- Email: [email protected]
Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser
Materials and Design 84 (2015) 173–177
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser Zhihui Zhang a,b,c, Shiyun Dong b, Yujiang Wang b, Binshi Xu a,b, Jinxiang Fang a,b, Peng He a,⁎ a b c
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China Department of Mechanical Engineering, Academy of Armored Force Technology, Changchun, Jilin 130117, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Received in revised form 11 June 2015 Accepted 13 June 2015 Available online 25 June 2015 Keywords: Super narrow gap Fiber laser Zr and Er micro-alloying Aluminum alloy
a b s t r a c t It's difficult to weld high strength thick plate since the groove is huge when using traditional arc welding, and the weld tends to be softened and large deformation could occur after multi-layer welding. All of these can affect the industrial application of high strength thick plate wielding. In this case, developing advanced welding technology and welding material is necessary to optimize the microstructure and performance of the welds. Fiber laser has many advantages such as good monochrome and high quality laser beam. In order to decrease the heat damage to the base metal from the welding heat source, low heat input is employed for welding thick plate. Fiber laser is applied in the welding of 20 mm thick Al–Zn–Mg–Cu alloy with super narrow gap filler wire. The microstructure comparison of Al–Mg–Mn alloy and Al–Mg–Mn–Zr–Er alloy welded joints reveals that a huge amount of fine equiaxed grains is formed in the weld zone of Zr and Er micro-alloying Al–Mg–Mn alloy welding wire and a great number of precipitation strengthening phases are precipitated in the weld zone after the heat treatment of welded joints in the entirety. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The welding of thick metallic plates usually involves wide-angle groove, multilayer, multipass, large amount of metal filling and tremendous residual stress. The narrow gap welding method for welding thick metal plates can-not only reduce the transverse section of the groove drastically and cut down the amount of weld metal deposition significantly, but also realize efficient welding with small weld heat input [1–3]. The commonly used narrow gap welding technologies include narrow gap submerged arc welding (SAW), narrow gap argon tungsten-arc welding, narrow gap electroslag welding and narrow gap laser welding. Nonetheless, super narrow gap fiber laser welding with filler wire is rarely reported. It applies linear heat input smaller than commonly adopted narrow gap welding, which can be minimized to 0.5 kj/mm. Fiber laser boasts the advantages of high quality light beam, perfect monochromaticity, short wave length and favorable technological adaptability [4,5]. Combining the edges of fiber laser and super narrow gap, it can reduce the heat losses upon the welding structure remarkably by welding heat source. As a result, an appropriate selection of welding materials can enhance the overall mechanical properties of welded joints. Aluminum alloy is the priority material for aircraft and spacecraft, which is extensively applied in the aerospace industry [6]. For example, 180 t thick aluminum plates are used in each airbus. However, in ⁎ Corresponding author. E-mail address: [email protected] (P. He).
http://dx.doi.org/10.1016/j.matdes.2015.06.087 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
practical application, the aluminum alloy thicker than or equal to 20 mm accounts for 50%. The purpose of the present work aims at adopting small power fiber laser for effective welding of 20 mm thick Al–Zn–Mg–Cu alloy, with single “I” pass groove whose horizontal width is 2.5 mm. Micro-alloying functions as a pivotal method to boost the properties of aluminum alloy. Studies indicate that the additions of trace rare earth elements into the aluminum alloy can remarkably refine the grains [7], and could purify the composition of alloy so as to improve the mechanical properties at last. The addition of Sc into the aluminum alloy can ostentatiously boost the strength of alloy while the low cost element Er shares similar functions with Sc in the aluminum alloy [8]. In order to compare the influence of different welding materials upon the microstructure of joints welded by super narrow gap fiber laser, the present study selects 5183 alloy and 5E06 as the welding wire. 2. Experimental procedures YLS-4000 YB-doped fiber laser with a wave-length of 1070 nm and maximum rate output power of 4000 W produced by IPG Photonics is applied in the experiment. During the welding, the laser head vertically deviates from the direction of the work piece by 10° to prevent reflective laser from damaging the focus lens. The motor execution system adopts two-dimensional welding machines. The experimental materials are 7A52 high strength aluminum alloy with a dimension of 160 mm × 80 mm × 20 mm. Filler wires with different chemical
174
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
Table 1 Chemical composition of 7A52, 5183 and 5E06 (wt.%). Alloy
Zn
Mg
Cu
Mn
Cr
Ti
Zr
Er
Fe
Si
Al
7A52 5183 5E06
4.2 – –
2.1 4.7 4.9
0.08 – –
0.30 0.8 0.7
0.16 – –
0.08 0.1 –
0.14 – 0.1
– – 0.3
≤0.30 – –
≤0.25 – –
Bal. Bal. Bal.
