Grain refinement and laser energy distribution during laser oscillating welding of Invar alloy

Materials and Design 186 (2020) 108195

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Grain refinement and laser energy distribution during laser oscillating welding of Invar alloy Zhenguo Jiang a, Xi Chen a, Hao Li b, Zhenglong Lei a,⁎, Yanbin Chen a, Shibo Wu a, Yuhua Wang b a b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China Shanghai Aircraft Manufacturing Co., Shanghai, 200436, China

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

• Grain refinement of Invar alloy was achieved by laser oscillating welding. • Effect of oscillation parameters on laser energy distribution was researched. • Microstructure and mechanical property of oscillated welds were investigated. • Established heat flux model based on energy distribution to describe temperature field and discuss solidification condition.

a r t i c l e

i n f o

Article history: Received 8 July 2019 Received in revised form 22 August 2019 Accepted 6 September 2019 Available online xxxx Keywords: Grain refinement Laser oscillating welding Invar alloy Solidification microstructure Finite element method

a b s t r a c t Laser oscillating welding method was employed to welding of Invar alloy so as to achieve grain refinement and decrease internal defects. Effects of frequency and amplitude of beam oscillation with sinusoidal trajectory on weld appearance and solidification microstructure were researched. The essential effect of beam oscillation could be attributed to change the laser energy deposition distribution on the processing surface. Optical metallographic and electron backscattered diffraction analysis results demonstrated that beam oscillation could achieve the transformation of solidification microstructure from oriented dendrite grain to equiaxed dendrite grain. Further, the application of beam oscillation might retard solidification cooling speed and strengthen the formation of γ -fiber texture that is conducive to the improvement of ductility. According to the acquisition of dynamic feature pictures with high-speed camera, the local rapid movement of laser spot resulted in a more stable keyhole without fluctuation and the intensification of melt flow. Temperature curves calculated by finite element method simulation showed that lateral laser oscillation made the temperature distribution of molten metals nearby the center of weld more uniform even negative temperature gradient. The comprehensive description of grain refinement mechanism caused by beam oscillation was presented on the basis of the experiment and simulation analyses. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Grain refinement has always been the cutting edge research of welding because substantial grain refinement of microstructure could improve most basic mechanical properties of engineering materials ⁎ Corresponding author. E-mail address: [email protected] (Z. Lei).

and enhance the ability to restrain the formation of defects during weld solidification [1,2]. Various physical or chemical technologies have been attempted to obtain and control grain refinement. Ultrasonic treatment could lead to a strongly turbulent melt that fleetly transfers heat and component throughout the molten melt under the action of acoustic streaming effect [3]. Effective grain refinement could be obtained by ultrasonic treatment in welding of magnesium alloy [1], 7075 aluminum alloy [4] and Ti–6Al–4V alloy [5]. Additionally, external

https://doi.org/10.1016/j.matdes.2019.108195 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Fig. 1. Schematic diagram: (a) experimental equipment used in the experiment, (b) sampling location and dimension of tensile specimen.

Fig. 2. Microstructure of Invar alloy base metal: (a) optical metallography, (b) inverse pole figure, (c) grain boundary misorientation angle, (d) ODF section figure.

magnetic field could effectively mix molten melt and decrease the temperature maldistribution by electromagnetic stirring. It has been put forward that electromagnetic stirring was beneficial to refine solidification structure, homogenize the composition of the weld and eliminate the formation of porosity defects [6,7]. Also, thermal diffusion and component diffusion were accompanied with each other in unsolidified melt. Consequently the thermal environment produced by ultrasonic treatment and electromagnetic stirring had a very low temperature gradient. However, the welding process with ultrasonic or electromagnetic stirring may be difficult in practical industrial applications for large or complex components. In addition to above methods, the addition of alloying elements such as boron [8], titanium [9], silicon [10] and rareearth element [11] to weld has been proposed for grain refinement. But the addition of extrinsic alloy elements would inevitably lead to changes in chemical composition of the weld, accompanied by

unpredictable changes of thermo-physical and mechanical properties of weld [12,13]. Fortunately, Kou et al. [14] proposed the idea of using heat source oscillation to optimize the solidification structure of weld. So far, welding heat source oscillation has become a common choice for most researchers to refine solidification structure and reduce the internal

Table 1 Experiment parameters used in laser rectilinear welding and laser oscillating welding. Laser power (kW)

Welding speed (mm/s)

Oscillating pattern

Oscillating amplitude (mm)

Oscillating frequency (Hz)

Line energy maximum (J/mm)

3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6

20 20 20 20 10 10 10 10

rectilinear sinusoidal sinusoidal sinusoidal rectilinear sinusoidal sinusoidal sinusoidal

0 4 4 4 0 2 4 6

0 5 10 20 0 80 80 80

180 103.6 57.7 38.1 360 109.7 75.2 60.9

Fig. 3. Major texture components displayed in ϕ2 = 45° section of ODF.

