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Review on process parameters effect on fatigue crack growth rate in friction stir welding
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ScienceDirect Materials Today: Proceedings 18 (2019) 3061–3070
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ICMPC-2019
Review on process parameters effect on fatigue crack growth rate in friction stir welding J. Dasa, S.R. Banikb, S.R.S.K. Reddyc, M.R. Sankara*, P.S. Robia a
Department of Mechanical Engineering, Indian Institute of Technology, Guwahati, Guwahati-781039, India b Department of Mechanical Engineering, National Institute of Technology, Silchar, Silchar-788010, India c Department of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies-AP, RK Valley-516330, India
Abstract With rapid growth in technology, friction stir welding has proven to be a highly beneficial welding procedure specially used in case of alloys composed of aluminium, titanium, magnesium, etc. on which conventional welding techniques turn out to be quite challenging. However, fatigue crack growth is a highly undesirable phenomenon which occurs during the welding process. This literature work is mainly intended to highlight the relation between various process parameters and fatigue crack growth and how they can be controlled for obtaining the best quality welds with maximum resistance to crack formation. Materials must also be chosen with care in order to maximize fatigue crack resistance. Main process parameters include tool geometry, tool rotational speed and welding speed. Other important factors consist of residual stresses, stress intensity factor, peening effect, etc. Their influences have been discussed in detail in the present paper. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Friction stir welding; fatigue crack growth; aluminium alloy; tool rotational speed; welding speed.
1. Introduction The solid state process of metal joining which is performed by an electrode (which can withstand welding heat), without fusion and filler material to join two facing workpieces is defined as friction stir welding (FSW). This process takes place within the solidus temperature line and hence the workpiece does not easily melt. That is the
* Corresponding author. Tel.: +91-361-2582684; fax: +91-361-2582699. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Nomenclature FSW FCGR CPR BM ω v
friction stir welding fatigue crack growth rate crack propagation rate base material/metal tool rotational speed welding/traverse speed
reason why it is an energy efficient process and conventionally known to be a green technology. Wrought or extruded aluminium is highly suitable for the FSW process. Since its inception in 1991, FSW has been widely utilized in manufacturing vehicle and aircraft components. [1-5]. Despite its wide usage, FSW itself has a few flaws of its own, fatigue crack growth (FCG) being one of them. However, researchers have studied FCG resistance of various friction stir welded alloys and have concluded that FSW process exhibits superior FCG resistance than other welding processes involving the use of electron beam and tungsten electrode [6, 7]. But in certain cases, FSW process shows low fatigue crack growth resistance. In an experiment, G. Padmanaban et al. (2011) examined the same and found that for AZ31B magnesium alloy, laser beam welding (LBW) process showed better FCG resistance than FSW [8]. In FSW, use of key-holes can notably lessen the fatigue life of the workpiece material [9]. FSW itself has undergone a lot of changes. In modified term it is known as friction stir processing (FSP) [10]. Now a days, ultrasonic vibration enhanced friction stir welding (UVeFSW) is being employed to enhance material flow and reduce the probability of crack initiation [11]. Various factors affect FCG rates and hence should be accurately controlled in order to attain welds of superior quality with the least fatigue crack formation. Their influences have been presented in detail in the following pages of this paper. 2. Working principle In FSW, a well profiled probe is inserted between the plates which are required to be joined. Figure 1(a) displays a simplified diagram of FSW process [12]. Due to frictional heat generation in the weld region, the workpiece material softens and plastic deformation of the material takes place [13]. This effect causes movement of material to the retreating side (RS) from the advancing side (AS), producing a solid-state metal joint [14]. However, the flow of material is predominantly determined by the base material (BM) characteristics, process parameters of FSW and dimensions of the tool [15]. FSW involves processes like extrusion, moulding, and stirring of the workpiece material [16]. Region of similar directions of tool rotation and welding forms the advancing side (AS), and the region of opposite directions forms the retreating side (RS). This difference results asymmetry in various mechanical properties in the two sides of the weld [17]. (a)
(b)
Fig. 1. (a) Schematic of FSW process, (b) Schematic of an FSW tool [12].
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3. Alloy Materials Most of the researchers use aluminium (Al) alloys in experiments involving FSW. The major properties possessed by Al are less weight to strength ratio, good electrical and thermal conductivity, high corrosion resistance, etc. Aluminium is significantly light in density as compared to steel and copper. Alloys composed of Al are mainly divided into 8 (1xxx to 8xxx) groups consisting of principle alloying elements like copper, manganese, silicon, magnesium and zinc. Al alloys of 2xxx and 6xxx groups are heat treatable alloys and hence are frequently used in FSW. The alloys joined using FSW may be similar or dissimilar. Examples include 2024-7075, 2198-T8, 6061-T6 and 6082-T6 Al alloys. Use of Al alloys reduces weight and costs upto 15 to 20 % [18]. Alloys mainly composed of magnesium (Mg), copper (Cu), steel (Fe) and titanium (Ti) also find varied applications in FSW. Magnesium has a lower density as compared to aluminium. Mg alloys are widely considered to be environment friendly and hence are good substitutes to Al alloys [19]. Copper possesses properties such as high thermal and electrical conductivity, and high melting point. Cu is a soft metal, therefore FSW can be easily performed on Cu alloys [20]. Principle alloying elements of Cu alloys include brass and zinc. Friction stir welded Cu alloys find huge applications in power transmission and manufacturing of electronic parts. 4. Effect of process parameters on fatigue crack growth rate 4.1 Tool Geometry Tool geometry forms a significant part of FSW. The major role of a tool in FSW is to localize heating and allow material flow. Experiments have revealed important dependence of FCGR on tool geometry. Figure 1(b) represents the diagrammatic view of an FSW tool [12]. S. N. F. Mokhtar et al. (2012) examined the FCG in both FSWed A6061 plates and non welded A6061 plates. The tool dimensions considered were 8 mm tool probe length, 3 mm probe radius and 6 mm shoulder radius. Observation showed the faster generation of cracks in FSWed plates as compared to non-welded plates. Also for the same tool geometry, FCGR was not influenced by the load levels of the components [21]. 