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Wear behaviour of hypereutectic Al–Si–Cu–Mg casting alloys with variable Mg contents
Wear 269 (2010) 684–692
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Wear journal homepage: www.elsevier.com/locate/wear
Wear behaviour of hypereutectic Al–Si–Cu–Mg casting alloys with variable Mg contents Alireza Hekmat-Ardakan a , Xichun Liu a , Frank Ajersch a,∗ , X.-Grant Chen b a b
École Polytechnique de Montréal, Dép. de Génie Chimique, P.O. Box 6079, Centre-ville, Montreal, Quebec, Canada H3C 3A7 Université du Québec à Chicoutimi (UQAC), Dép. des sciences appliquées, 555 boulevard de l’Université, Chicoutimi, Québec, Canada G7H 2B1
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Article history: Received 27 January 2010 Received in revised form 7 July 2010 Accepted 9 July 2010 Available online 17 July 2010 Keywords: Metal–matrix composite Three-body abrasion Wear testing Intermetallics Hardness Particle shape
a b s t r a c t The wear properties of A390 (Al–17Si–4.5Cu–0.5Mg, wt%) hypereutectic Al–Si alloy were compared to new alloys containing 6 and 10 wt% Mg. The wear behaviour was investigated using dry sand rubber wheel (DSRW) abrasive wear apparatus for these alloys in the as-cast condition and after T6 heat treatment. A transformation of hard and coarse primary Si particles to fine Mg2 Si particles with inferior hardness occurs with increasing Mg content. The finer particle size together with increasing solid fraction of the primary phase was found to be key factors in improving the wear resistance of alloys with high Mg content. In addition, the eutectic matrix microstructure of alloys with high Mg content showed considerable changes, particularly in size and morphology of the eutectic silicon which significantly contributes to the wear resistance. The wear test results showed improved wear resistance for alloys with high Mg content. The microstructure of the worn surface indicated that the intermetallic Mg2 Si particles in alloys with 6% and 10% Mg addition are more solidly bonded to the matrix compared to the coarse primary silicon particles in A390 alloy which can be pulled out from the matrix. The worn surface of the A390 alloy exhibits deep and non-uniform grooves contrary to shallow and uniform grooves for the high Mg content alloys, resulting in the improved resistance to wear. Although the T6 heat treatment improved the wear behaviour of each alloy, the ranking of the wear property remained the same as the as-cast alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hypereutectic Al–Si alloys such as A390 (Al–17Si–4.5Cu–0.5Mg) are used in applications that require high resistance to wear and corrosion, good mechanical properties, low thermal expansion and reduced density [1,2]. Their properties are of greatest interest to the automobile industry [3–5] for the production of fuel-efficient vehicles using light weight components produced from these alloys such as connecting rods, pistons, cylinder liners and engine blocks [6,7]. The good mechanical properties and high resistance to wear are essentially attributed to the presence of hard primary silicon particles distributed in the metal matrix [8]. The wear resistance can, however, also be affected by Si in the eutectic matrix. Hypereutectic aluminium–silicon alloys can be considered as in situ metal matrix composites (MMCs), where the primary silicon acts as the reinforcing phase [9]. This class of materials acquires its high strength and stiffness from the reinforcing phase and the damage tolerance and toughness is provided by the metal matrix [10]. However, the high latent heat and consequent long solidification
∗ Corresponding author. Tel.: +1 514 340 4711x4533; fax: +1 514 340 4468. E-mail address: [email protected] (F. Ajersch). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.07.007
time of hypereutectic aluminium–silicon alloys results in excessive growth of primary silicon particles as well as unfavourable shrinkage behaviour which adversely affects the use of these alloys [11]. Studies of Lasa and Rodriguez-Ibabe [12] focused on wear resistance of hypereutectic Al–Si–Cu–Mg alloys formed by various processing routes using a pin-on-disc method with different test speeds. Different forming processes such as permanent mould casting, thixoforming, squeeze casting and lost-foam casting result in different microstructural aspects, particularly in the size and distribution of primary Si particles. These are characterized by the number of Si particles per mm2 , mean particle size (Dp ) and the interparticle distance (). They have also shown that the wear behaviour strongly depends on the test speed. The tests carried out with low disc speed (0.089 m/s) strongly influence the coefficient of wear for alloys produced by different methods when compared to the tests with high disc speed (0.356 m/s). The diagrams of pin wear as a function of test distance indicated that the alloys produced by thixoforming resulted in a microstructure with the highest number of Si particles (∼866 part/mm2 ) as well as the lowest mean particle size (Dp = 11.6 m) with an interparticle distance of = 74.6 m and show no transition from mild wear (protective oxide layer) to severe wear. The protective oxide layer is broken and the wear
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Fig. 1. As-cast microstructure of Al–17%Si–4.5%Cu alloy with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg. The circled area presents the zone of intermetallic phases in the microstructure.
