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Production and properties of SiCp-reinforced aluminium alloy composites
Composites: Part A 34 (2003) 709–718 www.elsevier.com/locate/compositesa
Production and properties of SiCp-reinforced aluminium alloy composites Y. Sahina,*, M. Acılarb a
Department of Mechanical Education, Faculty of Technical Education, Gazi University, Besevler, Ankara, Turkey b Department of Metal Education, Faculty of Technical Education, Gazi University, Besevler, Ankara, Turkey Received 12 April 2002; revised 17 February 2003; accepted 2 April 2003
Abstract A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal-matrix composites (MMCs); B. Wear; E. Liquid metal infiltration
1. Introduction Aluminium matrix composites are among the most promising materials for wear and strucutural applications due to their low density, low cost and ease of fabrication of composite [1,2]. In recent years, much interest has centered on the use of SiC or Al2O3 particles [3 –6]. The processes for obtaining these composite materials include solid-state processes such as powder metallurgy (PM), where metal and ceramic powders are blended and hot-pressed [7], and liquid-state processes such as melt infiltration [8 – 12], compocasting, blending ceramic powder and molten aluminium and casting, melt stirring [13 – 19], pressurized infiltration and squeeze casting [20 –34]. For MMCs, the vortex method is cost-effective while PM is costly, but is suitable for small components. Pressure infiltration casting is a form of liquid infiltration which utilizes a pressurized inert gas to force liquid metal into a preform of the reinforcement material and was investigated by many * Corresponding author. E-mail address: [email protected] (Y. Sahin). 1359-835X/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1359-835X(03)00142-8
researchers [21 –28]. The advantages of pressure casting are high productivity and ease of fabrication, but the pressure casting machine and the die are expensive. Liquid metal routes are economically more viable, however, the non-wetting nature of many ceramics by molten aluminium was an obstacle [31,32] as it resulted in poor ceramic/metal interfaces and incomplete infiltration. This problem can be addressed, in part, through alloying additions, such as Mg and Li, and through the use of a reinforcement coating [33]. Squeeze casting provides good infiltration quality of chopped-preforms. However, stir casting is also restricted to composites with a low volume fraction although it was successfully used to prepare particle reinforced metals. Most of the researh work carried out in this field focused on melt stirring [14 – 22], presurized infiltration [20 – 28], and squeeze casting [29 – 34]. Very few studies have been performed using the vacuum infiltration method [35 – 39], even though both processing and equipment are simple. The purpose of this study was to develop a vacuum infiltration method for producing SiCp reinforced aluminium alloy composites. The microstructure, hardness, density and wear property of the composites were also investigated.
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2. Experimental details 2.1. Materials Metal matrix composites containing various volume fraction levels of SiC particles were produced by vacuum infiltration. For the production of MMCs, an Al – Si alloy was used as the matrix material while SiC powder with an average size of 149 or 80 mm was used as the reinforcement. The chemical composition of the Al alloy is shown in Table 1. In the infiltration experiments, the average grain sizes of Al and Mg powders were 2 100 and 2 250 mm, respectively. The oxidation process of SiC particles was carried out in an electric furnace. The SiC particles were heated to a pre-set temperature of 1200 8C for 3 h. 2.2. Infiltration process A new vacuum infiltration process was developed to produce MMCs with various volume fractions of SiC particulate. A schematic diagram of the infiltration equipment is shown in Fig. 1. The infiltration apparatus consisted of an electrical resistance furnace, which was used for melting the aluminium alloy, a temperature control unit, N2 tank and a vacuum pump unit. Three thousand kilograms of aluminium alloy was placed in a crucible and heated by means of the electrical furnace. N2 gas was passed over the compacts at about 1000 cc/min flow rate while the Al alloy was being melted. The steel tube that included the powder compact was inserted in the liquid metal after 60 s, whereupon the N2 flow was stopped. The infiltration time was variable for the different compacts. Samples were held at temperature for about 3 min to allow temperature uniformity between the liquid metal and the sample. Vacuum was applied continuously (80 –500 mmHg) during the experiments. As soon as the infiltration time reached the predetermined time, the asinfiltrated compact was removed from the melt and the vacuum pump turned off. All the infiltrated specimens were cooled to room temperature. The temperature was controlled by a K-type of thermocouple with an accuracy of ^ 2 8C. In this experimental study, the infiltration behaviour of the compacts was investigated on the various infiltration temperatures for establishing to the proper infiltration temperature, infiltration time and infiltration vacuum for each compact. The infiltration distance was measured for all the compacts. Infiltration temperatures of 600, 620, 640, 700, 750 and 800 8C were chosen for studying
Fig. 1. Schematic diagram of infiltration apparatus used for the experimental study.
the temperature effect on infiltration behaviour. The infiltration time was considered to begin immediately after the compact tube was inserted into the melt. 2.3. Preparation of compact The powder compacts prepared for infiltration are schematically shown in Fig. 2. A stainless steel tube measuring 10 mm outer diameter and 8 mm inner diameter and 200 mm in length was used as a sample holder. First of all, the bottom of steel tube was plugged with a porous filter and then another piece of alumina mat was used to prevent powder leakage during compaction. Mixed powders for each case were weighed according to the volume fraction of the particulates. The mixed powders of SiCp and pure Al, Mg powders were poured into the tube and gently tapped up to the powder height of 50 mm. Subsequently a piece of stainless steel and a 5 mm thick alumina mat were inserted
Table 1 The chemical composition of alloy used in the infiltration Elements Wt (%)
Si 9.42
Fe 0.38
Mn 0.431
Mg 0.36
Ti 0.10
Zn 0.06
Cu 0.05
Ni 0.04
Fig. 2. A schematic diagram of powder compact.
