Analysis and improvement of laser wire filling welding process stability with beam wobble

Analysis and improvement of laser wire filling welding process stability with beam wobble

Optics and Laser Technology 134 (2021) 106594 Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.c...

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Optics and Laser Technology 134 (2021) 106594

Contents lists available at ScienceDirect

Optics and Laser Technology journal homepage:

Full length article

Analysis and improvement of laser wire filling welding process stability with beam wobble


Junzhao Lia,b, Yibo Liua,b, Zuyang Zhena,b, Kexin Kanga,b, Peng Jina,b, Fuxiang Lia,b, Yue Liub, ⁎ Qingjie Suna,b, a b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, No.92 West Dazhi Street, Harbin 150001, China Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, No.2 Wenhuaxi Road, Weihai 264209, China


wobble promoted the transformation into heat conduction mode. • Beam stability and melt flow were improved by beam wobble. • Keyhole refinement in fusion zone and wider equiaxed grain zone were presented. • Grain • Welding of thick plates was achieved without porosity and lack of fusion. ARTICLE INFO


Keywords: Austenitic stainless steel Narrow gap welding Laser welding Beam wobble Process stability Microstructure evolution

The reasonable selection of laser welding modes, namely keyhole mode and heat conduction mode, is based on the practical application requirements. The results show that the heat flux rapidly decreases and beam moving velocity increases with the beam wobble amplitude of 2.0 mm and frequency of 150 Hz, promoting the transformation of keyhole mode to heat conduction mode. Wider and shallower weld morphology is obtained. The improvement of keyhole stability and modification of liquid metal fluidity promote the overflow of bubbles. In case of beam wobble welding, the movement and turbulence of weld pool driven by the beam wobble help to break the columnar crystals. Welding of 40-mm thick plates without incomplete fusion and pore defects is achieved by suitable beam wobble parameters.

1. Introduction Austenitic stainless steel with excellent mechanical properties and corrosion resistance has been widely used in some industrial structures, such as shipbuilding, nuclear power plant, pressure container and so on. For the manufacture of austenitic stainless steel component, high energy density laser beam welding enables to provide lower welding heat input and higher welding speed in comparison to conventional arc welding, which is conducive to improve performance properties of the welded joint [1]. However, the small laser spot requires precise preparation of weld groove to maintain stable welding process and the higher cooling rate has a significant effect on weld metallurgical process [2,3]. Laser wire filling welding process has an extensive application in additive manufacture, surface cladding and the connection of thick plate [4–6]. Suitable laser welding mode is selected according to the

welded material properties and specific application requirements [7–9]. For the welding of medium plate, the laser keyhole welding mode is desired to increase weld penetration. The heat conduction welding mode with shallow weld depth and wider width is proper for surface cladding and multi-pass narrow-gap welding process. However, the heat conduction mode is usually achieved by a larger laser output power and defocusing distance, expanding laser energy irradiation area and, however, reducing laser energy utilization. Besides, the wire feed speed is limited at laser conduction mode because more laser energy is required to enlarge weld molten pool. The weld reinforcement of each layer is smaller, which reduces welding efficiency and needs more weld layers to complete the welding of thick plate [10]. The accumulated welding thermal cycle produces a complex metallurgical reaction and a larger residual stress. Kuryntsev et al. used defocused laser beam welding to reduce requirements for preparation of edges and improve the stability of keyhole. Satisfactory welds without undercutting,

Corresponding author at: Harbin Institute of Technology at Weihai, No.2 Wenhuaxi Road, Weihai 264209, China. E-mail address: [email protected] (Q. Sun). Received 30 December 2019; Received in revised form 31 August 2020; Accepted 10 September 2020 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.

