Materials and Design 65 (2015) 726–736

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Materials and Design

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One-sided laser beam welding of autogenous T-joints for 6013-T4aluminium alloy

http://dx.doi.org/10.1016/j.matdes.2014.09.0550261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Universidade Federal de São Paulo, UNIFESP, RuaTalim, 330, São José dos Campos, SP 12230-280, Brazil. Tel.: +55 12 3309 9600, +5512 981447036.

E-mail addresses: aline.capella@unifesp.br, alinecapella@gmail.com(A.C. Oliveira).

A.C. Oliveira a,b,⇑, R.H.M. Siqueira a,b, R. Riva b, M.S.F. Lima b

a Instituto Tecnológico de Aeronáutica, ITA, Pça.Mal.Eduardo Gomes, 50, São José dos Campos, SP 1228-900, Brazilb Instituto de Estudos Avançados, IEAv-DCTA, Trevo Cel.Av.José.A.A.do Amarante, 1, São José dos Campos, SP 12228-001, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 July 2014Accepted 20 September 2014Available online 6 October 2014

Keywords:Laser beam weldingYb-fiber laserAutogenous T-jointsAluminium alloy

Autogenous T-joints for aluminium skin-stringer component performed by one-sided laser beam weldingprocess 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 toweld microstructural features. The joint mechanical behavior was evaluated concerning to the sheet roll-ing directions. It was observed that a precise control of the process parameters enabled to obtain weldbeads with acceptable dimensional and geometric characteristics and minimizing weld defects. Heliumshielding gas produced higher aspect ratio welds than those with pure argon. Although, pores wereobserved in the fusion zone, they represented only about 5% of the weld bead area. The optimal beampositioning should remain up to 0.2 mm relative to junction line, for seam angles between 10� and15�. 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 toskin rolling direction and parallel to the stringer rolling direction.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Laser beam welding has been studied, and even employed in theaeronautical industry, particularly for the joining of skin-stringerfuselage sections [1,2]. The traditional riveting joining method,although highly automatized, offers a small potential to increasingproduction rate or aircraft weight savings [2,3].

Some studies have demonstrated the laser joining technique asa possible replacement of riveting [4,5]. Usually a filler materialhas been used during laser welding to fill the gap existing at thejoint or improves the weld bead toughness [6]. Braun [7,8]analyzed the influence of the filler wire composition on the weld-ing process stability. He proposed a filler wire containing largeamounts of silicon for adjusting weld pool chemistry of theAA6000 alloys, ensuring the elimination of solidification cracks inthe fusion zone. Squillace and Prisco [9] also investigated the influ-ence of filler additions on micro and macro-mechanical behavior ofT-welded joints. Their results suggested the possibility to reducethe weakening at the heat affected zone of aluminium alloy welds

using a filler wire with high melting latent heat. However, laserwelding with filler wire has been considered an additional difficultfor industrial application, having many parameters and stringentrequirements for wire positioning [10]. According to Tao et al.[11], the feeding position, wire feeding direction, and wire feedingangle had significant influence on the laser welding process stabil-ity and on pore formation. Therefore, the use of filler wire can limitthe utilization of laser technology to joint skin-stringer compo-nents, since the process parameters are quite complex and defectgeneration must be strictly controlled.

Laser beam autogenous weld under T-joint configuration couldbe much simpler by minimizing the difficulties of the introductionof filler metal. One of drawbacks for the autogenous laser weldingis the presence of air gaps existing at the joint region, which canpromote defects such as concavity, sidewall fusion defects, rootsuck-up, and weld undercuts [10]. According to Salminen [6], inthe typical butt joint, the widest acceptable gap for autogenouslaser welding is usually 10% of the material thickness. Dawes[12] concludes that a gap width of 0.14 mm in sheet thickness of2 mm leads to concavity defect, reducing loading to 86%.

