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New technique of skin embedded wire double-sided laser beam welding
Optics & Laser Technology 91 (2017) 185–192
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Full length article
New technique of skin embedded wire double-sided laser beam welding
MARK
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Bing Han , Wang Tao, Yanbin Chen State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Skin embedded wire double-sided laser beam welding Al-Li alloys T-joint Hot crack Elements distribution
In the aircraft industry, double-sided laser beam welding is an approved method for producing skin-stringer Tjoints on aircraft fuselage panels. As for the welding of new generation aluminum-lithium alloys, however, this technique is limited because of high hot cracking susceptibility and strengthening elements’ uneven distributions within weld. In the present study, a new technique of skin embedded wire double-sided laser beam welding (LBW) has been developed to fabricate T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys using eutectic alloy AA4047 filler wire. Necessary dimension parameters of the novel groove were reasonably designed for achieving crack-free welds. Comparisons were made between the new technique welded T-joint and conventional T-joint mainly on microstructure, hot crack, elements distribution features and mechanical properties within weld. Excellent crack-free microstructure, uniform distribution of silicon and superior tensile properties within weld were found in the new skin embedded wire double-sided LBW T-joints.
1. Introduction Aluminum-lithium alloys are characterized by their low density, high strength and elastic modulus, excellent properties of anti-fatigue crack growth and corrosion resistance, improved ductility and toughness, and superb damage tolerance [1–3]. Since the first and second generations of Al-Li alloys presented some other characteristics that were found to be undesirable by aircraft manufacturers, with the progress of technology however, the third generation Al-Li alloys have received much attention for weight savings of aircrafts due to their more outstanding comprehensive performance than previous models. Among them, relatively new Al-Li alloys AA2060 and AA2099 are more promising candidates for fuselage panels of jetliners [4–7]. In Airbus Germany, double-sided laser beam welding (LBW) of skinstringer T-joints in lower fuselage areas partly instead of using the dominant riveting technique has led to a certain reduction of aircrafts’ weight. As already reported, T-joints composed of high strength Al alloys like AA2xxx, AA6xxx and AA7xxx alloys have been welded successfully without crack [8–11]. However, hot cracking sensitivity — one of Al-Li alloys typical weldability problems has threatened double-sided LBW further application greatly. What's worse, J. Enz et al. [12] found that, in compare with the inhomogeneous distribution of Li within the weld, the local loss of Si during welding caused hot cracking, whose influence on the mechanical properties of welds was supposed to be greater. Furthermore, it seemed difficult to improve elements’ uneven distributions effectively only by changing the welding parameters.
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In the present work, a new technique of skin embedded wire double-sided LBW was introduced to solve recent problems of hot cracking and elements’ inhomogeneous distributions. In order to change the way of filling material reasonably, a special arc groove was designed and machined on the skin panel so that the wire could be filled into the skin before welding. With the combined function of embedding wire within the skin before welding and double-sided filling wires during welding, a crack-free T-joint with more uniform distribution and higher content of Si could be obtained. 2. Materials and experimental procedure 2.0 mm thick Al-Li 2060-T8 laminated panels (600 mm×150 mm) and 2.0 mm thick Al-Li 2099-T83 extruded profiles (600 mm×28 mm) were used for the skin and stringer components, respectively. These wrought Al-Li alloys were especially developed for the aircraft industry by Alcan Inc., in particular for the lower shell fuselage applications [13]. The filler wire used was a commercially available eutectic alloy AA4047 wire of 1.2 mm in diameter produced by Maxal Inc. Chemical compositions of base metals (BMs) and filler wire are listed in Table 1. The skin embedded wire double-sided LBW involves several steps: pre-processing arc groove, embedding wire, and at last the doublesided LBW. Firstly, in pre-processing arc groove step, to avoid the generation of crack defect, an arc groove was accurately processed according to the special dimensional design. Critical sizes of the groove are illustrated in Fig. 1a. Secondly, in embedding wire step, AA4047
Corresponding author.
http://dx.doi.org/10.1016/j.optlastec.2016.12.023 Received 16 August 2016; Received in revised form 13 December 2016; Accepted 18 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
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Table 1 Chemical compositions of the base metals and filler wire (wt%).
Table 2 Welding parameters of the skin embedded wire double-sided LBW for T-joint.
Material
Cu
Si
Li
Zn
Mg
Mn
Zr
Ag
Al
Welding parameters
Values
2060 2099 4047
3.9 2.52 < 0.01
0.02 – 11.52
0.8 1.87 –
0.32 1.19 0.001
0.7 0.497 0.01
0.29 0.309 0.01
0.1 0.082 –
0.34 – –
Bal. Bal. Bal.
Laser power (P) Welding velocity (Vw) Wire feeding rate (Vfw) Incident beam angle (θ) Wire feeding angle (α) Wire feeding angle (β) Wire extension Focal position Shielding gas Shielding gas flow rate
3.2 kW 10 m/min 4.5 m/min 22° 22° 20° 8 mm Specimen surface Argon 15 L/min
wire was embedded into the groove by a clamping roller, as shown in Fig. 1b. Lastly, in the double-sided LBW step, the stringer was erected on the embedded wire on the skin by a mechanical clamping device. As depicted in Fig. 1c, the two fiber laser beams should be focused symmetrically onto two opposite positions along the stringer, respectively. The filler wire and shielding gas were delivered on the same plane as the laser beam and held at an angle of approximately 20° to the stringer in the leading and trailing directions, respectively. The adopted welding parameters are given in Table 2. In the last step, the new designed skin-stringer T-joint with embedded AA4047 wire was welded using two 6-axis industrial robots (KR-16W, KUKA Robot Group, Germany) which were connected to two 10 kW fiber lasers (YLS-10000, IPG Photonics Corp., Germany) and two wire feeders (KD-4010, Fronius International GmbH, Austria), respectively. The fiber lasers with an emission wavelength of 1.07 µm can deliver in continuous wave (CW) mode. The laser beam passed through a focusing mirror of 192 mm focus length and was finally focused as a spot of 0.26 mm in diameter. For the purpose of comparison, conventional double-sided LBW was additionally performed. The same welding configuration and parameters were also adopted as for the double-sided LBW without embedding wire. After welding, welds’ outer appearance and inner metallographic structure were detected by two optical microscopes (OLYMPUS SZX12 and OLYMPUS GX71). Selected welds were further analyzed by a scanning electron microscope (SEM, HITACHI S-3400N) on unetched microsections, and several positions within the weld zone were chosen to measure their local element distributions by an energy-dispersive Xray spectroscopy (EDS) fixed on SEM, as located in Fig. 2. Precipitation phases’ compositions were investigated by an X-ray diffraction apparatus (XRD, BRUKER D8 ADVANCE) and a differential scanning calorimetry facility (DSC, NETZSCH STA 449 F3). The local mechanical properties within the weld zone were tested at a strain rate of 0.5 mm/min using an INSTRON-5569 universal testing machine. The 1.0 mm thick flat specimens according to ASTM E8/ E8M-13a were extracted within the welds by an electrical-discharge machining and were parallel to the welding direction, as shown more detailed in Fig. 3. Both of the skin embedded wire and conventional double-sided LBW T-joints were tested by X-ray nondestructive testing with a range of 200 mm and an angle of 45° between the skin panel and X-ray path. The X-ray negatives were transformed into digital images by scanning
Fig. 2. Sketch map of the measuring points for the EDS analysis.
