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The effect of production parameters on microstructure and wear resistance of powder metallurgy Al–Al2O3 composite
Materials and Design 32 (2011) 1031–1038
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
The effect of production parameters on microstructure and wear resistance of powder metallurgy Al–Al2O3 composite Mehdi Rahimian a,*, Nader Parvin b, Naser Ehsani c a
Faculty of Engineering, Islamic Azad University-Semnan Branch, Semnan, Iran Faculty of Mining and Materials Engineering, Amirkabir University of Technology (AUT), Hafez Ave., Tehran, Iran c Faculty of Materials and Manufacturing Technology, Malek Ashtar University of Technology (MUT), Babayi Highway, Lavizan, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 23 March 2010 Accepted 13 July 2010 Available online 16 July 2010
a b s t r a c t Aluminum matrix composite is one of the most conventional types of metal matrix composites. This paper deals with the effect of production parameters on wear resistance of Al–Al2O3 composites. Alumina powder with a particle size of 12, 3 and 48 l and pure aluminum powder with particle size of 30 l were used. The amount of added alumina powder was up to 20%. Ball milling was utilized to blend the powders. The range of sintering temperature and time were 500, 550 and 600 °C and 30, 45, 60 and 90 min respectively. It was found that increasing sintering temperature results in increasing density, hardness and wear resistance and homogenization of the microstructure. However at certain sintering temperatures and time, considerable grain growth and reduction of hardness value occurred, leading to the degradation of wear resistance. The results showed that at high alumina content, relative density of the composite increases. However, after raising the particle size of alumina, relative density initially increases and then drops to lower values. Increasing weight percent of alumina powder leads to higher hardness and consequently improves the wear resistance of Al–Al2O3 composite. The use of fine alumina particles has a similar effect on hardness and the wear resistance. Finally, a finer grain size was observed, at high amount and low size of the reinforcement particle. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction There is an increasing trend towards using composite materials in order to achieve better performance in engineering materials. Thus, production and application of metal matrix composites (MMC) have increased in recent years [1]. Al–Al2O3 MMC, has been of greater importance due to superior mechanical properties and excellent wear resistance, under various applications [2]. Aluminum matrix composites have made numerous applications in aerospace, automotive, military and electronic industry due to low density, high toughness and high corrosion resistance [3,4]. Low wear resistance of pure aluminum is a serious drawback in using it in many applications. Addition of ceramic particles to aluminum matrix would improve the strength, hardness, wear resistance and corrosion resistance of the matrix [5,6]. Particle reinforcements are more favorable than fiber type, due to better control of microstructure and mechanical properties, by varying the size and the volume fraction of the reinforcement [3]. Al2O3 is the most popular among ceramic particle reinforcement after SiC particles. Al2O3 has higher thermal stability compared to SiC, * Corresponding author. Address: No. 87-1, Shohada 22, Shohada Street, Semnan 3513645575, Iran. Tel.: +98 9125322792. E-mail addresses: [email protected] (M. Rahimian), [email protected] (N. Parvin), [email protected] (N. Ehsani). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.07.016
since it does not react with the metal matrix at high temperatures and does not produce brittle phases [1]. Powder metallurgy is considered as a good technique in producing metal–matrix composites. An important advantage of this method is its low processing temperature compared to melting techniques. Therefore, interaction between the matrix and the reinforcement phases is prevented. On the other hand, good distribution of the reinforcing particles can be achieved [4]. Another advantage of powder metallurgy technique is in its ability to manufacture near net shape product at low cost [7]. In this research, uniaxial pressing was used to produce composite sample. The effects of sintering time and temperature, weight percent and size of reinforcement particles on wear properties, microstructure, relative density and hardness of samples were studied. The optimum condition of processing parameters and the key strengthening mechanisms can be extracted from this study.
2. Materials and experimental procedures In this investigation, aluminum powder with purity of 99.97% and the average particle size of 30 lm and three types of alumina powder with average particle size in the range of 3–48 lm were used in the range of 0–20 wt.%. The densities of aluminum and
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alumina powders were 2.7 and 3.97 g/cm3, respectively. Chemical composition of alumina powder is given in Table 1. Electron micrographs of the powders are presented in Fig. 1. Powders were weighted and then mixed in a planetary ball mill for 1 h under a rotation speed of 150 rpm. For this purpose, four balls made of WC-Co with a weight of 80 g each were used. Ball to powder ratio was 10:1 and liquid ethanol was used as a process control agent. The powders were dried at 100 °C before compaction. Samples were uniaxially pressed at 440 MPa. Glycerol was used to reduce friction and wear of the matrix. Raw samples were sintered in argon at temperatures of 500, 550 and 600 °C for the periods of 30, 45, 60 and 90 min. The microstructure was examined using Tescan VEGA-II XMU and Philips XL30 electron microscopes. Keller reagent was used as the etchant. Densities were measured according to ASTM B328 [8]. Variations in the grain size were investigated utilizing Clemex image analyser. Grain size number and grain area were measured using the software. Hardness was measured in the Brinnel scale with an indenter of 2.5 mm diameter and a 30 kgf force. Pin on disc test was conducted for 100, 200, 300 and 400 m distances. Wear test samples had a cylindrical shape with a diameter of 7 mm. The running in distance was set at 1500 m and the disc material was AISI4140 steel [9]. The applied load was 5 N equivalent to 0.129 MPa on the pin. Sliding Table 1 Chemical composition of Al2O3 powder. Compounds
wt.%
a alumina
93 0.7 1.7 1.2 3.4
Fe2O3 TiO2 CaO Other materials
speed was set at 1 m/s in all test conditions. Specimens were thoroughly cleaned in alcohol and dried prior to weighing. The wear behavior was based on three tests under identical conditions. 3. Results and discussion 3.1. Relative density The effect of sintering temperature and duration upon relative density of the specimens is illustrated in Fig. 2. Since the sintering temperature has a profound effect on diffusivity of the atoms and neck growth, the relative density is raised [10]. The diffusion coefficient and sintering temperature are related according to equation [11].