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spatter and pores were achieved [11]. Ning et al. conducted a study of multi-pass laser welding of ultra-high strength steel and found that the defocused laser beam was conducive to the filling of weld passes [10]. Metelkova et al. found that the suitable defocused distance could support conduction melting mode and greatly increase selective laser melting productivity [12]. Meanwhile, the weld pool morphologies led to a different microstructure due to the various cooling rate. Laser beam modulation, such as pulse profile, beam oscillation and beam positioning, is adapted to join advanced materials because of its specific interaction with the welded materials [13–16]. Among this, beam oscillation welding process was extensively applied to weld aluminum alloys/stainless steel with the advantage of regulating laser energy, inhibiting pore formation, refining grain size, and thus improving joint quality [17–22]. The laser beam wobble welding is similar to the swing MAG arc welding and swing TIG welding process for narrow-gap welding [23,24]. The energy was redistributed into the groove sidewalls to increase sidewalls penetration. Furthermore, the fluidity of liquid metal in weld pool was optimized to improve weld formation and facilitate the floating up of bubbles. Laser beam wobble technology has potential to improve narrow-gap laser wire filling welding adaptability for thick plate by its specific weld morphologies and process characteristics. Relevant studies, especially weld formation and microstructural evolution of 316L welded joint under various beam wobble conditions were rare. In this paper, the effects of laser beam wobble parameters and defocusing distance on weld formation features and process stability of laser welding 316L stainless steel were studied. Then the solidification process and microstructural evolution of 316L welded joint with beam wobble welding were investigated.

be oscillated up to a maximum frequency of 1000 Hz and amplitude of 2.0 mm. Circle wobble is considered as a suitable pattern to alleviate the formation possibility of pores [18]. The background light source as illuminator is adopted to capture weld pool behaviour and spatters, as seen in Fig. 1(b). The high speed camera is presented 60 degree with the welding direction. The high-speed camera system for the observation of keyhole feature is shown in Fig. 1(c). The keyhole stability during laser wire filling welding process is observed by stainless steel and heat resistant quartz glass. The keyhole evolution process and the formation of pores can be clearly recorded. The corresponding expression of laser spot center with the beam wobble type is presented in Eq. (1)

2. Experimental procedure

3.1. Weld cross-sectional morphology

Base metal and filler wire are SUS316L austenite stainless steel and ER316L wire with diameter of 1.0 mm. The welding experiments are conducted using a fiber laser system with a maximum available laser power of 6 kW, as seen in Fig. 1(a). The wavelength of the operating laser is 1070 nm, the BPP beam parameter product is 6.4 mm ▪ mrad and the beam radius is 0.24 mm at focal position. The IPG D50 weld head is used, which is controlled by the galvanometer scanners and can

The weld morphologies with various welding parameters are presented in Figs. 3 and 4. Compared to the conventional laser welding process, the weld cross-sectional morphologies show specific characteristics at different beam wobble amplitudes and frequencies. During the whole experiments, filler wire feeding rate of 4.5 m/min and welding speed of 0.3 m/min remain constant. At laser defocusing distance of +5 mm, the weld width increases and weld depth decreases

vt + a·sin(2 f ·t x (t ) = y (t ) a·sin(2 f · t )



with x(t) and y(t) denoting the instantaneously central position of laser spot. The welding speed is described by v. a and f are beam wobble amplitude and frequency, respectively. 40 mm 316L stainless steel is welded by multi-pass laser welding process using the optimized welding parameters. Metallurgical and mechanical samples are extracted from the weld. Microstructure of the welded joint is observed by OLYMPUS DSX 510 optical microscope (OM). The layered tensile and impact tests are conducted by WEW-300 universal testing machine and JBD-300Y impact testing machine at room temperature, respectively. Three replicates are performed and the average value is taken to evaluate the tensile strength. The sizes of the samples for tensile and impact testing are shown in Fig. 2. 3. Results and discussion

Fig. 1. (a) Schematic diagram of narrow-gap laser welding process; (b) high-speed camera acquisition system for welding pool; (c) for keyhole behaviour. 2

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Fig. 2. The sizes of samples for (a) tensile and (b) impact testing.