apparatus, and finally the pore defect characteristics were all extracted by the MATLAB software on computer. 3. Results and discussion 3.1. Macro- and microstructures of the welds Typical macroscopic appearance of the skin embedded wire doublesided LBW T-joint is shown in Fig. 4. In contrast with the macrograph of conventional double-sided LBW T-joint in Fig. 4b, most importantly, no hot crack could be observed on the skin embedded wire doublesided LBW T-joint, and no tiny spatter mark was found which means a more stable droplet transition during welding, as shown in Fig. 4a. Besides, some dark blocky deposits were observed on the weld of skin embedded wire double-sided LBW, and these deposits probably originated from the convergence of Si within the pool and coagulated mainly on the weld surface. Cross-sections of the entire welds welded by the conventional and skin embedded wire double-sided LBW are shown in Fig. 5, respec-
Fig. 1. Physical dimensions of the arc groove (a); embedded wire inside the groove (b) and used configuration for the skin embedded wire double-sided LBW of T-joints (c).
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Fig. 3. Schematic diagram of: the tensile specimens’ dimensions (a) and the sampling location (b).
skin embedded wire and conventional double-sided LBW were performed by XRD tests, as shown in Fig. 7. The XRD spectrums revealed that the T phases, TB phases and tiny T2 phases were identified by the appearance of their corresponding peaks in our conventional doublesided LBW weld. With introduction of the embedding AA4047 wire, however, intensities of all peaks referring to the T phase were enhanced synchronously. Consequently, the proportion of T phase within the weld welded by skin embedded wire double-sided LBW was increased. In addition, intensities of the peaks referring to the TB or T2 phase were all decreased. Consequently, promotion of the T phase and inhibition of the TB and the T2 phase within the weld could be achieved by the skin embedded wire double-sided LBW using AA4047 wire. The total intensity of the T phase could be assumed to be the sum of each relevant peak's intensity, that is:
tively. Apparently, tiny cavities still could be observed in the center of the weld welded by the conventional double-sided LBW (Fig. 5b), and these cavities could be the source of crack initiation or crack propagation path. By the skin embedded wire double-sided LBW, however, a kind of denser weld without cavity, incomplete fusion or crack could be obtained successfully (Fig. 5a). Optical microstructures of the skin embedded wire double-sided LBW T-joint are shown in Fig. 6. With appropriate fusion depth and weld width, original groove and embedded wire could be entirely covered and melted by pool, and no obvious crack, undercut or incomplete fusion was formed along the border of original groove (see Fig. 6a). Through the magnification, the weld microstructures exhibited mainly dendritic structures and three areas in terms of grain morphology occur, i.e. cellular dendrite zone (CDZ) located within the weld center (see Fig. 6b), parallel dendrite zone (PDZ) (see Fig. 6c) and nondendritic equiaxed zone (EQZ) along the fusion boundary (see Fig. 6d). Fine equiaxed grains in the EQZ were believed to have formed via a heterogeneous nucleation mechanism aided by Al3Zr and Al3(Li, Zr) precipitates [14]. Main precipitation of T (AlLiSi) phases, which show tetrahedron spatial structure, could be identified on the grain boundaries within the weld zone. This kind of phase has a cubic crystal structure, F-43 m, and a lattice parameter of 0.593 nm. Additionally, adjacent to the fusion boundary in heat affected zone (HAZ), a band of partially melted zone (PMZ) formed by planar crystallization and overaged zone (OZ) were shown in Fig. 6d. As a result of elements’ solid solution, no obvious precipitation could be observed within the PMZ, and this zone was proved to be the soften region of the T-joint. A kind of elongated TB (Al7Cu4Li) phases were observed along the grain boundaries within the OZ. Comparative studies on precipitated phases within the weld of the
IT = IT (111) + IT (220)+⋯+IT (511)
(1)
where IT is the general intensity of the T phase within the weld. Therefore, by Garvie-Nicholson equation [15], the volume fraction of the T phase within the weld should be given by:
fT =
IT ⋅100% IT + ITB + IT 2 + ISi + IAl
(2)
where fT is the volume fraction of the T phase within the weld. Half quantitatively, fT values within the welds obtained by the skin embedded wire and conventional double-sided LBW were calculated by above equations and compared with each other. As a consequence, fT value within the weld increased drastically from 2.84% (conventional double-sided LBW) to 6.57% (the skin embedded wire doublesided LBW), which half quantitatively proved that the T phase could be increased more than doubled through the skin embedded wire double-
Fig. 4. Macrographs of: the skin embedded wire double-sided LBW T-joint (a) and conventional double-sided LBW T-joint without embedding wire (b).
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Fig. 5. Cross-sections of the welds welded by: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
Fig. 6. Microstructures of the skin embedded wire double-sided LBW T-joint: cross-section of the entire weld (a); magnifications of the region I (b); the region II (c) and the region III (d).
the T phase, and peak D was associated with the melting of weld matrix. On the orange dotted line, however, peak C was dramatically enhanced, which means the ratio of the T phase had been largely increased within the weld by the skin embedded wire double-sided LBW. In addition, as a result of peak C enlarging, peak D was covered and disappeared totally.
