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Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing
Accepted Manuscript Title: Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing Authors: Luo Yi, Li Jinglong, Xu Jie, Zhu Liang, Han Jingtao, Zhang Chengyang PII: DOI: Reference:
S0924-0136(18)30194-8 https://doi.org/10.1016/j.jmatprotec.2018.04.047 PROTEC 15747
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
10-8-2017 14-4-2018 30-4-2018
Please cite this article as: Luo Y, Li J, Xu J, Zhu L, Han J, Zhang C, Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing, Journal of Materials Processing Tech. (2010), https://doi.org/10.1016/j.jmatprotec.2018.04.047 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.
Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing
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Luo Yia,b,c,*, Li Jinglongc, Xu Jiea,b, Zhu Lianga,b, Han Jingtaoa,b, Zhang Chengyanga,b
School of Material Science and Engineering, Chongqing University of Technology,
b
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Chongqing 400054, People’s Republic of China
Chongqing Municipal Engineering Research Center of Institutions of Higher
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Education for Special Welding Materials and Technology, Chongqing 400054,
State Key Laboratory of Solidification Processing, Northwestern Polytechnical
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People’s Republic of China
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University, Xi’an 710072, People’s Republic of China
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Corresponding author: Luo Yi
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E-mail address of the corresponding author: [email protected]
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Graphical Abstract
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Abstract
The wire-arc additive manufacturing (WAAM) process directly and locally adds
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materials to fabricate metal components in a layer-by-layer manner. In this study,
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pulsed arc and non-pulsed arc were used in WAAM process of aluminum alloys. Arc information in manufacturing was used to identify the droplet transfer in projected transfer mode. A calculation method was proposed to analyze the arc pulse effect on
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the droplet transfer. As a result, it was found that the pulsed arc can achieve higher droplet transfer frequency and the size of the droplet in the pulsed arc is smaller than that in the non-pulsed arc. In addition, the manufacturing efficiency in pulsed arc is higher than that in non-pulsed arc at the similar arc power because of high deposition 2
rate. Although the pulsed arc is more suitable for the WAAM of aluminum alloy than non-pulsed arc, the arc force acting on the droplet in pulsed arc is greater than that in non-pulsed arc. The arc force increases with the increase of the arc power, which is likely to cause the collapse of the materials accumulation layer. Therefore, it is
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long as the arc power meets the requirement of manufacturing efficiency.
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necessary to use a smaller pulsed arc power in WAAM process of aluminum alloys as
Keywords: Pulsed arc, Additive manufacturing, Metal droplet, Deposition rate, Arc
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force
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1 Introduction
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In comparison to traditional subtractive fabrication technologies, additive manufacturing (AM) is an additional technique for fabricating complex metal
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components in a layer-by-layer manner. AM can potentially reduce energy
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consumption and materials waste and numerous processes have been successfully established for conventional metals. In order to deposit metal droplets on a solid material surface to fabricate complex 3D structures, some typical AM processes were
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proposed, which provide design freedom and environmental/ecological advantages (Bikas et al., 2016). Visser et al. (2015) used a direct-write method named laser-induced forward transfer (LIFT) by drop-based deposition. Chao et al. (2013) and Yi et al. (2016) fabricated 3D complex components by metal micro-droplet 3
deposition manufacture (MDDM) method, which was considered as a novel effective and low cost method for fabricating metal parts. Cao et al. (2006) fabricated 3D parts made of pure aluminum by depositing molten aluminum droplets, layer-by-layer, via computer information transferred through the Internet in a real-time mode. These
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studies were classified into the fused deposition modeling (FDM) process, which was one of the AM techniques.
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Another AM technique is the wire-arc additive manufacturing (WAAM), which is a high-deposition rate process and may be used for manufacturing large-scale AM parts
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(Ding et al., 2015). WAAM process uses an electric arc, either the gas metal arc
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(GMA) or the gas tungsten arc (GTA), as a heat source and wire as feedstock. In
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addition to gravity, arc force plays a most important role in the deposition process of
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metal droplet. The arc force is composed of the electromagnetic force, the plasma
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flow force, the evaporation recoil force and the charged particle impact force. It is precisely because of the arc force, the metal droplet transfer process in WAAM is
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complex. Therefore, WAAM process is different from FDM process. But, due to its
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high deposition rate, environmental friendliness and cost-competitiveness, the WAAM process becomes more attractive (Ding et al., 2016). The stability of GTA was advantageous enough to be used as heat source to produce components in WAAM.