with the beam wobble amplitude and frequency. And a higher beam wobble frequency improves weld uniformity. However, the weld width reaches a maximum weld width at beam amplitude of 1.0 mm when laser defocusing distance is +20 mm and the weld width decreases at a higher beam wobble amplitude. Besides, the weld width decreases with beam wobble frequencies. The beam wobble has a less effect on weld morphologies at a larger defocusing distance. The change of weld morphologies with beam wobble is mainly attributed to the specific laser energy distribution. The laser beam wobble increases laser heating zone and decreases laser energy density of unit area, causing laser energy transfer to width direction. Thus the weld morphology is shallower and wider. Combined with larger defocusing distance and beam wobble amplitude, the laser density around the laser spot cannot reach the melting threshold of 316L base metal, resulting in the decrease of weld width and depth. The porosity by X-ray nondestructive testing with various welding conditions is shown in Fig. 4. It can be seen that the porosity in weld is closely related to laser welding mode and the porosity is significantly reduced with the decrease of weld depth. The welding mode is transformed from keyhole mode into heat conduction mode, as the beam wobble amplitude and frequency increases, thus avoiding the formation of keyhole-induced pores and, therefore, the porosity of the weld. When the beam amplitude increases to 2.0 mm and frequency reaches to 150 Hz, the pores in weld are completely eliminated and defect-free weld joint is obtained. During narrow gap laser welding process, the incomplete sidewall fusion defect can be avoided on the condition that the width of weld pool is larger than the groove gap. Enough weld penetration enables the connection of filling layers. With the application of beam wobble, the laser energy distribution is modified to affect the formation condition of weld pool. The laser peak intensity irradiated on materials decreases with the increase of laser defocusing distance due to the divergence of laser beam. As the laser defocusing distance increases, a higher laser power is required to improve the laser energy density, which increases the melting area of materials. The laser energy density is mainly affected by beam wobble amplitude, while beam wobble frequency influences laser energy continuity. Obviously, the laser energy intensity is

Fig. 4. The porosity and weld depth with various welding conditions (Laser power of 4.0 kW, wire feeding rate of 4.5 mm/min and welding speed of 0.3 m/ min).

weakened and divergent with beam wobble. A higher wobble frequency increases the moving velocity of laser beam, and meanwhile homogenizes the distribution of laser energy. The features of laser energy deposition and beam moving trajectories are presented in Fig. 5. It can be seen that the laser energy can be distributed into the edge of weld pool due to the transverse motion component of laser beam. The laser energy concentrates on a narrower area at smaller wobble amplitude, which has a negligible effect on weld formation. With the beam wobble, the velocity of laser beam is higher than the conventional laser welding. The velocity of laser beam increases with beam wobble amplitude and frequency. The distributed stripe heat source can be used to represent the temperature cycle with beam wobble. Suppose the deposited energy is evenly distributed within the beam wobble amplitude. When defocusing distance is +5 mm, the maximum heat flux decreased to 63% and 16% in the case of the beam wobble amplitude of 0.5 mm and 2.0 mm, respectively. The numerical result of heat flux is actually less than the actual value due to the overlapping of laser beam spot. Therefore, the increased beam moving velocity and decreased heat flux density are conducive to the transformation of keyhole welding mode to heat conduction mode. The weld width is wider and figure-shaped weld penetration can be avoided, which enable the complete fusion of sidewalls during narrow-gap laser welding process. 3.2. The welding process analysis The keyhole evolution processes of laser welding at various welding conditions are presented in Fig. 6. The keyhole evolution process is

Fig. 3. Effects of beam wobble parameters on weld width: (a) laser power and defocusing distance; (b) wobble amplitude and frequency at laser power of 3.0 kW and laser defocusing distance of +5 mm; (c) wobble amplitude and frequency at laser power of 4.0 kW and laser defocusing distance of +20 mm. 3

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Fig. 5. The schematic of modified heat source: (a) conventional laser welding; (b) laser beam wobble.

