Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0436-1

Microstructure and wear characteristics on Al alloy matrix composite reinforced

with Ni perform† Wonjo Park1, Cheolhong Park2, Hyungjin Kim3 and Sunchul Huh1,*

1Department of Precision and Mechanical Engineering, Institute of Marine Industry, Gyeongsang National University, Tongyeong, 650-160, Korea

2Department of Mechanical and Engineering, Graduate School, Gyeongsang National University, Tongyeong, 650-160, Korea 3Department of Mechanical System Engineering and Institute of Marine Industry, Gyeongsang National University, Tongyeong, 650-160, Korea

(Manuscript Received June 8, 2011; Revised March 2, 2012; Accepted March 12, 2012)

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Abstract Al based composite reinforced with Nickel is used for diesel engine piston, because the thermal properties, strength and corrosion re-

sistant are for better than Al alloy alone. For processing, the intermetallic compounds of Ni and Al improves wear resistance due to its high hardness. Existing process methods for MMC (metal matrix composite) using preform were manufactured under high-pressure. However, this causes deformation of the preform or weaknesses in the completed MMC. Low-pressure infiltration can prevent these problems, and there is an advantage of cost reduction in of production with small-scale of production equipment. In this study, the micro-structure and wear characteristics of Al-based composite with Ni preform as reinforcement with low-pressure infiltration was analyzed.

Keywords: Wear characteristics; Intermetallic compound; Perform; Low pressure infiltration; MMC ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Nowadays, transportation via automobiles, shipping aircraft etc. requires high speed and a large output of power. Therefore, the required parts have a tendency to increase mechanical characteristics. Of Special note are a considerable number of studies that have been conducted on development of new lightweight materials for internal combustion engine parts. In MMCs (metal matrix composites), interaction allows reinforc-ing material to mix with the base metal and disperse appropri-ately and, unlike alloy materials such MMCs possess not only the properties of the matrix metal but also the characteristics of the reinforcement material. Therefore, MMCs can fit the desired purpose, having attained mechanical properties which are difficult to acquire in single metal.

Among such composites, it is particularly the Al alloy ma-trix composites in which nickel – with its superior strength, corrosion resistance and thermal properties – is added in pre-form or powder form, exhibits greatly enhanced wear and corrosion resistance, and high temperature tolerance in com-parison to existing Al alloys. They are, therefore, currently used for pistons or piston rings in automobile diesel engines [1] and some sectors in global industry are conducting studies

towards application of composite materials with superior high-temperature tolerance, wear resistance and heat conductivity for use in special structural parts [2-4]. In view of the greater demand for such materials, it is expected that they will be applied to an even broader range of purposes [4].

Molten metal casting is used to manufacture such pistons, a method wherein the molten Al alloy is injected into the Ni preform. During the manufacturing process, a reaction occurs in the interface between the Ni and the Al matrix, causing their transformation into a hard intermetallic compound, thereby greatly improving wear resistance. The problem of exercising appropriate control over the interface reaction be-tween the reinforcement and the metal matrix is a factor that must be taken into constant consideration at every stage: from the manufacture of the material to its final use. Also, MMCs are usually manufactured using infiltration. Such pressuriza-tion causes a transformation in the preform (which is intended to facilitate distribution of the reinforcement), and thus causes deficiencies in the completed composite, such as a deteriora-tion of consistency in the reinforcement. In general, an infiltra-tion pressure of 60~80 MPa is used when squeeze casting during manufacture. Such high pressure necessitates large scale machinery and makes it difficult to cast composites in complex forms [5]. For this reason, it is necessary to consider manufacturing MMCs using low pressure infiltration that takes into account the strength of the preform. Recently many studies have been conducted on ways to reduce infiltration

*Corresponding author. Tel.: +82 55 772 9111, Fax.: +82 55 772 9119 E-mail address: schuh@.ac.kr

† Recommended by Editor Jai Hak Park © KSME & Springer 2012

1742 W. Park et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746

pressure [6]. This study applies the superior properties of nickel to

AC8A (a representative lightweight alloy matrix used in cast-ing), for utilization in pistons or piston rings of internal com-bustion and other engines. The AC8A metal matrix is made to penetrate and then induced to infiltrate the nickel reinforce-ment. During manufacture of the MMC, the pressure required for infiltration of the metal matrix into the reinforcement is lower than that used in previous Squeeze Casting methods. This is due to the structure of the molten metal being generally composed of the matrix metal, the precipitated substance and the intermetallic compound. The objective of this research is to observe microstructures and analyze the characteristics of intermetallic compounds that are generated by reaction of the reinforcement and the matrix metal. Moreover, the study’s scope extends to wear tests on composites through low-pressure infiltration under differing controlled conditions to study the characteristics of abrasion and friction.