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The thermal effect of GTA was shown by the details of the molten pool behavior and temperature field, which were important to microstructure (Wang et al., 2016). To increase the interpass temperature can facilitate phase transformation by extending the high temperature period and produce the desired microstructure (Ma et al., 2015). So 4
the GTA-WAAM process is capable of producing full density alloyed components with tensile properties that are comparable to powder metallurgy methods (Shen et al., 2016). The components produced by this method are reported to be efficient and less expensive than components produced by laser-based powder AM processes
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(Bermingham et al., 2015). In additions, it is important to succeed in obtaining repeatable and reproducible
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material with acceptable quality. The electric arc and metal droplet are important factors that affect the quality of the fabricated component in GMA-WAAM process. It
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is effective to improve the quality of manufacturing by sensing and analyzing the
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information of arc and droplet in GMA-WAAM process (Xiong et al., 2016). These
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informations include droplet size and mean value of droplet detachment frequency in
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GMA-WAAM process (Anzehaee et al., 2011). High-speed photography technology
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synchronized with the arc current and arc voltage signals is a classical experimental method to aid the understanding of the arc and droplet transfer behavior (Scotti et al.,
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2014). The phenomenological explanations based on arc physics are given to justify
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the main governing factors, such as arc force and wire feed rate, for the particular metal transfer characteristics (Yang et al., 2016). Generally, with the increase of arc current, the droplet diameter and mass decrease, while the plasma drag force and the
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gravity acting on the droplet decrease (Li et al., 2015). The transfer frequency of droplet increases with augment of wire feed rate or decrease of inductance correction value (Sun et al., 2015). In the former researches, the studies about the pulses effect on arc and droplet 5
transfer in WAAM process were insufficient. More studies focusd on numerical simulation of heat transfer in WAAM process (Foteinopoulos et al., 2018). But there was not enough experiment method and result to support the technological research. In this paper, an application of pulsed arc in WAAM process of aluminum alloys was
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proposed and the focus was the effect of pulsed arc on the metal droplet transfer behaviors. We proposed the experiment method and calculation method to analyze in
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section 2 and 3. Arc current, voltage and acoustic emission (AE) signals detected in
manufacturing were used as information source to identify droplet transfer. The
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feature information detected in experiment was used as variables to develop
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computational models and to analyze the droplet characteristics. Computational
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methods and models about droplet size, deposition rate and arc force were proposed to
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aid understanding of the arc pulse effect and predict manufacturing efficiency in
WAAM process.
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2 Experiment details
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section 4. As a result, this study provides a basis for the application of pulsed arc in
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The experiments were performed on an automatic arc processing system, as shown in Fig.1, and the characteristics of components in experiment system were provided in Table 1. In this system, wire metallic materials were melted and accumulated by
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GMA heat source. The filler wire was fed to the torch tip coaxially. The filler wire tip was melted to form metal droplets by the arc discharge caused between the wire tip and base plate. As the arc moved along the targeted path, accumulated metal droplets were deposited and solidified. A programmable 3-axis linear stage system was used to 6
move the workpiece during processing while the arc torch was kept stationary. Filler wire of aluminum alloy (ER-4043), which diameter was 1.2mm, was used in the experiments. Pure Ar was used as shielding gases and the gas flow rate was kept at 15 L/min. 6061 aluminum alloy with 8mm thickness was used as base plates in
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experiment. In view of the arc stability in electrode positive (EP) region, the direct-current electrode positive (DCEP) was used to govern the metal droplet
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dynamics. The value of arc current is the main factor determining the critical condition of projected transfer mode. Usually, the arc current needs to be larger and
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the arc voltage is matched with the arc current. Table 2 shows the processing
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parameters of pulsed arc in experiment. The arc current (I) was set as ranged from
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164A to 200A, the acr voltage (U) ranged from 22.6V to 27.0V and the pulse
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frequency ranged from 240Hz to 300Hz. The combination of these parameters can
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achieve projected transfer mode of metal droplet in pulsed arc. The non-pulsed arc needed a larger arc current than pulsed arc to realize the projected transfer mode. We
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try to make pulsed arc and non-pulsed arc work in similar projected transfer mode.