sided LBW. The influence of the skin embedded wire double-sided LBW on the T phase content could also be detected by the DSC curves. The DSC thermograms of the conventional and skin embedded wire double-sided LBW welds are plotted in Fig. 8, respectively. Four endothermic peaks could be found on the blue solid line, sited at 80.6 °C (peak A), 288.5 °C (peak B), 621.6 °C (peak C) and 644.6 °C (peak D). Recent results indicated that peak A was largely caused by the dissolution of GP (Cu) zone, peak B corresponded to the dissolution of the δ′ phase, peak C was due to the dissolution of
3.2. Local element distributions of the welds Comparison EDS tests on local element distributions between the 188
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elements in the weld of conventional double-sided LBW. Distributions of elements from the BMs like Cu, Zn and Mg were corresponding to their differences of contents in the skin (AA2060) and stringer (AA2099) materials, that is, Cu and Mg were more accumulated near the lower fusion line whereas higher content of Zn was measured near the upper fusion line. The content of Si, which was entirely brought into the weld by AA4047 filler wire and played a critical role on inhibition of cracks, also varied a lot under pool flow field effect. Areas near the weld surface (position a in Fig. 2) and in the center of the weld (positions b, c, e and f in Fig. 2) all showed a higher Si-content in comparison to the upper part (position d in Fig. 2) and lower part (positions g and h in Fig. 2) of the weld. Nonetheless, Sicontents within all test positions were still lower than 2 wt%, and led to a high hot cracking sensitivity. Lippold et al. [16] found that, to avoid hot cracking of the weld effectively, a Si-content of at least 2 wt% was needed. In the skin embedded wire double-sided LBW weld, however, distributions of elements, particularly Si and Cu, had been changed and improved dramatically. Taking advantages of the groove and embedded AA4047 wire, measured Si-contents showed an overall increase and all test results exceeded 2 wt%. Especially in original groove region and near the weld surface, Si-contents were all increased over 3 wt% and consequently led to a much lower hot cracking sensitivity. In contrast with the increase of Si, weld's other elements like Mg, Mn, Ag and especially Cu all showed overall decreasing trends, and these phenomena could be explained as the result of less skin material elements melting into the pool during the skin embedded wire double-sided LBW process. It is noteworthy that the decrease of skin's elements, especially Cu, will also probably reduce intergranular low melting eutectic and increase grain boundary melting point, so as to avoid hot cracking to a certain extent. Elemental distributions of Si within the welds of the skin embedded wire and conventional double-sided LBW were conducted by EDS mapping tests, as shown in Fig. 9. To corresponding with the above EDS spot scan results, Si was mainly concentrated near the outer surface and in the center of the weld welded by conventional doublesided LBW, however, much lower contents of Si were detected especially at the bottom of the weld, as shown in Fig. 9b. In the weld of the skin embedded wire double-sided LBW, however, an overall increase of Si was realized in the whole weld zone, and lack of Si especially at the bottom of the weld was improved very well, as shown in Fig. 9a. Besides EDS tests, the theoretical average contents Cth, Si/Cu of Si and Cu within the weld of the skin embedded wire double-sided LBW were also calculated by the use of Eq. (3), and this equation was deduced from weld's geometrical considerations. In view of differences between welding speed and filler wire feed rate, a sub-equation (4) was complementally introduced:
Fig. 7. X-ray diffraction spectrums of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Fig. 8. DSC thermograms of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Table 3 Contents of the alloying elements (wt%) at 8 positions within the skin embedded wire (conventional) double-sided LBW weld (according to Fig. 2) measured by EDS (not determinable *). Testing position
a b c d e f g h
Measured value for alloying element content Cu
Si
Zn
Mg
Mn
Ag
Al
1.50 (2.88) 1.63 (2.96) 1.77 (2.67) 2.21 (2.89) 1.65 (2.75) 1.63 (2.91) 1.59 (3.03) 1.76 (3.80)
3.88 (1.95) 3.00 (1.63) 2.97 (1.80) 2.80 (0.97) 2.49 (1.72) 2.97 (1.56) 3.01 (1.34) 3.73 (0.18)
0.73 (0.64) 0.64 (0.57) 0.81 (0.88) 0.86 (1.07) 0.62 (0.46) 0.82 (0.74) 0.43 (0.75) 0.34 (0.62)
0.13 (0.32) 0.16 (0.29) 0.16 (0.29) 0.28 (0.26) 0.29 (0.31) 0.22 (0.30) 0.24 (0.30) 0.23 (0.57)
0.15 (0.18) 0.21 (0.23) 0.17 (0.22) 0.28 (0.36) 0.11 (0.21) 0.19 (0.10) 0.07 (0.15) 0.20 (0.22)
* (*) 0.09 (0.14) 0.08 (0.24) * (*) 0.05 (0.25) 0.11 (0.22) 0.18 (0.24) 0.14 (0.27)
Bal.
Cth, Si / Cu =
Ask ⋅CSi / Cu, sk + Astr ⋅CSi / Cu, str + Apw ⋅CSi / Cu, pw + 2⋅Afw ⋅CSi / Cu, fw As
Bal.
(3)
Bal.
Afw =
Bal.
Afw ⋅vfw vw
=
π 2 vfw ⋅d fw⋅ 4 vw
with: Cth, Si/Cu Average Si/Cu-content in weld [%]. CSi/Cu, sk Si/Cu-content of skin material [%]. CSi/Cu, str Si/Cu-content of stringer material [%]. CSi/Cu, pw Si/Cu-content of embedded wire [%]. CSi/Cu, fw Si/Cu-content of filler wire [%]. Ask Cross-sectional area of melted skin in seam [m2]. Astr Cross-sectional area of melted stringer in seam [m2]. Apw Cross-sectional area of embedded wire [m2]. Afw Cross-sectional area of filler wire [m2]. As Cross-sectional area of seam [m2]. vfw Speed of filler wire [m/s]. vw Welding speed [m/s].
Bal. Bal. Bal. Bal.
skin embedded wire and conventional double-sided LBW welds were conducted at designated positions (see Fig. 2), as indicated in Table 3. Apparently, there was an inhomogeneous distribution of alloying 189
(4)
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Fig. 9. Elemental distributions of Si within the weld zone: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
dfw Diameter of filler wire [m]. Ideally, the calculated average content of Cu is 1.82 wt%, which is approximately agreed with the test results in Table 3. However, for a maximum Si-content of 3.88 wt% measured within the weld, a much higher theoretical average value of 5.38 wt% was calculated. This phenomenon also reveal that, different from Cu, a considerable loss of Si was occurred during the skin embedded wire double-sided LBW. With a lower boiling point of 2355 °C than Cu (2595 °C), Si was much easier to be evaporated out especially from high-temperature areas on the keyhole wall and molten pool surface. In addition, with a lower density of 2.35 g/cm3 than Al-matrix (2.7 g/cm3), Si was more tend to be floated up to the molten pool surface under buoyancy effect in welding.
near cracks, and original cracks could be connected with each other by new cracks in this typical stage until the specimen's whole fracture. In compare with the red curve above, however, no similar crack propagation stage was found on the blue curve of the skin embedded wire double-sided LBW T-joint, which proved that a crack-free weld could be obtained. In addition, a much higher tensile strength peak was observed on the blue curve, which means a larger scale enhancement on weld strength. In order to rule out the chance, five repetitions for each condition were tested. Average values of ultimate tensile strength (UTS), yield strength (YS) and percentage elongation (El) of the Tjoints are shown in Fig. 11, respectively. In compare with the average UTS (89.4 MPa), YS (62.1 MPa) and El (2.1%) of conventional LBW Tjoints, an overall improvement of UTS (179.8 MPa), YS (70.0 MPa) and El (4.8%) on the skin embedded wire double-sided LBW T-joint were tested. In order to study strengthening mechanism in the skin embedded wire double-sided LBW process, SEM fractographic examinations on local longitudinal tensile specimen of conventional double-sided LBW T-joint were performed, as shown in Fig. 12. Typical fracture characteristics of columnar dendrites with different orientations could be observed in macroscopic fracture morphology (see Fig. 12a), indicating that fracture was preferentially expanded on original hot cracks. Typical intergranular cracking appearances could be observed in region A and B. Grains were covered by liquid films of low melting eutectics, indicating that hot cracks could initiated not only in boundaries of parallel dendrite but also in boundaries of cellular dendrite (see
3.3. Local mechanical properties of the welds Different from previous longitudinal tensile tests of the T-joint, local tensile tests within the weld zone without BMs’ effect were performed. Comparative results of local longitudinal tensile tests between the skin embedded wire and conventional double-sided LBW T-joints are shown in Figs. 10 and 11. Rapid propagation of hot cracks could be identified after the maximum tensile strength on the engineering stress-strain curve of conventional double-sided LBW T-joint (as arrowed in Fig. 10). New cracks could be generated not only on the tips of hot cracks but also on the boundaries of columnar crystal
Fig. 11. Local longitudinal tensile properties within the weld zone of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Fig. 10. Engineering stress-strain curves during local longitudinal tensile testing of the skin embedded wire double-sided LBW T-joint in comparison to conventional doublesided LBW T-joint.