rate progressively increases during the test at low speed. The pin height during the wear test remains almost constant compared to alloys formed by other processing routes and the accelerated wear is attributed to cracking and fracture of the large silicon particles. In the present study, the microstructure of the hypereutectic alloy consists of fine primary particles dispersed in the matrix. This microstructure was produced by increasing the Mg content of A390 alloy and promoting the formation of fine intermetallic particles of Mg2 Si instead of coarse primary Si particles. Mg2 Si has a high melting temperature, a low density, a high hardness, a low thermal expansion coefficient, an equilibrium interface with excellent workability and a potential for cost reduction [13]. In terms of properties and solidification behaviour, great similarities exist between Mg2 Si and Si [14], except for the size of the precipitated particles. The present authors have previously carried out a thermodynamic evaluation [15] and microstructural evolution of A390 alloy and alloys with additions up to 10% Mg for conventional castings and for semisolid formed alloys [16,17]. It was observed that the increased Mg content resulted in the transformation of the primary Si particles to Mg2 Si intermetallic particles with a finer particle size as well as in the modification of the eutectic matrix, particularly for the eutectic Si phase. The microstructural features of these alloys with high Mg content should also influence wear behaviour of these alloys even though the measured hardness of Si particles is higher than Mg2 Si particles (Si = 1255 HV, Mg2 Si = 523 HV). The objective of present study is to investigate the wear behaviour of A390 alloy and alloys with addition of 6 and 10 wt% Mg in the as-cast (permanent mould) and in the T6 heat-treated conditions. 2. Experimental procedure 2.1. Materials The 6 and 10% Mg alloys were produced by adding AZ91 Mgbase alloy to A390 alloy. The chemical analysis of the test alloys
and AZ91 Mg alloy are presented in Table 1. In order to account for the oxidation loss, an additional amount of 20% Mg as AZ91 was added to the melt. The AZ91 alloy was wrapped in aluminium foil and added to the melt of A390 alloy at 750 ◦ C using an electric resistance furnace. Silicon and copper were also added to the melt in order to maintain the chemical composition of these elements at the same value as for the A390 composition. The melt was degassed with argon for 5 min with a flow rate 4 CFH (ft3 /h), and stirred for 2 min before pouring into a refractory coated permanent metallic mould preheated to ∼400 ◦ C to form tensile test bars. This standard mould was used to prepare rectangular samples measuring 25 mm × 75 mm with a 12-mm thickness which were then tested in the DSRW wear apparatus. The T6 heat treatment was carried out by solutionizing the ascast samples at 500 ◦ C for 1 h, followed by immediate quenching in normal water before aging at 175 ◦ C for 8 h. 2.2. Hardness test Macro-hardness measurements were carried out on each of the as-cast and T6 heat-treated samples using a Mitutoyo ATK-600 hardness machine with the Rockwell B method (100 kg load and 1/16 in. ball). The results of the average of eight measurements were then converted to the standard Vickers hardness (10 kg load with pyramid diamond indenter). Table 1 Chemical composition of test alloys and AZ91 Mg alloy (wt.%). Test alloy
Al
Si
Cu
Mg
Fe
Mn
Zn
P
Ti
A390 6% Mg 10% Mg
Bal. Bal. Bal.
16.7 16.83 17.14
4.58 4.12 4.32
0.58 6.08 9.74
0.32 0.28 0.25
0.02 0.04 0.05
0.01 0.015 0.02
0.0003 0.0002 0.0002
0.02 0.02 0.019
AZ91
Mg
Al
Mn
Zn
Si
Ni
Cu
Fe
Bal.
9.3
0.12
0.62
0.02
0.0006
0.0007
0.0046
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Fig. 2. Intermetallic phases in as-cast A390 (0.5% Mg) alloy: (a) optical microscopy, (b) SEM microscopy and (c) EDX analysis showing the intermetallic phases (1) Al5 FeSi (), (2) Cu2 Mg8 Si6 Al5 (Q) and (3) CuAl2 () of (b).
Table 2 The solid fraction of the solidified phases for 3 alloys in the as-cast condition. Alloys
0.5% Mg 6% Mg 10% Mg a
Phases (%) Primary Si
Mg2 Sia
CuAl2 ()
Cu2 Mg8 Si6 Al5 (Q)
Al5 FeSi ()
7.4 6.7 2.1
– 4.5 21.3
6.5 5.7 5.2
0.6 0.5 0.5
<0.1 <0.1 <0.1
The eutectic Mg2 Si is excluded from calculation.
Micro-hardness tests were also carried out for each of these samples using the Vickers diamond indent (Mitutoyo MT-2001 with Clemex image technology). A 100-gf load was applied in order to measure the average hardness of ␣-Al solid solution for both the as-cast and the heat-treated samples. 2.3. Wear test procedure A dry sand rubber wheel (DSRW) abrasive wear apparatus, model F-1510 FALEX, was used in this study. The apparatus and the test procedures are well-defined in the standard ASTM (G65-81)
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Fig. 3. Histograms of length distribution of Si and Mg2 Si particles in as-cast condition for: (a) primary Si in 0.5% Mg alloy, (b) primary Si in 6% Mg alloy, (c) Mg2 Si in 6% Mg alloy and (d) Mg2 Si in 10% Mg alloy.
abrasion test [18]. The basic ASTM machine consists of a rubberrimmed steel wheel, 228 mm × 12.7 mm wide; a sand hopper connected by a tube to a nozzle that allows a flow of 250–350 g/min with 50–70 mesh silica test sand. A revolution counter stops the drive motor after a set number of revolutions; and a weighted lever arm holds the specimen and generates a horizontal force against the wheel. The two test parameters, consisting of the sliding distance (based on wheel revolutions) and the specimen load, were set to 215.4 m (300 wheel revolutions) and 130 N, respectively. A Sartorius CP323S electronic precision balance with precision 0.001 g was used to measure the weight difference before and after the wear tests. The densities of the three alloys, measured by the Archimedes technique [19], were found to be 2.68, 2.58 and 2.51 g/cm3 for alloys with 0.5, 6 and 10% Mg additions, respectively.
2.5. Microstructural observation Metallographic specimens for the as-cast and the heat-treated samples were polished with abrasive paper up to 1200 followed by micropolishing using a monocrystal diamond suspension (0.5 m). All samples were etched with a 0.5% HF solution agent. Optical microscopy was carried out to characterize the microstructure of alloys and the Clemex software was used to determine the primary particle size and the solid fraction. The microstructure of alloys and the worn surface of wear samples were also analysed using a scanning electron microscopy (SEM, JOEL-840) coupled with energy dispersive X-ray (EDX) spectroscopy for elemental analysis.
2.4. Roughness measurement 3. Results and discussion The study of the worn surface was carried out using DEKTAK 3030ST contact profilometer apparatus. This profilometer can measure small surface variations using the vertical stylus displacement as a function of position. The stylus force was set 5 mg with 3 mm line scan perpendicular to wear direction. The total profile is then separated into roughness and waviness. The waviness filter removes very high frequency data from the raw (total) profile data since it can often be attributed to vibrations or to the presence of debris on the part surface. The roughness value was calculated by taking into account a 600-m cut-off length filter for waviness. The mean value of roughness (RA ) was measured by analysing a 3-mm line scan for each alloy at three different positions.