D ¼ D0 exp
Q RT
ð1Þ
D is the diffusion coefficient at temperature T, D0 is the self diffusion, Q is the activation energy, R is Boltzmann’s constant. Sintering time provides the necessary condition for diffusion of atoms leading to a reduction in pore volume and hence greater relative density as presented in Fig. 2 [11]. The effect of sintering temperature on the elevation of density has been reported before [7,12]. Moreover, the interaction of sintering time on reduction in density and micro structural evolution has been investigated [10,13]. Density and reinforcement interrelationship is shown in Fig. 3. The reason why density of aluminum composite material is lower than pure aluminum stems from reduced compressibility as a result of reinforcement additions, which in turn is due to the fact that alumina is inherently harder than aluminum, moreover, the presence of the reinforcements, act as a barrier network against consolidation during the sintering process. The melting point of alumina is 2054 °C.
Fig. 1. SEM micrographs of Al and Al2O3 powders (a) Al, (b) Al2O3 3 lm, (c) Al2O3 12 lm and (d) Al2O3 48 lm.
M. Rahimian et al. / Materials and Design 32 (2011) 1031–1038
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Fig. 2. Variation of the relative density as a function of sintering time and temperature.
The adverse effect of reinforcement on compressibility has been investigated [14,15]. As the average size of alumina particles rises from 3 to 12 lm, the relative density of the composite increases and then drops to lower values. This phenomenon may be attributed to low specific surface and high compressibility of the coarse particles. Since the average particle size of alumina (3–12 lm) is lower than that of aluminum (30 lm), the reinforcement particles
Fig. 3. Variation of the relative density as a function of Al2O3 particle size and amount.
are located in the empty space preferentially before compaction. In comparison, when the particle size of alumina of 48 lm is employed, the void among aluminum particles is not filled by the reinforcement, resulting in reduced relative density under such circumstances.
Fig. 4. SEM micrographs showing the effect of sintering time on microstructure of composites at 600 °C (a) 45, (b) 60 and (c) 90 min and the effect of sintering temperature on microstructure of composites after sintering for 45 min (d) 500, (e) 550 and (f) 600 °C.
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Atomic displacement can be determined from equation [11]
3.2. Microstructure changes 3.2.1. The effect of sintering on microstructure The effect of sintering time upon microstructure is illustrated in Fig. 4a–c, in specimens containing 10 wt.% alumina with average size of 12 lm. As cited in the literature, diffusion of the atoms is dependent on time and temperature, so that under proper conditions, homogenization and grain movement are activated. Grain growth was directly proportional to the sintering temperature and time, in which the values were in accordance with the literature [12,16].
Table 2 Variation of microstructure grain size and area as function of sintering time and temperature. Sintering temperature (°C)
Sintering time (min)
Grain size number ASTME112-96
Grain area (lm2)
600 600 600 500 550 600
45 60 90 45 45 45
8.4 8.6 6.8 6.62 8.7 8.4
820 960 1723 2057 730 820
pffiffiffiffiffiffi r ¼ 2:4 Dt
ð2Þ
where r represents the radial distance from the initial position of the atoms, D the diffusion coefficient, and t the sintering time. It can be seen that the atomic displacement is related to the square root of time, leading to grain coarsening in the microstructure (Table 2). Fig. 4d–f, illustrates the microstructure after sintering for 45 min, suggesting that the amount of porosity has been reduced. This may be attributed to the diffusion and displacement of the atoms activated by the sintering temperature and time as explained by Eqs. (1) and (2) [10]. 3.2.2. The effect of alumina on microstructure The effect of the amount of alumina on microstructure is shown in Fig. 5a–c. At high amount of the reinforcement, finer grains were observed in the microstructure. It seems that alumina particles act as a barrier against the movement of grain boundaries and hence retards grain growth. This phenomenon can be explained in more detail by equation
k¼
4ð1 f Þr 3f
ð3Þ
Fig. 5. SEM micrographs showing the effect of alumina particle content on microstructure of composite with 12 lm Al2O3 particle (a) 5% Al2O3, (b)10% Al2O3, (c) 20% Al2O3 and the effect of alumina particle size on microstructure of composite with 10 wt.% Al2O3 particle (d) 3 lm, (e) 12 lm and (f) 48 lm.
M. Rahimian et al. / Materials and Design 32 (2011) 1031–1038 Table 3 Variation of microstructure grain size and area as function of particle size and content of Al2O3. Alumina particle size (lm)
Alumina content (wt.%)
Grain size number ASTME112-96
Grain area (lm2)
12 12 12 3 12 48
5 10 20 10 10 10
7.4 8.7 9.2 9.5 8.7 7.8
1100 730 288 236 730 1004
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60 min when sintering at 600 °C, leads to reduced hardness values. For instance, hardness dropped from 67 HB to 61 HB when sintering time changed from 60 to 90 min. This may stem from grain growth mechanisms at high sintering temperatures. The effect of alumina size and content can be observed in Fig. 8. Since alumina is inherently harder than aluminum, its presence leads to a higher hardness in the composite [1,19], which can be analyzed by the rule of mixtures [17].