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Fig. 12. SEM images of the fracture during local longitudinal tensile testing of conventional double-sided LBW T-joint: macro morphology (a) and magnified views of marked regions A and B (b)-(c).
skin embedded wire double-sided LBW and conventional double-sided LBW have been investigated by X-ray nondestructive testing (NDT) technique, as shown in Fig. 14. In conventional double-sided LBW Tjoint, the pores’ number, minimum spacing and maximum diameter were tested to be 12, 3.2 mm and 0.60 mm respectively in a range of 200 mm, as shown in Fig. 14b. In the skin embedded wire double-sided LBW T-joint, the pores’ number, minimum spacing and maximum diameter were tested to be 11, 3.4 mm and 0.60 mm respectively in a same range, as shown in Fig. 14a. Apparently, no obvious negative effect on porosity defects was shown during the skin embedded wire double-sided LBW process.
Figs. 12b and 12c). On the other hand, abundant T phases and slight T2 phases could be observed on parallel dendrites (see Fig. 12b). As a result of Si reduction, however, only a small quantity of T phases could be observed on cellular dendrites (see Fig. 12c). In compare with the fracture features of conventional double-sided LBW T-joint, obvious dimpled morphology was detected on the fracture of skin embedded wire double-sided LBW T-joint, indicating typical transgranular fracture and extensive plastic deformation were generated before final fracture (see Fig. 13a). As microfractograph of region C shown in Fig. 13b, several secondary cracks and sporadic T2 phases were distinguished on the bottom of dimples. Microfractograph of region D, however, exhibited features associated with transgranular fracture in dendrites and intergranular fracture along dendritic boundaries (Fig. 13c). These two kinds of zones were randomly distributed on the fracture surface, and T phases could only be found on grain boundaries. As a result of the increase of Si content within the weld, more T phases were formed on grain boundaries. The grain boundary continuity could be broken by the inserted T phases, and because of T phase's high hardness, to bypass T phase was the only way to make hot cracks propagated along grain boundaries during welding. So, the grain boundary strength was going to be improved more effectively by increasing T phases. To sum up, by the skin embedded wire doublesided LBW, fracture mechanism within the weld zone had been converted from previous predominant intergranular fracture to more ideal inter- and transgranular mixed-mode fracture.
4. Conclusion (1) A new technique of skin embedded wire double-sided LBW had been introduced to manufacture the T-joints consisting of 2060T8/2099-T83 Al-Li alloys by AA4047 filler wire. Several steps were involved when using this technique: pre-processing arc groove, embedding wire and double-sided LBW. By the combine of new designed arc groove and matching optimized welding parameters, excellent Al-Li alloys T-joints without crack could be obtained ideally. (2) In compare with conventional double-sided LBW T-joints, calculated volume fraction (fT) of main grain boundary precipitation T (AlLiSi) phase was increased from previous 2.84% to present 6.57% within the typical weld welded by skin embedded wire double-sided LBW. Furthermore, measured Si-contents within the weld showed an overall increase from below 2 wt% to over 2 wt%, and distribution of Si was more uniform. In contrast with a higher
3.4. Porosity defects Porosity defects in two kinds of T-joints welded respectively by the
Fig. 13. SEM images of the fracture during local longitudinal tensile testing of the skin embedded wire double-sided LBW T-joint: macro morphology (a) and magnified views of marked regions C and D (b)-(c).
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Fig. 14. Porosity defects within the T-joints welded by: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
theoretical Si-content average value of 5.38 wt%, however, a considerable loss of Si was occurred during the skin embedded wire double-sided LBW. (3) In compare with conventional double-sided LBW weld, local mechanical properties within the weld welded by the skin embedded wire double-sided LBW all showed apparent improvements. In detail, average values of ultimate tensile strength, yield strength and percentage elongation were all improved from previous 89.4 MPa, 62.1 MPa and 2.1% to present 179.8 MPa, 70.0 MPa and 4.8%, respectively. Features of inter- and transgranular mixed-mode were identified on local tensile specimens of the skin embedded wire double-sided LBW. No obvious negative influence on porosity defects was found during the skin embedded wire double-sided LBW process.
[4] [5]
[6]
[7] [8]
[9]
[10]
Acknowledgments [11]
The authors would like to thank National Engineering Research Center of Commercial Aircraft Manufacturing for financial support of this project.
[12]
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new generation aluminum-lithium alloys for aerospace application, in: Proceedings of the 13th International Conference on Aluminum Alloys, 2012, pp. 425-430. R.J. Rioja, J. Liu, The evolution of Al-Li base products for aerospace and space applications, Metall. Mater. Trans. A 43 (2012) 3325–3337. H. Babel, J. Gibson, M. Tarkanian, C. Parrish, M. Prietto, A. Ordonez-Chu, H. Haberl, J. Kabisch, R. Clark, J. Ogren, O.S. Es Said, 2099 aluminum-lithium with key-locked inserts for aerospace applications, J. Mater. Eng. Perform. 16 (2007) 584–591. C. Giummara, B. Thomas, R.J. Rioja, New aluminium lithium alloys for aerospace applications, in: Proceedings of the Light Metals Technology onference, 2007, 〈http://www.alcoa.com〉. R.J. Rioja, J. Liu, The evolution of Al-Li base products for aerospace and space applications, Metall. Trans. A 43 (2012) 3325–3337. J. Enz, V. Khomenko, S. Riekehr, V. Ventzke, N. Huber, N. Kashaev, Single-sided laser beam welding of a dissimilar AA2024-AA7050 T-joint, Mater. Des. 76 (2015) 110–116. A. Prisco, F. Acerra, A. Squillace, G. Giorleo, C. Pirozzi, U. Prisco, F. Bellucci, LBW of similar and dissimilar skin-stringer joints. Part 1: process optimization and mechanical characterization, Adv. Mater. Res. 38 (2008) 306–319. Z.B. Yang, W. Tao, L.Q. Li, Y.B. Chen, F.Z. Li, Y.L. Zhang, Double-sided laser beam welded T-joints for aluminium aircraft fuselage panels: process, microstructure, and mechanical properties, Mater. Des. 33 (2012) 652–658. W. Tao, Z.B. Yang, Y.B. Chen, L.Q. Li, Z.G. Jiang, Y.L. Zhang, Double-sided fiber laser beam welding process of T-joints for aluminum aircraft fuselage panels: filler wire melting behavior, process stability, and their effects on porosity defects, Opt. Laser Technol. 52 (2013) 1–9. J. Enz, S. Riekehr, V. Ventzke, N. Kashaev, Influence of the Local Chemical Composition on the Mechanical Properties of Laser Beam Welded Al-Li Alloys, Phys. Proced. 39 (2012) 51–58. P. Lequeu, P. Lassince, T. Warner, Aluminum alloy development for the airbus A380-part 2, Adv. Mater. Process. 165 (2007) 41–44. A. Gutierrez, J.C. Lippold, A proposed mechanism for equiaxed grain formation along the fusion boundary in Al-Cu-Li alloys, Weld. J. 77 (1998) 123–132. R.C. Garvie, P.S. Nicholson, Phase analysis in zirconia system, J. Am. Ceram. Soc. 55 (1972) 303–305. T. Böllinghaus, J. Lippold, C.E. Cross, Hot Cracking Phenomena in Welds 3, Springer Verlag, 2011.
