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Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints
JMADE-02519; No of Pages 8 Materials and Design xxx (2016) xxx–xxx
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints Yanjun Li a,b,c, Yue Zhao a,b,c,⁎, Quan Li d, Aiping Wu a,b,c, Ruican Zhu d, Guoqing Wang e a
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, China d Capital Aerospace Machinery Company, Beijing 100076, China e China Academy of Launch Vehicle Technology, Beijing 100076, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The out-of-plane deformation in Ti2AlNb alloy joints produced by electron beam welding show a concaveconcave mode. • The weld shape varies significantly according to the focus current of the electron beam, and has four kinds of shapes. • The joint with a nail-shaped weld bead has the least angular distortion when the heat input is kept constant.
a r t i c l e
i n f o
Article history: Received 1 September 2016 Received in revised form 16 November 2016 Accepted 22 November 2016 Available online xxxx Keywords: Ti2AlNb alloy Welding distortion Numerical simulation Electron beam welding
a b s t r a c t Ti2AlNb alloy is an attractive material for advanced aerospace applications. Welding of the alloy can lead to severe distortion, influencing dimensional precision of the welded workpiece and structural integration. In this study, the effect of welding parameters on the weld shape of the Ti2AlNb alloy jointed by electron beam welding was investigated. A three-dimensional thermal-elastic-plastic finite element method was developed to simulate the welding distortion. The simulation results agreed with experimental measurements very well. It showed that the developed computational approach has sufficient accuracy and can be used to predict welding distortion. Because of the low longitudinal shrinkage force, the workpiece was bent to a concave-concave shape. When welding without fixture, the bead-on-plate joint has less distortion than butt weld. Also, it was found that focus current can significantly change the weld shape, resulting in various of transverse shrinkage distributions, consequently determining transverse bending deformation. When welding heat input is kept constant, the nailshaped weld made with a certain negative defocusing electron beam will have minimum angular distortion. © 2016 Published by Elsevier Ltd.
1. Introduction ⁎ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. E-mail address: [email protected] (Y. Zhao).
Over the past decades, Ti2AlNb-based alloys have attracted wide attention. The alloys contain a large amount of ordered orthorhombic O phase, which was firstly identified by Banerjee et al. in 1988. In addition
http://dx.doi.org/10.1016/j.matdes.2016.11.083 0264-1275/© 2016 Published by Elsevier Ltd.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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of the ordered orthorhombic O (Ti2AlNb) phase, the alloys include α2 (hexagonal, Ti3Al), and ordered B2 (body-centered cubic) phase as well. Generally, the contents of Al and Nb in the alloys are 22– 25 at.% and 20– 27 at.%, respectively. Ti-22Al-23Nb (at.%), Ti-22Al-25Nb (at.%), and Ti-22Al-27Nb (at.%) [1,2] are the typical Ti2AlNb alloys. Due to the high specific strength, attractive creep and corrosion resistance, and superior processability, the Ti2AlNb alloys show great potential as structural materials in aerospace and elevated temperature fields [3,4]. To fabricate high-quality joined structures, various welding or joining processes are being developed. The processes to be developed will extend the applications of the Ti2AlNb alloys. Up to now, several welding methods have been studied to investigate the weldabilities of the Ti2AlNb alloys, including diffusion bonding [5], linear friction welding [6,7], laser beam welding (LBW) [8–11] and electron beam welding (EBW) [12–14]. Zou et al. [5] have studied the microstructure and strength of the joints during transient liquid phase diffusion bonding of Ti-22Al-25Nb alloy. Chen et al. [6,7] have investigated the microstructure evolution and mechanical properties of linear friction welded Ti2AlNb alloy under as-weld and post-weld heat treatment conditions. Lei et al. [10,11] have investigated the microstructure evolution and tensile properties of laser welded Ti-22Al-27Nb and Ti-22Al-27Nb/TC4 joints, and Zhang [9] has studied the laser weldability of dissimilar Ti22Al-27Nb/TA15 alloys. The laser welded joints of this type of alloys showed low ductility at ambient and high temperatures due to the solidification columnar structure and the O phase precipitated in the B2 grain boundaries in the welds. EBW is preferable to joining titanium alloy, because it is in much clean vacuum chamber and has high energy density and relatively low heat input [15,16] that produces a weld with narrow-deep penetration, small heat affected zone, low distortion and residual stresses. Therefore, EBW has become a preferred method of the Ti2AlNb alloys welding. Tan et al. [12–14] has conducted dissimilar welding of Ti-22Al-25Nb and TC11 alloys using EBW. In their studies, hot work, such as isothermal deformation accompanied with heat treatment, has been employed to improve the microstructures and the mechanical properties of the welded joints. It should be noted that welded sheet structures are commonly used in aerospace industry. In the applications, weld distortion has become a serious problem to be solved, since high requirements on structural integrity and dimension accuracy of the structures need to be satisfied. However, previous studies have mainly focused on the microstructures and mechanical properties of the welded joint of the Ti2AlNb alloy. There has been little work done towards how welding parameters affecting the weld distortion. Therefore, it has great importance to study the welding induced distortion of Ti2AlNb alloy for promoting its application in aerospace structures. In this study, a three-dimensional (3D) numerical model based on thermal elastic-plastic finite element method was developed to simulate the welding distortion of EBW welded Ti2AlNb alloy plate with the thickness of 5 mm. Meanwhile, experiments were carried out to verify the numerical simulation results. The influence of joint types (beadon-plate joint and butt-joint) on the welding distortion was numerically studied. Further, the effects of focus current on weld bead geometry and welding distortion were also examined.
Fig. 1. The schematic of welding process.
Fig. 2. Welding fixture.