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Table 2 Volume fraction of powder compacts used for infiltration process Type of the specimen
A B C D E F G a
Volume fraction of compacts (%) SiCp (149 mm)
Al powder (2100 mm)
Mg powder (2250 mm)
Balance (Alloy matrix, %)
– 10 20 30 40 50 55a
50 40 30 20 10 – –
5 5 5 5 5 5 –
Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36 þ Mg5% Al-9.42 þ Mg-0.36 þ Mg5%
SiCp was oxidized at 1100 8C.
into the tube to act as a stopper. The volume fraction of compacts used for the infiltration process is given Table 2. For preparing the compacts, ball milling was used to mix the powders. In the mill process, various volume fractions of powders were mixed at various times to establish the optimal mixing time which was about 1 h. The steel tube contained the compacts, was placed vertically, and 35 g of weight was placed on the mixed powders in the steel tubes. Then, the samples were vibrated for about 3 min. As a result of this, the powders were forced slightly and homogeneously throughout the samples. Later another double-sided filter was inserted on the top of compacts while the alumina mat was also placed between the filters. Finally, a steel rod was inserted between stopper filter and the top end of the tube to prevent the powder from crumbling during infiltration. Seven kinds of mixed powders were used in these experiments as shown in Table 2. After infiltration, the volume fractions of SiCp in composites were 0, 10, 20, 30, 40, 50 and 55 vol%SiCp for infiltrating the compacts of A,B,C,…, and G, respectively. For F samples, 5 wt%Mg þ 9.42 wt%Si þ Mg%0.36 was used as a matrix while 50 wt%SiCp and 5 wt%Mg was also used as the composite. For G samples, SiC particles were oxidized at 1000 8C for a period of 3 h before starting the infiltration process. In this case the Al– Si alloy included 5 wt%Mg. The powder compact tube was further connected to a mechanical vacuum pump. Prior to inserting the powder compact into the melt, the vacuum pump was turned on and the alumina mat was removed. The characteristics of the powder compacts are also listed in Table 2. The density of pure aluminium and SiC particles was 2.7 and 3.2 g/cm3, respectively. 2.4. Metallography The distribution of SiCp and the infiltration extent of the Al alloy were characterised by optical microscopy. After infiltration, different locations were marked on the 10 mm long composite compacts. Seven specimens were taken at the locations of 10 mm from the bottom of the compacts.
Specimens for metallographic observation were prepared by grinding through 800 grit papers followed by polishing with 6 and 3 mm diamond paste. 2.5. Density and hardness measurement The density of the composites was obtained by the Archimedian principle of weighing small pieces cut from the composite disc first in air and then in water. The theoretical density of composite and alloy matrix specimens was then calculated according to the rule of mixtures. The porosity of the composites was also determined. The hardness of the composites and the matrix alloy was measured after polishing to a 3 mm finish. 2.6. Wear testing Dry sliding wear tests were performed on MMCs and the alloy matrix using a pin-on-disc type apparatus. The materials to be tested were cut into cylindrical shape from the composite plate by electrical discharge machining. The pins produced from the composites were 6.3 mm in diameter, and approximately 4.5 mm long with brass sleeves using an epoxy adhesive. This was followed by a polishing procedure using a grinding paper up to 800 grade. The pin was mounted in the specimen holder with the particle reinforced composite to the counterface. The discs were flatly ground to give a surface finish of approximately 0.15 mm (CLA). The pin and disc were cleaned ultrasonically and weighed in a micro-balance before each test, and then inserted into the machine. Tests were carried out at a sliding speed of 1.0 m/s under various loads of 12, 24, and 36 N at room temperature. In all tests, each experiment was repeated at least 6 times. After the test, the specimens were cleaned with acetone, dried and weighed again to determine the weight loss due to wear. The wear rates were calculated from the difference in weight of the specimens measured before and after the tests to the nearest 0.1 mg using an analytical balance, and then converted into volume loss known density data.
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3. Results and discussion 3.1. Infiltration behaviour of various compacts Infiltration temperature, vacuum, volume fraction of particle, matrix alloy and infiltration time are important process parameters for the fabrication of low volume fraction composites. Fig. 3a shows the variation of infiltration length versus infiltration temperature for the 10 vol% SiCp compacts at various infiltration times. Infiltration was observed to start after 120 and 60 sec for infiltration temperatures at 600 and 620 8C, respectively. The infiltration rate increased with increasing temperature up to 640 8C and then remained stable after the temperature increased. To improve the wetting ability of the solid by a molten metal, some conditions such as mutual solubility, reaction or formation of intermetallic compound is necessary. Therefore, an increase in infiltration temperature would be promoted by the reaction between Al powder and Al alloy melt contributing to an increase in infiltration rate. Similar results were also observed in a study of
Fig. 3. Variation of infiltration length versus infiltration temperature for the 10 vol% of SiCp compacts. Infiltration time (a), different application of vacuum (b).
Al-8.7%Si-2.6%Cu infiltration into Ni-coated SiC powder compact by Chung et al. [36] who indicated that the infiltration temperature of 640 8C was favourable for the fabrication of SiCp/Al composites. In their results, the powder compacts contained well-mixed Ni-coated SiCp and pure aluminium powders, and then were packed in quartz tubes. They showed that infiltration lengths increased up to the 6.0 cm when infiltration was performed at 680 8C but the infiltration time was 30 s. In an earlier work by same authors [37] matrix composites of aluminium with 5.9 wt%Si and 0.23 wt%Mg containing about 50 vol% Ni-coated SiCp were successfully infiltrated at above 700 8C, and X-ray mapping analysis showed that Ni and P segregated to the interface between the matrix and the carbide. Particulate Al composites reinforced with nickel-coated SiC or Al2O3 were fabricated at 900 8C in less than that 2 min while in the case of the as-received powders, no infiltration was observed after 20 min under similar experimental conditions [13]. Al-matrix composites containing SiC whiskers were fabricated by pressureless infiltration of liquid Al –Mg or Al– Si – Mg alloys at 830– 950 8C in the presence of N2 into a preform of nickel coated SiCw. The infiltration distance increased significantly with increasing temperature for a given alloy, but no infiltration occurred at 800 8C. The use of SiCw without the metal coating and the Al-10Mg matrix led to no infiltration up to 900 8C. In contrast, the use of coated SiCw and the same matrix led to infiltration at just 830 8C even without prior evacuation. Considering the results published by other authors [8,13] infiltration time was variable. For example, Xi et al. [8] infiltrated both SiC and C fibres with Al alloys for 600 s. However, in our case, the infiltration time of 180 s was much lower, but temperatures up to 800 8C were used. This is due to the increased amount of particles used in the composites and the different aluminium alloy. It indicates that rapid infiltration could be achieved for the test conditions. Fig. 3b shows the variation of infiltration length versus infiltration temperature for the 10 vol%SiCp compacts at different pressure conditions from 80 up to 200 mmHg for a period of 3 min. This figure also indicates that the infiltration length was lower under the application of 80 mmHg than that of the others, and increased with increasing vacuum pressure. For the 20 vol% composite, optimal vacuum was found to be at 200 mmHg. The melt can only penetrates the interspaces, the capillary resistance of which is equal to, or smaller than, the exerted vacuum. Candan et al. [22] showed that the time necessary to complete infiltration in the compacts decreased with increasing particle size and increasing applied pressure of 400– 900 kPa. The variation of infiltration length for the various composite compacts containing Al powder under different infiltration temperatures is shown in Fig. 4a. It is observed that the infiltration length for compacts containing Al powders and 10 vol% particles is higher than that of the compacts containing higher vol% particles when the tests
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Fig. 4. (a) Variation of infiltration length versus volume of particle infiltrated under 200 mmHg fixed vacuum pressure compacts at different temperatures. (b) Variation of infiltration length versus applied pressure infiltrated compacts under various vacuum pressures at 750 and 800 8C temperatures.
