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Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method
Accepted Manuscript Title: Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method Authors: Chao Chen, Chenglei Fan, Xiaoyu Cai, Sanbao Lin, Chunli Yang PII: DOI: Reference:
S0924-0136(19)30118-9 https://doi.org/10.1016/j.jmatprotec.2019.03.030 PROTEC 16170
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
1 November 2018 17 March 2019 21 March 2019
Please cite this article as: Chen C, Fan C, Cai X, Lin S, Yang C, Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method, Journal of Materials Processing Tech. (2019), https://doi.org/10.1016/j.jmatprotec.2019.03.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method Chao Chen, Chenglei Fan*, Xiaoyu Cai, Sanbao Lin, Chunli Yang State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology,
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Harbin, 150001, China
*Corresponding author: Dr. Chenglei Fan, Tel. +86-0451-86418895, Fax. +86-0451-
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86418714. E-mail address: [email protected]
Postal address: 912, State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, No. 92 Xidazhi Street, Harbin, China, 150001
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Abstract: The ultrasonic intensity was changed in different ultrasonic radiator heights
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(URH). Compared with conventional GMAW, the pulsed ultrasonic assisted gas metal
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arc welding (PU-GMAW) was an effective and newly-developed aluminum alloy welding method, because of its rapid droplet transfer and deep weld penetration. In PU-
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GMAW, the penetration and width of weld was increased due to the increase of the arc
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energy. When the URH was 20mm and 22mm, the droplet transfer mode of projected transfer changed to the mixed transition, which included projected transfer and shortcircuiting transfer. The hardness of welded joint increased with the effect of pulsed
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ultrasonic, and the columnar crystal zone width of weld seam decreased. Ultrasonic radiation force was a determinant factor in arc compression and droplet transfer
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acceleration. The microstructure improvement in the PU-GMAW was caused by ultrasonic vibration and ultrasonic cavitation. Keywords: Pulsed ultrasonic; Ultrasonic intensity; Gas metal arc welding; Droplet
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transfer; Weld formation; Microstructure
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Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method Abstract: The ultrasonic intensity was changed in different ultrasonic radiator heights URH. Compared with conventional GMAW, the pulsed ultrasonic assisted gas metal
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arc welding (PU-GMAW) was an effective and newly-developed aluminum alloy welding method, because of its rapid droplet transfer and deep weld penetration. In PU-
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GMAW, the penetration and width of weld was increased due to the increase of the arc energy. When the URH was 20mm and 22mm, the droplet transfer mode of projected
transfer changed to the mixed transition, which included projected transfer and short-
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circuiting transfer. The hardness of welded joint increased with the effect of pulsed
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ultrasonic, and the columnar crystal zone width of weld seam decreased. Ultrasonic
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radiation force was a determinant factor in arc compression and droplet transfer acceleration. The microstructure improvement in the PU-GMAW was caused by
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ultrasonic vibration and ultrasonic cavitation.
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Keywords: Pulsed ultrasonic; Ultrasonic intensity; Gas metal arc welding; Droplet transfer; Weld formation; Microstructure
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1. Introduction
In tungsten inert gas welding (TIG), Dai (2003) reported that ultrasonic was
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introduced into the weld pool by base metal. He found that the microstructure of welded joints was refined. However, the effect of ultrasonic on microstructure evolution was not discussed in detail. Chen et al. (2017a, 2017b) studied the mechanism of grain
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refinement and the effect of ultrasonic on pores behaviors. They found that the larger cavitation intensity, the smaller grain size. The larger the sound pressure in the weld pool, the faster the rising velocity of pores. WATANABE et al. (2010) presented a method of ultrasonic assisted gas metal arc welding. The ultrasonic was introduced into weld pool by welding wire and found that the degree of grain refinement was reduced when the TIG current increased to a certain value. Base on the ultrasonic cavitation 2
effect and acoustic radiation force, a method of adding coaxial ultrasound in arc welding was presented by Sun et al. (2009) and Fan et al. (2012). The arc was compressed and its energy density increased, which lead to the weld penetration increased. Thomsen (2006) reported a control system for manual pulsed Gas Metal Arc Welding which included an arc length controller and a metal transfer controller. Kanti K M and Rao P S (2008) presented that a back propagation neural network model was
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developed for controlling the droplet transfer. Waveform control method was proposed
by Chen et al. (2007) and Era et al. (2009). This method could output corresponding
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waveforms to control different droplet transfer mode. However, the method was easily
upset and has a small applicable range of process parameters. By using continuous ultrasonic controlled droplet transfer, which was reported by Fan et al. (2012). They
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indicated that the reducing droplet transfer cycle and improving welding quality were
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obtained, when ultrasound was introduced into the GMAW process.
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Xie et al. (2016) reported that the pulsed ultrasonic with a lower frequency of ~20Hz was applied in the arc welding process. However, the influence of pulsed
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ultrasonic parameters on welding process still has not been studied in detail. Few
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researchers had analyzed the effect of pulsed ultrasonic parameters on the microstructures of weld seam.
Al-Cu alloy is widely applied in aerospace, automobile manufacturing and rail
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transit, due to its advantages of low density, high strength-to-weight ratio and good corrosion resistance, as reported by Albertini G et al. (1997). Gas metal arc welding
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(GMAW) is one of the most commonly applied welding methods for aluminum alloy welding. Nevertheless, Yan et al. (2009) and Xu et al. (2007) indicated that the instability welding process and coarse microstructure would affect the quality of welded
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joints in GMAW. Adding pulsed ultrasonic in GMAW, which named as pulsed ultrasonic wave assisted GMAW (PU-GMAW), could solve those issues for aluminum alloy welding. In the PU-GMAW, an ultrasonic field exists between the ultrasonic radiator and base metal. The arc and droplet of GMAW were influenced by the ultrasonic field. The ultrasonic intensity was increased, the effect of ultrasonic on arc 3
and droplet was more obvious. Therefore, the ultrasonic intensity was one of the most important parameters in the PU-GMAW. The ultrasonic intensity changed with the change of ultrasonic radiator height (URH, as shown in Fig.1). In this work, the effect of ultrasonic intensity on the droplet transfer behavior, the weld formation and microstructure of bead welding joint in gas metal arc welding of Al-Cu alloy was
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investigated.