4.2 Tool rotational speed () and welding speed (v) Extensive research has been done to examine the influence of ω and v on FCGR in FSWed materials. Tool position also has an important effect on FCGR in FSWed alloys [22]. Yu E Ma et al. (2013) examined FCGR in friction stir welded (FSWed) 2198-T8 aluminium alloy plates for different welding parameter ratios (ω/v) in the range of 2 to 4 and observed similar FCGRs in the welded region [23]. P. Sivaraj et al. (2014) aimed at evaluating the FCGR of FSWed 7075-T651 aluminium alloy using of 250 rpm and v of 25 mm/min. They observed that for the selected process parameters, the FCGR in the Al alloy joint was noticeably higher than the parent material [24]. V. Richter-Trummer et al. (2016) examined the FCG behavior of FSWed AA2198-T8 Al-Li alloy plates. The considered welding parameters were downward force of 8.5 KN, of 1200 rpm, tilt angle of 0 and v of 500 mm/min. They observed that the samples welded perpendicular to the rolling orientation of the material could bear less number of fatigue cycles than the samples welded parallel to the same. This is where the FCG path was found to be symmetric [25]. G. D’Urso et al. (2014) examined the effect of FCG on FSWed 8 mm thick 6060-T6 Al alloy considering v and ω in the range of 117-683 mm/min and 838-1262 rpm, respectively. They came to the conclusion that fatigue life showed direct proportionality to the welding speed and the crack propagation rate was elevated as ω and v increased [26]. M. Besel et al. (2015) performed various experiments to observe the roles of different welding speeds on FCG behavior of Al-Mg-Sc 5024 alloy plates under fatigue loading. They used position controlled FSW at constant of 1200 rpm to butt join the plates using traverse speeds of 480, 600 and 729 mm/min. They concluded that low travel speeds of the tool induced high material flow at the thermo mechanically influenced region resulting in initiation of crack in the welded specimens [27]. H. Qu et al. (2007) examined the fatigue strength of Al 5083-O FSWed joints taking = 800 rpm and v = 100 mm/min and found 100% joint efficiency. However, the joint efficiency in fatigue strength had a large variance between 31% and 75% for ω of 500 - 800 rpm and v of 100 - 200 mm/min [28]. In another experimental study of the process parameters, the initiation of cracks was found at both
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bottom and top weld surfaces of FSWed AA6061-AA7050 alloys and the cracks moved in the direction of the joint center. For high and medium speeds of tool rotation, i.e. 410 and 340 rpm, crack initiation was not observed at the weld cross-sectional surface. It was concluded that with rise in tool speed, fatigue strength increased [29]. D.G. Moghadam et al. (2016) performed FSW on AA2024-T351 joints for different rotational and traverse speeds. They experimentally found that at v = 8 mm/min and = 400 rpm, the fatigue curve of BM lied above the welded specimen and also summarized that higher and v of the tool caused higher rates of crack propagation. At ω = 400 rpm, the CPR increased with increasing traverse speeds, but didn’t exceed the CPR in BM, which drew the conclusion that welded specimens showed better fatigue behavior for low values of ω [30]. Chao He et al. (2017) experimentally investigated very high cycle fatigue (VHCF) behavior of nugget zone (NZ) in AA7075 FSWed joint taking v =150 mm/min, = 300 rpm and tilt angle 8 and found that high welding and rotation speeds could create defects in the weld zone resulting in fatigue crack initiation at the bottom part of NZ [31]. H. Lombard et al. (2008) optimized the FSW process variables (rotational welding speeds) for maximum fatigue life with minimum defects in AA5083-H321. They observed that least chance of defects formation would occur for in the range of 615– 635 rpm [32]. 5. Microstructure analysis of fatigue crack growth rate In FSW, microstructure investigation reveals four zones, namely the stir zone (SZ) or nugget zone (NZ), heat affected zone (HAZ), flow arm zone (FAZ) and thermomechanically affected zone (TMAZ) [33]. Grain refinement highly influences the FCG behavior of FSWed Al alloys [34]. In recent experimentations, T. Okada et al. noticed considerable difference in grain size in NZ and BM. For FSWed 2024-T3 Al alloy, crack propagation was found to be less at the grain boundaries as compared to the inside grains. With increase in crack growth, grain boundary influence on the development of striations in the stir zone was observed to be insignificant [35]. N. Akhtar et al. studied the FCGRs in FSWed Al-Li alloy. Microstructure analysis revealed equiaxed grain formation in the welded nugget. This region also showed 2 to 3 times faster FCGRs with respect to the BM. TMAZ was found to possess the lowest microhardness [36]. Experimentation on 7178 Al alloy revealed little effects of grain size on FCGR. Rather for materials of larger grain size, over ageing improved resistance to crack growth to a great extent [37]. Researchers have observed difference in fatigue crack properties at various weld locations. Major reason behind this is the microstructural inhomogeneity [38, 39]. Figure 2 represents the development of crack within the weld zone of FSWed 6061-T6 Al alloy [40].
Fig. 2. Crack generation in the nugget zone (AA6061-T6) [40].
C. Deng et al. (2016) examined the outcome of microstructural heterogeneity on VHCF properties of AA7050T7451. They concluded that for both FSWed joint and BM, fatigue failure occurred at cycles exceeding 107. Fatigue failure was mainly observed on the AS of the FSWed joints (HAZ and TMAZ sites) [41]. Chao He et al. (2015)
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experimentally investigated the VHCF behaviors of FSWed AA7075-T651 of 10 mm thickness. They found that the VHCF range possessed only 35% fatigue strength of the BM. Due to particle change from TMAZ (intra-granular) to NZ (inter-granular), the NZ and TMAZ possessed different resistances to fatigue crack initiation [42]. P. Nelaturu et al. (2018) worked on FSP of A356 Al alloy to study how fatigue crack propagation and initiation was effected by inhomogeneous microstructures. By the investigation they revealed three possible locations for crack initiation of low life FSP fine samples. They also revealed that both propagation and initiation of cracks were extremely sensitive to microstructure [43]. A. Ali et al. (2008) worked on 2024-T351 Al alloy about crack initiation and coalescence and performed fatigue endurance tests of FSW taking a longitudinal specimen of 80 mm length and 60 mm width. They concluded that the crack coalescence and overlapping were caused by multiple crack initiation [44]. S. Shukla et al. (2018) investigated on a 7 mm thick 5024 Al alloy of three different microstructures, two ultrafine-grained (UFG210 and UFG410) and one fine-grained (FG) and came to the conclusion that with decrease in grain size, crack initiation and propagation rate increased. Also, FG alloy had noticeably less rate of propagation as compared to UFG alloys [45]. M. Muzvidziwa et al (2016) investigated FCP behavior of an FSWed Ti-6Al-4V alloy. Observations showed the microstructurally affected crack path profiles rationalized through consideration of Keq. Also the FCG behavior of the microstructurally evolved region could undesirably be affected by the β phase volume fraction [46]. Y.E. Ma (2011) compared the properties of FSWed 2195 and 2198 Al-Li alloys with traditional 2024 Al alloy. Conclusion was drawn that the former two alloys had superior resistance to damage. The difference in damage tolerance was mainly due to microstructure of the welded samples having local inhomogeneity [47]. By the experiment of A.T. Kermanidis et al. (2017), negative influence of HAZ microstructural gradient on FCGRs was observed for AA2024 in the K range of 11 to 25 MPam1/2, but FCGRs mainly remained unaffected by the gradient slope. They also observed higher CPRs in decreasing microstructure gradient compared to the increasing one [48]. 