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Therefore, the matching of arc current and arc voltage of the non-pulsed arc in Table 3 was greater than that of the pulsed arc as shown in Table 2. The dynamic waveforms of arc current and arc voltage in processing were recorded
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in real-time. Hall sensor was used to measure the arc current and voltage signals. Piezo-electric sensor was mounted on the surface of base plate, which was used to measure the AE signals in processing. A CCD camera was used to record the high-speed video (HSV) of manufacturing at 1000 fps. Multiple neutral-density filters 7
were used to capture the arc behavior and droplet transfer during welding. The use of a band-pass filter with a 625~650 nm range helped in achieving the desired object sharpness and separation. The arc current, voltage and AE signals were processed by preamplifier and signal conditioner, and then they were synchronized and transferred
according to arc current and voltage signals, and it was expressed as
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P(t ) U (t ) I(t )
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to computer by data acquisition unit. The arc power (P) was calclulated by computer
(1).
The arc power was used to indicate the combined effects of the arc current and
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voltage in this paper.
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3.1 Projected transfer mode of metal droplet
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3 Results and calculation
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The metal droplet transfer mode is gonverned by the matching of the arc current
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and voltage. As the arc current exceeds a certain critical value, the arc root homogenously covers the metal droplet and results in its rapid detachment by smaller
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droplet size. Fig.2 shows the process of droplets detachment, which is called projected
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transfer mode (PTM) of metal droplet. As long as the arc current exceeds the critical value and matches an appropriate voltage, the PTM will occur both in pulsed arc and in non-pulsed arc. The images in Fig.2a recorded the real process of droplet
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transferred in PTM. It takes only about 8 ms to finish the transfer of droplet. Of course, the droplet transfer cycle is different in different process conditions, and the impact of the pulse can not be ignored. Especially, the cycle of droplet transfer depends on the pulse frequency in pulsed arc mode. Fig.3 shows the image of the 8
metal droplet and filler wire, where h is the travel distance of metal droplet transferring to base plate, which is equal to the initial height of the droplet. Owing to the continuous melting of the solid portion of the filler wire, the metal droplets are provided by filler wire and transferred to base plate to realize deposition modelling.
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Because of the driving effect from the arc force and the gravity of droplet, the metal droplet induces impact force to base plate during transferring process. The AE energy
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is excitated by the impact force and releases AE signals. The droplet dynamics
governed by arc pulses are included in the information of the arc power signals and
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the AE signals.
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Fig.4a shows the waveforms of acquired voltage and current signals as pulsed arc,
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which consist of the same phase of voltage and current pulses. The average current of
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pulsed arc is 164A and the average voltage is 22.8V. The waveforms of arc voltage and current for non-pulsed arc are shown in Fig.4b, the arc current is set to 220A and
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the arc voltage is set to 25.8V. Compared with Fig4a, the output current and voltage of
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the non-pulsed arc is smoother. Thus, the impact of pulses on the metal droplet transition is included in the characteristics of signals. In order to make use of the common effect of current and voltage to describe the characteristics of the
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manufacturing process, the arc power signal is used as the source of information.
Fig.5a shows the synchronous waveforms of the arc power signals and the AE signals acquired in manufacturing process. The AE events caused by the impact of the 9
metal droplet transfer are clearly visible, which are called droplet events, as shown in Fig.5b. With the increase of the arc energy output at t1, the arc continues to burn and the filler wire is melted to form the droplet. When the arc energy output reaches the maximum value, the droplet is transferred into the molten pool under the affection of
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the pulsed arc force and gravity at t2. Subsequently, the arc energy output decreases continuously at the trailing edge of the pulse, but the arc continues to burn at t3.