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Fig. 4. Optimization of weld appearance: (a) joint form, (b) weld appearance without beam oscillation, (c) weld appearance with beam oscillation.

metallurgical defects of weld [15,16]. It is attractive and feasible to use welding heat source oscillation to refine grain and suppress defects because rapid local movement of heat source could not only promote liquid metal flow, but also realize more uniform loading of heat source. As a practical welding method, laser welding was often accompanied with some adverse effects due to molten-pool instabilities, such as humping effect on weld surface [17,18] and formation of porosity defects in weld [19,20], that could be avoided by using advanced technologies. With the development of laser oscillating welding method, it has been widely used to optimize temperature distribution of molten-pool [21], eliminate porosities caused by residual gas [22,23], improve the tolerance of assembly gap by widening the weld [24], and optimize weld morphology without humping defects [25]. Laser heat source could be oscillated periodically through scanning mirrors, either sinusoidal, circular or other complex oscillation tracks [26]. Lei et al. [27] pointed out that oscillation pattern had significant different effects on the microstructure, mechanical property and molten-pool flow. Actually, the fundamental effect of heat source oscillation could be attributed to the

variation of linear energy distribution caused by the variation of local effective welding speed. The cooling speed of molten-pool could be adjusted and predetermined by combining the oscillating motion of laser spot with the precise modulation of laser output power [26,28]. So far, the effects of laser beam oscillation on laser energy distribution, solidification microstructure and mechanical property of weld have not been investigated simultaneously. Invar alloy has been extensively used as a high reliability and stability material [29,30], however, research on Invar alloy welding by laser beam oscillation has not been carried out. As the most popular and efficient numerical method, finite element method has been rapidly extended to almost all fields of science and technology, such as dispersion [31], nonlinearity [32], low loss of fiber [33], temperature field of welding [34] etc. Here it was applied to simulate laser energy distribution of laser welding. In this work, effects of oscillation frequency (F) and amplitude (A) on weld appearance and microstructure evolution of Invar alloy during laser oscillating welding were investigated. The relationship among laser energy distribution, solidification microstructure and

Fig. 5. Weld appearance with different oscillation frequencies at the oscillation amplitude of 4 mm: (a) weld of F = 0 Hz, (b) weld of F = 5 Hz, (c) weld of F = 10 Hz, (d) weld of F = 20 Hz.

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Fig. 6. Weld appearance with different oscillation amplitudes at the oscillation frequency of 80 Hz: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm, (d) weld of A = 6.0 mm.

mechanical property of weld was minutely described. The temperature field of laser oscillating welding was determined based on the accurate heat source model and finite element method. The mechanism of grain refinement during laser oscillating welding was explored based on solidification theory. 2. Experiments The laser beam oscillation device used in this research was the ScanTracker laser processing head, produced by the Precitec GmbH & Co.KG. The optical components of the processing head mainly included a collimating lens with the focusing focal length of 200 mm, a galvanometer scanner driven by a direct-current (DC) motor and a focus lens with the focusing focal length of 300 mm. The processing head can realize two welding modes respectively, that is, the conventional laser rectilinear welding and laser oscillating welding with lateral beam oscillation. The highest amplitude of lateral oscillation was proportional to the focusing focal length. The highest frequency could reach above 200 Hz. According to the required width of the weld, the processing head enabled the oscillation amplitude of laser beam to be set as a certain value that was stable throughout welding process, and the oscillation frequency of laser beam could also be set to a desired value. The schematic diagram of experimental equipment is shown in Fig. 1 (a). The IPG YLR-10000 fiber laser with the wavelength of 1060 nm was adopted as the laser beam source. Experimental specimen was placed on an electro-mechanical device and driven to move linearly with a constant rate. The welding scanner and the high-speed camera were built above the sample in a stationary state. By using the high speed camera device manufactured by Optronis GmbH of Germany, dynamic variation characteristics of welding molten-pool were observed and recorded. The recording speed used in the welding process was at the frame rate of 5000 frames per second. To clearly observe the welding