enables to re-melt the bubbles that may have formed and promoted its overflow from keyhole. Besides, the recoil pressure acted on keyhole wall reduces, indicating the stable keyhole state. At higher beam wobble amplitude and lower wobble frequency (a = 2.0 mm, f = 20 Hz), the keyhole morphology presents an unstable state. The keyhole moves along the laser beam due to the slow moving velocity. The keyhole wall has a larger fluctuation and is contracted at the middle position. Therefore, the bottom position of keyhole is easily to collapse to form pores that trapped in the solidified weld metal. The keyhole depth evolution process corresponding to Fig. 6 is shown in Fig. 7(a–e) and relative variance is evaluated to show its volatility in Fig. 7(f). Compared to conventional laser welding process, the relative variation for laser beam welding are generally reduced except the weld with lower wobble frequencies which is attributed to the single observation direction. The lower beam wobble frequencies cause the slower moving velocity of laser beam along the welding trajectory, therefore the keyhole gradually vanishes and its depth decreases at the reversal position. The relative variation cannot really

closely related to the formation of pores in weld. For conventional laser welding with defocusing distance of +5 mm, a deep keyhole is formed and the keyhole rear wall experiences a larger fluctuation. The laser beam irradiates on the liquid metal in the keyhole and a larger amount of metal vapor is formed and impacts the keyhole wall, causing the deformation and rupture of keyhole and thus the formation of pores. Besides, the weld is narrower with larger reinforcement in conventional laser welding process, causing that the accumulation of melting filler wire produces a impact on keyhole wall. At higher beam wobble frequencies, the keyhole can remain a complete state from the observation direction due to the continuous keyhole evolution process. When the beam wobble amplitude is 0.5 mm, the keyhole width is contracted and keyhole depth slightly decreases, which is adverse to the overflow of bubbles. Due to the turbulence of weld pool, the keyhole rear wall produces fluctuation, decreasing the process stability. With beam wobble amplitude of 2.0 mm, the keyhole expands to increase keyhole width and the keyhole is shallower, which shortens the overflow distance of bubbles. Meanwhile, the rotating movement of laser beam

Fig. 6. The keyhole evolution processes for laser welding at various welding conditions. 4

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Fig. 7. (a–e) Keyhole evolution, (f) relative variation corresponding to Fig. 6.

reflect the keyhole evolution for laser welding process with lower wobble frequencies. At higher beam wobble amplitudes and frequencies, the keyhole depth during the whole welding process show a little fluctuation, helping to improve welding stability. Combined with porosity and weld depth in Fig. 3, the evolution process of keyhole is closely related to the formation of pores in welds. The filler wire metal introduces to weld pool at the edge of the keyhole, causing an obvious impact on keyhole morphology. The keyhole is deformed and even collapsed when metal vapor is trapped, as shown in Fig. 6(a). However, the enlarged keyhole size and transformation to heat conduction mode help to gas escape and improve welding adaptability. Fig. 8 shows the weld pool morphologies and cross sections with various welding conditions. It can be seen that the melt pool length is decreased, while weld pool width is increased with the increase of beam wobble amplitude and frequency. The weld pool length decreases from 18.7 mm with conventional laser welding to 16.1 mm with beam wobble amplitude of 2.0 mm and frequency of 150 Hz. The tracer particles (SiC with a diameter of 0.8 mm) are used to characterize the fluidity of weld pool surface for laser wire filling welding process, as seen in Fig. 9. For conventional laser welding process, the weld pool is

produced with the melting filler wire and base metal. The liquid metal flows to the rear area of weld pool at the effect of Marangoni convention. At smaller wobble amplitude with higher frequency or larger wobble amplitude with smaller frequency, the capability to drive weld pool is limited and the weld pool has a weak fluctuation, resulting in indistinctive change of weld pool and poor wettability of weld metal. Nearly no stirring effect is observed in weld pool and the fluidity condition is similar with that of conventional laser welding process. With the increase of beam wobble amplitude and frequency, a remarkable vortex is observed at the front of weld pool. The stirring effect on weld pool enhances and the liquid metal rotates as the laser beam moves. The flowing velocity of liquid weld metal increases and the flowing direction is turn to the edge of weld pool. The wettability of liquid weld metal on base metal is improved and melt pool width increases. Zhu et al. found that the vortexes increased the convective heat transfer to the sidewalls, and thus to increase the sidewall penetration [25]. However, a higher beam frequency of 300 Hz causes a fierce turbulence in weld pool, leading to an unstable weld pool state. The vortex area is limited and the tendency of liquid metal flowing to edge is weakened. The existing vortex on weld pool increase the existing time of pores at