6 mm and 20 mm, respectively. The welding conditions are shown in Table 1. After welding, the deflection (displacement in Z-direction) distribution was measured using a Global Plus 3D coordinate-measure-machine (CMM). During the measuring, the welded specimen was held horizontally, and a series of coordinate points with a spacing of 5 mm at top surface were measured along both longitudinal and transverse directions. Note that the plane defined by three points A, B, and C was designated as datum plane, as shown in Fig. 3, and the obtained coordinate data was processed by coordinate transformation so as to zero the data measured at the three points. Several welding experiments were conducted to study the effect of focus current on cross-sectioned weld geometry, and the welding conditions are shown in Table 2. Note that the electron beam focused on the top surface when the focus current was 2250 mA. For metallographic examinations of the welds, the specimens were transversely sectioned. After mounting, grinding and polishing, the specimens were etched using a special reagent (3 mL HF, 2 mL HNO3, 7 mL H2O2 and 20 mL H2O). The weld geometries obtained from the metallographic specimens were used to verify the accuracy of the calculated temperature field. 3. 3. Finite element analysis 3.1. Geometry configuration
2. Experimental procedure The base metal used in this study was hot-rolled Ti-22Al-25Nb sheet with the thickness of 5 mm. The nominal composition of the material is: Al 22 at.%, Nb 25 at.% and balanced with Ti. It was provided by Central Iron and Steel Research Institute, China. The specimens were cut from the sheet into 100 mm × 100 mm coupons. The bead-on-plate seam welds were made using GENOVA 98 model EBW machine. A schematic diagram of welding process can be seen in Fig. 1. A specially designed specimen clamping fixture is shown in Fig. 2. As indicated, the fixtures are comprised of base plate (stainless steel), backing plate (copper) and clamping plate (stainless steel), with the thicknesses of 20 mm,
Based on ABAQUS code, a sequentially coupled thermo-elastic-plastic finite element computational program was developed to simulate welding temperature field and deformation in EBW welded joint of Table 1 Welding condition used for weld distortion measurement. Parameter Accelerating voltage U (kV)
Electron current Ib (mA)
Assemble current If (mA)
Welding speed v (mm/min)
Work distance D (mm)
value
27
2260
1000
200
60
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 4. Schematic diagram of butt welding process.
linear heat equation: Fig. 3. Finite element model and boundary conditions for bead-on-plate welding.
ρc Ti2AlNb alloy plate. Fig. 3 shows the finite element model of bead-onplate welding processing used in this study. The dimensions of the finite element model are the same as that of the specimens used in the experiment. To balance the computing time and the calculation accuracy, element meshes are finer in the weld zone and its vicinity, while the meshes become coarser away from the weld zone. In the area within 0.5 mm from the weld centerline, the element size is 0.25 mm (length) × 0.1 mm (width), while, in the area 0.5 mm–3.5 mm away from the weld centerline, the element size is 0.5 mm (length) × (0.2– 1.0) mm (width). The number of division in the thickness direction is 10. The total number of nodes is 88,319, and the number of elements 79,500. The element types used in thermal analysis and mechanical analysis were DC3D8 and C3D8R [17], respectively. Compared to bead-on-plate welding, there exists a small gap between the two parts to be butt welded. The width of the gap significantly affects the degree of transverse and longitudinal deformations in the weld zone, possibly having a significant effect on weld distortion. Therefore, in this study, the weld distortion of bead-on-plate joint and butt joint were comparatively investigated using the numerical simulation method. As shown in Fig. 4, a 0.2 mm wide gap in the butt joint was assumed. The filling of the welding gap was simulated using the “element birth and death” technique. The elements representing the gap were “dead” at the beginning of calculation and then activated step by step as heat source moving close.
∂T ðx; y; z; t Þ ¼ −∇∙qðx; y; z; t Þ þ Q ðx; y; z; t Þ ∂t
ð1Þ
where ρ is the density of the materials (kg/m3), c is the specific heat capacity (J/(kg·°C)), T is the current temperature (°C), q is the heat flux vector (W/m2), ∇is the spatial gradient operator, and Q is the internal heat generation rate (W/m3). The non-linear isotropic constitutive equation of Fourier's heat conduction was employed: q ¼ −k∇T
ð2Þ
where k is the thermal conductivity (W/(m·°C)). In the present study, a combined moving heat source model was developed to simulate the welding processes. The combined heat source model was composed of a 2D Gaussian heat source and a 3D conical heat source with Gaussian distribution, as shown in Fig. 6. For the 2D Gaussian heat source, the heat flux q(x, y, 0) can be expressed by the following equation [18]:
qðx; y; 0Þ ¼
3φηQ πR2s
exp −3 x2 þ y2 =R2s
ð3Þ
where φ is the power ratio of 2D Gaussian heat source, Q is the power of electron beam, η is the efficiency of the process, Rs is effective radius of 2D Gaussian heat source. For the 3D conical heat source, the heat flux
3.2. Heat source and thermal analysis Under given welding conditions, analysis of heat flow in transient of welding was performed using the 3D finite element models with a moving heat source. Temperature-dependent thermo-physical properties of Ti2AlNb alloy is shown in Fig. 5. The thermal properties below 900 °C were provided by Central Iron and Steel Research Institute, China, while those above 900 °C were extrapolated. During welding, the transient temperature field is determined by solving the following non-
Table 2 Welding conditions used for different weld geometries. Case
Accelerating voltage U (kV)
Electron current Ib (mA)
Assemble current If (mA)
Welding speed v (mm/min)
Work distance D (mm)
Case 1 Case 2 Case 3 Case 4
60
25
2170 2190 2230 2270
1000
200
Fig. 5. Temperature dependent thermo-physical properties of Ti2AlNb alloy.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 6. Schematic diagram of combined heat source model.
q(x, y, z) can be expressed by the following equations [17]: qðx; y; zÞ ¼ q0 exp −
q0 ¼
Fig. 7. Temperature-dependent mechanical properties of Ti2AlNb alloy.
!
