Effect of Friction Stir Processing Process Parameters on the ...

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Effect of Friction Stir Processing Process Parameters on the Mechanical Properties of AZ31B Mg Alloy Ganta Venkateswarlu 1 – M Joseph Davidson2 – Pulla Sammaiah3 1

Department of Mechanical Engineering, SCCE Karimnagar, A.P, India, [email protected] Department of Mechanical Engineering, NIT Warangal, A.P, India-506004 , [email protected] Department of Mechanical Engineering, S.R. Engineering College, Warangal, A.P, India-506004, [email protected]

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Keywords

Abstract

Friction Stir Processing AZ31B Mg Alloy Grain Refinement Taguchi

In this investigation, the effect of friction stir processing (FSP) parameters such as rotational speed, traverse speed and tool tilt angle on the mechanical properties of tensile strength, hardness and impact energy of magnesium alloy AZ31B was studied. The experiments were carried out as per Taguchi parametric design concepts and an L9 orthogonal array was used to study the influence of various combinations of process parameters. Statistical optimization technique, ANOVA was used to determine the optimum levels and to find the significance of each process parameter. The results indicate that rotational speed (RS), and traverse speed (TS) are the most significant factors, followed by tilt angle (TA) in deciding the mechanical properties of friction stir processed magnesium alloy. In addition, mathematical models were developed to establish relationship between different process variables and mechanical properties.

Article

History

Received 26 November 2013 | Revised 8 April 2014 | Accepted 29 April 2014

Category

Professional Paper

Citation

Venkateswarlu G, Davidson M J, Sammaiah P (2014) Effect of Friction Stir Processing Process Parameters on the Mechanical Properties of AZ31B Mg Alloy. Journal of Manufacturing and Industrial Engineering, 1-2(13): 1-5, doi:10.12776/mie.v13i1-2.338

INTRODUCTION In recent years, the magnesium alloys have been widely used for structural components, especially in automotive industry due to its favourable characteristics of light weight, recycling and high specific strength [1]. However, the use of magnesium alloy has been strongly limited in forming because of its poor ductility and formability at room temperature as a result of hcp lattice structure. AZ31B magnesium alloy is commercially available in sheet form, and offers good properties. This alloy exhibits very limited ductility at room temperature. However, the recent results indicate that it is possible to improve the ductility and formability of magnesium sheet at elevated forming temperatures under certain conditions [2]. The results also proposed that improved mechanical properties can be attained by refining and homogenising the grain structure of the sheet. It can be done by a variety of processes like thermo-mechanical treatment (TMT), equal-channel angular process (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB), etc. All are complex and time consuming processes. Friction stir processing (FSP) is an emerging surface engineering technique based on the principles of friction stir welding (FSW), a solid state joining process, developed initially for aluminium alloys by The Welding Institute (TWI) of United Kingdom (UK) in 1991. FSP locally eliminates inherent casting defects and dramatically refines the grain structure, thereby improves properties of strength, ductility, formability etc [3-6]. Friction stir processing has been successfully applied to many aluminium alloys in improving mechanical properties [7-10]. But very limited work has been carried out on friction stir processing of magnesium alloys. Sato et al [11] determined the effect of FSP on microstructure of AZ91 magnesium alloy and observed that more grain refinement and homogenization when compared to the cast properties. http://dx.doi.org/10.12776/mie.v13i1-2.338

Darras et al [12] studied the effect of various friction stir processing parameters on the thermal histories and properties of commercial AZ31B-H24 magnesium alloy sheet. They refinement and homogenization of microstructure is more an observation in a single pass. Fine grain size can be obtained in a single pass friction stir processing through severe plastic deformation and control of heat input during processing [13]. More studies are required to investigate the influence of FSP process variables on the resulting microstructure and mechanical properties of magnesium alloy. In the present work, a statistical approach based on Taguchi and ANOVA techniques was adopted to determine the degree of importance of each process parameter on the variance of mechanical properties of friction stir processed magnesium alloy. Mathematical models have been developed between the process variables and its mechanical properties.

METHODOLOGY Taguchi proposed that the engineering optimization of a process should be carried out in three step approaches: the system design, the parameter design and the tolerance design [14].Taguchi method uses orthogonal arrays from design of experiments theory to study a large number of variables with a small number of experiments. The orthogonal arrays reduce the number of experimental configurations to be studied. Furthermore, the conclusions drawn from small scale experiments are valid over the entire experimental region spanned by the control factors and their settings [15]. Orthogonal arrays are not unique to Taguchi [16]. However, Taguchi has modified their use by providing tabulated sets of standard orthogonal arrays and corresponding linear graphs to fit specific projects [17]. The experimental results are then transformed into a signal-to-noise (S/N) ratio. It uses the S/N

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Venkateswarlu G, Davidson M J, Sammaiah P

Effect of Friction Stir Processing Process Parameters on the Mechanical Properties of AZ31B Mg Alloy

ratio as a measure of quality characteristics deviating from or nearing to the desired values.