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Obviously, the occurrence time of these droplets is corresponding to the peak duration
of the arc power pulse signal. This means that the metal droplet transfer occurs when
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the maximum energy of the pulsed arc is output. Also, an arc power pulse corresponds
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to a droplet transfer, which is called the one pulse one droplet mode. So, the process
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of metal droplet transfer is closely related to the pulse effect of the arc in the projected
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transfer mode.
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For non-pulsed arc, the process of metal droplet transfer is different. Fig.6a shows the correspondence between arc power signals and AE signals as non-pulsed arc. It
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can be seen that the evolution of arc energy output is completely different from the arc
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power signals in Fig.5a and Fig.6a. The output of the arc energy under the affection of pulse is pulsating, but the energy output of the non-pulsed arc is smoother. Because the waveform of the arc power signal is smooth, the metal droplet transfer in
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non-pulsed arc mode can not be identified directly by the waveform of arc power signal. But the droplet events in AE signals are very distinct as shown in Fig.6b, which reveals the information of droplet transfer in time domain. The characteristics such as transfer frequency and duration of metal droplet transfer in non-pulsed arc 10
mode can be distinguished by the information of droplet events. Thus, AE signals can be used as an information source to identify the metal droplet transfer. Due to the different energy characteristics of the non-pulsed arc output, the process of metal droplet transfer may show completely different characteristics. The arc energy output
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reaches the maximum value at t1, after which, the arc continues to burn and the filler wire gradually melts to form the droplet. When the droplet grows to a certain volume,
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the droplet is transferred into the molten pool under the affection of the non-pulsed arc force and gravity at t2. Subsequently, the arc continues to burn at t3. It can be seen
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from the above analysis that the droplet transfer is closely related to the arc force,
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whether it is a pulsed arc or a non-pulsed arc. In the following sections, the
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characteristic information of the droplet is calculated and analyzed according to the
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signals characteristics detected in experiment.
3.2 Calculation method
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In previous works and even in recent works, the dynamic characteristics of metal
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transfer were computed numerically (Choi et al., 1998; Tipi et al., 2015) and tested experimentally (Norrish et al., 2014). The droplet transfer process in projected transfer mode was illustrated clearly in these researches, which provided a good
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fundation for following research. Under the effect of surface tension, the metal droplet travels to base plate in the form of approximate spherical element after the detachment from wire tip. Thus, the metal droplet is approximated to be a sphere in the following calculation, which volume (VR) is calculated by 11
VR
4 R3 3
(2),
where R is the radius of the metal droplet. Assuming n droplets are transferred in a manufacturing cycle, the total volume (VT) of these droplets is
VT n VR
(3).
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The total mass of these droplets is
mT VT
(4),
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where mT is the total mass of droplets transferred from wire to base plate and ρ is the
density of droplet. The density of 4043 aluminum alloy wire in the molten state is
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2.38 g/cm3, which was used in the calculation. Assuming the mass of base plate is m0,
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the total mass of base plate and droplets transferred to base plate is mt, which can be
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measured separately by balance before and after WAAM processing. The mass of
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droplets transferred from wire to base plate is
(5).
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mT mt m0
Accordingly, the radius of droplet is calculated by Eq. (6). 1
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3 mt m0 3 R 4 n
(6)
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Where, n is the number of droplets transferred in a manufacturing cycle, which can be identified from the AE signals acquired in manufacturing process. mt and m0 can be
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obtained in experiment. Vf is the additive volume per second, which is called deposition rate and is calculated by V f VR f
(7).
Where, f is the transfer frequency of the metal droplet, which can be calculated from AE signals or arc power signals. According to Eq.(2), Eq.(6) and Eq.(7), the Vf is 12
Vf
f mt m0 n
(8).
Next, the arc force acting on the droplet is calculated from the perspective of energy conservation. Eq. (9) describes the relationship of energy conservation for a droplet transferred from wire tip to base plate.
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F s Ek E p
(9),
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where Ek is the kinetic energy of the motional droplet and Ep is the potential energy of
the droplet. F is the arc force acting on the droplet during the droplet transferred from wire tip to base plate. s is the travel distance of the motional droplet. This relationship
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1 m v2 m g h 2
(10),
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F s
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is expressed as
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where m is the mass of a droplet and v is the velocity of the droplet moving. h is the initial height of the droplet, which is equal to s. Although F and v in the Eq. (10) are
2 1 mv m g h 2
(11).
