One-sided laser beam welding of autogenous T-joints for 6013-T4 aluminium alloy

Accepted Manuscript One-sided laser beam welding of autogenous T-joints for 6013-T4 aluminium alloy A.C. Oliveira, R.H.M. Siqueira, R. Riva, M.S.F. Lima PII: DOI: Reference:

S0261-3069(14)00757-2 http://dx.doi.org/10.1016/j.matdes.2014.09.055 JMAD 6828

To appear in:

Materials and Design

Received Date: Accepted Date:

25 July 2014 20 September 2014

Please cite this article as: Oliveira, A.C., Siqueira, R.H.M., Riva, R., Lima, M.S.F., One-sided laser beam welding of autogenous T-joints for 6013-T4 aluminium alloy, Materials and Design (2014), doi: http://dx.doi.org/10.1016/ j.matdes.2014.09.055

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One-sided laser beam welding of autogenous T-joints for 6013-T4 aluminium alloy Oliveira, A.C.a,b,1*; Siqueira, R.H.M.a,b; Riva, R.b; Lima, M.S.F.b a

Instituto Tecnológico de Aeronáutica, ITA, Pça.Mal.Eduardo Gomes, 50, São José dos Campos, SP, 1228-900, Brazil b Instituto de Estudos Avançados, IEAv-DCTA, Trevo Cel.Av.José.A.A.do Amarante, 1, São José dos Campos, SP, 12228-001, Brazil Abstract Autogenous T-joints for aluminium skin-stringer component performed by one-sided laser beam welding process was conducted using a high power Yb-fiber laser. The influence of the shielding gas, seam angle, beam focal position, and beam positioning relative to weld centerline were investigated regarding to weld microstructural features. The joint mechanical behavior was evaluated concerning to the sheet rolling directions. It was observed that a precise control of the process parameters enabled to obtain weld beads with acceptable dimensional and geometric characteristics and minimizing weld defects. Helium shielding gas produced higher aspect ratio welds than those with pure argon. Although, pores were observed in the fusion zone, they represented only about 5% of the weld bead area. The optimal beam positioning should remain up to 0.2 mm relative to junction line, for seam angles between 10° and 15°. The weld mechanical behavior depended on the sheet rolling direction. Joint efficiency up to 85% were obtained after hoop tensile tests when the weld bead longitudinal-section was perpendicular to skin rolling direction and parallel to the stringer rolling direction. Keywords: laser beam welding; Yb-fiber laser; autogenous T-joints; aluminium alloy. 1. Introduction Laser beam welding has been studied, and even employed in the aeronautical industry, particularly for the joining of skin-stringer fuselage sections [1,2]. The traditional riveting joining method, although highly automatized, offers a small potential to increasing production rate or aircraft weight savings [2,3]. Some studies have demonstrated the laser joining technique as a possible replacement of riveting [4,5]. Usually a filler material has been used during laser welding to fill the gap existing at the joint or improves the weld bead toughness [6]. Braun [7,8] analyzed the influence of the filler wire composition on the welding process stability. He proposed a filler wire containing large amounts of silicon for adjusting weld pool chemistry of the AA6000 alloys, ensuring the elimination of solidification cracks in the fusion zone. Squillace and Prisco [9] also investigated the influence of filler additions on micro and macro-mechanical behavior of T-welded joints. Their results suggested 1

Permanent address: Universidade Federal de São Paulo, UNIFESP, Rua Talim, 330, São José dos Campos, SP, 12230-280, Brazil * Corresponding author: Tel.: +55 12 3309-9600 or +55 12 981447036. E-mail address: [email protected] or [email protected] (A.C.Oliveira) 1

