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Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded Aluminium Alloy 6063-O



A. Varun Kumar and K. Balachandar
 
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ABSTRACT

The effect of process parameters on metallurgical properties of friction stir welded aluminium alloy 6063-O was analysed in the present study. Samples were friction stir welded under tool rotational speed of 600, 800 and 1200 rpm and traversing speed of 0.6, 0.9 and 1.2 mm sec-1, with an axial load of 8000 kg constant for all trials and the process parameters were optimized by using Taguchi orthogonal array. Optical microstructure analysis were carried out to define the metallurgical properties at various zones of friction stir welded samples (Unaffected Base Material, Heat Affected Zone, Thermo Mechanically Affected Zone and Weld Nugget Zone) and scanning electron microscopy analysis (SEM) was carried out to determine the material flow path at the heat affected zone (HAZ) and weld nugget zone (WN) of friction stir welded samples. In present work two sets of experiments were carried out on AA 6063-O one with silicon carbide powder and without silicon carbide powder. Tool used for the FSW process is high carbon steel D3 (Heat treated 58-60 HRC), the SiC powders were inserted along the breadth of the plates by making a drill of 1 mm to a depth of 15 mm. Micro hardness survey was done across the weld regions using vickers hardness test. Correlation of micro hardness tests and metallurgical properties of the friction stir weldments were studied by optical microscope analysis, scanning electron microscopy analysis and the samples were chosen for (SEM) using grey relational analysis (GRA). It was observed that the sample 9 with SiC powders, welded with a traverse speed of 1.2 mm sec-1, tool speed of 1200 rpm and axial load of 8000 Kg showed the best behavior.

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  How to cite this article:

A. Varun Kumar and K. Balachandar, 2012. Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded Aluminium Alloy 6063-O. Journal of Applied Sciences, 12: 1255-1264.

DOI: 10.3923/jas.2012.1255.1264

URL: https://scialert.net/abstract/?doi=jas.2012.1255.1264
 
Received: March 10, 2012; Accepted: March 24, 2012; Published: June 30, 2012



INTRODUCTION

Friction Stir Welding (FSW) is a solid state joining technology is one of the environmental friendly fabrication techniques involving energy efficiency and versatility to provide satisfactory combination of microstructure and mechanical properties of the weld (Nandan et al., 2008). During processing, a non consumable tool attached with desired designed pin is inserted to butting edge of the plates to be joined. Tool shoulder should touch the plate surface. Under this condition tool is rotated and traversed along the bond line. Frictional heat is generated, material gets softened locally and plastic deformation of the work piece occurs. The process is suitable for joining plates and sheets; however it can be employed for pipes, hollow section and positional welding. Since its introduction, the process has application for low melting alloys which are difficult to be joined by conventional fusion welding. In this respect aluminum alloys need special attention for their specific properties like low melting point, high strength to weight ratio, low density, substantial ductility, adequate corrosion resistance, satisfactory cryogenic properties, non-magnetic nature, ease of forming and machining.

Fusion welding of Al alloy creates voids, hot cracking, distortion in shape, precipitate resolution and work hardening. In present work, we carried FSW on similar Aluminium Alloy by varying the process parameters of tool and traverse speed. FSW may be one of the feasible solution; however the, weldability of Al alloys during friction stir welding varies depending upon their as received condition. Joining of similar and dissimilar Al alloy can also be carried out by FSW with different process parameters (Yeni et al., 2008). For heat treatable Al alloys, FSW processing generates a softened region within the weld zone because of the dissolution of strengthening precipitates. For non-heat treatable Al alloys, softening is not observed, when alloys are not sensitive to strain hardening. However non-heat treatable and sensitive strain hardening aluminum alloys exhibit softening in weld zone owing to decrease in dislocation density.

EXPERIMENTAL PROCEDURE

Materials: The material used in the present investigation is Aluminium Alloy 6063-O and tool used for the process is High Carbon Steel D3 and their chemical compositions are given in the Table 1 and 2, respectively.