3 x2 þ y2
ð4Þ
ðRu þ ðRb −Ru Þjzj=H Þ2
9ηQ ð1−φÞ πHð1− expð−3ÞÞ R2u þ Ru Rb þ R2b
ð5Þ
where q0 is the heat flux on the centerline of the 3D heat source, Ru and Rb are the larger and smaller radii of the cone, H is the height of the cone. The parameters of the heat source are listed in Table 3. Besides, radiation and heat conduction from the workpiece to fixtures were responsible for the heat loss during welding process. Heat loss (qr) due to radiation was modeled using Stefan-Boltzman law: h i qr ¼ −εσ ðT S þ 273Þ4 −ðT 0 þ 273Þ4
ð6Þ
where ε is the emissivity and ε = 0.6 for Ti2AlNb alloy, σ = 5.67 × 10−8 W/m2 °C, Ts is the surface temperature of the plate and T0 is the ambient temperature. It is difficult to accurately determine heat loss (qb) through the heat conduction, because thermal contact resistance between the workpiece and fixture is unsteady. To simplify the analysis, in the present study, the heat loss was approximately modeled using the heat flux loss by convection, and calculated using the equations: qb ¼ βb ðT s −T 0 Þ
ð7Þ
βb ¼ αkSF =d
ð8Þ
kSF ¼ 2ks k f = ks þ k f
ð9Þ
where βb is an equivalent convection coefficient, kSF is the ideal heat conduction coefficient from the workpiece to fixture, α is the contact factor with an estimated value of 5% based on some inverse analysis, ks is the thermal conductivity of the workpiece, kf is the thermal conductivity of the fixture, d is the element size in thickness direction which is
Table 3 Parameters of heat source model. Parameter
φ
Rs/mm Ru/mm Rb/mm H/mm
For welding conditions in Table 1 For welding condition in Case 1 Table 2 Case 2 Case 3 Case 4
0.24 0.13 0.17 0.17 0.25
2.5 2.0 1.5 2.0 2.5
0.7 1.3 0.8 0.8 0.6
0.6 0.6 0.75 0.7 0.43
5.0 3.5 4.0 5.0 5.0
0.5 mm in the present model. The thermal conductivities of copper and stainless steel were considered as constant with the values of 390 W/ m °C and 14.3 W/m °C at ambient temperature, respectively, because the total heat input during welding was relatively low and did not substantially increase the temperature of the fixture.
3.3. Mechanical analysis The mechanical analysis was conducted using the computed temperatures of thermal analysis. The finite element models employed in mechanical analysis were the same as those used in thermal analysis, except for the element type and boundary conditions. Two kinds of restraint conditions were used in the EBW model, i.e. with and without welding fixture. In the case of without external constraint, displacement constraints were applied to point A, B, C to prevent rigid rotation and translation of the model, as arrows pointed in Fig. 3. For the model with welding fixture, in addition to the constraints at points A, B, C, the nodes in the clamped region at the surface of the workpiece were also restricted in Z-direction, and regained freedom after the workpiece was cooled down to room temperature. The alloy was presumed isotropic and Hooke's law was applied to the elastic region; while Von Mises criterion to the plastic region using the temperature-dependent mechanical properties [17]. Fig. 7 shows the temperature-dependent mechanical properties of the Ti2AlNb alloy. The mechanical properties below 900 °C were provided by Central Iron and Steel Research Institute, China, while those above 900 °C were extrapolated. During welding, the solid-phase transformation induced changes in the thermal physical properties and volume of the material were not evident, thus the influence of solid-phase transformation on the model is negligible. As shown in Table 4, four simulation cases (Case A, Case B, Case C and Case D) were performed to examine the difference between the numerical results from the two types of joints, namely, bead-on-plate joint (Case A and Case B) and butt joint (Case C and Case D). In the cases, the influence of welding fixture on the welding deformation was Table 4 Simulated cases. Case
Joint type
Welding fixtures
Case A Case B Case C Case D
Bead on plate Bead on plate Butt Butt
Yes No Yes No
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 8. Comparison of fusion zones between experiment and FEM.
considered. The welding processes were with welding fixtures in Case A and Case C, and without external constraints in Case B and Case D.
4. Results and discussion
Fig. 10. Deflection distribution along line EF.
4.1. Comparison between simulated and experimental results Fig. 8 shows the fusion zones computed by FEM and experimentally measured from in the EBW joint produced with the welding conditions in Table 1. Good agreement was achieved between the numerical simulation and experiment. It suggested that the numerical heat source model developed using FEM precisely represented realistic situation and can be used to predict the weld shape. Fig. 9 shows the simulated and measured contours of deflection (the displacement in Z direction) of the Ti2AlNb plate bead-on-plate welded with fixture (Case A). As may be seen, the simulated deflection distributions well agreed with the experimental result, except for a certain difference in start-up region of the weld. A transverse bending (angular distortion) and a little longitudinal bending were produced after welding, thus the workpiece showed a concave-concave mode deformation [19]. The maximum deflection was at the center of the weld seam, with a value of −0.46 mm in the simulation extremely approximating the measured value of −0.42 mm. Fig. 10 shows the deflection distributions computed by FEM and measured by experiment along line EF in Fig. 3. The deflection distribution obtained from the experiment agrees well with the result from FEM simulation. It can be concluded that the computational approach developed in this study has sufficient computing accuracy for prediction of workpiece deflection. In this study, further welding experiments were carried out and the corresponding weld shapes obtained. Also, the mechanical analysis was done using numerical simulation.
Watanable and Satoh [19] have observed the so-called concaveconcave, concave-convex and convex-concave types of buckling distortions in a series of bead-on thin-sheet welds. Deng et al. [20–22] have investigated welding deformation of thin-sheet joints using both experimental and numerical methods, under various welding conditions including welding sequence, heat input and welding methods. The studies concluded that the out-of-plane deformation mode (concaveconvex and convex-concave) is dependent on the angular distortion. The longitudinal shrinkage force (also called tendon force) made the plate bent to a convex-concave (concave-convex) shape when the angular distortion was small (large) enough. However, in this study, the concave-concave shaped deformation formed under a small angular distortion. This mode has never been discussed in the previous studies. To clarify the effect of the tendon force, body forces were applied to simulate the longitudinal shrinkage force, as shown in Fig. 11a. Fig. 11b shows the predefined deflection distribution, which is similar to welded joint. When the applied Tendon force was not large enough, the plate was still bent to a concave-concave shape, as shown in Fig. 11c. While the applied Tendon force was large enough, it would become the main factor controlling the deformation mode, and then the plate was bent to a convex-concave shape as seen in Fig. 11d. This result suggests that when the shrinkage force is relatively small, the final deformation is mainly angular distortion, presenting a concave-concave mode. While the shrinkage force is large enough, the final deformation can be a concave-convex or convex-concave mode. In EBW welded joint,
Fig. 9. Comparison of deflection distributions between simulation (a) and experiment (b).