6. Effect of temperature and heat treatment on fatigue crack growth rate T. H. Tra et al. (2012) examined the FCG behavior in FSWed AA6063-T5. The behavior of the fatigue cracks was observed for as welded (AW) along with post weld heat treated (PWHTed) friction stir welds at 27°C and 200 °C. They found that the FCP rate in the BM was lower in all circumstances as compared to FCP rate in the weld zone [49]. T. H. Tra also performed investigations on post weld heat treatments (PWHT) on FCG behavior in FSWed 5.0 mm thick AA 6063-T5 plate. By the experiments he was able to show that the FCGRs were sensitive to PWHTed solutions. He applied two PWHT solutions, post weld solution (PWS) 1 (aging at 175C/12hrs) which showed tiny improvement of FCG resistance, and PWS 2 (aging at 530C/1hr) which had a remarkable effect. His investigations also involved the use of elevated temperatures such as 200°C [50, 51]. M. N. Ilman et al. (2013) studied the FCGR of FSWed 2024-T3 Al alloy under transient thermal tensioning (TTT). They performed FCGR test by a fatigue experiment at a stable amplitude and 11 Hz frequency (f) taking 0.1 as the stress ratio (R). It was found that the TTT treatment improved the FCG resistance of the FSWs [52]. C. Sharma et al. (2014) examined the fatigue behavior of FSWed 5 mm thick AA7039 Al–Zn–Mg alloy in different temper conditions such as O, W and T6. The developed FSWed joints in the three above mentioned temper conditions showed fatigue life of 6–8 times than the BM. FSWed joints in T6 and O temper states resulted in lesser fatigue life than the W temper state [53]. 7. Effect of stress ratio (R) on fatigue crack growth rate C.D. Donne et al. (2000) investigated the FCP of defective and defect free FSWed AA2024-T3 and AA6013-T6 of 4 mm thickness. At high load ratios (R>0.5), Al alloys were less sensitive to the load ratio effects, hence the residual stresses had a very limited effect on the FCP [54]. M. Ericsson et al. (2005) investigated FCPs of an FSWed Al-Mg-Si 6082 alloy plate and found that at R=0.1 (low load ratio) and low ∆K value, the CPRs in the weld were more than the BM by 3-5 times, but at high R value (0.8) the weld and the parent material had the same crack propagation rate. Also, at a point close to the fracture, the weld CGR was about same for both elevated and reduced values of R and in the HAZ, the propagation rate lied between the weld and BM [55]. K. Katsuki et al. (2008) performed tension compression fatigue tests using FSWed AA5454 sheet specimens. The stress ratio taken was -1 and it was observed that CPR and the hardness of the material had an opposite effect on one another [56]. In an
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experiment on FSWed 5 mm thick 7xxx-T7 Al plate, with respect to BM a reduction of FCGR was observed in HAZ. It was also observed that in this case FCGR was higher for lower values of R [57]. Yu E. Ma et al. (2011) used C(T) and ESE(T) samples of AA 2195-T8 to perform FCG tests. Using ESE(T) specimens with R value of 0.1 they got 10 times faster FCGR in the BM with respect to that in the smallest samples (140 mm×40 mm). Also, taking R value of 0.6, they observed 2 and 4 times faster FCGR in the BM as compared to 185 mm×50 mm and 370 mm×100 mm samples respectively. On the other hand, BM possessed 5-10 times faster FCGRs as compared to weld zone of C(T) specimens. Figure 3 (a-b)shows the location of weld in both the specimens [58]. P.M.G.P. Moreira et al. (2012) performed SN (stress range vs. fatigue life) and fatigue tests of the BM and FSWed 2195-T8X aluminium-lithium alloy at room temperature. The stress ratios were taken as 0.1, 0.5 and 0.8. They observed that the FCP data showed notable R dependency for the BM, but this was not significant for the FSWed material [59]. E. Maggiolini et al. (2016) examined the effect of FCG path on FSWed AA6082-T6 taking different values of stress ratio (R) and observed that the loading conditions did not have a major impact on the region where the crack was initiated [60]. (b) (a)
Y
Y X
X
Weld
Fig. 3. Schematic representation of weld zone in (a) C(T) and (b) ESE(T) samples [58].
8. Effect of residual stress and stress intensity factor (ΔK) on fatigue crack growth rate In the late 2000s, researchers investigated the residual stress effects on FCGR. In an experimental study by G. Bussu et al. on 2024-T351 aluminium plates, it was concluded that in the tension residual stress fields, the cracks which propagated at an angle of 90 to weld line would show faster CGRs than in the compressive residual stress fields. In another observation tensile residual stress fields showed minimal change in the stress intensity factor (∆K) value. As residual stress value approached zero and moved towards compression, ΔK value changed significantly [61]. In another experiment, Y.E. Ma et al. (2011) designed C(T) samples of 2195-T8 Al-Li Alloy and then measured the FCGRs in those specimens. It was observed that FCGRs were lower in the nugget zone as compared to the BM. They also observed a significant impact of compressive residual stresses on FCGRs around the notch and how their control on cracks reduced with crack growth [62]. S. Kim et al. (2008) investigated the FCP behavior of 4 mm thick FSWed AA5083-H32 and AA6061-T651. The FCP behavior of the two samples were studied considering FCG either perpendicular or parallel to the DXZ for different ∆K values. For low ∆K values, the presence of FSW zone did not have a significant effect on the FCGRs. However, notable effects were observed for higher values of ∆K [63]. In a study on the FCP in FSWed AA2024-T351, L. Fratini et al. (2009) noticed that residual stress dominated the FCG behavior outside the weld. From a few FCG tests, it was noticed that based on initial notch position residual stresses would induce low CGRs [64]. K. Kuwayama et al. (2009) performed FCP tests on 2 mm thick FSWed AA2024-T3 and accelerated CPR was observed for high values of tensile residual stress [65]. Y. E. Ma et al. (2011) studied FCGRs in residual field using residual K (Kres) approaches. The presence of edge notch root in C(T) and ESE(T) specimens was found within a residual stress field causing compression thereby reducing effective ΔK (Keff) and CGRs [66]. John A. New et al. (2015) performed FCG tests at constant ΔK on FSWed AA2024-T351. In absence of residual stress field, CGR (da/dN) remained constant with crack length, whereas da/dN showed variable growth with the field of residual stress being present [67]. Many researchers like R. Citarella conducted a detailed comparison of experimental and numerical findings to scrutinize the effects of residual stresses on FCP and good agreement was found between the two [68].
