Mechanical behavior of friction stir welded joints of aeronautic aluminum alloy AA2024-T351

Transcription

1 Mechanical behavior of friction stir welded joints of aeronautic aluminum alloy AA2024-T351 Abstract This investigation work focus in the analysis of Friction Stir weldability of the aluminum alloy AA2024-T351, a heat treated aluminum alloy, typically used in the aeronautical construction, due to its good mechanical properties and corrosion resistance. Therefore, FSW parameters have been developed and an evaluation of the influence of various characteristic defects of FSW in the mechanical resistance, under static tensile and bending loads, and fatigue tests. The mechanical efficiency of welded joints with various types of defects has been established and compared to the base material and welded joints without defects. It has been shown that the most critical defects in FSW are the root defects, both kissing-bond and intermittent flaws. Keywords Friction Stir Welding; AA2024-T351; Welded joints defects; Fatigue 1) Introduction FSW was invented by The Welding Institute in 1991, and since the first industrial application in 1995 that shows to be a superior welding method in the joining of many engineering materials, with emphasis to aluminum alloys. The FSW process uses a non-consumable rotating tool, consisting of a shoulder, generally normal to the axis of rotation and a smaller diameter profiled pin, rotating at high speed which plunges into butting edges of the workpieces to be joined. The movement of the tool over the work pieces results in friction and plastic deformation that generates heat. As the rotating tool translates along the joint line, plasticized material is stirred and forged behind the trailing face of the pin where it consolidates to form a solid state weld [1; 2]. The aluminum alloys, like 2XXX series are, extensively used in the aeronautic industry to produce airplane components such as frame parts, bulkhead, longeron and so forth. The fastening and riveting method along with adhesives have dominated this industry over the past decades, due to the low weldability of the most used aluminum alloys (2XXX and 7XXX series). The elimination of fasteners and riveters would not just only reduce the manufacturing cost, but would also provide a considerable weight reduction in the airplanes [3;4]. In the vast majority of engineering practice, most aluminum structures are welded and mainly subject to fatigue loading. The fatigue leads to damage of structural parts even if the stress applied to the part is quite low. The main factors that influence fatigue resistance of welded structures are type of loading, microstructure and weld defects. Although FSW joints have better quality over fusion welding techniques, there are some typical defects that can be found in FS welds when the used parameters are not correct, or when small variations of the correct parameters occurs [5].

2 The major aims of this investigation are to establish and assess the influence of this type of defects in the mechanical resistance under static and dynamic loads and find correct parameters to weld the aluminum alloy AA2024-T351. In order to know the relative importance of the 3 most typical defects in FSW, over the mechanical behavior of FSW structures, 4 types of specimens were welded. Specimens without FSW typical defects, two kinds of root defects and one internal defect were compared with base material specimens over static efforts of tensile and bending tests, and over dynamic efforts for fatigue. In the fatigue tests, S-N curves and metallographic analysis of fractures surfaces has been done. The joints characterization under the different conditions was complemented with non-destructive control techniques, such as visual analysis and x-rays, and destructive techniques such as metallographic analysis and hardness. The fracture surface observation using SEM have been done to investigate the cause of fatigue failure. FSW Typical Defects The typical defects that used to occur in FSW butt joint are represented in Figure 1. The defects that are going be studied are: 1) Lack of penetration (Kissing-bonds); 2) Root flaws (weak or intermittent linking); 3) Voids on the advancing side. Figure 1 - Typical defects on FSW butt joint These defects result from, e.g., lack of penetration of the pin and non uniform vertical force along the material thickness, imperfect stir of materials during the process and inadequate surface preparation. For each type of defect a code name was given. The characterization of the defect types is described in Table 1. Defect Type Code 0 I II III Description Not defective Root defect type I (particles alignment) Root defect type II (kissing-bond) Table 1 - Establishment of the 4 different Defect Types included in the FSW joints produced Internal defects (voids)