HC ¼ Hm fm þ Hr fr
ð4Þ
The volume fraction of the reinforcement (f) and its radius (r), determine the distance apart the particles (k), and hence, prevent the movement of grain boundaries [17]. This can be confirmed in Fig. 5d–f, which presents the effect of alumina particle size on the microstructure after sintering at 550 °C for 45 min. At large particle size, the interparticle distance among the reinforcements is also increased leading to coarser grains. Also the obtaining results in Table 3 confirm this phenomenon [17]. The formation of intermediate layers at the reinforcement-base metal interface can have adverse effects on the mechanical properties of the composite. Fig. 6 depicts that little interaction has occurred between aluminum and alumina [18]. 3.3. Effect of processing parameters on hardness The effect of sintering temperature and time can be observed in Fig. 7. It should be noted that for both sintering at 600 and 550 °C result in higher hardness; however, the sintering time of more than
Fig. 8. The effect of Al2O3 particle size and amount on composite hardness.
Fig. 6. SEM micrograph illustrates the interface between Al and Al2O3 in composite.
Fig. 9. The effect of sintering time and temperature on composite hardness.
Fig. 7. Variation of the hardness as a function of sintering time and temperature.
Fig. 10. Variations in weight loss of the composites as a function of sintering time and temperature after 400 m sliding distance.
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Hm, HC and Hf are the hardness of the matrix, composite and reinforcement respectively. fm and fr are the volume fraction of the matrix and reinforcement respectively. It was observed from this study that by increasing the amount of alumina from 10% to 20%, hardness increased from 62 to 74 HB. According to Fig. 8 reducing the size of alumina showed the same trend [20]. Two opposing mechanisms take part in the overall hardness of Al–Al2O3 composites. Reinforcements increase the hardness value since they are inherently harder than the matrix. On the other hand, reduced densities have been reported under such circumstances due to lower compressibility of alumina [21,22]. Moreover, as the alumina particle size is reduced, according to Eq. (5), more stress is required to allow the dislocations to pass the reinforcements; hence, higher hardness values are achieved.
s0 ¼
Gb k
3.4. Wear behavior 3.4.1. The effect of sintering on wear resistance Wear tests were conducted on the composites with 10 wt.% alumina and with average size of 12 lm. Table 2, illustrates the changes in grain size at different sintering temperatures and times, which are more pronounced at high temperatures. In addition,
ð5Þ
s0 is the required stress of a dislocation to pass through a hard phase, G is the shear modulus of the material and b is the burgers vector of the dislocation [17]. Positive correlation between hardness and particle size of the reinforcement has been reported [1,23].
Fig. 12. Variations in wear volume loss of the composites as a function of (a) Al2O3 particle content and (b) Al2O3 particle size in different sliding distance.
Fig. 11. Variations of wear weight loss of the composites as a function of sintering temperature after sintering time of (a) 45, (b) 60 and (c) 90 min in different sliding distance.
Fig. 13. Variation of the wear rate of the composites as a function of (a) Al2O3 particle content and (b) Al2O3 particle size in different sliding distance.
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Fig. 14. SEM micrograph of worn surface of composites with (a, c) 5 wt.% and (b, d) 10 wt.% Al2O3.
grain growth has lead to low hardness in the composite. At elevated sintering temperatures, high hardness was achieved in the experiments, provided that it would not lead to grain growth in the microstructure (Fig. 9), which is the case for long sintering times. On the other hand, hardness and wear are closely inter-related, so that high hardness leads to reduced wear. Low grain size results in high hardness (Table 2 and Fig. 9) [17], which in turn reduces wear (Fig. 10). According to Table 2 increasing the sintering time from 60 to 90 min raised the grain area from 960 to 1723 lm2. This event leads to a reduction in hardness from 67 to 61 HB, thus wear weight loss increased 25%. Fig. 11a–c illustrate the weight loss of the specimens after a distance of 400 m, under different processing conditions. It can be seen that weight loss is low at high sintering temperatures and times. However, over-sintered specimens (90 min), exhibited higher wear due to reduced hardness as a result of grain growth under such circumstances (Fig. 11c). For instance, a reduction in weight loss of 26% was noted when the sintering temperature was raised by 50 degrees from 550 °C. However, this was 22% when sintering was performed at 600 °C under identical processing parameters [24,25].
[17,25]. Another point to consider is that increasing the amount of alumina from 10% to 20%, wear resistance did not improve as was originally expected. This phenomenon may be verified by the fact that relative density has been reduced and hence the reduced wear resistance. Figs. 12b and 13b illustrate the volume loss of Al–10Al2O3 composite indifferent alumina particle size and after various sliding distance. It shows that reducing the alumina particle size from 48 to 3 lm, volume loss increases by 16%. In comparison, reducing the particle size from 12 to 3 lm raised the volume loss by 9%. The results are comparable with earlier literature [27–34]. The improved wear resistance can be attributed to low roughness of coarse alumina particles [27,29]. Fig. 14 depicts the wear surface of the composite specimens containing from 5 to 10 wt.% alumina. It shows that flow wear by accumulated plastic shear flow has occurred. It is evident from the beach marks and the grooves that possible wear mechanism in this specimen is a mixture of abrasive–adhesive wear; however, the adhesive wear contribution in the sample containing 5 wt.% reinforcement is high. Galling has also occurred in certain regions; however, it has not been predominant. This phenomenon has been observed in the past literature [27–29,35].