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
New technique of skin embedded wire double-sided laser beam welding
MARK
⁎
Bing Han , Wang Tao, Yanbin Chen State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Skin embedded wire double-sided laser beam welding Al-Li alloys T-joint Hot crack Elements distribution
In the aircraft industry, double-sided laser beam welding is an approved method for producing skin-stringer Tjoints on aircraft fuselage panels. As for the welding of new generation aluminum-lithium alloys, however, this technique is limited because of high hot cracking susceptibility and strengthening elements’ uneven distributions within weld. In the present study, a new technique of skin embedded wire double-sided laser beam welding (LBW) has been developed to fabricate T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys using eutectic alloy AA4047 filler wire. Necessary dimension parameters of the novel groove were reasonably designed for achieving crack-free welds. Comparisons were made between the new technique welded T-joint and conventional T-joint mainly on microstructure, hot crack, elements distribution features and mechanical properties within weld. Excellent crack-free microstructure, uniform distribution of silicon and superior tensile properties within weld were found in the new skin embedded wire double-sided LBW T-joints.
1. Introduction Aluminum-lithium alloys are characterized by their low density, high strength and elastic modulus, excellent properties of anti-fatigue crack growth and corrosion resistance, improved ductility and toughness, and superb damage tolerance [1–3]. Since the first and second generations of Al-Li alloys presented some other characteristics that were found to be undesirable by aircraft manufacturers, with the progress of technology however, the third generation Al-Li alloys have received much attention for weight savings of aircrafts due to their more outstanding comprehensive performance than previous models. Among them, relatively new Al-Li alloys AA2060 and AA2099 are more promising candidates for fuselage panels of jetliners [4–7]. In Airbus Germany, double-sided laser beam welding (LBW) of skinstringer T-joints in lower fuselage areas partly instead of using the dominant riveting technique has led to a certain reduction of aircrafts’ weight. As already reported, T-joints composed of high strength Al alloys like AA2xxx, AA6xxx and AA7xxx alloys have been welded successfully without crack [8–11]. However, hot cracking sensitivity — one of Al-Li alloys typical weldability problems has threatened double-sided LBW further application greatly. What's worse, J. Enz et al. [12] found that, in compare with the inhomogeneous distribution of Li within the weld, the local loss of Si during welding caused hot cracking, whose influence on the mechanical properties of welds was supposed to be greater. Furthermore, it seemed difficult to improve elements’ uneven distributions effectively only by changing the welding parameters.
⁎
In the present work, a new technique of skin embedded wire double-sided LBW was introduced to solve recent problems of hot cracking and elements’ inhomogeneous distributions. In order to change the way of filling material reasonably, a special arc groove was designed and machined on the skin panel so that the wire could be filled into the skin before welding. With the combined function of embedding wire within the skin before welding and double-sided filling wires during welding, a crack-free T-joint with more uniform distribution and higher content of Si could be obtained. 2. Materials and experimental procedure 2.0 mm thick Al-Li 2060-T8 laminated panels (600 mm×150 mm) and 2.0 mm thick Al-Li 2099-T83 extruded profiles (600 mm×28 mm) were used for the skin and stringer components, respectively. These wrought Al-Li alloys were especially developed for the aircraft industry by Alcan Inc., in particular for the lower shell fuselage applications [13]. The filler wire used was a commercially available eutectic alloy AA4047 wire of 1.2 mm in diameter produced by Maxal Inc. Chemical compositions of base metals (BMs) and filler wire are listed in Table 1. The skin embedded wire double-sided LBW involves several steps: pre-processing arc groove, embedding wire, and at last the doublesided LBW. Firstly, in pre-processing arc groove step, to avoid the generation of crack defect, an arc groove was accurately processed according to the special dimensional design. Critical sizes of the groove are illustrated in Fig. 1a. Secondly, in embedding wire step, AA4047
Corresponding author.
http://dx.doi.org/10.1016/j.optlastec.2016.12.023 Received 16 August 2016; Received in revised form 13 December 2016; Accepted 18 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
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Table 1 Chemical compositions of the base metals and filler wire (wt%).
Table 2 Welding parameters of the skin embedded wire double-sided LBW for T-joint.
Material
Cu
Si
Li
Zn
Mg
Mn
Zr
Ag
Al
Welding parameters
Values
2060 2099 4047
3.9 2.52 < 0.01
0.02 – 11.52
0.8 1.87 –
0.32 1.19 0.001
0.7 0.497 0.01
0.29 0.309 0.01
0.1 0.082 –
0.34 – –
Bal. Bal. Bal.