compositions are used including 5183 welding wire and 5E06 welding wire. The diameter of both welding wires is 1.6 mm. The chemical compositions of the parent metal and the welding wires are listed in Table 1. Backing welding is used to penetrate the truncated edge and the entire welding process is completed in the manner of laser feeding from the bottom up. 50%He + 50%Ar gas is selected as the protective gas for welding. Mechanical polishing and electrolytic polishing are applied for specimens. An ISM-7001F scanning electron microscope (SEM), JEOL-2100 transmission electron microscopy (TEM) and FEI-F20 high resolution TEM are utilized to observe the microstructural morphology and precipitated phases in the weld zones. Corresponding supporting EDAX energy disperse spectroscopy is adopted for semi-quantitative analysis of the chemical composition. Three parallel samples are prepared for each processing state. The test value in each state is determined by the average value of the measurements. Hardness test is carried out 100 gf load for 10 s, with an HVS-1000 digital microhardness tester. Each test is conducted on both sides at an interval of 1 mm from the center of the transverse section of the weld. The value of hardness tested is the average of the three measurements. Thermal treatment is applied to the welded joints of 5E06 alloy after welding. In this process, samples are treated at 470 °C for 3 h in a nitrate oven and afterwards at 120 °C for 12 h artificial aging treatment in the thermostatic drier box. 3. Results and discussion Fig. 1 indicates the macroscopic morphology of the transverse section of welded joints. The groove is 3 mm in width and the upper and bottom widths of weld are consistently less than 4 mm. It can be seen that perfect fusion forms between the side walls of groove and the passes without defects such as holes, inclusion or cracks. The difficulties of narrow gap welding lie in the fusion of side walls. Defocused spots of the same size of groove gap are selected during welding, which can develop perfect fusion of side walls. The laser reflection composed of fusion curve and side walls becomes the primary reason for the fusion of side walls. In order to compare the differences of welded joints of diverse welding materials in morphology, backscattered-electron diffraction (EBSD) is utilized to observe the morphology of grains near the fusion line of the welded joints. Fig. 2(a) and (b) present the morphology
Fig. 1. Macroscopic schematic diagram of welded joints.
features of grains near the fusion line filled with 5183 welding wire and 5E06 welding wire, respectively. It is discovered that epitaxial solidification appears near the fusion line of weld filled with 5183 welding wire and coarse columnar crystals grow in the weld center [9]. The columnar crystals generate from the parent metal zone at the weld boundary. In contrast, fine equiaxed grains with small size and tiny width in random distribution occur in the fusion zone of 5E06 welding wire, indicating no epitaxial solidification occurs at the parent metal. Nucleation may take place on the grains of the matrix where the lattice in the liquid metal does not have to overcome any energy barrier, resulting in the so-called epitaxial solidification. Nevertheless, when the composition of weld metal is different from that of the matrix metal, or heterogeneous grains exist in the fusion bath, the mode of crystallization may be destroyed and these equiaxed fine grains hinder the growth of epitaxial crystals. Since homogeneous nucleation can hardly happen in weld solidification, the nucleation of equiaxed grains sources from heterogeneous nucleation and the degree of supercooling of the composition existing in the leading edge of the solid–liquid phases [10,11]. EBSD is used to observe the morphology of grains in the bond zone of welded joints with diverse passes. Compared with other areas, the microstructures in these zones go through twice heat cycle where the microstructural changes become more sophisticated and the tapping points become weak links. These zones can better reflect the mechanical properties of the entire welded joint. Fig. 2(c) and (d) respectively exhibit the morphology of grains in interlayer structures of weld zones of 5183 and 5E06 welding wires. Fig. 2(c) illustrates the existence of obvious interfacial transition zone between the passes and the coexistence of the coarse grain zone and the fine grain zone. The reason is that the previous layer will go through re-fusion and heat treatment when the latter layer is welded. Under the effects of high heat laser, the more distinctively the grains grow close to the fusion line, the more refined the grains are between the weld passes. It results from the fairly small super narrow gap fusion bath and the high degree of supercooling of laser welding. The thermogenesis of the following layer upon the previous layer is equivalent to short-term heat treatment. Twice re-crystallization both during the heating and the cooling generates notable grain refinement. Therefore, this zone boasts higher overall mechanical properties. Fig. 2(c) displays the columnar grains of an average size of 168 μm. The crystal in the re-fusion zone grows in a changed direction because of the directions of temperature gradient and grain growth shift with heat dissipation of multilayer weld fusion bath. Fig. 2(d) demonstrates the equiaxed grains. It is compared that the grains in the weld zone of Al–Mg–Mn alloy welding wire after Zr and Er micro-alloying are evidently refined, the grain size sharply shrinks to 36 μm and the grain morphology transforms from columnar crystal to equiaxed crystal. The weld and the casting share similar solidification processes, usually from the surface to the center and featuring controlled directional solidification. As the solidification zone gradually sways away from the fusion bath surface, the temperature gradient of the leading edge of the solid–liquid interface diminishes little by little, the solidification velocity gradually drops, the solute content increasingly rises and the constitutional supercooling zone significantly expands. As a result, the crystalline form turns from dendrite to equiaxed grain [12]. When the size and the amount of free crystal formed in the liquid phase in front of the solidification interface reach a certain value, the columnar crystal will be blocked in growth and morph into equiaxed crystal. Liquid-phase flow plays a decisive role in the formation of free crystal in front of the solidification interface. The cooling speed of fusion bath during laser welding is exceedingly high, which will further augment constitutional supercooling and refine the weld metal grains. The degree of supercooling is the crucial factor deciding the formation of equiaxed crystal [13,14]. Another reason for grain refinement is the effect of metamorphic element Zr and rare element Er. Gutierez [15] discovered that Al3Zr and Al3(Li, Zr) originally in disperse distribution near the boundary of the fusion bath remain and the atoms in the liquid metal can be ranked in the crystal morphology
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
175
Fig. 2. (a) EBSD analysis of the microstructure near the fusion line of 5183 welding wire. (b) EBSD analysis of the microstructure near the fusion line of 5E06 welding wire. (c) EBSD analysis of interlayer microstructure of weld zone of 5183 welding wire. (d) EBSD analysis of interlayer microstructure of weld zone of 5E06 welding wire.