molten-pool, a semiconductor laser produced by Cavitar Company of Finland with the wavelength of 808 nm and the highest power of 500 W was taken as an auxiliary light source. Establishing a threedimension Cartesian coordinates, the welding direction was set to X direction, and the oscillation direction of laser beam was set to Y direction, and the normal direction of the surface of workpiece was set to Z direction. These directions would be helpful to describe the heat source model and the molten-pool temperature distribution. Experimental material studied was Invar alloy produced by the French Aperam Alloys Co. LTD. Fig. 2 shows optical metallographic and EBSD microstructure figure of Invar alloy which has the single austenitic structure. The crystal orientation of the base material is randomly distributed. The experiment workpiece was prepared with the dimension of 100 mm × 200 mm × 19.5 mm. To ensure surface cleanliness, the protective layer of specimen surface was removed by milling machine, and specimen surface was washed with acetone solution before welding to eliminate surface pollution. The experiment parameters of laser rectilinear welding and laser oscillating welding are summarized in Table 1. In addition to welding experiment, the underlying relations of oscillation parameters were investigated by modeling and simulation. After welding, the welds of different parameters and substrate were randomly cut out, then prepared into metallographic specimens. After that, metallographic surfaces were burnished and corroded to reveal solidification microstructure. First of all, the surfaces of all metallurgical specimens were corroded with the mixed solution of 5% nitric acid and 95% industrial alcohol solution to reveal dendrite structures. The dendrite structures in the center and heat affected zone of weld were observed by VHX-1000 optical microscope. After that, the surfaces of these metallurgical specimens were electro-polished with the mixed solution of 20% perchloric acid and 80% acetone to reveal grain structures. The grain structures in the center and heat affected zone were characterized by electron backscatter diffraction analysis technology. Fig. 3

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Fig. 7. Laser energy distribution with different oscillation frequencies at the oscillation amplitude of 4.0 mm: (a) F = 0 Hz, (b) F = 5 Hz, (c) F = 10 Hz, (d) F = 20 Hz, (e) F = 40 Hz, (f) F = 80 Hz.

shows the relationship between important orientations of crystalline grain structure and texture to visualize and analyze the evolution of microstructure for alloys with cubic crystal structure [35]. According to the standard of ASTM E-8, tensile specimens were processed from the specific position to research the relationship between solidification microstructure and mechanical property. The schematic diagram of sampling location and dimension of tensile specimen is shown in Fig. 1(b). The tensile tests were carried out using AG-X Plus 250kN electron materials testing system with the tensile rate of 1.0 mm/min. The features of fracture morphology were analyzed by FEI Quanta 200 FEG device equipped with the scanning electron microscope.

3. Results and discussion The comparison of welds between laser welding without oscillating and laser oscillating welding is shown in Fig. 4. For the joint form in Fig. 4(a), there are the unfused area and a large number of porosities in welds of laser rectilinear welding, as shown in Fig. 4(b). Weld seam width can be adjusted to match joint gap and geometry during laser oscillating welding process due to a high-frequency pendulum motion of laser beam, as shown in Fig. 4(c). Welding quality often depends on the energy input per unit length and the lateral distribution of energy. The appearance, microstructure and mechanical property of welds can be optimized favorably owing to the changing of laser energy distribution. 3.1. Effects of oscillation frequency and amplitude on weld appearance Fig. 5 shows cross section, longitudinal section and surface profile of welds produced by laser rectilinear welding and laser oscillating welding with different oscillation

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Fig. 8. Laser energy distribution with different oscillation amplitudes at the oscillation frequency of 80 Hz: (a) A = 0 mm, (b) A = 0.5 mm, (c) A = 1.0 mm, (d) A = 2.0 mm, (e) A = 4.0 mm, (f) A = 6.0 mm.

frequencies. The laser power and welding speed are 3.6 kW and 1.2 m/min, respectively. It can be calculated that the welding line energy of laser rectilinear welding is 180 J/mm, which is used as a basis to calculate the variation of welding line energy during laser oscillating welding with different oscillation parameters, as shown in Table 1. The laser beam oscillation trajectory was sinusoidal curve. The oscillating amplitude is kept at 4.0 mm. For laser rectilinear welding, the appearance of welding has significant non-uniformity, and the maximum and minimum widths of weld fusion zone are about 4.0 mm and 1.0 mm respectively, as shown in Fig. 5(a). Due to the limited dimension of laser spot, it could be clearly seen that the fusion zone of longitudinal section and surface profile was discontinuous and periodical when the oscillation frequency is less than 20 Hz in Fig. 5 (b and c). Above all, the discontinuous and periodical appearance for welding should be avoided as far as possible, especially on longitudinal section. Otherwise the profile of weld cross section is irregularly changed depending on the sampling location. When the