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 11. Influence of tendon force on deflection distributions.
the final deformation presented the concave-concave mode because of its the low longitudinal shrinkage force. 4.2. Comparison between bead-on-plate joint and butt joint Fig. 12 shows the deflection distributions of different cases listed in Table 4, along the line EF in Fig. 3. The deflection distributions computed in Cases A and C indicated the angular distortions of bead-on-plate joint and butt joint with fixture were basically identical. By contrast, without fixture in Case B and D, the difference in angular distortion between the two kinds of joints was quite evident. The butt joint had a greater deformation than the bead-on-plate joint. In fact, the only difference between bead-on-plate joint and butt joint is the gap at the joint. The gap significantly influences the stiffness of the material adjacent to it, especially on the stiffness in transverse direction. Compared to the bead-onplate joint, the gap at the butt joint left it more degrees of freedom, causing the larger distortion. Nevertheless, when the fixture was applied, the effect of the gap on the stiffness of the workpiece was offset, resulting in the similar final deformations in the bead-on-plate joint
Fig. 12. Deflection distributions along line EF in different simulation cases.
(without gap) and butt joint (with gap). Therefore, the effect of the gap at the joint needs to be considered when the welding deformation of a butt joint without fixture is predicted using thermal elastic-plastic finite element method. 4.3. Influences of focus current on weld shape and welding distortion In order to investigate the influence of focus current on welding deformation, the welding experiments under various currents were performed and the welding processes were numerically simulated using the verified computational approach. The weld profiles obtained from the experients and numerial simulations are shown in Fig. 13. In the welding cases, the heat input was kept constant, and the fixture was not employed. As may be seen in Fig. 13, the weld shape significantly varies with the focus current. The welds possess four kinds of shapes: wedge-shape (Fig. 13a), bell-shape (Fig. 13b), nail-shape (Fig. 13c) and funnel-shape (Fig. 13d). To quantitatively analyze the effect of weld volume on the angular distortion, the fusion areas above and below the neutral axis in each case were measured. The values of the area differences between above and below the neutral axis in the cases 1 to 4 were 3.56 mm2, 2.37 mm2, 1.66 mm2 and 1.97 mm2, respectively. Fig. 14 shows the contours of deflection distributions calculated in the four cases with different focus currents. The out-of-plane deformation of all the four different cases had the same deformation mode, namely, concave-concave mode, which was featured by a large angular distortion and a slight longitudinal bending. However, there still existed unignorable differences between the deformation values of these four cases. The maximum deflections of Case 1 to 4 were − 1.00 mm, − 0.87 mm, − 0.60 mm and − 0.77 mm, respectively. The joint with wedge-shaped weld had the greatest deformation while that with nail-shaped weld had the smallest deformation. Fig. 15 shows the deflection distributions along line EF. This result shows the rank of the weld shapes in order of deformation values from highest to lowest is: wedge shape N bell shape N funnel shape N nail shape. In the study, the final welding deformations of the workpiece are mainly angular distortions. This is because the solidification shrinkage and thermal contraction of the weld metal in the transverse direction are relatively large. Also, the transverse shrinkage is not evenly distributed in thickness, but dependent on the weld shape. As shown in Fig. 13,
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 13. Weld profiles induced by EBW with different focus currents.
the welds tend to be wider at the top than at the bottom, causing more shrinkage at the top of the welds than at the bottom. As a result, the upward angular distortions are produced. Furthermore, the larger the differences between the areas of fusion zone above and below the neutral axis, the larger the angular distortion of the joint. The rank of the weld shapes in order of the area differences from highest to lowest is also: wedge shape N bell shape N funnel shape N nail shape. Fig. 16 shows the transverse shrinkages of the plate at the middle cross-section in the four simulation cases. The negative value represents the increased width of the plate. The weld shapes are also ranked in order of the differences between the transverse shrinkage on top surface and bottom surfaces from highest to lowest: wedge shape (Case 1) N bell shape
(Case 2) N funnel shape (Case 4) N nail shape(Case 3). The results demonstrate that focus current changes the weld shape, and resulting in various the transverse shrinkage distribution in thickness, consequently determines the deformation of transverse bending. Also, when the heat put is kept constant, the joint with a nail-shaped weld made with a certain negative defocusing has the least angular distortion. 5. Conclusions In this study, a thermal-elastic-plastic finite element method based on ABAQUS code was developed to simulate distortions of EBW welded Ti2AlNb alloy plates. The weld shapes observed in the experiments
Fig. 14. Deflection distributions under different focus currents.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Acknowledgment This work was supported by the Joint Innovation Fund for China Academy of Launch Vehicle Technology and Colleges (CALT201309). References
Fig. 15. Deflection distributions along line EF under different focus currents.
verified the calculated temperature fields, indicating the numerical simulation has sufficient accuracy. Also, the influences of a gap at joint and focus current on welding deformation were also studied numerically. Based on the simulated results and the experimental measurements, the following conclusions can be drawn. (1) The computational approach developed in this study can be successfully used for numerically simulating welding temperature field and distortions of EBW welded joint of the Ti2AlNb alloy. (2) Because of the low longitudinal shrinkage force produced by the EBW, the Ti2AlNb plate is bent to a concave-concave shape. When the longitudinal shrinkage force becomes large enough, the deformation shape changes to a convex-concave mode. (3) Since the gap at joint increases the degree of freedom, the final deformation of the butt welded joint (with a gap) without fixture is higher than that of the bead-on-plate joint (without gap). Therefore, the effect of the gap needs to be considered when the welding deformation of a butt joint without fixture is predicted using thermal elastic plastic finite element method. (4) The focus current significantly changes the weld shapes, resulting in various transverse shrinkage distributions, consequently determines the final deformation of the workpiece. When heat input is kept constant, the nail-shaped weld produced by a certain negative defocusing beam will have the least angular distortion.