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9. Other experiments involving fatigue crack growth rate Many researchers like O. Hatamleh investigated the impact of peening on the FCG behaviour of FSWed AA 7075-T7351 and AA 2195-T8 sheets at various peening conditions like shot peening (SP) and laser peeing (LP). By using three layered LP they found a notable decrease in FCG in comparison to the AW condition and unwelded BM, but shot peening could not influence FCG in FSW samples to a notable extent [69, 70]. J.A. Ronevich et al (2016) tested FSWed steel pipelines and measured FCGR vs K range in 21MPa hydrogen gas at f = 1 Hz and R = 0.5. They concluded that heat affected FCGRs of the BM were insignificantly smaller as compared to the welded region [71]. Researchers have studied the FCG simulation of various Al alloys using XFEM and FEM and confirmed their utility in successfully simulating a complex 3D geometry for FCG simulation [72, 73]. From the experiments performed by Y.E. Ma et al. on FCP in FSWed 2xxx Al-Li alloy, it was observed that for 1.6 mm thick 2198 M(T) specimens, CGRs were similar to the parent metal and for the 8 mm thick ESE(T) samples, the CGRs were noticeably retarded in notch region [74]. Lei Wang et al. (2016) performed various tests on 4 mm thick 2024-T4 Al alloy plates, including FCP tests in air, 3.5% NaCl solution and under pre-corrosion condition. From the results obtained, they concluded that 3.5% NaCl solution decreased the fatigue life upto 50% of the previous welded specimens and the corrosive environment decreased the same upto 64%. It was also found that the CGRs in BM were lesser as compared to the welded region in the three mentioned test conditions [75]. M. Zadeh et al. (2009) found good agreement between analytical and computational methods in investigating FCG of AA2024-T351 using boundary element method [76]. B. Zou et al. (2011) studied the growth of fatigue cracks in AA7075-T6 FSWed joints. They used different stress ratios for an FCG simulation using AFGROW and studied their influence on FCG. They observed that the RS of the HAZ showed the lowest FCP, whereas the zone located perpendicular to the welding seam showed the highest. It was concluded that FCP increased with increase in stress ratio [77]. 10. Conclusion In this review, mainly the influences of welding conditions on FCGR have been investigated. Also the effects of microstructure, residual stress, peening effect and many other factors have been studied and reviewed. Major conclusions drawn include large influence of tool rotational speed and welding speed on fatigue crack generation and propagation. Microstructure analysis reveals importance of grain size and how various regions of a workpiece undergo different FCG rates. Laser peening improves the quality of the material and reduces the chances of crack formation. Various numerical analysis show good acceptance between experimental and theoretical findings of FSW and how it improves the quality of the weld overall. Even though friction stir welding is highly useful when compared to conventional welding methods, fatigue crack propagation in FSWed materials is a major disadvantage and hence process parameters should be used in the most favorable range to reduce crack formation and improve the efficiency of the weld. Acknowledgement Authors are thankful to “ELSEVIER (Licence numbers: 4381301401731, 4381310435271, 4381310923427)” for providing the copy right permission to various figures in the current paper. References [1]
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[30] D.G. Moghadam, K. Farhangdoost, Influence of welding parameters on fracture toughness and fatigue crack growth rate in friction stir welded nugget of 2024-T351 aluminum alloy joints, Transactions of Nonferrous Metals Society of China, 26 (2016) 2567−2585. [31] C. He, K. Kitamura, K. Yang, Y. Liu, Q. Wang, Q. Chen, Very high cycle fatigue crack initiation mechanism in nugget zone of AA 7075 friction stir welded joint, Advances in Materials Science and Engineering, 2017 (2017) 1-10. [32] H. Lombard, D.G. Hattingh, A. Steuwer, M.N. James, Optimising FSW process parameters to minimise defects and maximise fatigue life in 5083-H321 aluminium alloy, Engineering Fracture Mechanics, 75 (2008) 341–354. [33] A.F. Goldestanesh, A. Ali, M. Bayat, Experimental and analytical studies of fatigue crack growth in peened after skimmed friction stir welded joint of 2024-T351, Key Engineering Materials, 462-463 (2011) 1212-1217. [34] S.J. Kim, Y.H. Jeong, H.J. Sohn, Probabilistic fatigue crack growth in compact tension specimens from FSWed 7075-T651 aluminium alloys, Applied Mechanics and Materials, 152-154 (2012) 293-296. [35] T. Okada, S. Machida, T. Nakamura, H. Tanaka, K. Kuwayama, M. Asakawa, Fatigue crack growth of friction-stir-welded aluminum alloy, Journal of Aircraft, 54 (2017) 737-746. [36] N. Akhtar, H. Jin, F. Jia, S.J. Wu, Fatigue crack growth rates in friction stir welding joints of AI-Li 2060-T8X alloy, 12th International Bhurban Conference on Applied Sciences & Technology (IBCAST), (2015) 6-13. [37] P.J.E. Forsyth, A.W. Bowen, The relationship between fatigue crack behaviour and microstructure in 7178 aluminium alloy, International Journal of Fatigue, 3 (1981) 17-25. [38] T.H. Tra, M. Seino, M. Sakaguchi, M. Okazaki, Fatigue crack propagation behaviour relevant to inhomogeneity in the friction stir weld, Journal of Solid Mechanics and Materials Engineering, 4 (2010) 840-848. [39] Á. Meilinger, J. Lukács, Characteristics of fatigue cracks propagating in different directions of FSW joints made of 5754-H22 and 6082-T6 alloys, Materials Science Forum, 794-796 (2014) 371-376. [40] P.M.G.P. Moreira, A.M.P. de Jesus, A.S. Ribeiro, P.M.S.T. de Castro, Fatigue crack growth in friction stir welds of 6082-T6 and 6061-T6 aluminium alloys: a comparison, Theoretical and Applied Fracture Mechanics, 50 (2008) 81-91. [41] C. Deng, H. Wang, B. Gong, X. Li, Z. Lei, Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds, International Journal of Fatigue, 83 (2016) 100-108. [42] C. He, Y. Liu, J. Dong, Q. Wang, D. Wanger, C. Bathias, Fatigue crack initiation behaviours throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue, International Journal of Fatigue, 81 (2015) 171–178. [43] P. Nelaturu, S. Jana, R.S. Mishra, G. Grant, B.E. Carlson, Influence of friction stir processing on the room temperature fatigue cracking mechanisms of A356 aluminum alloy, Materials Science & Engineering: A, 716 (2018) 165-178. [44] A. Ali, M.W. Brown, C.A. Rodopoulos, Modelling of crack coalescence in 2024-T351 Al alloy friction stir welded joints, International Journal of Fatigue, 30 (2008) 2030–2043. [45] S. Shukla, M. Komarasamy, R.S. Mishra, Grain size dependence of fatigue properties of friction stir processed ultrafine-grained Al-5024 alloy, International Journal of Fatigue, 109 (2018) 1-9. [46] M. Muzvidziwa, M. Okazaki, K. Suzuki, S. Hirano, Role of microstructure on the fatigue crack propagation behavior of a friction stir welded Ti-6Al-4V, Materials Science & Engineering: A, 652 (2016) 59-68. [47] Y.E. Ma, Fracture behavior and crack growth rate of Al-Li alloy friction stir welded joints, Advanced Materials Research, 1662-8985 (2011) 131-136. [48] A.T. Kermanidis, A. Tzamtzis, An experimental approach for estimating the effect of heat affected zone (HAZ) microstructural gradient on fatigue crack growth rate in aluminium alloy FSW, Materials Science & Engineering: A, 691 (2017) 110-120. [49] T.H. Tra, M. Okazaki, K. Suzuki, Fatigue crack propagation behavior in friction stir welding of AA6063-T5: roles of residual stress and microstructure, International Journal of Fatigue, 43 (2012) 23–29. [50] T.H. Tra, Effect of the post weld heat treatments on the fatigue crack growth behavior in friction stir welding of a heat treatable aluminum alloy, International Journal of Research in Engineering and Technology, 4 (2015) 6-10. [51] T.H. Tra, Fatigue crack growth at the representative zones in friction stir welding of a heat-treatable aluminium alloy at 2000 C, Vietnam Journal of Science and Technology, 56 (2018) 39-46. [52] M.N. Ilman, Kusmono, P.T. Iswanto, Fatigue crack growth rate behaviour of friction stir aluminium alloy AA2024-T3 welds under transient thermal tensioning, Materials and Design, 50 (2013) 235–243. [53] C. Sharma, D.K. Dwivedi, P. Kumar, Fatigue behavior of friction stir weld joints of Al–Zn–Mg alloy AA7039 developed using base metal in different temper condition, Materials and Design, 64 (2014) 334–344. [54] C.D. Donne, G. Biallas, T. Ghidini, G. Raimbeaux, Effect of weld imperfections and residual stresses on the fatigue crack propagation in friction stir welded joints, Conference: Gothenborg, Sveden, (2000) 1-10. [55] M. Ericsson, R. Sandstrom, Fatigue crack propagation in friction stir welded and parent AA6082, Materials Processing – Welding, 77 (2006) 450-455.