the possibility to reduce the weakening at the heat affected zone of aluminium alloy welds using a filler wire with high melting latent heat. However, laser welding with filler wire has been considered an additional difficult for industrial application, having many parameters and stringent requirements for wire positioning [10]. According to Tao et al [11], the feeding position, wire feeding direction, and wire feeding angle had significant influence on the laser welding process stability and on pore formation. Therefore, the use of filler wire can limit the utilization of laser technology to joint skinstringer components, since the process parameters are quite complex and defect generation must be strictly controlled. Laser beam autogenous weld under T-joint configuration could be much simpler by minimizing the difficulties of the introduction of filler metal. One of drawbacks for the autogenous laser welding is the presence of air gaps existing at the joint region, which can promote defects such as concavity, sidewall fusion defects, root suck-up, and weld undercuts [10]. According to Salminen [6], in the typical butt joint, the widest acceptable gap for autogenous laser welding is usually 10% of the material thickness. Dawes [12] concludes that a gap width of 0.14 mm in sheet thickness of 2 mm leads to concavity defect, reducing loading to 86%. Yang et al. [13] showed that, in the double-sided laser beam welding, parameters such as: incident beam position, beam angle, and beam separation distance affect strongly the metallurgical quality of the T-joints. Prisco et all [14] showed that the distribution of weld bead along of skin-stringer components influences the mechanical strength of the joint. According to their results, the melt region should not exceed 30% of skin thickness since an excessive penetration in this area promotes stress concentration favoring the crack propagation, especially in the heat affected zone. In this sense, the one-sided laser beam welding in the T-joint configuration promotes lower penetration depth of weld bead in the skin component. Additionally, one laser run induces less thermal damage (less heat input) which decreases possible part distortion. The main drawback refers to laser alignment that becomes more critical in order to ensure the junction between the components. Although the introduction of the filler metal and the double-sided laser beam configuration was clearly accepted, the available literature indicated the use of lasers with lower beam quality than the Yb-fiber laser of the present work. To the authors, there is lack of knowledge about one-sided laser beam welds of high-strength aluminium, under T-joints autogenous condition, using a high power Yb-fiber laser. In the present work, autogenous AA6013 T-joints in one-sided configuration were performed using a high power Yb-fiber laser. The main goal of this research is to evaluate the relationship between the welding parameters, the microstructure and the mechanical properties of the joints, in order to optimize the laser process for this aeronautic application. 2. Experimental details 2.1. Material and experimental setup

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Sheets of aluminium alloy, AA6013, with 1.6 mm thickness, received in room temperature aged T4 condition, were used. The chemical composition of the sheets is presented in Table 1. The 18 mm x 100 mm metal sheets were tightly clamped and fixed on a XYZ CNC moving table. Before processing, all sheets were cleaned with abrasive paper (SiC grit 600) and ethanol to remove the residual grease and any contaminants. The laser processing head was attached to a goniometer table, which allows changing the laser beam incidence angle from 0° to 900. Figure 1-a shows the schematic diagram of the welding experiment. Overlapped and Tshaped autogenous joints were conducted by varying the laser average power between 1000 and 1800 W and the welding speeds from 1.8 to 9 m/min. Argon or helium with a flow rate of 20 L/min were used as shielding gases. Additionally, T-shaped autogenous joints were conducted by varying the seam angle of 6° to 29° between the laser beam and the stringer specimen (Figure 1-b). The influence of the beam positioning on the weld bead quality also was evaluated, varying the beam positioning along of the weld longitudinal section between -0.5 mm and 0.40 mm relative to the joint centerline (joint line between the sheets), as shown in the Figure 1-c. 2.2. Microstructure analysis Metallographic analyses were performed on the weld bead cross-section by optical microscopy (OM). The specimens were grinded with abrasive paper (SiC grits of 240, 400, 600 and 1200, respectively), polished to a mirror finish (1.0 μm Al2O3 solution and colloidal SiO2) and chemically etched using Keller’s reagent (2 ml of HF, 1 ml of HNO3 and 88 ml of H2O). The penetration depth and the width of the weld bead, obtained under different process parameters, were analyzed by optical microscopy (MO). Image J software [15] was used to measure the dimensional characteristics in five transversal positions of the weld bead. Additionally, the amount of the pores presents in the weld cross-section was examined. Areas of the micro and macro porosities were measured and it subtracted from total area of the weld bead using the Image J software. 2.3 Mechanical testing In general, to check the mechanical resistance of aircraft fuselage panels, two kinds of monotonic quasi-static tensile tests have been considered [9,13]. The first tensile test applies a perpendicular stress to the contact plane between the stringer and skin and verifies the adhesion of the components. The second one applies a stress to skin perpendicularly to welding direction. This test aims simulating the resistance of the Tjoints to a circumferential stress (hoop stress) due to the pressurization of the cylindrical fuselage. Here, they will be referred as T-pull test and hoop tensile test, respectively. The specimens were cut from the welded plates, with dimensions of 18 mm x 72 mm for the T-pull tests, and 9 mm x 72 mm for the hoop tensile tests. The Figure 2-a shows the schematic drawing of the specimen fixed to tensile machine to the hoop tensile tests. To execute the T-pull tests, a home-made system to clamp the specimen was used (see Figure 2). The skin panel was tightened between the upper and lower plates of the clamping system via four bolts. A rectangular groove machined at the middle of the 3