Geometry of parts: The plates were machined according to the geometry of (200x150x6) mm. The tool designed with the following dimensions, diameter of the rod 18 mm, shoulder diameter 15 mm, pin diameter 4.8 mm, pin length has been set as 4.8 mm and the tool has been hardened to 58-60 HRC.

Experimental setup: A setup was designed and constructed according to Friction Stir Welding (FSW). The experiments were done by varying the traverse speed (mm sec-1), rotational speed (rpm) and axial load is kept constant as 8000 kg for all trials. Two sets of experiments were carried one with SiC powder and other set of experiment without SiC powder. The design of experiments was constructed by Taguchi’s orthogonal array design (Fig. 1).

Table 1: Chemical composition of AA 6063-O in (wt. %) (Alcoa, 2006)
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Table 2: Chemical composition of high carbon steel D3 in (wt.%), Latrobe specialty steel company (2006)
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Taguchi’s experimental design: The design of experiments (DOE) approach is important to determine the behavior of the objective function we are examining because it is able to identify which factors are more important. The choice of DOE depends mainly on the type of objectives and on the number of variables involved. This can be done with just a fraction of the runs, using only a “high” and “low” setting for each factor and some center points when necessary. Therefore DOE statistical techniques are especially useful in complex physical processes, such as welding (Vairis and Petousis, 2009). The experimental design proposed by Taguchi involves using orthogonal arrays to organize the parameters affecting the process and the levels at which they should be varied; it allows for the collection of the necessary data to determine which factors mostly affect product quality with a minimum amount of experimentation, thus saving time and resources. With 2 process parameters and 3 levels of each parameter we fall into L9 array (Table 3) from Taguchi’s orthogonal array selector (Taguchi, 1986). Process parameter table is shown in Table 4.

Sample preparation for optical microscopy study: The samples were prepared for optical microscopy study to the following dimensions (50x30x6) mm the samples were manually polished and etched by using Keller’s reagent of following concentrations (i) Conc. HNO3: 2.5 mL (ii) Conc. HCL: 1 mL (iii) Conc. HF: 1 drop (iv) H2O: 180 mL) (Cakir, 2008).

Sample preparation for SEM analysis: Similarly the friction stir welded samples were prepared for scanning electron microscopy analysis (SEM) to the following dimensions (5x1x1).

Grey relational analysis: The overall performance characteristic of the multiple response process depends on the calculated grey relational grade.

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 1: FSW process

Table 3: Orthogonal array
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Table 4: Friction stir welding process parameters
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

This approach converts a multiple response process optimization problem into a single response optimization situation with the objective function is overall Grey relational grade. The optimal parametric combination is then evaluated which would result highest grey relational grade. The optimal factor setting for maximizing overall grey relational grade can be performed by Taguchi method (Aydin et al., 2010).

Optimal samples for SEM using GRA: From the two sets of experiments one with SiC powders (Table 5) and without SiC powders (Table 6) the optimal samples were chosen for SEM analysis from larger the better expressions, because higher the hardness value gives better weld strength. From the calculated values of normalized values, grey relational coefficient, grey relational grade and rank in Table 5 and 6, it is observed that sample number 6 and 9 revealed to be the optimal samples for SEM analysis by correlating with the optical microstructures study and micro hardness values in the weld nugget zone along the cross section of weldments.