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[56] K. Katsuki, M. Gutensohn, M. Endo, D. Eifler, Observation of the behavior of fatigue cracks in friction stir welded aluminum alloy joints, Key Engineering Materials, 385-387 (2008) 797-800. [57] R. Muzzolini, J.C. Ehrstrom, N. Fuzier, A. Reynolds, K.K. Chan, E.I. Lim, Fatigue crack propagation and stable tearing in friction stir welded aluminium sheet, American Society for Investigative Pathology, (2006) 1-29. [58] Y.E. Ma, P. Staron, T. Fischer, P.E. Irving, Size effects on residual stress and fatigue crack growth in friction stir welded 2195-T8 aluminium– part I: experiments, International Journal of Fatigue, 33 (2011) 1417–1425. [59] P.M.G.P. Moreira, A.M.P. de Jesus, M.A.V. de Figueiredo, M. Windisch, G. Sinnema, P.M.S.T. de Castro, Fatigue and fracture behaviour of friction stir welded aluminium-lithium 2195, Theoretical and Applied Fracture Mechanics, 60 (2012) 1-9. [60] E. Maggiolini, R. Tovo, L. Susmel, M.N. James, D.G. Hattingh, Crack path and fracture analysis in FSW of small diameter 6082-T6 aluminium tubes under tension torsion loading, International Journal of Fatigue, 92 (2016) 478–487. [61] G. Bussu, P.E. Irving, The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024T351 aluminium joints, International Journal of Fatigue, 25 (2003) 77–88. [62] Y.E. Ma, L. Wang, B. Liu, Residual stress effects on crack growth in nugget of Al-Li alloy 2195-T8 friction stir welded joints, Applied Mechanics and Materials, 138-139 (2011) 646-650. [63] S. Kim, C.G. Lee, S.J. Kim, Fatigue crack propagation behavior of friction stir welded 5083-H32 and 6061-T651 aluminum alloys, Materials Science and Engineering: A, 478 (2008) 56–64. [64] L. Fratini, S. Pasta, P. Reynolds, Fatigue crack growth in 2024-T351 friction stir welded joints: longitudinal residual stress and microstructural effect, International Journal of Fatigue, 31 (2009) 495–500. [65] K. Kuwayama, M. Asakawa, T. Okada, T. Nakamura, S. Machida, S. Fujita, Fatigue crack propagation property of friction stir welded 2024-T3 aluminium alloy, 50th AIAA/ASME/ASCE/AHS/ASC structures, Structural Dynamics and Materials Conference, (2009) 1-7. [66] Y.E. Ma, P. Starson, T. Fischer, P.E. Irving, Size effects of residual stress and fatigue crack growth in friction stir welded 2195-T8 aluminium-part II: modelling, International Journal of Fatigue, 33 (2011) 1426–1434. [67] J.A. New, S.W. Smith, B.R. Seshadri, M.A. James, R.L. Brazill, R.W. Schultz, J.K. Donald, A. Blair, Characterization of residual stress effects on fatigue crack growth of a friction stir welded aluminum alloy, Journal of NASA/TM–2015-218685. [68] R. Citarella, P. Carlone, M. Lepore, R. Sepe, Hybrid technique to assess the fatigue performance of multiple cracked FSW joints, Engineering Fracture Mechanics, 162 (2016) 38–50. [69] O. Hatamleh, J. Lyons, R. Forman, Laser and shot peening effects on fatigue crack growth in friction stir welded 7075-T7351 aluminum alloy joints, International Journal of Fatigue, 29 (2007) 421–434. [70] O. Hatamleh, M. Hill, S. Forth, D. Garcia, Fatigue crack growth performance of peened friction stir welded 2195 aluminum alloy joints at elevated and cryogenic temperature, Materials Science and Engineering: A, 519 (2009) 61-69. [71] J.A. Ronevich, B.P. Somerday, Z. Feng, Hydrogen accelerated fatigue crack growth of friction stir welded X52 steel pipe, International Journal of Hydrogen Energy, 42 (2017) 4259-4268. [72] A. Kraedegh, W. Li, A. Sedmak, A. Grbovic, N. Trišović, Simulation of fatigue crack growth in A2024-T351 T-welded joint, Structural Integrity and Life, 17 (2017) 3-6. [73] A.F. Golestaneh, A. Ali, M. Bayat, Analytical and numerical investigation of fatigue crack growth in aluminium alloy, Key Engineering Materials, 462-463 (2011) 1050-1055. [74] Y.E. MA, P.E. Irving, T. Fischer, X. Zhang, G. Servetti, Effects of residual stresses on fatigue crack propagation in friction stir welded 2198-T8 and 2195-T8 Al-Li alloy joints, 12th International Conference on Fracture, 3 (2009) 1797-1806. [75] L. Wang, L. Hui, S. Zhou, L. Xu, B. He, Effect of corrosive environment on fatigue property and crack propagation behavior of Al 2024 friction stir weld, Transactions of Nonferrous Metals Society of China, 26 (2016) 2830−2837. [76] M. Zadeh, A. Ali, A.F. Golestaneh, B.B. Sahari, Three dimensional simulation of fatigue crack growth in friction stir welded joints of 2024T351 Al alloy, Journal of Scientific and Industrial Research, 68 (2009) 775-782. [77] B. Zou, X. Yang, J. Chen, Fatigue crack growth rates in friction stir welding joints of 7075-T6 Al alloy and fatigue life prediction based on AFGROW, Advanced Materials Research, 337 (2011) 507-510.
ScienceDirect Materials Today: Proceedings 18 (2019) 3061–3070
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ICMPC-2019
Review on process parameters effect on fatigue crack growth rate in friction stir welding J. Dasa, S.R. Banikb, S.R.S.K. Reddyc, M.R. Sankara*, P.S. Robia a
Department of Mechanical Engineering, Indian Institute of Technology, Guwahati, Guwahati-781039, India b Department of Mechanical Engineering, National Institute of Technology, Silchar, Silchar-788010, India c Department of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies-AP, RK Valley-516330, India
Abstract With rapid growth in technology, friction stir welding has proven to be a highly beneficial welding procedure specially used in case of alloys composed of aluminium, titanium, magnesium, etc. on which conventional welding techniques turn out to be quite challenging. However, fatigue crack growth is a highly undesirable phenomenon which occurs during the welding process. This literature work is mainly intended to highlight the relation between various process parameters and fatigue crack growth and how they can be controlled for obtaining the best quality welds with maximum resistance to crack formation. Materials must also be chosen with care in order to maximize fatigue crack resistance. Main process parameters include tool geometry, tool rotational speed and welding speed. Other important factors consist of residual stresses, stress intensity factor, peening effect, etc. Their influences have been discussed in detail in the present paper. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Friction stir welding; fatigue crack growth; aluminium alloy; tool rotational speed; welding speed.
