Turning Machinability of SiC Particle Reinforced Aluminum Alloy Composites

Abstract

The machining of SiC particle-reinforced aluminum alloy composites was performed using a lathe and PCD tools, and the effects of the particle size and the volume fraction of the SiC particles on the machinability was discussed. Composites were fabricated by squeeze casting. The range of the cutting force fluctuations during the cutting of the alloy became slightly narrower due to the reinforcement, while the average values increased due to the reinforcement. The resistance with larger volume fractions was higher. When comparing the composites with different SiC particle sizes within the same volume fraction, there was minimal variation in the resistance. The reinforcement decreased the surface roughness, achieving a finish closer to the theoretical surface roughness, indicating that the reinforcement reduces the formation of the built-up edge during the cutting. The roughness of the composites with 4 µm particles was lower than that of the composites with 25 µm particles. The flank wear of the tool after machining the composite with the 25 µm particles was more severe than that after machining the composites with the 4 µm particles. Larger particles in the composite would promote stress concentration leading to local fractures then the formation of partially agglomerated areas of the pulverized particles on the machined surface. Conversely, smaller particles would distribute the stress more evenly throughout the matrix, thus reducing the stress concentration. This would lead to a smoother surface and decrease in the tool wear.

Fig. 12 Schematic illustration of chip formation mechanism during cutting composite.

1. Introduction

Aluminum alloys are significantly used for many industrial applications due to their lightweight and excellent oxidation resistance. However, their use in components requiring high temperature strength, rigidity, wear resistance, and low thermal expansion is limited. To address these limitations, non-metallic fibers and particles have been proposed to reinforce aluminum alloys.

Silicon carbide (SiC) exhibits a high hardness, high stiffness and low thermal expansion, making the SiC fiber and particle widely used as the reinforcements in not only the academic but also the industrial fields. The microstructure [1–3], mechanical [4, 5] and thermal behaviors [6], and wear characteristics [7] of the SiC particle (SiC_p)-reinforced aluminum alloy composites have been investigated, as well as the composites reinforced with other materials such as carbon fiber [8–11], alumina fiber [12] and potassium titanate fiber [13]. The turning machinability is also an important property for finishing the products for the industrial application of the composites, and it has been investigated for pitch-based carbon fiber [14], alumina fiber [15, 16] and potassium titanate fiber [17]. The finishing of the composites with the large volume fraction of SiC_p would be more difficult due to the high hardness of the SiC_p. The turning machinability of the SiC_p-reinforced aluminum alloy composites, particularly the effects of particle size and volume fractions of the composites, remains unclear.

In this study, the aluminum alloy composites reinforced with the different sizes and volume fractions of the SiC_p were fabricated by the squeeze casting, and the effects of the size and volume fraction of the particles on the machinability were discussed.

2. Experimental Procedure

The JIS-AC8A (Al-12Si-1Cu-1Ni-1Mg alloy casting) was used as the matrix metal and SiC_p was used as the reinforcement. The properties [18] of the SiC_p are listed in Table 1. To fabricate the SiC_p preform, the SiC_p was mixed with a binder in a water bath. The mixture was then pressed into a mold at ambient temperature and dried. The volume fraction of the SiC_p in the preform was controlled to 30 and 50 vol%. In order to fabricate the preforms with 30 vol% SiC_p, the atomized 6061 aluminum powders with a 20 µm diameter were mixed with the SiC_p.

Table 1 Properties of SiC_p [18].

The composites were fabricated by squeeze casting. The preform was horizontally placed in a permanent mold, then molten AC8A was poured into the mold. A pressure of 100 MPa was mechanically applied and maintained until solidification was complete. The composites fabricated in this study are listed in Table 2.

Table 2 Volume fraction and average diameter of SiC_p in composites.

Test pieces with a diameter of 40 mm were machined from the composite, and their machinability during the turning operations was evaluated by machining the surface of the pieces on a lathe. The dimensions of the cutting tool used in this study and the cutting conditions are listed in Table 3. The cutting resistances (cutting force: F_x and feed force: F_y) were measured using an elastic disc-type tool dynamometer attached to the lathe. The roughness of the machined surface (maximum height of profile, R_z) was measured using a laser scanning digital microscope, and the morphology of the chips after machining was observed. The width of the flank wear of the tool was measured by observing the flank of the tools after machining the composites.

Table 3 Dimensions of the tool and cutting conditions used.