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Fig. 16. Transverse shrinkages of the middle cross-sections.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints Yanjun Li a,b,c, Yue Zhao a,b,c,⁎, Quan Li d, Aiping Wu a,b,c, Ruican Zhu d, Guoqing Wang e a
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, China d Capital Aerospace Machinery Company, Beijing 100076, China e China Academy of Launch Vehicle Technology, Beijing 100076, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The out-of-plane deformation in Ti2AlNb alloy joints produced by electron beam welding show a concaveconcave mode. • The weld shape varies significantly according to the focus current of the electron beam, and has four kinds of shapes. • The joint with a nail-shaped weld bead has the least angular distortion when the heat input is kept constant.
a r t i c l e
i n f o
Article history: Received 1 September 2016 Received in revised form 16 November 2016 Accepted 22 November 2016 Available online xxxx Keywords: Ti2AlNb alloy Welding distortion Numerical simulation Electron beam welding
a b s t r a c t Ti2AlNb alloy is an attractive material for advanced aerospace applications. Welding of the alloy can lead to severe distortion, influencing dimensional precision of the welded workpiece and structural integration. In this study, the effect of welding parameters on the weld shape of the Ti2AlNb alloy jointed by electron beam welding was investigated. A three-dimensional thermal-elastic-plastic finite element method was developed to simulate the welding distortion. The simulation results agreed with experimental measurements very well. It showed that the developed computational approach has sufficient accuracy and can be used to predict welding distortion. Because of the low longitudinal shrinkage force, the workpiece was bent to a concave-concave shape. When welding without fixture, the bead-on-plate joint has less distortion than butt weld. Also, it was found that focus current can significantly change the weld shape, resulting in various of transverse shrinkage distributions, consequently determining transverse bending deformation. When welding heat input is kept constant, the nailshaped weld made with a certain negative defocusing electron beam will have minimum angular distortion. © 2016 Published by Elsevier Ltd.
1. Introduction ⁎ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. E-mail address: [email protected] (Y. Zhao).
Over the past decades, Ti2AlNb-based alloys have attracted wide attention. The alloys contain a large amount of ordered orthorhombic O phase, which was firstly identified by Banerjee et al. in 1988. In addition
http://dx.doi.org/10.1016/j.matdes.2016.11.083 0264-1275/© 2016 Published by Elsevier Ltd.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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of the ordered orthorhombic O (Ti2AlNb) phase, the alloys include α2 (hexagonal, Ti3Al), and ordered B2 (body-centered cubic) phase as well. Generally, the contents of Al and Nb in the alloys are 22– 25 at.% and 20– 27 at.%, respectively. Ti-22Al-23Nb (at.%), Ti-22Al-25Nb (at.%), and Ti-22Al-27Nb (at.%) [1,2] are the typical Ti2AlNb alloys. Due to the high specific strength, attractive creep and corrosion resistance, and superior processability, the Ti2AlNb alloys show great potential as structural materials in aerospace and elevated temperature fields [3,4]. To fabricate high-quality joined structures, various welding or joining processes are being developed. The processes to be developed will extend the applications of the Ti2AlNb alloys. Up to now, several welding methods have been studied to investigate the weldabilities of the Ti2AlNb alloys, including diffusion bonding [5], linear friction welding [6,7], laser beam welding (LBW) [8–11] and electron beam welding (EBW) [12–14]. Zou et al. [5] have studied the microstructure and strength of the joints during transient liquid phase diffusion bonding of Ti-22Al-25Nb alloy. Chen et al. [6,7] have investigated the microstructure evolution and mechanical properties of linear friction welded Ti2AlNb alloy under as-weld and post-weld heat treatment conditions. Lei et al. [10,11] have investigated the microstructure evolution and tensile properties of laser welded Ti-22Al-27Nb and Ti-22Al-27Nb/TC4 joints, and Zhang [9] has studied the laser weldability of dissimilar Ti22Al-27Nb/TA15 alloys. The laser welded joints of this type of alloys showed low ductility at ambient and high temperatures due to the solidification columnar structure and the O phase precipitated in the B2 grain boundaries in the welds. EBW is preferable to joining titanium alloy, because it is in much clean vacuum chamber and has high energy density and relatively low heat input [15,16] that produces a weld with narrow-deep penetration, small heat affected zone, low distortion and residual stresses. Therefore, EBW has become a preferred method of the Ti2AlNb alloys welding. Tan et al. [12–14] has conducted dissimilar welding of Ti-22Al-25Nb and TC11 alloys using EBW. In their studies, hot work, such as isothermal deformation accompanied with heat treatment, has been employed to improve the microstructures and the mechanical properties of the welded joints. It should be noted that welded sheet structures are commonly used in aerospace industry. In the applications, weld distortion has become a serious problem to be solved, since high requirements on structural integrity and dimension accuracy of the structures need to be satisfied. However, previous studies have mainly focused on the microstructures and mechanical properties of the welded joint of the Ti2AlNb alloy. There has been little work done towards how welding parameters affecting the weld distortion. Therefore, it has great importance to study the welding induced distortion of Ti2AlNb alloy for promoting its application in aerospace structures. In this study, a three-dimensional (3D) numerical model based on thermal elastic-plastic finite element method was developed to simulate the welding distortion of EBW welded Ti2AlNb alloy plate with the thickness of 5 mm. Meanwhile, experiments were carried out to verify the numerical simulation results. The influence of joint types (beadon-plate joint and butt-joint) on the welding distortion was numerically studied. Further, the effects of focus current on weld bead geometry and welding distortion were also examined.
Fig. 1. The schematic of welding process.
Fig. 2. Welding fixture.