Larger the Better formula can be expressed as follows:

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Where,

xi (k) = Value after the Grey relational generation
min. yi (k) = The smallest value of yi(k) for the kth response
max. yi (k) = The largest value of yi(k) for the kth response

Table 5: Larger the better values of friction stir welded samples with SiC powders
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Table 6: Larger the better values of friction stir welded samples without SiC powders
Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Grey relational coefficient can be expressed as follows:

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Where,

Δ oi = xo (k) - xi (k)
ψ = 0.5
Δ min = xo (k) - xi (k)
Δ max = x0 (k) - xi (k)

Grey relational grade:

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O

Where,

yi = Grey relational grade
n = Number of process responses

Model Calculation Using Larger the Better Expression for Sample 6:

(NV) xik= [(41.4 – 41.4)]/[(62.2 – 41.4)]
xik= 0
(GRC) Øik= [(0 – 0.5 x 1)]/[(1-0) + 0.5]
Øik = 0.3333
(GRG) yi= [(0.3333)]/[(9)]
yi= 0.0370

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 2(a-g) : Optical microstructures of friction stir welded sample 6 without SiC powders, (a) HAZ: advancing side, (b) HAZ: retreating side, (c) TMAZ: advancing side, (d) TMAZ: retreating side, (e) UA: advancing side, (f) UA: retreating side and (g) Weld nugget

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 3(a-g): Optical Microstructures of friction stir welded sample 9 with SiC powders, (a) HAZ: Advancing side, (b) HAZ: retreating side, (c) TMAZ: Advancing side, (d) TMAZ: retreating side, (e) UA: Advancing side, (f) UA: retreating side and (g) Weld nugget

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 4(a-c): SEM Micrographs of friction stir welded sample 6 without SiC powders, (a) HAZ: Advancing side, (b) HAZ: retreating side and (c) Weld nugget

RESULTS AND DISCUSSION

Metallographic examination: Figure 2a-g shows various zones of friction stir welding samples consists of HAZ, TMAZ, UA and WN it can be observed in weld nugget (Fig. 2g) that the grains are less equiaxed when compared to that of unaffected material due to stirring process and no porosity formation in the nugget zone (Rodrigues et al., 2009; Reddy et al., 2007; Sato et al., 1999; Barcellona et al., 2006; Moreira et al., 2008; Shukla and Shah, 2010; Fonda and Bingert, 2004). The observed to be fine at the advancing side of HAZ (Fig. 2a) to that of retreating side due to the plasticized movement of material which takes place in the stirring process of FSW and also due to variation in the process parameters such as tool speed and traverse speed (Cavaliere et al., 2006; Cavaliere et al., 2009; Lorrain et al., 2010; Leal and Loureiro, 2008; Ghosh et al., 2010). In TMAZ zone there is slight variation of grain size observed due to the effect of adjacent side of nugget zone (Yeni et al., 2008; Cavaliere et al., 2005).

Figure 3a-g shows the optical microstructures of AA 6063-O which composed of SiC powders. The thermo-mechanically affected zone (TMAZ) Fig. 3c and d which is adjacent to the weld nugget either the retreating side orthe advancing side, has been plastically deformed and thermally affected. The distribution of the SiC powders in the nugget region is finer, suggesting that re-arrangement of the particles had taken place during friction stir welding due to the high deformation and stirring (Fig. 3g) (Uzun, 2007). The dark angular particles are defined to be SiC powders at various zones of friction stir welded samples (Hassan et al., 2009; Feng et al., 2008).

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 5(a-c): SEM micrographs of friction stir welded sample 9 with SiC powders (a) HAZ: Advancing side, (b) HAZ: retreating side and (c) Weld nugget

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 6: Micro hardness profile of friction stir welded samples without SiC powders

It is observed that the nonuniform distribution of SiC powders at the unaffected zone and after FSW process SiC powders in the nugget zone are significantly improved (Nandipati et al., 2010).

Scanning electron microscope (SEM) analysis: With the support of Grey Relational Analysis (GRA) the chosen optimal samples were analyzed with Scanning Electron Microscopy (SEM) to conclude the best optimal process parameters which, I have used for the FSW process. The SEM analysis were carried out over two samples (6 and 9) one without SiC powder and with SiC powder.

Voids in the weld nugget are generated owing to difference in material transport (Ghosh et al., 2010) due to lower traverse speed of the tool over the material (Fig. 4c). The weld nugget experiences high strain and is prone to recrystallization.