on the heterogeneous grains and grow to be equiaxed grains after heterogeneous crystal nucleation occurs. With the additions of metamorphic element Zr and rare element Er into the 5E06 welding wire, the second phase Al3Zr and Al3(Li, Zr) nanoparticles in disperse distribution inside the original grains of the welding wire share interface coherency with Al matrix; the lattice mismatch with Al crystal lattice is lower than 5%; with the high heat stability achieved, the particles become the heterogeneous nucleation core of α(Al) in the weld so as to stimulate nucleation of weld grains of relatively smaller sizes [16]. To define the mode of existence of Er and Zr in the weld zone, TEM is applied to observe the fine structure of precipitated phase in the weld zone. Fig. 3(a) depicts the high angle circular dark field image of the precipitated phase in the weld zone of 5E06 welding wire. It can be noticed that a few dislocation lines are formed in the weld zone. Fine second phase particles are visible near the dislocation, distributing around the dislocation lines, which can restrain dislocation and grain boundary migration. Fig. 3(b) reflects the high power morphology of precipitated phase. The EDS results of the precipitated phase in Fig. 3(b) bear witness that these precipitated phase particles contain Al, Mg, Zr and Er etc. which are judged to be the Al3(Zr,Er) phases [17]. These particles exercise sound pinning effects upon dislocation and subgrain boundary and play the functions of precipitation strengthening and inhibition of recrystallization. Fig. 3(c) demonstrates the high resolution morphology of ternary compound particles that have uniform crystal structures. The atomic planes are extremely bright in the core while rather dark in the adjacent counterpart. The atomic plane is in axial symmetry in the middle of the dark region while the dark region in the center is in the shape of symmetric bean. One of the cases is that Er and Zr inside the particles aggregate separately in uneven distribution and the dark region in the middle might be the region where Er aggregates. Since the atomic number of Er is comparatively high and the scattering of more electronic beams is generated, the contrast becomes dark. Conversely, the atomic number of Zr is small and the external light region might be the region where Zr aggregates. The contrast in the middle dark region of the bean shape is similar to that of the coherent particles
may result from the stress triggered by the different constants of crystal lattices in the internal and external layers. Meanwhile, the mismatching degree of Al3(Zr,Er) and aluminum matrix is quite small, which complies with the conditions for the sizes and structures of heterogeneous nucleus.
Fig. 3. HAADF STEM image and energy spectrum analysis of precipitated phase of weld. (a) Low power morphology of precipitated phase. (b) High power morphology of precipitated phase and EDS energy spectrum. (c) High power morphology of precipitated phase.
176
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
Fig. 4. (a) Dark field image of weld zone after heat treatment. (b) Bright field image of weld zone after heat treatment.
Selecting appropriate heat treatment technologies can change the microstructures of the welded joints so as to enhance the overall mechanical properties of the welded structures [18]. Er and Zr, widely studied to be the most effective modificator in aluminum alloy, and they cannot merely refine grains but also heighten the strength of alloy. The dominant reason is that during solidification the primary Al3(Zr,Er) phase firstly precipitated has identical lattice structures and similar lattice constants with Al solid solution which can be taken as the core of crystallization to refine the grains. Meanwhile, Er is easy to be solved in the matrix, supersaturated and decomposed from the solid solution when heated, precipitating fine disperse L12 Al3(Zr,Er) precipitated phase and strengthening the alloy [17]. In the subsequent aging process, plenty of precipitation strengthening phases are precipitated and the hardness of alloy is increased [19,20]. After welding the welded joints go through solid solution at 470 °C for 3 h and then aging at 120 °C for 12 h on the whole. Fig. 4(a) and (b) respectively represents the dark field image and the bright field image of the weld zone where quantities of disperse strengthening phases are precipitated and the function of precipitation strengthening is put into play. Fig. 5 indicates the horizontal microhardness of weld joint center. A comparison reveals that the hardness value of Al–Mg–Mn–Zr–Er alloy weld is higher than that filling ER5183 weld. As to Al–Mg–Mn–Zr–Er alloy, grain refinement of Er and Zr remarkably decreases the grain size in welded joint and accordingly increases the amount of grain boundary. It will more hinder the dislocation and enhance the resistance to plastic flow, which is manifested in the microhardness increase at the macro level and testifies the function of refined crystalline strengthening of
Zr and Er in aluminum alloy [17]. After thermal treatment of Al–Mg– Mn–Zr–Er alloy welded joints, the average microhardness of weld is 111HV, 11.4HV higher than that before thermal treatment. The analytical result is that the significantly increased precipitated phases in the weld metal after thermal treatment effectively hinder dislocation and obviously boost the hardness of weld metal. 4. Conclusions The paper adopts narrow gap groove to reliably weld the 20 mm thick 7A52 aluminum alloy plates with fiber laser and compares the microstructures and microhardnesses of ER5183 welded joints and Al–Mg–Mn–Zr–Er alloy welded joints. Specific conclusions of this work are drawn as follows: 1. KW-level laser welding of thick aluminum alloy plates is achieved with filler wire. The super narrow gap groove is no more than 3 mm, which lowers the weld metal deposition. 2. Weld grain can be refined by Er–Zr microalloying alloy weld. A large amount of equiaxed grains are formed in the weld. After overall thermal heating, plenty of phases are precipitated after strengthening in the weld. 3. After thermal treatment of Al–Mg–Mn–Zr–Er alloy welded joints, the average microhardness in the weld increases by 11.4HV. The analytical result of hardness enhancement is that precipitated phases sharply increase in weld metal after thermal treatment, which effectively hinders the dislocation and drastically increases the hardness of weld metal.