upper plate allows the stringer to pass through. Under this configuration, the stringer was subjected to a tension aiming to tear it off from the skin. The Figures 2-b and 2-c show, respectively, the schematic drawing of the specimen fixed to home-made system and the image of this home-made system coupled to tensile machine to the T-pull tests. The experiments were performed using a 100 kN universal tensile machine (EMICDL10000). 2.4 Laser beam characterization A high-power Yb-fiber laser with a 2 kW maximum power (IPG, YLS 2000) was used in the experiments. Using a process fiber of 10 m long and 100 μm diameter, the beam was focused by a lens with focal length of 160 mm. The laser beam profile was measured by using a modified rotating wire beam scanner based on the work of Lim and Steen [16]. An optical fiber of 150 mm long and 125 μm external diameter was coupled to a disk fixed on a continuous rotating axis. The fiber was positioned near the focus region of the laser beam without any attenuation optical element alloying measuring of the actual laser beam used on the welding experiments. A detector comprising of a lens with focal length of 50 mm and a photodiode coupled to an oscilloscope were located such that they measure the light reflected by the fiber surface. The radiation reflected by the optical fiber was delimited by vertical slit in front of the detector and all scattered radiation vertically reaches this detector. In the horizontal direction, the reflected radiation detected depends on the slit width and on the fiber-slit distance. The fiber displacement generates a signal in the detector that represents the vertical integrated laser beam profile. The beam radius was then estimated by knife-edge method where the laser beam radius w is obtained by measuring a beam clip width DC, which represents the distance between the points at which the power output was 10% and 90% of maximum value as defined by Siegman [17]. The beam radius w is related to the beam clip width by the Siegman expression: w= 1.561Dc/2. Figure 3 shows the Yb-fiber laser beam profile (solid line) obtained at the focal position (Figure 3-a), 1mm and 2 mm far from the focus (Figures 3-b and 3-c, respectively) using an average laser power of 1000 W. The dashed lines refer to the integrated beam profiles used to calculate the beam diameter according to the Siegman procedure [17]. The vertical dashed lines shown on Figure 3-a represents the 10 % and 90% clip points which define the beam clip width Dc. The laser beam profile was measured in different distances of the focal lens in order to obtain the beam propagation curve showed in Figure 4. With this data we estimated the laser beam waist w0= 50 ʅm, the laser beam quality factor M2= 9 which represents a beam parameter product (b.p.p.) of only 3 mm.mrad, and a laser beam depth of focus 2ZR= 1.6 mm [18]. 3. Results and discussion 3.1. Dimensional and metallographic characteristics of the weld bead Overlapped and T-shaped autogenous joints were performed by varying the laser power between 1000 and 1800 W and welding speed from 1.8 to 9 m/min. The main aim was to weld with acceptable dimensional characteristics accordingly to the state of the art, and with few defects. The weld cross-sections at different configurations are shown in Figures 5-a and 5-b, respectively. In general, the keyhole welding has been carried out with incident laser beam intensities on the surface of about 106 W/cm2. In the present 4

