density, hardness, microhardness of a phase, tensile strength and percentage elongation of Al–40Zn–Cu alloys as a function of copper content.
Fig. 4. Variation of friction coefficient of the alloys tested as a function of sliding distance.
Fig. 5. Temperature versus sliding distance curves for the alloys tested.
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Fig. 6. Plots of wear loss versus sliding distance of the alloys.
Fig. 7. The volume loss versus copper content for the alloys tested for a sliding distance of 907 km.
4. Discussion The microstructure of binary Al–40Zn alloy consisted of cored a dendrites and eutectoid a+Z phase (Fig. 2a). Addition of copper resulted in the formation of stable y (CuAl2) intermetallic phase in the interdendritic region of the ternary alloys (Fig. 2b–d). The size and volume fraction of the y phase increased as the copper content of the Al–40Zn–Cu alloys increased (Fig. 2b–d). It appears that replacing zinc with aluminium in Zn–Al–Cu system results in the formation of y phase in place of e particles. These observations are in agreement with the results of previous investigations and can be related to Al–Zn, Al–Cu and Al–Zn–Cu phase diagrams [15–19]. It was found that the hardness of the Al–40Zn–Cu alloys increased, but their percentage elongation decreased continuously with increasing copper content. However, the tensile strength of the alloys increased with increasing copper content up to 3%, above which the trend reversed (Fig. 3). These results may be explained in terms of microstructural features and solution strengthening mechanism. Addition of copper to the binary
Al–40Zn alloy results in solid solution strengthening of the a phase and formation of copper-rich y phase. Solid solution strengthening effect (solution of copper in a) causes an increase in both hardness and tensile strength of the ternary alloys. However, formation of hard and brittle y phase weakens the interdendritic regions of the alloys and gives rise to cracking tendency. Observations suggest that the solid solution strengthening is the dominant mechanism for the ternary alloys containing up to 3% copper, but weakening effect of y phase becomes more effective for the alloys containing more than 3% copper. Therefore, the tensile strength of the alloys increased with increasing copper content up to 3% Cu, above which the trend reversed, while their hardness increased continuously over the entire range of Cu concentration in the Al alloys (Fig. 3). Similar explanations have also been offered previously in the literature [6,12,20]. It was observed that the friction coefficient of alloys decreased in the initial stage of test run following a very sharp increase and reached almost constant levels after a sliding distance of approximately 10 km (Fig. 4). It was also observed that their temperature increased initially, followed by a decrease, and reached a steady-state value (Fig. 5). However, the wear loss of the alloys increased rapidly at the beginning of the test run and reached almost constant levels after a running distance of approximately 400 km (Fig. 6). These observations may be explained in terms of materials tribology and lubrication theory. At the beginning of the test run, the oil film is not stable and its thickness is not enough to separate the mating surface. During this stage, metal-to-metal contact takes place in part and gives rise to an increase in friction coefficient, temperature and wear loss of the alloys [21–23]. However, the increase in the friction coefficient was much sharper than that observed in the temperature. Therefore the friction coefficient reached its maximum value in a much shorter time than that observed for the temperature. This resulted in the formation of a phase difference between the curves of both parameters. As a result the temperature of the wear samples continued to increase, while their friction coefficient decreased in the initial stage of sliding. This may be explained according to the relations between the friction coefficient, frictional heat and temperature. It is well known that as the friction coefficient increases the amount of frictional heat increases and gives rise to an increase in temperature [21–23].
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Fig. 8. Wear surfaces of (a) Al–40Zn–1Cu, (b) Al–40Zn–3Cu, (c) Al–40Zn–5Cu alloys and (d) SAE 65 bronze tested for a sliding distance of 907 km.