4. Conclusions 3.4.2. The effect of alumina on wear resistance Figs. 12a and 13a illustrate the effect of alumina content with average size of 12 lm on wear resistance of the specimens. As alumina is introduced in the composite, the wear volume loss of the composite decreases by 31%, under identical test conditions [26,24]. Moreover, raising the amount of alumina from 10 to 20 wt.% volume loss was reduced to 11% (Fig. 12a). Raising the amount of alumina from 5 to 10 wt.% after a distance of 400 m, reduced wear rate by 33% (Fig. 13a). Since alumina is much harder than the aluminum matrix, higher wear resistance can be expected
1. Proper sintering temperature and time results in improved wear properties, however, excess sintering conditions deteriorates the wear properties due to grain growth and reduced hardness. For instance, increasing the sintering temperature from 550 to 600 °C lowered the weight loss by 7% after sintering for 45 min, as compared to 22% for that of 90 min. 2. Addition of alumina, considerably improves the wear properties of pure aluminum in all wear test distances. For instance, addition of 5 wt.% alumina with an average size of 12 lm,
1038
3.
4.
5.
6.
M. Rahimian et al. / Materials and Design 32 (2011) 1031–1038
reduced the weight loss of the composite by 31%. On the other hand, reducing particle size of alumina increased the wear rate. For example, the wear rate increases by 16% if alumina particle size reduces from 48 to 3 lm after a test run of 400 m. Elevated sintering temperatures help reduce porosity and enhance densification, whereas excess sintering time leads to grain coarsening. For instance, increasing the sintering time from 45 to 60 min, increased the grain area from 820 to 960 lm2 in Al–Al2O3 composite. High amounts of alumina lead to reduced relative porosity and, large alumina size raises the relative density initially and drops it later. The maximum density obtained was 99.95% which was achieved after sintering for 90 min at a sintering temperature of 600 °C. Increasing the amount and reducing the size of alumina promote high hardness in the composite. Maximum hardness of 79 HB was observed in the specimen containing 20 wt.% alumina with average particle size of 3 lm which was sintered for 45 min at 550 °C. Flow wear by accumulated plastic shear flow was probably predominant wear mechanism in more composite specimens.
References [1] Kok M. Production and mechanical properties of Al2O3 particle-reinforced 2024 aluminum alloy composites. J Mater Process Technol 2005;161:381–7. [2] Sevik H, Kurnaz SC. Properties of alumina particulate reinforced aluminum alloy produced by pressure die casting. Mater Des 2006;27:676–83. [3] Dobrzanski LA, Wlodarczyk A, Adamiak M. The structure and properties of PM composite materials based on EN AW-2124 aluminum alloy reinforced with the BN or Al2O3 ceramic particles. J Mater Process Technol 2006;175:186–91. [4] Torralba JM, daCost CE, Velasco F. P/M aluminum matrix composites: an overview. J Mater Process Technol 2003;133:203–6. [5] Torres B, Lieblich H, Ibanez J, Garcia-Escorial A. Mechanical properties of some PM aluminide and silicide reinforced 2124 aluminum matrix composites. Scripta Mater 2002;47:45. [6] Sahin Y, Murphy S. The effect of fibre orientation of the dry sliding wear of borsic- reinforced 2014 aluminum alloy. J Mater Sci 1996;34:5399–407. [7] Min KH, Kang SP, Kim DG, Kim YD. Sintering characteristic of Al2O3-reinforced 2xxx series Al composite powders. J Alloys Compd 2005;400:150–3. [8] Metals handbook, powder metallurgy. 9th ed. vol. 7. American Society for Metals; 1993. [9] Al-Qutub AM, Allam IM, Qureshi TW. Effect of sub-micron Al2O3 concentration on dry wear properties of 6061 aluminum based composite. J Mater Process Technol 2006;172:327–31. [10] German RM. Sintering theory and practice. New York: Wiley; 1996.