Acknowledgments The paper is financially supported by 973 Project of China, No. 2011CB013403. The authors are grateful to Ling Zhao, Qing Liu and Fengyuan Shu for valuable assistance they provided with various aspects of the research program. References
Fig. 5. Distribution of micro-hardness in cross section of the welded joints.
[1] K. Devendranath Ramkumar, G. Thiruvengatam, S.P. Sudharsan, Debidutta Mishra, N. Arivazhagan, R. Sridhar, Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750, Mater. Des. 60 (2014) 125–135. [2] Changheui Jang, Pyung-Yeon Cho, Minu Kim, Seung-Jin Oh, Jun-Seog Yang, Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds, Mater. Des. 31 (2010) 1862–1870. [3] L. Tan, L.J. Zhang, J.X. Zhang, D. Zhuang, Effect of geometric construction on residual stress distribution in designing a nuclear rotor joined by multipass narrow gap welding, Fusion Eng. Des. 89 (2014) 456–465.
Z. Zhang et al. / Materials and Design 84 (2015) 173–177 [4] M.J. Zhang, G.Y. Chen, Y. Zhou, S.H. Liao, Optimization of deep penetration laser welding of thick stainless steel with a 10 kW fiber laser, Mater. Des. 53 (2014) 568–576. [5] C.Y. Cui, X.G. Cui, X.D. Ren, T.T. Liu, J.D. Hu, Y.M. Wang, Microstructure and microhardness of fiber laser butt welded joint of stainless steel plates, Mater. Des. 49 (2013) 761–765. [6] Tolga Dursun, Costas Soutis, Recent developments in advanced aircraft aluminium alloys, Mater. Des. 56 (2014) 862–871. [7] V. Singh, K.S. Prasad, A.A. Gokhale, Effect of minor Sc additions on structure, age hardening and tensile properties of aluminium alloy AA8090 plate, Scr. Mater. 50 (2004) 903–908. [8] S.P. Wen, K.Y. Gao, Y. Li, H. Huang, Z.R. Nie, Synergetic effect of Er and Zr on the precipitation hardening of Al–Er–Zr alloy, Scr. Mater. 65 (2011) 592–595. [9] D. Turnbull, Formation of crystal nuclei in liquid metals, J. Appl. Phys. 21 (1950) 1022–1028. [10] Anastasios D. Kostrivas, John C. Lippold, Simulating weld-fusion boundary microstructures in aluminum alloys, JOM 56 (2004) 65–72. [11] T. Michel, L. Alvarez, J.-L. Sauvajol, R. Almairac, R. Aznar, O. Mathon, J.-L. Bantignies, E. Flahaut, Structural selective charge transfer in iodine-doped carbon nanotubes, J. Phys. Chem. Solids 67 (2006) 1190–1192. [12] G. Duggan, M. Tong, D.J. Browne, Modelling the creation and destruction of columnar and equiaxed zones during solidification and melting in multi-pass welding of steel, Comput. Mater. Sci. 97 (2015) 285–294.
177
[13] S. Witzke, J.P. Riquet, Columnar-equiaxed transition in Al–Cu alloy ingots, Acta Metall. 30 (1982) 1717–1722. [14] W. Kurz, C. Bezençon, M. Gäumann, Columnar to equiaxed transition in solidification processing, Sci. Technol. Adv. Mater. 2 (2001) 185–191. [15] A. Gutierez, J.C. Lippold, Proposed mechanism for equiaxed grain formation along the fusion boundary in aluminum–copper–lithium alloys, Weld. J. 77 (1998) 123s–132s. [16] H.Y. Li, Z.H. Gao, H. Yin, H.F. Jiang, X.J. Su, J. Bin, Effects of Er and Zr additions on precipitation and recrystallization of pure aluminum, Scr. Mater. 68 (2013) 59–62. [17] D.X. Yang, X.Y. Li, D.Y. He, H. Huang, Effect of minor Er and Zr on microstructure and mechanical properties of Al–Mg–Mn alloy (5083) welded joints, Mater. Sci. Eng. A 561 (2013) 226–231. [18] Mogammad W. Dewan, J.D. Liang, M.A. Wagab, Ayman M. Okeil, Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel, Mater. Des. 54 (2014) 6–13. [19] H. Wu, S.P. Wen, K.Y. Gao, H. Huang, W. Wang, Z.R. Nie, Effect of Er additions on the precipitation strengthening of Al–Hf alloys, Scr. Mater. 87 (2014) 5–8. [20] H.Y. Li, J. Bin, J.J. Liu, Z.H. Gao, X.C. Lu, Precipitation evolution and coarsening resistance at 400 °C of Al microalloyed with Zr and Er, Scr. Mater. 67 (2012) 73–76.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser Zhihui Zhang a,b,c, Shiyun Dong b, Yujiang Wang b, Binshi Xu a,b, Jinxiang Fang a,b, Peng He a,⁎ a b c
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China Department of Mechanical Engineering, Academy of Armored Force Technology, Changchun, Jilin 130117, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Received in revised form 11 June 2015 Accepted 13 June 2015 Available online 25 June 2015 Keywords: Super narrow gap Fiber laser Zr and Er micro-alloying Aluminum alloy