2. Materials and methods
The working system of PU-GMAW is shown in Fig.1. The system consists of two
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power sources: a welding power source and an ultrasonic power source. The ultrasonic
generator contains an ultrasonic radiator, an ultrasonic transducer and an ultrasonic horn. An ultrasonic field exists between the ultrasonic radiator and base metal. Therefore, the
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droplet transfer process could be affected by the ultrasonic field. Ultrasonic wave could
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be introduced into weld pool by particle medium (arc or shielding gas). In this work,
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the ultrasonic frequency was 20 kHz. The output frequency of the ultrasonic field was
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50Hz. The URH increased from 14mm to 24mm and its spacing was 2mm. The wire feed speed of 4.5m/min, the welding current of 95A and the welding speed of 7mm/s
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was employed. Argon (99.99%) with the flow rate of 20 L/min was adopted as shielding
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gas.
Fig.1 Working system of PU-GMAW The 2A14-T4 aluminum alloy plate with dimensions of 300mm×150mm×5mm was used as the base metal. A diameter of 1.2mm ER2319 welding wire was selected 4
as the filler metal. Table1 shows the chemical compositions of the base metal and filler metal. Before welding, the surface oxide film of base metal was removed by mechanical grinding. A high-speed camera with a frame rate of 2000 frame/s was carried out to capture the images of droplet transfer process. The cross-section of welded joints was obtained by the wire cutting electrical technique, and the cutting line was perpendicular to the direction of welding. Five samples were cut for each welding seam. The Keller
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solution (1mlHF, 1.5mlHCl, 2.5 ml HNO3, 95mlH2O) was adopted to etch the polished
specimens. The microstructure of bead welding joint was analyzed via an optical
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microscope. A software of Nano Measurer 1.2 was used to measure grain size. The hardness of bead welding joint was measured via a digital micro-hardness tester (HVS-
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1000), with a load of 200g and loading time of 15 s.
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3. Results
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3.1 Droplet transfer behaviors
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Fig.2 shows the droplet transfer processes. From Fig.2a, in the conventional GMAW the droplet transfer mode is projected transfer and its period is about 44.5ms.
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Compared with the conventional GMAW, the PU-GMAW process had many changes, such as the decrease of metal transfer period and arc length, as shown in Figs.2b-g.
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There are two droplet transfer modes in the PU-GMAW, including, the projected transfer and the mixed transition. The droplet transfer mode was the projected transfer
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when the URHs were 14mm, 16mm, 18mm and 24mm. The minimum period of droplet transfer was about 9ms at the URH of 18mm. A similar result had been reported by Fan et al. (2012) in the continuous ultrasonic wave assisted GMAW. They indicated that the
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metal transfer cycle in GMAW was decreased under the action of continuous ultrasound. However, they didn’t find that the projected transfer changed to the mixed transition when the URH changed to a certain value. In this work, the droplet transfer mode of the projected transfer changed to the mixed transition when the URHs were 20mm and 22mm. The mixed transition contained short-circuiting transfer and projected transfer. 5
0ms
3ms
0.5ms
3.6mm
44ms
5.5ms
3.6mm
3.6mm
44.5ms
3.6mm
3.6mm
3.6mm
(a) Conventional GMAW 0.5ms
3.6mm
4ms
14ms
3.6mm
3.6mm
20.5ms
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0.5ms
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(c) 16mm 0ms
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0ms
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(b) 14mm
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0ms
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(e) 20mm
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3.6mm
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(d) 18mm
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0ms
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(g) 24mm
24ms
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0.5ms
3.6mm
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(f) 22mm 0ms
3.6mm
27ms
3.6mm
3.6mm
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Fig.2 Droplet transfer process with different welding parameters
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Arc lengths with the different welding process are shown in Fig.3. Compared with
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the conventional GMAW, the arc length of PU-GMAW was shorten. The arc length was
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decreased with the increase of URH when the URH increased from 14mm to 22mm. The arc length was increased with the increase of URH when the URH increased from
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22mm to 24mm. Sun et al. (2009) indicated that the arc shape could be compressed by adding continuous ultrasonic in TIG welding. Fan et al. (2017) reported that arc
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constriction was obtained in continuous ultrasonic wave assisted GMAW of low carbon steel. They indicated that the shorter arc length, the larger ultrasonic intensity. Therefore,
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the ultrasonic resonant height of this paper was 22mm.
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Arc length/mm
6 5 4 3
1 0 14mm
16mm
18mm
20mm
Welding conditions
22mm
24mm
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MIG
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2
Fig.3 Arc length with different welding conditions 3.2 Weld formation
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Fig.4 shows the effect of metal transfer mode in the PU-GMAW on weld formation.
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Fig.4a shows the weld formation of conventional GMAW. The weld formation of 14mm
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URH is shown in Fig.4b. The droplet transfer mode is the projected transfer. The weld
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formation of 20mm URH is shown in Fig.4c. The droplet transfer mode is the mixed transition. Comparing the three weld formations, the weld penetrations in the PU-
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GMAW were increased than that of the conventional GMAW, especially the mixed transition mode. The widest weld width was obtained when the URH was 14mm. The
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weld reinforcement was the highest in the conventional GMAW. According to Fig.3, the arc energy of PU-GMAW was more concentrated than that of the conventional
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GMAW. Therefore, the PU-GMAW had more energy for melting base metal which led
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to the increase of the penetration and width of welding seam.
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(a)
(b) R W P
2mm (c)
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2mm
(d) 9
MIG 14mm 20mm
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Size/mm
8 7
4 3 2
0
2mm
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1
Weld reinforcement
Weld width
Weld penetration
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Geometric parameters of welding seam
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Fig.4 Weld formation
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3.3 Microstructure
Fig.5 shows the microstructure of welded joints. The base metal (BM) and heat
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affect zone (HAZ) have similar microstructures, which are columnar crystals along the rolling direction. There is a columnar crystal zone near the fusion line. The width of the
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columnar crystal zone is approximately 700μm~1000μm in conventional GMAW, as shown in Fig.5a. The widths of columnar crystal zone in the PU-GMAW were reduced
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obviously compared with the conventional GMAW. The width of columnar crystal zones is about 650μm when the URH is 14mm, as shown in Fig.5c. The width of columnar crystal zones is around 500μm when the URH is 20mm, as shown in Fig.5e.
