Production of functionally graded SiC/Al-Cu-Mg composite by centrifugal casting

1 Introduction

Metal matrix composites are important in engineering applications because of their high specific strength, modulus, and wear resistance [1–6]. Aluminum matrix composite materials are extensively used in many industrial applications because of their high specific modulus, strength, hardness, and excellent wear resistance [7].

Functionally graded (FG) metal matrix composites have a great significance for the use in the automobile, aerospace, and defense industries. An FG metal matrix composite whose reinforcement particle is volume fraction varies continuously from the inner to the outer sections of the particle. Therefore, their mechanical properties are different from the inner and outer regions [8].

Until recently, a number of studies on the FG metal matrix composites produced by centrifugal casting had silicon carbide (SiC), Al₂ O₃, TiC, and AlB₂ reinforcement particle in aluminum alloys [8–12]. The centrifugal casting technique is an effective method for producing FG composites, because centrifugal forces cause heavier ceramic particles in the liquid metal to be displaced toward the external zone of the casting [10, 11]. In addition, centrifugal casting promotes very good mould filling combined with a desired microstructural control, which usually results in improved overall mechanical properties. Also, centrifugal casting contributes to lower costs by reducing the number of production steps compared with surface modification and coating methods, which have to be performed separately [10, 11].

This work aims to segregate SiC particles in liquid Al-4% Cu-1.2% Mg alloy to the local region using 5 wt.% SiC/Al-4% Cu-1.2% Mg composites fabricated by compocasting to achieve uniformly distributed SiC/Al-4% Cu-1.2% Mg composites. The microstructural characteristics, Brinell hardness, and wear resistance of the sample along radial direction are presented in detail.

2 Materials and methods

The matrix alloy was an Al-Cu-Mg alloy whose composition is listed in Table 1. The reinforcement particles were chosen as SiC with 99.8% purity. The green variety SiC particles of 20 μm average particle size have been used as reinforcement whose size distribution is shown in Figure 1.

Figure 1

Particle size distribution of reinforced SiC.

Initially, the 5 wt.% SiC/Al-Cu-Mg composite melts are synthesized by liquid metal stir casting method and later shaped into a hollow mould in a horizontal centrifugal casting machine. Centrifugal action was then employed with the die containing a semisolid (Al-Cu-Mg_liquid+SiC_solid) composite to drive the solid SiC particles toward the outer region to produce an FG SiC/Al-Cu-Mg composite with an even higher volume percent reinforcement. The centrifugation process was carried out under a rotation speed of 600 rpm at 750°C. A schematic representation of the stir casting following the centrifugation process of the FG SiC/Al-Cu-Mg composite is shown in Figure 2. Two FG composite materials were produced in this way.

Figure 2

Schematic illustration of the SiC/Al-Cu-Mg composites’ production route.

For the microstructure analysis and hardness test, the samples were ground up to 2400 grid using SiC paper followed by polishing using 0.2 μm diamond paste. An Olympus optical microscope equipped with Clemex Vision image analysis system was used for the microstructural analysis. The Vickers microhardness of the composites was measured under 300 gf loads with 1 mm ranges on the composite from the outside to the inside as seen in Figure 1. The composites were solutionized at 540°C for 5 h followed by water quenching at 60°C and waiting at room temperature for 20 h, and aging was carried out at 183°C for 10 h.

The cylindrical pin specimens of 6 mm diameter and 10 mm length were taken from the composite for wear test as seen in Figure 2. The wear tests were carried out using a pin-on disk tester under a dry sliding condition in accordance with the ASTM G99-04 standards as explained in detail in a previous work [13]. The wear tests were carried out with a load of 50 N, sliding velocities of 1.25 m s^-1, and a sliding distance of 600 m. After each wear test, the weight mass loss in the pins was measured. The wear volume losses were calculated from the weight mass losses.

3 Results and discussion

The macroscopic photograph of the centrifugally casting composite is shown in Figure 3. The composites contain the external reinforced region, particle-free region, and inner reinforced region. Most of the SiC particles in the Al-Cu-Mg matrix are segregated toward the external circumference of the composite by centrifugal casting, and an SiC/Al-Cu-Mg composite with a high SiC particle volume fraction is obtained. The other SiC particles remain in the inner reinforced region of the composites. This result is in agreement with a previous work on the FG SiC-reinforced composite material [10, 14, 15].

Figure 3

Macroscopic photograph showing two FG SiC-reinforced composite materials.

Figure 4 shows the optical microstructures of a 5 wt.% SiC-reinforced composite material, which was produced by stir casting. SiC particles are dispersed uniformly in the aluminum matrix, except some congregated SiC particles, and only a few voids can be observed.

Figure 4

Microstructures of the SiC/Al-Cu-Mg composite produced by stir casting.