The microstructure of the three alloys in the as-cast condition is shown in Fig. 1a–c. The A390 base alloy, shown in Fig. 1a, exhibits a coarse polygonal structure of primary Si (fs(primary Si) = 7.4%) with a eutectic network of Al–Si phases together with the intermetallic phases, CuAl2 (), Cu2 Mg8 Si6 Al5 (Q) and Al5 FeSi (). The intermetallic phases and the EDX analyses are shown in Fig. 2. These intermetallic phases were also identified in the previous paper [16] using backscatter images. The 6% Mg alloy (Fig. 1b) is dominated by a distribution of both polygonal Si and dendritic-polygonal Mg2 Si (fs(primary Si) = 6.7%, fs(Mg2 Si) = 4.5%) and with a network of Al–Si–Mg2 Si eutectic phases.
Table 3 The mean length of Si and Mg2 Si particles for the as-cast Al–17%Si–4.5%Cu with 0.5%, 6% and 10% Mg. Alloy
Particle size Si Mean length (m)
A390 (0.5% Mg) 6 Mg 10 Mg
190.87 145.16 –
Mg2 Si Standard deviation (m) 73.21 52.46 –
Mean length (m)
Standard deviation (m)
– 88.31 95.38
– 32.99 43.21
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Fig. 4. The eutectic matrix network circled of as-cast alloys with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg.
Fig. 5. The eutectic matrix network of alloys with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg after T6 heat treatment.
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Fig. 6. The sphericity of eutectic Si for as-cast and after T6 heat treatment of alloys with 0.5% Mg, 6% Mg and 10% Mg.
Increasing the Mg content up to 10%, results in the precipitation of intermetallic particles of Mg2 Si as shown in Fig. 1c. The solid fraction of the Mg2 Si particles also increases considerably with the addition of 10% Mg, where fs(Primary Mg2 Si) = 21.3% and fs(Si) = 2.1%. Table 2 presents the analysis of phases for all three alloys. It was observed that the CuAl2 () is the most important intermetallic phase in all three alloys. Histograms of length distribution of Si and Mg2 Si particle are shown in Fig. 3 for the three alloys. It is observed that coarsest Si particles correspond to the A390 base alloy with some particles larger than 340 m. The mean length of Si particles decreases with addition of Mg as shown in Table 3. The coarsest Mg2 Si particles correspond to the alloy with 10% Mg content with some particles even larger than 425 m. The microstructure of 10% Mg alloy (Fig. 1c) shows that some dendritic Mg2 Si particles become interconnected and results in extremely large agglomerations. However, Table 3 indicates that the mean length of the Mg2 Si particles is significantly smaller than the Si particles. Fig. 4 compares the eutectic matrix network of the three alloys. The eutectic Si particles in A390 alloy are much larger in size, with a needle like shape and more separated than in the alloys with 6 and 10% Mg. The interparticle space () of eutectic Si was measured to be 9.79, 6.82 and 6.70 m for the as-cast alloys of 0.5% Mg, 6% Mg and 10% Mg, respectively. Eutectic Mg2 Si with Chinese script morphology also appears in the eutectic microstructure of high Mg content alloys. The T6 solution heat treatment promotes rounding of the eutectic Si particles particularly for high Mg content alloys (Fig. 5). Fig. 6 shows the sphericity of eutectic Si before and after heat treatment. The rounding of the eutectic phase after heat treatment is more significant for high Mg content alloys than for the base alloy. No effect on primary particles and intermetallic phases was observed in these samples as shown in Fig. 4c. The eutectic Mg2 Si is observed in high Mg alloys to form very fine particles as a result of the T6 heat treatment. Fig. 7 shows the values of the measured hardness of the three alloys for the as-cast condition and after the T6 heat treatment. The hardness of the as-cast alloys increases with addition of Mg which is attributed to the increase in solid fraction of the primary phases of Si + Mg2 Si from 7.4% to 23.4%. The changes in the microstructure of the eutectic network of matrix, particularly the increase of hardness of the ␣-Al solid solution, are also shown in Fig. 8. The T6 heat treatment provides substantial improvement in hardening of all alloys which is most significant for the 0.5% Mg alloy. The results in Fig. 7 also show that the hardness of the heat-treated samples is the lowest for the 6% Mg alloy. Increasing the Mg content to 10% again increases the hardness.
689
Fig. 7. Macro-hardness of alloys with 0.5% Mg, 6% Mg and 10% Mg in as-cast and after T6 heat treatment.
Fig. 8. Micro-hardness of ␣-Al solid solution for alloys with 0.5% Mg, 6% Mg and 10% Mg in as-cast and after T6 heat treatment.
3.1. Abrasive wear behaviour The wear test results are shown in Fig. 9, in units of volume loss of material. The wear rate was calculated to vary from 2.6 to 3.4 × 10−4 mm3 /mm. Fig. 10 shows the stereo macrographs of the worn surface of the three alloys.
Fig. 9. The volume loss of alloys with 0.5% Mg, 6% Mg and 10% Mg after wear test using dry sand rubber wheel (DSRW) apparatus for as-cast and T6 heat-treated alloys.
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of alloy is identified as key parameter because particles of smaller size reduce the stress concentration at the interface between hard primary particles and the matrix. This interface is susceptible to cracking and tends to increase with particle size [23]. 3.2. Microstructural analysis of alloys
Fig. 10. The macrographs of the worn surface of as-cast alloys with (A) 0.5% Mg, (B) 6% Mg and (C) 10% Mg.