2. Experimental procedure

2.1 Low pressure infiltration process

As for the casting material, AC7A~B and AC8A~C are most frequently applied, and though it is also possible to use AC4C, the latter is not favored because it can cause a darken-ing of color after surface processing. The former (and AC7C) retain a high-gloss even after chemical polishing, and are bright even after coating. The matrix material used in this study was AC8A (KS D 2330), which is an Al-Cu-Mg-Si composite material generally used for automobile and other internal combustion engines. Dooin Co., Ltd. of South Korea was the material supplier of the aluminum for casting. Table 1 shows the chemical composition of the AC8A product used as the matrix metal in this study. As for the reinforcement, we used a Nickel porous preform with 5% volume fraction (Vf), made of pure nickel, which allowed us to attain the required characteristics of wear resistance, thermal properties and cor-rosion resistance. And of special note, the reinforcement was Ni porous preform by Sumitomo Electric Toyama Co., Ltd., Japan, as shown in Fig. 1.

The principle of infiltration used in molten metal casting is shown in Fig. 1(a), where preform is placed in the cylinder mold and piston pressure is used to make the molten matrix metal penetrate into the preform. Fig. 1(b) shows the nickel preform with a diameter of 30 mm and a height of 10 mm, which was the porous preform used as the reinforcement in this experiment. The photo in Fig. 1(c) shows the completed MMC after infiltration. Although in squeeze casting the infil-tration pressure used in manufacture is generally high, our

study employed hydraulic cylinders and load cells for detect-ing the load, and the pressure used to induce penetration of the reinforcement by the matrix metal was limited to a maximum of 0.3 MPa. Meanwhile during the infiltration, we differenti-ated the temperature conditions during the composite infiltra-tion to 650, 700 and 750oC.

2.2 Experimental methods

Microstructure was observed using an optical microscope (Olympus BX60M). Because the molten structure is com-posed of the matrix metal, precipitated substances and inter-metallic compounds, it was difficult to examine the features revealed by the optical microscope. Therefore, element and quantitative analyses using the EPMA (Electron probe X-ray microanalyzer), the XRD (X-Ray Diffraction) and the SEM/EDX (Energy Dispersive X-ray Spectrometer) were done on the revealed structures in order to investigate their unique characteristics.

The wear test was conducted with a wear tester (R&B Fric-tion & Wear Tester PD-102) according to ASTM E8 regula-tions, using a Ball on Disc method. The diameter of the wear track was 7.5 mm, the counterpart material was an SUJ-2 ball (800 Hv) of high carbon chrome steel and the conditions as follows. In order to observe the wear properties according to infiltrated temperature, the load of the specimen was constant at 30N, the machine speed was kept at 50 rpm, the sliding time was 1 hour (3600 seconds), and a 2 minute pre-track was granted to impose identical conditions on each test specimen.

To study wear properties of the composites, we examined the friction coefficient and wear loss according to the infiltrated temperature, taking into consideration the microstructure and hardness. Also, an SEM (Scanning Electron Microscope) was used to analyze the worn surface after the wear test, in order to observe the wear track and wear phenomena, and additionally, we inquired the unique phenomena on the fractures.

3. Results and discussion

3.1 Microstructure of composites

Fig. 2 shows the observed microstructures according to the

Fig. 1. Infiltration principle of AC8A metal and Ni preform.

Table 1. Chemical compositions of AC8A casting alloy (wt %). Material Cu Si Fe Mn Ni Sn Al

Contents 0.99 12.2 0.38 0.12 0.02 0.02 Blan.

W. Park et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746 1743

respective infiltrated temperatures. Ni preform is easier to infiltrate than fiber composed preform and because it contains sufficient space for the matrix metal to penetrate. Therefore it allowed sufficient infiltration even with a pressure of just 0.3 MPa during the composite manufacture [7].

Moreover, observation of the structure confirmed that there were no pores in the composite caused by infiltration defects. Reviewing the features of the composite structures observed in the optical microscope at 500 magnifications, it is noted in Fig. 2 that the unique structures(A,B) are smaller than those found in composites manufactured at other temperatures and that microstructures are distributed constantly. In Fig. 2(b) and Fig. 2(c), brightly-colored structures(A) are more widely dis-tributed than Fig. 2(a). Also, an observation of the characteris-tics of the reinforcement indicates that as the infiltration tem-perature increases, the relative surface area of the reinforce-ment diminishes, thereby causing transformation in the boun-dary layer where it meets the matrix metal. Structures such as the dark-colored grains show uniformity in size and distribu-tion regardless of the infiltrated temperature. Although the brightly-colored grain structures were minutely and uniformly distributed at 650oC, 700oC and 750oC, the size of the distrib-uted grain had increased.