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F s
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transient, they are calculated using their mean values in the study, which is
Due to the small mass of a droplet, the potential energy is negligible. So, the Eq. (11)
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is simplified as
F s
2 1 mv 2
(12).
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Where,
v
s t
(13),
and t is the time of droplet moving from tip to substrate, which is not equal to the the cycle of the droplet transfer (T). According to the observation of the droplet transfer 13
process by high-speed video, t is approximately half of T. So, the relationship between t and f is expressed as
t
1 2f
(14).
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According to the above calculation, the average arc force acting on the droplet during the transfer process is
8 F s f 2 R3 3
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(15).
The above equations reveal a feasible experimental and analytical method to the characteristics of metal droplet transfer in GMA-WAAM. Eq.(6) can be used to
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calculate the droplet radius. mt, m0 and n are the variables in this equation, which
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reflects the average value of droplet radius in the whole experiment process. Eq.(8)
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can be used to calculate the deposition efficiency of metal droplet and Eq.(15) can be used to calculate the average arc force driven the droplet transfer. According to the
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calculation results, these equations are applied to analyze the characteristics of droplet
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transfer and regression models are developed in the following section, which closely relates to the manufacturing quality and efficiency in GMA-WAAM.
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4 Discussion
The characteristics of droplet transfer reflect the manufacturing efficiency and
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quality. Transfer frequency (f) is one of the characteristics of metal droplet transfer. The transfer frequency of droplet for pulsed arc can be distinguished from the time domain signals of arc power or AE. However, the transfer frequency of droplet for non-pulsed arc can only be identified from the time domain signals of AE in this study. Fig.7 shows the influence of the arc power to the transfer frequency. The transfer 14
frequency of droplet increases with the increase of arc power not only for non-pulsed arc but also for pulsed arc. Fig.8 shows the output current waveforms of the pulsed-arc and the non-pulsed arc in the similar current setting. Compared with the current of non-pulsed arc, the pulse current of pulsed arc is much larger than the
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critical current value of projected transfer mode. Under our experimental conditions, the critical current of the projected transfer mode is about 140A. It can be seen from
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Fig.8, because of the pulse effect, the current values of the pulsed-arc are much higher
than the critical current, but the current values of the non-pulsed arc are still lower
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than the critical current. So, the droplet transfer frequency in pulsed arc is much larger
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than that in non-pulsed arc as shown in Fig.7.
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The frequency characteristic of droplet transfer is closely related to the droplet size.
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In general, the droplet transfer frequency is higher, the droplet size is smaller. Fig.9
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shows the influence of the transfer frequency to the radius of droplet. The radius of droplet is calculated by Eq. (6). Because the pulse peak values of pulsed current are
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much greater than the values of non-pulsed current, the pulsed current produces larger
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electromagnetic force and the plasma flow force than the non-pulsed current. These forces act on the metal droplet and prevent the droplet growing up. Therefore, the droplet radius in pulsed arc is smaller than that in non-pulsed arc at the similar droplet
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transfer frequency. Eq. (16) can be used to describe the relationship between the transfer frequency and the droplet radius for non-pulsed arc. The relationship between the transfer frequency and the droplet radius for pulsed arc is described by Eq. (17). The values of R-Square of Eq. (16) and (17) are 0.997 and 0.946, respectively, which 15
indicates a high degree of correlation.
R 0.822 0.002 f 2.596 106 f 2
(16)
R 0.406 9.524 104 f 2.029 106 f 2
(17)
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According to Eq. (8), the deposition rate of manufacturing is calculated, which indicates the manufacturing efficiency. Fig.10 demonstrates that the deposition rate of
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droplet transfer in pulsed arc is higher than that in non-pulsed arc at the similar arc
power. Only to improve the arc power, the non-pulsed arc can achieve a higher
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manufacturing efficiency, but which will undoubtedly increase the heat input and lead
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to a greater thermal deformation. Eq. (18) and (19) are used to describe the influence
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of the arc power to the deposition rate in non-pulsed arc and pulsed arc, respectively.
