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Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant
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Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant ⁎
Junzhao Lia,b, Qingjie Suna,b, , Kexin Kangb, Zuyang Zhenb, Yibo Liua,b, Jicai Fenga,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 West Wenhua Road, Weihai 264209, China
H I GH L IG H T S
welding with filler wire was applied in multi-layer narrow-gap welding in vertical positions. • Laser process stability and weld defects were studied and compared in laser vertical-up and vertical-down welding. • The droplet transfer behavior and fluidity of molten pool were investigated for laser vertical welding. • The welding with beam oscillation could increase laser irradiated area and homogenize temperature in molten pool. • Laser • Weld formation was improved and 20-mm vertical welded joint was obtained without defects by laser welding.
A R T I C LE I N FO
A B S T R A C T
Keywords: Large structures Narrow gap welding Laser vertical welding Beam oscillation Process stability
Incomplete fusion, pore and melt sagging are common defects for multi-layer narrow-gap laser welding with filler wire in vertical positions for large and heavy structures. This investigation showed that vertical-up laser welding had a lower sensitivity to pores than vertical-down welding, whereas increasing welding speed could improve welding stability and reduce the formation possibility of pores. For laser vertical welding with hot wire, surface tension transfer mode could effectively guarantee pre-heating of filler wire and achieve a stable welding process. The keyhole stability and melt flow varied with welding directions. The melt sagging defect was usually observed and mainly caused by the concentrated laser energy, which easily led to discontinuous weld bead. Laser beam oscillation technology increased laser melting area and promoted wetting behavior of filler wire with base metal. The weld width increased and concave weld metal surface was obtained with suitable process parameters. Narrow-gap welding process could be carried out at a small defocusing position with a lower laser power, further decreasing tendency of melt slagging. The 20-mm defect-free joint was obtained by vertical-up and verticaldown narrow-gap laser oscillation welding with hot wire.
1. Introduction Mid-thick austentic stainless steels with good corrosion resistance and mechanical properties are widely used in the shipbuilding, nuclear plant and container manufacturing industries. Meanwhile, narrow-gap laser welding with filler wire (NG-LWFW) process has been increasingly adopted in manufacturing large engineering components because of narrower groove, lower welding heat input and higher productivity [1,2]. For the structures with complex shape, various welding positions, especially vertical welding (vertical-up and vertical-down) are required to accommodate the space orientations of the welds. However, welding process parameters and stability vary with welding positions, which further affects weld formation and quality. Thus, it is essential to study
⁎
the effect of welding positions on laser welding characteristics to successfully achieve high-quality welded joints. Welding in vertical position has a wide application, and sometimes is inevitable in manufacture of large and heavy structures. Several common issues, such as incomplete fusion, pores, instable droplet transfer and molten pool would occur in all-position welding of thick components [3–5]. Especially, multi-layer laser welding thick plate in vertical position is difficult due to the imbalances of hydrostatic pressure (gravity), surface tension and recoil pressure of metal vapor. However, for MAG and TIG arc welding, the arc force acting on molten pool can be used to balance the effect of gravity and the lower heat input promotes to maintain stable molten pool. Swing arc and rotating arc welding technology also have been developed to reduce linear
Corresponding author at: Harbin Institute of Technology at Weihai, No. 2 West Wenhua Road, Weihai 264209, China. E-mail address: [email protected] (Q. Sun).
https://doi.org/10.1016/j.optlastec.2019.105921 Received 18 May 2019; Received in revised form 12 September 2019; Accepted 21 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Junzhao Li, et al., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105921
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welding heat input [6]. Moreover, laser welding with characteristics of fast solidification rate and lower welding heat input is able to reduce the downward trend of molten pool and obtain the defect-free narrowgap welded joint. Most studies on flat and horizontal NG-LWFW of heavy structures have been advancing. During multi-layer narrow-gap welding, defocusing laser beam was usually adopted to increase laser irradiated area and avoid the incomplete fusion of groove sidewalls [7]. Dual laser beam welding and oscillation laser welding with filler wire were also studied to improve effective area of laser beam, which could solve the misalignment between single laser beam and filler wire [8,9]. The application of laser welding with hot wire (LWHW) process on horizontal narrow-gap welding was investigated and melt slagging defect was prevented due to the gravity direction [10]. Fujinaga et al. used rectangularly modulated laser beam with filler wire to all-position butt welding and found that the evaporation recoil force could reduce spatters [4]. Besides, Xu et al. investigated arc swing parameters on narrow gap MAG weld formation and demonstrated that weld pool sagging defect was mainly related to arc force impulse and arc linear energy [11]. Chang et al. found pores in vertical welding were mainly attributed to the collapsing keyhole and turbulent fluid flow of weld metal [12]. The autogenously laser vertical-up welding promoted the bubbles move out from the keyhole surface. Sohail et al. revealed that the molten metal flowed periodically, which caused gravity at different positions had little effect on flow behavior and bead geometry by numerical investigation [13]. However, investigations on narrow-gap laser vertical welding with filler wire were insufficient, particularly for the regulation of welding process stability. High speed camera has been widespread used to observe welding process characteristics such as plasma plume, droplet transfer mode and weld pool fluidity, which reflected the stability of welding process. Through observation of droplet transfer process in groove, the bead formation was more sensitive to beam position [2]. Zhang et al. observed the keyhole dynamics during laser welding through a high borosilicate glass. The hydrodynamics of keyhole wall affected weld formation and was sensitive to weld defects [14]. Zhao et al. indicated that the side shielding gas suppressed the laser-induced plasma and improved utilization of laser power. Plasma features was related to the gas flow rate and gas composition [15]. Nasstrom et al. studied the melt pool feature and process robustness of hot-wire laser welding and found that arc formation between wire tip and groove sidewalls greatly deteriorated welding stability [16]. Moreover, Yamazaki et al. found that the cold filler wire was intermittently heated and fused with laser beam oscillation. The wire components may become segregated in the bead, which destroyed the stable welding process and even affected the joint quality [9]. Beam oscillation technology could effectively suppress incomplete fusion of groove sidewalls. Above-mentioned researches indicate that narrow-gap laser vertical welding has great potential to improve industrial adaptability by its own features such as special energy flux distribution, weld morphologies and welding efficiency. Relevant studies about NG-LWHW mainly focused on flat position. However, existing researches on the positional NG-LWHW are still very limited and the inherent mechanism of laser welding process stability is still unavailable. Considering that welding of large structures in vertical position is sometimes inevitable, this paper aims to investigate the laser vertical welding feature, including vertical-up and vertical-down positions, and to reveal the influence of welding positions on process stability and joint quality.
Table 1 The chemical compositions and mechanical properties of 316L stainless steel alloy and filler wire (at.%).
316L ER316L
C
Mn
P
S
Si
Cr
Ni
Mo
Fe
Tensile strength
0.02 0.02
1.58 1.89
0.02 0.02
– 0.01
0.45 0.76
16.74 18.7
12.89 12.2
2.05 2.3
Bal. Bal.