Fig. 8. The melt pool morphologies and cross sections with various welding conditions. 5

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Fig. 9. The trajectories of tracer particles and fluidity of weld pool for laser welding: (a, b) conventional laser welding; (c, d) laser beam wobble welding.

the front of weld pool, which promotes the break and overflow of pores before solidification.

Obvious grain refinement in fusion zone and narrower columnar grain zone are presented with the applying laser beam wobble, resulting in the uniform distribution of ferrite phase. The movement of liquid metal in weld pool is enhanced to improve the element diffusion between dendrites and heat transfer ability, which is conducive to the homogeneous of microstructural composition and the brakeage of columnar crystals that grow from the fusion boundary towards weld central zone. As seen in Fig. 11, more skeletal-shaped ferrite phases are formed in the weld with beam wobble, improving the diversification of grain orientation. According to the solidification structure map in Fig. 12(a), the local solidification rate and temperature gradient result in the various grain structures of welded joint during solidification process [26]. In case of thermal steady condition, the solidification rate R is relevant to the welding speed and the angle between the surface normal of weld pool shape and welding direction. The laser beam wobble technology modifies the solidification rate by changing the shape of weld pool, namely weld pool width, length and the angle of solidification front. At the same time the stirring effect of weld pool caused by beam wobble

3.3. Microstructural analysis According to the Creq/Nieq of 316L austenitic stainless steel, the FA solidification mode is confirmed, which indicates that the alloy solidifies as primary ferrite and austenite forms during cooling process. The weld microstructure contains austenite and untransformed ferrite. Figs. 10 and 11 shows the microstructural images of 316L laser welded joint at various zones. The microstructure of 316L stainless steel is the austenite phase with small amount of untransformed ferrite phase. At the interface between of 316L base metal and fusion zone (fusion line zone), the grains grow towards the weld center along to the heat transfer direction and formed coarser columnar crystals. In the weld central zone, equiaxed crystals with larger size are formed due to the increased nucleation sites and lower temperature gradient. The equiaxed crystals zone is narrower. The untransformed ferrite phases remain in weld zone in the shape of lathy and skeletal morphologies.

Fig. 10. OM images for 316L laser welded joints: (a) test locations of weld microstructure; (b1–b3) conventional laser welding; (c1–c3) laser beam wobble welding. 6

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Fig. 11. SEM images of microstructure morphologies: (a) conventional laser welding; (b) laser beam wobble welding.

Fig. 12. (a) Solidification structure map; (b) Weld pool shape for conventional laser welding and beam wobble welding process [26].

Fig. 13. Weld morphologies of 40-mm welded joint: (a) joint with defects; (b) defects-free joint; (c) weld macro-morphology; (d) X-ray nondestructive results. Table 1 Ultimate tensile strength and impact energy for different layers of the welded joint. Samples position