Laser power (P) Welding velocity (Vw) Wire feeding rate (Vfw) Incident beam angle (θ) Wire feeding angle (α) Wire feeding angle (β) Wire extension Focal position Shielding gas Shielding gas flow rate
3.2 kW 10 m/min 4.5 m/min 22° 22° 20° 8 mm Specimen surface Argon 15 L/min
wire was embedded into the groove by a clamping roller, as shown in Fig. 1b. Lastly, in the double-sided LBW step, the stringer was erected on the embedded wire on the skin by a mechanical clamping device. As depicted in Fig. 1c, the two fiber laser beams should be focused symmetrically onto two opposite positions along the stringer, respectively. The filler wire and shielding gas were delivered on the same plane as the laser beam and held at an angle of approximately 20° to the stringer in the leading and trailing directions, respectively. The adopted welding parameters are given in Table 2. In the last step, the new designed skin-stringer T-joint with embedded AA4047 wire was welded using two 6-axis industrial robots (KR-16W, KUKA Robot Group, Germany) which were connected to two 10 kW fiber lasers (YLS-10000, IPG Photonics Corp., Germany) and two wire feeders (KD-4010, Fronius International GmbH, Austria), respectively. The fiber lasers with an emission wavelength of 1.07 µm can deliver in continuous wave (CW) mode. The laser beam passed through a focusing mirror of 192 mm focus length and was finally focused as a spot of 0.26 mm in diameter. For the purpose of comparison, conventional double-sided LBW was additionally performed. The same welding configuration and parameters were also adopted as for the double-sided LBW without embedding wire. After welding, welds’ outer appearance and inner metallographic structure were detected by two optical microscopes (OLYMPUS SZX12 and OLYMPUS GX71). Selected welds were further analyzed by a scanning electron microscope (SEM, HITACHI S-3400N) on unetched microsections, and several positions within the weld zone were chosen to measure their local element distributions by an energy-dispersive Xray spectroscopy (EDS) fixed on SEM, as located in Fig. 2. Precipitation phases’ compositions were investigated by an X-ray diffraction apparatus (XRD, BRUKER D8 ADVANCE) and a differential scanning calorimetry facility (DSC, NETZSCH STA 449 F3). The local mechanical properties within the weld zone were tested at a strain rate of 0.5 mm/min using an INSTRON-5569 universal testing machine. The 1.0 mm thick flat specimens according to ASTM E8/ E8M-13a were extracted within the welds by an electrical-discharge machining and were parallel to the welding direction, as shown more detailed in Fig. 3. Both of the skin embedded wire and conventional double-sided LBW T-joints were tested by X-ray nondestructive testing with a range of 200 mm and an angle of 45° between the skin panel and X-ray path. The X-ray negatives were transformed into digital images by scanning
Fig. 2. Sketch map of the measuring points for the EDS analysis.
apparatus, and finally the pore defect characteristics were all extracted by the MATLAB software on computer. 3. Results and discussion 3.1. Macro- and microstructures of the welds Typical macroscopic appearance of the skin embedded wire doublesided LBW T-joint is shown in Fig. 4. In contrast with the macrograph of conventional double-sided LBW T-joint in Fig. 4b, most importantly, no hot crack could be observed on the skin embedded wire doublesided LBW T-joint, and no tiny spatter mark was found which means a more stable droplet transition during welding, as shown in Fig. 4a. Besides, some dark blocky deposits were observed on the weld of skin embedded wire double-sided LBW, and these deposits probably originated from the convergence of Si within the pool and coagulated mainly on the weld surface. Cross-sections of the entire welds welded by the conventional and skin embedded wire double-sided LBW are shown in Fig. 5, respec-
Fig. 1. Physical dimensions of the arc groove (a); embedded wire inside the groove (b) and used configuration for the skin embedded wire double-sided LBW of T-joints (c).
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Fig. 3. Schematic diagram of: the tensile specimens’ dimensions (a) and the sampling location (b).
skin embedded wire and conventional double-sided LBW were performed by XRD tests, as shown in Fig. 7. The XRD spectrums revealed that the T phases, TB phases and tiny T2 phases were identified by the appearance of their corresponding peaks in our conventional doublesided LBW weld. With introduction of the embedding AA4047 wire, however, intensities of all peaks referring to the T phase were enhanced synchronously. Consequently, the proportion of T phase within the weld welded by skin embedded wire double-sided LBW was increased. In addition, intensities of the peaks referring to the TB or T2 phase were all decreased. Consequently, promotion of the T phase and inhibition of the TB and the T2 phase within the weld could be achieved by the skin embedded wire double-sided LBW using AA4047 wire. The total intensity of the T phase could be assumed to be the sum of each relevant peak's intensity, that is:
tively. Apparently, tiny cavities still could be observed in the center of the weld welded by the conventional double-sided LBW (Fig. 5b), and these cavities could be the source of crack initiation or crack propagation path. By the skin embedded wire double-sided LBW, however, a kind of denser weld without cavity, incomplete fusion or crack could be obtained successfully (Fig. 5a). Optical microstructures of the skin embedded wire double-sided LBW T-joint are shown in Fig. 6. With appropriate fusion depth and weld width, original groove and embedded wire could be entirely covered and melted by pool, and no obvious crack, undercut or incomplete fusion was formed along the border of original groove (see Fig. 6a). Through the magnification, the weld microstructures exhibited mainly dendritic structures and three areas in terms of grain morphology occur, i.e. cellular dendrite zone (CDZ) located within the weld center (see Fig. 6b), parallel dendrite zone (PDZ) (see Fig. 6c) and nondendritic equiaxed zone (EQZ) along the fusion boundary (see Fig. 6d). Fine equiaxed grains in the EQZ were believed to have formed via a heterogeneous nucleation mechanism aided by Al3Zr and Al3(Li, Zr) precipitates [14]. Main precipitation of T (AlLiSi) phases, which show tetrahedron spatial structure, could be identified on the grain boundaries within the weld zone. This kind of phase has a cubic crystal structure, F-43 m, and a lattice parameter of 0.593 nm. Additionally, adjacent to the fusion boundary in heat affected zone (HAZ), a band of partially melted zone (PMZ) formed by planar crystallization and overaged zone (OZ) were shown in Fig. 6d. As a result of elements’ solid solution, no obvious precipitation could be observed within the PMZ, and this zone was proved to be the soften region of the T-joint. A kind of elongated TB (Al7Cu4Li) phases were observed along the grain boundaries within the OZ. Comparative studies on precipitated phases within the weld of the
IT = IT (111) + IT (220)+⋯+IT (511)
(1)
where IT is the general intensity of the T phase within the weld. Therefore, by Garvie-Nicholson equation [15], the volume fraction of the T phase within the weld should be given by:
fT =
IT ⋅100% IT + ITB + IT 2 + ISi + IAl
(2)
where fT is the volume fraction of the T phase within the weld. Half quantitatively, fT values within the welds obtained by the skin embedded wire and conventional double-sided LBW were calculated by above equations and compared with each other. As a consequence, fT value within the weld increased drastically from 2.84% (conventional double-sided LBW) to 6.57% (the skin embedded wire doublesided LBW), which half quantitatively proved that the T phase could be increased more than doubled through the skin embedded wire double-
Fig. 4. Macrographs of: the skin embedded wire double-sided LBW T-joint (a) and conventional double-sided LBW T-joint without embedding wire (b).