716 MPa 570 MPa
Fig. 1. Scheme of the experimental setup for narrow gap laser welding.
process parameters for root pass welding of narrow gap welding. Beadon-plate vertical welding experiments were carried out to investigate the weld geometry, wire melting phenomena and molten pool feature with various process parameters. Then single-track of narrow-gap welding experiments were conducted to study the effect of laser welding parameters on a narrow groove, which provided the basis for narrow-gap welding parameter selection. Fig. 1 shows a scheme of the experimental setup for vertical NGLWHW process. Sample plates were automatically welded using a 6 kW IPG YLS-6000 fiber laser power source with a six-axis robotic welder. The wavelength of the operating laser was 1070 nm and the BPP beam parameter product was 6.4 mm mrad. The IPG D50 Wobble welding head consisted of a collimation unit with focal length 200 mm, a galvanometer scanner unit and an f-θ focusing unit with focal length 150 mm. The beam oscillation was controlled by the galvanometer scanner and could be oscillated up to maximum frequency of 1000 Hz and amplitude of 2 mm. A Fronius heater source was used to provide resistance heat for the filler wire. The hot-wire could improve the utilization factor of laser energy and make more laser energy to melt the base metal. To reveal the behavior of molten pool and the melting phenomena of filler wire, a high-speed video camera was used to discover the filler melting phenomena and molten pool behavior. A background laser (wavelength: 808 nm) was used for illumination and an interference filter was adopted. The welded sample plates were sectioned transversely relative to the welding direction and prepared for metallurgical examination. After etching in a solution of aqua regia (FeCl3: HCl: H2O = 10 g: 30 mL: 120 mL), microstructural observation was carried out by light microscope. Standard tensile coupons were performed with tensile rate of 2 mm/min for the purpose of determining the cross-weld tensile properties. The tensile samples with 3 mm thickness were cut from different layer positions. Three tensile samples were performed, and the average value was taken to evaluate the tensile strength. The fractured surfaces of tensile specimens were observed by scanning electron microscope.
2. Experimental methods and procedures Base metal was 316L stainless steel and the type of filler wire was ER316L of 1.0 mm in diameter. The chemical compositions and mechanical properties of the 316L stainless steel and ER316L filler wire are shown in Table 1. The elongation rate of 316L base material is 62%. Butt-welding process used 5.0 mm steel plate to optimize welding 2
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Table 2 The welding parameters for butt welded joints. Welding direction Vertical-up Vertical-up Vertical-up Vertical-up Vertical-up
welding welding welding welding welding
Vertical-down Vertical-down Vertical-down Vertical-down Vertical-down
(a) (b) (c) (d) (e)
welding welding welding welding welding
(a) (b) (c) (d) (e)
Laser power (W)
Welding speed (m/min)
Heat input (J/mm)
Weld feature
3000 3500 4200 3000 3000
1.2 1.2 1.2 0.8 1.0
150 175 210 225 180
Incomplete fusion Incomplete fusion Defect-free joint Defect-free joint Pore defect
3000 3000 4200 4200 4200
0.8 1.0 1.0 1.2 1.4
225 180 252 210 180
Pore defect Incomplete fusion Pore defect Bead collapse Defect-free joint
3. Results and discussion
increasing welding speed can significantly reduce the amount of pores for both vertical-up and vertical-down welding, which attributes to the stable molten pool and keyhole morphology. It can be seen that a larger laser power and a higher welding speed can improve weld quality and decrease pore tendency for vertical-down welded joint. Moreover, it can be observed that vertical-up welding needs a lower heat input for full penetration welding than vertical-down welding process. The hydrostatic force on keyhole wall was small for vertical-up welding process, expanding keyhole size. The inner keyhole could absorb more laser irradiated energy to increase weld penetration [12]. Through optimizing welding process parameters, the defects-free welded joints are obtained. Fig. 4 shows the plasma plume features for vertical-up and verticaldown welding process. The force behavior of keyhole in vertical welding includes, metal evaporation recoil force which promotes the formation and expansion of keyhole, hydrostatic pressure and surface tension of molten pool which pressurize keyhole to collapse. For vertical-up welding process, the molten pool is behind the laser spot position and the gravity direction is opposite to the welding direction. The keyhole expands and welding stability improves. The plasma feature experiences growing, persistence and attenuation process, indicating the evolution behavior of welding keyhole. The welding process is stable and a few spatters are formed. However, the plasma is in an unstable state for vertical-down welding process and more spatters are
3.1. Autogenously laser welding In order to investigate the welding parameters for root pass of narrow-gap vertical welding, the butt vertical welding of 5 mm plate is conducted. The weld formation, process stability and joint properties are studied to optimize the welding process. Table 2 shows the process parameters for vertical-up and vertical-down butt welding, and the weld surface topographies and cross sections of welded joints are presented in Figs. 2 and 3. Butt welding tests are performed at various laser powers and welding speeds with focal position of +2 mm in this study. From the weld surface topographies and cross sections, it can be seen that the incomplete fusion and pore are the main defects for the vertical welded joint. The pore defect is less sensitive for vertical-up welded joints than vertical-down welded joints. For vertical-up welding process, some bubbles can overflow from the interface between molten pool and wall surface of keyhole, reducing the amount of pores that retaining in the weld. However, for vertical-down welding, bubbles that float under the effect of buoyancy cannot timely move out of weld pool and form pores with weld metal solidifying. With increasing welding heat input, the incomplete fusion of welded joints can be avoided, while the larger molten pool easily collapses to undermine welding stability. Moreover,
Fig. 2. Cross sections of butt-welded joints by autogenously laser beam for vertical-up welding. 3
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Fig. 3. Cross sections of butt-welded joints by autogenously laser beam for vertical-down welding.
observed. The liquid metal of molten pool that is above keyhole flows under the gravity effect. In the case of slower welding speed, the large hydrostatic pressure of molten pool is inclined to block keyhole, which weakens welding process stability and cause the formation of pores. Besides, the higher solidification rate of laser welding process reduces the flow rate of bubbles, causing the pores exist in the weld. The tensile strength and elongation percentage of butt-welded joints for vertical-up welding and vertical-down welding are shown in Fig. 5. The optimal tensile strengths for both welded joints are approximate.
The maximum tensile strength are 681.8 MPa and 670.2 MPa, reaching 95.2% and 93.6% of base metal, respectively. Meanwhile, elongation percentage of vertical-up welded joint is slightly superior to verticaldown welded joint. The decreasing elongation percentage in verticaldown welded joint may be attributed to some existing micro-pores defect.
Fig. 4. The plasma plume features for (a-d) vertical-up welding and (e-h) vertical-down welding. 4
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Fig. 5. The tensile properties of welded joints: (a) vertical-up welded joints; (b) vertical-down welded joints.
frequency gradually decreases from plug-in mode, surface tension mode to globular mode. The droplet transfer modes of vertical-down welding are shown in Fig. 8. The surface tension transfer mode can achieve a stable welding process for both vertical-up and vertical-down welding process. The liquid filler wire can smoothly and stably transfer into weld pool. However, due to the gravity effect of molten pool, the liquid filler metal flows along the welding direction and covers the laser irradiated area. The higher hydrostatic pressure of molten pool causes the force imbalance and collapse of keyhole, causing an instable welding process with some pores and spatters. Increasing welding velocity or decreasing wire feeding speed may be an effective approach to reduce the hydrostatic pressure of molten pool. The smaller weld pool size can decrease the fluidity velocity of liquid metal and meanwhile alleviate the impact and turbulence on keyhole. The smaller filling amount decreases welding efficiency and increases welding process complexity. The fluidity of molten metal for vertical-up and vertical-down welding and the effect of beam oscillation on flowing behavior are
3.2. Process stability for LWHW The droplet transfer modes and hot-wire current curves of verticalup welding are shown in Figs. 6 and 7. The droplet transfer modes mainly depend on the intersection position between laser beam and filler wire. The transfer modes are divided into plug-in mode, surface tension mode and globular mode. For plug-in transfer mode, the filler wire is contacted with base metal and the hot-wire current is stable to preheat. However, this transfer mode requires high welding stability and easily causes melting fluctuate of filler wire. The surface tension transfer mode is the most stable welding process. The hot-wire current curve presents fluctuate feature and only the contact of filler wire and molten pool can achieve the preheating of filler wire. The globular transfer mode with a large droplet cannot achieve a stable welding process with more spatters. The droplet under the surface tension, evaporation recoil force and shielding gas blowing gas grows to a larger size, which cannot stably transfer into molten pool. The volatility of hot-wire current curve in Fig. 7 also indicates the droplet transfer
Fig. 6. The droplet transfer modes of vertical-up welding: (a) plug-in transfer mode; (b) surface tension transfer mode; (c) globular transfer mode. 5
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Fig. 7. The hot-wire current for various droplet transfer modes.