[11] Porter DA, Easterling KE. Phase transformations in metals and alloys. McGrawHill; 1980. [12] German RM. Powder metallurgy science. Princeton (NJ): Metal Powder Industries Federation; 1984. [13] Naranjo M, Rodr_Pguez JA, Herrera EJ. Sintering of Al/AlN composite powder obtained by gas–solid reaction milling. Scripta Mater 2003;49:65–9. [14] Shehata F, Fathy A, Abdelhameed M, Moustafa SF. Preparation and properties of Al2O3 nanoparticle reinforced copper matrix composites by in situ processing. Mater Des 2009;30:2756–62. [15] Slipenyuk A, Kuprin V, Milman Yu, Goncharuk V, Eckert J. Properties of P/M processed particle reinforced metal matrix composites specified by reinforcement concentration and matrix-to-reinforcement particle size ratio. Acta Mater 2006;54:157–66. [16] German RM. Powder metallurgy of iron and steel. New York: Wiley; 1998. [17] Dieter GE. Mechanical metallurgy. 3rd ed. McGraw-Hill; 1976. [18] Shorowordi KM, Laoui T, Haseeb ASMA, Celis JP, Froyen L. Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study. J Mater Process Technol 2003;142:738–43. [19] Kang YCh, Chan SL. Tensile properties of nanometric Al2O3 particulatereinforced aluminum matrix composites. Mater Chem Phys 2004;85:438–43. [20] Rahimian M, Ehsani N, Parvin N, Baharvandi HR. The effect of particle size, sintering temperature and sintering time on the properties of Al– Al2O3 composites, made by powder metallurgy. J Mater Process Technol 2009;209:5387–93. [21] Abouelmagd G. Hot deformation and wear resistance of P/M aluminium metal matrix composites. J Mater Process Technol 2004;155–156:1395–401. [22] Nlü BS. Investigation of tribological and mechanical properties Al2O3–SiC reinforced Al composites manufactured by casting or P/M method. Mater Des 2008;29:2002–8. [23] Sahin Y. Preparation and some properties of SiC particle reinforced aluminium alloy composites. Mater Des 2003;24:671–9. [24] Rahimian M, Ehsani N, Parvin N, Baharvandi HR. The effect of sintering temperature and the amount of reinforcement on the properties of Al–Al2O3 composite. Mater Des 2009;30:3333–7. [25] Mondal DP, Das S. High stress abrasive wear behavior of aluminum hard particle composites: effect of experimental parameters, particle size and volume fraction. Tribol Int 2006;39:470–8. [26] Sadık-nlü B. Investigation of tribological and mechanical properties Al2O3–SiC reinforced Al composites manufactured by casting or P/M method. Mater Des 2008;29:2002–8. [27] Kök M, Özdin K. Wear resistance of aluminum alloy and its composites reinforced by Al2O3 particles. J Mater Process Technol 2007;183:301–9. [28] Yilmaz SO. Comparison on abrasive wear of SiCrFe, CrFeC and Al2O3 reinforced Al2024 MMCs. Tribol Int 2007;40:441–52. [29] Yılmaz O, Buytoz S. Abrasive wear of Al2O3-reinforced aluminum-based MMCs. Compos Sci Technol 2001;61:2381–92. [30] AtR E. Mechanical properties and wear strengths in aluminum alumina composites. Mater Struct 1998;31:418–22. [31] Zhang ZF, Zhang LC, Mai YW. Particle effects on friction and wear of aluminum matrix composites. J Mater Sci 1995;30:5999–6004. [32] Kök M. Abrasive wear of Al2O3 particle reinforced 2024 aluminum alloy composites fabricated by vortex method. Composites Part A 2006;37:457–64. [33] Korkut MH. Microstructure and wear behavior of Al2024nSiFenAl2O3 composite. Tribol Int 2003;36:169–80. [34] Yang LJ. A methodology for the prediction of standard steady-state wear coefficient in an aluminium-based matrix composite reinforced with alumina particles. J Mater Process Technol 2005;162–163:139–48. [35] Stachowiak GW. Wear materials-mechanism and practice. Wiley; 2006.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
The effect of production parameters on microstructure and wear resistance of powder metallurgy Al–Al2O3 composite Mehdi Rahimian a,*, Nader Parvin b, Naser Ehsani c a
Faculty of Engineering, Islamic Azad University-Semnan Branch, Semnan, Iran Faculty of Mining and Materials Engineering, Amirkabir University of Technology (AUT), Hafez Ave., Tehran, Iran c Faculty of Materials and Manufacturing Technology, Malek Ashtar University of Technology (MUT), Babayi Highway, Lavizan, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 23 March 2010 Accepted 13 July 2010 Available online 16 July 2010
a b s t r a c t Aluminum matrix composite is one of the most conventional types of metal matrix composites. This paper deals with the effect of production parameters on wear resistance of Al–Al2O3 composites. Alumina powder with a particle size of 12, 3 and 48 l and pure aluminum powder with particle size of 30 l were used. The amount of added alumina powder was up to 20%. Ball milling was utilized to blend the powders. The range of sintering temperature and time were 500, 550 and 600 °C and 30, 45, 60 and 90 min respectively. It was found that increasing sintering temperature results in increasing density, hardness and wear resistance and homogenization of the microstructure. However at certain sintering temperatures and time, considerable grain growth and reduction of hardness value occurred, leading to the degradation of wear resistance. The results showed that at high alumina content, relative density of the composite increases. However, after raising the particle size of alumina, relative density initially increases and then drops to lower values. Increasing weight percent of alumina powder leads to higher hardness and consequently improves the wear resistance of Al–Al2O3 composite. The use of fine alumina particles has a similar effect on hardness and the wear resistance. Finally, a finer grain size was observed, at high amount and low size of the reinforcement particle. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction There is an increasing trend towards using composite materials in order to achieve better performance in engineering materials. Thus, production and application of metal matrix composites (MMC) have increased in recent years [1]. Al–Al2O3 MMC, has been of greater importance due to superior mechanical properties and excellent wear resistance, under various applications [2]. Aluminum matrix composites have made numerous applications in aerospace, automotive, military and electronic industry due to low density, high toughness and high corrosion resistance [3,4]. Low wear resistance of pure aluminum is a serious drawback in using it in many applications. Addition of ceramic particles to aluminum matrix would improve the strength, hardness, wear resistance and corrosion resistance of the matrix [5,6]. Particle reinforcements are more favorable than fiber type, due to better control of microstructure and mechanical properties, by varying the size and the volume fraction of the reinforcement [3]. Al2O3 is the most popular among ceramic particle reinforcement after SiC particles. Al2O3 has higher thermal stability compared to SiC, * Corresponding author. Address: No. 87-1, Shohada 22, Shohada Street, Semnan 3513645575, Iran. Tel.: +98 9125322792. E-mail addresses: [email protected] (M. Rahimian), [email protected] (N. Parvin), [email protected] (N. Ehsani). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.07.016
since it does not react with the metal matrix at high temperatures and does not produce brittle phases [1]. Powder metallurgy is considered as a good technique in producing metal–matrix composites. An important advantage of this method is its low processing temperature compared to melting techniques. Therefore, interaction between the matrix and the reinforcement phases is prevented. On the other hand, good distribution of the reinforcing particles can be achieved [4]. Another advantage of powder metallurgy technique is in its ability to manufacture near net shape product at low cost [7]. In this research, uniaxial pressing was used to produce composite sample. The effects of sintering time and temperature, weight percent and size of reinforcement particles on wear properties, microstructure, relative density and hardness of samples were studied. The optimum condition of processing parameters and the key strengthening mechanisms can be extracted from this study.