FSP process parameters The friction stir processing parameters such as the tool rotational speed, the traverse speed and the tool angle play a major role in influencing microstructure, and therefore the mechanical properties are increased. In the present investigation, three process parameters namely, tool rotational speed, traverse speed and tool tilt angle are considered. Pilot experiments were carried out using 4 mm thick rolled sheet of magnesium AZ31B to determine the working range of FSP process parameters. The chemical composition of the base metal (Mg AZ31B-O) used in this investigation is given in Table 1. Table 1 Chemical composition of Mg AZ31B-O (Wt %) Element Al Zn Mn Si Ni % Wt

3.02

0.89

0.29

0.026

.0009

Fe

Mg

0.0025

Balance

When the tool rotational speed is lower than 900 rpm, worm hole defect is observed due to insufficient heat generation and insufficient metal filling whereas a tunnel defect was found due to excessive heat generation when the rotational speed is higher than 1400 rpm. When the traverse speed is lower than 24 mm/min, pin holes are observed due to excessive heat generation and a tunnel defect is found due to insufficient heat input caused by inadequate flow of metal, when the traverse speed is greater than 40 mm/min. Defect free surface was obtained, for a tool tilt angle of 0 to 2o. Based on the above experiments, the range of process parameters is fixed as 9001400 rpm for rotational speed, 24-40 mm/min for traverse speed and 0-2o for tool tilt angle.

Selection of orthogonal array (OA) Experiments have been carried out using Taguchi’s L9 Orthogonal Array (OA) experimental design which consists of 9 combinations of rotational speed, traverse speed and tool tilt angle. It considers three process parameters (without interaction) to be varied in three discrete levels. As per Taguchi experimental design philosophy, a set of three levels assigned to each process parameter, has two degrees of freedom (DOF). This gives a total of 6 DOF for three process parameters. Thus we have a total of 8 DOF for the factors for the present experiments. The nearest three level orthogonal array available satisfying the criterion of selecting the OA is L9 having 8 DOF. The experimental design is shown in Table 2. Table 2 Experimental design Tensile strength(N/m m2)

Micro hardness( Hv) (Kgf/mm2)

Impa ct ener gy (J)

0

190

78

3.0

32

1

237

83

4.5

Rotatio nal speed (RPM)

Travers e speed (mm/mi n)

Tilt angle (Degre e)

900

24

900 900

40

2

222

81

6.0

1150

24

1

239

85

3.0

1150

32

2

242

83.6

4.0

1150

40

0

226

87

2.5

1400

24

2

181

76.1

2.0

1400

32

0

196

83

3.5

1400

40

1

179

81

4.0

215

79.0

4.0 4.0

Parent material

2

EXPERIMENTAL WORK In this investigation, the rolled sheet of magnesium AZ31B alloy was used as the base material, which was supplied by Xi’an Yuchen Metal Products Co., Ltd, China. The sheet of 4 mm thickness was cut into the required size (150 x 100 mm) using milling machine. The friction stir processing was done on vertical milling machine with the position of the tool fixed relative to the surface of the sheet as shown in Fig.1. The work piece was firmly clamped to the bed and a specially made tool was plunged in to the selected area of the material sheet for sufficient time in order to plasticize around the pin. After adequate plasticization, the tool is traversed across the surface of the material for a single pass. The entire sheet was processed with 7-8 overlapped processes. A non-consumable taper threaded tool made of high carbon steel H13 with a shoulder diameter of 18 mm and 6 mm pin diameter of length 3 mm was used. The FSP experiments were conducted on the sheet in rolling direction as per the selected orthogonal array.

Figure 1 Frictions stir processing set up

Tensile testing was carried out on a universal testing machine to find the tensile strength and average of three test results were used to analyse the tensile strength of the samples that were cut along the processing direction as per ASTM E8M04 guidelines. Vickers micro hardness measurement was done using Vickers micro hardness tester with a load of 0.5 kgf and a dwell period of 10 seconds. Three FSPed specimens with dimensions 3 × 10 × 55 mm were prepared for charpy notch impact tests. The specimens for microstructural analysis were sectioned to the required size and then polished using different grades of emery papers and etched with a standard reagent made of 4.2 g picric acid, 10 ml acetic acid, 10 ml diluted water, and 70 ml ethanol. The microstructure of the base material and processed samples was examined by optical microscope.