Fig. 8. The droplet transfer modes of vertical-down welding: (a) plug-in transfer mode; (b) surface tension transfer mode.
Fig. 9. The fluidity of molten pool for vertical-up welding with filler wire: (a, b) without beam oscillation; (c, d) with beam oscillation.
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Fig. 10. The fluidity of molten pool for vertical-down welding with filler wire: (a, b) without beam oscillation; (c, d) with beam oscillation.
Fig. 11. The schematic diagram of molten pool and keyhole: (a) vertical-up welding; (b) vertical-down welding.
shown in Figs. 9 and 10. During vertical welding, the liquid metal flows downward and the velocity of liquid weld metal increases by the gravity. The molten pool is narrow and elongates for conventional LWHW process because of the concentrated laser energy and gravity effect. The weld metal temperature behind keyhole is lower, which increases surface tension of weld metal and reduce the wetting behavior. The higher solidification rate of liquid metal and gravity effect prevent the backflow of liquid metal, causing the discontinuous weld bead formation. The weld with higher reinforcement and narrower width is observed. Pei et al. studied the decreasing temperature of the liquid metal behind at the rear of keyhole was the mainly reason to cause humping formation in high-speed laser flat welding. Adapting dual beam could increase the spreading ability of liquid metal and suppress the nucleation of humping by decreasing temperature gradient of weld pool [17]. Laser beam oscillation can increase laser irradiated area and reduce the laser linear energy density, which improves the heat dissipation capability and increases the viscosity of weld metal. The periodical movement of laser beam can modify fluidity of molten pool and promote the liquid metal flow into side area of weld pool, improving wetting behavior of filler wire and base metal. The temperature of weld pool becomes more even, which decreases cooling rate of weld metal and favors the fusion of groove sidewall. The weld pool under the beam oscillation is wider and shorter, suppressing the downward flowing of weld metal and promoting wetting behavior. Moreover, beam oscillation enlarges the area of keyhole and decreases
hydrostatic pressure of molten pool to keyhole, which can reduce the tendency of keyhole collapse and improve welding stability. The bubbles that are formed in weld pool are found to be deflated by beam oscillation [18]. Compared to conventional vertical-up and verticaldown welding process, it can be clear seen that vertical-up welding has a greater tendency of melt sagging, while vertical-down welding tends to form pores in welds. The main effect is caused by the size of the melt pool and time needed for it to solidify. For vertical-up welding process, the melt pool is flowing away from the laser heating area and solidifying much faster than in vertical down. The liquid cannot flow back and the hump bead easily occurs. The weld with beam oscillation is wider and continuous through the regulation of weld pool, which is considered as an effective way to eliminate these defects. The process stability for various welding positions can be explained by the force analysis of molten pool. The schematic diagram of forces on molten pool is shown in Fig. 11. There are four forces acting on the weld pool, namely the gravity (G), surface tension (Fα), shielding gas blow force (Fb) and the supporting force of solidified metal (N). Among these forces, the effects of gravity (G) and surface tension (Fα) play a decisive role in the flow of liquid weld metal. The gravity (G) is always downwards, which promotes the down flow of weld metal and destabilizes welding stability. The surface tension (Fα) promotes liquid metal flow from high-temperature area to low-temperature area. For verticalup welding, the shielding gas blow force (Fb) and the supporting force of solidified metal (N) hinder the down flow of liquid metal, while the
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Fig. 12. Cross sections for vertical-up narrow-gap welded joints: (a1-e1) laser power; (a2-e2) oscillation amplitude; (a3-e3) wire feeding speed; (a4-e4) focal position.
surface tension (Fα) facilitates the metal downward flow. However, the surface tension force (Fα) counteracts the resultant force of gravity (G) and shielding gas blow force (Fb) for vertical-down welding. Through force analysis of molten pool, it can be concluded that the stability of vertical-up welding is poor, which verifies the weld formation in above section. Beam oscillation homogenizes the temperature in weld pool, leading to the reduction of surface tension (Fα) that caused by temperature gradient. Moreover, the larger weld width increases the supporting force of solidified metal (N). Therefore, the beam oscillation technology restrains the downward flowing of welding pool. Besides the melt sagging defect, the weld pore is also a common defect for vertical welding process. The schematic diagrams of molten pool and keyhole of vertical-up and vertical-down are shown in Fig. 10. In the case of filler wire leading process, the welding direction changes the relative position of laser keyhole and molten pool. For vertical-up welding process, beam oscillation and gravity enlarge keyhole size,
which benefits laser beam arrive the keyhole inside. Keyhole morphology is stable and weld surface quality is improved. However, the liquid molten metal under the gravity flows downward, causing the wall surface of keyhole produces large fluctuate for vertical-down welding. Keyhole is contracted and laser beam energy is unable to maintain a stable keyhole state, resulting in imbalance force of keyhole. Peng et al. found that the filling amount of filler metal increased hydrostatic pressure (gravity) of liquid molten metal, which decreased keyhole stability. The keyhole tended to collapse due to the imbalance force of keyhole wall in flat position [19]. The keyhole collapse will lead to the formation of bubbles in weld. The bubbles cannot overflow and form pores as a results of high solidification rate of molten metal. It can be seen that process stability of vertical-up welding is better than vertical-down welding during LWHW.
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Fig. 13. Weld geometries for vertical-up narrow-gap welded joints: (a1-e1) laser power; (a2-e2) oscillation amplitude; (a3-e3) wire feeding speed; (a4-e4) focal position.
under the effect of gravity tends to flow downward to cause unstable welding process and deteriorates weld formation. Besides, higher wire feed speed absorbs more laser energy to cause fluctuation of laser energy into keyhole, which also affects weld process stability. The larger focal position decreases laser energy density, causing a shallow weld penetration. Larger focal position with beam oscillation greatly increases laser spot, which leads to interference between laser beam and sidewalls in narrower groove. Moreover, it can be seen that all pores present circular morphology with a large size from weld cross section, which is a typically process pore that is caused by unstable welding process and keyhole collapse. The keyhole was blocked and the entrapped shielding gas could not overflow from the weld pool, increasing the forming tendency of pores. The cross-sections and weld geometries for vertical-down welded joints with various laser powers and wire feeding speeds are shown in Fig. 14. It can be clearly seen that all the welds present concave shape, indicating the filler wire sufficiently wets the groove sidewalls. In Fig. 14(a1), the weld penetration is deep and some pores are observed for conventional laser welding. The welds are shallower and wider with beam oscillation. Increasing laser power promotes interlayer fusion, while a higher wire feeding speed deteriorates welding stability and some pores are formed. Moreover, compared to vertical-up welds, the welds are shallower and wider for vertical-down welds, which is because the melt sagging makes the keyhole contracted. Therefore, lower laser energy is absorbed and conducted in deep orientation. Due to the
3.3. Narrow-gap weld geometry characteristics Fig. 12 shows the weld cross section for narrow-gap vertical-up welded joints and the weld geometries, such as weld width, penetration and reinforcement, are shown in Fig. 13. Laser power, oscillation amplitude and focal position decide the relative laser energy density, while wire feeding speed decides the filling amount. Xu et al. found that the swing arc could control the stability of weld pool by dispersing arc energy, improving heat dissipation capacity and increasing sidewall fusion during all-position welding process [6]. Thus a stable molten pool can be achieved through controlling welding heat input and molten pool size. With the increase of laser power, the welding mode is transformed from conduction mode to deep penetration mode. Weld penetration increases, while weld width basically remains unchanged because the laser energy transfers to the depth direction. However, when laser power is lower than 4200 W, the shallower weld penetration is easily to cause incomplete fusion between weld passes. Higher laser power of 4700 W easily leads to the formation of undercut and pore defects. Increasing oscillation amplitudes can decrease laser energy density and increase laser irradiated area. Therefore, weld penetration decreases and width increases. Moreover, the stirring effect of laser beam oscillation in molten pool promotes the concave weld morphology, indicating good wetting behavior between filler wire and groove sidewalls. Higher wire feeding speed increases wire filling efficiency, meanwhile enlarges the molten pool size. The molten pool 9
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Fig. 14. Cross sections for vertical-down narrow-gap welded joints: (a1-e1) laser power; (a2-e2) wire feeding speed; (a3) Weld geometries in various laser powers; (a4) Weld geometries in various wire feeding speeds. Table 3 The welding parameters for narrow-gap vertical-up welded joint. Welding pass (No.)