2. Materials and experimental procedures In this investigation, aluminum powder with purity of 99.97% and the average particle size of 30 lm and three types of alumina powder with average particle size in the range of 3–48 lm were used in the range of 0–20 wt.%. The densities of aluminum and
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alumina powders were 2.7 and 3.97 g/cm3, respectively. Chemical composition of alumina powder is given in Table 1. Electron micrographs of the powders are presented in Fig. 1. Powders were weighted and then mixed in a planetary ball mill for 1 h under a rotation speed of 150 rpm. For this purpose, four balls made of WC-Co with a weight of 80 g each were used. Ball to powder ratio was 10:1 and liquid ethanol was used as a process control agent. The powders were dried at 100 °C before compaction. Samples were uniaxially pressed at 440 MPa. Glycerol was used to reduce friction and wear of the matrix. Raw samples were sintered in argon at temperatures of 500, 550 and 600 °C for the periods of 30, 45, 60 and 90 min. The microstructure was examined using Tescan VEGA-II XMU and Philips XL30 electron microscopes. Keller reagent was used as the etchant. Densities were measured according to ASTM B328 [8]. Variations in the grain size were investigated utilizing Clemex image analyser. Grain size number and grain area were measured using the software. Hardness was measured in the Brinnel scale with an indenter of 2.5 mm diameter and a 30 kgf force. Pin on disc test was conducted for 100, 200, 300 and 400 m distances. Wear test samples had a cylindrical shape with a diameter of 7 mm. The running in distance was set at 1500 m and the disc material was AISI4140 steel [9]. The applied load was 5 N equivalent to 0.129 MPa on the pin. Sliding Table 1 Chemical composition of Al2O3 powder. Compounds
wt.%
a alumina
93 0.7 1.7 1.2 3.4
Fe2O3 TiO2 CaO Other materials
speed was set at 1 m/s in all test conditions. Specimens were thoroughly cleaned in alcohol and dried prior to weighing. The wear behavior was based on three tests under identical conditions. 3. Results and discussion 3.1. Relative density The effect of sintering temperature and duration upon relative density of the specimens is illustrated in Fig. 2. Since the sintering temperature has a profound effect on diffusivity of the atoms and neck growth, the relative density is raised [10]. The diffusion coefficient and sintering temperature are related according to equation [11].
D ¼ D0 exp
Q RT
ð1Þ
D is the diffusion coefficient at temperature T, D0 is the self diffusion, Q is the activation energy, R is Boltzmann’s constant. Sintering time provides the necessary condition for diffusion of atoms leading to a reduction in pore volume and hence greater relative density as presented in Fig. 2 [11]. The effect of sintering temperature on the elevation of density has been reported before [7,12]. Moreover, the interaction of sintering time on reduction in density and micro structural evolution has been investigated [10,13]. Density and reinforcement interrelationship is shown in Fig. 3. The reason why density of aluminum composite material is lower than pure aluminum stems from reduced compressibility as a result of reinforcement additions, which in turn is due to the fact that alumina is inherently harder than aluminum, moreover, the presence of the reinforcements, act as a barrier network against consolidation during the sintering process. The melting point of alumina is 2054 °C.
Fig. 1. SEM micrographs of Al and Al2O3 powders (a) Al, (b) Al2O3 3 lm, (c) Al2O3 12 lm and (d) Al2O3 48 lm.
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Fig. 2. Variation of the relative density as a function of sintering time and temperature.
The adverse effect of reinforcement on compressibility has been investigated [14,15]. As the average size of alumina particles rises from 3 to 12 lm, the relative density of the composite increases and then drops to lower values. This phenomenon may be attributed to low specific surface and high compressibility of the coarse particles. Since the average particle size of alumina (3–12 lm) is lower than that of aluminum (30 lm), the reinforcement particles
Fig. 3. Variation of the relative density as a function of Al2O3 particle size and amount.
are located in the empty space preferentially before compaction. In comparison, when the particle size of alumina of 48 lm is employed, the void among aluminum particles is not filled by the reinforcement, resulting in reduced relative density under such circumstances.