However, the decrease in the friction coefficient does not reduce the amount of frictional heat; it only reduces the rate of its increase. Since the measured temperature indicates the level of frictional heat released, it continues to increase while the friction coefficient decreases. After the initial period, as the oil film thickness increases, metal-to-metal contact area would decrease and reach a minimum level after a certain range of sliding distance. As a result, friction coefficient, temperature and wear loss of the samples become almost constant. It was found that the wear loss of the alloys decreased with increasing copper content up to 3%, but increased above this level (Fig. 7). These observations may be explained in terms of microstructure and mechanical properties of the alloys. As mentioned earlier, the hardness and tensile strength of the Al–40Zn–Cu alloys increased with increasing copper content, but the tendency in the tensile strength reversed after 3% Cu. Despite increasing hardness above 3% Cu, wherein the tensile strength decreased, the wear resistance deteriorated. This signifies that tensile strength is more dominating in controlling the sliding wear behaviour of the alloys than that of the hardness. The increase in the wear loss and the decrease in the tensile strength and elongation of the alloys containing more than 3% Cu may be attributed to the predominant adverse effect of cracking tendency over strengthening caused by intermetallic y particles. Therefore, wear resistance (inverse of wear loss) and tensile strength are expected to increase with increasing copper content of the alloys up to 3% Cu and decrease above this level. The wear surfaces of Al–40Zn–Cu alloys were characterized by smearing and fine scratches (Fig. 8a–c). Smearing was caused by back transfer of contacting surface material from the disc surface to the sample surface, while the fine scratches are produced by the ploughing action of hard y phase and oxidised wear particles formed during sliding [8,10,24]. However, deep scratches and limited amount of smearing were observed on the wear surface of
SAE 65 bronze sample (Fig. 8d). The scratches may be related to the ploughing action of hard d phase particles of SAE 65 bronze. If these particles become loose due to wear, they act as abrasive particles during sliding. Limited amount of smearing observed on the wear surface of the bronze samples may result from removal and back transfer of material from contacting surfaces. These observations suggest that adhesion is the dominant wear mechanism for Al–40Zn–Cu alloys, while abrasion and adhesion both control the process of material removal significantly during the wear of the SAE 65 bronze. Accordingly, the Al–40Zn–Cu alloys displayed higher wear resistance than the SAE 65 bronze (Fig. 6).
5. Conclusions 1. The microstructure of Al–40Zn alloy consisted of aluminiumrich a dendrites and eutectoid a+Z phases. In addition to these phases, y (CuAl2) particles formed in the interdendritic regions of the copper containing ternary alloys. 2. Hardness of Al–40Zn–Cu alloys increased continuously with increasing copper content, but percentage elongation showed a reverse trend. The tensile strength of these alloys increased with increasing copper content up to 3%, but above this level it decreased as the copper content increased. This was related to two opposite effects: solid solution hardening of a phase and weakening effect through cracking tendency due to y phase. The latter is more effective at copper contents beyond 3%. 3. As the sliding distance increased, the friction coefficient, temperature and volume loss of alloys reached constant levels following an initial decrease in friction coefficient, an initial increase and a decrease in temperature and only an initial increase in volume loss. 4. The volume loss of Al–40Zn–Cu alloys decreased with increasing copper content up to 3%, but showed an increase above this
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level. Therefore, among these alloys, the highest tensile strength and wear resistance (inverse of volume loss) were attained by the Al–40Zn–3Cu alloy. 5. The wear loss of the Al–40Zn–Cu alloys was found to be inversely proportional to their tensile strength. However, it showed a similar change with percentage elongation up to 3% Cu, above which the trend reversed and hardness produced a mixed effect on it. 6. Adhesion was found to be the most effective wear mechanism for the Al–40Zn–Cu alloys, but both abrasion and adhesion operated considerably to cause the wear of the SAE 65 bronze.
Acknowledgments This work was supported by the Karadeniz Technical University under a research contract (No. 2003.112.003.1). The authors would like to thank all the technicians in the Materials Division and Machine Shop of the Mechanical Engineering Department of the University for their valuable help. References [1] Gervais E, Levert H, Bess M. The development of a family of zinc-base foundry alloys. AFS Trans 1980;88:183–94. [2] Murphy S, Savas-kan T. Comparative wear behaviour of Zn–Al based alloys in an automotive engine application. Wear 1984;98:151–61. [3] Savas- kan T, Murphy S. Mechanical properties and lubricated wear of Zn–25Albased alloys. Wear 1987;116:221–4. [4] Lee PP, Savas-kan T, Laufer E. Wear resistance and microstructure of Zn–Al–Si and Zn–Al–Cu alloys. Wear 1987;117:79–89. [5] Zhu YH, Yan B, Huang W. Bearing wear resistance of monotectoid Zn–Al based alloy (ZA-35). Mater Sci Technol. 1995;11:109–13. [6] Prasad BK, Patwardhan AK, Yegneswaran AH. Characterization of the wear response of a modified zinc-based alloy vis-a`-vis a conventional zinc-based alloy and a bearing bronze at a high sliding speed. Metall Mater Trans A 1996;27A:3513–23.