Laser power (kW)
Welding speed (m/min)
Wire feeding speed (m/min)
Focal position (mm)
Oscillation amplitude (mm)
Root pass Filler passes
4.5 4.2–4.7
1.2 0.3
– 3.8–4.2
+5 +20
– 1.5–2.0
Table 4 The welding parameters for narrow-gap vertical-down welded joint. Welding pass (No.)
Laser power (kW)
Welding speed (m/min)
Wire feeding speed (m/min)
Focal position (mm)
Oscillation amplitude (mm)
Root pass Filler passes
4.5 4.5–4.9
1.4 0.3
– 4.0–4.2
+5 +20
– 1.5–2.0
weld cross section is presented in Fig. 15. The groove root of 5.0 mm, single groove bottom width of 1.8 mm and groove angle of 3 degree are designed. The multi-layer welded joint includes one root pass and five filler passes. Each laser filler pass is about 3.5–4.0 mm.
shallower welds, the pore tendency in vertical-down welding is lower than vertical-up welding process. The optimum welding process parameters of narrow-gap welding 20-mm plate in this study are shown in Table 3 and Table 4. And the 10
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which is attributed to the weld microstructure by various welding heat input. The tensile strength and elongation percentage of filler passes are closely approximate, while the tensile strength of middle layer is slightly higher than upper layer. This is because of the different grain sizes between filler layers caused by various thermal cycles. The multi thermal cycle modifies grain growth orientation and some finer grains are found in the overlapped zones. Zhang et al. considered that the different mechanical properties at various layers were closely related to the grain size [20]. The bottom layer has the highest tensile strength and elongation percentage because the tensile samples of bottom layer are located at autogenously laser welding zone. However, it can be seen that higher tensile strength of bottom layer is lower than butt-welded joints in Section 3.1, which is attributed to the effect of next weld thermal cycle on weld microstructural evolution during multi-layer welding process. From Fig. 17, it is clear that the elongation rate of bottom layer is much higher than that of upper layer and middle layer. The reason can be explained from fracture morphologies that ductile fracture mode occurs during tensile test and the dimples are larger and deeper in bottom layer in Fig. 18. Moreover, cleavage fracture with some trapezoidal steps is observed in upper and middle layer, which leads to the decrease of elongation percentage. 4. Conclusions
Fig. 15. The weld cross section of 20 mm thick welded joint: (a) vertical-up welding; (b) vertical-down welding.
1. Vertical-up laser welding has lower sensitivity to pores than verticaldown welding. Increasing welding speed could improve welding stability and reduce the formation possibility of pores. 2. Surface tension transfer mode can achieve effective preheating of filler wire and a stable welding process. Beam oscillation increases laser irradiated area and homogenizes temperature of weld pool, which promotes wetting behavior of weld metal and decreases the tendency of melt sagging during vertical welding. 3. The 20-mm defect-free joint is obtained by vertical-up and verticaldown narrow-gap laser oscillation welding with hot wire. The tensile strength and elongation percentage are increased from face layer, middle layer and bottom layer, which is attributed to the weld microstructure by various welding heat input. Ductile fracture mode
Form Fig. 16, it can be observed that the transverse shrinkage of vertical-down welded joint is larger than that of vertical-up welded joint due to the different welding heat inputs. The filling metal has the ability to resist the transverse shrinkage, thus the groove width is basically unchanged after the fourth weld pass. The larger groove shrinkage and higher wire feeding speed improve the wire filling efficiency to increase weld reinforcement of each layer for vertical-down welding. The tensile strength and elongation percentage of different layers are shown in Fig. 17. Both the tensile strength and elongation percentage are increased from face layer, middle layer and bottom layer,
Fig. 16. The transverse shrinkage of groove and remaining groove depth during multi-layer welding. 11
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Fig. 17. The tensile strength and elongation percentage of 20 mm thick welded joint: (a) vertical-up welded joint; (b) vertical-down welded joint.
Fig. 18. The fracture morphologies of tensile samples: (a-c) vertical-up welded joint; (d-f) vertical-down welded joint.
occurs for all tensile samples with dimples.
welding positions, J Mater. Process Tech. 213 (2013) 1640–1652. [4] S. Fujinaga, R. Ohashi, T. Urakami, S. Katayama, A. Matsunawa, Development of an all-position YAG laser butt welding process with addition of filler wire, Weld. Int. 19 (2005) 441–446. [5] R. Li, T. Wang, C. Wang, F. Yan, X.Y. Shao, X.Y. Hu, J.M. Li, A study of narrow gap laser welding for thick plates using the multi-layer and multi-pass method, Opt. Laser Technol. 64 (2014) 172–183. [6] W.H. Xu, S.B. Lin, C.L. Fan, C.L. Yang, Prediction and optimization of weld bead geometry in oscillating arc narrow gap all-position GMA welding, Int. J Adv. Manuf. Tech. 79 (2015) 183–196. [7] W.X. Yang, J.J. Xin, C. Fang, W.H. Dai, J. Wei, J.F. Wu, Y.T. Song, Microstructure and mechanical properties of ultra-narrow gap laser weld joint of 100 mm-thick SUS304 steel plates, J Mater. Process. Tech. 265 (2019) 130–137. [8] G. Ma, L. Li, Y. Chen, Effects of beam configurations on wire melting and transfer behaviors in dual beam laser welding with filler wire, Opt. Laser Technol. 91 (2017) 138–148. [9] Y. Yamazaki, Y. Abe, Y. Hioki, M. Nakatani, A. Kitagawa, K. Nakata, Fundamental study of narrow-gap welding with oscillation laser beam, Weld. Int. 30 (2016) 699–707. [10] J. Sun, K. Feng, K. Zhang, B. Guo, E. Jiang, P. Nie, J. Huang, Z. Li, Fiber laser welding of thick AISI 304 plate in a horizontal (2G) butt joint configuration, Mater. Des. 118 (2017) 53–65. [11] G. Xu, L. Lin, J. Wang, J. Zhu, P. Li, Study of weld formation in swing arc narrow gap vertical GMA welding by numerical modeling and experiment, Int. J Adv. Manuf. Tech. 96 (2018) 1905–1917. [12] B. Chang, Z. Yuan, H. Pu, H. Li, H. Cheng, D. Du, J. Shan, Study of gravity effects on titanium laser welding in the vertical position, Materials 10 (2017) 1031. [13] M. Sohail, S.W. Han, S.J. Na, A. Gumenyuk, M. Rethmeier, Numerical investigation of energy input characteristics for high-power fiber laser welding at different positions, Int. J Adv. Manuf. Tech. 80 (2015) 931–946. [14] M. Zhang, G. Chen, Y. Zhou, S. Li, Direct observation of keyhole characteristics in deep penetration laser welding with a 10 kW fiber laser, Opt. Express 21 (2013) 19997.