3 2) Experimental Details 2.1) Material The base material used in the present study was rolled sheets of AA2024-T351 with 4mm thickness. The material has been delivered in sheets with dimensions of 500mm x 250mm. All the welds were performed in a butt-joint arrangement. The chemical composition and mechanical properties are presented in table 1. Elements Cr Cu Fe Mg Mn Si Ti Zn Al Chemical composition Max Max Max 0.5 Max 0.15 Max 0.25 remain Tensile yield strength 324MPa Ultimate tensile strength 469MPa Elongation at break 20% Table 2 - Chemical composition and mechanical properties of AA2024-T ) Welding conditions The original rolled sheets have been divided in plates with dimensions of 250mm x 100mm, and were welded parallel to the rolling direction in a conventional milling machine. For all the welds the rotation speed was 710rpm, travel speed of 224mm.min -1 and a tilt angle of 0.5º. The used tool was modular dual tool allowing different combinations of shoulders, pins and different pin lengths. The different tools, with the different pin lengths, used to do the welds are schemed in Table 3. Shoulder Pin FSW tool assembled Pin length Defect type produced Tool 1 3.8mm 0 3.6mm I 3.3mm II Tool 2 3.9mm III Table 3 The different tools, with the different pin lengths, used to produce the 4 different Defect Types For purposes of fast analyses in situ, and to minimize the possibility of deviations from the steady state, 50mm at the beginning and about 30mm at the end of the welding plates, were removed. From each welded plate, one tensile specimen, one bending specimen and four fatigue specimens were cut out. 2.3) Metallographic Analysis The metallographic analysis allows to characterize the influence of the welding process in the microstructure in the base material. Distribution of the precipitates in the different zones of the weld and the changes of grain size resulting from the FSW process are investigated. Cross sections of the welds perpendicular to the welding direction were prepared using a sequence of grit paper of 240, 320, 600, 800, 1000, 2400 and 4000 mesh, and then polished on a microcloth using a diamond suspension of 6µm

4 diamond. The specimens were etched using Keller s reagent, and after were washed in distilled water. Finally the specimens were observed using an Olympus CK40M optic microscope. 2.4) Fatigue Testing The fatigue tests were carried out in a servo-hydraulic testing machine Instron, model 8874 equipped with a load cell of 25kN. The weld was transverse to the stress axis in the S-N specimen. A sinusoidal load time function was used, with the stress ratio R of 0.1. Oscillation was set between 4 14 Hz. 3) Results 3.1) Microstructure In Figure 2 is possible to see the macrostructure of the FS weld joint. The microstructure of a joint welded by the FSW process normally is divided in four distinct zones: a) the dynamically recrystallized zone, also known as nugget; b) the thermo-mechanically affected zone (TMAZ); c) heat affected zone (HAZ); d) base material (BM). Figure 2 Macrograph of the cross section perpendicular to the welding direction of an etched Defect Type 0 specimen The weld nugget (in the middle of Figure 3) is characterized by a fine grain structure due to dynamic recrystallization. The TMAZ, in Figure 3, is the zone in the transition between the weld nugget and the HAZ. This zone is characterized by a highly plastic deformed grain structure due to the FSW tool. In this zone the heat and the mechanical deformation were not enough to dynamically recrystalize the grain structure, originating elongated grains around the weld nugget. The HAZ, in Figure 4, extends approximately to the shoulder diameter for friction stir welds. The HAZ is similar to heat affected zone resulting from conventional fusion welding processes and is characterized by no material flow and relatively low temperatures as compared to the weld nugget during welding. Depending on the base material, its initial heat treatment, and the proximity to the centerline, precipitate coarsening, precipitate dissolution, recovery, recrystallization or grain growth can occur in the HAZ. Finally the BM shown in Figure 5 is the zone in which is contained the unaffected material that is remote from the weld and although it may have experienced a thermal cycle from the process, this has little effect on microstructure or mechanical properties. Figure 3 Microstructure of the TMAZ and weld nugget of the AA2024 T351 welds