[7] Pandey JP, Prasad BK. Sliding wear response of a zinc-based alloy compared to a copper-based alloy. Metall Mater Trans A 1998;29A:1245–55. [8] Prasad BK, Patwardhan AK, Yegneswaran AH. Wear characteristics of a zincbased alloy compared with a conventional bearing bronze under mixed lubrication conditions: Effects of material and test parameters. Can Metall Q 2001;40(2):193–210. [9] Savas-kan T, Pu¨rc- ek G, Murphy S. Sliding wear of cast zinc-based alloy bearings under static and dynamic loading conditions. Wear 2002;252:693–703. [10] Pu¨rc- ek G, Savas-kan T, Ku¨c- u¨ko¨merog˘lu T, Murphy S. Dry sliding friction and wear properties of zinc-based alloys. Wear 2002;252:894–901. [11] Savas-kan T, Pu¨rc- ek G, Hekimog˘lu AP. Effect of copper content on the mechanical and tribological properties of ZnAl27-based alloys. Tribol Lett 2003;15(3):257–63. [12] Savas-kan T, Hekimog˘lu AP, Pu¨rc- ek G. Effect of copper content on the mechanical and sliding wear properties of monotectoid-based zinc–aluminium–copper alloys. Tribol Int 2004;37:45–50. [13] Savas-kan T, Aydıner A. Effects of silicon content on the mechanical and tribological properties monotectoid-based zinc–aluminium–silicon alloys. Wear 2004;257:377–88. [14] Savas-kan T, Bican O. Effects of silicon content on the microstructural features and mechanical and sliding wear properties of Zn–40Al–2Cu–(0–5)Si alloys. Mater Sci Eng A 2005;404:259–69. [15] Savas-kan T, Torul O, Cuvalci H. Examination of microstructure and mechanical properties of zinc-aluminium alloys. In: Metall. Congr. (Chamber of Turkish Metallurgical Engineers). Ankara, Turkey; 1988. p. 784–98. [16] Bektas-og˘lu A, Savas- kan T. An investigation of dry sliding wear properties of Zn–60Al–(1–5)Cu alloys. Eng Mach (Chamber of Turkish Mechanical Engineers) 2005;46(544):31–9. [17] Murphy S. Solid-phase reactions in the low-copper part of the Al–Cu–Zn system. Z Metallkd 1980;71:96–102. [18] Mondal DP, Das S, Rajput V. Effect of zinc concentration and experimental parameters on high stress abrasive wear behaviour of Al–Zn alloys: a factorial design approach. Mater Sci Eng A 2005;406:24–33. [19] Baker H, editor. Alloy phase diagrams. ASM handbook, Vol. 3, 1992. [20] Savas-kan T, Turhal MS- . Relationships between cooling rate, copper content and mechanical properties of monotectoid based Zn–Al–Cu alloys. Mater Charact 2003;51:259–70. [21] Halling J. Principle of tribology. London: Macmillan Education Ltd; 1989. [22] Hutchings IM. Tribology: friction and wear of engineering materials. London: Edward Arnold; 1992. [23] Bowdwen FP, Tabor D. The friction and lubrication of solids. Oxford: Clarendon Press; 2001. [24] Prasad BK. Effects of heat treatment on the partially lubricated sliding wear behaviour of a zinc-based alloy. Mater Trans JIM 1999;40(7):578–85.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Mechanical and tribological properties of Al– 40Zn– Cu alloys Yasin Alemdag˘, Temel Savas- kan Mechanical Engineering Department, Karadeniz Technical University, Kanuni Campus, 61080 Trabzon, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 9 August 2007 Received in revised form 15 April 2008 Accepted 18 April 2008 Available online 6 June 2008
One binary Al–40Zn and five ternary Al–40Zn–Cu alloys with different copper contents were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated in ascast state. Friction and wear properties of the ternary alloys were studied using a conforming block-ondisc type tester. The results obtained were compared with those of SAE 65 bearing bronze. The microstructure of Al–40Zn–Cu alloys consisted of aluminium-rich a dendrites surrounded by eutectoid a+Z phases and y (CuAl2) particles. Hardness of the ternary alloys increased continuously with increasing copper content, but their tensile