Figure 5 shows the microstructures of the FG SiC-reinforced composite material taken at different regions from the external direction (x). As seen in Figure 5A and F, the cross-section of the FG composite along the radial direction of the composite is divided into the external reinforced region, middle free particle region, and inner reinforced region, and the distribution of SiC particles and the microstructure of the matrix alloy are different in these three zones. Figure 5A and B shows that the SiC particles in the Al-Cu-Mg melt segregate to the external region after centrifugal casting, and the SiC particles in the composite taken at 1 and 5 mm have a uniform distribution. A comparison of the distribution pattern of SiC particles in the composite in Figure 5C shows that there is a sharp transition between the external reinforced region and the particle-free region in the Al-Cu-Mg matrix. This is in agreement with a previous work on the FG composite material [10, 14, 15]. As seen in Figure 5D and E, there are no SiC particles, and thin, granular θ-CuAl₂ and β-Al₃ Mg₂ phases are present among primary (Al) phases. In addition, in Figure 5F, it can be observed in composite materials at 55 mm that there are many voids and congregated SiC particles as seen in Figure 4.

Figure 5

Microstructure image of the composite located at different regions from external to the internal zone: (A) 1 mm, (B) 3 mm, (C) 4 mm, (D) x=7 mm, (E) x=15 mm, and (F) x=55 mm.

Figure 6 shows the graded distribution of SiC particles in the Al-Cu-Mg matrix. The external region of the composite shows a higher concentration of SiC particles than the internal region of the composite. The image analysis results depicted in Figure 4 shows that the external region of the composite contains a maximum of 27 vol.% SiC followed by a graded and reduced SiC volume percentage of 26 and 22 at 3 and 4 mm away from the outer periphery, respectively. After 5 mm, the volume fraction drops sharply reaching zero.

Figure 6

Graded distribution of SiC particles from the external to the internal region in the FG composite.

The hardness values measured from the external to the internal region for as-cast and heat-treated composite are shown in Figure 7. As expected, the maximum hardness values are observed at the external region of the composite due to the presence of a higher volume fraction of SiC particles. In the as-cast composite, the external zone hardness having a composition of 27% SiC is 230 Hv compared to 90–100 Hv for the particle-free zone. The hardness profile of the heat-treated composite is different from that of the as-cast composite. The hardness of the heat-treated composite is higher than that of the as-cast composite. In the heat-treated composite, the particle-enriched region is shown with a maximum hardness of 232–255 Hv compared to the hardness of the internal region with 90–100 Hv.

Figure 7

Variation in hardness from the external region of the as-cast and heat-treated SiC/Al-Cu-Mg FG composites.

Compared with the microstructure of the as-cast and heat-treated composites, it is inferred that lots of SiC particles, whose hardness are much higher than that of the matrix, segregated in the external region, resulting in a higher hardness in the external region. The hardness in the internal region without SiC particles is the lowest. However, the hardness is increased a little with the increase of SiC particles in the external region of the as-cast and heat-treated composites.

Figure 8 shows the average cumulative wear volume loss from the wear tests carried out with two sets of samples from the external and internal regions under a 50 N applied load and 600 m sliding distance. It has been identified that the wear volume loss of the external region is lower than that of the internal region. As seen in Figure 8, the cumulative wear volume loss decreased up to six times by adding 25 vol.% SiC particles within the Al-Cu-Mg matrix alloy. This is in agreement with a previous work on the metal matrix composites [16, 17]. An increase in the wear resistance of the composite by the high volume percent reinforcement phase can be attributed to the higher hardness values of SiC particles compared to the matrix alloy. Similarly, a 600% increase in the wear resistance is as high as expected when compared to the relative hardness of the external and internal regions of the composite material itself.

Figure 8

Average cumulative wear volume loss of the composite.

Figure 9 shows the variation of the coefficient of friction with the sliding distance for the composite having 25 vol.% SiC particles and SiC particles free in the regions, respectively. Figure 9 suggests that 600 m of the total sliding distance can be considered as a run-in period in which the coefficient of friction changes from 0.41 to approximately 0.61, by adding 25 vol.% SiC particles in the Al-Cu-Mg matrix alloy. The friction pattern shown in Figure 9 is similar with the results reported in some earlier works [16, 17].

Figure 9

Friction coefficient versus sliding distance for the external and internal regions of the composite.

4 Conclusions

This study investigated the production and wear properties of an FG SiC-reinforced composite that was produced by stir casting followed by centrifugal casting. The conclusions from this work can be summarized as follows.

An FG SiC/Al-Cu-Mg composite with a graded distribution of SiC particles near the outer periphery of the casting has been successfully processed by the centrifugal casting method. The cross-section of the FG SiC/Al-Cu-Mg composite along the radial direction of the composite is divided into the external reinforced region, middle free particle region, and inner reinforced region, and the distribution of the SiC particles and the microstructure of the matrix alloy are different in these three zones.

The maximum hardness values were observed at the external region of the composite materials due to the presence of a higher volume fraction of SiC particles. In the as-cast composite, the hardness of the Al-Cu-Mg matrix increased almost three times as 90–230 Hv by increasing the SiC volume content to 27 vol.%.

The average cumulative weight loss of the FG SiC/Al-Cu-Mg composite in the external region was lower than that in the particle-free region. It was shown that the average cumulative weight loss of the Al-Cu-Mg matrix decreased by six times with the addition of 25 vol.% SiC particles. The friction coefficient of the Al-Cu-Mg matrix alloy changes from 0.41 to approximately 0.61 by adding 25 vol.% SiC particles.