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A larger temperature gradient (G) was the main cause of columnar crystal zone in the weld metal. The temperature gradient in PU-GMAW maybe decreased due to more concentrated arc energy. Krajewski A et al. (2012) and Chen et al. (2017) indicated that the dendrite crystal could be broken under the action of ultrasonic stir and ultrasonic cavitation. Therefore, the crystal fragmentation was another reason for the decrease of columnar crystal zone in the PU-GMAW. The microstructure of weld metal consists of 9
the columnar crystals, dendrite crystals and equiaxed grains in the conventional GMAW, as shown in Fig.5b. The top of weld seam has a columnar crystal zone with the width of 60~70μm. Figs.5d and f show the weld microstructure with the URH of 14mm and 20mm, respectively. Compared with the conventional GMAW, the number and size of columnar crystal and dendrite crystal was decreased in the PU-GMAW. A comparison
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gradually disappeared and the size of equiaxed grains was smaller.
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of Fig.5d and Fig.5f, with the increase of the ultrasonic intensity, the columnar crystal
Fig.5 Microstructures of welding seam with the different welding processes
3.4 Hardness profile Fig.6 shows the hardness profiles of welded joints in the different welding process. The black arrow in Fig.6 shows the direction of hardness measurements. The hardness 10
profile tendencies of different joints were the same. The hardness of HAZ was lower than that of BM under three different welding parameters. Because the microstructure was affected by the welding thermal cycle, the grain of HAZ grown, which was larger than that of base metal, as reported by Li et al. (2018). In weld metal (WM), the hardness profiles of PU-GMAW had a smaller fluctuation than that of conventional GMAW, especially the hardness of weld reinforcement. When the URH was 20mm, the
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hardness obtained in the weld metal was increased by 10Hv compared with the
conventional GMAW. The hardness of weld seam in the PU-GMAW was improved by
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A
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grain refining according to Hall-Petch relation.
Fig.6 Hardness profile of welded joints
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4. Discussion
4.1 Mechanism of droplet transfer changing Compared with the conventional GMAW, the decrease of arc length and the
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increase of droplet transfer frequency were found in the PU-GMAW. The schematic diagram of pulsed ultrasonic effecting on arc and droplet transfer in the PU-GMAW is shown in Fig.7. Fig.7 (a) shows the arc and droplet in the conventional GMAW. In the PU-GMAW, the arc length is compressed under the action of the ultrasonic radiation force (Fu), as shown in Fig.7 (b). Riera-Francod S E et al. (2000) and Gallego-Juarez 11
JA et al. (2010) showed that the ultrasonic wave could improve the particle collision frequency. Therefore, the frequency of plasma collision could be increased by adding pulse ultrasound. Owing to the increase of plasma collision frequency, the plasma heat dissipation rate increased. According to the principle of minimum voltage, the arc generated more heat, when the arc heat dissipation increased. The process would cause the arc compression. Fig.7(c) shows the force analysis of droplet in droplet flying
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process. During the process of droplet flying, the action of the plasma flowing force on
droplet could be overlooked. In the vertical direction, the GMAW droplet only
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influenced by the gravity (G). The process of droplet flying was affected via the gravity and ultrasonic radiation force (Fu) in the PU-GMAW. Droplet flying acceleration (a) formula is given as follow: 𝐹
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a = 𝑚𝑟
(1)
was the gravity (G) (𝐺 = 𝑚𝑔, g is the gravitational acceleration, which is a
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GMAW
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Where Fr is the resultant force of substance, m is the mass. In the GMAW, the Fr-
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constant.), so the aGMAW is the g. In the PU-GMAW, the Fr-pu-GMAW is expressed by formula (2), the apu-GMAW is calculated by equation (3). Therefore, the acceleration of
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droplet flying in PU-GMAW was faster than that of GMAW.
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𝐹𝑟−𝑝𝑢−𝑚𝑖𝑔 = 𝐺 + 𝐹𝑢
(2)
𝐹
𝑎𝑝𝑢−𝑚𝑖𝑔 = 𝑔 + 𝑚𝑢
3)
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The acoustic radiation force calculated using the following formula. 𝐹𝑢 = −𝑔𝑟𝑎𝑑𝑣0 𝑃̅ 2
𝜌𝑐 2
0
2 𝑠 𝑐𝑠
𝑣0 = 2𝜋𝑟𝑠3 [3𝜌𝑖𝑛𝑐 2 (1 − 𝜌
𝐼 = 𝑃2
)−
(4) 2 ̅𝑖𝑛 𝜌0 𝑉 2𝜌𝑠 −2𝜌
2
( 2𝜌
𝑠 +𝜌
)]
(5) (6)
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Where v0 is the time-averaged potential, rs is the particle radius in the acoustic field, 2 𝑃̅𝑖𝑛 is the mean square value of the sound pressure (P), ρ0 is the density of the acoustic
2 field medium; c is the sound velocity, cs is the material’s sound velocity, 𝑉̅𝑖𝑛 is the
mean square value of the vibration speed (V), ρs is the material density. In this work, the ultrasonic frequency and ultrasonic power were constant value, so the particle 12
vibration speed was a constant value. The relation between the sound intensity (I) and sound pressure is given in formula (6). From formulas (5) and (6), at the same conditions, v0 was increased with the increase of I. Xie et al. (2016) pointed out that the ultrasonic intensity changed with the increase of URH. Figs.7 d-f show the schematic of arc and droplet transfer in PU-GMAW with the different URH. The degree of arc compression increased gradually when the HRU increased from height1 to height2. The
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arc shape expanded gradually when the HRU increased from height2 to height3. The height2 should be the resonant height of ultrasonic. Fan et al. (2017) indicated that the
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shorter arc length, the larger ultrasonic intensity. In this work, when the URH was
22mm, the arc length was shortest than others, as shown in Fig.3. This means that the resonant height was 22mm. (c)
(b) PU-MIG Arc
Droplet
Arc
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Fu
Base metal
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Base metal (e)
Ultrasonic radiator
Ultrasonic radiator
Height 2
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Height 1
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(d)
(f)
G
Fur
Base metal
Ultrasonic radiator Height 3
Base metal
Base metal
URH=22mm
URH=24mm
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Base metal
Droplet MIG PU-MIG
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(a)
URH=16mm
Fig.7 Schematic diagram of the arc and droplet transfer in GMAW and PU-GMAW
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4.2 Mechanism of microstructure evolution Commonly, the microstructure of aluminum alloy welded joints consist of
columnar crystals, dendrites and equiaxed crystals in fusion welding. From Fig.5, the weld seam microstructure was changed in the PU-GMAW. The weld seam solidification was affected by ultrasonic cavitation and acoustic streaming when the ultrasonic wave 13