Fig. 9 clearly shows that the wear rate decreases with increasing the Mg content for both as-cast and heat-treated samples. After T6 heat treatment the wear resistance of all samples is greatly improved. Harun et al. [20] and Ott et al. [21] have also found that the wear resistance of Al–Si alloys is affected by heat treatment in a favorable way. Improvement in the yield strength obtained after heat treatment was also known to delay or inhibit wear [22]. Nevertheless, the effect of hardness on wear behaviour is not evident as the wear rate of the heat-treated alloys improves with increasing Mg content in spite of lower hardness. This is in contrast with the wear behaviour of the as-cast samples, where the hardness increases with addition of Mg. However, the wear behaviour can be directly related to the hardness for a specific alloy composition when the specific alloy wear behaviour is compared in the as-cast and the heat-treated conditions. In this case, the microstructure, particularly the size and morphology of the primary phase, does not change for these two conditions. For different compositions, the change in size of the primary phases rather than the hardness
Fig. 11 shows SEM images of the worn surface of the as-cast alloy sample for different Mg contents. The images are consistent with the wear test results shown in Fig. 9 and with the microstructure of the alloys shown previously in Fig. 1, where the average size of the primary silicon particles in base alloy (A390) is almost 1.8 times bigger than the average size of the Mg2 Si intermetallic particles for alloys with high Mg content. It was also observed that the silicon particles for the high Mg alloys are surrounded by Mg2 Si particles resulting in a better bonding of Si particles to the matrix than for the case of bonding of individual silicon particles in A390 alloy. Therefore, the stress imposed during the wear test of A390 provokes the fracture of coarse silicon particles which can be pulled out from the matrix and therefore give rise to deep wear grooves with a non-uniform distribution as shown in Fig. 11a. The grooves depicted on the worn surface of the high Mg content alloys (Fig. 11b and c) are shallow and uniform, indicating that the primary Mg2 Si tends to remain in the matrix. The mean value of roughness (RA ) was calculated to be 3.9 ± 0.4, 2.8 ± 0.3 and 2.1 ± 0.1 m for the 0.5%, 6% and 10% Mg alloys, respectively. The high roughness value of the worn surface of the base alloy implies the presence of deep and non-homogeneous grooves on the surface compared to the worn surface of high Mg content alloys. As a result, the value of roughness is indicative of the high wear resistance of alloys with higher Mg content. It was also observed that the hardness of matrix increases and the eutectic silicon becomes finer with the addition of Mg. The reduction of the tendency to cracking also tends to distribute the wear load more effectively [24]. As a result, matrix of the high Mg alloys is more effective in resisting wear than that of the base
Fig. 11. SEM images of worn surface of the as-cast alloys with (a) 0.5% Mg, (b) 6% Mg and (c) 10% Mg after the wear test.
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Fig. 12. Optical and SEM images of the worn surface of as-cast alloys with 0.5% Mg (a and b) and 10%Mg alloys (c and d).
alloy, where the debris of silicon particles or even the silica sands adversely affect the soft matrix and leads to substantial wear. Fig. 12a–d shows the wear characteristics at the surface of the 0.5% and the 10%Mg as-cast alloys using optical and SEM images. The debonding of large primary Si at their interface with the matrix of 0.5% Mg alloy can be observed in Fig. 12a at low and in Fig. 12b at high magnifications. The inferior wear properties may be attributed to this effect. Pulling out the entire or part of primary silicon as well as the presence of deep and non-homogeneous grooves in the matrix of the 0.5% Mg alloy implies that the matrix of the base alloy is more susceptible to wear than the alloy with high Mg content. This is supported by the fact that matrix of the base alloy is softer and finer than the other alloys due to the change of size and morphology of the eutectic Si phase. The appearance of the eutectic Mg2 Si and the increase of hardness of the ␣-Al solid solution in the alloys with high Mg content also contribute to this effect. Therefore, the debonding of the primary Si particle dispersed in softer matrix affects the wear rate to a greater extent when compared to the fragmented and cracked primary Mg2 Si in harder matrix. Fig. 12c shows the primary Mg2 Si particles in 10% Mg to be fragmented and also exhibit cracks (Fig. 12d) which enhances the wear resistance when compared to the large plastic deformation caused by primary Si particles in the base alloy. Qin et al. [10] have also noticed these cracks in Mg2 Si particles at the worn surface of Mg2 Si/Al composite after wear test. The T6 heat treatment improves the wear resistance of the all tested alloys. It was also observed that the wear ranking of the heat-treated alloys is the same as for the case of the as-cast alloys. These results can be explained by the fact that the wear resistance is dependant on the size and morphology of the primary as well as the eutectic phases and both changes should be taken into account.
4. Conclusions Wear properties of hypereutectic cast alloys such as A390 alloy and similar alloys with 6 and 10% Mg content were measured. The
improvement of wear characteristics for alloy with high Mg content was attributed to the transformation of the coarse primary Si particles to intermetallic Mg2 Si with a finer particle size. In addition, the solid fraction of the primary phase increases significantly with addition of Mg. The eutectic matrix microstructure of alloys with high Mg content also showed pronounced changes, particularly in size and morphology of the eutectic silicon with higher hardness which contribute to the wear properties. The wear test results indicate that the smaller intermetallic Mg2 Si particles for alloys with 6 and 10% Mg are more easily fragmented but remain attached to the matrix when compared to the coarse primary silicon in the base alloy (A390). The larger primary silicon particles are more easily detached from matrix and the debris of silicon particles become embedded in the matrix, resulting in deep and non-uniform grooves due to the softer matrix. This is contrary to shallow and uniform grooves for high Mg content alloys with the harder matrix. It is therefore concluded that both changes in the morphological aspects of the primary particles and the eutectic matrix should be considered in explanation of wear results. The T6 heat treatment improved the wear behaviour of each alloy and showed the same ranking of the wear property as the as-cast alloys. Acknowledgments The authors gratefully acknowledge the financial support from the Fonds Quebecois de Recherche sur la Nature et les Technologies (FQRNT) and Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors also thank Mr. Mario Patry for carring out the wear tests at UQAC. References [1] G. Timmermans, L. Froyen, Fretting wear behaviour of hypereutectic P/M Al–Si in oil environment, Wear 230 (1999) 105–117. [2] P.K. Rohatgi, R. Asthana, S. Das, Solidification, structure and properties of cast metal–ceramic particle composites, Int. Met. Rev. 31 (1986) 115–139. [3] N. Saka, A.M. Eleiche, N.P. Suh, Wear of metals at high sliding speeds, Wear 44 (1977) 105–125.
