Effect of Si Content on Turning Machinability of Al Si Binary Alloy Castings

Effect of Si Content on Turning Machinability of Al-Si Binary Alloy Castings

Masatsugu Kamiya

_{, Takao Yakou, Tomohiro Sasaki and Yoshiki Nagatsuma}

Department of Mechanical Engineering and Materials Science, Yokohama National University, Yokohama 240-8501, Japan

The effect of eutectic Si or primary Si on the machinability of Al-Si alloy castings, where eutectic Si or primary Si was served to improve the chip breakability were investigated. To enhance chip breakability, eutectic Si made the chips thin, and cracks that formed in primary Si during machining acted as nuclei for chip breaking. Eutectic Si had a stronger effect on surface roughness than primary Si, and eutectic Si reduced the adhesion on the cutting edge. The decrease in adhesion on the cutting edge led to a corresponding decrease in surface roughness. The cracking of primary Si was responsible for the increase in surface roughness in hypereutectic alloys. Tool wear increased with increasing amount of eutectic Si. In hypereutectic alloys, tool wear was accelerated by the contact between the tool and cracked primary Si.

[doi:10.2320/matertrans.L-MRA2007886]

(Received September 11, 2007; Accepted November 24, 2007; Published January 30, 2008)

Keywords: aluminum-silicon alloy, machinability, chip breaking, surface roughness, tool wear

1. Introduction

Among the elements added to free-cutting aluminum (Al) alloys to replace Pb, viz., Si, Ni, Fe or Mn, Si has been found to be the most effective for chip breakability.1,2)_{Moreover, it} has been reported that the fracture of primary Si which is located near the cutting edge during machining caused chip breaking and increased the tool wear in hypereutectic cast Al-Si alloys.3,4) Since cast Al-Si alloys are widely used as components for engines, and machining is needed after casting to obtain a certain precision, several studies have been conducted on the machinability of cast Al-Si alloys.3) However, most of these studies have investigated tool wear as a function of machinability. Since cast Al-Si alloys have great chip breakability, the investigations did not focus on the chip breaking mechanism in cast Al-Si alloys.

The purpose of this study is to experimentally investigate the effect of Si particles on machinability of cast Al-Si alloys, focusing on chip breakability, as determined through turning tests on alloys covering a composition range from hypoeu-tectic to hypereuhypoeu-tectic.

2. Experimental Procedure

The specimens used in this study were seven kinds of cast Al-Si binary alloys with the chemical compositions shown in Fig. 1. These specimens were prepared by melting of 99.99% pure Al and an Al-24.7%Si master alloy (all compositions in this paper are given in mass% unless otherwise stated) using electric furnace in air and were casted into a steel mold at 1033 K. Bars 45 mm in diameter and 75 mm in length were obtained. The heat treatment was performed at 733 K for 15 h in order to obtain granular eutectic Si having the same morphology primary Si. Meanwhile, cast Al-Si alloys in mass production were modified to improve their mechanical properties through refinement of Si particles. Non-modified materials were used in this study to obtain data for alloys containing large Si particles. The chemical compositions of

specimens are shown in Table 1. Throughout this paper, these specimens are referred to by the summary shown in Table 1.

Figure 2 shows optical micrographs of the specimens. Eutectic Si and large aggregated primary Si were observed in hypereutectic alloys containing above 15% Si. The average particle size of primary Si in 15%Si, 20%Si and

Al-25%Si was 30, 120 and 150mm, respectively; i.e., it

increased with increasing Si content. Figure 3 is a magnified image of the eutectic Si in Fig. 2. The amount of granular eutectic Si increased as the Si content increased within Al-12%Si. However, the amount of granular eutectic Si is

0 5 10 15 20 25 30

300 400 500 600 700 800 900 1000

(a) (b) (c) (d) (e) (f) (g)

Temperature, T (

°

C)

Si content (mass%)

Fig. 1 Phase diagram of the Al-Si system5)_{and chemical composition of} specimens used in the experiment.

Table 1 Si content of specimens measured by an x-ray spectroscope.