3. Results and Discussion

3.1 Microstructure and hardness

Figure 1 shows the optical micrographs of the unreinforced AC8A alloy and composites. In the unreinforced alloy, the eutectic silicon, the grey phase in the microstructure, was dispersed. In the composites, the SiC_p, which appears to be dark and with an irregular shape, was dispersed in the alloy matrix. In the composite 4–30 (Fig. 1(b)), a light circle area was observed in the matrix due to the residual 6061 aluminum powders mixed in the preform.

Fig. 1

Optical micrographs of unreinforced AC8A alloy and composites. (online color)

The Vickers hardness values of the unreinforced AC8A and composites were shown in Fig. 2. The hardness of the AC8A alloy increased due to the reinforcement. Reinforcement with the larger particle leads to a lower hardness with the same volume fraction of particles.

Fig. 2

Vickers hardness comparison of machining unreinforced AC8A alloy and composites.

3.2 Cutting resistance

Figure 3 shows the waveforms of the cutting force (F_x) while machining the unreinforced AC8A alloy and composites at a cutting speed (v) of 50 m/min, a feed rate (f) of 0.2 mm/rev, and a cutting depth (t) of 1.0 mm. The range of F_x fluctuations for the composite seems to be slightly narrower compared to that for the unreinforced alloy. The range of the fluctuations for the composites with higher SiC_p volume fractions (4–50 and 25–50) seems to be further narrower. Differences in the range between the composites with 4 and 25 µm SiC_ps were not observed.

Fig. 3

Functional waveforms of cutting resistance (F_x) which machining unreinforced AC8A alloy and composites (v = 50 m/min, t = 1.0 mm, f = 0.2 mm/rev).

Similar trends were observed for the F_y, overall the values were lower than F_x.

Similar trends were observed under the other cutting conditions. Although the F_x and F_y increased with higher f and values t, there was little change with variations in v.

Figure 4 shows the average cutting resistance (F_x and F_y) during cutting the unreinforced AC8A alloy and composites at a v of 50 m/min, f of 0.2 mm/rev, and t of 1.0 mm. The cutting resistance of the AC8A alloy increased due to the dispersion of SiC_p. When comparing the composites with different SiC_p sizes within the same volume fraction (4–30 and 25–30, 4–50 and 25–50), there is minimal variation in the cutting resistance. The cutting resistance for the composites with larger particle volume fractions was higher, especially for F_x (Fig. 4(a)).

Fig. 4

Comparison of average cutting resistance of each sample (v = 50 m/min, t = 1.0 mm, f = 0.2 mm/rev).

3.3 Machined surface

Figure 5 illustrates the waveform diagrams of the surface roughness when the machining unreinforced AC8A alloy and composites at a v of 50 m/min, f of 0.2 mm/rev, and t of 1.0 mm. The unreinforced AC8A alloy exhibits a mixture of small and relatively large waves on its surface. The composites tend to show smaller waveforms overall. Notably, with a high volume of SiC_p, specifically the fine particles (4–50), the overall waveforms became smaller and the semicircular shape of the tool edge became distinctly visible. When comparing samples with a high volume fraction of SiC_p (4–50 and 25–50) to those with a lower SiC_p volume fraction (4–30 and 25–30), the latter tends to exhibit small coarser waves.

Fig. 5

Waveform diagrams of the finished surface roughness of materials at the cutting conditions are v = 50 m/min, f = 0.2 mm/rev, t = 1.0 mm.

Figure 6 shows the waveform diagrams of the surface roughness at the v of 50 m/min, f of 0.1 mm/rev, and t of 0.2 mm. Similar to the higher f and t values (Fig. 5), the characteristic of the AC8A alloy exhibits a mixture of small and relatively large waves. Furthermore, due to the dispersion of SiC_p, the overall wave pattern tends to diminish in magnitude. Since t was relatively low, no clear semicircular patterns corresponding to the tool tip were observed in the overall waveform.

Fig. 6

Waveform diagrams of the finished surface roughness of materials at the cutting conditions are v = 50 m/min, f = 0.1 mm/rev, t = 0.2 mm.

Figure 7 shows a comparison of the R_z among the specimens (v = 50 m/min, f = 0.2 mm/rev). In every cutting condition, the reinforcement with SiC_p leads to a reduction in the R_z of the AC8A alloy. Experimental values of the R_z of the composites got closer to the theoretical roughness value R_th, which can be geometrically calculated from the nose radius of the cutting tool (r) and f [19]. It can be written as

  

\begin{equation} R_{\text{th}} = \frac{f^{2}}{8r} \end{equation} (1)

Fig. 7

Comparison of surface roughness (R_z) after machining the unreinforced alloy and composites and the theoretical roughness (R_th) at the cutting conditions are v = 50 m/min, f = 0.2 mm/rev.

