Experimental investigation during wire electric discharge cutting of SiCp/6061 aluminum metal matrix composite.

Shandilya, Pragya ; Jain, Pramod Kumar ; Misra, Joy Prakash 等

1. INTRODUCTION

MMCs have become increasingly important in modern industrial applications due to their properties such as high strength-to-weight ratio, high toughness, lower value of coefficient of thermal expansion, good wear resistance, and capability of operating at elevated temperature [Miracle D.B. (2005); Taha M.A. (2001)]. Using traditional machining processes to machine hard composite materials causes very high tool wear due to abrasive nature of reinforcing particles [Yan B.H. & Wang C.C. (1993)] thus shortening the cutting tool life. Although, nontraditional machining techniques such as water jet machining (WJM) and laser beam machining (LBM) have been used in past [Muller F. et. al. (2000); Grabowski A. et. al. (2006) ] but their machining equipment is expensive and height of the workpiece is a constraint in using these processes. In this context, wire electric discharge machining (WEDM) seems to be a better choice for machining the MMCs because it offers easy control and has capability of machining intricate complex shapes WEDM is a type of thermal advanced machining process capable of accurately machining the parts having varying hardness, complex shapes and sharp edges that are difficult to machine by other traditional and nontraditional machining processes. Lot of research work has been on WEDM but, very few investigations on WEDM of MMC have been done [Patil N.G. et. al. (2006); Saha P. et. al. (2009)] and particularly there is lack of published literature on WEDM of Al-based MMCs. This paper presents the experimental findings on the wire electric discharge cutting (WEDC) of 6061 aluminum MMC reinforced with silicon carbide particulates (i.e. Si[C.sub.p]/6061 Al). In the present work effect of servo voltage (SV) and pulse-on time ([T.sub.ON]) on the quality of cut in terms of average cutting speed and microstructure of the cut surface has been evaluated to bracket range of input parameters for carrying out further research on WEDC of Si[C.sub.p]/6061 aluminum MMC.

2. EXPERIMENTATION

In this experiment, three types of 6061 aluminum based MMCs made by stir casting and having 5%, 7.5% and 10% SiC particles (by weight) as reinforcement were used as the workpieces. The workpieces were of rectangular shape having a thickness of 6 mm. Table 1 shows the chemical compositions of the matrix of the MMCs used in this study.

The experiments were conducted on the ECOCUT WEDM Machine from Electronica India Pvt Ltd. A diffused brass wire of 0.25 mm diameter was used as the cutting tool. The deionized water was used as dielectric. The input parameters were varied during the experiments namely servo voltage (SV) in the range of 70-100 V, and pulse-on time ([T.sub.ON]) in the range of 1-10 [micro]s to study their effects on the average cutting speed and microstructure of the cut surface. The experiments were conducted using one-factor-at-a-time experiment strategy in which only one input parameter was varied while all others input parameters were kept constant at the middle point of their respective range. Average cutting speed was calculated by dividing the length of cut by the total cutting time which was recorded using a stop watch having a least count of 0.01 seconds while, the microstructure of the cut surface was examined using SEM.

3. EXPERIMENTAL RESULTS AND ANALYSIS

Machining characteristics. Fig. 1(a) shows the effect of voltage on the average cutting speed indicating that as the average cutting speed decreases continuously with increase in the voltage for all three MMCs and that at any value of the voltage, the average cutting speed is maximum for the MMC with 7.5% Si[C.sub.p]. The frequency of wire breakage increases significantly when the voltage reaches less than 70 V. Fig. 1(b) depicts the variation of the average cutting speed with the pulse-on time indicating that the average cutting speed increases continuously with increase in the pulse-on time for all three percentage of Si[C.sub.p] and its value being highest for the MMC with 7.5% Si[C.sub.p] at any value of the pulse-on time. Surface Morphology. SEM of the cut surface by WEDC was conducted to evaluate the effect of machining parameters on the surface textures of the 6061 Al based MMC reinforced with 7.5% SiC particles. Figs 2 (a)-(d) show the SEM images of the cut surfaces for the end values of the bracketed input parameters (i.e. voltage at 70 and 90 V; and [T.sub.ON] 1 and 4 us). It is clear that the morphology of the WEDC surface was dependent on the applied voltage and pulse-on time. The WEDC surface abounds with the craters and ridged surface. The craters and ridge-rich surface were formed by the melted material. When lower voltages (Fig. 2(a)), lower pulse-on time (Fig. 2(c)) are used, the surface characteristics had minor hillocks and valley. When the voltage and pulse-on time increases the cut surface exhibits deeper craters and ridge-rich surfaces.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

4. CONCLUSIONS

From the preliminary investigations on WEDC of 6061 aluminum alloy based MMC reinforced with three different percentage of the SiC particles it can be conclude that to achieve higher value of the average cutting speed, lower value of voltage and higher value of pulse-on time should be used. At any given value of any of the two input parameters the MMC with 7.5% Si[C.sub.p] gives the highest value of the average cutting speed among the % of the SiC particles considered in this work. Through analysis of SEM, it has been observed that the depth of micro-voids and micro-cracks increases as the voltage and pulse-on time increase. In future the study can be extended using different work material, process parameters and performance measures.

5. REFERENCES

Grabowski, A.; Nowak, M. & Sleziona, J. (2006). Laser cutting of an Al Si alloy/ Si[C.sub.p] composites theory and experiments. Journal of Achievements in Materials and Manufacturing Engg. Vol., 17., (12), pp. 61-6.

Miracle, D.B.; (2005). Metal matrix composites-from science to technological significance, Composite Science and Technology, Vol., 65, pp. 2526-2540

Muller, F.; & Monaghan, J. (2000). Non- conventional machining particle metal matrix composite, International Journal of Machine tool and Manufacture, Vol., 40, (9) pp. 1351-1366

Patil, N.G.; & Brahmankar, P.K. (2006): Some investigations into wire electro-discharge machining performance of Al/Si[C.sub.p] composites, Int. Journal of Machining and Machinability of Materials, Vol., 1, (4), pp. 412-431

Saha, P.; Tarafdar, D. & Pal, S. K. (2009), Modeling of wire-electro- discharge machining of TiC/Fe in situ etal matrix composite using normalized RBFN with enhanced K means clustering technique, Int. Journal of Advance Manufacturing and Technology, Vol., 43,pp. 107-116

Taha, M.A.; (2001). Practicalization of cast metal matrix composites (MMCs), Mater Des, Vol. 22, pp. 431-441

Yan, B.H.; & Wang, C.C. (1993): Machinability of SiC particle reinforced aluminum alloy composite material, Journal of Japan Institute Light Metals, Vol., 43 (4), pp. 187-192

Tab. 1 Chemical composition of Al6061 alloy

Si 0.4-0.8
Fe 0.7 max
Cu 0.15-0.40
Mn 0.15 max
Mg 0.8-1.2
Cr 0.04-0.35
Zn 0.25 max
Ti 0.15 max
Other elements 0.15 total, 0.05 max each
Al REM