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Fig. 5. Cross-sections of the welds welded by: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
Fig. 6. Microstructures of the skin embedded wire double-sided LBW T-joint: cross-section of the entire weld (a); magnifications of the region I (b); the region II (c) and the region III (d).
the T phase, and peak D was associated with the melting of weld matrix. On the orange dotted line, however, peak C was dramatically enhanced, which means the ratio of the T phase had been largely increased within the weld by the skin embedded wire double-sided LBW. In addition, as a result of peak C enlarging, peak D was covered and disappeared totally.
sided LBW. The influence of the skin embedded wire double-sided LBW on the T phase content could also be detected by the DSC curves. The DSC thermograms of the conventional and skin embedded wire double-sided LBW welds are plotted in Fig. 8, respectively. Four endothermic peaks could be found on the blue solid line, sited at 80.6 °C (peak A), 288.5 °C (peak B), 621.6 °C (peak C) and 644.6 °C (peak D). Recent results indicated that peak A was largely caused by the dissolution of GP (Cu) zone, peak B corresponded to the dissolution of the δ′ phase, peak C was due to the dissolution of
3.2. Local element distributions of the welds Comparison EDS tests on local element distributions between the 188
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elements in the weld of conventional double-sided LBW. Distributions of elements from the BMs like Cu, Zn and Mg were corresponding to their differences of contents in the skin (AA2060) and stringer (AA2099) materials, that is, Cu and Mg were more accumulated near the lower fusion line whereas higher content of Zn was measured near the upper fusion line. The content of Si, which was entirely brought into the weld by AA4047 filler wire and played a critical role on inhibition of cracks, also varied a lot under pool flow field effect. Areas near the weld surface (position a in Fig. 2) and in the center of the weld (positions b, c, e and f in Fig. 2) all showed a higher Si-content in comparison to the upper part (position d in Fig. 2) and lower part (positions g and h in Fig. 2) of the weld. Nonetheless, Sicontents within all test positions were still lower than 2 wt%, and led to a high hot cracking sensitivity. Lippold et al. [16] found that, to avoid hot cracking of the weld effectively, a Si-content of at least 2 wt% was needed. In the skin embedded wire double-sided LBW weld, however, distributions of elements, particularly Si and Cu, had been changed and improved dramatically. Taking advantages of the groove and embedded AA4047 wire, measured Si-contents showed an overall increase and all test results exceeded 2 wt%. Especially in original groove region and near the weld surface, Si-contents were all increased over 3 wt% and consequently led to a much lower hot cracking sensitivity. In contrast with the increase of Si, weld's other elements like Mg, Mn, Ag and especially Cu all showed overall decreasing trends, and these phenomena could be explained as the result of less skin material elements melting into the pool during the skin embedded wire double-sided LBW process. It is noteworthy that the decrease of skin's elements, especially Cu, will also probably reduce intergranular low melting eutectic and increase grain boundary melting point, so as to avoid hot cracking to a certain extent. Elemental distributions of Si within the welds of the skin embedded wire and conventional double-sided LBW were conducted by EDS mapping tests, as shown in Fig. 9. To corresponding with the above EDS spot scan results, Si was mainly concentrated near the outer surface and in the center of the weld welded by conventional doublesided LBW, however, much lower contents of Si were detected especially at the bottom of the weld, as shown in Fig. 9b. In the weld of the skin embedded wire double-sided LBW, however, an overall increase of Si was realized in the whole weld zone, and lack of Si especially at the bottom of the weld was improved very well, as shown in Fig. 9a. Besides EDS tests, the theoretical average contents Cth, Si/Cu of Si and Cu within the weld of the skin embedded wire double-sided LBW were also calculated by the use of Eq. (3), and this equation was deduced from weld's geometrical considerations. In view of differences between welding speed and filler wire feed rate, a sub-equation (4) was complementally introduced:
Fig. 7. X-ray diffraction spectrums of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Fig. 8. DSC thermograms of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Table 3 Contents of the alloying elements (wt%) at 8 positions within the skin embedded wire (conventional) double-sided LBW weld (according to Fig. 2) measured by EDS (not determinable *). Testing position
a b c d e f g h
Measured value for alloying element content Cu
Si
Zn
Mg
Mn
Ag
Al
1.50 (2.88) 1.63 (2.96) 1.77 (2.67) 2.21 (2.89) 1.65 (2.75) 1.63 (2.91) 1.59 (3.03) 1.76 (3.80)
3.88 (1.95) 3.00 (1.63) 2.97 (1.80) 2.80 (0.97) 2.49 (1.72) 2.97 (1.56) 3.01 (1.34) 3.73 (0.18)
0.73 (0.64) 0.64 (0.57) 0.81 (0.88) 0.86 (1.07) 0.62 (0.46) 0.82 (0.74) 0.43 (0.75) 0.34 (0.62)
0.13 (0.32) 0.16 (0.29) 0.16 (0.29) 0.28 (0.26) 0.29 (0.31) 0.22 (0.30) 0.24 (0.30) 0.23 (0.57)
0.15 (0.18) 0.21 (0.23) 0.17 (0.22) 0.28 (0.36) 0.11 (0.21) 0.19 (0.10) 0.07 (0.15) 0.20 (0.22)
* (*) 0.09 (0.14) 0.08 (0.24) * (*) 0.05 (0.25) 0.11 (0.22) 0.18 (0.24) 0.14 (0.27)
Bal.
Cth, Si / Cu =
Ask ⋅CSi / Cu, sk + Astr ⋅CSi / Cu, str + Apw ⋅CSi / Cu, pw + 2⋅Afw ⋅CSi / Cu, fw As
Bal.
(3)
Bal.
Afw =
Bal.
Afw ⋅vfw vw
=
π 2 vfw ⋅d fw⋅ 4 vw
with: Cth, Si/Cu Average Si/Cu-content in weld [%]. CSi/Cu, sk Si/Cu-content of skin material [%]. CSi/Cu, str Si/Cu-content of stringer material [%]. CSi/Cu, pw Si/Cu-content of embedded wire [%]. CSi/Cu, fw Si/Cu-content of filler wire [%]. Ask Cross-sectional area of melted skin in seam [m2]. Astr Cross-sectional area of melted stringer in seam [m2]. Apw Cross-sectional area of embedded wire [m2]. Afw Cross-sectional area of filler wire [m2]. As Cross-sectional area of seam [m2]. vfw Speed of filler wire [m/s]. vw Welding speed [m/s].