oscillation frequency is greater than 20 Hz, continuous and symmetric weld appearance in the cross section, longitudinal section and weld surface profile can be obtained, as shown in Fig. 5(d). Fig. 6 shows that the effect of oscillation amplitude on weld appearance in laser welding of Invar alloy. The oscillation trajectory is still sinusoidal curve. The oscillation frequency is kept at 80 Hz. Then the laser power and welding speed are 3.6 kW and 0.6 m/min, respectively. The rectilinear welds with different welding speed all have a lot of porosity defects, as shown in Figs. 5(a) and Fig. 6(a). As the oscillation amplitude of laser spot is 2.0 mm, it is found that lateral oscillation of laser spot perpendicular to welding direction is beneficial to reduce rather than completely avoid the formation of porosity in Invar alloy weld, as shown in Fig. 6 (b). On average, the quantity and size of internal porosity are significantly less than that of the rectilinear welds without oscillation. It should be noted that

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Fig. 9. Variation trend before and after laser beam oscillation: (a) fusion zone dimension variation with oscillation amplitude (F = 80 Hz), (b) ratio coefficient variation with oscillation frequency (A = 4.0 mm), (c) ratio coefficient variation with oscillation amplitude (F = 80 Hz).

when the oscillation amplitude is more than 4.0 mm the oscillated welds of Invar alloy are free from porosity defects in Fig. 6(c and d). By comparing weld appearance in Fig. 6(a–d), the fusion zone width of laser oscillating welding increases linearly with the increase of oscillation amplitude. On the contrary, the maximum depth of fusion zone tends to decrease gradually. However, accompanying the increase of oscillation amplitude from 0 mm to 6.0 mm the total area of weld fusion zone under the same heat input condition does not decrease, but slightly increases to be exact, then eventually tends to be stable, as shown as in Fig. 9 (a). 3.2. Effect of oscillation frequency and amplitude on laser energy distribution During laser oscillating welding, the spatial-temporal motion pattern of the laser spot could be represented by the function of welding speed, oscillation frequency and amplitude. The inherent relationship between welding parameters and laser energy deposition distribution could be achieved through the modeling and simulation calculation method. Furthermore, the heat flux model could be further established accurately according to the result of laser energy distribution. According to the three-dimension Cartesian coordinates in Fig. 1, the moving track of laser spot in laser rectilinear welding can be expressed by Eq. (1), and the moving track of laser spot in laser oscillating welding can be expressed by Eq. (2) [27,36]: X ðt Þ ¼ X 0 þ V  t Y ðt Þ ¼ Y 0

ð1Þ

X ðt Þ ¼ X 0 þ V  tY ðt Þ ¼ Y 0 þ A  sinð2  π  F  t Þ

ð2Þ

Thereinto X 0 and Y 0 are initial position coordinates of laser spot, V is moving speed of substrate, A is the oscillation amplitude and F is the oscillation frequency, t is the laser welding duration.

Since the energy distribution of initial motionless laser spot is approximately Gaussian distribution [37], which can be expressed by Eq. (3): I ¼ Im  e−α  X 2 =R20  e−α  Y 2 =R20

ð3Þ

Thereinto Im is the maximum energy density of beam spot, α is the energy concentration coefficient, and R0 is the effective radius of laser spot. Using the normalized and dimensionless representation [26], the transient intensity distribution of initial motionless laser spot can be expressed by Eq. (4): Q¼

I ð X; Y Þ ¼ e−α  X 2 =R20  e−αY 2=R20 Im

ð4Þ

thereinto Q is the normalized energy intensity distribution. Combining with the motion equation of laser spot, the laser energy deposition distribution can be described by the integral Eq. (5): ZT E¼

e−αX ðtÞ 2=R20  e−αY ðtÞ 2=R20 dt

ð5Þ

0

thereinto E is the laser energy deposition distribution during the laser spot movement, T is the integral duration. According to the motion Eq. (1), the integral operation is performed by Eq. (5). The energy deposition distribution of laser rectilinear welding during the duration of 0 to T can be acquired in Fig. 7(a). Therein, V is 10 mm/s as well as T is 1.0 s, and the heat source radius of laser beam is approximate to 0.5 mm. According to the motion Eq. (2), the integral operation is performed by Eq. (5). The energy deposition distribution of laser

Fig. 10. Dendrite structure morphology of welds with different oscillation amplitude based on optical micrographs: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm, (d) weld of A = 6.0 mm.