Yang et al. [13] showed that, in the double-sided laser beamwelding, parameters such as: incident beam position, beam angle,and beam separation distance affect strongly the metallurgicalquality of the T-joints. Prisco et al. [14] showed that the distribu-tion of weld bead along of skin-stringer components influences

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the mechanical strength of the joint. According to their results, themelt region should not exceed 30% of skin thickness since anexcessive penetration in this area promotes stress concentrationfavoring the crack propagation, especially in the heat affected zone.In this sense, the one-sided laser beam welding in the T-joint con-figuration promotes lower penetration depth of weld bead in theskin component. Additionally, one laser run induces less thermaldamage (less heat input) which decreases possible part distortion.The main drawback refers to laser alignment that becomes morecritical 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 availableliterature indicated the use of lasers with lower beam quality thanthe Yb-fiber laser of the present work. To the authors, there is lackof knowledge about one-sided laser beam welds of high-strengthaluminium, under T-joints autogenous condition, using a highpower Yb-fiber laser.

In the present work, autogenous AA6013 T-joints in one-sidedconfiguration were performed using a high power Yb-fiber laser.The main goal of this research is to evaluate the relationshipbetween the welding parameters, the microstructure and themechanical properties of the joints, in order to optimize the laserprocess for this aeronautic application.

2. Experimental details

2.1. Material and experimental setup

Sheets of aluminium alloy, AA6013, with 1.6 mm thickness,received in room temperature aged T4 condition, were used. Thechemical composition of the sheets is presented in Table 1. The18 mm � 100 mm metal sheets were tightly clamped and fixedon a XYZ CNC moving table. Before processing, all sheets werecleaned with abrasive paper (SiC grit 600) and ethanol to removethe residual grease and any contaminants. The laser processinghead was attached to a goniometer table, which allows changingthe laser beam incidence angle from 0� to 90�.

Fig. 1a shows the schematic diagram of the welding experi-ment. Overlapped and T-shaped autogenous joints were conductedby varying the laser average power between 1000 and 1800 W andthe welding speeds from 1.8 to 9 m/min. Argon or helium with aflow rate of 20 L/min were used as shielding gases. Additionally,T-shaped autogenous joints were conducted by varying the seamangle of 6–29� between the laser beam and the stringer specimen(Fig. 1b). The influence of the beam positioning on the weld beadquality also was evaluated, varying the beam positioning along ofthe weld longitudinal section between �0.5 mm and 0.40 mm rel-ative to the joint centerline (joint line between the sheets), asshown in Fig. 1c.

2.2. Microstructure analysis

Metallographic analyses were performed on the weld beadcross-section by optical microscopy (OM). The specimens weregrinded with abrasive paper (SiC grits of 240, 400, 600 and 1200,respectively), polished to a mirror finish (1.0 lm Al2O3 solutionand colloidal SiO2) and chemically etched using Keller’s reagent(2 ml of HF, 1 ml of HNO3 and 88 ml of H2O).

Table 1Chemical composition of AA6013-T4 (%).

Mg Si Cu

0.94 ± 0.05 0.62 ± 0.02 0.82 ± 0.02

The penetration depth and the width of the weld bead, obtainedunder different process parameters, were analyzed by opticalmicroscopy (MO). Image J software [15] was used to measure thedimensional characteristics in five transversal positions of the weldbead. Additionally, the amount of the pores presents in the weldcross-section was examined. Areas of the micro and macro poros-ities were measured and it subtracted from total area of the weldbead using the Image J software.

2.3. Mechanical testing

In general, to check the mechanical resistance of aircraft fuse-lage panels, two kinds of monotonic quasi-static tensile tests havebeen considered [9,13]. The first tensile test applies a perpendicu-lar stress to the contact plane between the stringer and skin andverifies the adhesion of the components. The second one appliesa stress to skin perpendicularly to welding direction. This test aimssimulating the resistance of the T-joints 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 dimen-sions of 18 mm � 72 mm for the T-pull tests, and 9 mm � 72 mmfor the hoop tensile tests. Fig. 2a shows the schematic drawing ofthe specimen fixed to tensile machine to the hoop tensile tests.To execute the T-pull tests, a home-made system to clamp thespecimen was used (see Fig. 2). The skin panel was tightenedbetween the upper and lower plates of the clamping system viafour bolts. A rectangular groove machined at the middle of theupper plate allows the stringer to pass through. Under this config-uration, the stringer was subjected to a tension aiming to tear it offfrom the skin. Fig. 2b and c shows, respectively, the schematicdrawing of the specimen fixed to home-made system and theimage of this home-made system coupled to tensile machine tothe T-pull tests. The experiments were performed using a 100 kNuniversal tensile machine (EMIC-DL10000).