Fig. 4. SEM micrographs showing the effect of sintering time on microstructure of composites at 600 °C (a) 45, (b) 60 and (c) 90 min and the effect of sintering temperature on microstructure of composites after sintering for 45 min (d) 500, (e) 550 and (f) 600 °C.
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Atomic displacement can be determined from equation [11]
3.2. Microstructure changes 3.2.1. The effect of sintering on microstructure The effect of sintering time upon microstructure is illustrated in Fig. 4a–c, in specimens containing 10 wt.% alumina with average size of 12 lm. As cited in the literature, diffusion of the atoms is dependent on time and temperature, so that under proper conditions, homogenization and grain movement are activated. Grain growth was directly proportional to the sintering temperature and time, in which the values were in accordance with the literature [12,16].
Table 2 Variation of microstructure grain size and area as function of sintering time and temperature. Sintering temperature (°C)
Sintering time (min)
Grain size number ASTME112-96
Grain area (lm2)
600 600 600 500 550 600
45 60 90 45 45 45
8.4 8.6 6.8 6.62 8.7 8.4
820 960 1723 2057 730 820
pffiffiffiffiffiffi r ¼ 2:4 Dt
ð2Þ
where r represents the radial distance from the initial position of the atoms, D the diffusion coefficient, and t the sintering time. It can be seen that the atomic displacement is related to the square root of time, leading to grain coarsening in the microstructure (Table 2). Fig. 4d–f, illustrates the microstructure after sintering for 45 min, suggesting that the amount of porosity has been reduced. This may be attributed to the diffusion and displacement of the atoms activated by the sintering temperature and time as explained by Eqs. (1) and (2) [10]. 3.2.2. The effect of alumina on microstructure The effect of the amount of alumina on microstructure is shown in Fig. 5a–c. At high amount of the reinforcement, finer grains were observed in the microstructure. It seems that alumina particles act as a barrier against the movement of grain boundaries and hence retards grain growth. This phenomenon can be explained in more detail by equation
k¼
4ð1 f Þr 3f
ð3Þ
Fig. 5. SEM micrographs showing the effect of alumina particle content on microstructure of composite with 12 lm Al2O3 particle (a) 5% Al2O3, (b)10% Al2O3, (c) 20% Al2O3 and the effect of alumina particle size on microstructure of composite with 10 wt.% Al2O3 particle (d) 3 lm, (e) 12 lm and (f) 48 lm.
M. Rahimian et al. / Materials and Design 32 (2011) 1031–1038 Table 3 Variation of microstructure grain size and area as function of particle size and content of Al2O3. Alumina particle size (lm)
Alumina content (wt.%)
Grain size number ASTME112-96
Grain area (lm2)
12 12 12 3 12 48
5 10 20 10 10 10
7.4 8.7 9.2 9.5 8.7 7.8
1100 730 288 236 730 1004
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60 min when sintering at 600 °C, leads to reduced hardness values. For instance, hardness dropped from 67 HB to 61 HB when sintering time changed from 60 to 90 min. This may stem from grain growth mechanisms at high sintering temperatures. The effect of alumina size and content can be observed in Fig. 8. Since alumina is inherently harder than aluminum, its presence leads to a higher hardness in the composite [1,19], which can be analyzed by the rule of mixtures [17].
HC ¼ Hm fm þ Hr fr
ð4Þ
The volume fraction of the reinforcement (f) and its radius (r), determine the distance apart the particles (k), and hence, prevent the movement of grain boundaries [17]. This can be confirmed in Fig. 5d–f, which presents the effect of alumina particle size on the microstructure after sintering at 550 °C for 45 min. At large particle size, the interparticle distance among the reinforcements is also increased leading to coarser grains. Also the obtaining results in Table 3 confirm this phenomenon [17]. The formation of intermediate layers at the reinforcement-base metal interface can have adverse effects on the mechanical properties of the composite. Fig. 6 depicts that little interaction has occurred between aluminum and alumina [18]. 3.3. Effect of processing parameters on hardness The effect of sintering temperature and time can be observed in Fig. 7. It should be noted that for both sintering at 600 and 550 °C result in higher hardness; however, the sintering time of more than
Fig. 8. The effect of Al2O3 particle size and amount on composite hardness.
Fig. 6. SEM micrograph illustrates the interface between Al and Al2O3 in composite.
Fig. 9. The effect of sintering time and temperature on composite hardness.
Fig. 7. Variation of the hardness as a function of sintering time and temperature.
Fig. 10. Variations in weight loss of the composites as a function of sintering time and temperature after 400 m sliding distance.
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Hm, HC and Hf are the hardness of the matrix, composite and reinforcement respectively. fm and fr are the volume fraction of the matrix and reinforcement respectively. It was observed from this study that by increasing the amount of alumina from 10% to 20%, hardness increased from 62 to 74 HB. According to Fig. 8 reducing the size of alumina showed the same trend [20]. Two opposing mechanisms take part in the overall hardness of Al–Al2O3 composites. Reinforcements increase the hardness value since they are inherently harder than the matrix. On the other hand, reduced densities have been reported under such circumstances due to lower compressibility of alumina [21,22]. Moreover, as the alumina particle size is reduced, according to Eq. (5), more stress is required to allow the dislocations to pass the reinforcements; hence, higher hardness values are achieved.
s0 ¼
Gb k
3.4. Wear behavior 3.4.1. The effect of sintering on wear resistance Wear tests were conducted on the composites with 10 wt.% alumina and with average size of 12 lm. Table 2, illustrates the changes in grain size at different sintering temperatures and times, which are more pronounced at high temperatures. In addition,
ð5Þ
s0 is the required stress of a dislocation to pass through a hard phase, G is the shear modulus of the material and b is the burgers vector of the dislocation [17]. Positive correlation between hardness and particle size of the reinforcement has been reported [1,23].