strength decreased above 3% Cu. Friction coefficient and temperature of the Al–40Zn–Cu alloys and bronze increased in the initial period of run. This was followed by a reduction in the properties and attainment of constant levels afterwards. However, volume loss of the alloys increased rapidly at the beginning of the test run and reached almost constant levels after a sliding distance of approximately 400 km. The Al–40Zn–Cu alloys were found to be much superior to the SAE 65 bronze, as far as their wear resistance is concerned. Among the alloys tested, highest strength and wear resistance were obtained with the Al–40Zn–3Cu alloy. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Al–40Zn–Cu alloys Effects of Cu content Mechanical and tribological properties
1. Introduction As a result of extensive research work carried out over the past, a number of zinc-based commercial alloys having eutectic (Zn–5Al), eutectoid (Zn–22Al) and monotectoid (Zn–40Al) compositions have been developed. The alloys showed better performance than either bronze or cast iron used in different engineering and tribological applications [1–14]. Zinc-based monotectoid alloys containing copper and silicon have been found to be superior to the alloys based on either eutectic or eutectoid compositions, as far as their mechanical and tribological properties are concerned [9,10,13,14]. Among the zinc-based ternary alloys, best mechanical properties and wear performance were obtained with the Zn–40Al–2Cu alloy [9,10,12]. However, only a few studies have been carried out on the evaluation of mechanical and tribological properties of aluminium-based ternary alloys containing zinc and small amounts of copper [15,16]. According to the results of these investigations, hardness, tensile strength and wear resistance of the Al-based ternary alloys increased with decreasing zinc and increasing copper contents [15,16]. Among the copper containing Al–Zn alloys, highest hardness and tensile strength were obtained with Al–40Zn–Cu alloys [15]. This indicates that the binary Al–40Zn alloy can be taken as the basis for preparing and investigating ternary
Corresponding author. Tel.: +90 462 377 2986; fax: +90 462 325 5526.
E-mail address: [email protected] (T. Savas- kan). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.04.008
Al–Zn–Cu alloys. However, the effects of copper content on the microstructure and properties of Al–40Zn–Cu alloys have not been fully understood. Especially, no work has been carried out pertaining to the lubricated friction and wear properties of these alloys. Therefore, the purpose of this work was to investigate the microstructure, mechanical properties and lubricated friction and wear behaviour of Al–40Zn–Cu casting alloys and to determine their most suitable chemical composition for tribological applications.
2. Experimental studies 2.1. Preparation of alloys, chemical composition and microstructure One binary Al–40Zn and five ternary Al–40Zn–Cu alloys were prepared by permanent mould casting. Commercially pure aluminium (99.7%), high-purity zinc (99.99%) and electrolytic copper (99.99%) were used for the preparation of the alloys. The alloys were melted in an electric furnace and poured at a temperature of approximately 650 1C into a permanent mould at room temperature. The mould had a conical shape with length 196 mm, internal diameter at the bottom 60 mm, and internal diameter at the top 72 mm. For comparing the wear behaviour of the Al–40Zn–Cu alloys with that of a conventional bearing material, a cylindrical ingot of centrifugally cast SAE 65 bronze (CuSn12) with a diameter of 60 mm was obtained from a commercial source.
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Fig. 1. Schematic diagram of the conforming block-on-disc test machine.
