The Effect of Process Parameters on Quality Characteristics in the Drilling of Aluminium–Metal Matrix Composites ^†

Abstract

Present work focusses on investigating the effect of process parameters such as feed rate and spindle speed on quality characteristics of the hole, i.e., surface roughness (Ra) and circularity at entry and exit in the drilling of aluminium (Al) 6061 reinforced with different volume fraction of silicon nitride (Si₃N₄). Optimum parameters for Ra and circularity of hole at entry and exit are obtained as feed rate at 0.125 mm/rev, spindle speed at 300 rpm, diameter of drill at 8 mm, and % Vol. of Si₃N₄ at 5%. Using Analysis of Variance (ANOVA), we observed that spindle speed is the most influential parameter followed by feed rate.

1. Introduction

Al 6061 is largely employed in heavy-duty structural components of automobile, aerospace and defense systems because of its good strength-to-weight ratio. Mechanical properties of Al 6061 such as tensile strength, microhardness, ballistic resistance, etc., can be enhanced by reinforcing with ceramic materials such as metal oxides, nitrides and carbides [1,2]. Si₃N₄ is a metal nitride that provides excellent creep properties and tensile strength with a reduction in ductility when it is reinforced in appropriate proportion with Al 6061 endowed this combination for structural applications [3,4]. Previous studies have shown that Al 6061 reinforced with Si₃N₄ enhances corrosion resistance, wear resistance and metallurgical stability at high temperatures making this composite suitable for high wear and thermal-resistant applications [5]. Stir casting is the liquid state fabrication technique found to be a reliable production method for Al-Si₃N₄ MMC offers good mechanical and tribological properties [3,4,5,6,7]. Stir casting produces outstanding characteristics in Al-Si₃N₄ with regulated process conditions, and its low material density of 3.3 g/cm³ and heat conductivity of 170–230 W/mk make it excellent for applications where weight is an issue [3,8]. The presence of agglomerated reinforcing components in the matrix causes a non-uniform metallurgical structure, which leads to poor machinability of Al-MMCs during drilling operations, resulting in a shorter tool life, poor surface integrity, higher energy consumption, increased cutting force and so on [9]. Due to the poor quality of the hole created during drilling, mechanical assemblies are susceptible to failure over time. Selection of the correct volume proportion of Si₃N₄ in the Al matrix phase plays an important role in improving the machinability of the Al-Si₃N₄ composite. Levels of process parameters significantly affect the machining zone temperature while drilling the workpiece [10]. Only a few researchers have carried out an investigation on the effect of the volume proportion of Si₃N4 in Al MMCs on the quality of drilling operations. Surface roughness, circularity and cylindricity are greatly influenced by the volume proportion of Si₃N₄ [11]. Drilling process parameters such as spindle speed and feed rate are the most obvious influence on the quality of the hole. Spindle speed, then feed, has the greatest effect on the thrust force [12]. Cutting velocity has the second-greatest impact on the surface after feed rate [13]. But, none of the work has suggested the standardized process parameters in the drilling of Al-MMC, in particular Al-Si₃N₄ composites. It becomes necessary to study the significant effect of process parameters on the quality characteristics of a hole before announcing them as standard parameters. Taguchi method is the easiest and most deliberate statistical approach to evaluate the effect of cutting parameters on output variables [14,15]. An analytical statistical technique known as ANOVA is used to quantify the relative importance of each factor on the objective function [11,16]. The relative contribution of each control factor to the output response is effectively determined through ANOVA so that it helps in obtaining the optimized parameter for the desired output [17,18]. Hence, the current study aims to investigate the effect of cutting parameters such as feed rate and spindle speed at various levels on the surface roughness and circularity of the hole in the drilling of Al 6061 reinforced with different volume proportions of Si₃N₄ MMC, fabricated using stir casting method.

2. Materials and Methods/Methodology

2.1. Material

In the present study, Al 6061 is selected as matrix material and α-phase Si₃N₄ as reinforcement. The properties of Al 6061 and Si₃N₄ are mentioned in

Table 1. Properties of Al6061 and Si₃N₄ (α-phase).