are carried out under 200 mmHg pressure. It means that the rate of infiltration and quality of infiltration are affected by the volume fraction of particles. It also reveals that infiltration length depends upon the experimental conditions and the volume fraction of the reinforcement. At all test temperatures, the infiltration length was 50 mm under 200 mmHg pressure for the 10 vol% composite and it decreased to 4.5 mm for the 20 vol% composite under the same pressure at 600 8C. However, infiltration length was 48 mm for the 30 vol% composite at 700 8C. This phenomenon is attributed to the fact that the porosity in the compact with the lower volume fraction of the SiCp is higher than that of the compact with the higher volume fraction of SiCp in the present experiment (Fig. 8b) because of the increasing melt temperature. In addition, no infiltration was achieved for the 30 vol% composites when tested at 600 8C. The infiltration lengths are 22, 34, 43, 48 and 50 mm when infiltration was performed at 620, 640, 660, 700 and 750 8C for the 30 vol% SiCp reinforced composites, respectively. This effect might have resulted from the reduction in viscosity of the melt with increasing temperature. Surface modification of the reinforcement by coating or passive oxidation technique has been successful to some extent in preventing the detrimental interfacial reaction and enhancing the material wettabilities. This also agrees with the results of the previous reports. Maxwell et al.
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[25] measured the infiltration distance versus time for Al into SiCp compacts at 670 8C and found that an incubation time had to be exceeded before infiltration could start. Alonso et al. [27] reported that the infiltration distance of pure Al into SiC or TiCp compacts at 750 8C was proportional to incubation time ðt1=2 Þ with various pressures. The infiltration process was performed at 200 mmHg pressure under different temperatures up to 800 8C. For the accomplishment of the full infiltration of higher volume fraction of composite compacts, higher temperature and vacuum application is required. The sensitivity of the kinetics of infiltration to particle volume fraction is a result of the dual role of an increasing particle volume fraction in decreasing the spacing between particles and in increasing the amount of solid formed per unit volume of compact. Fig. 4b also shows the variation of infiltration length with applied vacuum pressure. Tests were performed on the 50%SiC compact at temperatures of 750 and 800 8C. This figure revealed that infiltration length increased sharply with increasing pressure. It rose almost linearly up to 400 mmHg pressure at both temperatures, and remained stable over this pressure. This is the result of the increase of the penetrating ability of the melt as the vacuum pressure increased during the infiltration. However, the rapid increase of the applied pressure caused more and more interspaces to be penetrated and the flow velocity of the melt gradually to be reduced. The porosity indicated that at low temperature, low vacuum, and non-homogeneous distribution could be observed unless the optimal processing conditions obtained, especially for 10, 20 and 30% particle-reinforced compacts. However, the process parameters should be optimised and the optimal processing parameters determined in producing MMCs containing different volume fractions of particles are shown in Table 3. As shown in the table, full infiltration occurred at these experimental test conditions. For example, full infiltration of the 55 vol% SiCp composite appeared at 800 8C and 180 s infiltration time under the 500 mmHg pressure. Infiltration not only depends on the volume fraction but also temperature and alloy. It is well-known that the wetting angle of molten Al on SiC can be substantially reduced as the temperature increases above 1000 8C, and the wetting angle can be reduced further when Si or Mg are present in the alloy [8]. Therefore, to achieve spontaneous infiltration, the wetting angle must be brought down substantially below 908. Fig. 5 shows the variation of infiltration length versus infiltration time for the 50 vol% particle reinforced compacts. For the production of the MMCs, 5 wt% Mg was introduced into the Al– Si alloy at a temperature of 800 8C, and it was compared to the alloy without addition of Mg. The infiltration appeared to start after 3 s and the infiltration time and rate increased to 50 mm after 10 s time and thereafter remained stable. It means that the infiltration rate increased as the Mg was used. It is presumably due to the improvement in wettability of the alloy. However, no infiltration was observed in the absence of Mg even after
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Table 3 The optimal infiltration processes parameters in producing the MMCs Volume fraction of particles (%)
Types of the alloy
Infiltration temperature (8C)
Infiltration time (s)
Applied vacuum (mmHg)
10%SiCp 20%SiCp 30%SiCp 40%SiCp 50%SiCp 55%SiCpa
Al –Si Al –Si Al –Si Al –Si Al –Si þ 5 wtMg Al –Si þ 5 wtMg
640 700 700 800 800 800
120 120 180 180 180 180
300 400 400 500 500 500
a
SiCp was oxidized at 1100 8C.
an infiltration time of 60 s and the infiltration started at about 300 s. Other studies indicated that the Mg addition into the metal matrix is useful in improving the wettability of the composites. For example, Oh et al. [40] found a slightly improved wettability when increasing the magnesium content from 2 to 4.5%. Bardal et al. [41] studied the wettability and the interfacial reaction products in the AlSiMg surface oxidized SiC system. It was found that the oxidized SiC is more easily wetted than as produced SiC. Transmission electron microscopic observation showed that at the interface between SiC and the Al matrix, Mg –Al spinel crystallities are generally found at high densities. Previous work carried out by Canul et al. [34] showed that Mg reduced the surface tension and the contact angle between Al and SiC, and thus its presence in the alloy was essential in pressureless infiltrating SiCp preforms with Al alloys. Silicon, as an alloying element in Al, retards the kinetics of the chemical reactions that resulted in the formation of the unwanted carbides of Al4C3 and Al4SiC4. Fig. 5b shows the variation of infiltration length versus Mg content in the given alloy for SiCp and oxidized-SiCp composite compacts. It is obvious that no infiltration was observed for the non-oxidized-SiCp when the 4 wt% Mg content in the alloy was used. However, the infiltration length reached 50 mm when 5 wt% Mg was used with the same particles. For the oxidized-SiCp, it was again 50 mm when 3 wt%Mg was used and remained stable thereafter. The decreases in the surface tension with an increase of Mg content in Al –Mg alloy melts were reported with other researchers [22,26]. Surface tension decreased from 1.12 to 1.02 N m21 with the addition of 8 wt%Mg to Al for nonoxidized surface at a temperature of 700 8C. SiCw reinforced Al was fabricated by vacuum infiltration of liquid aluminium into a porous whisker preform under argon gas pressure, using an infiltration temperature of 665 8C [41]. In their work, the reinforcement volume fraction was lower than that of our case, the volume of whiskers ranging from 11 to 37%.