a b s t r a c t It's difficult to weld high strength thick plate since the groove is huge when using traditional arc welding, and the weld tends to be softened and large deformation could occur after multi-layer welding. All of these can affect the industrial application of high strength thick plate wielding. In this case, developing advanced welding technology and welding material is necessary to optimize the microstructure and performance of the welds. Fiber laser has many advantages such as good monochrome and high quality laser beam. In order to decrease the heat damage to the base metal from the welding heat source, low heat input is employed for welding thick plate. Fiber laser is applied in the welding of 20 mm thick Al–Zn–Mg–Cu alloy with super narrow gap filler wire. The microstructure comparison of Al–Mg–Mn alloy and Al–Mg–Mn–Zr–Er alloy welded joints reveals that a huge amount of fine equiaxed grains is formed in the weld zone of Zr and Er micro-alloying Al–Mg–Mn alloy welding wire and a great number of precipitation strengthening phases are precipitated in the weld zone after the heat treatment of welded joints in the entirety. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The welding of thick metallic plates usually involves wide-angle groove, multilayer, multipass, large amount of metal filling and tremendous residual stress. The narrow gap welding method for welding thick metal plates can-not only reduce the transverse section of the groove drastically and cut down the amount of weld metal deposition significantly, but also realize efficient welding with small weld heat input [1–3]. The commonly used narrow gap welding technologies include narrow gap submerged arc welding (SAW), narrow gap argon tungsten-arc welding, narrow gap electroslag welding and narrow gap laser welding. Nonetheless, super narrow gap fiber laser welding with filler wire is rarely reported. It applies linear heat input smaller than commonly adopted narrow gap welding, which can be minimized to 0.5 kj/mm. Fiber laser boasts the advantages of high quality light beam, perfect monochromaticity, short wave length and favorable technological adaptability [4,5]. Combining the edges of fiber laser and super narrow gap, it can reduce the heat losses upon the welding structure remarkably by welding heat source. As a result, an appropriate selection of welding materials can enhance the overall mechanical properties of welded joints. Aluminum alloy is the priority material for aircraft and spacecraft, which is extensively applied in the aerospace industry [6]. For example, 180 t thick aluminum plates are used in each airbus. However, in ⁎ Corresponding author. E-mail address: [email protected] (P. He).
http://dx.doi.org/10.1016/j.matdes.2015.06.087 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
practical application, the aluminum alloy thicker than or equal to 20 mm accounts for 50%. The purpose of the present work aims at adopting small power fiber laser for effective welding of 20 mm thick Al–Zn–Mg–Cu alloy, with single “I” pass groove whose horizontal width is 2.5 mm. Micro-alloying functions as a pivotal method to boost the properties of aluminum alloy. Studies indicate that the additions of trace rare earth elements into the aluminum alloy can remarkably refine the grains [7], and could purify the composition of alloy so as to improve the mechanical properties at last. The addition of Sc into the aluminum alloy can ostentatiously boost the strength of alloy while the low cost element Er shares similar functions with Sc in the aluminum alloy [8]. In order to compare the influence of different welding materials upon the microstructure of joints welded by super narrow gap fiber laser, the present study selects 5183 alloy and 5E06 as the welding wire. 2. Experimental procedures YLS-4000 YB-doped fiber laser with a wave-length of 1070 nm and maximum rate output power of 4000 W produced by IPG Photonics is applied in the experiment. During the welding, the laser head vertically deviates from the direction of the work piece by 10° to prevent reflective laser from damaging the focus lens. The motor execution system adopts two-dimensional welding machines. The experimental materials are 7A52 high strength aluminum alloy with a dimension of 160 mm × 80 mm × 20 mm. Filler wires with different chemical
174
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
Table 1 Chemical composition of 7A52, 5183 and 5E06 (wt.%). Alloy
Zn
Mg
Cu
Mn
Cr
Ti
Zr
Er
Fe
Si
Al
7A52 5183 5E06
4.2 – –
2.1 4.7 4.9
0.08 – –
0.30 0.8 0.7
0.16 – –
0.08 0.1 –
0.14 – 0.1
– – 0.3
≤0.30 – –
≤0.25 – –
Bal. Bal. Bal.