study, surface incident intensity values between 1-5.107 W/cm2 were employed. Under these power levels, the vapor pressure had enough thrust to move the melt upwards during the welding inhibiting the collapse of the keyhole. This additional thrust could be understood as a driven force decreasing pore formation dynamics. Similarly to the results observed by Braun [7,8], the fusion zone exhibits a fine cellular dendritic solidification structure with many equiaxed grains in the weld centerline. According to studies [19], the equiaxed grains tend to decrease the cracking susceptibility and to improve the weld mechanical properties. A partial melting zone is adjacent to the fusion boundaries and its width is the order of 50 μm in all analyzed conditions (Figure 6). Although this heat-affected zone (HAZ) seemed to be composed by liquation cracks, this is not true. The darkening of HAZ grain boundaries are due to the extended etching time necessary to reveal the aluminium microconstituents. This region has been considered as result of heating up the area surrounding the fusion zone to temperatures between the eutectic temperature and the liquidus of the alloy [7]. Another analyzed aspect refers to presence of pores in the weld bead. Some factors such as inadequate shielding gas, presence of surface contaminants, hydrogen trapping in the melt, vaporization of alloy elements, and keyhole collapse [20,21,22] could promote process instability, contributing to the formation of the micro and macropores. Figure 7 presents the effective area of the weld cross-section and the related specific porosity at different welding speeds. The results evidenced a maximum pore area of about 0.1 mm2, which represents about 5% of the weld bead area. This low porosity has been related to high quality of the laser beam [23], because its low divergence and small focal diameter produced more stable welding conditions. Thus, the Yb-fiber laser quality with focal radius of order 50 ђm and M2 = 9 contribute to pores reduction along of the weld bead. To get an overview of the weld dimension in each experimental condition, the penetration depth (Fig. 8-a) and the width (Fig. 8-b) of the weld beads were measured. The results show that the weld dimensions decrease with the increase of welding speed for a given laser average power. 3.2. Influence of shielding gas The use of argon or helium as shielding gases was compared in the welding experiments. Both gases have been commonly used for laser welding of the aluminium alloys [24,25], but their different physical properties could influence the weld bead quality. The shielding gas influence could be associated to three main physic phenomena occurred in the material surface during the welding process: the energy absorption by plasma on the surface, the scattering of laser radiation by ejected particles, and gas lens effect due caused by the temperature gradients near the surface [26]. Greese et al [27] showed the shield gas type has a minor influence on the welding process when using a 1.06 um Nd-YAG laser because its radiation is less absorbed by the ejected vapor. However, in this work, as it is shown on Figure 9, the penetration depth of the welds using helium promoted deeper weld beads than those obtained with argon, under the same experimental conditions. We believe the influence of the shielding gas on the Yb-fiber laser welding could be attributed to its better beam quality. In this case, the laser beam intensity is much more sensitive to any disturbance caused by a temperature gradient or particle scattering. So the use of Helium which has a better thermal conductivity could explain the results shown of Figure 9. 3.3. Influence of the seam angle 5