Image for - Effect of Welding Parameters on Metallurgical Properties of Friction Stir Welded 
  Aluminium Alloy 6063-O
Fig. 7: Micro-hardness profile of friction stir welded samples with SiC powders

The micrographs of the friction stir welding plates reveal that the grain refinements at the HAZ of advancing side and it is observed that the material flow carried from the retreating side to advancing side of HAZ (Fig. 4a and b) (Karthikeyan and Kumar, 2011).

The weld nugget reveals homogenous distributions of SiC powder and there are no pores or voids observed in the nugget due to maximum tool speed and traverse speed (Fig. 5c) (Uzun, 2007; Rao and Das, 2011). The grain size in HAZ of advancing side (Fig. 5a) is fine than that in nugget zone, the HAZ zone to be termed as weakest zone of the friction stir welding process (Nandipati et al., 2010). It is also observed that the material flow from the retreating side of the HAZ (Fig. 5b) to the advancing side of HAZ due to plasticized flow of materials which leads to deposition of SiC powder in the advancing side.

Micro hardness test: The micro hardness test was carried across the cross sections of the friction stir welded samples to define the hardness of various weld zones with a load of 100 gf. From the Fig. 6 and 7 shown below which shows two sets of experiments one without SiC powder and other set with SiC powder. We can observe that the weld nugget i.e., center of the weld has higher hardness values when compared to that of other zones of weld regions (Unaffected Material, Heat Affected Zone and Thermo Mechanically Affected Zone). The increase in micro hardness at the nugget zone is caused by intensive stirring process. And the hardness values are observed to be similar at the other regions due to friction stir process i.e., plasticized flow of materials both at the advancing and retreating sides of friction stir welded samples (Adamowski and Szkodo, 2007).

CONCLUSION

Friction Stir Welding (FSW) over Aluminium Alloy 6063-O without and with SiC powders was successfully carried out. The Optical microstructure study were made on the following zones (i) Unaffected Base Material (UA) (ii) Heat Affected Zone (HAZ) (iii) Thermo Mechanically Affected Zone (TMAZ) both Advancing Side (AS), Retreating Side (RS) and (iv) Weld Nugget Zone (WNZ). Micro Hardness Values at the weld nugget zone were observed to be higher in the SiC powders inserted plates of AA 6063-O compared to that of other set of experiments carried over the plates of AA 6063-O without SiC powders, due to the distribution of SiC powders at the nugget. Optical Microstructure Analysis and Scanning Electron Microscope Analysis (SEM) clearly shows that the weld nugget zone consists of relatively better distributions of SiC powders due to stirring process in FSW. Thus, from the correlation of Optical Microstructure Analysis, Scanning Electron Microscope Analysis (SEM), Grey Relational Analysis (GRA) and Micro Hardness Values it is revealed that sample 9 with SiC powders were found to be optimum which gives higher hardness value at the weld nugget zone and no voids were observed in the nugget with a traverse speed of 1.2 mm sec-1, tool speed of 1200 rpm and axial load of 8000 Kg.

ABBREVIATIONS

AA : Aluminium alloy
FSW : Friction stir welding
HAZ : Heat affected zone
TMAZ : Thermomechanically affected zone
UA : Unaffected Zone
WN : Weld nugget zone
AS : Advancing side
RS : Retreating side
SiC : Silicon carbide powder
SEM : Scanning electron microscope
TS : Traverse speed (mm sec-1)
TS : Tool speed (rpm)
NV : Normalized values
GRA : Grey relational analysis
GRC : Grey relational coefficient
GRD : Grey relational grade
R : Rank

ACKNOWLEDGMENTS

The authors gratefully thank the technical support of Dr. K. Prasad Rao, Dr. G. Phanikumar and Dr. H. Khalid Rafi for providing the equipment to conduct experiments at “Department of Metal Joining Laboratory IIT Madras”, Chennai.

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