6 mm and 20 mm, respectively. The welding conditions are shown in Table 1. After welding, the deflection (displacement in Z-direction) distribution was measured using a Global Plus 3D coordinate-measure-machine (CMM). During the measuring, the welded specimen was held horizontally, and a series of coordinate points with a spacing of 5 mm at top surface were measured along both longitudinal and transverse directions. Note that the plane defined by three points A, B, and C was designated as datum plane, as shown in Fig. 3, and the obtained coordinate data was processed by coordinate transformation so as to zero the data measured at the three points. Several welding experiments were conducted to study the effect of focus current on cross-sectioned weld geometry, and the welding conditions are shown in Table 2. Note that the electron beam focused on the top surface when the focus current was 2250 mA. For metallographic examinations of the welds, the specimens were transversely sectioned. After mounting, grinding and polishing, the specimens were etched using a special reagent (3 mL HF, 2 mL HNO3, 7 mL H2O2 and 20 mL H2O). The weld geometries obtained from the metallographic specimens were used to verify the accuracy of the calculated temperature field. 3. 3. Finite element analysis 3.1. Geometry configuration
2. Experimental procedure The base metal used in this study was hot-rolled Ti-22Al-25Nb sheet with the thickness of 5 mm. The nominal composition of the material is: Al 22 at.%, Nb 25 at.% and balanced with Ti. It was provided by Central Iron and Steel Research Institute, China. The specimens were cut from the sheet into 100 mm × 100 mm coupons. The bead-on-plate seam welds were made using GENOVA 98 model EBW machine. A schematic diagram of welding process can be seen in Fig. 1. A specially designed specimen clamping fixture is shown in Fig. 2. As indicated, the fixtures are comprised of base plate (stainless steel), backing plate (copper) and clamping plate (stainless steel), with the thicknesses of 20 mm,
Based on ABAQUS code, a sequentially coupled thermo-elastic-plastic finite element computational program was developed to simulate welding temperature field and deformation in EBW welded joint of Table 1 Welding condition used for weld distortion measurement. Parameter Accelerating voltage U (kV)
Electron current Ib (mA)
Assemble current If (mA)
Welding speed v (mm/min)
Work distance D (mm)
value
27
2260
1000
200
60
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 4. Schematic diagram of butt welding process.
linear heat equation: Fig. 3. Finite element model and boundary conditions for bead-on-plate welding.
ρc Ti2AlNb alloy plate. Fig. 3 shows the finite element model of bead-onplate welding processing used in this study. The dimensions of the finite element model are the same as that of the specimens used in the experiment. To balance the computing time and the calculation accuracy, element meshes are finer in the weld zone and its vicinity, while the meshes become coarser away from the weld zone. In the area within 0.5 mm from the weld centerline, the element size is 0.25 mm (length) × 0.1 mm (width), while, in the area 0.5 mm–3.5 mm away from the weld centerline, the element size is 0.5 mm (length) × (0.2– 1.0) mm (width). The number of division in the thickness direction is 10. The total number of nodes is 88,319, and the number of elements 79,500. The element types used in thermal analysis and mechanical analysis were DC3D8 and C3D8R [17], respectively. Compared to bead-on-plate welding, there exists a small gap between the two parts to be butt welded. The width of the gap significantly affects the degree of transverse and longitudinal deformations in the weld zone, possibly having a significant effect on weld distortion. Therefore, in this study, the weld distortion of bead-on-plate joint and butt joint were comparatively investigated using the numerical simulation method. As shown in Fig. 4, a 0.2 mm wide gap in the butt joint was assumed. The filling of the welding gap was simulated using the “element birth and death” technique. The elements representing the gap were “dead” at the beginning of calculation and then activated step by step as heat source moving close.
∂T ðx; y; z; t Þ ¼ −∇∙qðx; y; z; t Þ þ Q ðx; y; z; t Þ ∂t
ð1Þ
where ρ is the density of the materials (kg/m3), c is the specific heat capacity (J/(kg·°C)), T is the current temperature (°C), q is the heat flux vector (W/m2), ∇is the spatial gradient operator, and Q is the internal heat generation rate (W/m3). The non-linear isotropic constitutive equation of Fourier's heat conduction was employed: q ¼ −k∇T
ð2Þ
where k is the thermal conductivity (W/(m·°C)). In the present study, a combined moving heat source model was developed to simulate the welding processes. The combined heat source model was composed of a 2D Gaussian heat source and a 3D conical heat source with Gaussian distribution, as shown in Fig. 6. For the 2D Gaussian heat source, the heat flux q(x, y, 0) can be expressed by the following equation [18]:
qðx; y; 0Þ ¼
3φηQ πR2s
exp −3 x2 þ y2 =R2s
ð3Þ
where φ is the power ratio of 2D Gaussian heat source, Q is the power of electron beam, η is the efficiency of the process, Rs is effective radius of 2D Gaussian heat source. For the 3D conical heat source, the heat flux
3.2. Heat source and thermal analysis Under given welding conditions, analysis of heat flow in transient of welding was performed using the 3D finite element models with a moving heat source. Temperature-dependent thermo-physical properties of Ti2AlNb alloy is shown in Fig. 5. The thermal properties below 900 °C were provided by Central Iron and Steel Research Institute, China, while those above 900 °C were extrapolated. During welding, the transient temperature field is determined by solving the following non-
Table 2 Welding conditions used for different weld geometries. Case
Accelerating voltage U (kV)
Electron current Ib (mA)
Assemble current If (mA)
Welding speed v (mm/min)
Work distance D (mm)
Case 1 Case 2 Case 3 Case 4
60
25
2170 2190 2230 2270
1000
200
Fig. 5. Temperature dependent thermo-physical properties of Ti2AlNb alloy.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 6. Schematic diagram of combined heat source model.
q(x, y, z) can be expressed by the following equations [17]: qðx; y; zÞ ¼ q0 exp −
q0 ¼
Fig. 7. Temperature-dependent mechanical properties of Ti2AlNb alloy.
!
