Investigation of Nondestructive Testing Methods for Friction Stir Welding
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metals Review Investigation of Nondestructive Testing Methods for Friction Stir Welding Hossein Taheri 1,* , Margaret Kilpatrick 2, Matthew Norvalls 3, Warren J. Harper 3, Lucas W. Koester 4, Timothy Bigelow 3,4 and Leonard J. Bond 3,5 1 Department of Manufacturing Engineering, Georgia Southern University, Statesboro, GA 30460, USA 2 Department of Mechanical Engineering, Georgia Southern University, Statesboro, GA 30460, USA; [email protected] 3 Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA; [email protected] (M.N.); [email protected] (W.J.H.); [email protected] (T.B.); [email protected] (L.J.B.) 4 Center for Nondestructive Evaluation (CNDE), Iowa State University, Ames, IA 50011, USA; [email protected] 5 Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA * Correspondence: [email protected]; Tel.: +1-912-478-7463 Received: 15 April 2019; Accepted: 23 May 2019; Published: 29 May 2019 Abstract: Friction stir welding is a method of materials processing that enables the joining of similar and dissimilar materials. The process, as originally designed by The Welding Institute (TWI), provides a unique approach to manufacturing—where materials can be joined in many designs and still retain mechanical properties that are similar to, or greater than, other forms of welding. This process is not free of defects that can alter, limit, and occasionally render the resulting weld unusable. Most common amongst these defects are kissing bonds, wormholes and cracks that are often hidden from visual inspection. To identify these defects, various nondestructive testing methods are being used. This paper presents background to the process of friction stir welding and identifies major process parameters that affect the weld properties, the origin, and types of defects that can occur, and potential nondestructive methods for ex-situ detection and in-situ identification of these potential defects, which can then allow for corrective action to be taken. Keywords: friction stir welding (FSW); defects; mechanical properties; nondestructive testing & evaluation (NDT&E); in-line monitoring 1. Introduction Friction stir welding (FSW) is a joining process that was first demonstrated by The Welding Institute (TWI) of Great Britain in 1991 [1]. Since that time, FSW use has soared, and by the end of 2007, TWI had issued 200 licenses for the process. Its applications continue to grow. In addition, approximately nineteen hundred patent applications have also been filed relating to aspects of FSW [2,3]. The popularity of the process can be related to the multiple advantages that FSW offers, when compared with other jointing modalities, including the ability to join vastly different metals, when performed correctly, achieve minimal defect creation, maintain much higher material strength along the bonds than typical for other joints, and provide smooth surfaces after joining. In performing the FSW process it typically involves two metals clamped on a rigid surface that serves as an anvil together and a rotating shoulder, a mechanical “stirring” device that resembles a drill bit. The anvil serves to react the downward pressure, i.e., the plunge force, during the FSW process. By forcing the rotating shoulder into the materials along the weld interface, a frictional force Metals 2019, 9, 624; doi:10.3390/met9060624 www.mdpi.com/journal/metals Metals 2019, 9, x; doi: FOR PEER REVIEW 2 of 22 Metals 2019, 9, 624 2 of 22 is generated due to the high speeds and maintained downward pressure of the rotating shoulder against the metal plates. The resulting frictional heating creates a softened zone which is mechanically is generated due to the high speeds and maintained downward pressure of the rotating shoulder plasticized at the location of joining (Figure 1). The welding tool is simultaneously rotated and moved against the metal plates. The resulting frictional heating creates a softened zone which is mechanically along the desired weld line, blending the materials along the path. The resulting weld is typically plasticized at the location of joining (Figure1). The welding tool is simultaneously rotated and stronger than that given by traditional fusion welding methods because it is formed at a lower moved along the desired weld line, blending the materials along the path. The resulting weld is temperature, which minimizes a heat-affected zone and this also reduces distortion and resulting typically stronger than that given by traditional fusion welding methods because it is formed at a residual stress. In addition, FSW is an environmentally friendly process, as it does not use shielding lower temperature, which minimizes a heat-affected zone and this also reduces distortion and resulting gas or filler material and it involves minimal energy input. residual stress. In addition, FSW is an environmentally friendly process, as it does not use shielding gas or filler material and it involves minimal energy input. FigureFigure 1. 