Symbol in Fig. 1 Specimen Si (mass%)

(a) Al-2%Si 2.31

(b) Al-5%Si 5.64

(c) Al-10%Si 10.31

(d) Al-12%Si 12.74

(e) Al-15%Si 14.51

(f) Al-20%Si 21.80

(g) Al-25%Si 24.29

*1_{This Paper was Originally Published in Japanese in J. JILM57(2007)} 191–196.

slightly decreased, while the particle size of granular eutectic Si is increased when primary Si was observed in hyper-eutectic alloys.

Turning tests were carried out on a lathe using a carbide cutting tool (K10). The cutting tool had the following

geometry: back rake angle 0_{, side rake angle 5, back}

clearance angle 6_{, side clearance angle 6, end cutting edge} angle 30_{, side cutting edge angle 0, and corner radius} 0.4 mm. The tests were operated using a feed rate of 0.1 mm/ rev, a cutting depth of 0.5 mm and a cutting speed between 0.5 and 2 m/s. The range of cutting speed covered conditions under which a relatively strong adhesion leads to problems during machining of Al alloys. All experiments were performed under dry cutting conditions, and most tests were repeated at least twice to ensure reproducibility.

3. Results

3.1 Relation between Si content and chip breakability Figure 4 shows photographs of chips machined at various cutting speeds. The chips of Al-2%Si and Al-5%Si were relatively long. However, the chips became shorter with decreasing cutting speed and increasing Si content. The chips of hypereutectic alloys were fragmented into a piece at all cutting speeds.

To evaluate the chip breakability from the chip shape in these photographs, ‘‘chip breaking factor’’,Nwas defined, as a parameter of chip breakability determined by measuring the

number of chips contained in a 100 g sample of chips. According to Asanoet al.,6)_{the chip breakability is optimum} in the range104_<N<105_{. Figure 5 gives the variation ofN} with Si content. In the region of Si contents below 12%, theN ranges below 104_{. However, for hypereutectic alloys} con-taining above 15% Si,N rapidly exceeded104_.

3.2 Relation between Si content and machined surface Micrographs of surfaces machined at a cutting speed of 0.5 m/s are shown in Fig. 6. Feedmarks were observed on the machined surfaces of all the alloys. The surface roughness was significantly large in Al-2%Si and Al-5%Si, and significantly small in Al-12%Si. The surface roughness became large again in Al-20%Si and Al-25%Si. Figure 7 shows the variation of surface roughnessRawith Si content. At all cutting speeds, arithmetical mean deviation of the assessed profile (Ra) decreased with increasing Si content, reaching a minimum at 12% Si which corresponded to the eutectic alloy composition. Ra increased as the Si content increased above 12%.

3.3 Relation between Si content and tool wear

Figure 8 shows SEM images of the flank face of cutting tip after removal of the adhesion on the cutting edge by NaOH. In the machining of Al-10%Si containing eutectic Si, the worn surfaces were relatively smooth. On the other hand, worn surfaces with deep grooves were observed in the case of Al-15%Si containing eutectic Si and primary Si. The

(a) Al-2%Si (b) Al-5%Si

(c) Al-10%Si (d) Al-12%Si

(e) Al-15%Si (f) Al-20%Si

(g) Al-25%Si

100 µm

Fig. 2 Optical micrographs of specimens.

(a) Al-2%Si (b) Al-5%Si

(c) Al-10%Si (d) Al-12%Si

(e) Al-15%Si (f) Al-20%Si

(g) Al-25%Si

20 µm

variation in maximum flank wear VBmax with Si content after machining to 400 m is shown in Fig. 9. The flank wear tends to increase with increasing Si content. However, the VBmax of hypereutectic alloys was larger than that of hypoeutectic alloys, and the rate of increase of VBmax as a function of Si content was higher in hypereutectic alloys.