RESULTS AND DISCUSSIONS Influence of parameters on mechanical properties It can be observed from the Table 1 that the mechanical properties have been considerably improved by friction stir processing. The mechanical properties are found to be lower at lower rotational speed (900 rpm). This is due to the insufficient heat generation resulting in poor plasticisation zone and insufficient deformation by poor stirring action and insufficient deformation by the tool pin that may be reasons to decrease in properties . The resulting properties of material can be improved by providing sufficient heat to plasticise the material in order to ensure complete deformation with proper material flow and grain refinement through dynamic recrystallisation using higher rotational speeds. Again from Table 1, it is clear that the tensile strength and hardness has been improved at rotational speed of 1150 rpm, as the required heat input coupled with stirring to the material has resulted in greater http://dx.doi.org/10.12776/mie.v13i1-2.338


refinement of the grains. Further increase in the rotational speed (1400 rpm) has resulted in higher temperatures in the stirred zone than the optimal, leading to grain growth that ultimately has decreased the tensile strength as well as hardness. Therefore, optimal rotational speed must be used to avoid grain growth and incomplete deformation. The best mechanical properties were obtained at a rotational speed of 1150 rpm. The variations in the micro hardness values at various distances from the nugget centre line is observed at all the conditions except where higher heat is generated, the stir zone (SZ) exhibits higher average hardness than the base metal due to the formation of homogeneous equiaxed fine grains. The micro hardness values at the region away from the nugget zone are found to be closer to the parent material hardness. This is due to the insufficient deformation and thermal exposure in thermo-mechanical affected zone (TMAZ) on advancing side (AS) and retreating side (RS) side. This tendency suggests that the significant increase of hardness is due to mainly the grain refinement. The relationship between the hardness in the stirred zone and its grain size was examined. The hardness followed the HallPetch relationship. The impact energy decreased with increase in rotational speed and is higher for lower rotational speed. Traverse speed also affects the mechanical properties of processed material. A lower traverse speed (24 mm/min) results slower cooling rate that allows sufficient time for grain growth leading to lower tensile strength and hardness. The time of the exposure of processed area to frictional heat generated from the rotating tool is controlled to limit the amount of grain growth. There is an optimum value of traverse speed to obtain better mechanical properties. In the present work, the best results were obtained at a traverse speed of 32 mm/min and for all the rotational speeds. Similar effect was observed for impact energy. Small increment on tilt angle helps to improve the properties as it increases the homogenisation of material with better stirring action.

STATISTICAL ANALYSIS The statistical analysis of the data was done in three phases. In the first phase, ANOVA was done to find the effect of process parameters and their contribution to responses, in the second phase, the relationships between the responses and the friction stir processed parameters were established and in the third phase, multi objective optimisation of process parameters were done.

Analysis of Variance ANOVA (analysis of variance) is a statistical technique for determining the degree of difference or similarity between two or more groups of data. It is based on the comparison of the average value of common components. The percentage contribution of various process parameters to the selected performance characteristic can be estimated by ANOVA. Taguchi recommended a logarithmic transformation of mean square deviation called signal-to-noise ratio (S/N ratio) for analysis of the results. Signal-to-noise ratio (SNR) is utilized to measure the deviation of quality characteristic from the target. In this investigation, the S/N ratio was chosen according to the criterion, the “larger-the-better” in order to maximize the responses. The S/N ratio for the “larger-the-better” target for all the responses was calculated as follows. The formula used for computing S/N ratio is given below.

http://dx.doi.org/10.12776/mie.v13i1-2.338


Larger the better: S/ N ratio (η) = -10 log10

1 n 1 n i 0 Y i2

(1)

Where n is the number of experiments (for one set of parameters n=1) and Yi is the response for ith experiment. The experimental results were transformed into signal-to-noise (S/N) ratio using statistical software. The S/N ratio values of all levels are calculated for all properties and presented in Tables 35. The main effects plots for S/N ratio of tensile strength, micro hardness and impact energy are shown in Fig 2. Larger S/N ratio corresponds to better quality characteristics. Therefore, the optimal level of process parameter is the level of highest S/N ratio [16]. Table 3 S/N Tensile strength Level

RS

TS

TA

1

46.74

46.10

46.24

2

47.34

47.02

46.69

3

46.33

46.33

46.52

Delta

1.98

0.92

0.44

Rank

1

2

3

Table 4 S/N Micro hardness Level

RS

TS

TA

1

38.02

37.98

38.37

2

38.64

38.47

38.38

3

38.06

38.27

37.97

Delta

0.62

0.49

0.41

Rank

1

2

3

Level

RS

TS

TA

1

12.72

8.36

9.46

2

9.84

11.99

11.54

3

9.64

11.85

11.20

Delta

3.07

3.62

2.088

Rank

2

1

3

Table 5 S/N Impact energy

Table 6 shows the results of ANOVA with the properties of tensile strength, impact energy and micro hardness. It is observed that the rotational speed (73.27%) and traverse speed (17.03%) have more statistical influence on the tensile strength. The tool tilt angle (5.814%) presents low percentage of statistical significance contribution on tensile strength variation. The rotational speed has 52.15% contribution, traverse speed has 25.37% contribution and tool tilt angle has 14.98% contribution to micro hardness. The traverse speed (35.61%) has most statistical influence on the variance of impact energy followed by rotational speed (31.23%) and tool tilt angle (15.11%).

