Friction stir welding, A Survey

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International Journal on Theoretical and Applied Research in Mechanical Engineering (IJTARME)

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ISSN (Print): 2319-3182, Volume -7, Issue-2-3, 2018 26

1Amandeep Singh, ²Neel Kanth Grover, ³Vikas Chawla, ⁴Harinder Grover

1,2,3,4 IKGPTU, Kapurthala,

Email: ¹[email protected], ²[email protected],³[email protected], ⁴[email protected]

Abstract— Friction stir welding is solid state welding process in which materials are joined by application of friction welding produced by introducing a rotating tool into the base plates to be joined. FSW is finding its wide application in the aerospace and marine industries due to high strength weld production along with good corrosion resistance characteristics of the weldment. In the present paper a brief study has been done on the various research trends in the friction stir welding.

Index Terms—FSW (Friction stir welding), HAZ (Heat affected zone), TMAZ (Thermo-mechanically affected zone).

I. INTRODUCTION

Friction stir welding (FSW) is a solid–state welding process, [1–3] comprising of a rotating tool with a shoulder and a pin, moves alongside the butting edges of two tightly held plates placed on an assistance plate. The shoulder makes strong contact with the upper surface of the work–piece. High heat produced due to friction at the shoulder surface and to a small amount at the pin surface, softens the work piece material being welded. High plastic deformation and drift of this plasticized metal happen as the tool is moves along the welding direction.

Material is transferred from the front face of the tool to the back edge where it is settled into a joint. Friction stir welding (FSW) is a solid state method invented by the welding institute (TWI) (Thomas, 1991) and now become key welding method for joining of aluminum. Aluminum alloys are widely used in automobile industries, aerospace, ship construction, in offshore structures, in train wagons and trams, and in bridge constructions due to its light weight and higher strength to weight ratio.

II. PROCESS PARAMETERS

For FSW, process parameters plays very important role, basic process parameters are, tool rotation (rpm) in clockwise or anticlockwise direction and welding speed (mm/min) along the line of joint. The tool rotation result in mixing and stirring of plate material in vicinity of the rotating pin and shoulder and the transverse movement of tool transferred the stirred plastic material from the

advancing edge to the trailing edge of the pin and resulted in welded joint. Higher rate of tool rotation results in high heat generation due to high friction heating and result in extreme stirring and mixing of material. However, it should be remembered that frictional combination of work piece and tool surface is key factor that govern the heating rate. So, a lonely high tool rotation rate does not yields in high heating as increasing tool rotation rate will change the coefficient of friction at interface. In adding to the tool rotation rate, few significant process parameters are tool traverse speed and tilt angle or the angle of spindle with respect to the work piece surface. An appropriate tilt of the tool helps in proper handling of stirred material by the shoulder of the tool and efficiently transportation of this stirred material to the back of the pin profile from the front of the pin as the tool moves forward.

Further, the plunge depth, insertion length of the pin into the work pieces (also called target depth) is deciding factor for production of sound welds. The insertion depth of pin is linked with the pin length. Fig 1 is showing the various parameters. When the insertion depth is too small, the shoulder of tool does come in contact with the work piece surface. Thus, rotating shoulder is not capable of handling the stirred material properly to the back of the pin, resulting in producing the welds with surface groove or inner channel. When the plunge depth is too deep, the tool’s shoulder plunges into the work piece generating unnecessary flash. In this case, a considerably concave weld is produced, leading to local thinning of the welded plates [4-8].

Figure 1FSW parameters [20]

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III. TOOL MATERIALS AND GEOMETRY

Tool geometry is the most significant aspect of process expansion. The tool geometry is crucial in material movement and deciding factor of the traverse speed at which FSW can be conducted. An FSW tool primarily consists of a pin and shoulder. The tool has two major functions: (a) local heating, and (b) material movement.