Properties	Al 6061	Si3N4 (α-Phase)
Density	2.7 gm/cm3	3.44 gm/cm3
Melting Point	585 °C	1900 °C
Ultimate Tensile Strength	310 MPa	525 MPa
Young’s Modulus	68.9 MPa	297 GPa
Thermal Conductivity	167 W/m-K	30 W/m-K
Specific Heat	897 J/Kg-K	1.1 J/g-°C
Hardness	92 BHN	162 HRC
Dielectric Constant	-	10.5

.

2.2. Stir Casting Process

The stir casting process is the most economical and simple method to fabricate MMCs [18,19,20]. Al 6061 is reinforced with Si₃N₄ in three different volume proportions (i.e., 0%, 5% and 10%). Three different composite specimens are prepared to the size of the mould 52 mm × 51 mm × 20 mm using the stir casting process as shown in Figure 1a,b. The compositions of the specimens are mentioned in

Table 2. Composition of composite specimens.

Specimen No.	Composition
1	Al6061
2	Al6061 + 5% Vol. of Si3N4
3	Al6061 + 10% Vol. of Si3N4

.

2.3. Hardness Test

The hardness of the composites is measured using a Rockwell hardness tester, as shown in Figure 2.

2.4. Planning of Experiments

Experiments are planned using Taguchi’s technique in Minitab software 18.1. Four control factors of three levels are selected for the study; details of the control factors and their levels are mentioned in

Table 3. Hardness of the composite workpieces.

Specimen No.	Average RHN
1	43.4
2	45.6
3	48.4

. Taguchi’s L27 standard orthogonal array is selected for 4 control factors at 3 levels to conduct the experiment.

2.5. Drilling of Al-Si₃N₄ Composite Specimens

Al 6061 MMC reinforced with 0%, 5% and 10% volume proportion of Si₃N₄. Drilling is performed on the composite workpieces using special purpose automatic radial drilling machine as shown in Figure 3a. The cutting parameters are set based on Taguchi’s L27 standard orthogonal array. HSS drill bits of 6 mm, 8 mm and 10 mm diameter are used for the drilling process as shown in Figure 3b.

2.6. Surface Roughness and Circularity Measurement

The surface roughness of each hole is measured using Surfcom Flex with a sampling length of 4 mm, as shown in Figure 4a. Circularity at entry and exit of the hole is measured using a Tool Maker’s Microscope with the lowest count of 0.001 mm as shown in Figure 4b. The diameter of the hole is measured both along the X and Y axes and measurements are used to calculate the circularity of the hole based on the two-point method using Equation (1).

$$\mathbf{C}\mathbf{i}\mathbf{r}\mathbf{c}\mathbf{u}\mathbf{l}\mathbf{a}\mathbf{r}\mathbf{i}\mathbf{t}\mathbf{y}=\frac{\mathbf{M}\mathbf{a}\mathbf{x}\left(\mathbf{D}\mathbf{i}\mathbf{a} \mathbf{X},\mathbf{D}\mathbf{i}\mathbf{a} \mathbf{Y}\right)-\mathbf{M}\mathbf{i}\mathbf{n}(\mathbf{D}\mathbf{i}\mathbf{a} \mathbf{X},\mathbf{D}\mathbf{i}\mathbf{a} \mathbf{Y})}{2}$$

(1)

3. Results

3.1. Hardness

The measured hardness of the composite workpieces is tabulated as shown in Table 3.

3.2. Taguchi Analysis

Taguchi’s statistical approach gives the way to know the effect of control factors under consideration on the Ra and circularity of drilled holes [21]. Experimental runs according to Taguchi’s L27 orthogonal array and results such as surface roughness (Ra) and circularity at entry and exit of the hole are tabulated as shown in

Table 4. Taguchi’s L27 orthogonal array with responses.