The effect of the SiC_p size on the R_z of the composites was clearly observed when the cutting depth was large (t = 1.0 mm) (Fig. 7(a)); the R_z of the composites with 4 µm SiC_p (4–30 and 4–50) was smaller than that of the composites with 25 µm SiC_p (25–30 and 25–50).

Figure 8 shows the appearance of the chips generated from the specimens (v = 50 m/min, f = 0.2 mm/rev). The unreinforced alloy chips are relatively short, whereas those generated from the composites are slightly longer when t was 1.0 mm (Fig. 8(a)). In comparison to the unreinforced alloy, the chips from the composites generally do not exhibit curling.

Fig. 8

Observation of chip shape of unreinforced AC8A alloy and composites (v = 50 m/min, f = 0.2 mm/rev).

Figure 9 is enlarged views of the chip and elemental distribution in the chips from the unreinforced alloy and composites (v = 50 m/min, f = 0.2 mm/rev, t = 1.0 mm), showing that the chips undergo repeated shear deformations during the cutting. For the unreinforced alloy, this deformation leads to fragmentation of the eutectic silicon, which was originally dispersed in the matrix, into small pieces (Fig. 9(a)). Similarly, for the composites, SiC_p is pulverized into fine fragments due to the repeated shear deformation, and these fragments are randomly distributed throughout the matrix (Figs. 9(b) and (c)); even for the composites with a larger (25 µm) SiC_p, the size of the fine SiC_p fragments in the chip seem to be under 10 µm (Fig. 9(c)). However, the machined surface of the chips from the composites, especially 4–50 (a low-mag. image in Fig. 9(b)), seems to be smoother than that from the unreinforced alloy. Although the surface of the 25–50 was smoother than the unreinforced alloy (low-mag. images in Figs. 9(a), (c)), the area consisting of the agglomerated pulverized particles was partially observed, as indicated by the square in Fig. 9(c). Based on Figs. 8 and 9, the chips from the unreinforced alloy and the composites can be classified as a “flow type” and “saw-toothed type”, respectively [20, 21].

Fig. 9

Chip morphology and elemental distribution in the chips from the unreinforced alloy and composites (v = 50 m/min, f = 0.2 mm/rev, t = 1.0 mm).

3.4 Tool wear

Figure 10 shows the appearance of the tool edge after cutting the composites for 45 min (v = 50 m/min, f = 0.1 mm/rev, t = 0.2 mm). Flank wear was clearly observed after cutting the composites under these cutting conditions. No accretion of the composites on the tool was observed.

Fig. 10

Appearance of the tool edge after cutting the composites for 45 min (v = 50 m/min, f = 0.1 mm/rev, t = 0.2 mm).

Figure 11 shows the relationship between the flank wear of the tool after cutting the composites and the cutting distance (v = 50 m/min, f = 0.1 mm/rev, t = 0.2 mm). It was clearly observed that the flank wear of the tool after machining the composite 25–50 was more severe than that after machining the composite 4–50, indicating that the small particles reduced the flank wear.

Fig. 11

Flank wear of the tool after cutting the composites and the cutting distance (v = 50 m/min, f = 0.1 mm/rev, t = 0.2 mm).

3.5 Discussion

First, the effects of the SiC_p reinforcement on the cutting resistance and surface roughness of the alloy were discussed. Under every cutting condition in this study, the range of the cutting force fluctuations during the cutting of the aluminum alloy got slightly narrower due to the reinforcement (Fig. 3), while the average values increased due to the SiC_p reinforcement (Fig. 4). Hard SiC_p (Table 1) reinforcement tends to increase the cutting resistance due to the abrasive nature of the particles leading to elevated cutting forces. This effect is particularly pronounced at the higher volume fractions. Dispersion of the hard and brittle SiC_p would promote the localized stress concentration and/or interfacial fractures, which cause severe friction and impact on the cutting edge of the tool. As shown in Fig. 9, for the composites, not only the eutectic silicon, which was originally distributed in the unreinforced AC8A alloy, but also the SiC_p are pulverized into fine fragments due to the repeated shear deformation, and these fragments are distributed in the matrix leading to the formation of the “saw-toothed type” chips. As Nakayama [20, 21] reported that this type of chip is formed due to the generation of cracks formed in the brittle materials, the same phenomenon would occur for the SiC_p-reinforced composites in this study.