4. Discussion

4.1 Effect of Si content on machinability

Figure 10 shows micrographs of the cross-section of chips generated at cutting speed of 0.5 m/s. Fractured primary Si can be observed in hypereutectic alloys with a Si content above 15%. It has been reported that the fracture of primary Si located near the cutting edge during machining accelerates tool wear and chip breaking in hypereutectic Al-Si alloys.3,4) Results of this study are in reasonable agreement with these findings. However, to our knowledge, it has not been reported that the surface roughness reaches a minimum at the eutectic composition and that the surface roughness increases at Si

(a) Al-2%Si

(b) Al-5%Si

V=0.5 m/s V=1.0 m/s V=2.0 m/s

(c) Al-10%Si

(d) Al-12%Si

(e) Al-15%Si

(f) Al-20%Si

(g) Al-25%Si

Fig. 4 Chip shapes.

Si content (mass%) x104

Chip breaking factor, N (number of chips/100g)

0 5 10 15 20 25 30

0 1 2 3 4 5

V=0.5m/s V=1.0m/s V=2.0m/s

Fig. 5 Relationship between Si content and chip breaking factor.

(a) Al-2%Si (b) Al-5%Si

(c) Al-10%Si (d) Al-12%Si

(e) Al-15%Si (f) Al-20%Si

(g) Al-25%Si

100 µm

cutting direction

feed direction

Fig. 6 Surface after machined at 0.5 m/s.

0 5 10 15 20 25 30

Si content (mass%)

Surface roughness, Ra (

µ

m)

0 5 10 15

V=0.5m/s V=1.0m/s V=2.0m/s

contents above or below that composition. In hypoeutectic alloys, it is found that chip breakability and surface finish improve with increasing Si content, but tool life decreases. These may be attributed to the increase in the amount of eutectic Si with increasing Si content, as described later. On the other hand, it is found that chip breakability improves with increasing Si content, but surface finish and tool life worsen in hypereutectic alloys. It is believed that the main reason for these observations is the fracture of primary Si near the cutting edge.

4.2 Influence of eutectic Si and primary Si on machi-nability

The experiments in this study were performed using materials containing of particles of two different sizes:

eutectic Si and primary Si. Discussion about the influence of these particles on chip breakability, surface roughness and tool wear should be considered.

Al-10%Si Al-15%Si

V=0.5 m/s

V=1.0 m/s

V=2.0 m/s

10 µm

Fig. 8 SEM micrographs of cutting tools after machining to 400 m.

Flank wear, VBmax (

µ

m)

(a) V=0.5 m/s

0 20 40 60 80 100

Flank wear, VBmax (

µ

m)

(b) V=1.0 m/s

0 20 40 60 80 100

Si content (mass%)

Flank wear, VBmax (

µ

m)

l

(c) V=2.0 m/s

0 5 10 15 20 25 30

0 20 40 60 80 100

Fig. 9 Relationship between Si content and flank wear after machining to 400 m.

500µm

(a) (b) (c) (d)

(e) (f) (g)

4.2.1 Chip breakability

As described in Section 4.1, during machining, large primary Si easily fractured, forming nuclei of chip breaking. Figure 11 shows eutectic Si in the cross-section of chips generated at a cutting speed of 0.5 m/s. No fractured eutectic Si particles were observed in any of the cases as long as they were subjected to a large plastic strain. The variation in the thickness,t, and curl radius,r, of the chip with Si content is

shown in Fig. 12 and Fig. 13, respectively; t and r both

decreased with increasing Si content. On the other hand, the Vickers hardness of alloys increases with increasing Si content even in hypoeutectic alloys with no primary Si, as shown in Fig. 14. This suggests that the eutectic Si is attributable to the increase in the hardness of the matrix. It has been reported that the shear angle during machining increased as the hardness of the work material increased whose hardness was changed by heat treatment.7)The chip thickness decreased with increasing shear angle.8)It is found that the eutectic Si increased the shear angle, and thus

decreasing t. Moreover, the increase in the hardness of the matrix decreased the adhesion on the rake face of the cutting tool, and thus the chip could be easily removed from the cutting tool, which decreased the curl radius of the chip. These led to the growth of the nuclei of fracture generated by cracking of primary Si, resulting in chip breaking. Thus, it is believed that eutectic Si indirectly promotes chip breaking. 4.2.2 Surface roughness

As shown in Fig. 7, surface roughness Ra showed a

minimum at the eutectic composition. Figure 15 is a photo-graph of cutting tools after being machined at a cutting speed of 0.5 m/s. It was observed that the adhesion on the cutting tool decreased with increasing Si content for Al-12%Si. The extent of adhesion correlated strongly to the surface rough-ness. Moreover, the amount of eutectic Si increased with increasing Si content below Si content of 12%. These findings suggest that an increase in eutectic Si decreased the adhesion on the cutting edge, and hence decreasing the surface roughness.