compositions are used including 5183 welding wire and 5E06 welding wire. The diameter of both welding wires is 1.6 mm. The chemical compositions of the parent metal and the welding wires are listed in Table 1. Backing welding is used to penetrate the truncated edge and the entire welding process is completed in the manner of laser feeding from the bottom up. 50%He + 50%Ar gas is selected as the protective gas for welding. Mechanical polishing and electrolytic polishing are applied for specimens. An ISM-7001F scanning electron microscope (SEM), JEOL-2100 transmission electron microscopy (TEM) and FEI-F20 high resolution TEM are utilized to observe the microstructural morphology and precipitated phases in the weld zones. Corresponding supporting EDAX energy disperse spectroscopy is adopted for semi-quantitative analysis of the chemical composition. Three parallel samples are prepared for each processing state. The test value in each state is determined by the average value of the measurements. Hardness test is carried out 100 gf load for 10 s, with an HVS-1000 digital microhardness tester. Each test is conducted on both sides at an interval of 1 mm from the center of the transverse section of the weld. The value of hardness tested is the average of the three measurements. Thermal treatment is applied to the welded joints of 5E06 alloy after welding. In this process, samples are treated at 470 °C for 3 h in a nitrate oven and afterwards at 120 °C for 12 h artificial aging treatment in the thermostatic drier box. 3. Results and discussion Fig. 1 indicates the macroscopic morphology of the transverse section of welded joints. The groove is 3 mm in width and the upper and bottom widths of weld are consistently less than 4 mm. It can be seen that perfect fusion forms between the side walls of groove and the passes without defects such as holes, inclusion or cracks. The difficulties of narrow gap welding lie in the fusion of side walls. Defocused spots of the same size of groove gap are selected during welding, which can develop perfect fusion of side walls. The laser reflection composed of fusion curve and side walls becomes the primary reason for the fusion of side walls. In order to compare the differences of welded joints of diverse welding materials in morphology, backscattered-electron diffraction (EBSD) is utilized to observe the morphology of grains near the fusion line of the welded joints. Fig. 2(a) and (b) present the morphology
Fig. 1. Macroscopic schematic diagram of welded joints.
features of grains near the fusion line filled with 5183 welding wire and 5E06 welding wire, respectively. It is discovered that epitaxial solidification appears near the fusion line of weld filled with 5183 welding wire and coarse columnar crystals grow in the weld center [9]. The columnar crystals generate from the parent metal zone at the weld boundary. In contrast, fine equiaxed grains with small size and tiny width in random distribution occur in the fusion zone of 5E06 welding wire, indicating no epitaxial solidification occurs at the parent metal. Nucleation may take place on the grains of the matrix where the lattice in the liquid metal does not have to overcome any energy barrier, resulting in the so-called epitaxial solidification. Nevertheless, when the composition of weld metal is different from that of the matrix metal, or heterogeneous grains exist in the fusion bath, the mode of crystallization may be destroyed and these equiaxed fine grains hinder the growth of epitaxial crystals. Since homogeneous nucleation can hardly happen in weld solidification, the nucleation of equiaxed grains sources from heterogeneous nucleation and the degree of supercooling of the composition existing in the leading edge of the solid–liquid phases [10,11]. EBSD is used to observe the morphology of grains in the bond zone of welded joints with diverse passes. Compared with other areas, the microstructures in these zones go through twice heat cycle where the microstructural changes become more sophisticated and the tapping points become weak links. These zones can better reflect the mechanical properties of the entire welded joint. Fig. 2(c) and (d) respectively exhibit the morphology of grains in interlayer structures of weld zones of 5183 and 5E06 welding wires. Fig. 2(c) illustrates the existence of obvious interfacial transition zone between the passes and the coexistence of the coarse grain zone and the fine grain zone. The reason is that the previous layer will go through re-fusion and heat treatment when the latter layer is welded. Under the effects of high heat laser, the more distinctively the grains grow close to the fusion line, the more refined the grains are between the weld passes. It results from the fairly small super narrow gap fusion bath and the high degree of supercooling of laser welding. The thermogenesis of the following layer upon the previous layer is equivalent to short-term heat treatment. Twice re-crystallization both during the heating and the cooling generates notable grain refinement. Therefore, this zone boasts higher overall mechanical properties. Fig. 2(c) displays the columnar grains of an average size of 168 μm. The crystal in the re-fusion zone grows in a changed direction because of the directions of temperature gradient and grain growth shift with heat dissipation of multilayer weld fusion bath. Fig. 2(d) demonstrates the equiaxed grains. It is compared that the grains in the weld zone of Al–Mg–Mn alloy welding wire after Zr and Er micro-alloying are evidently refined, the grain size sharply shrinks to 36 μm and the grain morphology transforms from columnar crystal to equiaxed crystal. The weld and the casting share similar solidification processes, usually from the surface to the center and featuring controlled directional solidification. As the solidification zone gradually sways away from the fusion bath surface, the temperature gradient of the leading edge of the solid–liquid interface diminishes little by little, the solidification velocity gradually drops, the solute content increasingly rises and the constitutional supercooling zone significantly expands. As a result, the crystalline form turns from dendrite to equiaxed grain [12]. When the size and the amount of free crystal formed in the liquid phase in front of the solidification interface reach a certain value, the columnar crystal will be blocked in growth and morph into equiaxed crystal. Liquid-phase flow plays a decisive role in the formation of free crystal in front of the solidification interface. The cooling speed of fusion bath during laser welding is exceedingly high, which will further augment constitutional supercooling and refine the weld metal grains. The degree of supercooling is the crucial factor deciding the formation of equiaxed crystal [13,14]. Another reason for grain refinement is the effect of metamorphic element Zr and rare element Er. Gutierez [15] discovered that Al3Zr and Al3(Li, Zr) originally in disperse distribution near the boundary of the fusion bath remain and the atoms in the liquid metal can be ranked in the crystal morphology
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
175
Fig. 2. (a) EBSD analysis of the microstructure near the fusion line of 5183 welding wire. (b) EBSD analysis of the microstructure near the fusion line of 5E06 welding wire. (c) EBSD analysis of interlayer microstructure of weld zone of 5183 welding wire. (d) EBSD analysis of interlayer microstructure of weld zone of 5E06 welding wire.