3 x2 þ y2
ð4Þ
ðRu þ ðRb −Ru Þjzj=H Þ2
9ηQ ð1−φÞ πHð1− expð−3ÞÞ R2u þ Ru Rb þ R2b
ð5Þ
where q0 is the heat flux on the centerline of the 3D heat source, Ru and Rb are the larger and smaller radii of the cone, H is the height of the cone. The parameters of the heat source are listed in Table 3. Besides, radiation and heat conduction from the workpiece to fixtures were responsible for the heat loss during welding process. Heat loss (qr) due to radiation was modeled using Stefan-Boltzman law: h i qr ¼ −εσ ðT S þ 273Þ4 −ðT 0 þ 273Þ4
ð6Þ
where ε is the emissivity and ε = 0.6 for Ti2AlNb alloy, σ = 5.67 × 10−8 W/m2 °C, Ts is the surface temperature of the plate and T0 is the ambient temperature. It is difficult to accurately determine heat loss (qb) through the heat conduction, because thermal contact resistance between the workpiece and fixture is unsteady. To simplify the analysis, in the present study, the heat loss was approximately modeled using the heat flux loss by convection, and calculated using the equations: qb ¼ βb ðT s −T 0 Þ
ð7Þ
βb ¼ αkSF =d
ð8Þ
kSF ¼ 2ks k f = ks þ k f
ð9Þ
where βb is an equivalent convection coefficient, kSF is the ideal heat conduction coefficient from the workpiece to fixture, α is the contact factor with an estimated value of 5% based on some inverse analysis, ks is the thermal conductivity of the workpiece, kf is the thermal conductivity of the fixture, d is the element size in thickness direction which is
Table 3 Parameters of heat source model. Parameter
φ
Rs/mm Ru/mm Rb/mm H/mm
For welding conditions in Table 1 For welding condition in Case 1 Table 2 Case 2 Case 3 Case 4
0.24 0.13 0.17 0.17 0.25
2.5 2.0 1.5 2.0 2.5
0.7 1.3 0.8 0.8 0.6
0.6 0.6 0.75 0.7 0.43
5.0 3.5 4.0 5.0 5.0
0.5 mm in the present model. The thermal conductivities of copper and stainless steel were considered as constant with the values of 390 W/ m °C and 14.3 W/m °C at ambient temperature, respectively, because the total heat input during welding was relatively low and did not substantially increase the temperature of the fixture.
3.3. Mechanical analysis The mechanical analysis was conducted using the computed temperatures of thermal analysis. The finite element models employed in mechanical analysis were the same as those used in thermal analysis, except for the element type and boundary conditions. Two kinds of restraint conditions were used in the EBW model, i.e. with and without welding fixture. In the case of without external constraint, displacement constraints were applied to point A, B, C to prevent rigid rotation and translation of the model, as arrows pointed in Fig. 3. For the model with welding fixture, in addition to the constraints at points A, B, C, the nodes in the clamped region at the surface of the workpiece were also restricted in Z-direction, and regained freedom after the workpiece was cooled down to room temperature. The alloy was presumed isotropic and Hooke's law was applied to the elastic region; while Von Mises criterion to the plastic region using the temperature-dependent mechanical properties [17]. Fig. 7 shows the temperature-dependent mechanical properties of the Ti2AlNb alloy. The mechanical properties below 900 °C were provided by Central Iron and Steel Research Institute, China, while those above 900 °C were extrapolated. During welding, the solid-phase transformation induced changes in the thermal physical properties and volume of the material were not evident, thus the influence of solid-phase transformation on the model is negligible. As shown in Table 4, four simulation cases (Case A, Case B, Case C and Case D) were performed to examine the difference between the numerical results from the two types of joints, namely, bead-on-plate joint (Case A and Case B) and butt joint (Case C and Case D). In the cases, the influence of welding fixture on the welding deformation was Table 4 Simulated cases. Case
Joint type
Welding fixtures
Case A Case B Case C Case D
Bead on plate Bead on plate Butt Butt
Yes No Yes No
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 8. Comparison of fusion zones between experiment and FEM.
considered. The welding processes were with welding fixtures in Case A and Case C, and without external constraints in Case B and Case D.
4. Results and discussion
Fig. 10. Deflection distribution along line EF.
4.1. Comparison between simulated and experimental results Fig. 8 shows the fusion zones computed by FEM and experimentally measured from in the EBW joint produced with the welding conditions in Table 1. Good agreement was achieved between the numerical simulation and experiment. It suggested that the numerical heat source model developed using FEM precisely represented realistic situation and can be used to predict the weld shape. Fig. 9 shows the simulated and measured contours of deflection (the displacement in Z direction) of the Ti2AlNb plate bead-on-plate welded with fixture (Case A). As may be seen, the simulated deflection distributions well agreed with the experimental result, except for a certain difference in start-up region of the weld. A transverse bending (angular distortion) and a little longitudinal bending were produced after welding, thus the workpiece showed a concave-concave mode deformation [19]. The maximum deflection was at the center of the weld seam, with a value of −0.46 mm in the simulation extremely approximating the measured value of −0.42 mm. Fig. 10 shows the deflection distributions computed by FEM and measured by experiment along line EF in Fig. 3. The deflection distribution obtained from the experiment agrees well with the result from FEM simulation. It can be concluded that the computational approach developed in this study has sufficient computing accuracy for prediction of workpiece deflection. In this study, further welding experiments were carried out and the corresponding weld shapes obtained. Also, the mechanical analysis was done using numerical simulation.
Watanable and Satoh [19] have observed the so-called concaveconcave, concave-convex and convex-concave types of buckling distortions in a series of bead-on thin-sheet welds. Deng et al. [20–22] have investigated welding deformation of thin-sheet joints using both experimental and numerical methods, under various welding conditions including welding sequence, heat input and welding methods. The studies concluded that the out-of-plane deformation mode (concaveconvex and convex-concave) is dependent on the angular distortion. The longitudinal shrinkage force (also called tendon force) made the plate bent to a convex-concave (concave-convex) shape when the angular distortion was small (large) enough. However, in this study, the concave-concave shaped deformation formed under a small angular distortion. This mode has never been discussed in the previous studies. To clarify the effect of the tendon force, body forces were applied to simulate the longitudinal shrinkage force, as shown in Fig. 11a. Fig. 11b shows the predefined deflection distribution, which is similar to welded joint. When the applied Tendon force was not large enough, the plate was still bent to a concave-concave shape, as shown in Fig. 11c. While the applied Tendon force was large enough, it would become the main factor controlling the deformation mode, and then the plate was bent to a convex-concave shape as seen in Fig. 11d. This result suggests that when the shrinkage force is relatively small, the final deformation is mainly angular distortion, presenting a concave-concave mode. While the shrinkage force is large enough, the final deformation can be a concave-convex or convex-concave mode. In EBW welded joint,
Fig. 9. Comparison of deflection distributions between simulation (a) and experiment (b).