1. Introduction The solid state process of metal joining which is performed by an electrode (which can withstand welding heat), without fusion and filler material to join two facing workpieces is defined as friction stir welding (FSW). This process takes place within the solidus temperature line and hence the workpiece does not easily melt. That is the
* Corresponding author. Tel.: +91-361-2582684; fax: +91-361-2582699. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Nomenclature FSW FCGR CPR BM ω v
friction stir welding fatigue crack growth rate crack propagation rate base material/metal tool rotational speed welding/traverse speed
reason why it is an energy efficient process and conventionally known to be a green technology. Wrought or extruded aluminium is highly suitable for the FSW process. Since its inception in 1991, FSW has been widely utilized in manufacturing vehicle and aircraft components. [1-5]. Despite its wide usage, FSW itself has a few flaws of its own, fatigue crack growth (FCG) being one of them. However, researchers have studied FCG resistance of various friction stir welded alloys and have concluded that FSW process exhibits superior FCG resistance than other welding processes involving the use of electron beam and tungsten electrode [6, 7]. But in certain cases, FSW process shows low fatigue crack growth resistance. In an experiment, G. Padmanaban et al. (2011) examined the same and found that for AZ31B magnesium alloy, laser beam welding (LBW) process showed better FCG resistance than FSW [8]. In FSW, use of key-holes can notably lessen the fatigue life of the workpiece material [9]. FSW itself has undergone a lot of changes. In modified term it is known as friction stir processing (FSP) [10]. Now a days, ultrasonic vibration enhanced friction stir welding (UVeFSW) is being employed to enhance material flow and reduce the probability of crack initiation [11]. Various factors affect FCG rates and hence should be accurately controlled in order to attain welds of superior quality with the least fatigue crack formation. Their influences have been presented in detail in the following pages of this paper. 2. Working principle In FSW, a well profiled probe is inserted between the plates which are required to be joined. Figure 1(a) displays a simplified diagram of FSW process [12]. Due to frictional heat generation in the weld region, the workpiece material softens and plastic deformation of the material takes place [13]. This effect causes movement of material to the retreating side (RS) from the advancing side (AS), producing a solid-state metal joint [14]. However, the flow of material is predominantly determined by the base material (BM) characteristics, process parameters of FSW and dimensions of the tool [15]. FSW involves processes like extrusion, moulding, and stirring of the workpiece material [16]. Region of similar directions of tool rotation and welding forms the advancing side (AS), and the region of opposite directions forms the retreating side (RS). This difference results asymmetry in various mechanical properties in the two sides of the weld [17]. (a)
(b)
Fig. 1. (a) Schematic of FSW process, (b) Schematic of an FSW tool [12].
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3. Alloy Materials Most of the researchers use aluminium (Al) alloys in experiments involving FSW. The major properties possessed by Al are less weight to strength ratio, good electrical and thermal conductivity, high corrosion resistance, etc. Aluminium is significantly light in density as compared to steel and copper. Alloys composed of Al are mainly divided into 8 (1xxx to 8xxx) groups consisting of principle alloying elements like copper, manganese, silicon, magnesium and zinc. Al alloys of 2xxx and 6xxx groups are heat treatable alloys and hence are frequently used in FSW. The alloys joined using FSW may be similar or dissimilar. Examples include 2024-7075, 2198-T8, 6061-T6 and 6082-T6 Al alloys. Use of Al alloys reduces weight and costs upto 15 to 20 % [18]. Alloys mainly composed of magnesium (Mg), copper (Cu), steel (Fe) and titanium (Ti) also find varied applications in FSW. Magnesium has a lower density as compared to aluminium. Mg alloys are widely considered to be environment friendly and hence are good substitutes to Al alloys [19]. Copper possesses properties such as high thermal and electrical conductivity, and high melting point. Cu is a soft metal, therefore FSW can be easily performed on Cu alloys [20]. Principle alloying elements of Cu alloys include brass and zinc. Friction stir welded Cu alloys find huge applications in power transmission and manufacturing of electronic parts. 4. Effect of process parameters on fatigue crack growth rate 4.1 Tool Geometry Tool geometry forms a significant part of FSW. The major role of a tool in FSW is to localize heating and allow material flow. Experiments have revealed important dependence of FCGR on tool geometry. Figure 1(b) represents the diagrammatic view of an FSW tool [12]. S. N. F. Mokhtar et al. (2012) examined the FCG in both FSWed A6061 plates and non welded A6061 plates. The tool dimensions considered were 8 mm tool probe length, 3 mm probe radius and 6 mm shoulder radius. Observation showed the faster generation of cracks in FSWed plates as compared to non-welded plates. Also for the same tool geometry, FCGR was not influenced by the load levels of the components [21]. 4.2 Tool rotational speed () and welding speed (v) Extensive research has been done to examine the influence of ω and v on FCGR in FSWed materials. Tool position also has an important effect on FCGR in FSWed alloys [22]. Yu E Ma et al. (2013) examined FCGR in friction stir welded (FSWed) 2198-T8 aluminium alloy plates for different welding parameter ratios (ω/v) in the range of 2 to 4 and observed similar FCGRs in the welded region [23]. P. Sivaraj et al. (2014) aimed at evaluating the FCGR of FSWed 7075-T651 aluminium alloy using of 250 rpm and v of 25 mm/min. They observed that for the selected process parameters, the FCGR in the Al alloy joint was noticeably higher than the parent material [24]. V. Richter-Trummer et al. (2016) examined the FCG behavior of FSWed AA2198-T8 Al-Li alloy plates. The considered welding parameters were downward force of 8.5 KN, of 1200 rpm, tilt angle of 0 and v of 500 mm/min. They observed that the samples welded perpendicular to the rolling orientation of the material could bear less number of fatigue cycles than the samples welded parallel to the same. This is where the FCG path was found to be symmetric [25]. G. D’Urso et al. (2014) examined the effect of FCG on FSWed 8 mm thick 6060-T6 Al alloy considering v and ω in the range of 117-683 mm/min and 838-1262 rpm, respectively. They came to the conclusion that fatigue life showed direct proportionality to the welding speed and the crack propagation rate was elevated as ω and v increased [26]. M. Besel et al. (2015) performed various experiments to observe the roles of different welding speeds on FCG behavior of Al-Mg-Sc 5024 alloy plates under fatigue loading. They used position controlled FSW at constant of 1200 rpm to butt join the plates using traverse speeds of 480, 600 and 729 mm/min. They concluded that low travel speeds of the tool induced high material flow at the thermo mechanically influenced region resulting in initiation of crack in the welded specimens [27]. H. Qu et al. (2007) examined the fatigue strength of Al 5083-O FSWed joints taking = 800 rpm and v = 100 mm/min and found 100% joint efficiency. However, the joint efficiency in fatigue strength had a large variance between 31% and 75% for ω of 500 - 800 rpm and v of 100 - 200 mm/min [28]. In another experimental study of the process parameters, the initiation of cracks was found at both