Chemical compositions of the alloys were determined by atomic absorption analysis. Metallographic samples of the aluminium-based alloys and bronze were prepared using standard techniques and etched in sodium hydroxide and ferric chloride solutions, respectively. The microstructure of the alloys was examined using both optical and scanning electron microscopy (SEM). 2.2. Physical and mechanical tests Density of the alloys was determined by measuring their volume and mass. Brinell hardness was measured using a 2.5 mm diameter ball indenter at a load of 62.5 kgf. Vickers microhardness of different phases of the alloys was also measured at a load of 50 gf. Tensile properties of the alloys were studied using specimens with 8 mm gauge diameter and 40 mm gauge length at a strain rate of 6.25 10 3 s 1. The hardness and tensile test results were determined by taking the average of at least three readings. 2.3. Friction and wear tests The friction and wear tests were carried out using a conforming block-on-disc test machine shown in Fig. 1. The machine consists of a disc, a block (specimen) and its mounting system, a speed control unit, a loading system and friction force and temperature measuring systems. The disc having a diameter of 15070.01 mm was fabricated with SAE 4140 steel (0.41% C, 0.90% Cu, 0.14% Mo, 0.90% Mn, 0.20% Si and balance Fe) and hardened to 5571 HRC. Samples 10 mm 15 mm 35 mm were prepared from the alloy castings and one end was machined with a 149.670.05 mm diameter cutter so that the resulting curved surface conformed exactly to the edge of the disc. The contact surface area was calculated to be 150.2 mm2. Friction and wear tests were performed at a constant pressure of 6 MPa and a sliding speed of 1.5 m s 1. Lubrication was provided by dropping SAE 20W/50 oil onto the revolving disc at a rate of 1.5 cm3 h 1. The friction force was measured using a load-cell and the friction coefficient was calculated by dividing the frictional force by normal load. Friction test for each sample was carried out for 12 h corresponding to a sliding distance of 64.8 km. The temperature of the samples was monitored by inserting a copper–nickel thermocouple in a hole at a distance of 1.5 mm
Table 1 Chemical composition of the alloys Alloy
Al–40Zn Al–40Zn–1Cu Al–40Zn–2Cu Al–40Zn–3Cu Al–40Zn–4Cu Al–40Zn–5Cu
Chemical composition (wt%) Al
Zn
Cu
61.2 59.1 57.3 56.9 56.6 55.3
38.8 39.7 40.5 40.0 39.3 39.5
– 1.2 2.2 3.1 4.1 5.2
from the rubbing surface. Each wear sample was ultrasonically cleaned and weighed before the wear test using a balance with the accuracy of 0.01 mg. The disc was cleaned with solvents to remove oil traces or other surface contaminants before each wear test. The sample was removed during the tests at an interval of 24 h corresponding to a sliding distance of 129.6 km, cleaned in solvents and reweighed to determine the weight loss. However, the first two readings were taken at an interval of 12 h to determine the initial wear behaviour of the alloys more accurately. The mass loss of the samples was converted into volume or wear loss using the measured density values of the alloys. Each wear test was carried out for 168 h corresponding to a sliding distance 907 km. The wear surface of the samples was examined with SEM.
3. Results 3.1. Chemical composition and microstructure The chemical composition of alloys is shown in Table 1. The microstructure of the binary alloy consisted of aluminium-rich a dendrites surrounded by eutectoid a+Z phase, Fig. 2a. In addition to these phases, y (CuAl2) particles formed in the interdendritic regions of the ternary Al–40Zn–Cu alloys (Fig. 2b–d). The y particles coarsened with increasing copper content of the alloy system. The microstructure of the SAE 65 bronze was observed to consist of copper-rich a dendrites and eutectoid a+d phases (Fig. 2e). The phases in the alloys and the bronze were identified
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Fig. 2. Microstructures of (a) Al–40Zn, (b) Al–40Zn–1Cu, (c) Al–40Zn–3Cu, (d) Al–40Zn–5Cu alloys and (e) SAE 65 bronze (CuSn12).
by optical and electron microscopy, energy dispersive analysis and X-ray diffraction work. 3.2. Physical and mechanical test results The change of density, hardness, tensile strength and elongation of the aluminium-based alloys and microhardness of a phase as a function of copper content are shown in Fig. 3. The density and hardness of the ternary alloys increased continuously with increasing copper content up to 3%. However, when the copper content exceeded 3%, the tensile strength of the alloys decreased. The microhardness of the a phase also increased with increasing copper content up to 3% and became constant above this level. However, the percentage elongation of the alloys decreased continuously with increasing copper content. 3.3. Friction and wear test results The variation of friction coefficient and temperature of the ternary Al–40Zn–Cu alloys and SAE 65 bronze as a function of sliding distance shown in Figs. 4 and 5. The friction coefficient of the alloys attained constant levels following an initial decrease.