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Fig. 11. Grain structure morphology of welds with different oscillation amplitude based on EBSD images: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm, (d) weld of A = 6.0 mm.

oscillating welding with different oscillation frequencies during the duration of 0–1.0 s can be acquired in Fig. 7(b–f). The oscillation amplitude A is the value of 4 mm. Similarly, the energy deposition distribution of laser oscillating welding with different oscillation amplitudes during the duration of 0–1.0 s can be illustrated in Fig. 8(a–f). The oscillation frequency F is the value of 80 Hz. By this means, the effects of oscillation frequency and amplitude on laser energy distribution can be observed and researched intuitively. The essential characteristic of laser oscillating welding with sinusoidal trajectory can be regarded as the superposition of uniform linear motion in X direction and pendulum motion in Y direction. The energy input per unit length can be adjusted and determined by welding speed, laser power, oscillation frequency and amplitude. For a specific welding speed and laser power, the maximum energy input per unit length decreases with the increase of oscillation frequency and amplitude, corresponding to the diminution in weld penetration, as shown in Fig. 9(b and c). It should be noted that as the oscillating frequency exceeds a certain value proportional to the welding speed, the maximum energy input per unit length will tend to be stable in Fig. 9(b).

3.3. Microstructure and mechanical property of welds with different laser energy distribution The solidification microstructure of weld, especially the dendrite morphology and dimension, is determined by the thermal environment condition of molten-pool. Fig. 10 illustrates dendrite structures of weld center and heat affected zone. It is easily seen that there is a significant difference in dendrite morphology and dimension between the rectilinear weld and the oscillated weld. The rectilinear weld mainly produces long-range oriented dendrites accompanied with some short-range oriented dendrites, as shown in Fig. 10(a). These oriented dendrites grow rapidly from the boundary of weld and converge at the centerline of the weld. The morphology of most oriented dendrites are usually continuous and long-range. During laser oscillating welding, the growth of oriented dendrites is obstructed, and the dendrite morphology is obviously dispersive and short-range, as shown in Fig. 10(b–d). In addition, the dimension of dendrite structures decrease further with the increase of oscillation amplitude. To quantitatively characterize grain refinement, Fig. 11 shows the electron back scatter diffraction (EBSD) images of welds made by laser rectilinear welding and laser oscillating welding. Due to the competitive dendrite growth characteristics for cubic crystal alloys, the coarse columnar grains are produced with the preferential growth direction of b100N. For rectilinear welds without beam oscillation, the growth of oriented columnar grain structures usually starts at the boundary of the weld and ends at the centerline of the weld, as shown in Fig. 11(a). As oscillation amplitude is 2.0 mm, some isotropic crystalline grains are appeared near the centerline of weld, as shown in Fig. 11(b). As oscillation amplitude is 4.0 mm or 6.0 mm, solidification microstructure of welds is characterized by equiaxed crystalline grains, rather than oriented columnar crystalline grains, as shown in Fig. 11(c and d).

Fig. 12 shows the quantitative comparisons of grain area size and misorientation angle in weld center zone. The average grain area of rectilinear weld without oscillation is 82070.8 μm2. The average grain area of oscillated weld with 2.0 mm oscillation amplitude is 38% smaller than that of rectilinear weld. Particularly, the average grain area sizes are refined by 50.1% and 54.6% respectively in the oscillated weld with 4.0 mm and 6.0 mm oscillation amplitude. For polycrystalline materials, crystal boundaries of grain can be divided into small-angle grain boundaries and high-angle grain boundaries. The smallangle grain boundaries are the grain boundary with misorientation angle less than 15°, which can be regarded as a series of dislocations. The high-angle grain boundaries are the grain boundary with misorientation angle greater than 15°, which can hinder the propagation of brittle cracks [38]. Due to the steep temperature gradient and rapid cooling speed in conventional laser welding, the solidification process of molten-pool is in a severe non-equilibrium state which usually produces a large number of small-angle grain boundaries [39]. Instead, it is worth noting that laser welding with beam oscillation can optimize the grain boundary and increase the proportion and amount of high-angle grain boundaries that is advantageous to the weld performance. To represent the evolution of the microstructure, Fig. 13 shows the inverse pole figure (IPF) images and ϕ2 ¼ 45 ° orientation distribution function (ODF) sections of welds with different amplitudes. Referring to ODF section in Fig. 2, the initial texture of Invar alloy base metal is characterized by γ -fiber feature but the pole density is very low. It is clearly seen that laser welding methods with different energy distribution have a significant impact on the transformation of solidification microstructure texture. For weld of laser rectilinear welding in Fig. 13(a and b), the solidification microstructure is almost entirely represented by λ -fiber texture. However, for oscillated weld with amplitude of 2.0 mm in Fig. 13(c and d), the solidification microstructure is composed of high-density Goss-fiber texture and relatively low-density λ -fiber texture. When oscillation amplitude is 4.0 mm or 6.0 mm, the solidification texture of oscillated weld is composed of high-density γ -fiber texture and low-density λ -fiber texture, as shown in Fig. 13(e–h). Therefore, laser oscillating welding method can not only refine grain, but also optimize solidification microstructure with strong γ -fiber texture feature that is conducive to the improvement of the ductility [40]. The formation of high-density γ -fiber texture feature may be related to the reduction of molten-pool cooling speed during laser oscillating welding [41]. Fig. 14 shows the tensile strength of Invar alloy substrate and welds, indicating the relationship between solidification microstructure and mechanical property of welds with different energy distribution. During tensile test, stress-strain curve of the elastic phase is almost the same, after in the elastic-plastic stage begins to show a significantly different trend. The ultimate tensile strength of welds without beam oscillation is about 365 MPa, about 82% of Invar alloy substrate. However, the average ultimate tensile strengths of welds with the oscillation amplitude of 2.0 mm and 4.0 mm are respectively 393 MPa and 398 MPa, about 90% of Invar alloy substrate. It can be observed that ultimate tensile strength of oscillated weld is