1st layer

2nd layer

3rd layer

4th layer

5th layer

6th layer

Base metal

UTS/MPa Elongation rate/% Samples position Impact energy/J

593 39.8 Top layer 213

594 41.2

595 40.3 Middle layer 209

592 40.7

595 40.4 Bottom layer 207

596 41.5

607 42.8 Base metal 287


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affects the fluidity of weld pool and local temperature gradient. Therefore, the solidification behaviour of welds, such as grain size, orientation and phase proportion, will be optimized. The morphologies of weld pool for conventional laser welding and beam wobble welding process are extracted from Fig. 8 and shown in Fig. 12(b). The welding velocity is kept constant during welding process. The width of weld pool is increased and the length is decreased, causing the decrease of angle α with beam wobble and thus the increase of solidification rate R. The beam wobble modifies point heat source to linear heat source as shown in Fig. 5, which homogenizes laser energy density in the whole irradiated zone in comparison to conventional Gaussian heat source distribution. The inverse movement of the laser beam reheats melting metal at the side of melt pool and alleviates cooling process. It leads to an extension of temperature field and thus a uniform distribution of temperature in the melt pool. The tailing edge angle of melt pool is increased while melt pool length is decreased. The local temperature gradient in melt pool is deceased [26]. Therefore, the combined effect of increased solidification rate R and decreased local temperature gradient promotes the formation of equiaxed dendritic in welded zone.

metal. As the beam wobble applies, the wider keyhole and stirring effect in weld pool promote the overflow of bubbles. (3) Obvious grain refinement in fusion zone and wider equiaxed grain zone are presented with the applying laser beam wobble. The columnar crystals are broken due to the movement and turbulence of weld pool. The formation of skeletal ferrite phases in welded zone is increased. (4) Welding of 40-mm thick plates without incomplete fusion and pore defect is achieved by optimizing laser beam wobble welding parameters by an autogenously laser root pass and eleven laser filler passes. The tensile strength and elongation percentage of welded joint is up to the level of base metal. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement

3.4. Multi-pass laser welding of thick plate

This project is supported by the National Natural Science Foundation of China (Grant No. U1960102, 51705103, 51475104).

The weld morphologies of 40-mm thick plate are shown in Fig. 13. From Fig. 13(a), the obviously incomplete sidewalls fusion defect is existed during multi-layer welding process because of the smaller laser irradiated zone. The groove sidewall cannot receive enough laser energy to guarantee effectively bonding with liquid weld metal. The incomplete sidewall fusion defect severely reduces weld quality and damages joint performance. Furthermore, a large amount of pores and incomplete interlayer fusion defect are observed, which is attributed to the process instability and insufficient laser energy [27]. The shielding gas may be trapped in weld pool and is unable to overflow until weld solidification process. Besides, the close and collapse of instable keyhole during welding process is another reason to cause pores defect. The pores in weld reduce joint loading area and thus impair joint practical performance. Through optimizing laser beam wobble welding process parameters, the defect-free welded joint with thickness of 40 mm is achieved, as shown in Fig. 13(b). The weld joint is welded by an autogenously laser root pass and eleven laser filler passes. The root face of 5.0 mm is welded with laser power of 3.0 kW, welding speed of 0.8 m/ min and defocusing distance of 5.0 mm, while the parameters of filler passes are laser power of 4.2 kW, welding speed of 0.3 m/min, wire feeding speed of 4.5 m/min, defocusing distance of 20 mm, wobble amplitude of 2.0 mm and frequency of 150 Hz. No incomplete fusion and pore defects are observed in weld cross section in Fig. 13(c, d). Table 1 shows the tensile strength and impact energy of layered welded joint. The tensile strengths show a minor fluctuation at various layers and all tensile samples are fractured at the base metal. An increasing tendency of tensile strength is presented from top layer to bottom layer. Typical dimple morphology with plastic deformation in the fracture surface is featured, which indicates excellent weld toughness. The heat affected zone of laser welded joint is so narrower that the impact samples of HAZ zone cannot be fabricated. The impact toughness of weld is lower than base metal due to the higher ferrite content in weld. The impact absorbed energy at top layer of welded joint is higher in comparison to other positions.

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4. Conclusions (1) Suitable beam wobble parameter can achieve the same effect as the laser beam defocusing distance at a lower laser heat input. Wider and shallower weld morphology is obtained with the beam wobble amplitude of 2.0 mm and frequency of 150 Hz, which is conducive to avoid incomplete sidewall fusion and finger-shaped penetration. (2) The inhibition mechanism of pores can be attributed to the improvement of keyhole stability and modified fluidity of liquid 8

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