5 Figure 4 Microstructure of the HAZ of the AA2024 T351 welds Figure 5 Microstructure of the BM zone of the AA2024 T351 welds In the figure 6 it can be seen micrographs of all the Defect Types studied in this work. In the Defect Type I, Figure 6.a), is possible to see some oxide layers (in a darker tone), as it was expected. The Defect Type II micrographs, Figure 6.a) and 6.b), show a weld nugget with a smaller height, not enclosing all the thickness of the plates, and are encircled by a similar granular structure to the TMAZ, due to plastic deformation that was induced to the material under the pin. The height of this defect has an average height of 0.4mm. In this Defect Type a zone is distinguished easily where the parts only are strongly leaned, with the surface of interface slightly deformed and another zone slightly deformed, where although already exists linking, the oxides have not been destroyed. The Defects Type III, Figure 6.d) and 6.e), appears in the advancing side, under the weld nugget, next to the border with the TMAZ. This zone has a clear influence of the pin in the stirred material. The nugget form is also different in this type of defect, because of the form of the pin that was different in this Defect type. Figure 6 Different types of defects studied: a) Defect Type I; b) Defect Type II; c) Defect Type II; d) Defect Type III; e) Defect Type III

6 3.2) Hardness Vickers hardness tests were performed in order to obtain important information about the mechanical properties along the welded zone. The measures were done with an indentation load of 0.5kg, in a specimen used for metallographic analysis, along the center of the welding, and 0.5mm from the top and the base of the welded specimen. The results are presented in Figure 7. Figure 7 Vickers hardness tests in the centre, top and base of the welds From the results obtained the following comments can be done: The BM hardness is higher in the base and top, with a medium value of 150HV, than in the centre with a medium value of 140 HV, due to the higher deformation induced by the rolling process, being higher than any welding zone; The lowest hardness is located in the retreating side, in the TMAZ to HAZ interface, because it is the hottest zone without dynamic recrystallization or grain deformation; The highest values of hardness, in the weld nugget, are found in the base and top. 3.3) Tensile and Bending Tests In order to analyze the results of tensile and bending tests, two analyses criteria s were used for each one of the tests. For the tensile tests EGRET (eficiência global de resistência estática à tracção) that means global efficiency to tensile static resistance, and EGREF (eficiência global de resistência estática á flexão) which means global efficiency to bending static resistance. For the tensile tests: Ei; σ 0,2i; σmáxi; Ai; Teni Mechanical properties of the welded joint i (respectively Young Modulus, Yield Strength, Ultimate strength, Elongation at break and Toughness ); EMB; σ 0,2MB; σmáxmb; AMB; TenMB Mechanical properties of the BM;

7 CE; Cσ 0,2; Cσ máx; CA; CTen Constants corresponding to the ponderation coeficients atributted to each one of the five mechanical properties in the EGRET factor with the values of respectively 0.05, 0.28,0.33, 0.11 and The results are schemed in Figure 8.a). For the bending tests: Fi; εi; Ei Mechanical properties of the welded joint I (respectively maximum load, dislocation at maximum load and Energy absorbed at break); FMB, εmb e EMB Mechanical properties of the BM; CF, Cε e CE Constants corresponding to the ponderation coeficients atributted to each one of the five mechanical properties in the EGREF factor with the values of respectively 0.25, 0.25 and 0.5. The results are schemed in Figure 8.b). 3.4) Fatigue Figure 8 EGRET and EGREF factors for tensile and bending tests respectively Figure 9 Fatigue resistance for BM and the 4 defect type conditions. a) S-N curve; b) Number of cycles efficiency relatively to the BM performance