Fig. 12. Variations in wear volume loss of the composites as a function of (a) Al2O3 particle content and (b) Al2O3 particle size in different sliding distance.
Fig. 11. Variations of wear weight loss of the composites as a function of sintering temperature after sintering time of (a) 45, (b) 60 and (c) 90 min in different sliding distance.
Fig. 13. Variation of the wear rate of the composites as a function of (a) Al2O3 particle content and (b) Al2O3 particle size in different sliding distance.
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Fig. 14. SEM micrograph of worn surface of composites with (a, c) 5 wt.% and (b, d) 10 wt.% Al2O3.
grain growth has lead to low hardness in the composite. At elevated sintering temperatures, high hardness was achieved in the experiments, provided that it would not lead to grain growth in the microstructure (Fig. 9), which is the case for long sintering times. On the other hand, hardness and wear are closely inter-related, so that high hardness leads to reduced wear. Low grain size results in high hardness (Table 2 and Fig. 9) [17], which in turn reduces wear (Fig. 10). According to Table 2 increasing the sintering time from 60 to 90 min raised the grain area from 960 to 1723 lm2. This event leads to a reduction in hardness from 67 to 61 HB, thus wear weight loss increased 25%. Fig. 11a–c illustrate the weight loss of the specimens after a distance of 400 m, under different processing conditions. It can be seen that weight loss is low at high sintering temperatures and times. However, over-sintered specimens (90 min), exhibited higher wear due to reduced hardness as a result of grain growth under such circumstances (Fig. 11c). For instance, a reduction in weight loss of 26% was noted when the sintering temperature was raised by 50 degrees from 550 °C. However, this was 22% when sintering was performed at 600 °C under identical processing parameters [24,25].
[17,25]. Another point to consider is that increasing the amount of alumina from 10% to 20%, wear resistance did not improve as was originally expected. This phenomenon may be verified by the fact that relative density has been reduced and hence the reduced wear resistance. Figs. 12b and 13b illustrate the volume loss of Al–10Al2O3 composite indifferent alumina particle size and after various sliding distance. It shows that reducing the alumina particle size from 48 to 3 lm, volume loss increases by 16%. In comparison, reducing the particle size from 12 to 3 lm raised the volume loss by 9%. The results are comparable with earlier literature [27–34]. The improved wear resistance can be attributed to low roughness of coarse alumina particles [27,29]. Fig. 14 depicts the wear surface of the composite specimens containing from 5 to 10 wt.% alumina. It shows that flow wear by accumulated plastic shear flow has occurred. It is evident from the beach marks and the grooves that possible wear mechanism in this specimen is a mixture of abrasive–adhesive wear; however, the adhesive wear contribution in the sample containing 5 wt.% reinforcement is high. Galling has also occurred in certain regions; however, it has not been predominant. This phenomenon has been observed in the past literature [27–29,35].
4. Conclusions 3.4.2. The effect of alumina on wear resistance Figs. 12a and 13a illustrate the effect of alumina content with average size of 12 lm on wear resistance of the specimens. As alumina is introduced in the composite, the wear volume loss of the composite decreases by 31%, under identical test conditions [26,24]. Moreover, raising the amount of alumina from 10 to 20 wt.% volume loss was reduced to 11% (Fig. 12a). Raising the amount of alumina from 5 to 10 wt.% after a distance of 400 m, reduced wear rate by 33% (Fig. 13a). Since alumina is much harder than the aluminum matrix, higher wear resistance can be expected
1. Proper sintering temperature and time results in improved wear properties, however, excess sintering conditions deteriorates the wear properties due to grain growth and reduced hardness. For instance, increasing the sintering temperature from 550 to 600 °C lowered the weight loss by 7% after sintering for 45 min, as compared to 22% for that of 90 min. 2. Addition of alumina, considerably improves the wear properties of pure aluminum in all wear test distances. For instance, addition of 5 wt.% alumina with an average size of 12 lm,
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3.
4.
5.
6.
M. Rahimian et al. / Materials and Design 32 (2011) 1031–1038
reduced the weight loss of the composite by 31%. On the other hand, reducing particle size of alumina increased the wear rate. For example, the wear rate increases by 16% if alumina particle size reduces from 48 to 3 lm after a test run of 400 m. Elevated sintering temperatures help reduce porosity and enhance densification, whereas excess sintering time leads to grain coarsening. For instance, increasing the sintering time from 45 to 60 min, increased the grain area from 820 to 960 lm2 in Al–Al2O3 composite. High amounts of alumina lead to reduced relative porosity and, large alumina size raises the relative density initially and drops it later. The maximum density obtained was 99.95% which was achieved after sintering for 90 min at a sintering temperature of 600 °C. Increasing the amount and reducing the size of alumina promote high hardness in the composite. Maximum hardness of 79 HB was observed in the specimen containing 20 wt.% alumina with average particle size of 3 lm which was sintered for 45 min at 550 °C. Flow wear by accumulated plastic shear flow was probably predominant wear mechanism in more composite specimens.
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