However, their temperature increased initially, followed by a decrease and a steady-state value (Fig. 5). In addition, friction coefficient and temperature of the SAE 65 bronze were considerably higher than those of the Al–40Zn–Cu alloys. Volume or wear loss of the alloys is plotted as a function of sliding distance in Fig. 6. The volume loss rapidly increased in the beginning of the tests and reached constant levels after a sliding distance of approximately 400 km. Among the alloys, lowest volume loss was obtained for the Al–40Zn–3Cu alloy, while the SAE 65 bronze showed the highest. The volume loss of the ternary Al–40Zn–Cu alloys as a function of copper content is shown in Fig. 7. The volume loss of the alloys decreased with increasing copper content up to 3%, but the trend reversed above this level. Wear surfaces of three representative Al–40Zn–Cu alloys and the SAE 65 bronze are shown in Fig. 8a–d, respectively. The wear surfaces of Al–40Zn–Cu alloys are characterized by fine scratches and extensive smearing (Fig. 8a–c), while deep scratches and limited smearing were observed to be the main features of the wear surfaces of SAE 65 bronze (Fig. 8d). These observations indicate that smearing was the most effective wear mechanism for the Al–40Zn–Cu alloys, whereas scratches in addition to smearing were responsible for the wear of the SAE 65 bronze.
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Fig. 3. The change of density, hardness, microhardness of a phase, tensile strength and percentage elongation of Al–40Zn–Cu alloys as a function of copper content.
Fig. 4. Variation of friction coefficient of the alloys tested as a function of sliding distance.
Fig. 5. Temperature versus sliding distance curves for the alloys tested.
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Fig. 6. Plots of wear loss versus sliding distance of the alloys.
Fig. 7. The volume loss versus copper content for the alloys tested for a sliding distance of 907 km.
4. Discussion The microstructure of binary Al–40Zn alloy consisted of cored a dendrites and eutectoid a+Z phase (Fig. 2a). Addition of copper resulted in the formation of stable y (CuAl2) intermetallic phase in the interdendritic region of the ternary alloys (Fig. 2b–d). The size and volume fraction of the y phase increased as the copper content of the Al–40Zn–Cu alloys increased (Fig. 2b–d). It appears that replacing zinc with aluminium in Zn–Al–Cu system results in the formation of y phase in place of e particles. These observations are in agreement with the results of previous investigations and can be related to Al–Zn, Al–Cu and Al–Zn–Cu phase diagrams [15–19]. It was found that the hardness of the Al–40Zn–Cu alloys increased, but their percentage elongation decreased continuously with increasing copper content. However, the tensile strength of the alloys increased with increasing copper content up to 3%, above which the trend reversed (Fig. 3). These results may be explained in terms of microstructural features and solution strengthening mechanism. Addition of copper to the binary
Al–40Zn alloy results in solid solution strengthening of the a phase and formation of copper-rich y phase. Solid solution strengthening effect (solution of copper in a) causes an increase in both hardness and tensile strength of the ternary alloys. However, formation of hard and brittle y phase weakens the interdendritic regions of the alloys and gives rise to cracking tendency. Observations suggest that the solid solution strengthening is the dominant mechanism for the ternary alloys containing up to 3% copper, but weakening effect of y phase becomes more effective for the alloys containing more than 3% copper. Therefore, the tensile strength of the alloys increased with increasing copper content up to 3% Cu, above which the trend reversed, while their hardness increased continuously over the entire range of Cu concentration in the Al alloys (Fig. 3). Similar explanations have also been offered previously in the literature [6,12,20]. It was observed that the friction coefficient of alloys decreased in the initial stage of test run following a very sharp increase and reached almost constant levels after a sliding distance of approximately 10 km (Fig. 4). It was also observed that their temperature increased initially, followed by a decrease, and reached a steady-state value (Fig. 5). However, the wear loss of the alloys increased rapidly at the beginning of the test run and reached almost constant levels after a running distance of approximately 400 km (Fig. 6). These observations may be explained in terms of materials tribology and lubrication theory. At the beginning of the test run, the oil film is not stable and its thickness is not enough to separate the mating surface. During this stage, metal-to-metal contact takes place in part and gives rise to an increase in friction coefficient, temperature and wear loss of the alloys [21–23]. However, the increase in the friction coefficient was much sharper than that observed in the temperature. Therefore the friction coefficient reached its maximum value in a much shorter time than that observed for the temperature. This resulted in the formation of a phase difference between the curves of both parameters. As a result the temperature of the wear samples continued to increase, while their friction coefficient decreased in the initial stage of sliding. This may be explained according to the relations between the friction coefficient, frictional heat and temperature. It is well known that as the friction coefficient increases the amount of frictional heat increases and gives rise to an increase in temperature [21–23].