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Fig. 12. Comparison of grain size and misorientation angle distribution: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm, (d) weld of A = 6.0 mm.

superior to that of conventional rectilinear weld. The highest strain of oscillated weld is about 48%, but that of rectilinear weld without oscillation is only about 25%. Fig. 15 shows the fracture surface morphology of tensile specimens. In Fig. 15(a and b), the tensile fracture of base metal is an oblique plane and shows many great dimples morphology with obvious tearing corrugate. In Fig. 15(c and d), the fracture morphology of rectilinear weld is a vertical plane relative to the direction of stress and shows a small number of dimples. Meanwhile, fracture surface has cleavage planes which is cracked and formed under the action of tensile normal stress. Particularly, there are a lot of porosity defects on the fracture surface of

weld without beam oscillation. In Fig. 15(e–h), the tensile fracture of oscillate weld is an oblique plane with a lot of tiny dimples. Fracture interface changes from a vertical plane to an oblique plane relative to the direction of stress with the increase of oscillation amplitude, indicating the increase of weld ductility. 3.4. Effect of beam oscillation on solidification condition of molten-pool Fig. 16 illustrates the surface dynamic behavior of the molten-pool at five evenly spaced moments during an oscillation period, which are recorded by high speed camera

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Fig. 13. Comparison of inverse pole figure (IPF) and orientation distribution function (ODF): (a–b) weld of A = 0 mm, (c–d) weld of A = 2.0 mm, (e–f) weld of A = 4.0 mm, (g–h) weld of A = 6.0 mm.

Fig. 14. Stress-strain curves of the substrate and welds.

during laser oscillating welding. The welding parameters used are consistent with that of the welds shown in Fig. 6. Thereinto, the yellow solid arrow illustrates the moving direction of the laser spot, and the yellow dashed arrow illustrates the moving direction of forced convection flow. For the laser rectilinear welding in Fig. 16(a), the movement velocity of laser spot relative to the Invar alloy substrate is a relatively low certain value. Since the metallic vapor recoil pressure caused by high-density laser energy is strong and changeable, which is also the dominant factor in formation of keyhole, the keyhole is unstable even under the thermal stability condition of conventional laser rectilinear welding. Serious fluctuations surrounding the keyhole are observed, accompanied by metal spatter ejecting out from vapor keyhole. It can be easily observed in Fig. 16(b–d) that laser oscillating welding can significantly change the geometric dimension of the molten-pool in both X direction and Y direction with the variation of oscillation amplitude. Meanwhile, the vapor keyhole becomes very stable with no fluctuation or spatter. Owing to the local high speed movement of vapor keyhole driven by beam oscillation, excessive evaporation of local material can be avoided maintaining a stable keyhole. For a given oscillation frequency, the movement velocity of the laser spot is proportional to the oscillation amplitude, hence the action of forced convection increases synchronously with the oscillation amplitude. As the laser spot moves to the reversal location of oscillation track that is the second and fourth instant in one oscillation cycle period, the flow of melting metals is obstructed by unmelted substrate resulting in a melt raised area, then melting metals begin to flow in the opposite direction. Under the action of forced convection, the violent flow of molten metals in molten-pool can improve the capacity of filling and reduce the formation of defects.