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bottom and top weld surfaces of FSWed AA6061-AA7050 alloys and the cracks moved in the direction of the joint center. For high and medium speeds of tool rotation, i.e. 410 and 340 rpm, crack initiation was not observed at the weld cross-sectional surface. It was concluded that with rise in tool speed, fatigue strength increased [29]. D.G. Moghadam et al. (2016) performed FSW on AA2024-T351 joints for different rotational and traverse speeds. They experimentally found that at v = 8 mm/min and = 400 rpm, the fatigue curve of BM lied above the welded specimen and also summarized that higher and v of the tool caused higher rates of crack propagation. At ω = 400 rpm, the CPR increased with increasing traverse speeds, but didn’t exceed the CPR in BM, which drew the conclusion that welded specimens showed better fatigue behavior for low values of ω [30]. Chao He et al. (2017) experimentally investigated very high cycle fatigue (VHCF) behavior of nugget zone (NZ) in AA7075 FSWed joint taking v =150 mm/min, = 300 rpm and tilt angle 8 and found that high welding and rotation speeds could create defects in the weld zone resulting in fatigue crack initiation at the bottom part of NZ [31]. H. Lombard et al. (2008) optimized the FSW process variables (rotational welding speeds) for maximum fatigue life with minimum defects in AA5083-H321. They observed that least chance of defects formation would occur for in the range of 615– 635 rpm [32]. 5. Microstructure analysis of fatigue crack growth rate In FSW, microstructure investigation reveals four zones, namely the stir zone (SZ) or nugget zone (NZ), heat affected zone (HAZ), flow arm zone (FAZ) and thermomechanically affected zone (TMAZ) [33]. Grain refinement highly influences the FCG behavior of FSWed Al alloys [34]. In recent experimentations, T. Okada et al. noticed considerable difference in grain size in NZ and BM. For FSWed 2024-T3 Al alloy, crack propagation was found to be less at the grain boundaries as compared to the inside grains. With increase in crack growth, grain boundary influence on the development of striations in the stir zone was observed to be insignificant [35]. N. Akhtar et al. studied the FCGRs in FSWed Al-Li alloy. Microstructure analysis revealed equiaxed grain formation in the welded nugget. This region also showed 2 to 3 times faster FCGRs with respect to the BM. TMAZ was found to possess the lowest microhardness [36]. Experimentation on 7178 Al alloy revealed little effects of grain size on FCGR. Rather for materials of larger grain size, over ageing improved resistance to crack growth to a great extent [37]. Researchers have observed difference in fatigue crack properties at various weld locations. Major reason behind this is the microstructural inhomogeneity [38, 39]. Figure 2 represents the development of crack within the weld zone of FSWed 6061-T6 Al alloy [40].
Fig. 2. Crack generation in the nugget zone (AA6061-T6) [40].
C. Deng et al. (2016) examined the outcome of microstructural heterogeneity on VHCF properties of AA7050T7451. They concluded that for both FSWed joint and BM, fatigue failure occurred at cycles exceeding 107. Fatigue failure was mainly observed on the AS of the FSWed joints (HAZ and TMAZ sites) [41]. Chao He et al. (2015)
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experimentally investigated the VHCF behaviors of FSWed AA7075-T651 of 10 mm thickness. They found that the VHCF range possessed only 35% fatigue strength of the BM. Due to particle change from TMAZ (intra-granular) to NZ (inter-granular), the NZ and TMAZ possessed different resistances to fatigue crack initiation [42]. P. Nelaturu et al. (2018) worked on FSP of A356 Al alloy to study how fatigue crack propagation and initiation was effected by inhomogeneous microstructures. By the investigation they revealed three possible locations for crack initiation of low life FSP fine samples. They also revealed that both propagation and initiation of cracks were extremely sensitive to microstructure [43]. A. Ali et al. (2008) worked on 2024-T351 Al alloy about crack initiation and coalescence and performed fatigue endurance tests of FSW taking a longitudinal specimen of 80 mm length and 60 mm width. They concluded that the crack coalescence and overlapping were caused by multiple crack initiation [44]. S. Shukla et al. (2018) investigated on a 7 mm thick 5024 Al alloy of three different microstructures, two ultrafine-grained (UFG210 and UFG410) and one fine-grained (FG) and came to the conclusion that with decrease in grain size, crack initiation and propagation rate increased. Also, FG alloy had noticeably less rate of propagation as compared to UFG alloys [45]. M. Muzvidziwa et al (2016) investigated FCP behavior of an FSWed Ti-6Al-4V alloy. Observations showed the microstructurally affected crack path profiles rationalized through consideration of Keq. Also the FCG behavior of the microstructurally evolved region could undesirably be affected by the β phase volume fraction [46]. Y.E. Ma (2011) compared the properties of FSWed 2195 and 2198 Al-Li alloys with traditional 2024 Al alloy. Conclusion was drawn that the former two alloys had superior resistance to damage. The difference in damage tolerance was mainly due to microstructure of the welded samples having local inhomogeneity [47]. By the experiment of A.T. Kermanidis et al. (2017), negative influence of HAZ microstructural gradient on FCGRs was observed for AA2024 in the K range of 11 to 25 MPam1/2, but FCGRs mainly remained unaffected by the gradient slope. They also observed higher CPRs in decreasing microstructure gradient compared to the increasing one [48]. 6. Effect of temperature and heat treatment on fatigue crack growth rate T. H. Tra et al. (2012) examined the FCG behavior in FSWed AA6063-T5. The behavior of the fatigue cracks was observed for as welded (AW) along with post weld heat treated (PWHTed) friction stir welds at 27°C and 200 °C. They found that the FCP rate in the BM was lower in all circumstances as compared to FCP rate in the weld zone [49]. T. H. Tra also performed investigations on post weld heat treatments (PWHT) on FCG behavior in FSWed 5.0 mm thick AA 6063-T5 plate. By the experiments he was able to show that the FCGRs were sensitive to PWHTed solutions. He applied two PWHT solutions, post weld solution (PWS) 1 (aging at 175C/12hrs) which showed tiny improvement of FCG resistance, and PWS 2 (aging at 530C/1hr) which had a remarkable effect. His investigations also involved the use of elevated temperatures such as 200°C [50, 51]. M. N. Ilman et al. (2013) studied the FCGR of FSWed 2024-T3 Al alloy under transient thermal tensioning (TTT). They performed FCGR test by a fatigue experiment at a stable amplitude and 11 Hz frequency (f) taking 0.1 as the stress ratio (R). It was found that the TTT treatment improved the FCG resistance of the FSWs [52]. C. Sharma et al. (2014) examined the fatigue behavior of FSWed 5 mm thick AA7039 Al–Zn–Mg alloy in different temper conditions such as O, W and T6. The developed FSWed joints in the three above mentioned temper conditions showed fatigue life of 6–8 times than the BM. FSWed joints in T6 and O temper states resulted in lesser fatigue life than the W temper state [53]. 7. Effect of stress ratio (R) on fatigue crack growth rate C.D. Donne et al. (2000) investigated the FCP of defective and defect free FSWed AA2024-T3 and AA6013-T6 of 4 mm thickness. At high load ratios (R>0.5), Al alloys were less sensitive to the load ratio effects, hence the residual stresses had a very limited effect on the FCP [54]. M. Ericsson et al. (2005) investigated FCPs of an FSWed Al-Mg-Si 6082 alloy plate and found that at R=0.1 (low load ratio) and low ∆K value, the CPRs in the weld were more than the BM by 3-5 times, but at high R value (0.8) the weld and the parent material had the same crack propagation rate. Also, at a point close to the fracture, the weld CGR was about same for both elevated and reduced values of R and in the HAZ, the propagation rate lied between the weld and BM [55]. K. Katsuki et al. (2008) performed tension compression fatigue tests using FSWed AA5454 sheet specimens. The stress ratio taken was -1 and it was observed that CPR and the hardness of the material had an opposite effect on one another [56]. In an
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experiment on FSWed 5 mm thick 7xxx-T7 Al plate, with respect to BM a reduction of FCGR was observed in HAZ. It was also observed that in this case FCGR was higher for lower values of R [57]. Yu E. Ma et al. (2011) used C(T) and ESE(T) samples of AA 2195-T8 to perform FCG tests. Using ESE(T) specimens with R value of 0.1 they got 10 times faster FCGR in the BM with respect to that in the smallest samples (140 mm×40 mm). Also, taking R value of 0.6, they observed 2 and 4 times faster FCGR in the BM as compared to 185 mm×50 mm and 370 mm×100 mm samples respectively. On the other hand, BM possessed 5-10 times faster FCGRs as compared to weld zone of C(T) specimens. Figure 3 (a-b)shows the location of weld in both the specimens [58]. P.M.G.P. Moreira et al. (2012) performed SN (stress range vs. fatigue life) and fatigue tests of the BM and FSWed 2195-T8X aluminium-lithium alloy at room temperature. The stress ratios were taken as 0.1, 0.5 and 0.8. They observed that the FCP data showed notable R dependency for the BM, but this was not significant for the FSWed material [59]. E. Maggiolini et al. (2016) examined the effect of FCG path on FSWed AA6082-T6 taking different values of stress ratio (R) and observed that the loading conditions did not have a major impact on the region where the crack was initiated [60]. (b) (a)
Y
Y X
X
Weld
Fig. 3. Schematic representation of weld zone in (a) C(T) and (b) ESE(T) samples [58].