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Wear journal homepage: www.elsevier.com/locate/wear
Wear behaviour of hypereutectic Al–Si–Cu–Mg casting alloys with variable Mg contents Alireza Hekmat-Ardakan a , Xichun Liu a , Frank Ajersch a,∗ , X.-Grant Chen b a b
École Polytechnique de Montréal, Dép. de Génie Chimique, P.O. Box 6079, Centre-ville, Montreal, Quebec, Canada H3C 3A7 Université du Québec à Chicoutimi (UQAC), Dép. des sciences appliquées, 555 boulevard de l’Université, Chicoutimi, Québec, Canada G7H 2B1
a r t i c l e
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Article history: Received 27 January 2010 Received in revised form 7 July 2010 Accepted 9 July 2010 Available online 17 July 2010 Keywords: Metal–matrix composite Three-body abrasion Wear testing Intermetallics Hardness Particle shape
a b s t r a c t The wear properties of A390 (Al–17Si–4.5Cu–0.5Mg, wt%) hypereutectic Al–Si alloy were compared to new alloys containing 6 and 10 wt% Mg. The wear behaviour was investigated using dry sand rubber wheel (DSRW) abrasive wear apparatus for these alloys in the as-cast condition and after T6 heat treatment. A transformation of hard and coarse primary Si particles to fine Mg2 Si particles with inferior hardness occurs with increasing Mg content. The finer particle size together with increasing solid fraction of the primary phase was found to be key factors in improving the wear resistance of alloys with high Mg content. In addition, the eutectic matrix microstructure of alloys with high Mg content showed considerable changes, particularly in size and morphology of the eutectic silicon which significantly contributes to the wear resistance. The wear test results showed improved wear resistance for alloys with high Mg content. The microstructure of the worn surface indicated that the intermetallic Mg2 Si particles in alloys with 6% and 10% Mg addition are more solidly bonded to the matrix compared to the coarse primary silicon particles in A390 alloy which can be pulled out from the matrix. The worn surface of the A390 alloy exhibits deep and non-uniform grooves contrary to shallow and uniform grooves for the high Mg content alloys, resulting in the improved resistance to wear. Although the T6 heat treatment improved the wear behaviour of each alloy, the ranking of the wear property remained the same as the as-cast alloys. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hypereutectic Al–Si alloys such as A390 (Al–17Si–4.5Cu–0.5Mg) are used in applications that require high resistance to wear and corrosion, good mechanical properties, low thermal expansion and reduced density [1,2]. Their properties are of greatest interest to the automobile industry [3–5] for the production of fuel-efficient vehicles using light weight components produced from these alloys such as connecting rods, pistons, cylinder liners and engine blocks [6,7]. The good mechanical properties and high resistance to wear are essentially attributed to the presence of hard primary silicon particles distributed in the metal matrix [8]. The wear resistance can, however, also be affected by Si in the eutectic matrix. Hypereutectic aluminium–silicon alloys can be considered as in situ metal matrix composites (MMCs), where the primary silicon acts as the reinforcing phase [9]. This class of materials acquires its high strength and stiffness from the reinforcing phase and the damage tolerance and toughness is provided by the metal matrix [10]. However, the high latent heat and consequent long solidification
∗ Corresponding author. Tel.: +1 514 340 4711x4533; fax: +1 514 340 4468. E-mail address: [email protected] (F. Ajersch). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.07.007
time of hypereutectic aluminium–silicon alloys results in excessive growth of primary silicon particles as well as unfavourable shrinkage behaviour which adversely affects the use of these alloys [11]. Studies of Lasa and Rodriguez-Ibabe [12] focused on wear resistance of hypereutectic Al–Si–Cu–Mg alloys formed by various processing routes using a pin-on-disc method with different test speeds. Different forming processes such as permanent mould casting, thixoforming, squeeze casting and lost-foam casting result in different microstructural aspects, particularly in the size and distribution of primary Si particles. These are characterized by the number of Si particles per mm2 , mean particle size (Dp ) and the interparticle distance (). They have also shown that the wear behaviour strongly depends on the test speed. The tests carried out with low disc speed (0.089 m/s) strongly influence the coefficient of wear for alloys produced by different methods when compared to the tests with high disc speed (0.356 m/s). The diagrams of pin wear as a function of test distance indicated that the alloys produced by thixoforming resulted in a microstructure with the highest number of Si particles (∼866 part/mm2 ) as well as the lowest mean particle size (Dp = 11.6 m) with an interparticle distance of = 74.6 m and show no transition from mild wear (protective oxide layer) to severe wear. The protective oxide layer is broken and the wear
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Fig. 1. As-cast microstructure of Al–17%Si–4.5%Cu alloy with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg. The circled area presents the zone of intermetallic phases in the microstructure.