Fig. 15. Fracture morphology of tensile tested samples: (a–b) substrate, (c–d) weld of A = 0 mm, (e–f) weld of A = 2.0 mm, (g–h) weld of A = 4.0 mm.

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Fig. 16. Surface dynamic behavior of the molten-pool and keyhole at five evenly spaced moments in an oscillation period: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm, (d) weld of A = 6.0 mm.

According to the research on the internal morphology of laser welding molten-pool, the lower rear wall of vapor keyhole normally had a bulge area in laser rectilinear welding process. As the laser gradually moved forward, the keyhole would contract over the bulge, then bulge would separate from the keyhole and remain in the molten-pool as a new pore [42]. However, the dimension of keyhole and bulge was significantly smaller and stable during laser oscillating welding process reducing the probability of pore defects [22]. In the case that the formation of porosity defects cannot be avoided in laser rectilinear welding, laser oscillating welding with the appropriate frequency and amplitude has great potential to achieve porosity-free weld. During lateral laser oscillating welding, the melt flow ahead of molten-pool front is disturbed by the local high speed movement of laser spot leading to violent convection flow. However, the forced convection is forceless at the tail of molten-pool that is transitional zone between the already solidified crystalline grain and the molten metals. Taking into account the convection flow of melt can further reduce the temperature gradient, the appearance of fine equiaxed dendrite is probably related to the uniform distribution of laser energy caused by the oscillation of laser heat flux. For laser oscillating welding with sinusoidal track at a high oscillation frequency, it can be assumed that the laser energy distribution along the Y direction is fixed in one cycle period for the given oscillation amplitude, obeying a definite mathematical distribution, as shown in Figs. 7 and 8. The three-dimensional heat flux model is considered as the combination of surface heat flux model and volumetric heat flux model. The variation of mathematical function for laser energy distribution along the Y direction depends on the oscillation amplitude, which can be calculated by curve fitting. Concretely, the surface heat flux model of laser rectilinear and oscillating welding can be expressed by Eq. (6): IS ðX; YÞ ¼ IS  F S ðY Þ  e−α  X 2 =R2S

ð6Þ

Then, the volumetric heat flux model of laser rectilinear and oscillating welding can be expressed by Eq. (7): IV ðX; Y; ZÞ ¼ IV  F V ðY Þ  e−α  X 2 =R2V  e−β  Z

ð7Þ

where, I S and I V are the maximum energy flux of surface heat flux and volumetric heat flux respectively, F S ðY Þ and F V ðY Þ are the mathematical function of laser energy distribution along the Y direction of surface heat flux and volumetric heat flux respectively, and RS and RV are the effective radius of surface heat flux and volumetric heat flux respectively, α is the energy concentration coefficient, and β is the energy flux attenuation coefficient

in the Z direction of volumetric heat flux. According to the calculated results of the above equation, the three-dimensional heat flux models of laser rectilinear welding and laser oscillating welding with lateral sinusoidal trajectory can be illustrated in Fig. 17. Based on the established laser heat flux model, the local temperature distribution is investigated numerically by finite element analysis method. The moltenpool temperature distributions under different welding methods can be illustrated in Fig. 18. Comparison of simulation and experimental results shows that the simulated moltenpool morphology is highly consistent with the weld morphology obtained by the experiment. The total energy input of all welds is the same. By comparing the temperature curves at different locations of molten-pool, it can be easily seen that lateral oscillation of laser heat flux has a significant impact on the temperature distribution of moltenpool. The highest temperature of laser spot heating position decreases gradually with the increase of oscillation amplitude owing to the reduction of the transient laser energy deposition. As shown in Fig. 18(a), since the highest-density laser energy acts always on the centerline of molten-pool, melting metals near the centerline of molten-pool have the maximum temperature, which is probably the dominant cause of the generation of keyhole fluctuation and welding spatter. Meanwhile, the molten-pool of laser rectilinear welding would be provided with the highest temperature gradient owing to the minimum geometric dimension of molten-pool. For oscillated welds with 2.0 mm or 4.0 mm oscillation amplitude, the temperature gradient decreases significantly due to the reduction of the maximum temperature and the enlargement in geometric dimension of moltenpool. Particularly, it can be easily observed from Fig. 18(b and c) that melting metals near the central region of molten-pool have the same temperature even a negative temperature gradient. Based on the solidification theory of Kurz et al. [43], the schematic summary of single-phase solidification morphologies is illustrated in Fig. 19. For one alloy, the temperature gradient G and solidification rate R are the main variables to determine the crystalline form of solidification microstructure. The ratio of G/R represented by the bands running from the lower left to the upper right defines the regional division of dendrite morphology. The orange band defines the conversion of the cellular crystal structure into the oriented dendrite crystal structure. The green band defines the conversion of the oriented dendrite crystal structure into the equiaxed dendrite crystal structure. So dendrite morphology of solidification structure can be adjusted to some degree by changing the temperature gradient and solidification rate. Under the thermal stability condition in welding process, the maximum local rate of crystal solidification in molten-pool is equal to welding