Because Ni/AC8A composites contain a variety of charac-

teristics, it is impossible to detect the characteristics of each respective structure using only the optical microscope. To examine each unique structure and its characteristics, therefore, we employed XRD (X-ray diffraction) and additional SEM/EDX quantitative analysis to observe the microstruc-tures. The substances that were revealed through XRD analy-sis in Fig. 3 included Al, Al3Ni, Si and also Al compounds. In particular, the Al3Ni phase is an intermetallic compound [8] created by molten reaction or during the manufacture of com-posites by casting, and as can be seen in the Al-Ni phase dia-gram, it is generated when a small quantity of Al reacts with the Ni element at a relatively low temperature [9]. In general, the Al3Ni phase has excellent hardness and strength but is very brittle [10]. Therefore, when the grain size of Al3Ni increases, its brittleness may be inversely negative on the composite. This issue of strength, which is dependent on grain size in conjunction with the characteristic of the intermetallic com-pound, is another matter that should be taken into considera-tion when manufacturing composites [11].

The Al and Si phase are the elements which are most abun-dantly contained in the matrix metal AC8A and are judged to be precipitated substances generated by melting.

SEM/EDX analysis was used to examine each unique structure based on the substances revealed by XRD analysis and to study the transformation of the intermetallic com-pounds or precipitated substances dependent on temperature conditions. Fig. 4 is the results of the EDX analysis that was conducted on the observed unique structures. We analyzed the structure of the dark-colored grains, the structure of the rela-tively minutely and the structure of the uniformly-distributed bright-colored grains as well as the boundary layer around the reinforcement. Quantitative analysis Fig. 4(a) led us to con-clude that the dark-colored [A] structures revealed in the opti-cal microscope are composed solely of the Si phase, and were precipitated in the matrix metal during melting. These struc-tures were uniformly distributed regardless of the infiltrated temperature, and grain size was also relatively uniform. Hard-

(650 oC×100)

(700 oC×500)

(750 oC×100)

(650 oC×500)

(700 oC×100) (750 oC×500) Fig. 2. SEM micrographs of composites on infiltrated temperature.

Fig. 3. The X-ray diffraction patterns of Ni/AC8A composites byXRD.

1744 W. Park et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746

ness value was around 800 Hv, which is extremely higher than the 110 Hv of the matrix metal. Although it is thus character-ized by great hardness, the fact that it fractures under 0.5 N and the form of the pressurized trace all indicate that it is also characterized by brittleness. Fig. 4(b) quantitative analysis of the [B] structure of the brightly-colored grains led us to con-clude that they are composed of the Al phase and the Ni phase and are an intermetallic compound generated during manufac-ture of the composite. In observing the characteristics depend-ent on infiltrated temperature, we confirmed that in contrast to the smaller grain size at 650oC, grain size increased as the infiltrated temperature increased.

The hardness value of this structure was relatively high at 680 Hv, and was observed to be brittle based on the form of the pressurized trace and the fact that it fractured at 0.5 N. There were no specific phase distributions the boundary layer between the reinforcement and the matrix metal. As the infil-trated temperature increased, the content of the Ni phase also increased and was observed as a thickening mixed layer of Al and Ni phases. This was also confirmed through transforma-tions of the Ni reinforcement according to the temperature changes.

Metal-based composite material can attain properties and features superior to other composites, because of the genera-tion of intermetallic compounds by interface reactions and precipitated substances during melting. It is necessary to con-sider ways to control their reactions appropriately during composite manufacture.

3.2 Micro-Vickers hardness of Ni/AC8A composites

Fig. 5 shows that with the increasing temperature, the hard-ness of Si phase becomes high, and the maximum average value is 1104 HV when infiltrated at 750oC.

Al3Ni phase has the similar tendency, maximum average value is 762 HV when infiltrated at 750oC, but the matrix metal AC8A at room temperature, Vickers hardness average value is 99.7 HV. So Ni/AC8A composite should has high hardness because the existence of high hardness Si phase and Al3Ni phase when the infiltration temperature increased.

Hardness is not only an important index of material per-formance, but also display other mechanical behavior of this material, such as strength, strength of material also become high, as hardness increased.

3.3 Wear characteristics of Ni/AC8A composites

To observe the influence of the intermetallic compounds or precipitated substances created on the MMC by the molten metal casting, we compared the wear characteristics of the Ni/AC8A composites according to the infiltrated temperature. In particular, wear characteristics of the composite were as-sessed by the friction coefficient and wear loss. Fig. 6 shows the friction coefficient derived by conducting a wear experiment for 60 minutes. The friction coefficient formed an unstable curve for 20-30 minutes, and became relatively stable after 30

(a) Dark-color[A]

(b) Brightly-color[B]

Fig. 4. The results of EDX analysis in Ni/AC8A composite.

Fig. 5. Vickers hardness of Al3Ni and Si phase in Ni/AC8A composites.

Fig. 6. Friction coefficient of Ni/AC8A composites at different infil-trated temperatures.