rate progressively increases during the test at low speed. The pin height during the wear test remains almost constant compared to alloys formed by other processing routes and the accelerated wear is attributed to cracking and fracture of the large silicon particles. In the present study, the microstructure of the hypereutectic alloy consists of fine primary particles dispersed in the matrix. This microstructure was produced by increasing the Mg content of A390 alloy and promoting the formation of fine intermetallic particles of Mg2 Si instead of coarse primary Si particles. Mg2 Si has a high melting temperature, a low density, a high hardness, a low thermal expansion coefficient, an equilibrium interface with excellent workability and a potential for cost reduction [13]. In terms of properties and solidification behaviour, great similarities exist between Mg2 Si and Si [14], except for the size of the precipitated particles. The present authors have previously carried out a thermodynamic evaluation [15] and microstructural evolution of A390 alloy and alloys with additions up to 10% Mg for conventional castings and for semisolid formed alloys [16,17]. It was observed that the increased Mg content resulted in the transformation of the primary Si particles to Mg2 Si intermetallic particles with a finer particle size as well as in the modification of the eutectic matrix, particularly for the eutectic Si phase. The microstructural features of these alloys with high Mg content should also influence wear behaviour of these alloys even though the measured hardness of Si particles is higher than Mg2 Si particles (Si = 1255 HV, Mg2 Si = 523 HV). The objective of present study is to investigate the wear behaviour of A390 alloy and alloys with addition of 6 and 10 wt% Mg in the as-cast (permanent mould) and in the T6 heat-treated conditions. 2. Experimental procedure 2.1. Materials The 6 and 10% Mg alloys were produced by adding AZ91 Mgbase alloy to A390 alloy. The chemical analysis of the test alloys
and AZ91 Mg alloy are presented in Table 1. In order to account for the oxidation loss, an additional amount of 20% Mg as AZ91 was added to the melt. The AZ91 alloy was wrapped in aluminium foil and added to the melt of A390 alloy at 750 ◦ C using an electric resistance furnace. Silicon and copper were also added to the melt in order to maintain the chemical composition of these elements at the same value as for the A390 composition. The melt was degassed with argon for 5 min with a flow rate 4 CFH (ft3 /h), and stirred for 2 min before pouring into a refractory coated permanent metallic mould preheated to ∼400 ◦ C to form tensile test bars. This standard mould was used to prepare rectangular samples measuring 25 mm × 75 mm with a 12-mm thickness which were then tested in the DSRW wear apparatus. The T6 heat treatment was carried out by solutionizing the ascast samples at 500 ◦ C for 1 h, followed by immediate quenching in normal water before aging at 175 ◦ C for 8 h. 2.2. Hardness test Macro-hardness measurements were carried out on each of the as-cast and T6 heat-treated samples using a Mitutoyo ATK-600 hardness machine with the Rockwell B method (100 kg load and 1/16 in. ball). The results of the average of eight measurements were then converted to the standard Vickers hardness (10 kg load with pyramid diamond indenter). Table 1 Chemical composition of test alloys and AZ91 Mg alloy (wt.%). Test alloy
Al
Si
Cu
Mg
Fe
Mn
Zn
P
Ti
A390 6% Mg 10% Mg
Bal. Bal. Bal.
16.7 16.83 17.14
4.58 4.12 4.32
0.58 6.08 9.74
0.32 0.28 0.25
0.02 0.04 0.05
0.01 0.015 0.02
0.0003 0.0002 0.0002
0.02 0.02 0.019
AZ91
Mg
Al
Mn
Zn
Si
Ni
Cu
Fe
Bal.
9.3
0.12
0.62
0.02
0.0006
0.0007
0.0046
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Fig. 2. Intermetallic phases in as-cast A390 (0.5% Mg) alloy: (a) optical microscopy, (b) SEM microscopy and (c) EDX analysis showing the intermetallic phases (1) Al5 FeSi (), (2) Cu2 Mg8 Si6 Al5 (Q) and (3) CuAl2 () of (b).
Table 2 The solid fraction of the solidified phases for 3 alloys in the as-cast condition. Alloys
0.5% Mg 6% Mg 10% Mg a
Phases (%) Primary Si
Mg2 Sia
CuAl2 ()
Cu2 Mg8 Si6 Al5 (Q)
Al5 FeSi ()
7.4 6.7 2.1
– 4.5 21.3
6.5 5.7 5.2
0.6 0.5 0.5
<0.1 <0.1 <0.1
The eutectic Mg2 Si is excluded from calculation.
Micro-hardness tests were also carried out for each of these samples using the Vickers diamond indent (Mitutoyo MT-2001 with Clemex image technology). A 100-gf load was applied in order to measure the average hardness of ␣-Al solid solution for both the as-cast and the heat-treated samples. 2.3. Wear test procedure A dry sand rubber wheel (DSRW) abrasive wear apparatus, model F-1510 FALEX, was used in this study. The apparatus and the test procedures are well-defined in the standard ASTM (G65-81)
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Fig. 3. Histograms of length distribution of Si and Mg2 Si particles in as-cast condition for: (a) primary Si in 0.5% Mg alloy, (b) primary Si in 6% Mg alloy, (c) Mg2 Si in 6% Mg alloy and (d) Mg2 Si in 10% Mg alloy.
abrasion test [18]. The basic ASTM machine consists of a rubberrimmed steel wheel, 228 mm × 12.7 mm wide; a sand hopper connected by a tube to a nozzle that allows a flow of 250–350 g/min with 50–70 mesh silica test sand. A revolution counter stops the drive motor after a set number of revolutions; and a weighted lever arm holds the specimen and generates a horizontal force against the wheel. The two test parameters, consisting of the sliding distance (based on wheel revolutions) and the specimen load, were set to 215.4 m (300 wheel revolutions) and 130 N, respectively. A Sartorius CP323S electronic precision balance with precision 0.001 g was used to measure the weight difference before and after the wear tests. The densities of the three alloys, measured by the Archimedes technique [19], were found to be 2.68, 2.58 and 2.51 g/cm3 for alloys with 0.5, 6 and 10% Mg additions, respectively.
2.5. Microstructural observation Metallographic specimens for the as-cast and the heat-treated samples were polished with abrasive paper up to 1200 followed by micropolishing using a monocrystal diamond suspension (0.5 m). All samples were etched with a 0.5% HF solution agent. Optical microscopy was carried out to characterize the microstructure of alloys and the Clemex software was used to determine the primary particle size and the solid fraction. The microstructure of alloys and the worn surface of wear samples were also analysed using a scanning electron microscopy (SEM, JOEL-840) coupled with energy dispersive X-ray (EDX) spectroscopy for elemental analysis.
2.4. Roughness measurement 3. Results and discussion The study of the worn surface was carried out using DEKTAK 3030ST contact profilometer apparatus. This profilometer can measure small surface variations using the vertical stylus displacement as a function of position. The stylus force was set 5 mg with 3 mm line scan perpendicular to wear direction. The total profile is then separated into roughness and waviness. The waviness filter removes very high frequency data from the raw (total) profile data since it can often be attributed to vibrations or to the presence of debris on the part surface. The roughness value was calculated by taking into account a 600-m cut-off length filter for waviness. The mean value of roughness (RA ) was measured by analysing a 3-mm line scan for each alloy at three different positions.
