SiC surface composites fabricated via Friction Stir Processing (FSP)

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ScienceDirect Materials Today: Proceedings 3 (2016) 4151–4156

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ICMRA 2016

Process parameters optimization for enhanced microhardness of AA 6061/ SiC surface composites fabricated via Friction Stir Processing (FSP) Sandeep Rathee1*, Sachin Maheshwari1, Arshad Noor Siddiquee2, Manu Srivastava1, Satish Kumar Sharma1 1

Division of Manufacturing Processes and Automation Engineering, Netaji Subhas Institute of Technology, New Delhi, India 2 Department of Mechanical Engineering Jamia Millia Islamia University, New Delhi, India

Abstract In this research work, Taguchi’s experimental design is adopted for achieving maximum micro-hardness values for AA6061/SiC surface composites fabricated using friction stir processing. This was achieved by optimizing friction stir processing parameters. Three process parameters namely tool rotational speed, tool traverse speed and tool tilt angle with three levels each were chosen to maximize microhardness values. Nine experiments were performed strictly as per Taguchi L9 orthogonal array. After single pass friction stir processing, microhardness of all the samples were tested on Vickers digital microhardness tester. The obtained results were analyzed with the help of signal to noise (S/N) ratio. The effect of all three parameters on microhardness values was studied. The analysis of signal to noise (S/N) ratio showed that maximum micro hardness was achieved when tool rotational speed, traverse speed and tilt angle were selected as 1400 rpm, 50 mm/min and 2.5˚ respectively.

© 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016. Keywords: Friction Stir Processing; Microhardness; Taguchi Orthogonal Array; process parameters.

* Corresponding author. Tel.: 08745819112; E-mail address:rathee8 @gmail.com

2214-7853© 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016.

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1. Introduction It is well known that aluminium alloys are realistic candidates for various structural applications in marine, automobile and aerospace industries due to their high strength to weight ratio, low weight density and high corrosion resistance [1]. But for several purposes these are not sufficiently stiff and strong and their reinforcement becomes necessary. The reinforced aluminium metal matrix composites exhibit high specific modulus, improved mechanical and tribological characteristics [1-3]. But the addition of hard phase reinforcement makes them brittle [4]. Also, the useful life of components often depends on their surface properties such as wear resistance and hardness. So, in most of cases surface composites are prepared which combines a tough metallic matrix with a hard ceramic reinforcement. However, the dispersion of reinforcement particles on metal surface and the control of its dispersal are more difficult to attain by conventional surface modification techniques [5, 6]. FSP is a newly developed solid state processing technique which is simply an alteration of friction stir welding process which was invented at The Welding Institute (TWI) in 1991. In its basic operation, a non consumable rotating tool with a specially designed pin and shoulder is plunged in to the monolithic plate and traversed in the defined direction to cover up the desired area of interest. Friction between tool and workpiece results in localized heating that softens and plasticizes the workpiece material. During this process, material undergoes intense plastic deformation which results in significant grain refinement. Though, FSP is basically advanced as a grain refinement technique for microstructural and surface properties improvement and removing various defects in cast products, yet it has emerged as an attractive process/route for fabricating the surface metal matrix composites [7]. Mishra et al. [7] done maiden work on surface composite fabrication using Al 5083 alloy as base material and SiC as reinforcement. In this work, a maximum microhardness of 173 Hv was achieved using 27 vol% of SiC particles which is almost double of the microhardness of base metal (85 Hv). After this work, lot of work on surface composite fabrication is reported. Initially FSP was used to modify aluminium alloys but with the passage of time FSP gains shining role in modification of other alloys like magnesium alloys [8], copper alloys [9], titanium alloys [10] and even steel [11]. Tool rotational speed and tool traverse speed are identified as the most influential process parameter of FSP which play a significant role in uniform distribution of reinforcement particles, grain refinement and heat generation during the composite fabrication process via FSP [12]. However, tilt angle of tool also affects the dispersion of reinforcement particles. Aluminium alloy 6061-T6 has wide applications in aerospace, automobile, marine sectors. In this study, the surface composites of SiC reinforced Al 6061 alloys are produced using FSP by evaluating the effect of three process parameters namely tool rotational speed, traverse speed and tilt angle on microhardness of fabricated surface composites. Taguchi technique and MiniTab software is used for experiment design and interpretation of experimental data.