Declaration of Competing Interest None. Acknowledgments This project is supported by National Key Research and Development Program of China (2016YFB0300602) and the National Natural Science Foundation of China (Grant No. U1960102, 51705103, 51475104). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105921. References [1] J.C. Feng, D.W. Rathod, M.J. Roy, J.A. Francis, W. Guo, N.M. Irvine, A.N. Vasileiou, Y.L. Sun, M.C. Smith, L. Li, An evaluation of multipass narrow gap laser welding as a candidate process for the manufacture of nuclear pressure vessels, Int. J Pres. Ves. Pip. 157 (2017) 43–50. [2] Y. Zhao, S. Ma, J. Huang, Y. Wu, Narrow-gap laser welding using filler wire of thick steel plates, Int. J Adv. Manuf. Tech. 93 (2017) 2955–2962. [3] D.W. Cho, S.J. Na, M.H. Cho, J.S. Lee, A study on V-groove GMAW for various
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means of laser beam oscillation during remote welding of AlMgSi, Opt. Lasers Eng. 108 (2018) 68–77. [19] J. Peng, S. Hu, X. Wang, J. Wang, F. Zhang, Effect of filler metal on three-dimensional transient behavior of keyholes and molten pools in laser welding, Chinese J. Laser 45 (2018) 1–10. [20] X. Zhang, G. Mi, L. Chen, P. Jiang, X. Shao, C. Wang, Microstructure and performance of hybrid laser-arc welded 40 mm thick 316 L steel plates, J Mater. Process. Tech. 259 (2018) 312–319.
[15] Y. Zhao, K. Zhu, Q. Ma, Q. Shang, J. Huang, D. Yang, Plasma behavior and control with small diameter assisting gas nozzle during CO2 laser welding, J Mater. Process. Tech. 237 (2016) 208–215. [16] J. Nasstrom, J. Frostevarg, A.F.H. Kaplan, Multipass laser hot-wire welding Morphology and process robustness, J. Laser Appl. 29 (2017) 1–7. [17] Y. Pei, A. Wu, J. Shan, J. Ren, Investigation of humping formation based on melt flow analysis in high-speed laser welding process, Acta Metall. Sin. 49 (2013) 725–730. [18] F. Fetzer, M. Sommer, R. Weber, J.P. Weberpals, T. Graf, Reduction of pores by
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Full length article
Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant ⁎
Junzhao Lia,b, Qingjie Suna,b, , Kexin Kangb, Zuyang Zhenb, Yibo Liua,b, Jicai Fenga,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 West Wenhua Road, Weihai 264209, China
H I GH L IG H T S
welding with filler wire was applied in multi-layer narrow-gap welding in vertical positions. • Laser process stability and weld defects were studied and compared in laser vertical-up and vertical-down welding. • The droplet transfer behavior and fluidity of molten pool were investigated for laser vertical welding. • The welding with beam oscillation could increase laser irradiated area and homogenize temperature in molten pool. • Laser • Weld formation was improved and 20-mm vertical welded joint was obtained without defects by laser welding.
A R T I C LE I N FO
A B S T R A C T
Keywords: Large structures Narrow gap welding Laser vertical welding Beam oscillation Process stability
Incomplete fusion, pore and melt sagging are common defects for multi-layer narrow-gap laser welding with filler wire in vertical positions for large and heavy structures. This investigation showed that vertical-up laser welding had a lower sensitivity to pores than vertical-down welding, whereas increasing welding speed could improve welding stability and reduce the formation possibility of pores. For laser vertical welding with hot wire, surface tension transfer mode could effectively guarantee pre-heating of filler wire and achieve a stable welding process. The keyhole stability and melt flow varied with welding directions. The melt sagging defect was usually observed and mainly caused by the concentrated laser energy, which easily led to discontinuous weld bead. Laser beam oscillation technology increased laser melting area and promoted wetting behavior of filler wire with base metal. The weld width increased and concave weld metal surface was obtained with suitable process parameters. Narrow-gap welding process could be carried out at a small defocusing position with a lower laser power, further decreasing tendency of melt slagging. The 20-mm defect-free joint was obtained by vertical-up and verticaldown narrow-gap laser oscillation welding with hot wire.
1. Introduction Mid-thick austentic stainless steels with good corrosion resistance and mechanical properties are widely used in the shipbuilding, nuclear plant and container manufacturing industries. Meanwhile, narrow-gap laser welding with filler wire (NG-LWFW) process has been increasingly adopted in manufacturing large engineering components because of narrower groove, lower welding heat input and higher productivity [1,2]. For the structures with complex shape, various welding positions, especially vertical welding (vertical-up and vertical-down) are required to accommodate the space orientations of the welds. However, welding process parameters and stability vary with welding positions, which further affects weld formation and quality. Thus, it is essential to study
⁎
the effect of welding positions on laser welding characteristics to successfully achieve high-quality welded joints. Welding in vertical position has a wide application, and sometimes is inevitable in manufacture of large and heavy structures. Several common issues, such as incomplete fusion, pores, instable droplet transfer and molten pool would occur in all-position welding of thick components [3–5]. Especially, multi-layer laser welding thick plate in vertical position is difficult due to the imbalances of hydrostatic pressure (gravity), surface tension and recoil pressure of metal vapor. However, for MAG and TIG arc welding, the arc force acting on molten pool can be used to balance the effect of gravity and the lower heat input promotes to maintain stable molten pool. Swing arc and rotating arc welding technology also have been developed to reduce linear
Corresponding author at: Harbin Institute of Technology at Weihai, No. 2 West Wenhua Road, Weihai 264209, China. E-mail address: [email protected] (Q. Sun).
https://doi.org/10.1016/j.optlastec.2019.105921 Received 18 May 2019; Received in revised form 12 September 2019; Accepted 21 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Junzhao Li, et al., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105921
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welding heat input [6]. Moreover, laser welding with characteristics of fast solidification rate and lower welding heat input is able to reduce the downward trend of molten pool and obtain the defect-free narrowgap welded joint. Most studies on flat and horizontal NG-LWFW of heavy structures have been advancing. During multi-layer narrow-gap welding, defocusing laser beam was usually adopted to increase laser irradiated area and avoid the incomplete fusion of groove sidewalls [7]. Dual laser beam welding and oscillation laser welding with filler wire were also studied to improve effective area of laser beam, which could solve the misalignment between single laser beam and filler wire [8,9]. The application of laser welding with hot wire (LWHW) process on horizontal narrow-gap welding was investigated and melt slagging defect was prevented due to the gravity direction [10]. Fujinaga et al. used rectangularly modulated laser beam with filler wire to all-position butt welding and found that the evaporation recoil force could reduce spatters [4]. Besides, Xu et al. investigated arc swing parameters on narrow gap MAG weld formation and demonstrated that weld pool sagging defect was mainly related to arc force impulse and arc linear energy [11]. Chang et al. found pores in vertical welding were mainly attributed to the collapsing keyhole and turbulent fluid flow of weld metal [12]. The autogenously laser vertical-up welding promoted the bubbles move out from the keyhole surface. Sohail et al. revealed that the molten metal flowed periodically, which caused gravity at different positions had little effect on flow behavior and bead geometry by numerical investigation [13]. However, investigations on narrow-gap laser vertical welding with filler wire were insufficient, particularly for the regulation of welding process stability. High speed camera has been widespread used to observe welding process characteristics such as plasma plume, droplet transfer mode and weld pool fluidity, which reflected the stability of welding process. Through observation of droplet transfer process in groove, the bead formation was more sensitive to beam position [2]. Zhang et al. observed the keyhole dynamics during laser welding through a high borosilicate glass. The hydrodynamics of keyhole wall affected weld formation and was sensitive to weld defects [14]. Zhao et al. indicated that the side shielding gas suppressed the laser-induced plasma and improved utilization of laser power. Plasma features was related to the gas flow rate and gas composition [15]. Nasstrom et al. studied the melt pool feature and process robustness of hot-wire laser welding and found that arc formation between wire tip and groove sidewalls greatly deteriorated welding stability [16]. Moreover, Yamazaki et al. found that the cold filler wire was intermittently heated and fused with laser beam oscillation. The wire components may become segregated in the bead, which destroyed the stable welding process and even affected the joint quality [9]. Beam oscillation technology could effectively suppress incomplete fusion of groove sidewalls. Above-mentioned researches indicate that narrow-gap laser vertical welding has great potential to improve industrial adaptability by its own features such as special energy flux distribution, weld morphologies and welding efficiency. Relevant studies about NG-LWHW mainly focused on flat position. However, existing researches on the positional NG-LWHW are still very limited and the inherent mechanism of laser welding process stability is still unavailable. Considering that welding of large structures in vertical position is sometimes inevitable, this paper aims to investigate the laser vertical welding feature, including vertical-up and vertical-down positions, and to reveal the influence of welding positions on process stability and joint quality.