was added in GMAW. In this work, the ultrasonic was introduced into the weld pool by the arc and shielding gas. The original crystallization behavior (columnar crystal and dendrites) was broken as shown in Fig.8. Introducing ultrasonic in weld pool could produce the ultrasonic cavitation. Cavitation bubble formed by the tensile stresses characteristic of the half-period of rare faction, these bubble continue to grow by inertia until they implode under the action of compressing stresses during the compression
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half-period, thus producing a high pressure pulse in weld pool. Jian et al. (2006) reported that the grain size of aluminum A356 alloy with the ultrasonic vibration was
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refined under the action of ultrasonic cavitation. Acoustic streaming was produced in
molten pool with an acoustic pressure gradient, which could promote the microstructure evolution of weld seam. Wang et al. (2017) reported that direct observation of Al-35Cu
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alloy fragmentation and detachment confirms that the acoustic cavitation and streaming
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flow play a crucial role in fragmentation of the intermetallic dendrites. Under the action
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of the ultrasonic cavitation and acoustic streaming, the number and size of columnar crystal and dendrite crystal decreased in PU-GMAW. Ultrasonic cavitation produced
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the high pressure pulse (Pmax) which can be expressed by formula (7)
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𝑃𝑚𝑎𝑥 =
𝛾
𝑃(𝛾−1) 𝛾−1 𝑃𝑣 [ 𝑃 ] 𝑣
(7)
Where 𝑃𝑚𝑎𝑥 is the maximum pressure, 𝑃𝑣 is the vapor pressure of cavitation bubble
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and γ is the ratio of specific heat. Chen et al (2016) reported that the 𝑃𝑚𝑎𝑥 was increased with the increase of P. According to formulas (6) and (7), it can be found that
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the 𝑃𝑚𝑎𝑥 was a direct proportion to I. Intensity increment collapsed the bubbles that bring great pressure pulse, which increased the influence of ultrasonic on weld pool. With the increase of ultrasonic intensity, the effective degree of ultrasonic stirring and
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ultrasonic cavitation on weld pool increased. Therefore, when the URH was 20mm, the microstructure varying of weld seam was more obvious than the URH of 14mm, as shown in Fig.5.
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Fig.8 Schematic diagram of the grains refinement in PU-GMAW
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5. CONCLUSIONS
(1) The conventional GMAW processes were improved under the action of pulsed ultrasonic. The arc length and droplet transfer cycle in the PU-GMAW decreased
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compared with the GMAW. The droplet transfer mode of the projected transfer changed
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to the mixed transition which included the short-circuiting transfer and the projected
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transfer when the URHs were 20mm and 22mm.
(2) Compared with the conventional GMAW, the weld appearances of PU-GMAW were
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enhanced. The deepest weld penetration was obtained in the mixed transition when the
the URH was 14mm.
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URH was 20mm. The widest weld width was obtained in the projected transfer when
(3) The microstructure and property of weld seam in the PU-GMAW were improved
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compared with the conventional GMAW. The columnar crystal gradually disappeared and the size of equiaxed grains in weld metal was smaller when the URH was 20mm.
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The hardness of the weld metal of PU-GMAW was increased by 10Hv. (4) Ultrasonic radiation force was the main cause of the arc compression and droplet transfer acceleration. The microstructure improvement in the weld metal was caused by
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ultrasonic cavitation.
Acknowledgment This study is financially supported by the National Science Foundation of China under Grant No. 51675130.
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Welding Institution. 28(2), 38-42.
[15] Li P., Nie F., Dong H., Li S., Yang G., Zhang H, 2018. Pulse MIG Welding of 6061-
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T6/A356-T6 Aluminum Alloy Dissimilar T-joint. J Mater Eng Perform. 1-10.
[16] Riera-Franco d S E, Gallego-Juarez J A, Rodriguez-Corral G, 2000.Application of high-power ultrasound to enhance fluid/solid particle separation processes.
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Ultrasonics. 38(1), 642-646.
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[17] Sun Q J, Lin S B, Yang C L, Zhao G Q, 2009. Penetration increase of AISI 304
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using ultrasonic assisted tungsten inert gas welding. Sci Technol Weld Joi. 14(8), 765-767.
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[18] Watanabe T, Shiroki M, Yanagisawa A, Sasaki T, 2010. Improvement of
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mechanical properties of ferritic stainless steel weld metal by ultrasonic vibration. J Mater Process Tech. 210(12), 1646-1651. [19] Xie W, Fan C, Yang C, Lin S., 2016. Effect of acoustic field parameters on arc
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acoustic binding during ultrasonic wave-assisted arc welding. Ultrason Sonochem. 29,476-484.
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[20] Xie W F, Fan C L, Yang C L, Lin S, 2016.Pulsed Ultrasonic Wave Assisted GMAW of 7A52 Aluminum Alloy. Weld J. 95(7), 239S-247S.
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[21] Yan J, Zeng X, Gao M, Lai J, Lin T, 2009. Effect of welding wires on microstructure and mechanical properties of 2A12 aluminum alloy in CO2, laserMIG hybrid welding. Appl Surf Sci. 255(16), 7307-7313.
17
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Fig.1 Working system of PU-GMAW
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Fig.2 Droplet transfer process with different welding parameters
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3.6mm
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Arc length/mm
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18mm
20mm
22mm
24mm
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Welding conditions
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Fig.3 Arc length with different welding conditions
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9
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7
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Size/mm
8
MIG 14mm 20mm
4 3
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2 1
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Fig.4 Welding seam formation
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0
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Fig.5 Microstructures of welded joints with the different welding processes
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Fig.6 The hardness distribution of welded joints
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Fig.7 Schematic diagram of the arc and droplet transfer in GMAW and PU-GMAW
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Fig.8 Schematic diagram of the grains refinement in PU-GMAW
Table 1 Chemical compositions of base metal and filler metal (Wt. %) Cu
Mg
Si
Zr
Mn
Fe
2A14-T4
4.48
1.68
1.24
0.81
0.59
0.43
ER2319
4.33
0.91
0.62
0.29
0.70
--
N
A M ED PT CC E A
26
V
Ti
Zn
Al
0.29
0.15
--
Bal.
0.33
0.55
0.26
Bal.