on the heterogeneous grains and grow to be equiaxed grains after heterogeneous crystal nucleation occurs. With the additions of metamorphic element Zr and rare element Er into the 5E06 welding wire, the second phase Al3Zr and Al3(Li, Zr) nanoparticles in disperse distribution inside the original grains of the welding wire share interface coherency with Al matrix; the lattice mismatch with Al crystal lattice is lower than 5%; with the high heat stability achieved, the particles become the heterogeneous nucleation core of α(Al) in the weld so as to stimulate nucleation of weld grains of relatively smaller sizes [16]. To define the mode of existence of Er and Zr in the weld zone, TEM is applied to observe the fine structure of precipitated phase in the weld zone. Fig. 3(a) depicts the high angle circular dark field image of the precipitated phase in the weld zone of 5E06 welding wire. It can be noticed that a few dislocation lines are formed in the weld zone. Fine second phase particles are visible near the dislocation, distributing around the dislocation lines, which can restrain dislocation and grain boundary migration. Fig. 3(b) reflects the high power morphology of precipitated phase. The EDS results of the precipitated phase in Fig. 3(b) bear witness that these precipitated phase particles contain Al, Mg, Zr and Er etc. which are judged to be the Al3(Zr,Er) phases [17]. These particles exercise sound pinning effects upon dislocation and subgrain boundary and play the functions of precipitation strengthening and inhibition of recrystallization. Fig. 3(c) demonstrates the high resolution morphology of ternary compound particles that have uniform crystal structures. The atomic planes are extremely bright in the core while rather dark in the adjacent counterpart. The atomic plane is in axial symmetry in the middle of the dark region while the dark region in the center is in the shape of symmetric bean. One of the cases is that Er and Zr inside the particles aggregate separately in uneven distribution and the dark region in the middle might be the region where Er aggregates. Since the atomic number of Er is comparatively high and the scattering of more electronic beams is generated, the contrast becomes dark. Conversely, the atomic number of Zr is small and the external light region might be the region where Zr aggregates. The contrast in the middle dark region of the bean shape is similar to that of the coherent particles
may result from the stress triggered by the different constants of crystal lattices in the internal and external layers. Meanwhile, the mismatching degree of Al3(Zr,Er) and aluminum matrix is quite small, which complies with the conditions for the sizes and structures of heterogeneous nucleus.
Fig. 3. HAADF STEM image and energy spectrum analysis of precipitated phase of weld. (a) Low power morphology of precipitated phase. (b) High power morphology of precipitated phase and EDS energy spectrum. (c) High power morphology of precipitated phase.
176
Z. Zhang et al. / Materials and Design 84 (2015) 173–177
Fig. 4. (a) Dark field image of weld zone after heat treatment. (b) Bright field image of weld zone after heat treatment.
Selecting appropriate heat treatment technologies can change the microstructures of the welded joints so as to enhance the overall mechanical properties of the welded structures [18]. Er and Zr, widely studied to be the most effective modificator in aluminum alloy, and they cannot merely refine grains but also heighten the strength of alloy. The dominant reason is that during solidification the primary Al3(Zr,Er) phase firstly precipitated has identical lattice structures and similar lattice constants with Al solid solution which can be taken as the core of crystallization to refine the grains. Meanwhile, Er is easy to be solved in the matrix, supersaturated and decomposed from the solid solution when heated, precipitating fine disperse L12 Al3(Zr,Er) precipitated phase and strengthening the alloy [17]. In the subsequent aging process, plenty of precipitation strengthening phases are precipitated and the hardness of alloy is increased [19,20]. After welding the welded joints go through solid solution at 470 °C for 3 h and then aging at 120 °C for 12 h on the whole. Fig. 4(a) and (b) respectively represents the dark field image and the bright field image of the weld zone where quantities of disperse strengthening phases are precipitated and the function of precipitation strengthening is put into play. Fig. 5 indicates the horizontal microhardness of weld joint center. A comparison reveals that the hardness value of Al–Mg–Mn–Zr–Er alloy weld is higher than that filling ER5183 weld. As to Al–Mg–Mn–Zr–Er alloy, grain refinement of Er and Zr remarkably decreases the grain size in welded joint and accordingly increases the amount of grain boundary. It will more hinder the dislocation and enhance the resistance to plastic flow, which is manifested in the microhardness increase at the macro level and testifies the function of refined crystalline strengthening of
Zr and Er in aluminum alloy [17]. After thermal treatment of Al–Mg– Mn–Zr–Er alloy welded joints, the average microhardness of weld is 111HV, 11.4HV higher than that before thermal treatment. The analytical result is that the significantly increased precipitated phases in the weld metal after thermal treatment effectively hinder dislocation and obviously boost the hardness of weld metal. 4. Conclusions The paper adopts narrow gap groove to reliably weld the 20 mm thick 7A52 aluminum alloy plates with fiber laser and compares the microstructures and microhardnesses of ER5183 welded joints and Al–Mg–Mn–Zr–Er alloy welded joints. Specific conclusions of this work are drawn as follows: 1. KW-level laser welding of thick aluminum alloy plates is achieved with filler wire. The super narrow gap groove is no more than 3 mm, which lowers the weld metal deposition. 2. Weld grain can be refined by Er–Zr microalloying alloy weld. A large amount of equiaxed grains are formed in the weld. After overall thermal heating, plenty of phases are precipitated after strengthening in the weld. 3. After thermal treatment of Al–Mg–Mn–Zr–Er alloy welded joints, the average microhardness in the weld increases by 11.4HV. The analytical result of hardness enhancement is that precipitated phases sharply increase in weld metal after thermal treatment, which effectively hinders the dislocation and drastically increases the hardness of weld metal.