As shown in Figs. 6, 7 and 9(b), (c), the reinforcement with SiC_p promoted the surface smoothness and decreased the roughness of the alloy, leading to the R_z values close to R_th (Fig. 7). These results indicated that the reinforcement reduces the formation of the built-up edge during the cutting. One of the authors [14–17] reported that the reinforcement with the nonmetal materials decreased the formation of the built-up edge and decreased the R_z values, and the similar phenomenon would also occur in this study.

Subsequently, the effects of the SiC_p size on the machinability of the alloy was discussed. As shown in Fig. 2, reinforcement with the larger particles led to the lower hardness with the same volume fraction of particles. A Vickers hardness test was carried out under a static load using a square pyramid-diamond indenter. Although the cutting test was carried out using a diamond (PCD) tool with a nose radius, it was carried out under a dynamic load. During the cutting process, the SiC_p in the composites were pulverized into fine fragments due to the repeated shear deformation, and these fine fragments were randomly distributed throughout the matrix leading to the formation of the “saw-toothed type” chips (Figs. 9(b) and (c)), even for the composites with a larger (25 µm) SiC_p. Therefore, the chip formation process after the SiC_p pulverization for the composites with 25 µm SiC_p would be similar to that for the composites with 4 µm SiC_p. This is why the significant differences in the fluctuations range of the cutting force between the 4 and 25 µm SiC_ps (Fig. 3), and the difference in the average cutting force was not observed (Fig. 4). For the composite with the fine particles, specifically a high volume of SiC_p (4–50), the overall waveforms of the surface roughness became smaller and the semicircular shape of the tool edge became distinctly visible (Fig. 5). In addition, the machined surface of the chips from the composite 4–50 (Fig. 9(b)) seems to be smoother than that from the composite 25–50 (Fig. 9(c)). This would be due to the severe pulverization of the larger (25 µm) SiC_p.

In this study, it should be noted that the machined surface of the composite with 4 µm SiC_p was smoother than that of the composite with 25 µm SiC_p, and the tool wear against the 4 µm SiC_p was less than that of the composite with 25 µm SiC_p. Based on these important facts, the chip formation mechanism during cutting the composite in this study was considered using the schematic illustration as shown in Fig. 12. As indicated earlier, the reinforcement decreased the formation of the built-up edge which acted as a protective material of the tool, leading to the direct contact between the tool and the SiC_p dispersed in the composite. This direct contact would promote the pulverization of SiC_p by the tool. Larger particles tend to concentrate the stress, thus increasing the potential for local fractures (Fig. 12(a)). This would lead to the formation of partially agglomerated area of the pulverized particles observed on the machined surface (Fig. 9(c)), then leading to the unclear semicircular shape of the tool edge in the waveform diagrams of the surface roughness (Fig. 5) and the increase in the surface roughness (Fig. 7(b)). This severe stress concentration would also promote the tool wear (Figs. 10 and 11). Conversely, smaller particles would distribute the stress more evenly throughout the ductile aluminum alloy matrix, thus reducing the stress concentration in the particle. This would lead to a smoother surface, then leading to the clear-semicircular shape of the tool edge in the waveform diagrams of the surface roughness (Fig. 5) and the decrease in the tool wear due to the hard and brittle particles.

Fig. 12

Schematic illustration of chip formation mechanism during cutting composite.

4. Conclusions

1. (1)    The SiC_p reinforcement slightly decreased the range of the cutting force fluctuations during the cutting of the alloy. The average cutting force values increased due to the reinforcement, and the resistance with higher SiC_p volume fractions was higher. When comparing the composites with different SiC_p sizes within the same volume fraction, there is minimal variation in the resistance.
2. (2)    With a high SiC_p volume, specifically the fine particles, the overall waveforms of the surface roughness became smaller and the semicircular shape of the tool edge became distinctly visible. The reinforcement decreased the surface roughness, achieving a finish closer to the theoretical surface roughness, indicating that the reinforcement reduces the formation of the built-up edge during the cutting. The roughness of the composites with 4 µm SiC_p was lower than that of the composites with 25 µm SiC_p within the same volume fraction.
3. (3)    The flank wear of the tool after machining the composite with 25 µm SiC_p was more severe than that after machining the composites with 4 µm SiC_p.
4. (4)    Based on these results, the cutting mechanism of the composite can be concluded as follows; larger particles in the composite promoted the stress concentration, leading to the local fractures then the formation of a partially agglomerated area of the pulverized particles on the machined surface. Conversely, smaller particles distributed the stress more evenly throughout the ductile aluminum alloy matrix, thus reducing the stress concentration, then leading to the smoother surface and decrease in the tool wear.

Acknowledgments

The Kindai Joint Research Center provided the opportunity for operating the SEM and EPMA instruments.

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