Table 1 The chemical compositions and mechanical properties of 316L stainless steel alloy and filler wire (at.%).
316L ER316L
C
Mn
P
S
Si
Cr
Ni
Mo
Fe
Tensile strength
0.02 0.02
1.58 1.89
0.02 0.02
– 0.01
0.45 0.76
16.74 18.7
12.89 12.2
2.05 2.3
Bal. Bal.
716 MPa 570 MPa
Fig. 1. Scheme of the experimental setup for narrow gap laser welding.
process parameters for root pass welding of narrow gap welding. Beadon-plate vertical welding experiments were carried out to investigate the weld geometry, wire melting phenomena and molten pool feature with various process parameters. Then single-track of narrow-gap welding experiments were conducted to study the effect of laser welding parameters on a narrow groove, which provided the basis for narrow-gap welding parameter selection. Fig. 1 shows a scheme of the experimental setup for vertical NGLWHW process. Sample plates were automatically welded using a 6 kW IPG YLS-6000 fiber laser power source with a six-axis robotic welder. The wavelength of the operating laser was 1070 nm and the BPP beam parameter product was 6.4 mm mrad. The IPG D50 Wobble welding head consisted of a collimation unit with focal length 200 mm, a galvanometer scanner unit and an f-θ focusing unit with focal length 150 mm. The beam oscillation was controlled by the galvanometer scanner and could be oscillated up to maximum frequency of 1000 Hz and amplitude of 2 mm. A Fronius heater source was used to provide resistance heat for the filler wire. The hot-wire could improve the utilization factor of laser energy and make more laser energy to melt the base metal. To reveal the behavior of molten pool and the melting phenomena of filler wire, a high-speed video camera was used to discover the filler melting phenomena and molten pool behavior. A background laser (wavelength: 808 nm) was used for illumination and an interference filter was adopted. The welded sample plates were sectioned transversely relative to the welding direction and prepared for metallurgical examination. After etching in a solution of aqua regia (FeCl3: HCl: H2O = 10 g: 30 mL: 120 mL), microstructural observation was carried out by light microscope. Standard tensile coupons were performed with tensile rate of 2 mm/min for the purpose of determining the cross-weld tensile properties. The tensile samples with 3 mm thickness were cut from different layer positions. Three tensile samples were performed, and the average value was taken to evaluate the tensile strength. The fractured surfaces of tensile specimens were observed by scanning electron microscope.
2. Experimental methods and procedures Base metal was 316L stainless steel and the type of filler wire was ER316L of 1.0 mm in diameter. The chemical compositions and mechanical properties of the 316L stainless steel and ER316L filler wire are shown in Table 1. The elongation rate of 316L base material is 62%. Butt-welding process used 5.0 mm steel plate to optimize welding 2
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Table 2 The welding parameters for butt welded joints. Welding direction Vertical-up Vertical-up Vertical-up Vertical-up Vertical-up
welding welding welding welding welding
Vertical-down Vertical-down Vertical-down Vertical-down Vertical-down
(a) (b) (c) (d) (e)
welding welding welding welding welding
(a) (b) (c) (d) (e)
Laser power (W)
Welding speed (m/min)
Heat input (J/mm)
Weld feature
3000 3500 4200 3000 3000
1.2 1.2 1.2 0.8 1.0
150 175 210 225 180
Incomplete fusion Incomplete fusion Defect-free joint Defect-free joint Pore defect
3000 3000 4200 4200 4200
0.8 1.0 1.0 1.2 1.4
225 180 252 210 180
Pore defect Incomplete fusion Pore defect Bead collapse Defect-free joint
3. Results and discussion
increasing welding speed can significantly reduce the amount of pores for both vertical-up and vertical-down welding, which attributes to the stable molten pool and keyhole morphology. It can be seen that a larger laser power and a higher welding speed can improve weld quality and decrease pore tendency for vertical-down welded joint. Moreover, it can be observed that vertical-up welding needs a lower heat input for full penetration welding than vertical-down welding process. The hydrostatic force on keyhole wall was small for vertical-up welding process, expanding keyhole size. The inner keyhole could absorb more laser irradiated energy to increase weld penetration [12]. Through optimizing welding process parameters, the defects-free welded joints are obtained. Fig. 4 shows the plasma plume features for vertical-up and verticaldown welding process. The force behavior of keyhole in vertical welding includes, metal evaporation recoil force which promotes the formation and expansion of keyhole, hydrostatic pressure and surface tension of molten pool which pressurize keyhole to collapse. For vertical-up welding process, the molten pool is behind the laser spot position and the gravity direction is opposite to the welding direction. The keyhole expands and welding stability improves. The plasma feature experiences growing, persistence and attenuation process, indicating the evolution behavior of welding keyhole. The welding process is stable and a few spatters are formed. However, the plasma is in an unstable state for vertical-down welding process and more spatters are
3.1. Autogenously laser welding In order to investigate the welding parameters for root pass of narrow-gap vertical welding, the butt vertical welding of 5 mm plate is conducted. The weld formation, process stability and joint properties are studied to optimize the welding process. Table 2 shows the process parameters for vertical-up and vertical-down butt welding, and the weld surface topographies and cross sections of welded joints are presented in Figs. 2 and 3. Butt welding tests are performed at various laser powers and welding speeds with focal position of +2 mm in this study. From the weld surface topographies and cross sections, it can be seen that the incomplete fusion and pore are the main defects for the vertical welded joint. The pore defect is less sensitive for vertical-up welded joints than vertical-down welded joints. For vertical-up welding process, some bubbles can overflow from the interface between molten pool and wall surface of keyhole, reducing the amount of pores that retaining in the weld. However, for vertical-down welding, bubbles that float under the effect of buoyancy cannot timely move out of weld pool and form pores with weld metal solidifying. With increasing welding heat input, the incomplete fusion of welded joints can be avoided, while the larger molten pool easily collapses to undermine welding stability. Moreover,
Fig. 2. Cross sections of butt-welded joints by autogenously laser beam for vertical-up welding. 3
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Fig. 3. Cross sections of butt-welded joints by autogenously laser beam for vertical-down welding.
observed. The liquid metal of molten pool that is above keyhole flows under the gravity effect. In the case of slower welding speed, the large hydrostatic pressure of molten pool is inclined to block keyhole, which weakens welding process stability and cause the formation of pores. Besides, the higher solidification rate of laser welding process reduces the flow rate of bubbles, causing the pores exist in the weld. The tensile strength and elongation percentage of butt-welded joints for vertical-up welding and vertical-down welding are shown in Fig. 5. The optimal tensile strengths for both welded joints are approximate.
The maximum tensile strength are 681.8 MPa and 670.2 MPa, reaching 95.2% and 93.6% of base metal, respectively. Meanwhile, elongation percentage of vertical-up welded joint is slightly superior to verticaldown welded joint. The decreasing elongation percentage in verticaldown welded joint may be attributed to some existing micro-pores defect.