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 of10 m long and 100 lm diameter, the beam was focused by a lenswith focal length of 160 mm.

The laser beam profile was measured by using a modified rotat-ing wire beam scanner based on the work of Lim and Steen [16]. Anoptical fiber of 150 mm long and 125 lm external diameter wascoupled to a disk fixed on a continuous rotating axis. The fiberwas positioned near the focus region of the laser beam withoutany attenuation optical element alloying measuring of the actuallaser beam used on the welding experiments. A detector compris-ing of a lens with focal length of 50 mm and a photodiode coupledto an oscilloscope were located such that they measure the lightreflected by the fiber surface. The radiation reflected by the opticalfiber was delimited by vertical slit in front of the detector and allscattered radiation vertically reaches this detector. In the horizon-tal direction, the reflected radiation detected depends on the slitwidth and on the fiber-slit distance. The fiber displacementgenerates a signal in the detector that represents the vertical inte-grated laser beam profile. The beam radius was then estimated by

Mn Fe Others, total

0.27 ± 0.04 0.20 ± 0.01 0.06 ± 0.01

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 longitudinalsection.

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.

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knife-edge method where the laser beam radius w is obtained bymeasuring a beam clip width DC, which represents the distancebetween 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.

Fig. 3 shows the Yb-fiber laser beam profile (solid line) obtainedat the focal position (Fig. 3a), 1 mm and 2 mm far from the focus

Fig. 3. Yb-fiber laser beam profile (solid line) obtained between the (a) focalposition (b) 1 mm and (c) 2 mm far from this one where the dashed lines representthe integrated beam profile.

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(Fig. 3b and c, respectively) using an average laser power of1000 W. The dashed lines refer to the integrated beam profilesused to calculate the beam diameter according to the Siegman pro-cedure [17]. The vertical dashed lines shown in Fig. 3a representsthe 10% and 90% clip points which define the beam clip width Dc.

The laser beam profile was measured in different distances ofthe focal lens in order to obtain the beam propagation curveshowed in Fig. 4. With this data we estimated the laser beam waistw0 = 50 lm, the laser beam quality factor M2 = 9 which representsa beam parameter product (b.p.p.) of only 3 mm.mrad, and a laserbeam depth of focus 2ZR = 1.6 mm [18].

Fig. 4. Beam radius (w) measurement at different positions (z) in respect to thefocal position (z0 = 160 mm). ZR is the Rayleigh distance.

3. Results and discussion

3.1. Dimensional and metallographic characteristics of the weld bead

Overlapped and T-shaped autogenous joints were performed byvarying the laser power between 1000 and 1800 W and weldingspeed from 1.8 to 9 m/min. The main aim was to weld withacceptable dimensional characteristics accordingly to the state ofthe art, and with few defects. The weld cross-sections at differentconfigurations are shown in Fig. 5a and b, respectively. In general,the keyhole welding has been carried out with incident laser beamintensities on the surface of about 106 W/cm2. In the present study,surface incident intensity values between 1 and 5 � 107 W/cm2

were employed. Under these power levels, the vapor pressurehad enough thrust to move the melt upwards during the weldinginhibiting the collapse of the keyhole. This additional thrust couldbe understood as a driven force decreasing pore formationdynamics.