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Elements
S0924-0136(19)30118-9 https://doi.org/10.1016/j.jmatprotec.2019.03.030 PROTEC 16170
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
1 November 2018 17 March 2019 21 March 2019
Please cite this article as: Chen C, Fan C, Cai X, Lin S, Yang C, Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method, Journal of Materials Processing Tech. (2019), https://doi.org/10.1016/j.jmatprotec.2019.03.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method Chao Chen, Chenglei Fan*, Xiaoyu Cai, Sanbao Lin, Chunli Yang State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology,
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Harbin, 150001, China
*Corresponding author: Dr. Chenglei Fan, Tel. +86-0451-86418895, Fax. +86-0451-
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86418714. E-mail address: [email protected]
Postal address: 912, State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, No. 92 Xidazhi Street, Harbin, China, 150001
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Abstract: The ultrasonic intensity was changed in different ultrasonic radiator heights
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(URH). Compared with conventional GMAW, the pulsed ultrasonic assisted gas metal
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arc welding (PU-GMAW) was an effective and newly-developed aluminum alloy welding method, because of its rapid droplet transfer and deep weld penetration. In PU-
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GMAW, the penetration and width of weld was increased due to the increase of the arc
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energy. When the URH was 20mm and 22mm, the droplet transfer mode of projected transfer changed to the mixed transition, which included projected transfer and shortcircuiting transfer. The hardness of welded joint increased with the effect of pulsed
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ultrasonic, and the columnar crystal zone width of weld seam decreased. Ultrasonic radiation force was a determinant factor in arc compression and droplet transfer
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acceleration. The microstructure improvement in the PU-GMAW was caused by ultrasonic vibration and ultrasonic cavitation. Keywords: Pulsed ultrasonic; Ultrasonic intensity; Gas metal arc welding; Droplet
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transfer; Weld formation; Microstructure
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Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method Abstract: The ultrasonic intensity was changed in different ultrasonic radiator heights URH. Compared with conventional GMAW, the pulsed ultrasonic assisted gas metal
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arc welding (PU-GMAW) was an effective and newly-developed aluminum alloy welding method, because of its rapid droplet transfer and deep weld penetration. In PU-
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GMAW, the penetration and width of weld was increased due to the increase of the arc energy. When the URH was 20mm and 22mm, the droplet transfer mode of projected
transfer changed to the mixed transition, which included projected transfer and short-
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circuiting transfer. The hardness of welded joint increased with the effect of pulsed
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ultrasonic, and the columnar crystal zone width of weld seam decreased. Ultrasonic
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radiation force was a determinant factor in arc compression and droplet transfer acceleration. The microstructure improvement in the PU-GMAW was caused by
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ultrasonic vibration and ultrasonic cavitation.
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Keywords: Pulsed ultrasonic; Ultrasonic intensity; Gas metal arc welding; Droplet transfer; Weld formation; Microstructure
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1. Introduction
In tungsten inert gas welding (TIG), Dai (2003) reported that ultrasonic was
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introduced into the weld pool by base metal. He found that the microstructure of welded joints was refined. However, the effect of ultrasonic on microstructure evolution was not discussed in detail. Chen et al. (2017a, 2017b) studied the mechanism of grain
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refinement and the effect of ultrasonic on pores behaviors. They found that the larger cavitation intensity, the smaller grain size. The larger the sound pressure in the weld pool, the faster the rising velocity of pores. WATANABE et al. (2010) presented a method of ultrasonic assisted gas metal arc welding. The ultrasonic was introduced into weld pool by welding wire and found that the degree of grain refinement was reduced when the TIG current increased to a certain value. Base on the ultrasonic cavitation 2
effect and acoustic radiation force, a method of adding coaxial ultrasound in arc welding was presented by Sun et al. (2009) and Fan et al. (2012). The arc was compressed and its energy density increased, which lead to the weld penetration increased. Thomsen (2006) reported a control system for manual pulsed Gas Metal Arc Welding which included an arc length controller and a metal transfer controller. Kanti K M and Rao P S (2008) presented that a back propagation neural network model was
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developed for controlling the droplet transfer. Waveform control method was proposed
by Chen et al. (2007) and Era et al. (2009). This method could output corresponding
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waveforms to control different droplet transfer mode. However, the method was easily
upset and has a small applicable range of process parameters. By using continuous ultrasonic controlled droplet transfer, which was reported by Fan et al. (2012). They
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indicated that the reducing droplet transfer cycle and improving welding quality were
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obtained, when ultrasound was introduced into the GMAW process.
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Xie et al. (2016) reported that the pulsed ultrasonic with a lower frequency of ~20Hz was applied in the arc welding process. However, the influence of pulsed
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ultrasonic parameters on welding process still has not been studied in detail. Few
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researchers had analyzed the effect of pulsed ultrasonic parameters on the microstructures of weld seam.
Al-Cu alloy is widely applied in aerospace, automobile manufacturing and rail
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transit, due to its advantages of low density, high strength-to-weight ratio and good corrosion resistance, as reported by Albertini G et al. (1997). Gas metal arc welding
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(GMAW) is one of the most commonly applied welding methods for aluminum alloy welding. Nevertheless, Yan et al. (2009) and Xu et al. (2007) indicated that the instability welding process and coarse microstructure would affect the quality of welded
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joints in GMAW. Adding pulsed ultrasonic in GMAW, which named as pulsed ultrasonic wave assisted GMAW (PU-GMAW), could solve those issues for aluminum alloy welding. In the PU-GMAW, an ultrasonic field exists between the ultrasonic radiator and base metal. The arc and droplet of GMAW were influenced by the ultrasonic field. The ultrasonic intensity was increased, the effect of ultrasonic on arc 3
and droplet was more obvious. Therefore, the ultrasonic intensity was one of the most important parameters in the PU-GMAW. The ultrasonic intensity changed with the change of ultrasonic radiator height (URH, as shown in Fig.1). In this work, the effect of ultrasonic intensity on the droplet transfer behavior, the weld formation and microstructure of bead welding joint in gas metal arc welding of Al-Cu alloy was
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investigated.
2. Materials and methods
The working system of PU-GMAW is shown in Fig.1. The system consists of two
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power sources: a welding power source and an ultrasonic power source. The ultrasonic
generator contains an ultrasonic radiator, an ultrasonic transducer and an ultrasonic horn. An ultrasonic field exists between the ultrasonic radiator and base metal. Therefore, the
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droplet transfer process could be affected by the ultrasonic field. Ultrasonic wave could
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be introduced into weld pool by particle medium (arc or shielding gas). In this work,
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the ultrasonic frequency was 20 kHz. The output frequency of the ultrasonic field was
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50Hz. The URH increased from 14mm to 24mm and its spacing was 2mm. The wire feed speed of 4.5m/min, the welding current of 95A and the welding speed of 7mm/s
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was employed. Argon (99.99%) with the flow rate of 20 L/min was adopted as shielding
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gas.