Fig. 4. The plasma plume features for (a-d) vertical-up welding and (e-h) vertical-down welding. 4
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Fig. 5. The tensile properties of welded joints: (a) vertical-up welded joints; (b) vertical-down welded joints.
frequency gradually decreases from plug-in mode, surface tension mode to globular mode. The droplet transfer modes of vertical-down welding are shown in Fig. 8. The surface tension transfer mode can achieve a stable welding process for both vertical-up and vertical-down welding process. The liquid filler wire can smoothly and stably transfer into weld pool. However, due to the gravity effect of molten pool, the liquid filler metal flows along the welding direction and covers the laser irradiated area. The higher hydrostatic pressure of molten pool causes the force imbalance and collapse of keyhole, causing an instable welding process with some pores and spatters. Increasing welding velocity or decreasing wire feeding speed may be an effective approach to reduce the hydrostatic pressure of molten pool. The smaller weld pool size can decrease the fluidity velocity of liquid metal and meanwhile alleviate the impact and turbulence on keyhole. The smaller filling amount decreases welding efficiency and increases welding process complexity. The fluidity of molten metal for vertical-up and vertical-down welding and the effect of beam oscillation on flowing behavior are
3.2. Process stability for LWHW The droplet transfer modes and hot-wire current curves of verticalup welding are shown in Figs. 6 and 7. The droplet transfer modes mainly depend on the intersection position between laser beam and filler wire. The transfer modes are divided into plug-in mode, surface tension mode and globular mode. For plug-in transfer mode, the filler wire is contacted with base metal and the hot-wire current is stable to preheat. However, this transfer mode requires high welding stability and easily causes melting fluctuate of filler wire. The surface tension transfer mode is the most stable welding process. The hot-wire current curve presents fluctuate feature and only the contact of filler wire and molten pool can achieve the preheating of filler wire. The globular transfer mode with a large droplet cannot achieve a stable welding process with more spatters. The droplet under the surface tension, evaporation recoil force and shielding gas blowing gas grows to a larger size, which cannot stably transfer into molten pool. The volatility of hot-wire current curve in Fig. 7 also indicates the droplet transfer
Fig. 6. The droplet transfer modes of vertical-up welding: (a) plug-in transfer mode; (b) surface tension transfer mode; (c) globular transfer mode. 5
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Fig. 7. The hot-wire current for various droplet transfer modes.
Fig. 8. The droplet transfer modes of vertical-down welding: (a) plug-in transfer mode; (b) surface tension transfer mode.
Fig. 9. The fluidity of molten pool for vertical-up welding with filler wire: (a, b) without beam oscillation; (c, d) with beam oscillation.
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Fig. 10. The fluidity of molten pool for vertical-down welding with filler wire: (a, b) without beam oscillation; (c, d) with beam oscillation.
Fig. 11. The schematic diagram of molten pool and keyhole: (a) vertical-up welding; (b) vertical-down welding.
shown in Figs. 9 and 10. During vertical welding, the liquid metal flows downward and the velocity of liquid weld metal increases by the gravity. The molten pool is narrow and elongates for conventional LWHW process because of the concentrated laser energy and gravity effect. The weld metal temperature behind keyhole is lower, which increases surface tension of weld metal and reduce the wetting behavior. The higher solidification rate of liquid metal and gravity effect prevent the backflow of liquid metal, causing the discontinuous weld bead formation. The weld with higher reinforcement and narrower width is observed. Pei et al. studied the decreasing temperature of the liquid metal behind at the rear of keyhole was the mainly reason to cause humping formation in high-speed laser flat welding. Adapting dual beam could increase the spreading ability of liquid metal and suppress the nucleation of humping by decreasing temperature gradient of weld pool [17]. Laser beam oscillation can increase laser irradiated area and reduce the laser linear energy density, which improves the heat dissipation capability and increases the viscosity of weld metal. The periodical movement of laser beam can modify fluidity of molten pool and promote the liquid metal flow into side area of weld pool, improving wetting behavior of filler wire and base metal. The temperature of weld pool becomes more even, which decreases cooling rate of weld metal and favors the fusion of groove sidewall. The weld pool under the beam oscillation is wider and shorter, suppressing the downward flowing of weld metal and promoting wetting behavior. Moreover, beam oscillation enlarges the area of keyhole and decreases
hydrostatic pressure of molten pool to keyhole, which can reduce the tendency of keyhole collapse and improve welding stability. The bubbles that are formed in weld pool are found to be deflated by beam oscillation [18]. Compared to conventional vertical-up and verticaldown welding process, it can be clear seen that vertical-up welding has a greater tendency of melt sagging, while vertical-down welding tends to form pores in welds. The main effect is caused by the size of the melt pool and time needed for it to solidify. For vertical-up welding process, the melt pool is flowing away from the laser heating area and solidifying much faster than in vertical down. The liquid cannot flow back and the hump bead easily occurs. The weld with beam oscillation is wider and continuous through the regulation of weld pool, which is considered as an effective way to eliminate these defects. The process stability for various welding positions can be explained by the force analysis of molten pool. The schematic diagram of forces on molten pool is shown in Fig. 11. There are four forces acting on the weld pool, namely the gravity (G), surface tension (Fα), shielding gas blow force (Fb) and the supporting force of solidified metal (N). Among these forces, the effects of gravity (G) and surface tension (Fα) play a decisive role in the flow of liquid weld metal. The gravity (G) is always downwards, which promotes the down flow of weld metal and destabilizes welding stability. The surface tension (Fα) promotes liquid metal flow from high-temperature area to low-temperature area. For verticalup welding, the shielding gas blow force (Fb) and the supporting force of solidified metal (N) hinder the down flow of liquid metal, while the
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Fig. 12. Cross sections for vertical-up narrow-gap welded joints: (a1-e1) laser power; (a2-e2) oscillation amplitude; (a3-e3) wire feeding speed; (a4-e4) focal position.
surface tension (Fα) facilitates the metal downward flow. However, the surface tension force (Fα) counteracts the resultant force of gravity (G) and shielding gas blow force (Fb) for vertical-down welding. Through force analysis of molten pool, it can be concluded that the stability of vertical-up welding is poor, which verifies the weld formation in above section. Beam oscillation homogenizes the temperature in weld pool, leading to the reduction of surface tension (Fα) that caused by temperature gradient. Moreover, the larger weld width increases the supporting force of solidified metal (N). Therefore, the beam oscillation technology restrains the downward flowing of welding pool. Besides the melt sagging defect, the weld pore is also a common defect for vertical welding process. The schematic diagrams of molten pool and keyhole of vertical-up and vertical-down are shown in Fig. 10. In the case of filler wire leading process, the welding direction changes the relative position of laser keyhole and molten pool. For vertical-up welding process, beam oscillation and gravity enlarge keyhole size,
which benefits laser beam arrive the keyhole inside. Keyhole morphology is stable and weld surface quality is improved. However, the liquid molten metal under the gravity flows downward, causing the wall surface of keyhole produces large fluctuate for vertical-down welding. Keyhole is contracted and laser beam energy is unable to maintain a stable keyhole state, resulting in imbalance force of keyhole. Peng et al. found that the filling amount of filler metal increased hydrostatic pressure (gravity) of liquid molten metal, which decreased keyhole stability. The keyhole tended to collapse due to the imbalance force of keyhole wall in flat position [19]. The keyhole collapse will lead to the formation of bubbles in weld. The bubbles cannot overflow and form pores as a results of high solidification rate of molten metal. It can be seen that process stability of vertical-up welding is better than vertical-down welding during LWHW.