Fig. 1. (a) SEM image of as-received SiC powder;

(b) XRD image of as-received SiC powder

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2. Experimental Procedure The base material employed in this work is a 5 mm thick aluminium 6061-T6 alloy sheet. The sheet was cut in plate form of 60 mm width and 200 mm length. The composition (weight %) of base material is 0.85 % Mg, 0.68 % Si, 0.22% Cu, 0.07% Zn, 0.05% Ti, 0.032% Mn, 0.06% Cr and remaining aluminium. In order to produce surface composites, a square groove with 2mm depth and 2 mm width was m achined out in the middle of the plate along the length direction. Then, SiC powder was packed in the groove so that the groove was completely filled. After that, the groove opening was initially closed by means of a pinless tool (as shown in fig. 3 a) in order to prevent the sputtering of powder during FSP. Finally, FSP was performed at room temperature with a tool having pin on a modified vertical milling machine. The tool made of H-13 steel having of a threaded pin profile with shoulder diameter of 20 mm, pin diameter of 6 mm and pin height of 2.5 mm was used for final experimentation. Nine experiments were performed according to Taguchi Design L9 orthogonal array with 3 columns and 3 rows (Table 1) along with Mini Tab software. The parameters under consideration were selected as: tool rotational speed (rpm), tool traverse speed (mm/min) and tool tilt angle (˚). The selected parameters, their range and array of experimentation are listed in Tables 1 and 2 respectively. The working range of selected parameters was find out by trial experiments. After single pass FSP, the specimens were cut in a direction perpendicular to the direction of FSP route. These samples are then polished using the varying grit sequence of emery grade papers. Microhardness measurement was performed using a Vickers digital microhardness tester under a load of 10 N and dwell time of 15 seconds. The hardness values are taken along a straight line 1.5 mm below the surface in the cross-sections of the specimens. The horizontal distance between two consecutive indentations is 2mm, but only nugget zone reading are taken in this study.

a

Fig. 2. Schematic illustration of FSP process

Fig. 3. (a) Pinless tool;

b

(b) Tool with threaded pin

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Table 1: Working range of selected parameters Symbol Parameters

Levels (1)

(2)

(3)

C1

Tool rotational speed (rpm)

900

1120

1400

C2

Tool traverse speed (mm/min)

31.5

40

50

C3

Tilt angle (˚)

1.5

2

2.5

Table 2: Experimental layout L9 (34) orthogonal array Exp. No. C1 C2 C3

Micro-hardness readings (Hv)

Mean

S/N Ratio

1

1

1

1

96.9

97

96.95

39.73096

2

2

2

2

99.8

99

99.40

3

3

3

3

115.6

116

115.80

4

1

2

3

97.5

96

96.75

5

2

3

1

101.2

100.23

100.715

6

3

1

2

99

99.6

99.30

7

1

3

2

102.78

102.14

102.46

8

2

1

3

98.13

98.79

98.46

9

3

2

1

99.23

98.12

98.675

39.94773 41.27417 39.71302 40.06188 39.93898 40.21109 39.8652 39.88414

Main Effects Plot for Means Data Means

Tool rotational speed

Tool traverse speed

106 104

Mean of Means

102 100 98 1

2 Tilt angle

3

1

2

3

1

106 104 102 100 98

Fig. 4. Main Effect Plot for Mean of hardness values

2

3

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3. Results and discussion 3.1 Signal to Noise ratio (S/N) Analysis Signal to Noise (S/N) ratio is estimated for each process parameter level. Since a higher value of microhardness is desirable so higher-the-better quality characteristic corresponding to a higher S/N ratio corresponds to optimal process parameter level. A framework to assess significance of process parameters is provided and thus optimal combination of process parameters can be predicted. S/N ratio for each selected parameter was calculated for variance minimization in micro hardness values. S/N ratio basically discloses experiment output sensitiveness to the noise factors. Choice of proper S/N ratio is based on in-depth process understanding. For this study, the-higher-thebetter criterion is chosen for optimal response and is calculated according to Equation 1 [13]. .............…. Equation (1) In the above equation, Yi represents the response value at ith level. Also, n is the number of repetitions which is equal to 3 in the present case. Table 2 presents the experimental results by employing Taguchi’s standard orthogonal array. The highest hardness was achieved when C1, C2 and C3 were chosen at combination of tool rotational speed of 1400 rpm, tool traverse speed of 50 mm/min and tool tilt angle of 2.5 degrees levels respectively. Figure 4 and Figure 5 clearly demonstrate the effect of process parameters C1, C2 and C3 on hardness and S/N ratio of responses.

Main Effects Plot for SN ratios Data Means

Tool rotational speed

40.6

Tool traverse speed

40.4

Mean of SN ratios

40.2 40.0 1

2

3

1

Tilt angle

40.6 40.4 40.2 40.0 1

2

3

Signal-to-noise: Larger is better Fig.5. Main Effect Plots for Signal to Noise Ratios

2

3

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3.2 Microhardness The average microhardness hardness of as received 6061-T6 aluminium alloy was 94 Hv. The microhardness values in all the fabricated surface composites were found higher than the base metal as shown in Table 2. The increase in microhardness values is due to combined effect of (i) hard nature of reinforcement particles; (ii) grain refinement and uniform dispersion of reinforcement particles; (iii) pinning of grain boundaries. 4. Conclusions The practical benefit of this study is to identify the most optimal combination of process parameters affecting microhardness of AA6061/SiC surface composite by performing minimum number of experiments using Taguchi technique. The obtained results can be summarized as follows: 1. Optimal results are obtained for a combination of tool rotational speed 1400 rpm, tool traverse speed 50 mm/min, tool tilt angle of 2.5 degrees respectively. 2. Maximum hardness of about 116 Hv was achieved at nugget zone which is much higher than the hardness of base metal (94 Hv). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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