Fig.1 Working system of PU-GMAW The 2A14-T4 aluminum alloy plate with dimensions of 300mm×150mm×5mm was used as the base metal. A diameter of 1.2mm ER2319 welding wire was selected 4
as the filler metal. Table1 shows the chemical compositions of the base metal and filler metal. Before welding, the surface oxide film of base metal was removed by mechanical grinding. A high-speed camera with a frame rate of 2000 frame/s was carried out to capture the images of droplet transfer process. The cross-section of welded joints was obtained by the wire cutting electrical technique, and the cutting line was perpendicular to the direction of welding. Five samples were cut for each welding seam. The Keller
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solution (1mlHF, 1.5mlHCl, 2.5 ml HNO3, 95mlH2O) was adopted to etch the polished
specimens. The microstructure of bead welding joint was analyzed via an optical
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microscope. A software of Nano Measurer 1.2 was used to measure grain size. The hardness of bead welding joint was measured via a digital micro-hardness tester (HVS-
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1000), with a load of 200g and loading time of 15 s.
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3. Results
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3.1 Droplet transfer behaviors
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Fig.2 shows the droplet transfer processes. From Fig.2a, in the conventional GMAW the droplet transfer mode is projected transfer and its period is about 44.5ms.
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Compared with the conventional GMAW, the PU-GMAW process had many changes, such as the decrease of metal transfer period and arc length, as shown in Figs.2b-g.
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There are two droplet transfer modes in the PU-GMAW, including, the projected transfer and the mixed transition. The droplet transfer mode was the projected transfer
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when the URHs were 14mm, 16mm, 18mm and 24mm. The minimum period of droplet transfer was about 9ms at the URH of 18mm. A similar result had been reported by Fan et al. (2012) in the continuous ultrasonic wave assisted GMAW. They indicated that the
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metal transfer cycle in GMAW was decreased under the action of continuous ultrasound. However, they didn’t find that the projected transfer changed to the mixed transition when the URH changed to a certain value. In this work, the droplet transfer mode of the projected transfer changed to the mixed transition when the URHs were 20mm and 22mm. The mixed transition contained short-circuiting transfer and projected transfer. 5
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Fig.2 Droplet transfer process with different welding parameters
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Arc lengths with the different welding process are shown in Fig.3. Compared with
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the conventional GMAW, the arc length of PU-GMAW was shorten. The arc length was
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decreased with the increase of URH when the URH increased from 14mm to 22mm. The arc length was increased with the increase of URH when the URH increased from
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22mm to 24mm. Sun et al. (2009) indicated that the arc shape could be compressed by adding continuous ultrasonic in TIG welding. Fan et al. (2017) reported that arc
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constriction was obtained in continuous ultrasonic wave assisted GMAW of low carbon steel. They indicated that the shorter arc length, the larger ultrasonic intensity. Therefore,
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the ultrasonic resonant height of this paper was 22mm.
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Fig.3 Arc length with different welding conditions 3.2 Weld formation
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Fig.4 shows the effect of metal transfer mode in the PU-GMAW on weld formation.
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Fig.4a shows the weld formation of conventional GMAW. The weld formation of 14mm
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URH is shown in Fig.4b. The droplet transfer mode is the projected transfer. The weld
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formation of 20mm URH is shown in Fig.4c. The droplet transfer mode is the mixed transition. Comparing the three weld formations, the weld penetrations in the PU-
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GMAW were increased than that of the conventional GMAW, especially the mixed transition mode. The widest weld width was obtained when the URH was 14mm. The
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weld reinforcement was the highest in the conventional GMAW. According to Fig.3, the arc energy of PU-GMAW was more concentrated than that of the conventional
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GMAW. Therefore, the PU-GMAW had more energy for melting base metal which led
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to the increase of the penetration and width of welding seam.
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(a)
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2mm (c)
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MIG 14mm 20mm
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4 3 2
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Geometric parameters of welding seam
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Fig.4 Weld formation
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3.3 Microstructure
Fig.5 shows the microstructure of welded joints. The base metal (BM) and heat
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affect zone (HAZ) have similar microstructures, which are columnar crystals along the rolling direction. There is a columnar crystal zone near the fusion line. The width of the
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columnar crystal zone is approximately 700μm~1000μm in conventional GMAW, as shown in Fig.5a. The widths of columnar crystal zone in the PU-GMAW were reduced
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obviously compared with the conventional GMAW. The width of columnar crystal zones is about 650μm when the URH is 14mm, as shown in Fig.5c. The width of columnar crystal zones is around 500μm when the URH is 20mm, as shown in Fig.5e.
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A larger temperature gradient (G) was the main cause of columnar crystal zone in the weld metal. The temperature gradient in PU-GMAW maybe decreased due to more concentrated arc energy. Krajewski A et al. (2012) and Chen et al. (2017) indicated that the dendrite crystal could be broken under the action of ultrasonic stir and ultrasonic cavitation. Therefore, the crystal fragmentation was another reason for the decrease of columnar crystal zone in the PU-GMAW. The microstructure of weld metal consists of 9
the columnar crystals, dendrite crystals and equiaxed grains in the conventional GMAW, as shown in Fig.5b. The top of weld seam has a columnar crystal zone with the width of 60~70μm. Figs.5d and f show the weld microstructure with the URH of 14mm and 20mm, respectively. Compared with the conventional GMAW, the number and size of columnar crystal and dendrite crystal was decreased in the PU-GMAW. A comparison
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A
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gradually disappeared and the size of equiaxed grains was smaller.
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of Fig.5d and Fig.5f, with the increase of the ultrasonic intensity, the columnar crystal
Fig.5 Microstructures of welding seam with the different welding processes
3.4 Hardness profile Fig.6 shows the hardness profiles of welded joints in the different welding process. The black arrow in Fig.6 shows the direction of hardness measurements. The hardness 10
profile tendencies of different joints were the same. The hardness of HAZ was lower than that of BM under three different welding parameters. Because the microstructure was affected by the welding thermal cycle, the grain of HAZ grown, which was larger than that of base metal, as reported by Li et al. (2018). In weld metal (WM), the hardness profiles of PU-GMAW had a smaller fluctuation than that of conventional GMAW, especially the hardness of weld reinforcement. When the URH was 20mm, the
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hardness obtained in the weld metal was increased by 10Hv compared with the
conventional GMAW. The hardness of weld seam in the PU-GMAW was improved by
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grain refining according to Hall-Petch relation.