were obtained from the optimum process parameters carried out on the products. The distribution of SiCp in these composites is uniform. Some pores were present in these composites, especially for the 10 vol% of particulate. During infiltration, the molten metal preferentially penetrated into the large interspaces and separated the particle crowded area and the small interspaces from the outside. From that moment, air or gas entrapment occurred. But no large pores were observed for the higher volume fraction composites. These results show that this infiltration process could be used to produce MMCs when the volume fraction was less than that of the 50% SiC particles because the vacuum method was used over 50% SiC volume fraction, in general. When the specimen was infiltrated at 680 8C, the dendritic structure was more obvious in the 10 vol% SiCp
3.2. Microstructures The optical micrographs of aluminium composites reinforced with 10, 20, 30, 40 and 50 vol% of SiCp are shown in Fig. 6a – f, respectively. These microstructures
Fig. 5. (a) Variation of infiltration length versus infiltration time for the 50 vol% of SiCp compacts infiltrated at 800 8C. (b) Variation of infiltration length versus Mg content in the alloy for the unoxidized-SiCp and oxidizedSiCp compacts.
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Fig. 6. Optical micrographs of various volume fraction of Al alloy composites reinforced with. (a) 10 vol% SiCp £ 50, (b) 20 vol% SiCp £ 50, (c) 30 vol% SiCp £ 50, (d) 40 vol% SiCp £ 50, (e) 50 vol% SiCp £ 50, (f) 50 vol% SiCp £ 1000.
composite because of the melting of the pure aluminium powders (Fig. 6a). The existence of pores is attributed to the high viscosity of the melt because more surface area occurred in the sample. The microstructure of the higher volume fraction of composite specimens infiltrated at 800 8C indicated the optimum infiltration conditions. No large pores existed in these specimens (Fig. 6c – e). This might be due to the improvement of flowing behaviour when the Al– Si alloy and Mg powders were used, and application of higher vacuum pressure. Fig. 6f shows a higher magnification of Fig. 6e, indicating a good bonding
interface between the matrix and SiC without evidence of cavities. 3.3. Hardness and density The variation of hardness of the composites with volume percent of particles is shown in Fig. 7. The hardness of the MMCs increased more or less linearly with the volume fraction of particles in the alloy matrix due to the increase of the ceramic phase. A higher hardness was also associated with a lower porosity, as shown in Fig. 8a and b.
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amount of porosity in the composites was observed with increasing SiC volume fraction because of increasing amount of vacuum pressure up to 500 mmHg, as evidenced by Fig. 4b. 3.4. Wear rate
The measured density of the composites is shown in Fig. 8a as a function of SiC volume fraction. As shown in the figure, the density of the composite increased with increasing SiC volume fraction. The porosity of the composite decreased with increasing SiC volume fraction, as shown in Fig. 8b. It is evident that the porosity level was higher for the 10 vol% of SiCp reinforced composite, as observed in the micrographs, which is related to the flowing behaviour of the composite. It also could be the result of lower and constant vacuum pressure. This figure indicates that a decreased
The wear testing carried out for various volume fractions of composites is shown here. Tests were performed at different loads under a sliding speed of 1.0 m s21. The average volumetric wear rates for composites and matrix alloy were determined, and are illustrated graphically in Figs. 9 and 10 as a function of load and volume fraction of particle, respectively. Fig. 9 shows the variation of volumetric wear rates for the particulate composites and matrix alloy as a function of applied load. It is observed that the wear rate of the matrix alloy was more than that of the composite at all load conditions. At this fixed speed under different loads, the wear rate increased as the normal load increased. This figure further revealed that the volumetric wear rate increased non-linearly with load and showed approximately concave behaviour. However, it was very sharp for the matrix alloy while a low tendency was observed for the MMCS. For the matrix alloy, the soft asperities were easily deformed and sheared under the repeated loading conditions while the hard asperities on the counterface or hard particles between the sliding surface plough and cut the soft surfaces, and resulted in giving more damage to the surface. However, for the composites, this might be a result of the SiC particles in the matrix breaking on exposure at the rubbing surface because of the changing in the surface characteristics of the composite samples, but SiC particles in the matrix alloy improved its thermal stability of the sample. Fig. 10 shows the variation of volumetric wear rates for the composites as a function of volume fraction of particle.
Fig. 8. (a) Variation of experimental density with vol% particle, (b) Variation of porosity with vol% particle.
Fig. 9. Average volumetric wear rates for SiCp reinforced composites as a function of load.
Fig. 7. Variation of hardness with vol% particle.
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on the infiltration temperature used. The temperatures used ranged from 640 –800 8C. The infiltration rate increased with increasing infiltration time. 3. Microstructural examination showed that the SiCp distributions were homogeneous. The hardness and wear resistance of the composites also increased with increasing volume fraction of particulate. The hardness and density of the composite increased linearly with increasing particulate content, however, porosity level decreased with increasing particulate content. 4. Dry sliding wear properties of the Al alloy improved significantly by the addition of SiC particles into the matrix alloy under all test conditions. The wear resistance of the composites increased significantly up to 10% SiC addition and remained stable thereafter. The wear rate of composite also appeared to increase nonlinearly with load.
Fig. 10. Average volumetric wear rates for SiCp reinforced composites as a function of volume fraction.
A considerable reduction in wear was observed for the 10 vol% of SiC particulate composite, compared to the unreinforced alloy. Fig. 10 also indicates that critical volume fraction of particles occurred in the composites. The wear rate decreased with increasing volume fraction, but the increase in the volume fraction beyond 10% produced only a small decrease in the wear rate, however, wear was then insensitive to particle content to the maximum content of 40%. Similar results were observed in previous work carried by Sahin [2,3] where continuous fibres were used as the reinforcement and the critical volume fraction of fibre was found to be about 16%.
4. Conclusions The following conclusions can be drawn from the present study on the production and properties of SiCp/Al – Si alloy composites. 1. A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of SiCp reinforcements. Al-9.42 wt%Si0.36 wt%Mg alloy matrix composites containing up to 55 vol% SiC particles were successfully produced by this method. 2. The infiltration behaviour was affected by various parametres such as temperature, time, alloy composition, oxidation of particles and volume fraction of particles. The higher volume fraction composites could be achieved when vacuum between 400 –500 mmHg pressure was applied while the lower volume fraction of composites was infiltrated at 200 mmHg pressure. The infiltration times were between 120 and 180 s depending
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[35] Kun Y, Dollhopf V, Kochendorfer R. Compos Sci Technol 1993;46: 1–6. [36] Chung W-S, Chang S-Y, Lin SJ. J Mater Res Soc 1999;14(3):803 –10. [37] Chung W-S, Lin S-J. Mater Res Bull 1996;31(12):1437–47. [38] Lee KB, Kwon H. Scripta Materialia 1997;36(8):847–52. [39] Chiou JM, Chung DDL. J Mater Sci 1991;26:2583 –8. [40] Oh S-Y, Cornie A, Russel KC. Ceram Engng Sci Proc 1987;8:912. [41] Bardal A. Mater Sci Engng 1992;A159:119.