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 11. Influence of tendon force on deflection distributions.
the final deformation presented the concave-concave mode because of its the low longitudinal shrinkage force. 4.2. Comparison between bead-on-plate joint and butt joint Fig. 12 shows the deflection distributions of different cases listed in Table 4, along the line EF in Fig. 3. The deflection distributions computed in Cases A and C indicated the angular distortions of bead-on-plate joint and butt joint with fixture were basically identical. By contrast, without fixture in Case B and D, the difference in angular distortion between the two kinds of joints was quite evident. The butt joint had a greater deformation than the bead-on-plate joint. In fact, the only difference between bead-on-plate joint and butt joint is the gap at the joint. The gap significantly influences the stiffness of the material adjacent to it, especially on the stiffness in transverse direction. Compared to the bead-onplate joint, the gap at the butt joint left it more degrees of freedom, causing the larger distortion. Nevertheless, when the fixture was applied, the effect of the gap on the stiffness of the workpiece was offset, resulting in the similar final deformations in the bead-on-plate joint
Fig. 12. Deflection distributions along line EF in different simulation cases.
(without gap) and butt joint (with gap). Therefore, the effect of the gap at the joint needs to be considered when the welding deformation of a butt joint without fixture is predicted using thermal elastic-plastic finite element method. 4.3. Influences of focus current on weld shape and welding distortion In order to investigate the influence of focus current on welding deformation, the welding experiments under various currents were performed and the welding processes were numerically simulated using the verified computational approach. The weld profiles obtained from the experients and numerial simulations are shown in Fig. 13. In the welding cases, the heat input was kept constant, and the fixture was not employed. As may be seen in Fig. 13, the weld shape significantly varies with the focus current. The welds possess four kinds of shapes: wedge-shape (Fig. 13a), bell-shape (Fig. 13b), nail-shape (Fig. 13c) and funnel-shape (Fig. 13d). To quantitatively analyze the effect of weld volume on the angular distortion, the fusion areas above and below the neutral axis in each case were measured. The values of the area differences between above and below the neutral axis in the cases 1 to 4 were 3.56 mm2, 2.37 mm2, 1.66 mm2 and 1.97 mm2, respectively. Fig. 14 shows the contours of deflection distributions calculated in the four cases with different focus currents. The out-of-plane deformation of all the four different cases had the same deformation mode, namely, concave-concave mode, which was featured by a large angular distortion and a slight longitudinal bending. However, there still existed unignorable differences between the deformation values of these four cases. The maximum deflections of Case 1 to 4 were − 1.00 mm, − 0.87 mm, − 0.60 mm and − 0.77 mm, respectively. The joint with wedge-shaped weld had the greatest deformation while that with nail-shaped weld had the smallest deformation. Fig. 15 shows the deflection distributions along line EF. This result shows the rank of the weld shapes in order of deformation values from highest to lowest is: wedge shape N bell shape N funnel shape N nail shape. In the study, the final welding deformations of the workpiece are mainly angular distortions. This is because the solidification shrinkage and thermal contraction of the weld metal in the transverse direction are relatively large. Also, the transverse shrinkage is not evenly distributed in thickness, but dependent on the weld shape. As shown in Fig. 13,
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Fig. 13. Weld profiles induced by EBW with different focus currents.
the welds tend to be wider at the top than at the bottom, causing more shrinkage at the top of the welds than at the bottom. As a result, the upward angular distortions are produced. Furthermore, the larger the differences between the areas of fusion zone above and below the neutral axis, the larger the angular distortion of the joint. The rank of the weld shapes in order of the area differences from highest to lowest is also: wedge shape N bell shape N funnel shape N nail shape. Fig. 16 shows the transverse shrinkages of the plate at the middle cross-section in the four simulation cases. The negative value represents the increased width of the plate. The weld shapes are also ranked in order of the differences between the transverse shrinkage on top surface and bottom surfaces from highest to lowest: wedge shape (Case 1) N bell shape
(Case 2) N funnel shape (Case 4) N nail shape(Case 3). The results demonstrate that focus current changes the weld shape, and resulting in various the transverse shrinkage distribution in thickness, consequently determines the deformation of transverse bending. Also, when the heat put is kept constant, the joint with a nail-shaped weld made with a certain negative defocusing has the least angular distortion. 5. Conclusions In this study, a thermal-elastic-plastic finite element method based on ABAQUS code was developed to simulate distortions of EBW welded Ti2AlNb alloy plates. The weld shapes observed in the experiments
Fig. 14. Deflection distributions under different focus currents.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083
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Y. Li et al. / Materials and Design xxx (2016) xxx–xxx
Acknowledgment This work was supported by the Joint Innovation Fund for China Academy of Launch Vehicle Technology and Colleges (CALT201309). References
Fig. 15. Deflection distributions along line EF under different focus currents.
verified the calculated temperature fields, indicating the numerical simulation has sufficient accuracy. Also, the influences of a gap at joint and focus current on welding deformation were also studied numerically. Based on the simulated results and the experimental measurements, the following conclusions can be drawn. (1) The computational approach developed in this study can be successfully used for numerically simulating welding temperature field and distortions of EBW welded joint of the Ti2AlNb alloy. (2) Because of the low longitudinal shrinkage force produced by the EBW, the Ti2AlNb plate is bent to a concave-concave shape. When the longitudinal shrinkage force becomes large enough, the deformation shape changes to a convex-concave mode. (3) Since the gap at joint increases the degree of freedom, the final deformation of the butt welded joint (with a gap) without fixture is higher than that of the bead-on-plate joint (without gap). Therefore, the effect of the gap needs to be considered when the welding deformation of a butt joint without fixture is predicted using thermal elastic plastic finite element method. (4) The focus current significantly changes the weld shapes, resulting in various transverse shrinkage distributions, consequently determines the final deformation of the workpiece. When heat input is kept constant, the nail-shaped weld produced by a certain negative defocusing beam will have the least angular distortion.
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Fig. 16. Transverse shrinkages of the middle cross-sections.
Please cite this article as: Y. Li, et al., Effects of welding condition on weld shape and distortion in electron beam welded Ti2AlNb alloy joints, Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.11.083