W. Park et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746 1745

minutes. The latter values have been organized into the graph. Table 2 shows the friction coefficient and wear loss data for

each specimen. The friction coefficients confirm the wear resistant nature of MMCs reinforced with Ni preform, which were superior to the Al alloy (AC8A) specimens. The com-posites were also better than AC8A in terms of wear loss, as the precipitated substances and intermetallic compounds were revealed, in observation of the composite structure, to be of significant hardness, which contributed to its surface wear-resistant properties [12]. It can be seen from this experiment that the lower temperature of 650oC led to superior wear resis-tance, and as the infiltrated temperature increased at the time of MMC manufacture, the wear resistance tended to deterio-rate. These wear test results appear to have been influenced by the grain size of the brittle intermetallic compound which reacted according to the temperature. Compared to the dense structure at 650oC, the precipitated substances (such as the brittle phases Si, Al3Ni compounds) began to crystallize and increase in size as the infiltrated temperature increased, result-ing in the decrease in wear resistance.

A more detailed causation needs to be identified through observation of the wear fractures and additional analysis. Fig. 7 shows the fracture forms of the wear track after the SEM wear test. Due to the Al matrix metal, the main component, the overall fracture was observed to be in the form of abrasive wear behavior [13], and the degree of wear loss could easily be confirmed by the interval of track widths according to the infiltration temperature.

The fractures also allowed observation of the influence of intermetallic compounds created by the molten casting, as shown in Fig. 8. We studied the defective factors in the wear properties through a SEM/EDS quantitative analysis of spots with unique forms and spots with abrasion wear in the ob-served wear scratch. While the scratches of the composites infiltrated at 650oC were relatively narrow and stable, scratches in the MMCs infiltrated at 700oC and 750oC, were unstable, revealing structures that were cracked or had fallen off from the load of the counterpart material, due to the abra-sion wear of the matrix metal and the brittleness of the inter-metallic compounds. The reason the friction coefficient was unstable could be identified easily through the wear scratch, and this was also judged to be the reason for the wear loss increment. EDS analysis allowed us to observe that area [A] with its unique structure was the structure of Al3Ni which showed brittle characteristics during the structure observation in Fig. 8(a), and that substance [B] of the abrasive wear area mostly contained the properties of the matrix metal in Fig. 8(b).

Table 2. Wear characteristics of specimens according to infiltration temperatures.

Infiltration temperature Wear characteristics

AC8A 650oC 700oC 750oC

Coefficient of friction 0.203 0.104 0.114 0.140

Wear loss (mg) 7.9 3.3 4.3 5.2

Fig. 7. Photography of specimen surface after wear test.

(a) A position

(b) B position

Fig. 8. SEM/EDX analysis of fracture surface after wear test.

1746 W. Park et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1741~1746

Although the wear-resistance properties of the composites are better than that of the matrix metal due to the precipitated substances created during molten casting and the intermetallic compounds generated by reactions, if the generated substances are brittle and the grain size increases, this tends to lead to a decrease in the wear-resistance. It is therefore necessary to exercise appropriate control over interface reaction of the in-termetallic compounds when manufacturing composites.

4. Conclusions

Ni/AC8A composites were manufactured using the low pressure infiltration method and studied characteristics of the composites. The findings are summarized as follows:

(1) The microstructures of the MMC revealed that there were differences in the precipitated or generated substances according to infiltration temperature. While the compound structures created at 650oC were minutely distributed, the size of crystals forming in the substances or compounds increased with the infiltration temperature.

(2) The molten reaction precipitated the Si phase as well as the intermetallic compound Al3Ni, and while these structures had high degrees of hardness, they were also brittle, causing them to crack easily even at a pressurized load of 0.5N. It was also observed that as the infiltration temperature increased, the grain size of Al3Ni grew in size. As the infiltration tempera-ture increases, the reinforcement disappears in the molten reaction, leading to a decrease in the amount of pure Ni that had been added as reinforcement.

(3) Wear test results of Ni/AC8A composites revealed that its wear resistance properties are better than matrix AC8A, although as the infiltrated temperature increases, the wear properties get worse. The fractures in the matrix metal (from abrasive wear) showed that the brittle structures cracked under the load of the counterpart material, which led to unstable friction coefficients and an increase in wear loss.

Acknowledgment

This research was financially supported by the Ministry of Education, Science Technology (MEST) and Korea Institute for Advancement of Technology (KIAT) through the Human Resource Training Project for Regional Innovation and Sec-ond-Phase of BK (Brain Korea) 21 Project.

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Wonjo Park was born in 1952, Korea. Prof. Park graduated from Dong-A Uni-versity, Korea, as PHD majored in Me-chanical Engineering. In present, Prof. Park is holding a post as professor in Gyeong-Sang National University, Ko-rea.