Runs	Feed Rate (mm/rev)	Spindle Speed (rpm)	Drill Dia. (mm)	% Vol. of Si3N4	Surface Roughness (Ra) in Microns	Circularity at Entry (mm)	Circularity at Exit (mm)
1	0.125	300	6	0	5.123	0.0098	0.0088
2	0.125	300	8	5	4.101	0.0083	0.0073
3	0.125	300	10	10	4.494	0.0095	0.0085
4	0.125	580	6	0	7.706	0.0107	0.0103
5	0.125	580	8	5	6.887	0.0102	0.0101
6	0.125	580	10	10	6.996	0.0104	0.0102
7	0.125	1160	6	0	9.142	0.0159	0.0149
8	0.125	1160	8	5	8.456	0.0132	0.0129
9	0.125	1160	10	10	8.772	0.0136	0.0124
10	0.575	580	6	10	8.457	0.0120	0.0109
11	0.575	580	8	0	9.547	0.0128	0.0119
12	0.575	580	10	5	7.772	0.0113	0.0110
13	0.575	1160	6	10	9.253	0.0148	0.0139
14	0.575	1160	8	0	9.612	0.0166	0.0157
15	0.575	1160	10	5	8.567	0.0145	0.0141
16	0.575	300	6	10	5.604	0.0106	0.0105
17	0.575	300	8	0	5.964	0.0110	0.0109
18	0.575	300	10	5	5.199	0.0101	0.0101
19	1.25	1160	6	5	9.418	0.0154	0.0150
20	1.25	1160	8	10	7.732	0.0163	0.0154
21	1.25	1160	10	0	9.732	0.0192	0.0182
22	1.25	300	6	5	5.769	0.0134	0.0129
23	1.25	300	8	10	6.084	0.0128	0.0130
24	1.25	300	10	0	6.785	0.0135	0.0134
25	1.25	580	6	5	7.352	0.0116	0.0112
26	1.25	580	8	10	7.657	0.0109	0.0109
27	1.25	580	10	0	9.306	0.0121	0.0121

. Holes created on the workpieces by drilling are shown in Figure 5.

The influence of control factors on the responses is investigated using Taguchi’s technique, where experimental results of each run are transformed into S/N ratio values. The smaller of the better condition is employed in the analysis of S/N ratios of all the responses. The response table for S/N ratios for Ra is mentioned in

Table 5. Response table for S/N ratios for Ra.

Level	Feed Rate (mm/rev)	Spindle Speed (rpm)	Drill Dia. (mm)	% Vol. of Si3N4
1	−16.39	−14.65	−17.34	−17.96
2	−17.6	−17.97	−17.05	−16.71
3	−17.66	−19.03	−17.26	−16.98
Delta	1.27	4.38	0.28	1.26
Rank	2	1	4	3

. The response table for S/N ratios for circularity at the entry and exit of holes is mentioned in

Table 6. Response table for S/N ratios for circularity at entry.

Level	Feed Rate (mm/rev)	Spindle Speed (rpm)	Drill Dia. (mm)	% Vol. of Si3N4
1	39.13	39.29	38.06	37.59
2	38.08	38.93	38.28	38.57
3	37.26	36.25	38.13	38.31
Delta	1.86	3.04	0.23	0.99
Rank	2	1	4	3

and

Table 7. Response table for S/N ratios for circularity at exit.

Level	Feed Rate (mm/rev)	Spindle Speed (rpm)	Drill Dia. (mm)	% Vol. of Si3N4
1	39.7	39.67	38.52	37.99
2	38.42	39.22	38.62	38.87
3	37.47	36.69	38.45	38.72
Delta	2.23	2.98	0.17	0.89
Rank	2	1	4	3

, respectively.

3.3. ANOVA

The significant contribution of feed rate, spindle speed, % Vol. of S₃N₄ and diameter of drill bit on Ra and circularity of the hole at entry and exit is determined by ANOVA. ANOVA for S/N ratios and the effect of process parameters on the responses are shown in

Table 8. Analysis of variance for SN ratios (Ra).

Responses	Ra	Circularity at Entry	Circularity at Exit
Source	% Contribution	% Contribution	% Contribution
Feed Rate (mm/rev)	15.29	20.075	26.87
Spindle Speed (rpm)	56.57	63.579	55.39
Drill Dia. (mm)	0.65	0.310	0.16
% Vol. of Si3N4	13.09	6.013	4.84
Residual Error	14.40	10.023	12.75
Total	100	100	100

.

