Introduction

VM process is distinctly different from other sand casting processes as the process requires no binders for holding the sand grains together in the mould [1]. The vacuum inside the mould results in a net pressure pushing in, holding the sand rigidly in the shape of the pattern, even after the pattern is removed. In this process sand thermal conductivity is lower and metal fluidity is improved [2]. However solidification time is slower. Fine surface finish and excellent dimensional accuracy, no moisture related defects, no cost for binders, excellent sand permeability, and no toxic fumes from burning the binders are key advantages of VM [3, 4]. The literature review reveals that lot of work has been reported on optimization of VM process [5, 6]. Some researchers have highlighted development of MMC by VM [7, 8]. But hitherto very less has been reported on process capability analysis of Al-Al2O3 based MMC with VM. So, in the present work effort has been made to understand the process capability of VM process for development of Al-Al2O3 based MMC. Singh [8] outlined a Taguchi based model for development of Al-5 %Al2O3 based MMC. In the present work, this model has been used further for process capability analysis of VM. Figures 1 and 2 shows schematic and 3D view of VM machine used for present study.

Fig. 1
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Schematic of VM machine [8]

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Fig. 2
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3D view of VM machine [8]

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For preparation of MMC commercially pure Al (≥99 %) was melted in a silicon-graphite crucible by an induction furnace. The composition of Al with Al2O3 was fixed as Al-5 %Al2O3. Al was preheated up to a temperature of 450 °C and particles of Al2O3 up to a temperature of 1,100 °C in core drying oven. Crucible used for pouring of composite slurry in the mould was also heated up to 760 °C. The stir caster was mounted on the furnace with the help of four legs. Mild steel was chosen as stirrer and impeller material. It has been used to obtain an output of 600 rpm.

Table 1 shows input parameters and their levels (based upon Taguchi L9 OA) for process capability analysis of VM process.

Table 1 Input parameter and their levels [8]
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Based upon Table 1, Fig. 3 and Table 2 respectively shows contribution of input parameters on dimensional accuracy and macro model of dimensional accuracy for Al-5 %Al2O3 MMC in VM.

Fig. 3
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Contribution of various input parameters for dimensional accuracy of MMC in VM

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Table 2 Macro model of dimensional accuracy for Al-5 % Al2O3 MMC in VM [8]
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These optimized settings (Ref. Table 2) has been used for process capability analysis. There are three sections in this paper. Following this introduction section, process capability analysis has been highlighted. Conclusions are drawn in last section followed by references.

Process Capability Analysis

Figure 4 shows dimensions of benchmark used for study. The input parameters were kept constant based upon Table 2. The CMM machine was used to measure the critical dimensions of the specimens. Table 3 shows measured dimension for critical dimensions 15, 60, 70 and 80 mm.

Fig. 4
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Benchmark dimension

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Table 3 Measured dimensions for final experimentation
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The result of the dimensional measurement have been used to evaluate the tolerance unit ‘n’ that drives starting from the standard tolerance factor ‘i’, defined in standard UNI EN 20286-I(1995). The standard value of tolerance was evaluated by considering the standard tolerance factor i (μm) as: i = 0.45 × D1/3 ± 0.001 × D, Where ‘D’ is the geometric mean of the range of nominal size in mm. In fact, the standard tolerance are not evaluated separately for each nominal size, but for a range of nominal size, for the generic nominal dimension DJN, the number of tolerance unit n is evaluated as follows: n = 1,000 (DJN−DJM)/i, Where DJM is the measured dimension. Tolerance is expressed as a multiple of ‘i’. Table 4 shows the classification of different IT grade according to UNI EN 20286-I (1995) for D1=60.00 mm. Similarly IT grades for D2, D3 and D4 were calculated, which are consistent according to ISO standard UNI EN 20286-I (1995).

Table 4 IT grades for D1 = 60 mm
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The data collected for the nominal dimensions D1, D2, D3 and D4 shown in Table 2 has been used for process capability analysis. Figures 5, 6, 7 shows R chart, X chart and process capability histogram for nominal dimension D1. Table 5 shows values of Cp, Cpk and other data for nominal dimension D1. For Cpk value of 1.5, the area under normal curve is 0.999993198 and non conforming ppm is 6.8016. Similarly Cp and Cpk values for other dimensions (D2, D3 and D4) were calculated. The value of Cpk for all critical dimensions is >1.33. The results of study suggest that VM process lies in ±4.5 sigma (σ) limits as per as dimensional accuracy of MMC is concerned.

Fig. 5
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R chart for the dimension nominal dimension D1

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Fig. 6
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X chart for nominal dimension D1

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Fig. 7
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Process capability study histogram for nominal dimension D1

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Table 5 statistical analysis for nominal dimension D1
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Conclusions

The result of study suggests that VM is a highly capable process. It is observed that the value of Cpk for all the four critical dimensions were >1.33. As Cpk values of 1.33 or greater are considered to be industry benchmarks, so this process will produce conforming products as long as it remains in statistical control.

The tolerance grades of the MMC produced are consistent with the permissible range of tolerance grades (IT grades) as per ISO standard UNI EN 20286-I (1995). So it is concluded that the parts produced by VM are acceptable in terms of accuracy as per industrial requirements.