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Fig. 17. Three-dimensional heat flux mode: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm.

speed represented by the red horizontal band in Fig. 19. Therefore, adjusting the temperature gradient of molten-pool is the most practical and reliable method for weld grain refinement. Fig. 20 shows the schematic description of conditions for the establishment of solidification structure during laser rectilinear welding and laser oscillating welding. Based on the constitutive supercooling theory of liquid-solid interface stability [44], a liquid-solid two-phase zone is formed ahead of solidification front as the constitutive supercooling condition is reached. In this zone, the processes of spontaneous crystallization nucleation, crystal growth, and dendrite evolution would occur. Above all, the constitutive supercooling acting as a thermodynamic protection prevents the newly nucleated dendrites from being easily melted by thermal convection surrounding these newborn dendrites. To ensure the stable existence of newborn dendrites, the temperature surrounding growing dendrites needs to be retained below fusing point, which means the temperature gradient in which molten-pool must be very low. As the temperature gradient reduces the constitutive undercooling region of molten-pool increases gradually. It should be noted that the flat temperature gradient means that most of molten metals would be undercooled at the same time. If a great many new equiaxed dendrites are produced simultaneously in supercooling liquid region, the growth of oriented dendrite would be blocked leading to the transformation from the oriented columnar dendrite grain to equiaxed dendrite grain.

4. Conclusions The research results prove that the weld appearance, solidification microstructure morphology and mechanical property are profoundly interrelated with the laser energy distribution. The conclusions are as follows:

(1) Invar alloy welds of laser rectilinear welding frequently occur porosity and hump defects. These defects are effectively suppressed by applying lateral laser oscillating welding with appropriate oscillation frequency and amplitude. (2) The essential effect of beam oscillation can be attributed to changing the deposition distribution of laser energy on the processing surface. The steady-state energy distribution is determined by the oscillation amplitude. The larger the oscillation amplitude is, the lower the maximum energy input per unit length becomes. (3) Remarkable grain refinement can be produced in Invar alloy welds by laser oscillating welding. Average grain area sizes are refined by 38.0%, 50.1% and 54.6% in the oscillated weld with 2.0 mm, 4.0 mm and 6.0 mm oscillation amplitude, respectively. Oscillated weld microstructures produce high-density γ -fiber texture. (4) The local rapid movement of laser spot during laser oscillating welding results in a more stable keyhole without fluctuation and the intensification of melt flow. (5) Based on simulation calculation of laser energy distribution, the three-dimensional heat flux models with different parameters are accurately established. Temperature curves indicate that lateral laser oscillation make the temperature distribution of the melt nearby the center of weld more uniform even negative

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Fig. 18. Effect of beam oscillation on temperature distribution of molten-pool: (a) weld of A = 0 mm, (b) weld of A = 2.0 mm, (c) weld of A = 4.0 mm.

temperature gradient. (6) The flat temperature gradient caused by laser oscillating welding provides the survival condition for the growth of equiaxed dendrite.

CRediT authorship contribution statement Zhenguo Jiang: Conceptualization, Methodology, Visualization, Writing - original draft, Data curation. Xi Chen: Software, Investigation. Hao Li: Resources, Funding acquisition. Zhenglong Lei: Writing - review & editing, Resources, Supervision. Yanbin Chen: Resources, Project administration. Shibo Wu: Software, Investigation. Yuhua Wang: Resources, Funding acquisition. Acknowledgements

Fig. 19. Schematic summary of single-phase solidification morphologies.

The authors gratefully acknowledge the financial support given by the National Key R&D Program of China (2017YFB1301600) and Annual Plan for absorption and Innovation of imported Technology in Shanghai (Research and Industrial Application of Key Technologies in

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Fig. 20. Schematic description of the conditions for the establishment of solidification structure: (a) solidification condition of laser rectilinear welding, (b) solidification condition of laser oscillating welding.

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