Production and properties of SiCp-reinforced aluminium alloy composites Y. Sahina,*, M. Acılarb a
Department of Mechanical Education, Faculty of Technical Education, Gazi University, Besevler, Ankara, Turkey b Department of Metal Education, Faculty of Technical Education, Gazi University, Besevler, Ankara, Turkey Received 12 April 2002; revised 17 February 2003; accepted 2 April 2003
Abstract A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal-matrix composites (MMCs); B. Wear; E. Liquid metal infiltration
1. Introduction Aluminium matrix composites are among the most promising materials for wear and strucutural applications due to their low density, low cost and ease of fabrication of composite [1,2]. In recent years, much interest has centered on the use of SiC or Al2O3 particles [3 –6]. The processes for obtaining these composite materials include solid-state processes such as powder metallurgy (PM), where metal and ceramic powders are blended and hot-pressed [7], and liquid-state processes such as melt infiltration [8 – 12], compocasting, blending ceramic powder and molten aluminium and casting, melt stirring [13 – 19], pressurized infiltration and squeeze casting [20 –34]. For MMCs, the vortex method is cost-effective while PM is costly, but is suitable for small components. Pressure infiltration casting is a form of liquid infiltration which utilizes a pressurized inert gas to force liquid metal into a preform of the reinforcement material and was investigated by many * Corresponding author. E-mail address: [email protected] (Y. Sahin). 1359-835X/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1359-835X(03)00142-8
researchers [21 –28]. The advantages of pressure casting are high productivity and ease of fabrication, but the pressure casting machine and the die are expensive. Liquid metal routes are economically more viable, however, the non-wetting nature of many ceramics by molten aluminium was an obstacle [31,32] as it resulted in poor ceramic/metal interfaces and incomplete infiltration. This problem can be addressed, in part, through alloying additions, such as Mg and Li, and through the use of a reinforcement coating [33]. Squeeze casting provides good infiltration quality of chopped-preforms. However, stir casting is also restricted to composites with a low volume fraction although it was successfully used to prepare particle reinforced metals. Most of the researh work carried out in this field focused on melt stirring [14 – 22], presurized infiltration [20 – 28], and squeeze casting [29 – 34]. Very few studies have been performed using the vacuum infiltration method [35 – 39], even though both processing and equipment are simple. The purpose of this study was to develop a vacuum infiltration method for producing SiCp reinforced aluminium alloy composites. The microstructure, hardness, density and wear property of the composites were also investigated.
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2. Experimental details 2.1. Materials Metal matrix composites containing various volume fraction levels of SiC particles were produced by vacuum infiltration. For the production of MMCs, an Al – Si alloy was used as the matrix material while SiC powder with an average size of 149 or 80 mm was used as the reinforcement. The chemical composition of the Al alloy is shown in Table 1. In the infiltration experiments, the average grain sizes of Al and Mg powders were 2 100 and 2 250 mm, respectively. The oxidation process of SiC particles was carried out in an electric furnace. The SiC particles were heated to a pre-set temperature of 1200 8C for 3 h. 2.2. Infiltration process A new vacuum infiltration process was developed to produce MMCs with various volume fractions of SiC particulate. A schematic diagram of the infiltration equipment is shown in Fig. 1. The infiltration apparatus consisted of an electrical resistance furnace, which was used for melting the aluminium alloy, a temperature control unit, N2 tank and a vacuum pump unit. Three thousand kilograms of aluminium alloy was placed in a crucible and heated by means of the electrical furnace. N2 gas was passed over the compacts at about 1000 cc/min flow rate while the Al alloy was being melted. The steel tube that included the powder compact was inserted in the liquid metal after 60 s, whereupon the N2 flow was stopped. The infiltration time was variable for the different compacts. Samples were held at temperature for about 3 min to allow temperature uniformity between the liquid metal and the sample. Vacuum was applied continuously (80 –500 mmHg) during the experiments. As soon as the infiltration time reached the predetermined time, the asinfiltrated compact was removed from the melt and the vacuum pump turned off. All the infiltrated specimens were cooled to room temperature. The temperature was controlled by a K-type of thermocouple with an accuracy of ^ 2 8C. In this experimental study, the infiltration behaviour of the compacts was investigated on the various infiltration temperatures for establishing to the proper infiltration temperature, infiltration time and infiltration vacuum for each compact. The infiltration distance was measured for all the compacts. Infiltration temperatures of 600, 620, 640, 700, 750 and 800 8C were chosen for studying
Fig. 1. Schematic diagram of infiltration apparatus used for the experimental study.
the temperature effect on infiltration behaviour. The infiltration time was considered to begin immediately after the compact tube was inserted into the melt. 2.3. Preparation of compact The powder compacts prepared for infiltration are schematically shown in Fig. 2. A stainless steel tube measuring 10 mm outer diameter and 8 mm inner diameter and 200 mm in length was used as a sample holder. First of all, the bottom of steel tube was plugged with a porous filter and then another piece of alumina mat was used to prevent powder leakage during compaction. Mixed powders for each case were weighed according to the volume fraction of the particulates. The mixed powders of SiCp and pure Al, Mg powders were poured into the tube and gently tapped up to the powder height of 50 mm. Subsequently a piece of stainless steel and a 5 mm thick alumina mat were inserted
Table 1 The chemical composition of alloy used in the infiltration Elements Wt (%)
Si 9.42
Fe 0.38
Mn 0.431
Mg 0.36
Ti 0.10
Zn 0.06
Cu 0.05
Ni 0.04
Fig. 2. A schematic diagram of powder compact.