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The values of R-Square of Eq. (18) and (19) are 0.995 and 0.947, respectively, which
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indicates a high degree of correlation. Therefore, these two models can be used for the prediction of the manufacturing efficiency. On the premise of determining the arc
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power, the manufacturing efficiency can be evaluated by the deposition efficiency
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according to these models.
V f 189.328 134.055 P 12.427 P 2
(18)
V f 104.694 85.351 P 5.821 P 2
(19)
By the above analysis, it is found that the pulse arc is beneficial to improve the
efficiency of additive manufacturing. Also, the production efficiency can be further improved with the increase of arc power. However, the increase of arc power will increase the heat input, and lead to a greater thermal deformation that can not be 16
ignored, even though the heat input of the pulsed arc is smaller than non-pulsed arc at the same conditions. In addition, the arc force acting on the droplet is also affected by the pulsed arc. Fig.11 shows the arc force acting on the metal droplet in pulsed arc and non-pulsed arc, which is calculated by Eq. (15). It is clear that, under the affection
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of pulsed arc, greater arc force will act on the droplets transferred by projected transfer mode. The arc force increases with increasing arc power in both pulsed arc
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and non-pulsed arc. Eq. (20) and (21) are used to describe the arc force acting on the metal droplet in non-pulsed arc and pulsed arc affected by arc power, respectively.
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From the R-Square values of equations, the degree of correlation is high.
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F 0.334 0.164 P 0.036 P 2
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F 0.207 0.021 P 0.010 P 2
(20) (21)
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According to the preliminary results, we believe that too large arc force acting on
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the droplet transfer is likely to cause the collapse of the materials deposition layer, as shown in Fig.11. When the arc force is small, the formation of the deposition layer is
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uniform. When the arc force is the largest, the deposition layer shows obvious
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collapse phenomenon. Therefore, the manufacturing efficiency and arc power are the factors that need to be considered. Pulsed arc is more suitable for the wire-arc additive manufacturing of aluminum alloy than the non-pulsed arc. As long as the arc power
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meets the requirements of manufacturing efficiency, it is necessary to use a smaller pulsed arc power.
As a result, the above sections provide experimental and computational methods to 17
analyze the effect of arc pulse on droplet transfer and deposition efficiency. Computational models about droplet size, deposition rate and arc force were proposed. The experimental results include the required parameter information for the computational models. The calculation results of models were used to develop
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regression models of manufacturing process. The correlation, which is reflected by the value of R-Square, proves the validity of the regression models. This method in
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the study actually provides a new way for the research of WAAM process. 5 Conclusions
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(1) Pulsed arc can achieve higher droplet transfer frequency than non-pulsed arc.
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Due to the pulse effect, the size of the droplet in the pulsed arc is smaller than the
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droplet size in the non-pulsed arc at the similar droplet transfer frequency in projected
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transfer mode.
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(2) According to the deposition rate of droplet transfer, the manufacturing efficiency in pulsed arc is higher than that in non-pulsed arc at the similar arc power.
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The regression models of deposition rate (Eq.(18) and Eq.(19)) established in the
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study have a high correlation and can be used to estimate the manufacturing efficiency.
(3) In view of the manufacturing efficiency and heat input controlling, the pulsed
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arc is more suitable for the wire-arc additive manufacturing of aluminum alloy than non-pulsed arc. Under the affection of pulsed arc, greater arc force will act on the droplet during the transfer process in projected transfer mode, which is not conducive to the deposition forming of aluminum alloy. Moreover, the arc force increases with 18
the increase of the arc power. Therefore, as long as the arc power meets the requirement of manufacturing efficiency, it is necessary to use a smaller pulsed arc power.
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Acknowledgements This work was supported by Basic Science and Frontier Technology Research Project
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of Chongqing Science and Technology Commission of China (Grant No.
cstc2017jcyjBX0065 and No. cstc2015jcyjA60009) and the fund of the State Key
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Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201717).
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Materials Processing Technology 214, 2488-2496.
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Shen, C., Pan, Z.X., Cuiuri, D., Dong, B.S., Li, H.J., 2016. In-depth study of the
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mechanical properties for Fe3Al based iron aluminide fabricated using the
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wire-arc additive manufacturing process. Materials Science and Engineering A
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669, 118-126.