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Figure 3 Optimization Values Table 7 Values of process parameters for the optimization of responses Tensile

Micro

Strength

hardness

Impact RS

TS

1126

TA

35

(N/mm2)

(Kgf/mm2)

250

87.8

1

% energy(J) 4.06

91.30

Table 8 Comparison of results Properties of FSP

Predicted results

Experimental results

250

246

Impact energy (J)

4.06

4.5

Hardness Kg/mm2

87.8

89.12

Tensile strength (MPa)

Figure 2 Comparison of main effects plots for S/N ratio of tensile strength, micro hardness and impact energy Table 6 ANOVA results % Source

DF

Contribution

%Contribution

%Contribution

Tensile

Impact energy

Micro hardness

31.23 35.61 15.11 18.05 100

52.15 25.37 14.98 7.50 100

strength RS TS TA Error Total

2 2 2 2 8

73.27 17.03 4.63 5.07 100

Multi objective optimisation The optimal process parameters obtained by multiple-response optimization are shown in Table 7 and Fig 3. For the optimized values of the process parameters, it is 91.30% desirable to get the tensile strength of 250 N/mm2 , micro hardness of 87.8 Kgf/mm2 and impact energy of 4.06 J. Any other combination of the process parameters will either be statistically less reliable or give poor results of at least one of the responses. The analysis was done by using the MINITAB 15 computer software. Confirmation experiments were conducted for the above mentioned optimized process condition and the results are tabulated in Table 8.

Mathematical Model Many statistical techniques are concerned with prediction of response. One such technique, which is widely used, is multiple regression analysis .Regression analysis is a statistical technique used to find relationships between variables for the purpose of predicting intermediate values within the range of the level. The use of a single independent variable is known as simple regression analysis, while the use of two or more independent variables is called multiple regression analysis. In this investigation, the relationship among process parameters for a given responses or out comes was modelled. Non-linear regression models are developed based on the experimental results to predict the mechanical properties such as tensile strength, yield strength, percentage of elongation and hardness. It is found that a second order polynomial curve fits the experimental values well. Regression coefficients ‘a0’, ‘a1’ , ‘a2’, ‘a3’, ‘a4’, ‘a5’ and ‘c’ are computed using statistical software, MINITAB 15. The dependent variable is expressed as a function of the process variables as given below. Dependent Variable (σu, E, Hv) = f (RS, TS, TA) Where σu – tensile strength, Hv – micro hardness, RS- rotational speed , TS- traverse speed, TA- tool angle. Dependent Variable = (a0 x RS) + (a1xTS)+ (a2xTA) + {a3x(RS)2} +{a4x(TS)2} + {a5x(TA)2} + c (2) The equations obtained through the above mentioned analysis is as follows σu

=

1.08227RS 2

+

20TS

+

2

20.1667TA-4.98667E-04RS -

2

0.307292TS -8.16667TA -670.898 4


Effect of Friction Stir Processing Process Parameters on the Mechanical Properties of AZ31B Mg Alloy 2

R = 94.93%

(3) 2

Hv = 0.177213RS + 2.05625TS + 1.8833TA-7.76000E-05RS -

[11]

2

0.0289063-1.55000TA -50.2360 2

R =92.51%

(4) 2

E=-0.0272RS+0.677083TS+1.16667TA+1.06667E-05RS 2

[12] [13]

2

0.00911458TS -0.3333TA + 7.78444 2

R =81.95%

(5)

[14]

CONCLUSIONS

[15]

In the present work, an attempt has been made to study the effect of FSP process parameters on the mechanical properties of friction stir processed AZ31B magnesium alloy. The following important conclusions are derived from this investigation. 1. AZ31B magnesium alloy was successfully friction stir processed without any macro level defect under the following range of process parameters: tool rotational speed of 900 -1400 rpm, traverse speed of 24-40 mm/min, and the tool tilt angle of 0-20. 2. Multi objective optimisation of the FSP process parameters was done and maximum mechanical properties were obtained at a rotational speed of 1126 rpm, at a traverse speed of 35 mm/min and at a tool angle of 10. 3. ANOVA analysis test was conducted to determine the significance of each FSP process parameter on the mechanical properties. It is found that tool rotational speed has the highest statistical influence on tensile strength and hardness, whereas, for impact energy, traverse speed has the strongest influence followed by traverse speed. 4. Regression models were developed to predict the mechanical properties for various tool rotational speeds, traverse speeds and tool angles without requiring experimental tests. The validity of the model developed was proved with an experimental test.

[16] [17] [18] [19] [20]


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