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Fig. 13. Weld geometries for vertical-up narrow-gap welded joints: (a1-e1) laser power; (a2-e2) oscillation amplitude; (a3-e3) wire feeding speed; (a4-e4) focal position.
under the effect of gravity tends to flow downward to cause unstable welding process and deteriorates weld formation. Besides, higher wire feed speed absorbs more laser energy to cause fluctuation of laser energy into keyhole, which also affects weld process stability. The larger focal position decreases laser energy density, causing a shallow weld penetration. Larger focal position with beam oscillation greatly increases laser spot, which leads to interference between laser beam and sidewalls in narrower groove. Moreover, it can be seen that all pores present circular morphology with a large size from weld cross section, which is a typically process pore that is caused by unstable welding process and keyhole collapse. The keyhole was blocked and the entrapped shielding gas could not overflow from the weld pool, increasing the forming tendency of pores. The cross-sections and weld geometries for vertical-down welded joints with various laser powers and wire feeding speeds are shown in Fig. 14. It can be clearly seen that all the welds present concave shape, indicating the filler wire sufficiently wets the groove sidewalls. In Fig. 14(a1), the weld penetration is deep and some pores are observed for conventional laser welding. The welds are shallower and wider with beam oscillation. Increasing laser power promotes interlayer fusion, while a higher wire feeding speed deteriorates welding stability and some pores are formed. Moreover, compared to vertical-up welds, the welds are shallower and wider for vertical-down welds, which is because the melt sagging makes the keyhole contracted. Therefore, lower laser energy is absorbed and conducted in deep orientation. Due to the
3.3. Narrow-gap weld geometry characteristics Fig. 12 shows the weld cross section for narrow-gap vertical-up welded joints and the weld geometries, such as weld width, penetration and reinforcement, are shown in Fig. 13. Laser power, oscillation amplitude and focal position decide the relative laser energy density, while wire feeding speed decides the filling amount. Xu et al. found that the swing arc could control the stability of weld pool by dispersing arc energy, improving heat dissipation capacity and increasing sidewall fusion during all-position welding process [6]. Thus a stable molten pool can be achieved through controlling welding heat input and molten pool size. With the increase of laser power, the welding mode is transformed from conduction mode to deep penetration mode. Weld penetration increases, while weld width basically remains unchanged because the laser energy transfers to the depth direction. However, when laser power is lower than 4200 W, the shallower weld penetration is easily to cause incomplete fusion between weld passes. Higher laser power of 4700 W easily leads to the formation of undercut and pore defects. Increasing oscillation amplitudes can decrease laser energy density and increase laser irradiated area. Therefore, weld penetration decreases and width increases. Moreover, the stirring effect of laser beam oscillation in molten pool promotes the concave weld morphology, indicating good wetting behavior between filler wire and groove sidewalls. Higher wire feeding speed increases wire filling efficiency, meanwhile enlarges the molten pool size. The molten pool 9
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Fig. 14. Cross sections for vertical-down narrow-gap welded joints: (a1-e1) laser power; (a2-e2) wire feeding speed; (a3) Weld geometries in various laser powers; (a4) Weld geometries in various wire feeding speeds. Table 3 The welding parameters for narrow-gap vertical-up welded joint. Welding pass (No.)
Laser power (kW)
Welding speed (m/min)
Wire feeding speed (m/min)
Focal position (mm)
Oscillation amplitude (mm)
Root pass Filler passes
4.5 4.2–4.7
1.2 0.3
– 3.8–4.2
+5 +20
– 1.5–2.0
Table 4 The welding parameters for narrow-gap vertical-down welded joint. Welding pass (No.)
Laser power (kW)
Welding speed (m/min)
Wire feeding speed (m/min)
Focal position (mm)
Oscillation amplitude (mm)
Root pass Filler passes
4.5 4.5–4.9
1.4 0.3
– 4.0–4.2
+5 +20
– 1.5–2.0
weld cross section is presented in Fig. 15. The groove root of 5.0 mm, single groove bottom width of 1.8 mm and groove angle of 3 degree are designed. The multi-layer welded joint includes one root pass and five filler passes. Each laser filler pass is about 3.5–4.0 mm.
shallower welds, the pore tendency in vertical-down welding is lower than vertical-up welding process. The optimum welding process parameters of narrow-gap welding 20-mm plate in this study are shown in Table 3 and Table 4. And the 10
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which is attributed to the weld microstructure by various welding heat input. The tensile strength and elongation percentage of filler passes are closely approximate, while the tensile strength of middle layer is slightly higher than upper layer. This is because of the different grain sizes between filler layers caused by various thermal cycles. The multi thermal cycle modifies grain growth orientation and some finer grains are found in the overlapped zones. Zhang et al. considered that the different mechanical properties at various layers were closely related to the grain size [20]. The bottom layer has the highest tensile strength and elongation percentage because the tensile samples of bottom layer are located at autogenously laser welding zone. However, it can be seen that higher tensile strength of bottom layer is lower than butt-welded joints in Section 3.1, which is attributed to the effect of next weld thermal cycle on weld microstructural evolution during multi-layer welding process. From Fig. 17, it is clear that the elongation rate of bottom layer is much higher than that of upper layer and middle layer. The reason can be explained from fracture morphologies that ductile fracture mode occurs during tensile test and the dimples are larger and deeper in bottom layer in Fig. 18. Moreover, cleavage fracture with some trapezoidal steps is observed in upper and middle layer, which leads to the decrease of elongation percentage. 4. Conclusions
Fig. 15. The weld cross section of 20 mm thick welded joint: (a) vertical-up welding; (b) vertical-down welding.
1. Vertical-up laser welding has lower sensitivity to pores than verticaldown welding. Increasing welding speed could improve welding stability and reduce the formation possibility of pores. 2. Surface tension transfer mode can achieve effective preheating of filler wire and a stable welding process. Beam oscillation increases laser irradiated area and homogenizes temperature of weld pool, which promotes wetting behavior of weld metal and decreases the tendency of melt sagging during vertical welding. 3. The 20-mm defect-free joint is obtained by vertical-up and verticaldown narrow-gap laser oscillation welding with hot wire. The tensile strength and elongation percentage are increased from face layer, middle layer and bottom layer, which is attributed to the weld microstructure by various welding heat input. Ductile fracture mode
Form Fig. 16, it can be observed that the transverse shrinkage of vertical-down welded joint is larger than that of vertical-up welded joint due to the different welding heat inputs. The filling metal has the ability to resist the transverse shrinkage, thus the groove width is basically unchanged after the fourth weld pass. The larger groove shrinkage and higher wire feeding speed improve the wire filling efficiency to increase weld reinforcement of each layer for vertical-down welding. The tensile strength and elongation percentage of different layers are shown in Fig. 17. Both the tensile strength and elongation percentage are increased from face layer, middle layer and bottom layer,
Fig. 16. The transverse shrinkage of groove and remaining groove depth during multi-layer welding. 11
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Fig. 17. The tensile strength and elongation percentage of 20 mm thick welded joint: (a) vertical-up welded joint; (b) vertical-down welded joint.
Fig. 18. The fracture morphologies of tensile samples: (a-c) vertical-up welded joint; (d-f) vertical-down welded joint.
occurs for all tensile samples with dimples.
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Declaration of Competing Interest None. Acknowledgments This project is supported by National Key Research and Development Program of China (2016YFB0300602) and the National Natural Science Foundation of China (Grant No. U1960102, 51705103, 51475104). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105921. References [1] J.C. Feng, D.W. Rathod, M.J. Roy, J.A. Francis, W. Guo, N.M. Irvine, A.N. Vasileiou, Y.L. Sun, M.C. Smith, L. Li, An evaluation of multipass narrow gap laser welding as a candidate process for the manufacture of nuclear pressure vessels, Int. J Pres. Ves. Pip. 157 (2017) 43–50. [2] Y. Zhao, S. Ma, J. Huang, Y. Wu, Narrow-gap laser welding using filler wire of thick steel plates, Int. J Adv. Manuf. Tech. 93 (2017) 2955–2962. [3] D.W. Cho, S.J. Na, M.H. Cho, J.S. Lee, A study on V-groove GMAW for various
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