Sun, Z., Lv, Y.H., Xu, B.S., Liu, Y.X., Lin, J.J., Wang, K.B., 2015. Investigation of
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droplet transfer behaviours in cold metal transfer (CMT) process on welding
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Ti-6Al-4V alloy. International Journal of Advanced Manufacturing Technology 80, 2007-2014.
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Tipi, A.R.D., Sani, S.K.H., Pariz, N., 2015. Improving the dynamic metal transfer model of gas metal arc welding (GMAW) process. International Journal of Advanced Manufacturing Technology 76, 657-668. Wang, J.F., Sun, Q.J., Wang, H., Liu, J.P., Feng, J.C., 2016. Effect of location on microstructure and mechanical properties of additive layer manufactured Inconel 21
625 using gas tungsten arc welding. Materials Science and Engineering A 676, 395-405. Visser, C.W., Pohl, R., Sun, C., Römer, G.W., Veld, B.H., Lohse, D., 2015. Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer. Advanced Materials
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27, 4087-4092. Xiong, J., Yin, Z.Q., Zhang, W.H., 2016. Closed-loop control of variable layer width
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for thin-walled parts in wire and arc additive manufacturing. Journal of Materials Processing Technology 233, 100-106.
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Yang, D.Q., He, C.J., Zhang, G.J., 2016. Forming characteristics of thin-wall steel
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parts by double electrode GMAW based additive manufacturing. Journal of
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Materials Processing Technology 227, 153-160.
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Yi, H., Qi, L.H., Luo, J., Jiang, Y.Y., Deng, W.W., 2016. Pinhole formation from
A
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194103-194107.
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liquid metal microdroplets impact on solid surfaces. Applied Physics Letters 108,
22
Figure captions: Fig.1 Schematic diagram of the experiment system. Fig.2 Schematic diagram of the projected transfer mode of metal droplet. Fig.3
Image of filler wire and metal droplet.
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Fig.4 Waveforms of arc voltage and current as (a) pulsed arc and (b) non-pulsed arc. Fig.5 Correspondence between arc power signals and AE signals as pulsed arc, (a)
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the correspondence from 7.00s to 7.06s and (b) highlight of T1 time interval in (a).
Fig.6 Correspondence between arc power signals and AE signals as non-pulsed arc,
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(a) the correspondence from 2.40s to 2.48s and (b) highlight of T2 time interval in (a). Influence of the arc power to the transfer frequency.
Fig.8
Illustration of critical current for pulsed-arc and non-pulsed arc.
Fig.9
Influence of the transfer frequency to the radius of droplet.
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Fig.7
Comparison of the deposition rate in manufacturing.
Fig.11
Comparison of the arc force acting on the metal droplet in pulsed arc and
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non-pulsed arc.
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Fig.10
23
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(b)
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A
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(a)
Fig.1
Illustration of the experiment system, (a) schematic diagram and (b) the photo
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of the experimental setup.
24
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(a)
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(b)
Fig.2 Process of droplets detachment in projected transfer mode, (a) images of arc
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and droplet and (b) schematic diagram of the projected transfer mode.
25
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Image of filler wire and metal droplet.
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Fig.3
(a)
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(b)
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Fig.4 Waveforms of arc voltage and current as (a) pulsed arc and (b) non-pulsed arc.
(a)
27
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(b)
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Fig.5 Correspondence between arc power signals and AE signals as pulsed arc, (a)
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the correspondence from 7.00s to 7.06s and (b) highlight of T1 time interval in (a).
28
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A
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(a)
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(b)
Fig.6 Correspondence between arc power signals and AE signals as non-pulsed arc,
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(a) the correspondence from 2.40s to 2.48s and (b) highlight of T2 time interval in (a).
29
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Influence of the arc power to the transfer frequency.
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Fig.7
Illustration of critical current for pulsed-arc and non-pulsed arc.
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Fig.8
30
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Influence of the transfer frequency to the radius of droplet.
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Fig.9
Fig.10
Comparison of the deposition rate in manufacturing.