1. FrictionFriction Stir Stir Welding Welding (FSW) (FSW) tool tool and and system system setup. setup. SinceSince FSW FSW is is typically typically implemented implemented in in an an auto automatedmated process, process, when when using using correctly correctly designed designed toolstools and and parameters, parameters, defects defects should should not not occur. occur. Howe However,ver, if the if theprocess process is incorrectly is incorrectly controlled, controlled, the resultingthe resulting quality quality of the of theweld weld can can be bedegraded. degraded. For For any any material material joining joining technique—No technique—No process process is is perfect,perfect, and and defects defects can can potentially potentially occur. occur. For For FS FSWW wormholes, wormholes, kissing kissing bonds, bonds, and and defects defects caused caused by lackbylack of penetration of penetration are the are typical the typical defects defects of current of current concern concern in industry in industry [4,5]. [With4,5]. the With use the of use FSW of soaring,FSW soaring, there thereis a need is a need for nondestructive for nondestructive evalua evaluationtion (NDE) (NDE) processes processes that that are are superior superior to to those those currentlycurrently available available in in the the market market [6] [6] to to provide provide adequate adequate quality quality control control [7], [7], particularly particularly for for safety safety criticalcritical applications. applications. TheThe nondestructive nondestructive testing testing (NDT) (NDT) demands demands by byindu industrystry when when FSW FSW is used is used are that are fast that and fast cost- and efficientcost-effi cientmethods methods are provided are provided to assess to assess the theweld weld quality. quality. Although Although welds welds are are generally generally of of high high quality,quality, some some heterogeneity heterogeneity may may arise arise due due to to the the im improperproper stirring stirring of of the the parent parent material, material, lack lack of of penetrationpenetration of of the the tool tool pin, pin, poor poor choice choice of of tool tool pi pinn design design or or improper improper choice choice of of the the process process parameter parameter window.window. When When defects defects do do occur, occur, they they are are very very di differentfferent in in form form from from those those typically typically found found in in a a conventionalconventional fusion fusion welding welding process. process. AA further further constraint, constraint, in in terms terms of of inspection inspection needs, needs, is is that that the the FSW FSW method method is is typically typically a a high-end high-end fabricationfabrication method. method. Destructive Destructive evaluation evaluation of ofsuch such welds welds in most in most cases cases is not is notrecommended recommended for evaluatingfor evaluating quality quality since they since are they costly, are in costly, terms inof termslost items, of lost and items,time consuming and time to consuming conduct [8]. to Whenconduct used [8]. Whendestructive used destructiveexamination examination techniques techniques generally generally involve involve bending bending tests tests and and metallography/macrographs.metallography/macrographs. In In thes thesee techniques, techniques, the the welded welded samples samples are are removed removed from from the the original original weldingwelding surface, surface, but but such such samples samples only only provide provide da datata for for the the region region where where measurements, measurements, such such as as micrographs,micrographs, are taken.taken. InIn general,general, evaluation evaluation of of weld weld quality quality is mostis most commonly commonly performed performed post-weld post- weldusing using conventional conventional non-destructive non-destructive testing testing methods, methods, suchas such X-rays, as X-rays, ultrasonic ultrasonic testing, testing, eddy current eddy currentand dye and penetrant, dye penetrant, although although the later the are later limited are to limited detecting to detecting surface defects. surface The defects. specific The type, specific size, Metals 2019, 9, 624 3 of 22 shape and orientation of a defect all affect the detectability and characterization of the specific anomaly, and this depends on the specific nondestructive method used [7,9]. Several articles have reviewed different aspects of FSW, such as the