8 From the analysis of the fatigue results, shown in Figure 9.a), is possible to see the good quality of the welds without defects, with a fatigue behavior near from the one of the BM. In the welded specimens with root defects, defect type I and defect type II, show a higher loss of fatigue resistance, being the defect type II, the one whose has worst fatigue properties. In the defect type III, the void defect, the gravity of this defect on the loss of properties over fatigue loading, is lesser than the root defects as it can be seen by the S-N curve. In the Figure 9.b) it is possible to see the efficiency in number of cycles relatively to the BM, and the performance of all the type of welds has bad results in high stress amplitudes. In all the welds the results get better when the stress amplitude became smaller. In the case of welds with defect type 0 the efficiency is higher than one in when the stress amplitudes has values smaller than 70MPa. 3.5) Fractography Analyzing the fracture zone of the welds, not all of them fractured in the same zone. The welds with defect type 0 fractured over the HAZ in the advancing side, the defect type I and II fractured in the weld root over the HAZ and the welds with defect type III fractured in the HAZ, in the defect, over the advancing side. The crack initiated, in the defective welds type 0, in multiple zones over the striates in the top of the weld, left by the tool and propagated over the HAZ. In the defective welds type I and II, the crack initiated in the weld root and propagated over the nugget. Finally, in the defective welds type III, the crack initiated over the defect in the advancing side and propagated over two directions (root and top) in the HAZ. In figure 10.a) is shown the typical cleavage plans in the vicinity of the top surface where the crack was initiated. It s possible to see to that there should be various nucleation sites over the fracture zone. In the Figure 10.b) is shown the zone of ductile fracture with typical dimple characteristic. There are no second phase particles visible in this zone, because the fracture happened in the HAZ, and in this zone might have happened precipitate coalescence due to high temperatures reached. Figure 10 SEM fractography of defect type 0: a) Localization of cleavage plans in the vicinity of the striates; b) Zone of final rupture In Figure 11.a) are shown the defect type I, and in Figure 12.a) the defect type II, in their crack initiation and propagation zone. It is possible to see the higher length of the root defect type II. In Figures 11.b) and 12.b) is shown the zone of ductile fracture.

9 In figure 13.a) is shown the nucleation site, which is the internal void. Figure 13.a) also shows the deformation pattern in the vicinity of the internal void of the specimens with defect type III. The ductile fracture zone, shown in Figure 13.b) is similar to the one of defect type 0. Figure 11 SEM fractography of defect type I illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) Zone of final rupture Figure 12 - SEM fractography of defect type II illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) Zone of final rupture Figure 13 SEM fractography of defect type III illustrating: a) Localization of cleavage plans in the vicinity of the defect; b) detail of a); c) Zone of final rupture

10 4) Conclusions The fatigue and mechanical properties of the friction stir welded joints, with and without defects and base material AA2024-T351 alloy were investigated. The following conclusions can be drawn: The several defects have a considerable effect in the mechanical properties of the welds, affecting the performance of those, in tensile and bending tests. This effect was higher in root defects (defect type I and II), than in defect type III. This defect seems not to play a major role in static tests; The welds have hardness values smaller than the BM (135 HV 155HV). The smaller value is found in the interface between the TMAZ and HAZ (123HV) in the retreating side. This is the zone where the specimens break in the tensile and bending tests, in welds with defect type 0 (no defects). The weld nugget has values smaller than the BM but higher than the TMAZ and HAZ (130HV 144HV); Over fatigue loading all the defect types play a major role in the weld performance. The BM has the better fatigue resistance, followed by defect type 0. The other types of defects lead to a decrease of resistance. In the end all the different material and defective conditions are ordered in decrescendo order of the level obtained for all the mechanical properties that were studied: 1. Base material 2. Welded joints without defects 3. Welded joints with internal defects (voids) 4. Weld joints with root defect of particles and oxides alignment 5. Welded joints with root defect (kissing-bond) 5) References [1] Vasco Godinho, Rudolf Zettler, Silvia Lomolino, Friction stir weld zone evaluation by image analysing techniques, Outubro de 2003; [2] VILAÇA, P.; Fundamentos do Processos de Soldadura por Fricção Linear: Análise Experimental e Modelação Analítica, PhD Thesis, IST, 2003; [3] Shusheng Di, Xinqi Yang, Guohong Luanb, Bo Jian; Comparative study on fatigue properties between AA2024-T4 friction stir welds and base materials; China;2006; [4] M. Ericsson, R. Sandstro m; Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG; Department of Materials Science and Engineering and Brinell Centre, Royal Institute of Technology (KTH), Sweden; 2002; [5] Caizhi Zhou, Xinqi Yang, Guohong Luan; Effect of oxide array on the fatigue property of friction stir welds; School of Materials Science and Engineering, Tianjin University, China; 2005; [6] Santos, Telmo; Vilaça, P; Quintino L. ; Freitas M. ; Advances in NDT techniques for friction stir welding; IDMEC, IST, Technical University of Lisbon, Lisboa, Portugal; [7] Vilaça, P; Pépe, Nuno; Quintino L. ; Metallurgical and corrosion features of friction stir welding of AA5083-H111; IST, Technical University of Lisbon, Lisboa, Portugal;

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