Similarly to the results observed by Braun [7,8], the fusion zoneexhibits a fine cellular dendritic solidification structure with manyequiaxed grains in the weld centerline. According to studies [19],the equiaxed grains tend to decrease the cracking susceptibilityand to improve the weld mechanical properties. A partial meltingzone is adjacent to the fusion boundaries and its width is the orderof 50 lm in all analyzed conditions (Fig. 6). Although thisheat-affected zone (HAZ) seemed to be composed by liquationcracks, this is not true. The darkening of HAZ grain boundariesare due to the extended etching time necessary to reveal thealuminium microconstituents. This region has been considered asresult of heating up the area surrounding the fusion zone totemperatures between the eutectic temperature and the liquidusof the alloy [7].

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. 8. (a) Penetration depth and (b) width of the weld bead in different weldingspeeds.

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Another analyzed aspect refers to presence of pores in the weldbead. Some factors such as inadequate shielding gas, presence ofsurface contaminants, hydrogen trapping in the melt, vaporizationof alloy elements, and keyhole collapse [20–22] could promoteprocess instability, contributing to the formation of the microand macropores. Fig. 7 presents the effective area of the weldcross-section and the related specific porosity at different weldingspeeds. The results evidenced a maximum pore area of about0.1 mm2, which represents about 5% of the weld bead area. Thislow porosity has been related to high quality of the laser beam[23], because its low divergence and small focal diameter producedmore stable welding conditions. Thus, the Yb-fiber laser qualitywith focal radius of order 50 lm and M2 = 9 contribute to poresreduction along of the weld bead.

To get an overview of the weld dimension in each experimentalcondition, the penetration depth (Fig. 8a) and the width (Fig. 8b) ofthe weld beads were measured. The results show that the welddimensions decrease with the increase of welding speed for a givenlaser average power.

3.2. Influence of shielding gas

The use of argon or helium as shielding gases was compared inthe welding experiments. Both gases have been commonly used forlaser welding of the aluminium alloys [24,25], but their differentphysical properties could influence the weld bead quality.

The shielding gas influence could be associated to three mainphysic phenomena occurred in the material surface during thewelding process: the energy absorption by plasma on the surface,the scattering of laser radiation by ejected particles, and gas lenseffect due caused by the temperature gradients near the surface[26]. Greese et al. [27] showed the shield gas type has a minorinfluence on the welding process when using a 1.06 um Nd-YAGlaser because its radiation is less absorbed by the ejected vapor.

Fig. 7. Weld cross-section area and the porosity formed for different weldingspeeds.

Fig. 9. Penetration depth of the weld bead using argon and helium as shieldinggases.

Fig. 10. Cross-section of the weld beads made with seam angle of 6–29�.

Fig. 11. Weld size characteristics with focal spot from 2.5 mm above or belowworkpiece surface.

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However, in this work, as it is shown in Fig. 9, the penetrationdepth of the welds using helium promoted deeper weld beads thanthose obtained with argon, under the same experimental conditions.

We believe the influence of the shielding gas on the Yb-fiberlaser welding could be attributed to its better beam quality. In thiscase, the laser beam intensity is much more sensitive to any distur-bance caused by a temperature gradient or particle scattering. Sothe use of Helium which has a better thermal conductivity couldexplain the results shown of Fig. 9.

3.3. Influence of the seam angle

The variation of seam angle on the sample provides data aboutthe weld bead transversal shape between the skin-stringer compo-nents. According to the literature [14], the weld bead in the skinregion should not exceed 30% of its thickness. The non-complianceof this criterion affects the mechanical strength of the welded

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.

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component, particularly due to the presence of heat-affected zone(HAZ) which concentrates stresses and becomes the region suscep-tible to crack propagation. Moreover, the penetration depth of theweld bead should surpass the stringer thickness. The insufficientweld penetration lack in the stringer contributes to decrease ofweld resistance. Fig. 10 presents the transversal section of the weldbeads by varying the seam angle between 6� and 29�. The anglesbetween 10� and 15� produce weld beads with adequate melt dis-tribution at the skin-stringer intersection. High angles generatewelds with penetration depth beyond 30% of skin thickness. Thegaps also influenced the process instability, generating macroporesin 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,

Table 2Sheet rolling direction relative to weld bead longitudinal-section.