The variation of seam angle on the sample provides data about the weld bead transversal shape between the skin-stringer components. According to the literature [14], the weld bead in the skin region should not exceed 30% of its thickness. The non-compliance of this criterion affects the mechanical strength of the welded component, particularly due to the presence of heat-affected zone (HAZ) which concentrates stresses and becomes the region susceptible to crack propagation. Moreover, the penetration depth of the weld bead should surpass the stringer thickness. The insufficient weld penetration lack in the stringer contributes to decrease of weld resistance. Figure 10 presents the transversal section of the weld beads by varying the seam angle between 6° to 29°. The angles between 10° and 15° produce weld beads with adequate melt distribution at the skinstringer intersection. High angles generate welds with penetration depth beyond 30% of skin thickness. The gaps also influenced the process instability, generating macropores in the melted region. 3.4. Influence of the beam focal position on the weld bead penetration T-joint process requires the study of the beam focal position (z) on the sample. Particularly for the processing of aluminium alloys, some studies [2,7] showed the variation of the penetration depth with the changes of the focal position at the workpiece surface (z = 0). Figure 11 presents the weld size by varying of beam focal position from 2.5 mm to -2.5 mm (focal spot 2.5 mm above or below workpiece surface) to a fixed seam angle of 15°. The results shown that the weld sizes are constants to defocusing of 1 mm, i.e., depth penetration and width of the weld bead remaining with constant values up to a defocusing of 1 mm (z = -1 mm). This aspect is related with the beam quality of the Yb-fiber laser of this work which has a depth of focus of 1.6 mm. In this distance, the laser beam intensity remains almost constant and the variation of the beam focal position relative to the material surface presents smaller influence on the weld dimensions. It is important to accentuate the fact this large depth of focus was obtained with a beam diameter of only 0.1 mm and therefore, the laser intensity was very high (~107 W/cm2). The Yb-fiber laser presents the best beam quality at moderate powers in comparison with other welding lasers sources [28]. For instance, the diode or lamp pumped NdYAG lasers has a minimum b.p.p. of 12 mm.mrad (M2 = 35) which is about four times the b.p.p. of the Yb-fiber laser used in this work [28]. As a consequence, the 4 kW NdYAG laser shown in [28] had to be focused with a beam diameter of 0.6 mm in order to achieve a depth of focus compatible with the required weld depth. In spite of higher NdYAG laser power, its beam intensity was only 106 W/cm2 which shows clearly the beam quality effect on the laser welding process. 3.5. Influence of the beam positioning relative to the joint centerline Welding experiments were carried out to evaluate the influence of the beam positioning relative to weld centerline. Analyses were performed by varying the beam positioning along of the weld longitudinal section, maintaining fixed the seam angle in 15° and the beam focus at the surface (z = 0). Figure 12 shows the distribution of weld transversal section considering eight different positions (P1 to P8) of the beam on the sample from a zero point (P0), located in the joint line between the sheets. The beam positioning, between -0.10 mm to 0.05 mm, P4 and P5, produces weld beads with high quality without severe defects. In other beam positions, defects such as pores and cracks increase significantly due to presence of the gap between the sheets, which also generated keyhole instability during the welding.