Fig.6 Hardness profile of welded joints
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4. Discussion
4.1 Mechanism of droplet transfer changing Compared with the conventional GMAW, the decrease of arc length and the
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increase of droplet transfer frequency were found in the PU-GMAW. The schematic diagram of pulsed ultrasonic effecting on arc and droplet transfer in the PU-GMAW is shown in Fig.7. Fig.7 (a) shows the arc and droplet in the conventional GMAW. In the PU-GMAW, the arc length is compressed under the action of the ultrasonic radiation force (Fu), as shown in Fig.7 (b). Riera-Francod S E et al. (2000) and Gallego-Juarez 11
JA et al. (2010) showed that the ultrasonic wave could improve the particle collision frequency. Therefore, the frequency of plasma collision could be increased by adding pulse ultrasound. Owing to the increase of plasma collision frequency, the plasma heat dissipation rate increased. According to the principle of minimum voltage, the arc generated more heat, when the arc heat dissipation increased. The process would cause the arc compression. Fig.7(c) shows the force analysis of droplet in droplet flying
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process. During the process of droplet flying, the action of the plasma flowing force on
droplet could be overlooked. In the vertical direction, the GMAW droplet only
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influenced by the gravity (G). The process of droplet flying was affected via the gravity and ultrasonic radiation force (Fu) in the PU-GMAW. Droplet flying acceleration (a) formula is given as follow: 𝐹
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a = 𝑚𝑟
(1)
was the gravity (G) (𝐺 = 𝑚𝑔, g is the gravitational acceleration, which is a
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GMAW
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Where Fr is the resultant force of substance, m is the mass. In the GMAW, the Fr-
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constant.), so the aGMAW is the g. In the PU-GMAW, the Fr-pu-GMAW is expressed by formula (2), the apu-GMAW is calculated by equation (3). Therefore, the acceleration of
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droplet flying in PU-GMAW was faster than that of GMAW.
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𝐹𝑟−𝑝𝑢−𝑚𝑖𝑔 = 𝐺 + 𝐹𝑢
(2)
𝐹
𝑎𝑝𝑢−𝑚𝑖𝑔 = 𝑔 + 𝑚𝑢
3)
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The acoustic radiation force calculated using the following formula. 𝐹𝑢 = −𝑔𝑟𝑎𝑑𝑣0 𝑃̅ 2
𝜌𝑐 2
0
2 𝑠 𝑐𝑠
𝑣0 = 2𝜋𝑟𝑠3 [3𝜌𝑖𝑛𝑐 2 (1 − 𝜌
𝐼 = 𝑃2
)−
(4) 2 ̅𝑖𝑛 𝜌0 𝑉 2𝜌𝑠 −2𝜌
2
( 2𝜌
𝑠 +𝜌
)]
(5) (6)
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Where v0 is the time-averaged potential, rs is the particle radius in the acoustic field, 2 𝑃̅𝑖𝑛 is the mean square value of the sound pressure (P), ρ0 is the density of the acoustic
2 field medium; c is the sound velocity, cs is the material’s sound velocity, 𝑉̅𝑖𝑛 is the
mean square value of the vibration speed (V), ρs is the material density. In this work, the ultrasonic frequency and ultrasonic power were constant value, so the particle 12
vibration speed was a constant value. The relation between the sound intensity (I) and sound pressure is given in formula (6). From formulas (5) and (6), at the same conditions, v0 was increased with the increase of I. Xie et al. (2016) pointed out that the ultrasonic intensity changed with the increase of URH. Figs.7 d-f show the schematic of arc and droplet transfer in PU-GMAW with the different URH. The degree of arc compression increased gradually when the HRU increased from height1 to height2. The
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arc shape expanded gradually when the HRU increased from height2 to height3. The height2 should be the resonant height of ultrasonic. Fan et al. (2017) indicated that the
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shorter arc length, the larger ultrasonic intensity. In this work, when the URH was
22mm, the arc length was shortest than others, as shown in Fig.3. This means that the resonant height was 22mm. (c)
(b) PU-MIG Arc
Droplet
Arc
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Fu
Base metal
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Base metal (e)
Ultrasonic radiator
Ultrasonic radiator
Height 2
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Height 1
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(d)
(f)
G
Fur
Base metal
Ultrasonic radiator Height 3
Base metal
Base metal
URH=22mm
URH=24mm
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Base metal
Droplet MIG PU-MIG
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(a)
URH=16mm
Fig.7 Schematic diagram of the arc and droplet transfer in GMAW and PU-GMAW
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4.2 Mechanism of microstructure evolution Commonly, the microstructure of aluminum alloy welded joints consist of
columnar crystals, dendrites and equiaxed crystals in fusion welding. From Fig.5, the weld seam microstructure was changed in the PU-GMAW. The weld seam solidification was affected by ultrasonic cavitation and acoustic streaming when the ultrasonic wave 13
was added in GMAW. In this work, the ultrasonic was introduced into the weld pool by the arc and shielding gas. The original crystallization behavior (columnar crystal and dendrites) was broken as shown in Fig.8. Introducing ultrasonic in weld pool could produce the ultrasonic cavitation. Cavitation bubble formed by the tensile stresses characteristic of the half-period of rare faction, these bubble continue to grow by inertia until they implode under the action of compressing stresses during the compression
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half-period, thus producing a high pressure pulse in weld pool. Jian et al. (2006) reported that the grain size of aluminum A356 alloy with the ultrasonic vibration was
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refined under the action of ultrasonic cavitation. Acoustic streaming was produced in
molten pool with an acoustic pressure gradient, which could promote the microstructure evolution of weld seam. Wang et al. (2017) reported that direct observation of Al-35Cu
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alloy fragmentation and detachment confirms that the acoustic cavitation and streaming
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flow play a crucial role in fragmentation of the intermetallic dendrites. Under the action
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of the ultrasonic cavitation and acoustic streaming, the number and size of columnar crystal and dendrite crystal decreased in PU-GMAW. Ultrasonic cavitation produced
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the high pressure pulse (Pmax) which can be expressed by formula (7)
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𝑃𝑚𝑎𝑥 =
𝛾
𝑃(𝛾−1) 𝛾−1 𝑃𝑣 [ 𝑃 ] 𝑣
(7)
Where 𝑃𝑚𝑎𝑥 is the maximum pressure, 𝑃𝑣 is the vapor pressure of cavitation bubble
PT
and γ is the ratio of specific heat. Chen et al (2016) reported that the 𝑃𝑚𝑎𝑥 was increased with the increase of P. According to formulas (6) and (7), it can be found that
CC E
the 𝑃𝑚𝑎𝑥 was a direct proportion to I. Intensity increment collapsed the bubbles that bring great pressure pulse, which increased the influence of ultrasonic on weld pool. With the increase of ultrasonic intensity, the effective degree of ultrasonic stirring and
A
ultrasonic cavitation on weld pool increased. Therefore, when the URH was 20mm, the microstructure varying of weld seam was more obvious than the URH of 14mm, as shown in Fig.5.