Acknowledgments The paper is financially supported by 973 Project of China, No. 2011CB013403. The authors are grateful to Ling Zhao, Qing Liu and Fengyuan Shu for valuable assistance they provided with various aspects of the research program. References
Fig. 5. Distribution of micro-hardness in cross section of the welded joints.
[1] K. Devendranath Ramkumar, G. Thiruvengatam, S.P. Sudharsan, Debidutta Mishra, N. Arivazhagan, R. Sridhar, Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750, Mater. Des. 60 (2014) 125–135. [2] Changheui Jang, Pyung-Yeon Cho, Minu Kim, Seung-Jin Oh, Jun-Seog Yang, Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds, Mater. Des. 31 (2010) 1862–1870. [3] L. Tan, L.J. Zhang, J.X. Zhang, D. Zhuang, Effect of geometric construction on residual stress distribution in designing a nuclear rotor joined by multipass narrow gap welding, Fusion Eng. Des. 89 (2014) 456–465.
Z. Zhang et al. / Materials and Design 84 (2015) 173–177 [4] M.J. Zhang, G.Y. Chen, Y. Zhou, S.H. Liao, Optimization of deep penetration laser welding of thick stainless steel with a 10 kW fiber laser, Mater. Des. 53 (2014) 568–576. [5] C.Y. Cui, X.G. Cui, X.D. Ren, T.T. Liu, J.D. Hu, Y.M. Wang, Microstructure and microhardness of fiber laser butt welded joint of stainless steel plates, Mater. Des. 49 (2013) 761–765. [6] Tolga Dursun, Costas Soutis, Recent developments in advanced aircraft aluminium alloys, Mater. Des. 56 (2014) 862–871. [7] V. Singh, K.S. Prasad, A.A. Gokhale, Effect of minor Sc additions on structure, age hardening and tensile properties of aluminium alloy AA8090 plate, Scr. Mater. 50 (2004) 903–908. [8] S.P. Wen, K.Y. Gao, Y. Li, H. Huang, Z.R. Nie, Synergetic effect of Er and Zr on the precipitation hardening of Al–Er–Zr alloy, Scr. Mater. 65 (2011) 592–595. [9] D. Turnbull, Formation of crystal nuclei in liquid metals, J. Appl. Phys. 21 (1950) 1022–1028. [10] Anastasios D. Kostrivas, John C. Lippold, Simulating weld-fusion boundary microstructures in aluminum alloys, JOM 56 (2004) 65–72. [11] T. Michel, L. Alvarez, J.-L. Sauvajol, R. Almairac, R. Aznar, O. Mathon, J.-L. Bantignies, E. Flahaut, Structural selective charge transfer in iodine-doped carbon nanotubes, J. Phys. Chem. Solids 67 (2006) 1190–1192. [12] G. Duggan, M. Tong, D.J. Browne, Modelling the creation and destruction of columnar and equiaxed zones during solidification and melting in multi-pass welding of steel, Comput. Mater. Sci. 97 (2015) 285–294.
177
[13] S. Witzke, J.P. Riquet, Columnar-equiaxed transition in Al–Cu alloy ingots, Acta Metall. 30 (1982) 1717–1722. [14] W. Kurz, C. Bezençon, M. Gäumann, Columnar to equiaxed transition in solidification processing, Sci. Technol. Adv. Mater. 2 (2001) 185–191. [15] A. Gutierez, J.C. Lippold, Proposed mechanism for equiaxed grain formation along the fusion boundary in aluminum–copper–lithium alloys, Weld. J. 77 (1998) 123s–132s. [16] H.Y. Li, Z.H. Gao, H. Yin, H.F. Jiang, X.J. Su, J. Bin, Effects of Er and Zr additions on precipitation and recrystallization of pure aluminum, Scr. Mater. 68 (2013) 59–62. [17] D.X. Yang, X.Y. Li, D.Y. He, H. Huang, Effect of minor Er and Zr on microstructure and mechanical properties of Al–Mg–Mn alloy (5083) welded joints, Mater. Sci. Eng. A 561 (2013) 226–231. [18] Mogammad W. Dewan, J.D. Liang, M.A. Wagab, Ayman M. Okeil, Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel, Mater. Des. 54 (2014) 6–13. [19] H. Wu, S.P. Wen, K.Y. Gao, H. Huang, W. Wang, Z.R. Nie, Effect of Er additions on the precipitation strengthening of Al–Hf alloys, Scr. Mater. 87 (2014) 5–8. [20] H.Y. Li, J. Bin, J.J. Liu, Z.H. Gao, X.C. Lu, Precipitation evolution and coarsening resistance at 400 °C of Al microalloyed with Zr and Er, Scr. Mater. 67 (2012) 73–76.