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It was also analyzed the influence of the beam positioning on the weld sizes (Figure 13). The variation of beam positioning does not influence s the weld size, excepting at the extreme beam positions, such as -0.50 mm and 0.40 mm (P1 and P8, in Figure 12), where the welds are uncompleted. This means that the molten volume remained constant, despite of the beam positioning variation. However, the welding quality was greatly affected by weld bead transversal distribution at the skin-stringer joint. Thus, the beam positioning relative to weld centerline should not exceed 0.2 mm. 3.6. Mechanical behavior of skin-stringer joint Two monotonic quasi-static tensile tests were carried out on the welded T-joints, T-pull test and hoop tensile test. In the both experiments, the joint strength were evaluated according to influence of the seam angle (10° and 15°) and the sheet rolling direction as depicted in Table 2 and results are presented in Table 3. The results show that the sheet rolling direction did not influence the T-pull tensile strength, for seam angles of 10° and 15°. Optical microscopy of the fractured surfaces after T-pull tests (Figure 14) shown that the fracture occurs at weld bead central region. In fact, the weld microstructure became completely different of its original microstructure, inhibiting the lamination effect on the mechanical behavior. The hoop tensile tests are shown in the Figure 15, considering seam angles of 10° (Figure 15-a) and 15° (Figure 15-b), respectively. An improvement of the ductility and mechanical strength are observed with longitudinal-section perpendicular to skin rolling direction and parallel to stringer rolling direction (series B, Table 2). Although this aspect must be confirmed by additional analyzes, the results indicate some influence of the sheet rolling direction on the weld behavior mechanical by hoop tensile tests. The joint efficiency was obtained by the ratio between UTS (Ultimate Tensile Strength) of the welded and of the base material (reference value, 319 MPa). The joint efficiency of the weld submitted to T-pull test is about of 50%, once the tensile strength of the weld bead remains approximately constant in all experimental conditions. On the other hand, the joint efficiency obtained of the weld bead submitted to hoop tensile test presents some variation depending on the experimental conditions. The joint efficiency reached 85% for the better welding condition (series B, Table 2). 4. Conclusions The experimental results reported in this work provide the main aspects of the aluminium alloy T-joints performed by high power Yb-fiber laser. It has been concluded: • Autogenous T-joint welds of the 6013-T4 alloy sheet were produced by onesided laser welding process, simulating the skin-string component of an aircraft. • A partial melting zone was observed adjacent to the fusion boundary, which extends few micrometers into the heat-affected zone (HAZ), depending on experimental conditions, but any crack was been originated on it. The main weld defect was porosity in the fusion zone. The maximum pore area was 0.1 mm2, representing only about 5% of the weld bead area. • Welding process using helium as shielding gas produced deeper welds when compared to welds performed using argon, under same experimental conditions. • The beam focal position on the sample can be varied up to 1 mm below the upper surface without affecting the weld sizes. Unlike, the focal positioning relative to weld centerline showed a critical parameter in the weld quality. Its variation should not exceed 0.2 mm relative to weld centerline. The seam angle 7

•

on the sample should be between 10° and 15°, considering the beam diameter of 0.1 mm. The mechanical behavior of the welded components was related to sheet rolling direction. Weld beads with higher values of ductility and mechanical strength were obtained when its longitudinal-section was perpendicular to skin rolling direction and parallel to stringer rolling direction. In this condition, joint efficiency up to 85% was obtained of welds submitted to hoop tensile test.

Acknowledgements Financial support from FAPESP (process n° 2007-03910-7) is gratefully acknowledged. References [1] SCHUBERT, E.; KLASSEN, M.; ZERNER, I.; WALZ, C.; SEPOLD, G. Lightweight structures produced by laser beam joining for future applications in automobile and aerospace industry. Journal of Materials Processing Technology, v. 115, p. 2-8, 2008. [2] RIVA, R.; LIMA, M.S.F.; OLIVEIRA, A.C. Soldagem a laser de estruturas aeronáuticas. Metalurgia & Materiais, v. 65, p. 48-50, jan/fev.2009. [3] TRAVESSA, D. N.; RONDON, V.; NETO, V. P. Aspectos da competitividade do alumínio em estruturas aeronáuticas. Metalurgia & Materiais, v. 65, p. 45-47, jan/fev.2009. [4] Braun, R.; Donne, C.D.; Staniek, G. Laser beam welding and friction stir welding of 6013-T6 aluminium alloy sheet. Mat.-wiss.u.Werkstofftech Journal, v.31, p. 1017-1026, 2000. [5] Badini, C.; Pavese, M.; Fino, P.; Biamino, S. Laser beam welding of dissimilar aluminium alloys of 2000 and 7000 series: effects of post-welding thermal treatments on T joint strength. Science and Technology of Welding and Joining 2009; 14:484-492. [6] Salminen, A. The filler wire – laser beam interaction during laser welding with low alloyed steel filler wire. Mechanika 2010; 4 (84):67-74. [7] Braun, R. Nd:YAG laser butt welding of AA6013 using silicon and magnesium containing filler powders. Materials Science and Engineering A 2006; 426:250-262. [8] Braun, R. Laser beam welding of Al-Mg-Si-Cu alloy 6013 sheet using silicon rich aluminium filler powders. Materials Science and Technology 2005; 21:133-140. ȏͻȐSquillace, A.; Prisco, U. Influence of filler material on micro-and-macro-mechanical behaviour of laser-beam-welded T-joint for aerospace applications. Mat. Res. 2013; 16(5):1106-1112. [10] Dilthey, U.; Fuest, D.; Scheller,W. Laser welding with filler. Optical and Quantum Electronics 1995; 27:1181-1191.