14
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Fig.8 Schematic diagram of the grains refinement in PU-GMAW
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5. CONCLUSIONS
(1) The conventional GMAW processes were improved under the action of pulsed ultrasonic. The arc length and droplet transfer cycle in the PU-GMAW decreased
U
compared with the GMAW. The droplet transfer mode of the projected transfer changed
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to the mixed transition which included the short-circuiting transfer and the projected
A
transfer when the URHs were 20mm and 22mm.
(2) Compared with the conventional GMAW, the weld appearances of PU-GMAW were
M
enhanced. The deepest weld penetration was obtained in the mixed transition when the
the URH was 14mm.
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URH was 20mm. The widest weld width was obtained in the projected transfer when
(3) The microstructure and property of weld seam in the PU-GMAW were improved
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compared with the conventional GMAW. The columnar crystal gradually disappeared and the size of equiaxed grains in weld metal was smaller when the URH was 20mm.
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The hardness of the weld metal of PU-GMAW was increased by 10Hv. (4) Ultrasonic radiation force was the main cause of the arc compression and droplet transfer acceleration. The microstructure improvement in the weld metal was caused by
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ultrasonic cavitation.
Acknowledgment This study is financially supported by the National Science Foundation of China under Grant No. 51675130.
References 15
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[4] Chen Q, Lin S, Yang C, Fan C, Ge H, 2017b. Grain fragmentation in ultrasonicassisted TIG weld of pure aluminum. Ultrason Sonochem. 39, 403-413.
[5] Chen H, Zeng M, Cao B, 2007. Current waveform control system of high-speed
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[6] Dai W L., 2003. Effects of high-intensity ultrasonic-wave emission on the weldability of aluminum alloy 7075-T6. Mater Lett. 57(16), 2447-2454.
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[16] Riera-Franco d S E, Gallego-Juarez J A, Rodriguez-Corral G, 2000.Application of high-power ultrasound to enhance fluid/solid particle separation processes.
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Ultrasonics. 38(1), 642-646.
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[17] Sun Q J, Lin S B, Yang C L, Zhao G Q, 2009. Penetration increase of AISI 304
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[18] Watanabe T, Shiroki M, Yanagisawa A, Sasaki T, 2010. Improvement of
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mechanical properties of ferritic stainless steel weld metal by ultrasonic vibration. J Mater Process Tech. 210(12), 1646-1651. [19] Xie W, Fan C, Yang C, Lin S., 2016. Effect of acoustic field parameters on arc
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acoustic binding during ultrasonic wave-assisted arc welding. Ultrason Sonochem. 29,476-484.
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[20] Xie W F, Fan C L, Yang C L, Lin S, 2016.Pulsed Ultrasonic Wave Assisted GMAW of 7A52 Aluminum Alloy. Weld J. 95(7), 239S-247S.
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[21] Yan J, Zeng X, Gao M, Lai J, Lin T, 2009. Effect of welding wires on microstructure and mechanical properties of 2A12 aluminum alloy in CO2, laserMIG hybrid welding. Appl Surf Sci. 255(16), 7307-7313.
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A
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Fig.1 Working system of PU-GMAW
18
0ms
3ms
0.5ms
3.6mm
44ms
5.5ms
3.6mm
3.6mm
44.5ms
3.6mm
3.6mm
3.6mm
(a) Conventional GMAW 0.5ms
3.6mm
4ms
14ms
3.6mm
3.6mm
20.5ms
21ms
3.6mm
3.6mm
0.5ms
3.6mm
3.5ms
8.5ms
3.6mm
3.6mm
3.6mm
3ms
15ms
3.6mm
3.6mm
5.5ms
8.5ms
9ms
3.6mm
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A
0.5ms
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(c) 16mm 0ms
14.5ms
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0ms
3.6mm
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(b) 14mm
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0ms
3.6mm
3.6mm
3.6mm
3.6mm
3.6mm
0.5ms
3.6mm
26.5ms
3.6mm
27ms
11.5ms
3.6mm
(e) 20mm
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16.5ms
3.6mm
3.6mm
27.5ms
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3.6mm
6.5ms
3.6mm
3.6mm
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21.5ms
4ms
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0ms
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(d) 18mm
28ms
3.6mm
3.6mm
28.5ms
3.6mm
3.6mm
0ms
0.5ms
3.6mm
11ms
1ms
1.5ms
3.6mm
3.6mm
16ms
3.6mm
21ms
5ms
3.6mm
3.6mm
3.6mm
3.6mm
26ms
5.5ms
26.5ms
3.6mm
3.6mm
3.6mm
8.5ms
18.5ms
3.6mm
3.6mm
23.5ms
3.6mm
(g) 24mm
24ms
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0.5ms
3.6mm
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(f) 22mm 0ms
3.6mm
27ms
3.6mm
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PT
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A
N
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Fig.2 Droplet transfer process with different welding parameters
20
3.6mm
8 7
Arc length/mm
6 5 4 3
1 0 MIG
14mm
16mm
18mm
20mm
22mm
24mm
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Welding conditions
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2
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A
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Fig.3 Arc length with different welding conditions
21
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9
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7
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Size/mm
8
MIG 14mm 20mm
4 3
PT
2 1
Weld reinforcement
Weld width
Weld penetration
Geometric parameters of welding seam
Fig.4 Welding seam formation
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0
22
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Fig.5 Microstructures of welded joints with the different welding processes
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A
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Fig.6 The hardness distribution of welded joints
24
IP T SC R U N A M ED
A
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Fig.7 Schematic diagram of the arc and droplet transfer in GMAW and PU-GMAW
25
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Fig.8 Schematic diagram of the grains refinement in PU-GMAW
Table 1 Chemical compositions of base metal and filler metal (Wt. %) Cu
Mg
Si
Zr
Mn
Fe
2A14-T4
4.48
1.68
1.24
0.81
0.59
0.43
ER2319
4.33
0.91
0.62
0.29
0.70
--
N
A M ED PT CC E A
26
V
Ti
Zn
Al
0.29
0.15
--
Bal.
0.33
0.55
0.26
Bal.
U
Elements