8. Effect of residual stress and stress intensity factor (ΔK) on fatigue crack growth rate In the late 2000s, researchers investigated the residual stress effects on FCGR. In an experimental study by G. Bussu et al. on 2024-T351 aluminium plates, it was concluded that in the tension residual stress fields, the cracks which propagated at an angle of 90 to weld line would show faster CGRs than in the compressive residual stress fields. In another observation tensile residual stress fields showed minimal change in the stress intensity factor (∆K) value. As residual stress value approached zero and moved towards compression, ΔK value changed significantly [61]. In another experiment, Y.E. Ma et al. (2011) designed C(T) samples of 2195-T8 Al-Li Alloy and then measured the FCGRs in those specimens. It was observed that FCGRs were lower in the nugget zone as compared to the BM. They also observed a significant impact of compressive residual stresses on FCGRs around the notch and how their control on cracks reduced with crack growth [62]. S. Kim et al. (2008) investigated the FCP behavior of 4 mm thick FSWed AA5083-H32 and AA6061-T651. The FCP behavior of the two samples were studied considering FCG either perpendicular or parallel to the DXZ for different ∆K values. For low ∆K values, the presence of FSW zone did not have a significant effect on the FCGRs. However, notable effects were observed for higher values of ∆K [63]. In a study on the FCP in FSWed AA2024-T351, L. Fratini et al. (2009) noticed that residual stress dominated the FCG behavior outside the weld. From a few FCG tests, it was noticed that based on initial notch position residual stresses would induce low CGRs [64]. K. Kuwayama et al. (2009) performed FCP tests on 2 mm thick FSWed AA2024-T3 and accelerated CPR was observed for high values of tensile residual stress [65]. Y. E. Ma et al. (2011) studied FCGRs in residual field using residual K (Kres) approaches. The presence of edge notch root in C(T) and ESE(T) specimens was found within a residual stress field causing compression thereby reducing effective ΔK (Keff) and CGRs [66]. John A. New et al. (2015) performed FCG tests at constant ΔK on FSWed AA2024-T351. In absence of residual stress field, CGR (da/dN) remained constant with crack length, whereas da/dN showed variable growth with the field of residual stress being present [67]. Many researchers like R. Citarella conducted a detailed comparison of experimental and numerical findings to scrutinize the effects of residual stresses on FCP and good agreement was found between the two [68].
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9. Other experiments involving fatigue crack growth rate Many researchers like O. Hatamleh investigated the impact of peening on the FCG behaviour of FSWed AA 7075-T7351 and AA 2195-T8 sheets at various peening conditions like shot peening (SP) and laser peeing (LP). By using three layered LP they found a notable decrease in FCG in comparison to the AW condition and unwelded BM, but shot peening could not influence FCG in FSW samples to a notable extent [69, 70]. J.A. Ronevich et al (2016) tested FSWed steel pipelines and measured FCGR vs K range in 21MPa hydrogen gas at f = 1 Hz and R = 0.5. They concluded that heat affected FCGRs of the BM were insignificantly smaller as compared to the welded region [71]. Researchers have studied the FCG simulation of various Al alloys using XFEM and FEM and confirmed their utility in successfully simulating a complex 3D geometry for FCG simulation [72, 73]. From the experiments performed by Y.E. Ma et al. on FCP in FSWed 2xxx Al-Li alloy, it was observed that for 1.6 mm thick 2198 M(T) specimens, CGRs were similar to the parent metal and for the 8 mm thick ESE(T) samples, the CGRs were noticeably retarded in notch region [74]. Lei Wang et al. (2016) performed various tests on 4 mm thick 2024-T4 Al alloy plates, including FCP tests in air, 3.5% NaCl solution and under pre-corrosion condition. From the results obtained, they concluded that 3.5% NaCl solution decreased the fatigue life upto 50% of the previous welded specimens and the corrosive environment decreased the same upto 64%. It was also found that the CGRs in BM were lesser as compared to the welded region in the three mentioned test conditions [75]. M. Zadeh et al. (2009) found good agreement between analytical and computational methods in investigating FCG of AA2024-T351 using boundary element method [76]. B. Zou et al. (2011) studied the growth of fatigue cracks in AA7075-T6 FSWed joints. They used different stress ratios for an FCG simulation using AFGROW and studied their influence on FCG. They observed that the RS of the HAZ showed the lowest FCP, whereas the zone located perpendicular to the welding seam showed the highest. It was concluded that FCP increased with increase in stress ratio [77]. 10. Conclusion In this review, mainly the influences of welding conditions on FCGR have been investigated. Also the effects of microstructure, residual stress, peening effect and many other factors have been studied and reviewed. Major conclusions drawn include large influence of tool rotational speed and welding speed on fatigue crack generation and propagation. Microstructure analysis reveals importance of grain size and how various regions of a workpiece undergo different FCG rates. Laser peening improves the quality of the material and reduces the chances of crack formation. Various numerical analysis show good acceptance between experimental and theoretical findings of FSW and how it improves the quality of the weld overall. Even though friction stir welding is highly useful when compared to conventional welding methods, fatigue crack propagation in FSWed materials is a major disadvantage and hence process parameters should be used in the most favorable range to reduce crack formation and improve the efficiency of the weld. Acknowledgement Authors are thankful to “ELSEVIER (Licence numbers: 4381301401731, 4381310435271, 4381310923427)” for providing the copy right permission to various figures in the current paper. References [1]
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