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Table 2 Volume fraction of powder compacts used for infiltration process Type of the specimen
A B C D E F G a
Volume fraction of compacts (%) SiCp (149 mm)
Al powder (2100 mm)
Mg powder (2250 mm)
Balance (Alloy matrix, %)
– 10 20 30 40 50 55a
50 40 30 20 10 – –
5 5 5 5 5 5 –
Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36% Al-9.42 þ Mg-0.36 þ Mg5% Al-9.42 þ Mg-0.36 þ Mg5%
SiCp was oxidized at 1100 8C.
into the tube to act as a stopper. The volume fraction of compacts used for the infiltration process is given Table 2. For preparing the compacts, ball milling was used to mix the powders. In the mill process, various volume fractions of powders were mixed at various times to establish the optimal mixing time which was about 1 h. The steel tube contained the compacts, was placed vertically, and 35 g of weight was placed on the mixed powders in the steel tubes. Then, the samples were vibrated for about 3 min. As a result of this, the powders were forced slightly and homogeneously throughout the samples. Later another double-sided filter was inserted on the top of compacts while the alumina mat was also placed between the filters. Finally, a steel rod was inserted between stopper filter and the top end of the tube to prevent the powder from crumbling during infiltration. Seven kinds of mixed powders were used in these experiments as shown in Table 2. After infiltration, the volume fractions of SiCp in composites were 0, 10, 20, 30, 40, 50 and 55 vol%SiCp for infiltrating the compacts of A,B,C,…, and G, respectively. For F samples, 5 wt%Mg þ 9.42 wt%Si þ Mg%0.36 was used as a matrix while 50 wt%SiCp and 5 wt%Mg was also used as the composite. For G samples, SiC particles were oxidized at 1000 8C for a period of 3 h before starting the infiltration process. In this case the Al– Si alloy included 5 wt%Mg. The powder compact tube was further connected to a mechanical vacuum pump. Prior to inserting the powder compact into the melt, the vacuum pump was turned on and the alumina mat was removed. The characteristics of the powder compacts are also listed in Table 2. The density of pure aluminium and SiC particles was 2.7 and 3.2 g/cm3, respectively. 2.4. Metallography The distribution of SiCp and the infiltration extent of the Al alloy were characterised by optical microscopy. After infiltration, different locations were marked on the 10 mm long composite compacts. Seven specimens were taken at the locations of 10 mm from the bottom of the compacts.
Specimens for metallographic observation were prepared by grinding through 800 grit papers followed by polishing with 6 and 3 mm diamond paste. 2.5. Density and hardness measurement The density of the composites was obtained by the Archimedian principle of weighing small pieces cut from the composite disc first in air and then in water. The theoretical density of composite and alloy matrix specimens was then calculated according to the rule of mixtures. The porosity of the composites was also determined. The hardness of the composites and the matrix alloy was measured after polishing to a 3 mm finish. 2.6. Wear testing Dry sliding wear tests were performed on MMCs and the alloy matrix using a pin-on-disc type apparatus. The materials to be tested were cut into cylindrical shape from the composite plate by electrical discharge machining. The pins produced from the composites were 6.3 mm in diameter, and approximately 4.5 mm long with brass sleeves using an epoxy adhesive. This was followed by a polishing procedure using a grinding paper up to 800 grade. The pin was mounted in the specimen holder with the particle reinforced composite to the counterface. The discs were flatly ground to give a surface finish of approximately 0.15 mm (CLA). The pin and disc were cleaned ultrasonically and weighed in a micro-balance before each test, and then inserted into the machine. Tests were carried out at a sliding speed of 1.0 m/s under various loads of 12, 24, and 36 N at room temperature. In all tests, each experiment was repeated at least 6 times. After the test, the specimens were cleaned with acetone, dried and weighed again to determine the weight loss due to wear. The wear rates were calculated from the difference in weight of the specimens measured before and after the tests to the nearest 0.1 mg using an analytical balance, and then converted into volume loss known density data.
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3. Results and discussion 3.1. Infiltration behaviour of various compacts Infiltration temperature, vacuum, volume fraction of particle, matrix alloy and infiltration time are important process parameters for the fabrication of low volume fraction composites. Fig. 3a shows the variation of infiltration length versus infiltration temperature for the 10 vol% SiCp compacts at various infiltration times. Infiltration was observed to start after 120 and 60 sec for infiltration temperatures at 600 and 620 8C, respectively. The infiltration rate increased with increasing temperature up to 640 8C and then remained stable after the temperature increased. To improve the wetting ability of the solid by a molten metal, some conditions such as mutual solubility, reaction or formation of intermetallic compound is necessary. Therefore, an increase in infiltration temperature would be promoted by the reaction between Al powder and Al alloy melt contributing to an increase in infiltration rate. Similar results were also observed in a study of
Fig. 3. Variation of infiltration length versus infiltration temperature for the 10 vol% of SiCp compacts. Infiltration time (a), different application of vacuum (b).
Al-8.7%Si-2.6%Cu infiltration into Ni-coated SiC powder compact by Chung et al. [36] who indicated that the infiltration temperature of 640 8C was favourable for the fabrication of SiCp/Al composites. In their results, the powder compacts contained well-mixed Ni-coated SiCp and pure aluminium powders, and then were packed in quartz tubes. They showed that infiltration lengths increased up to the 6.0 cm when infiltration was performed at 680 8C but the infiltration time was 30 s. In an earlier work by same authors [37] matrix composites of aluminium with 5.9 wt%Si and 0.23 wt%Mg containing about 50 vol% Ni-coated SiCp were successfully infiltrated at above 700 8C, and X-ray mapping analysis showed that Ni and P segregated to the interface between the matrix and the carbide. Particulate Al composites reinforced with nickel-coated SiC or Al2O3 were fabricated at 900 8C in less than that 2 min while in the case of the as-received powders, no infiltration was observed after 20 min under similar experimental conditions [13]. Al-matrix composites containing SiC whiskers were fabricated by pressureless infiltration of liquid Al –Mg or Al– Si – Mg alloys at 830– 950 8C in the presence of N2 into a preform of nickel coated SiCw. The infiltration distance increased significantly with increasing temperature for a given alloy, but no infiltration occurred at 800 8C. The use of SiCw without the metal coating and the Al-10Mg matrix led to no infiltration up to 900 8C. In contrast, the use of coated SiCw and the same matrix led to infiltration at just 830 8C even without prior evacuation. Considering the results published by other authors [8,13] infiltration time was variable. For example, Xi et al. [8] infiltrated both SiC and C fibres with Al alloys for 600 s. However, in our case, the infiltration time of 180 s was much lower, but temperatures up to 800 8C were used. This is due to the increased amount of particles used in the composites and the different aluminium alloy. It indicates that rapid infiltration could be achieved for the test conditions. Fig. 3b shows the variation of infiltration length versus infiltration temperature for the 10 vol%SiCp compacts at different pressure conditions from 80 up to 200 mmHg for a period of 3 min. This figure also indicates that the infiltration length was lower under the application of 80 mmHg than that of the others, and increased with increasing vacuum pressure. For the 20 vol% composite, optimal vacuum was found to be at 200 mmHg. The melt can only penetrates the interspaces, the capillary resistance of which is equal to, or smaller than, the exerted vacuum. Candan et al. [22] showed that the time necessary to complete infiltration in the compacts decreased with increasing particle size and increasing applied pressure of 400– 900 kPa. The variation of infiltration length for the various composite compacts containing Al powder under different infiltration temperatures is shown in Fig. 4a. It is observed that the infiltration length for compacts containing Al powders and 10 vol% particles is higher than that of the compacts containing higher vol% particles when the tests
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Fig. 4. (a) Variation of infiltration length versus volume of particle infiltrated under 200 mmHg fixed vacuum pressure compacts at different temperatures. (b) Variation of infiltration length versus applied pressure infiltrated compacts under various vacuum pressures at 750 and 800 8C temperatures.