4. Discussions

4.1. Effect of Spindle Speed and Feed Rate on Ra and Circularity

From the experiment, it is clearly evident that the Ra value significantly increases as the spindle speed increases from 300 rpm to 1160 rpm. The average Ra value increases from 6.853 μ to 7.775 μ with an increase in feed rate from 300 mm/rev to 580 mm/rev, and further decreases to 7.75 μ with an increase in feed rate from 580 mm/rev to 1160 mm/rev. variation of Ra with spindle speed and feed rate is shown in Figure 6a. The combined effect of feed rate and spindle speed on Ra is shown in Figure 6b. It is evident from Figure 6a that better Ra is obtained at lower feed (0.125 mm) and speed (300 rpm). The circularity of the hole at entry and exit deteriorates as the spindle speed increases from 300 rpm to 580 rpm and further deteriorates more at 1160 rpm. Circularity is significantly affected by spindle speed as compared to feed rate. The circularity of the hole at entry and exit worsens as the feed rate increases from 0.125 mm/rev to 0.575 mm/rev., and further deteriorates as the feed rate increases from 0.575 mm/rev to 1.25 mm/rev. Circularity at exit is better than at entry of the hole due to excessive thrust force. The variation of circularity at the entry and exit of the hole with spindle speed and feed rate is shown in Figure 7.

4.2. Effect of Diameter of Drill Bit and % Vol. of Si₃N₄ Reinforcement on Ra

The effect of the diameter of the drill bit on Ra is shown in Figure 8a. The diameter of the drill bit has no significant effect on Ra, and a superior Ra is attained at 8 mm dia. Ra of the hole surface improves with Si₃N₄ reinforcement up to 5% and deteriorates further with Si₃N₄ reinforcement up to 10%. The effect of Si₃N₄ reinforcement on Ra is shown in Figure 8b.

4.3. ANOVA

Table 3 shows that hardness increases with an increase in % vol. of Si₃N₄ reinforcement in Al-Si₃N₄ MMC and maximum hardness of 48.4 RHN is obtained at 10% vol. of Si₃N₄. The main effects for Ra and circularity at entry and exit are plotted in Figure 9 and Figure 10a,b. Figure 9 shows the effect of process parameters on Ra. It reveals that optimum parameters for Ra are obtained as feed rate at level 1 (0.125 mm/rev), spindle speed at level 1 (300 rpm), the diameter of drill at level 2 (8 mm) and % Vol. of Si₃N₄ at level 2 (5%). Figure 10a shows the effect of process parameters on the circularity of the hole at entry. Figure 10b shows the effect of process parameters on the circularity of the hole at the exit. It reveals that optimum parameters for circularity of the hole at entry and exit and are obtained as feed rate at level 1 (0.125 mm/rev), spindle speed at level 1 (300 rpm), the diameter of drill at level 2 (8 mm) and % Vol. of Si₃N₄ at level 2 (5%).

The ANOVA results in Table 8 reveal the significant contribution of each parameter on Ra. Spindle speed has more effect on Ra with its contribution of 56. 57%. The histogram for Ra is plotted in Figure 11. ANOVA for S/N ratios in Table 8 shows the relative contribution of process parameters on circularity at both the entry and exit of the hole, respectively. Spindle speed has more effect on circularity with its contribution of 63.579% at entry and 55.39% at the exit of the hole. The histogram for circularity at the entry and exit of the hole is plotted in Figure 12a,b, respectively. Confirmation experiments are carried out for the optimum combination of process parameters and are in good agreement with the responses with an error of 1%.

4.4. Microscopic Study of Drilled Hole

Figure 13a,b show a scanning electron microscopy (SEM) image of the drilled hole with optimum parameters. Figure 13a,b shows the aggregation of Si₃N₄ and matrix burn, which result in roughness on the surface of the drilled hole.

5. Conclusions

This paper presents the effect of process parameters, such as feed rate and spindle speed on quality characteristics of the hole such as surface roughness (Ra), and circularity at entry and exit in the drilling of Al6061, MMC reinforced with Si₃N₄ at three different volume proportions (0%, 5% and 10%), fabricated using the stir casting technique. Conclusions drawn from the results and discussions are as follows:

• Optimum parameters for Ra, circularity of hole at entry and exit are obtained as feed rate at level 1 (0.125 mm/rev), spindle speed at level 1 (300 rpm), diameter of drill at level 2 (8 mm), and % Vol. of Si₃N₄ at level 2 (5%);
• From ANOVA results, it is concluded that spindle speed is the most influential parameter on Ra and circularity of hole at entry and exit followed by feed rate.