Series Skin-stringer component Description

A Weld bead longitudinal-section perpendicular to skinand stringer rolling directions

B Weld bead longitudinal-section perpendicular to skin rolling directionand parallel to the stringer rolling direction

C Weld bead longitudinal-section parallel to skin rolling directionand perpendicular to the stringer rolling direction

D Weld bead longitudinal-section parallel to skin andstringer rolling directions

Table 3Tensile properties of the 6013-T4 joints.

Rolling directiona Ultimate tensile strength (MPa) Joint efficiency

T-pull test Hoop tensile test T-pull test (%) Hoop tensile test (%)

10� 15� 10� 15�

A 151 ± 6 155 ± 2 218 ± 10 231 ± 13 48 78B 152 ± 10 151 ± 4 272 ± 13 278 ± 11 47 86C 149 ± 8 159 ± 4 236 ± 6 260 ± 7 48 78D 154 ± 9 159 ± 7 227.7 ± 13 258 ± 15 49 76

a See Table 2.

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Fig. 14. Facture at the weld bead central region after the T-pull test.

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some studies [2,7] showed the variation of the penetration depthwith the changes of the focal position at the workpiece surface(z = 0). Fig. 11 presents the weld size by varying of beam focal posi-tion from 2.5 mm to �2.5 mm (focal spot 2.5 mm above or belowworkpiece surface) to a fixed seam angle of 15�. The results shownthat the weld sizes are constants to defocusing of 1 mm, i.e., depthpenetration and width of the weld bead remaining with constantvalues up to a defocusing of 1 mm (z = �1 mm).

Fig. 15. Hoop tensile tests in different rolling directions,

This aspect is related with the beam quality of the Yb-fiber laserof this work which has a depth of focus of 1.6 mm. In this distance,the laser beam intensity remains almost constant and the variationof the beam focal position relative to the material surface presentssmaller influence on the weld dimensions. It is important to accen-tuate the fact this large depth of focus was obtained with a beamdiameter of only 0.1 mm and therefore, the laser intensity was veryhigh (�107 W/cm2).

The Yb-fiber laser presents the best beam quality at moderatepowers in comparison with other welding lasers sources [28]. Forinstance, the diode or lamp pumped Nd-YAG lasers has a minimumb.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, the4 kW Nd-YAG laser shown in [28] had to be focused with a beamdiameter of 0.6 mm in order to achieve a depth of focus compatiblewith the required weld depth. In spite of higher Nd-YAG laserpower, its beam intensity was only 106 W/cm2 which shows clearlythe 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 influenceof the beam positioning relative to weld centerline. Analyses wereperformed by varying the beam positioning along of the weld lon-gitudinal section, maintaining fixed the seam angle in 15� and thebeam focus at the surface (z = 0). Fig. 12 shows the distribution ofweld transversal section considering eight different positions (P1

considering the two seam angles (a) 10� and (b) 15�.

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to P8) of the beam on the sample from a zero point (P0), located inthe joint line between the sheets. The beam positioning, between�0.10 mm to 0.05 mm, P4 and P5, produces weld beads with highquality without severe defects. In other beam positions, defectssuch as pores and cracks increase significantly due to presence ofthe gap between the sheets, which also generated keyhole instabil-ity during the welding.

It was also analyzed the influence of the beam positioning onthe weld sizes (Fig. 13). The variation of beam positioning doesnot influence s the weld size, excepting at the extreme beampositions, such as �0.50 mm and 0.40 mm (P1 and P8, inFig. 12), where the welds are uncompleted. This means that themolten volume remained constant, despite of the beam position-ing variation. However, the welding quality was greatly affectedby weld bead transversal distribution at the skin-stringer joint.Thus, the beam positioning relative to weld centerline shouldnot exceed 0.2 mm.

3.6. Mechanical behavior of skin-stringer joint

Two monotonic quasi-static tensile tests were carried out onthe welded T-joints, T-pull test and hoop tensile test. In the bothexperiments, the joint strength were evaluated according to influ-ence of the seam angle (10� and 15�) and the sheet rolling directionas depicted in Table 2 and results are presented in Table 3.