are carried out under 200 mmHg pressure. It means that the rate of infiltration and quality of infiltration are affected by the volume fraction of particles. It also reveals that infiltration length depends upon the experimental conditions and the volume fraction of the reinforcement. At all test temperatures, the infiltration length was 50 mm under 200 mmHg pressure for the 10 vol% composite and it decreased to 4.5 mm for the 20 vol% composite under the same pressure at 600 8C. However, infiltration length was 48 mm for the 30 vol% composite at 700 8C. This phenomenon is attributed to the fact that the porosity in the compact with the lower volume fraction of the SiCp is higher than that of the compact with the higher volume fraction of SiCp in the present experiment (Fig. 8b) because of the increasing melt temperature. In addition, no infiltration was achieved for the 30 vol% composites when tested at 600 8C. The infiltration lengths are 22, 34, 43, 48 and 50 mm when infiltration was performed at 620, 640, 660, 700 and 750 8C for the 30 vol% SiCp reinforced composites, respectively. This effect might have resulted from the reduction in viscosity of the melt with increasing temperature. Surface modification of the reinforcement by coating or passive oxidation technique has been successful to some extent in preventing the detrimental interfacial reaction and enhancing the material wettabilities. This also agrees with the results of the previous reports. Maxwell et al.
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[25] measured the infiltration distance versus time for Al into SiCp compacts at 670 8C and found that an incubation time had to be exceeded before infiltration could start. Alonso et al. [27] reported that the infiltration distance of pure Al into SiC or TiCp compacts at 750 8C was proportional to incubation time ðt1=2 Þ with various pressures. The infiltration process was performed at 200 mmHg pressure under different temperatures up to 800 8C. For the accomplishment of the full infiltration of higher volume fraction of composite compacts, higher temperature and vacuum application is required. The sensitivity of the kinetics of infiltration to particle volume fraction is a result of the dual role of an increasing particle volume fraction in decreasing the spacing between particles and in increasing the amount of solid formed per unit volume of compact. Fig. 4b also shows the variation of infiltration length with applied vacuum pressure. Tests were performed on the 50%SiC compact at temperatures of 750 and 800 8C. This figure revealed that infiltration length increased sharply with increasing pressure. It rose almost linearly up to 400 mmHg pressure at both temperatures, and remained stable over this pressure. This is the result of the increase of the penetrating ability of the melt as the vacuum pressure increased during the infiltration. However, the rapid increase of the applied pressure caused more and more interspaces to be penetrated and the flow velocity of the melt gradually to be reduced. The porosity indicated that at low temperature, low vacuum, and non-homogeneous distribution could be observed unless the optimal processing conditions obtained, especially for 10, 20 and 30% particle-reinforced compacts. However, the process parameters should be optimised and the optimal processing parameters determined in producing MMCs containing different volume fractions of particles are shown in Table 3. As shown in the table, full infiltration occurred at these experimental test conditions. For example, full infiltration of the 55 vol% SiCp composite appeared at 800 8C and 180 s infiltration time under the 500 mmHg pressure. Infiltration not only depends on the volume fraction but also temperature and alloy. It is well-known that the wetting angle of molten Al on SiC can be substantially reduced as the temperature increases above 1000 8C, and the wetting angle can be reduced further when Si or Mg are present in the alloy [8]. Therefore, to achieve spontaneous infiltration, the wetting angle must be brought down substantially below 908. Fig. 5 shows the variation of infiltration length versus infiltration time for the 50 vol% particle reinforced compacts. For the production of the MMCs, 5 wt% Mg was introduced into the Al– Si alloy at a temperature of 800 8C, and it was compared to the alloy without addition of Mg. The infiltration appeared to start after 3 s and the infiltration time and rate increased to 50 mm after 10 s time and thereafter remained stable. It means that the infiltration rate increased as the Mg was used. It is presumably due to the improvement in wettability of the alloy. However, no infiltration was observed in the absence of Mg even after
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Table 3 The optimal infiltration processes parameters in producing the MMCs Volume fraction of particles (%)
Types of the alloy
Infiltration temperature (8C)
Infiltration time (s)
Applied vacuum (mmHg)
10%SiCp 20%SiCp 30%SiCp 40%SiCp 50%SiCp 55%SiCpa
Al –Si Al –Si Al –Si Al –Si Al –Si þ 5 wtMg Al –Si þ 5 wtMg
640 700 700 800 800 800
120 120 180 180 180 180
300 400 400 500 500 500
a
SiCp was oxidized at 1100 8C.
an infiltration time of 60 s and the infiltration started at about 300 s. Other studies indicated that the Mg addition into the metal matrix is useful in improving the wettability of the composites. For example, Oh et al. [40] found a slightly improved wettability when increasing the magnesium content from 2 to 4.5%. Bardal et al. [41] studied the wettability and the interfacial reaction products in the AlSiMg surface oxidized SiC system. It was found that the oxidized SiC is more easily wetted than as produced SiC. Transmission electron microscopic observation showed that at the interface between SiC and the Al matrix, Mg –Al spinel crystallities are generally found at high densities. Previous work carried out by Canul et al. [34] showed that Mg reduced the surface tension and the contact angle between Al and SiC, and thus its presence in the alloy was essential in pressureless infiltrating SiCp preforms with Al alloys. Silicon, as an alloying element in Al, retards the kinetics of the chemical reactions that resulted in the formation of the unwanted carbides of Al4C3 and Al4SiC4. Fig. 5b shows the variation of infiltration length versus Mg content in the given alloy for SiCp and oxidized-SiCp composite compacts. It is obvious that no infiltration was observed for the non-oxidized-SiCp when the 4 wt% Mg content in the alloy was used. However, the infiltration length reached 50 mm when 5 wt% Mg was used with the same particles. For the oxidized-SiCp, it was again 50 mm when 3 wt%Mg was used and remained stable thereafter. The decreases in the surface tension with an increase of Mg content in Al –Mg alloy melts were reported with other researchers [22,26]. Surface tension decreased from 1.12 to 1.02 N m21 with the addition of 8 wt%Mg to Al for nonoxidized surface at a temperature of 700 8C. SiCw reinforced Al was fabricated by vacuum infiltration of liquid aluminium into a porous whisker preform under argon gas pressure, using an infiltration temperature of 665 8C [41]. In their work, the reinforcement volume fraction was lower than that of our case, the volume of whiskers ranging from 11 to 37%.