At the starting of the welding process as tool plunge into the work piece, heating produced primarily due to friction between the work piece and the tool pin. As the material deformation start additional heating produced. The tool is plunged into the work piece until the shoulder come in contact with the work piece. A great amount of heat is evolved as shoulder rub with the work piece surface due to friction. The pin profile and shoulder diameters are crucial aspect for the heat generation. The shoulder also provides confinement for the heated volume of material.

Other main function of the tool is stir and transportation of the plasticized material around the weld. Tool design governs, the characteristics of the weld produced in term of, strength, properties and microstructure as well as process load. Fig 2 is showing various tool geometries.

Initially cylindrical and square pin profiles are used in FSW. With the development of the process and better understanding of the material movement during the process, large number of the tool geometry are practiced by researcher. Complex tool geometries are also come in picture to regulate material flow, mixing and lower process loads [9-13].

Figure 2 Tool Geometries [19]

IV. MICROSTRUCTURE EVOLUTION

Based on microstructural characterization of the FSW weldment, There are three different zones, nugget zone, thermo-mechanically affected zone (TMAZ), and

heat-affected zone (HAZ), have been identified A. Nugget zone

Extreme heating and plastic deformation FSW process result in generation of a recrystallized fine-grained microstructure at the centre of the stirred zone. This zone is named as nugget zone. Generally an onion ring structure exists in the nugget zone. There are very less dislocated grains in the core of the recrystallized grains.

However, some Researcher concluded that there are very small recrystallized grains of the nugget zone contain high dense sub-boundaries, dislocation and sub grains. The boundary between the base plate and nugget zone is comparatively dispersed on the trailing side of the tool, but fairly sharp on the advancing side of the tool.

B. Thermo-mechanically affected zone

Next zone after the nugget zone is thermo-mechanically affected zone (TMAZ) between the parent material and the nugget zone. The TMAZ expose to both, deformation and heating during FSW process. The TMAZ is primarily characterized by extremely deformed structure. The base metal elongated grains were deformed in an upward flowing pattern around the nugget zone. Although the TMAZ experienced plastic deformation, recrystallization did not occur in this zone due to insufficient deformation strain. However, dissolution of some precipitates was observed in the TMAZ, due to high-temperature exposure during FSW/FSP. The extent of dissolution, of course, depends on the thermal cycle experienced by TMAZ.

Furthermore, there is detection of high density sub-boundaries in the TMAZ. The different zones are shown in fig 3.

Figure 3 Different welding zone of FSW [14]

C. Heat-affected zone

The next zone to the TMAZ is a heat-affected zone (HAZ). This zone also experienced the thermal cycle, but does not suffer any plastic deformation. According to Mahoney et al. [14] the temperature attained in the HAZ is above 250^oC for a heat-treatable aluminum alloy. The grain structure in HAZ is almost same the parent material.

However, precipitate structure is greatly affected by the

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thermal exposure above 250^oC. Recently, Jata et al. [18]

studied the outcome of friction stir welding on microstructure of 7050Al-T7451 aluminum alloy.

Research reported that FSW process has very little effect on the size of the sub grains in the HAZ; it simply leads to coarsening of the strengthening precipitates and there is hike of the precipitate-free zone (PFZ) by a factor of 5.

Like observation was also reported by Su et al. [16] in a thorough TEM investigation on FSW 7050Al-T651. The coarsening of precipitates and widening of PFZs is apparent. Likewise, Heinz and Skrotzki [17] also detected major coarsening of the precipitates in the HAZ of FSW 6013Al.

V. FSW DEFECT

An incorrect rate of heating leads to creation of FSW defects like, surface grooves, kissing bonds ,lack of penetration, voids, excessive flash, lack of fusion, nugget collapse, tunnels and surface galling. These defects are categorized as volumetric flaws and weld line flaws.

Based on the heat produced, different welding defects are produced during welding process. These welding defects depend upon main parameters of FSW, like, tool rotational rate (TR), the welding or transverse speed (WS), and the tilt angle. In order to obtain the good weld quality, proper optimization of these parameters is necessary. The area exterior to it can be separated into three areas related with the following defects.