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Fig. 8. Wear surfaces of (a) Al–40Zn–1Cu, (b) Al–40Zn–3Cu, (c) Al–40Zn–5Cu alloys and (d) SAE 65 bronze tested for a sliding distance of 907 km.
However, the decrease in the friction coefficient does not reduce the amount of frictional heat; it only reduces the rate of its increase. Since the measured temperature indicates the level of frictional heat released, it continues to increase while the friction coefficient decreases. After the initial period, as the oil film thickness increases, metal-to-metal contact area would decrease and reach a minimum level after a certain range of sliding distance. As a result, friction coefficient, temperature and wear loss of the samples become almost constant. It was found that the wear loss of the alloys decreased with increasing copper content up to 3%, but increased above this level (Fig. 7). These observations may be explained in terms of microstructure and mechanical properties of the alloys. As mentioned earlier, the hardness and tensile strength of the Al–40Zn–Cu alloys increased with increasing copper content, but the tendency in the tensile strength reversed after 3% Cu. Despite increasing hardness above 3% Cu, wherein the tensile strength decreased, the wear resistance deteriorated. This signifies that tensile strength is more dominating in controlling the sliding wear behaviour of the alloys than that of the hardness. The increase in the wear loss and the decrease in the tensile strength and elongation of the alloys containing more than 3% Cu may be attributed to the predominant adverse effect of cracking tendency over strengthening caused by intermetallic y particles. Therefore, wear resistance (inverse of wear loss) and tensile strength are expected to increase with increasing copper content of the alloys up to 3% Cu and decrease above this level. The wear surfaces of Al–40Zn–Cu alloys were characterized by smearing and fine scratches (Fig. 8a–c). Smearing was caused by back transfer of contacting surface material from the disc surface to the sample surface, while the fine scratches are produced by the ploughing action of hard y phase and oxidised wear particles formed during sliding [8,10,24]. However, deep scratches and limited amount of smearing were observed on the wear surface of
SAE 65 bronze sample (Fig. 8d). The scratches may be related to the ploughing action of hard d phase particles of SAE 65 bronze. If these particles become loose due to wear, they act as abrasive particles during sliding. Limited amount of smearing observed on the wear surface of the bronze samples may result from removal and back transfer of material from contacting surfaces. These observations suggest that adhesion is the dominant wear mechanism for Al–40Zn–Cu alloys, while abrasion and adhesion both control the process of material removal significantly during the wear of the SAE 65 bronze. Accordingly, the Al–40Zn–Cu alloys displayed higher wear resistance than the SAE 65 bronze (Fig. 6).
5. Conclusions 1. The microstructure of Al–40Zn alloy consisted of aluminiumrich a dendrites and eutectoid a+Z phases. In addition to these phases, y (CuAl2) particles formed in the interdendritic regions of the copper containing ternary alloys. 2. Hardness of Al–40Zn–Cu alloys increased continuously with increasing copper content, but percentage elongation showed a reverse trend. The tensile strength of these alloys increased with increasing copper content up to 3%, but above this level it decreased as the copper content increased. This was related to two opposite effects: solid solution hardening of a phase and weakening effect through cracking tendency due to y phase. The latter is more effective at copper contents beyond 3%. 3. As the sliding distance increased, the friction coefficient, temperature and volume loss of alloys reached constant levels following an initial decrease in friction coefficient, an initial increase and a decrease in temperature and only an initial increase in volume loss. 4. The volume loss of Al–40Zn–Cu alloys decreased with increasing copper content up to 3%, but showed an increase above this
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level. Therefore, among these alloys, the highest tensile strength and wear resistance (inverse of volume loss) were attained by the Al–40Zn–3Cu alloy. 5. The wear loss of the Al–40Zn–Cu alloys was found to be inversely proportional to their tensile strength. However, it showed a similar change with percentage elongation up to 3% Cu, above which the trend reversed and hardness produced a mixed effect on it. 6. Adhesion was found to be the most effective wear mechanism for the Al–40Zn–Cu alloys, but both abrasion and adhesion operated considerably to cause the wear of the SAE 65 bronze.
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