In contrast, it is observed that the primary Si near the machined surface were cracked in hypereutectic alloys, as shown in the cross sectional micrographs of the machined surface in Fig. 16. It is belived that the increase in the surface roughness was due to the dent caused by the cracking of primary Si.

4.2.3 Tool wear

As shown in Fig. 9, tool wear increased with increasing Si content. Tool wear was particularly large in the cutting of

(a) Al-5%Si

(b) Al-12%Si

(c) Al-25%Si

20 µm

Fig. 11 Eutectic Si particles in chips.

Si content (mass%)

Thickness of chip, t (mm)

0 5 10 15 20 25 30

0 0.5 1.0 1.5

V=0.5 m/s V=1.0 m/s V=2.0 m/s

Fig. 12 Relationship between Si content and thickness of chip.

Si content (mass%)

Curl radius of chip, r (mm)

0 5 10 15 20 25 30

0 5 10 15 20

V=0.5 m/s V=1.0 m/s V=2.0 m/s

Fig. 13 Relationship between Si content and curl radius of chip.

Si content (mass%)

Vickers hardness, HV10

0 5 10 15 20 25 30

0 50 100 150

hypereutectic alloys. The increase in the amount of eutectic Si increased the tool wear. Moreover, tool wear was accelerated by the contact between the tool and fractured primary Si.

5. Conclusions

An analysis of the results of this study led to the following conclusions:

(1) With increasing Si content, chip breakability and surface finish improved, while tool life worsened for the eutectic composition. For the hypereutectic compo-sition, chip breakability improved further with increas-ing Si content, while surface finish and tool life became inferior.

(2) Eutectic Si made the chips thin, and cracked primary Si acted as nuclei of chip breaking for chip breakability. Since eutectic Si reduced the adhesion on the cutting tool, the effect of eutectic Si on surface roughness was larger than that of primary Si. On the other hand, the cracking of primary Si was responsible for the increase of surface roughness in hypereutectic alloys. Tool wear

increased with increasing amount of eutectic Si. In hypereutectic alloys, tool wear was accelerated by the contact between the tool and fractured primary Si.

REFERENCES

1) S. Yoshihara and M. Hirano: Abstracts of the 1997 Autumn Meeting of the Japan Inst. Light Metals (1999) pp. 179–180.

2) S. Yoshihara, M. Hirano, S. Yoshihara and M. Hirano: J. JILM51(2001) 238–241 (in Japanese).

3) e.g. S. Hori, H. Kato and T. Kaji: J. JILM 20 (1970) 410–414 (in Japanese).

4) Y. Hasegawa and S. Hanasaki: J. JILM38(1988) 546–551 (in Japanese). 5) ASM International:ASM Handbook vol. 3 Alloy Phase Diagrams. 6) K. Asano and A. Fujiwara: J. JILM21(1971) 578–588 (in Japanese). 7) S. Yamada, H. Mizutani, K. Notoya and A. Takayanagi: J. JILM43

(1993) 206–212 (in Japanese).

8) E. M. Trent:Metal cutting, (Butterworth Heinemann, 2000) pp. 21–55. (a) Al-2%Si (b) Al-5%Si

(c) Al-10%Si (d) Al-12%Si

(e) Al-15%Si (f) Al-20%Si

(g) Al-25%Si

500 µm

Fig. 15 SEM micrographs of cutting tools after machining.

100µm (a) Al-2%Si

(b) Al-5%Si

(d) Al-12%Si

(e) Al-15%Si

(f) Al-20%Si

(g) Al-25%Si (c) Al-10%Si

cutting direction