The results show that the sheet rolling direction did not influ-ence the T-pull tensile strength, for seam angles of 10� and 15�.Optical microscopy of the fractured surfaces after T-pull tests(Fig. 14) shown that the fracture occurs at weld bead centralregion. In fact, the weld microstructure became completely differ-ent of its original microstructure, inhibiting the lamination effecton the mechanical behavior.

The hoop tensile tests are shown in Fig. 15, considering seamangles of 10� (Fig. 15a) and 15� (Fig. 15b), respectively. Animprovement of the ductility and mechanical strength areobserved with longitudinal-section perpendicular to skin rollingdirection and parallel to stringer rolling direction (series B, Table 2).Although this aspect must be confirmed by additional analyzes, theresults indicate some influence of the sheet rolling direction on theweld 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 submit-ted to T-pull test is about of 50%, once the tensile strength of theweld bead remains approximately constant in all experimentalconditions. On the other hand, the joint efficiency obtained of theweld bead submitted to hoop tensile test presents some variationdepending on the experimental conditions. The joint efficiencyreached 85% for the better welding condition (series B, Table 2).

4. Conclusions

The experimental results reported in this work provide themain aspects of the aluminium alloy T-joints performed by highpower Yb-fiber laser. It has been concluded:

� Autogenous T-joint welds of the 6013-T4 alloy sheet wereproduced by one-sided laser welding process, simulating theskin-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 defectwas porosity in the fusion zone. The maximum pore area was0.1 mm2, representing only about 5% of the weld bead area.

� Welding process using helium as shielding gas produced deeperwelds when compared to welds performed using argon, undersame 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 showeda critical parameter in the weld quality. Its variation should notexceed 0.2 mm relative to weld centerline. The seam angle onthe sample should be between 10� and 15�, considering thebeam diameter of 0.1 mm.� The mechanical behavior of the welded components was

related to sheet rolling direction. Weld beads with higher valuesof ductility and mechanical strength were obtained when itslongitudinal-section was perpendicular to skin rolling directionand parallel to stringer rolling direction. In this condition, jointefficiency up to 85% was obtained of welds submitted to hooptensile test.

Acknowledgement

Financial support from FAPESP (Process No. 2007-03910-7) isgratefully acknowledged.

References

[1] Schubert E, Klassen M, Zerner I, Walz C, Sepold G. Light-weight structuresproduced by laser beam joining for future applications in automobile andaerospace industry. J Mater Process Technol 2008;115:2–8.

[2] Riva R, Lima MSF, Oliveira AC. Soldagem a laser de estruturas aeronáuticas.Metal Mater 2009;65(jan/fev):48–50.

[3] Travessa DN, Rondon V, Neto VP. Aspectos da competitividade do alumínio emestruturas aeronáuticas. Metal Mater 2009;65(jan/fev):45–7.

[4] Braun R, Donne CD, Staniek G. Laser beam welding and friction stir welding of6013-T6 aluminium alloy sheet. Mat.-wiss.u.Werkstofftech J2000;31:1017–26.

[5] Badini C, Pavese M, Fino P, Biamino S. Laser beam welding of dissimilaraluminium alloys of 2000 and 7000 series: effects of post-welding thermaltreatments on T joint strength. Sci Technol Weld Joining 2009;14:484–92.

[6] Salminen A. The filler wire – laser beam interaction during laser welding withlow alloyed steel filler wire. Mechanika 2010;4(84):67–74.

[7] Braun R. Nd:YAG laser butt welding of AA6013 using silicon and magnesiumcontaining filler powders. Mater Sci Eng, A 2006;426:250–62.

[8] Braun R. Laser beam welding of Al–Mg–Si–Cu alloy 6013 sheet using siliconrich aluminium filler powders. Mater Sci Technol 2005;21:133–40.

[9] Squillace A, Prisco U. Influence of filler material on micro-and-macro-mechanical behaviour of laser-beam-welded T-joint for aerospaceapplications. Mat Res 2013;16(5):1106–12.