1. A great amount of flash due to excessive heat input during the process.

2. Lower heat generation during the process results in cavity defect.

3. Inappropriate stirring yields in cavity defect.

A distinctive existence of flash by the excessive heat input is an unacceptable by visual appearance even with acceptable weld strength. This welding defect can be easily removed by milling on the same machine [15].

VI. CONCLUSION

Friction stir welding is solid state welding, results in excellent mechanical and chemical properties as compared to other welding process. It is widely used in ship building and aerospace industries.

REFERENCES

[1] N. Zhang, X. Cao, S. Larose, P. Wanjara, "Review of tools for friction stir welding and processing", Canadian Metallurgical Quarterly, Vol. 51, 2012.

[2] K. Reshad Seighalani, M.K. Besharati Givi, A.M.

Nasiri, P. Bahemmat, "Investigations on the

Effects of the Tool Material", Geometry, and Tilt Angle on Friction Stir Welding of Pure Titanium, Journal of Materials Engineering and Performance, Vol. 19 (7) , October 2010, pp.

955–962.

[3] R. Nandan, T. DebRoy, H. K. D. H.

Bhadeshia,"Recent Advances in Friction Stir Welding – Process, Weldment Structure and Properties", Progress in Materials Science 53, 2008 pp. 980-1023.

[4] R. S. Mishra, M. W. Mahoney,"Friction stir welding and processing Materials Park", OH, ASM International, 2007.

[5] R.S. Mishraa, Z.Y. Ma,"Friction stir welding and processing", Materials Science and Engineering R 50, 2005, pp. 1–78.

[6] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A.

Spurling, C.C. Bampton, Scripta Mater. 36 (1997) 69.

[7] G. Liu, L.E. Murr, C.S. Niou, J.C. McClure, F.R.

Vega, Scripta Mater. 37 (1997) 355.

[8] K.V. Jata, S.L. Semiatin, Scripta Mater. 43 (2000) 743.

[9] S. Benavides, Y. Li, L.E. Murr, D. Brown, J.C.

McClure, Scripta Mater. 41 (1999) 809.

[10] L.E. Murr, Y. Li, R.D. Flores, E.A. Trillo, Mater.

Res. Innovat. 2 (1998) 150.

[11] W.M. Thomas, E.D. Nicholas, S.D. Smith, in: S.K.

Das, J.G. Kaufman, T.J. Lienert (Eds.), Aluminum 2001— Proceedings of the TMS 2001 Aluminum Automotive and Joining Sessions, TMS, 2001, p.

213.

[12] W.M. Thomas, K.I. Johnson, C.S. Wiesner, Adv.

Eng. Mater. 5 (2003) 485.

[13] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419.

[14] M.W. Mahoney, C.G. Rhodes, J.G. Flintoff, R.A.

Spurling, W.H. Bingel, Metall. Mater. Trans. A 29 (1998) 1955.

[15] Y.S. Sato, H. Kokawa, M. Enmoto, S. Jogan, Metall. Mater. Trans. A 30 (1999) 2429.

[16] J.Q. Su, T.W. Nelson, R.S. Mishra, M.W.

Mahoney, Acta Mater. 51 (2003) 713.

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[17] B. Heinz, B. Skrotzki, Metall. Mater. Trans. B 33 (6) (2002) 489.

[18] K.V. Jata, K.K. Sankaran, J.J. Ruschau, Metall.

Mater. Trans. A 31 (2000) 2181.

[19] Y. N. Zhang, X. Cao, S. Larose, P.

Wanjara,"Review of tools for friction stir welding and processing", Canadian Metallurgical Quarterly, Vol. 51, 2012.

[20] K. Reshad Seighalani, M.K. Besharati Givi, A.M.

Nasiri, P. Bahemmat, "Investigations on the Effects of the Tool Material", Geometry, and Tilt Angle on Friction Stir Welding of Pure Titanium, Journal of Materials Engineering and Performance, Vol. 19 (7) , October 2010, pp.

955–962.

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