The microstructure of the three alloys in the as-cast condition is shown in Fig. 1a–c. The A390 base alloy, shown in Fig. 1a, exhibits a coarse polygonal structure of primary Si (fs(primary Si) = 7.4%) with a eutectic network of Al–Si phases together with the intermetallic phases, CuAl2 (), Cu2 Mg8 Si6 Al5 (Q) and Al5 FeSi (). The intermetallic phases and the EDX analyses are shown in Fig. 2. These intermetallic phases were also identified in the previous paper [16] using backscatter images. The 6% Mg alloy (Fig. 1b) is dominated by a distribution of both polygonal Si and dendritic-polygonal Mg2 Si (fs(primary Si) = 6.7%, fs(Mg2 Si) = 4.5%) and with a network of Al–Si–Mg2 Si eutectic phases.
Table 3 The mean length of Si and Mg2 Si particles for the as-cast Al–17%Si–4.5%Cu with 0.5%, 6% and 10% Mg. Alloy
Particle size Si Mean length (m)
A390 (0.5% Mg) 6 Mg 10 Mg
190.87 145.16 –
Mg2 Si Standard deviation (m) 73.21 52.46 –
Mean length (m)
Standard deviation (m)
– 88.31 95.38
– 32.99 43.21
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Fig. 4. The eutectic matrix network circled of as-cast alloys with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg.
Fig. 5. The eutectic matrix network of alloys with (a) 0.5% Mg (A390), (b) 6% Mg and (c) 10% Mg after T6 heat treatment.
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Fig. 6. The sphericity of eutectic Si for as-cast and after T6 heat treatment of alloys with 0.5% Mg, 6% Mg and 10% Mg.
Increasing the Mg content up to 10%, results in the precipitation of intermetallic particles of Mg2 Si as shown in Fig. 1c. The solid fraction of the Mg2 Si particles also increases considerably with the addition of 10% Mg, where fs(Primary Mg2 Si) = 21.3% and fs(Si) = 2.1%. Table 2 presents the analysis of phases for all three alloys. It was observed that the CuAl2 () is the most important intermetallic phase in all three alloys. Histograms of length distribution of Si and Mg2 Si particle are shown in Fig. 3 for the three alloys. It is observed that coarsest Si particles correspond to the A390 base alloy with some particles larger than 340 m. The mean length of Si particles decreases with addition of Mg as shown in Table 3. The coarsest Mg2 Si particles correspond to the alloy with 10% Mg content with some particles even larger than 425 m. The microstructure of 10% Mg alloy (Fig. 1c) shows that some dendritic Mg2 Si particles become interconnected and results in extremely large agglomerations. However, Table 3 indicates that the mean length of the Mg2 Si particles is significantly smaller than the Si particles. Fig. 4 compares the eutectic matrix network of the three alloys. The eutectic Si particles in A390 alloy are much larger in size, with a needle like shape and more separated than in the alloys with 6 and 10% Mg. The interparticle space () of eutectic Si was measured to be 9.79, 6.82 and 6.70 m for the as-cast alloys of 0.5% Mg, 6% Mg and 10% Mg, respectively. Eutectic Mg2 Si with Chinese script morphology also appears in the eutectic microstructure of high Mg content alloys. The T6 solution heat treatment promotes rounding of the eutectic Si particles particularly for high Mg content alloys (Fig. 5). Fig. 6 shows the sphericity of eutectic Si before and after heat treatment. The rounding of the eutectic phase after heat treatment is more significant for high Mg content alloys than for the base alloy. No effect on primary particles and intermetallic phases was observed in these samples as shown in Fig. 4c. The eutectic Mg2 Si is observed in high Mg alloys to form very fine particles as a result of the T6 heat treatment. Fig. 7 shows the values of the measured hardness of the three alloys for the as-cast condition and after the T6 heat treatment. The hardness of the as-cast alloys increases with addition of Mg which is attributed to the increase in solid fraction of the primary phases of Si + Mg2 Si from 7.4% to 23.4%. The changes in the microstructure of the eutectic network of matrix, particularly the increase of hardness of the ␣-Al solid solution, are also shown in Fig. 8. The T6 heat treatment provides substantial improvement in hardening of all alloys which is most significant for the 0.5% Mg alloy. The results in Fig. 7 also show that the hardness of the heat-treated samples is the lowest for the 6% Mg alloy. Increasing the Mg content to 10% again increases the hardness.
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Fig. 7. Macro-hardness of alloys with 0.5% Mg, 6% Mg and 10% Mg in as-cast and after T6 heat treatment.
Fig. 8. Micro-hardness of ␣-Al solid solution for alloys with 0.5% Mg, 6% Mg and 10% Mg in as-cast and after T6 heat treatment.
3.1. Abrasive wear behaviour The wear test results are shown in Fig. 9, in units of volume loss of material. The wear rate was calculated to vary from 2.6 to 3.4 × 10−4 mm3 /mm. Fig. 10 shows the stereo macrographs of the worn surface of the three alloys.
Fig. 9. The volume loss of alloys with 0.5% Mg, 6% Mg and 10% Mg after wear test using dry sand rubber wheel (DSRW) apparatus for as-cast and T6 heat-treated alloys.
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of alloy is identified as key parameter because particles of smaller size reduce the stress concentration at the interface between hard primary particles and the matrix. This interface is susceptible to cracking and tends to increase with particle size [23]. 3.2. Microstructural analysis of alloys
Fig. 10. The macrographs of the worn surface of as-cast alloys with (A) 0.5% Mg, (B) 6% Mg and (C) 10% Mg.