[10] Dilthey U, Fuest D, Scheller W. Laser welding with filler. Opt Quant Electron1995;27:1181–91.

[11] Tao W, Yang Z, Chen Y, Li L, Jiang Z, Zhang Y. Double-sided laser beam weldingprocess of T-joints for aluminum aircraft fuselage panels: filler wire meltingbehavior, process stability, and their effects on porosity defects. Opt LaserTechnol 2013;52:1–9.

[12] Dawes CJ. ICALEO ‘85, 4th international congress on applications of lasers andelectro-optics, San Francisco; 1985. p. 73.

[13] Yang ZB, Tao W, Li LQ, Chen YB, Li FZ, Zhang YL. Double-sided laser beamwelded T-joints for aluminum aircraft fuselage panels: process,microstructure, and mechanical properties. Mater Des 2012;33:652–8.

[14] Prisco A, Troiano G, Acerra F, Bellucci BF, Squillace A, Prisco U. LBW of similarand dissimilar skin-stringer joints. Part I: Process optimization and mechanicalcharacterization. Adv Mater Res 2008;38:306–19.

[15] IMAGEJ. Image processing and analysis in JAVA. <http://rsbweb.nih.gov/ij/>[access 10.03.08].

[16] LIM GC, STEEN WM. The measurement of the temporal and spatial powerdistribution of a high powered CO2 laser beam. Opt Laser Technol1982:149–53. June.

[17 Siegman AE, Sasnett MW, Johnston Jr TF. Choice of clip level for beam widthmeasurements using knife-edge techniques. IEEE J Quantum Electron1991;27(4). April.

[18] Oliveira AC, Siqueira RHM, Lima MSF, Riva R. A simple model for optimizinglaser welding properties. In: The XIX international symposium on high powerlaser 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. Metall MaterTrans A 1999;30A:449–55.

736 A.C. Oliveira et al. / Materials and Design 65 (2015) 726–736

[20] Ream SL. Laser welding efficiency and cost: CO2, YAG, Fiber, and Disc. In:International congress on applications of laser & electro-optics, 23th, 2004.Proceedings. . .; 2004. p. 28–32.

[21] Pastor M, Zhao H, Martukanitz RP, Debroy T. Porosity, underfill andmagnesium loss during continuous wave Nd:YAG laser welding of thinplates of aluminum alloys 5182 and 5754. Weld Res Suppl1999;78(6):207s–16s.

[22] Haboudou A, Peyre P, Vannes AB, Peix G. Reduction of porosity contentgenerated during Nd:YAG laser welding of A356 and AA5083 aluminiumalloys. Mater Sci Eng, A 2003;363:40–52.

[23] Weberpals J, Dausinger F, Göbel G, Brenner B. Role of strong focusability on thewelding process. J Laser Appl 2007;19(4):252–8.

[24] Thomy C, Seefeld T, Vollrstsen F. Applications of high power fiber lasers forjoining of steel and aluminum alloys. In: International WLT – conference onlasers in manufacturing, 3rd, 2005, Munich. Proceedings. . .; 2005. p. 27–32.

[25] Verhaeghe G, Allen C, Hilton P. Achieving low-porosity laser welds in 12.7 mmthickness aerospace aluminium using a Yb-fiber laser. In: International WLT –conference on lasers in manufacturing, 4th, 2007, Munich. Proceedings. . .;2007. p. 17–23.

[26] Steen WM, Mazumder J. Laser material processing. 4nd ed. Springer VerlagLondon Limited; 2010.

[27] Greses J, Barlow CY, Steen WM, Hilton PA. Spectroscopic studies of plume/plasma in different gas environments. In: ICALEO 2001 Proceedings,Jacksonville, LIA, Orlando; October 2001. paper 808.

[28] Verhaeghe G, Hilton P. Battle of sources – using a high-power Yb-fibre laser forwelding steel and aluminium. In: International WLT – conference on lasers inmanufacturing, 3rd, 2005, Munich. Proceedings. . .; 2005. p. 33–8.