Bal. Bal. Bal. Bal.
skin embedded wire and conventional double-sided LBW welds were conducted at designated positions (see Fig. 2), as indicated in Table 3. Apparently, there was an inhomogeneous distribution of alloying 189
(4)
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Fig. 9. Elemental distributions of Si within the weld zone: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
dfw Diameter of filler wire [m]. Ideally, the calculated average content of Cu is 1.82 wt%, which is approximately agreed with the test results in Table 3. However, for a maximum Si-content of 3.88 wt% measured within the weld, a much higher theoretical average value of 5.38 wt% was calculated. This phenomenon also reveal that, different from Cu, a considerable loss of Si was occurred during the skin embedded wire double-sided LBW. With a lower boiling point of 2355 °C than Cu (2595 °C), Si was much easier to be evaporated out especially from high-temperature areas on the keyhole wall and molten pool surface. In addition, with a lower density of 2.35 g/cm3 than Al-matrix (2.7 g/cm3), Si was more tend to be floated up to the molten pool surface under buoyancy effect in welding.
near cracks, and original cracks could be connected with each other by new cracks in this typical stage until the specimen's whole fracture. In compare with the red curve above, however, no similar crack propagation stage was found on the blue curve of the skin embedded wire double-sided LBW T-joint, which proved that a crack-free weld could be obtained. In addition, a much higher tensile strength peak was observed on the blue curve, which means a larger scale enhancement on weld strength. In order to rule out the chance, five repetitions for each condition were tested. Average values of ultimate tensile strength (UTS), yield strength (YS) and percentage elongation (El) of the Tjoints are shown in Fig. 11, respectively. In compare with the average UTS (89.4 MPa), YS (62.1 MPa) and El (2.1%) of conventional LBW Tjoints, an overall improvement of UTS (179.8 MPa), YS (70.0 MPa) and El (4.8%) on the skin embedded wire double-sided LBW T-joint were tested. In order to study strengthening mechanism in the skin embedded wire double-sided LBW process, SEM fractographic examinations on local longitudinal tensile specimen of conventional double-sided LBW T-joint were performed, as shown in Fig. 12. Typical fracture characteristics of columnar dendrites with different orientations could be observed in macroscopic fracture morphology (see Fig. 12a), indicating that fracture was preferentially expanded on original hot cracks. Typical intergranular cracking appearances could be observed in region A and B. Grains were covered by liquid films of low melting eutectics, indicating that hot cracks could initiated not only in boundaries of parallel dendrite but also in boundaries of cellular dendrite (see
3.3. Local mechanical properties of the welds Different from previous longitudinal tensile tests of the T-joint, local tensile tests within the weld zone without BMs’ effect were performed. Comparative results of local longitudinal tensile tests between the skin embedded wire and conventional double-sided LBW T-joints are shown in Figs. 10 and 11. Rapid propagation of hot cracks could be identified after the maximum tensile strength on the engineering stress-strain curve of conventional double-sided LBW T-joint (as arrowed in Fig. 10). New cracks could be generated not only on the tips of hot cracks but also on the boundaries of columnar crystal
Fig. 11. Local longitudinal tensile properties within the weld zone of the skin embedded wire double-sided LBW T-joint in comparison to conventional double-sided LBW T-joint.
Fig. 10. Engineering stress-strain curves during local longitudinal tensile testing of the skin embedded wire double-sided LBW T-joint in comparison to conventional doublesided LBW T-joint.
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Fig. 12. SEM images of the fracture during local longitudinal tensile testing of conventional double-sided LBW T-joint: macro morphology (a) and magnified views of marked regions A and B (b)-(c).
skin embedded wire double-sided LBW and conventional double-sided LBW have been investigated by X-ray nondestructive testing (NDT) technique, as shown in Fig. 14. In conventional double-sided LBW Tjoint, the pores’ number, minimum spacing and maximum diameter were tested to be 12, 3.2 mm and 0.60 mm respectively in a range of 200 mm, as shown in Fig. 14b. In the skin embedded wire double-sided LBW T-joint, the pores’ number, minimum spacing and maximum diameter were tested to be 11, 3.4 mm and 0.60 mm respectively in a same range, as shown in Fig. 14a. Apparently, no obvious negative effect on porosity defects was shown during the skin embedded wire double-sided LBW process.
Figs. 12b and 12c). On the other hand, abundant T phases and slight T2 phases could be observed on parallel dendrites (see Fig. 12b). As a result of Si reduction, however, only a small quantity of T phases could be observed on cellular dendrites (see Fig. 12c). In compare with the fracture features of conventional double-sided LBW T-joint, obvious dimpled morphology was detected on the fracture of skin embedded wire double-sided LBW T-joint, indicating typical transgranular fracture and extensive plastic deformation were generated before final fracture (see Fig. 13a). As microfractograph of region C shown in Fig. 13b, several secondary cracks and sporadic T2 phases were distinguished on the bottom of dimples. Microfractograph of region D, however, exhibited features associated with transgranular fracture in dendrites and intergranular fracture along dendritic boundaries (Fig. 13c). These two kinds of zones were randomly distributed on the fracture surface, and T phases could only be found on grain boundaries. As a result of the increase of Si content within the weld, more T phases were formed on grain boundaries. The grain boundary continuity could be broken by the inserted T phases, and because of T phase's high hardness, to bypass T phase was the only way to make hot cracks propagated along grain boundaries during welding. So, the grain boundary strength was going to be improved more effectively by increasing T phases. To sum up, by the skin embedded wire doublesided LBW, fracture mechanism within the weld zone had been converted from previous predominant intergranular fracture to more ideal inter- and transgranular mixed-mode fracture.
4. Conclusion (1) A new technique of skin embedded wire double-sided LBW had been introduced to manufacture the T-joints consisting of 2060T8/2099-T83 Al-Li alloys by AA4047 filler wire. Several steps were involved when using this technique: pre-processing arc groove, embedding wire and double-sided LBW. By the combine of new designed arc groove and matching optimized welding parameters, excellent Al-Li alloys T-joints without crack could be obtained ideally. (2) In compare with conventional double-sided LBW T-joints, calculated volume fraction (fT) of main grain boundary precipitation T (AlLiSi) phase was increased from previous 2.84% to present 6.57% within the typical weld welded by skin embedded wire double-sided LBW. Furthermore, measured Si-contents within the weld showed an overall increase from below 2 wt% to over 2 wt%, and distribution of Si was more uniform. In contrast with a higher
3.4. Porosity defects Porosity defects in two kinds of T-joints welded respectively by the
Fig. 13. SEM images of the fracture during local longitudinal tensile testing of the skin embedded wire double-sided LBW T-joint: macro morphology (a) and magnified views of marked regions C and D (b)-(c).
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Fig. 14. Porosity defects within the T-joints welded by: the skin embedded wire double-sided LBW (a) and conventional double-sided LBW (b).
theoretical Si-content average value of 5.38 wt%, however, a considerable loss of Si was occurred during the skin embedded wire double-sided LBW. (3) In compare with conventional double-sided LBW weld, local mechanical properties within the weld welded by the skin embedded wire double-sided LBW all showed apparent improvements. In detail, average values of ultimate tensile strength, yield strength and percentage elongation were all improved from previous 89.4 MPa, 62.1 MPa and 2.1% to present 179.8 MPa, 70.0 MPa and 4.8%, respectively. Features of inter- and transgranular mixed-mode were identified on local tensile specimens of the skin embedded wire double-sided LBW. No obvious negative influence on porosity defects was found during the skin embedded wire double-sided LBW process.
[4] [5]
[6]
[7] [8]
[9]
[10]
Acknowledgments [11]
The authors would like to thank National Engineering Research Center of Commercial Aircraft Manufacturing for financial support of this project.
[12]
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