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[11] Tao, W.; Yang, Z.; Chen, Y.; Li, L.; Jiang, Z.; Zhang, Y. Double-sided laser beam welding process of T-joints for aluminum aircraft fuselage panels: Filler wire melting behavior, process stability, and their effects on porosity defects. Optics & Laser Technology 2013; 52:1-9. [12] Dawes, C.J. ICALEO '85, 4th International Congress on Applications of Lasers and Electro-Optics, San Francisco, 1985, p. 73. [13] Yang, Z.B.; Tao, W.; Li, L.Q.; Chen, Y.B.; Li, F.Z.; Zhang, Y.L. Double-sided laser beam welded T-joints for aluminum aircraft fuselage panels: Process, microstructure, and mechanical properties. Materials and Design 2012; 33:652-658. [14] Prisco, A.; Troiano, G.; Acerra, F.; Bellucci, B.F.; Squillace, A.; Prisco, U. LBW of similar and dissimilar skin-stringer joints, part I: process optimization and mechanical characterization. Advanced Materials Research 2008; 38:306-319. [15] IMAGEJ. Image Processing and Analysis in JAVA. Available in: http://rsbweb.nih.gov/ij/. Acess: 10th March 2008. [16] LIM, G.C. and STEEN, W. M.The measurement of the temporal and spatial oer distribution of a high powered CO2 laser beam. Optics Laser Technology, june 1982,149-153 [17] Siegman, A. E., Sasnett, M.W., & Johnston, T.F., Jr. (1991, April). Choice of clip level for beam width measurements using knife-edge techniques. IEEE Journal of Quantum Electronics, Vol. 27, No. 4 [18] Oliveira, A.C.; Siqueira, R.H.M.; Lima, M.S.F.; Riva, R. A simple model for optimizing laser welding properties. In: The XIX International Symposium on High Power Laser Systems and Applications, 2012, Istambul. Book of Abstracts XIX HPLS&A. Istambul: Tübital MAM, 2012. p.100. [19] Rappaz, M.; Drezet, J.-M.; Gremaud, M. A New Hot-Tearing Criterion, Metallurgical And Materials Transactions A Volume 30A, 1999, P. 449-455 [20] S.L. Ream. Laser welding efficiency and cost: CO2, YAG, Fiber, and Disc. In: International Congress on Applications of Laser&Electro-Optics, 23th , 2004. Proceedings…, p. 28-32, 2004. [21] Pastor, M.; Zhao, H.; Martukanitz, R. P.; Debroy, T. Porosity, underfill and magnesium loss during continuous wave Nd:YAG laser welding of thin plates of aluminum alloys 5182 and 5754. Welding Research Supplement 1999; 78(6):207s-216s. [22] Haboudou, A.; Peyre, P.; Vannes, A.B.; Peix, G. Reduction of porosity content generated during Nd:YAG laser welding of A356 and AA5083 aluminium alloys. Materials Science and Engineering A 2003; 363:40-52. [23] Weberpals, J.; Dausinger, F.; Göbel, G.; Brenner, B. Role of strong focusability on the welding process. Journal of Laser Applications 2007; 19(4):252-258.