were obtained from the optimum process parameters carried out on the products. The distribution of SiCp in these composites is uniform. Some pores were present in these composites, especially for the 10 vol% of particulate. During infiltration, the molten metal preferentially penetrated into the large interspaces and separated the particle crowded area and the small interspaces from the outside. From that moment, air or gas entrapment occurred. But no large pores were observed for the higher volume fraction composites. These results show that this infiltration process could be used to produce MMCs when the volume fraction was less than that of the 50% SiC particles because the vacuum method was used over 50% SiC volume fraction, in general. When the specimen was infiltrated at 680 8C, the dendritic structure was more obvious in the 10 vol% SiCp
3.2. Microstructures The optical micrographs of aluminium composites reinforced with 10, 20, 30, 40 and 50 vol% of SiCp are shown in Fig. 6a – f, respectively. These microstructures
Fig. 5. (a) Variation of infiltration length versus infiltration time for the 50 vol% of SiCp compacts infiltrated at 800 8C. (b) Variation of infiltration length versus Mg content in the alloy for the unoxidized-SiCp and oxidizedSiCp compacts.
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Fig. 6. Optical micrographs of various volume fraction of Al alloy composites reinforced with. (a) 10 vol% SiCp £ 50, (b) 20 vol% SiCp £ 50, (c) 30 vol% SiCp £ 50, (d) 40 vol% SiCp £ 50, (e) 50 vol% SiCp £ 50, (f) 50 vol% SiCp £ 1000.
composite because of the melting of the pure aluminium powders (Fig. 6a). The existence of pores is attributed to the high viscosity of the melt because more surface area occurred in the sample. The microstructure of the higher volume fraction of composite specimens infiltrated at 800 8C indicated the optimum infiltration conditions. No large pores existed in these specimens (Fig. 6c – e). This might be due to the improvement of flowing behaviour when the Al– Si alloy and Mg powders were used, and application of higher vacuum pressure. Fig. 6f shows a higher magnification of Fig. 6e, indicating a good bonding
interface between the matrix and SiC without evidence of cavities. 3.3. Hardness and density The variation of hardness of the composites with volume percent of particles is shown in Fig. 7. The hardness of the MMCs increased more or less linearly with the volume fraction of particles in the alloy matrix due to the increase of the ceramic phase. A higher hardness was also associated with a lower porosity, as shown in Fig. 8a and b.
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amount of porosity in the composites was observed with increasing SiC volume fraction because of increasing amount of vacuum pressure up to 500 mmHg, as evidenced by Fig. 4b. 3.4. Wear rate
The measured density of the composites is shown in Fig. 8a as a function of SiC volume fraction. As shown in the figure, the density of the composite increased with increasing SiC volume fraction. The porosity of the composite decreased with increasing SiC volume fraction, as shown in Fig. 8b. It is evident that the porosity level was higher for the 10 vol% of SiCp reinforced composite, as observed in the micrographs, which is related to the flowing behaviour of the composite. It also could be the result of lower and constant vacuum pressure. This figure indicates that a decreased
The wear testing carried out for various volume fractions of composites is shown here. Tests were performed at different loads under a sliding speed of 1.0 m s21. The average volumetric wear rates for composites and matrix alloy were determined, and are illustrated graphically in Figs. 9 and 10 as a function of load and volume fraction of particle, respectively. Fig. 9 shows the variation of volumetric wear rates for the particulate composites and matrix alloy as a function of applied load. It is observed that the wear rate of the matrix alloy was more than that of the composite at all load conditions. At this fixed speed under different loads, the wear rate increased as the normal load increased. This figure further revealed that the volumetric wear rate increased non-linearly with load and showed approximately concave behaviour. However, it was very sharp for the matrix alloy while a low tendency was observed for the MMCS. For the matrix alloy, the soft asperities were easily deformed and sheared under the repeated loading conditions while the hard asperities on the counterface or hard particles between the sliding surface plough and cut the soft surfaces, and resulted in giving more damage to the surface. However, for the composites, this might be a result of the SiC particles in the matrix breaking on exposure at the rubbing surface because of the changing in the surface characteristics of the composite samples, but SiC particles in the matrix alloy improved its thermal stability of the sample. Fig. 10 shows the variation of volumetric wear rates for the composites as a function of volume fraction of particle.
Fig. 8. (a) Variation of experimental density with vol% particle, (b) Variation of porosity with vol% particle.
Fig. 9. Average volumetric wear rates for SiCp reinforced composites as a function of load.
Fig. 7. Variation of hardness with vol% particle.
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on the infiltration temperature used. The temperatures used ranged from 640 –800 8C. The infiltration rate increased with increasing infiltration time. 3. Microstructural examination showed that the SiCp distributions were homogeneous. The hardness and wear resistance of the composites also increased with increasing volume fraction of particulate. The hardness and density of the composite increased linearly with increasing particulate content, however, porosity level decreased with increasing particulate content. 4. Dry sliding wear properties of the Al alloy improved significantly by the addition of SiC particles into the matrix alloy under all test conditions. The wear resistance of the composites increased significantly up to 10% SiC addition and remained stable thereafter. The wear rate of composite also appeared to increase nonlinearly with load.
Fig. 10. Average volumetric wear rates for SiCp reinforced composites as a function of volume fraction.
A considerable reduction in wear was observed for the 10 vol% of SiC particulate composite, compared to the unreinforced alloy. Fig. 10 also indicates that critical volume fraction of particles occurred in the composites. The wear rate decreased with increasing volume fraction, but the increase in the volume fraction beyond 10% produced only a small decrease in the wear rate, however, wear was then insensitive to particle content to the maximum content of 40%. Similar results were observed in previous work carried by Sahin [2,3] where continuous fibres were used as the reinforcement and the critical volume fraction of fibre was found to be about 16%.
4. Conclusions The following conclusions can be drawn from the present study on the production and properties of SiCp/Al – Si alloy composites. 1. A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of SiCp reinforcements. Al-9.42 wt%Si0.36 wt%Mg alloy matrix composites containing up to 55 vol% SiC particles were successfully produced by this method. 2. The infiltration behaviour was affected by various parametres such as temperature, time, alloy composition, oxidation of particles and volume fraction of particles. The higher volume fraction composites could be achieved when vacuum between 400 –500 mmHg pressure was applied while the lower volume fraction of composites was infiltrated at 200 mmHg pressure. The infiltration times were between 120 and 180 s depending
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