Fig. 9 clearly shows that the wear rate decreases with increasing the Mg content for both as-cast and heat-treated samples. After T6 heat treatment the wear resistance of all samples is greatly improved. Harun et al. [20] and Ott et al. [21] have also found that the wear resistance of Al–Si alloys is affected by heat treatment in a favorable way. Improvement in the yield strength obtained after heat treatment was also known to delay or inhibit wear [22]. Nevertheless, the effect of hardness on wear behaviour is not evident as the wear rate of the heat-treated alloys improves with increasing Mg content in spite of lower hardness. This is in contrast with the wear behaviour of the as-cast samples, where the hardness increases with addition of Mg. However, the wear behaviour can be directly related to the hardness for a specific alloy composition when the specific alloy wear behaviour is compared in the as-cast and the heat-treated conditions. In this case, the microstructure, particularly the size and morphology of the primary phase, does not change for these two conditions. For different compositions, the change in size of the primary phases rather than the hardness
Fig. 11 shows SEM images of the worn surface of the as-cast alloy sample for different Mg contents. The images are consistent with the wear test results shown in Fig. 9 and with the microstructure of the alloys shown previously in Fig. 1, where the average size of the primary silicon particles in base alloy (A390) is almost 1.8 times bigger than the average size of the Mg2 Si intermetallic particles for alloys with high Mg content. It was also observed that the silicon particles for the high Mg alloys are surrounded by Mg2 Si particles resulting in a better bonding of Si particles to the matrix than for the case of bonding of individual silicon particles in A390 alloy. Therefore, the stress imposed during the wear test of A390 provokes the fracture of coarse silicon particles which can be pulled out from the matrix and therefore give rise to deep wear grooves with a non-uniform distribution as shown in Fig. 11a. The grooves depicted on the worn surface of the high Mg content alloys (Fig. 11b and c) are shallow and uniform, indicating that the primary Mg2 Si tends to remain in the matrix. The mean value of roughness (RA ) was calculated to be 3.9 ± 0.4, 2.8 ± 0.3 and 2.1 ± 0.1 m for the 0.5%, 6% and 10% Mg alloys, respectively. The high roughness value of the worn surface of the base alloy implies the presence of deep and non-homogeneous grooves on the surface compared to the worn surface of high Mg content alloys. As a result, the value of roughness is indicative of the high wear resistance of alloys with higher Mg content. It was also observed that the hardness of matrix increases and the eutectic silicon becomes finer with the addition of Mg. The reduction of the tendency to cracking also tends to distribute the wear load more effectively [24]. As a result, matrix of the high Mg alloys is more effective in resisting wear than that of the base
Fig. 11. SEM images of worn surface of the as-cast alloys with (a) 0.5% Mg, (b) 6% Mg and (c) 10% Mg after the wear test.
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Fig. 12. Optical and SEM images of the worn surface of as-cast alloys with 0.5% Mg (a and b) and 10%Mg alloys (c and d).
alloy, where the debris of silicon particles or even the silica sands adversely affect the soft matrix and leads to substantial wear. Fig. 12a–d shows the wear characteristics at the surface of the 0.5% and the 10%Mg as-cast alloys using optical and SEM images. The debonding of large primary Si at their interface with the matrix of 0.5% Mg alloy can be observed in Fig. 12a at low and in Fig. 12b at high magnifications. The inferior wear properties may be attributed to this effect. Pulling out the entire or part of primary silicon as well as the presence of deep and non-homogeneous grooves in the matrix of the 0.5% Mg alloy implies that the matrix of the base alloy is more susceptible to wear than the alloy with high Mg content. This is supported by the fact that matrix of the base alloy is softer and finer than the other alloys due to the change of size and morphology of the eutectic Si phase. The appearance of the eutectic Mg2 Si and the increase of hardness of the ␣-Al solid solution in the alloys with high Mg content also contribute to this effect. Therefore, the debonding of the primary Si particle dispersed in softer matrix affects the wear rate to a greater extent when compared to the fragmented and cracked primary Mg2 Si in harder matrix. Fig. 12c shows the primary Mg2 Si particles in 10% Mg to be fragmented and also exhibit cracks (Fig. 12d) which enhances the wear resistance when compared to the large plastic deformation caused by primary Si particles in the base alloy. Qin et al. [10] have also noticed these cracks in Mg2 Si particles at the worn surface of Mg2 Si/Al composite after wear test. The T6 heat treatment improves the wear resistance of the all tested alloys. It was also observed that the wear ranking of the heat-treated alloys is the same as for the case of the as-cast alloys. These results can be explained by the fact that the wear resistance is dependant on the size and morphology of the primary as well as the eutectic phases and both changes should be taken into account.
4. Conclusions Wear properties of hypereutectic cast alloys such as A390 alloy and similar alloys with 6 and 10% Mg content were measured. The
improvement of wear characteristics for alloy with high Mg content was attributed to the transformation of the coarse primary Si particles to intermetallic Mg2 Si with a finer particle size. In addition, the solid fraction of the primary phase increases significantly with addition of Mg. The eutectic matrix microstructure of alloys with high Mg content also showed pronounced changes, particularly in size and morphology of the eutectic silicon with higher hardness which contribute to the wear properties. The wear test results indicate that the smaller intermetallic Mg2 Si particles for alloys with 6 and 10% Mg are more easily fragmented but remain attached to the matrix when compared to the coarse primary silicon in the base alloy (A390). The larger primary silicon particles are more easily detached from matrix and the debris of silicon particles become embedded in the matrix, resulting in deep and non-uniform grooves due to the softer matrix. This is contrary to shallow and uniform grooves for high Mg content alloys with the harder matrix. It is therefore concluded that both changes in the morphological aspects of the primary particles and the eutectic matrix should be considered in explanation of wear results. The T6 heat treatment improved the wear behaviour of each alloy and showed the same ranking of the wear property as the as-cast alloys. Acknowledgments The authors gratefully acknowledge the financial support from the Fonds Quebecois de Recherche sur la Nature et les Technologies (FQRNT) and Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors also thank Mr. Mario Patry for carring out the wear tests at UQAC. References [1] G. Timmermans, L. Froyen, Fretting wear behaviour of hypereutectic P/M Al–Si in oil environment, Wear 230 (1999) 105–117. [2] P.K. Rohatgi, R. Asthana, S. Das, Solidification, structure and properties of cast metal–ceramic particle composites, Int. Met. Rev. 31 (1986) 115–139. [3] N. Saka, A.M. Eleiche, N.P. Suh, Wear of metals at high sliding speeds, Wear 44 (1977) 105–125.
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