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[24] Thomy, C.; Seefeld, T.; Vollrstsen, F. Applications of high power fiber lasers for joining of steel and aluminum alloys. In: International WLT – Conference on Lasers in Manufacturing, 3rd, 2005, Munich. Proceedings…27-32, 2005. [25] Verhaeghe, G.; Allen, C.; Hilton, P. Achieving low-porosity laser welds in 12.7 mm thickness aerospace aluminium using a Yb-fiber laser. In: International WLT – Conference on Lasers in Manufacturing, 4th, 2007, Munich. Proceedings…17-23, 2007. [26] Steen, W. M.; Mazumder, J. Laser material processing, 4nd, Ed.Springer-Verlag London Limited,2010. [27] Greses, J.; Barlow, C.Y., Steen, W.M.; Hilton, P.A. Spectroscopic studies of plume/plasma in different gas environments. In: ICALEO 2001 Proceedings, Jacksonville, October 2001, LIA, Orlando, paper 808. [28] Verhaeghe G.; Hilton P. Battle of sources – using a high-power Yb-fibre laser for welding steel and aluminium. In: International WLT – Conference on Lasers in Manufacturing, 3rd, 2005, Munich. Proceedings…,33-38, 2005.

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Figure Captions: Fig.1: (a) Schematic diagram of the welding experiment, showing the (b) variation of the seam angle and (c) the variation of beam positioning along of the weld longitudinal section. Fig.2: (a) Schematic drawings of the specimen fixed (a) to tensile machine to the hoop tensile tests and (b) to home-made system to T-pull tests, showing the system (c) coupled to tensile machine to these tests. Fig.3: Yb-fiber laser beam profile (solid line) obtained between the (a) focal position (b) 1mm and (c) 2 mm far from this one where the dashed lines represent the integrated beam profile. Fig.4: Beam radius (w) measurement at different positions (z) in respect to the focal position (z0=160 mm). ZR is the Rayleigh distance. Fig.5: Cross-section from the (a) overlap and (b) T-shaped autogenous welds. Fig.6: HAZ partially melted zone adjacent to fusion boundary. Fig.7: Weld cross-section area and the porosity formed for different welding speeds. Fig.8: (a) Penetration depth and (b) width of the weld bead in different welding speeds. Fig.9: Penetration depth of the weld bead using argon and helium as shielding gases. Fig.10: Cross-section of the weld beads made with seam angle of 6° to 29°. Fig.11: Weld size characteristics with focal spot from 2.5 mm above or below workpiece surface. Fig.12: Cross-sections of the weld bead considering eight different positions of beam positioning on the sample. Fig.13: Influence of beam positioning on the weld sizes. Fig.14: Facture at the weld bead central region after the T-pull test. Fig.15: Hoop tensile tests in different rolling directions, considering the two seam angles (a) 10° and (b) 15°.

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Table Captions: Table 1: Chemical composition of AA6013-T4 (%), Table 2: Sheet rolling direction relative to weld bead longitudinal-section. Table 3: Tensile properties of the 6013-T4 joints.

12

Mg

Si

Cu

Mn

Fe

Others, total 0.94±0.05 0.62±0.02 0.82±0.02 0.27±0.04 0.20±0.01 0.06±0.01

13

Series

Skin-stringer component

Description

A

Weld bead longitudinalsection perpendicular to skin and stringer rolling directions.

B

Weld bead longitudinalsection perpendicular to skin rolling direction and parallel to the stringer rolling direction.

C

Weld bead longitudinalsection parallel to skin rolling direction and perpendicular to the stringer rolling direction.

D

Weld bead longitudinalsection parallel to skin and stringer rolling directions.

14

Rolling Direction*

Ultimate tensile strength (MPa) T-pull test

Hoop tensile test

Joint efficiency T-pull test

Hoop tensile test

A

10° 151±6

15° 155±2

10° 218±10

15° 231±13

48%

78%

B

152±10

151±4

272±13

278±11

47%

86%

C

149±8

159±4

236±6

260±7

48%

78%

D

154±9

159±7

227.7±13

258±15

49%

76%

* See Table 2.

Highlights ͻ

We report autogenous T-joints of the 6013-T4 produced by one-sided laser welding.

ͻ

It is investigated the influence of the process parameters on the weld features.

ͻ

Beam focal positioning on the sample is a critical parameter in the weld quality.

ͻ

Tensile strength has some dependence with sheet rolling direction jointed by laser.



15

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Figure 15