31
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Comparison of the arc force acting on the metal droplet in pulsed arc and
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Fig.11
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non-pulsed arc.
32
Characteristics
MIG/MAG power source
DC power system
Filler wire
Coaxial feeding
3-axis linear stage system
Programmable system
Arc torch
Keeping stationary
Hall sensor
Arc current and voltage measurement
Piezo-electric sensor
AE signals measurement
CCD camera
1000 fps frame rate
Band-pass filter
625~650 nm range
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N A M ED PT CC E A
33
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Components
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Table 1 Components of the experimental setup
Table 2
Experimental parameters of pulsed arc. Current
Voltage
Frequency
Travel velocity
Shield gas flow rate
I/A
U/V
f /Hz
v/mm·s-1
Q/L·min-1
1
164
22.8
240
2
172
23.4
260
3
180
23.8
280
10
15
4
188
24.2
290
5
200
24.8
300
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A
N
U
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NO.
34
Table 3
Experimental parameters of non-pulsed arc. Current
Voltage
Frequency
Travel velocity
Shield gas flow rate
I/A
U/V
f /Hz
v/mm·s-1
Q/L·min-1
1
178
22.6
2
186
24.4
3
204
25.4
0
10
15
4
220
25.8
5
230
27.0
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A
N
U
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NO.
35
S0924-0136(18)30194-8 https://doi.org/10.1016/j.jmatprotec.2018.04.047 PROTEC 15747
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
10-8-2017 14-4-2018 30-4-2018
Please cite this article as: Luo Y, Li J, Xu J, Zhu L, Han J, Zhang C, Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing, Journal of Materials Processing Tech. (2010), https://doi.org/10.1016/j.jmatprotec.2018.04.047 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.
Influence of pulsed arc on the metal droplet deposited by projected transfer mode in wire-arc additive manufacturing
a
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Luo Yia,b,c,*, Li Jinglongc, Xu Jiea,b, Zhu Lianga,b, Han Jingtaoa,b, Zhang Chengyanga,b
School of Material Science and Engineering, Chongqing University of Technology,
b
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Chongqing 400054, People’s Republic of China
Chongqing Municipal Engineering Research Center of Institutions of Higher
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Education for Special Welding Materials and Technology, Chongqing 400054,
State Key Laboratory of Solidification Processing, Northwestern Polytechnical
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c
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People’s Republic of China
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University, Xi’an 710072, People’s Republic of China
*
Corresponding author: Luo Yi
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E-mail address of the corresponding author: [email protected]
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Graphical Abstract
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Abstract
The wire-arc additive manufacturing (WAAM) process directly and locally adds
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materials to fabricate metal components in a layer-by-layer manner. In this study,
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pulsed arc and non-pulsed arc were used in WAAM process of aluminum alloys. Arc information in manufacturing was used to identify the droplet transfer in projected transfer mode. A calculation method was proposed to analyze the arc pulse effect on
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the droplet transfer. As a result, it was found that the pulsed arc can achieve higher droplet transfer frequency and the size of the droplet in the pulsed arc is smaller than that in the non-pulsed arc. In addition, the manufacturing efficiency in pulsed arc is higher than that in non-pulsed arc at the similar arc power because of high deposition 2
rate. Although the pulsed arc is more suitable for the WAAM of aluminum alloy than non-pulsed arc, the arc force acting on the droplet in pulsed arc is greater than that in non-pulsed arc. The arc force increases with the increase of the arc power, which is likely to cause the collapse of the materials accumulation layer. Therefore, it is
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long as the arc power meets the requirement of manufacturing efficiency.
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necessary to use a smaller pulsed arc power in WAAM process of aluminum alloys as
Keywords: Pulsed arc, Additive manufacturing, Metal droplet, Deposition rate, Arc
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force
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1 Introduction
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In comparison to traditional subtractive fabrication technologies, additive manufacturing (AM) is an additional technique for fabricating complex metal
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components in a layer-by-layer manner. AM can potentially reduce energy
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consumption and materials waste and numerous processes have been successfully established for conventional metals. In order to deposit metal droplets on a solid material surface to fabricate complex 3D structures, some typical AM processes were
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proposed, which provide design freedom and environmental/ecological advantages (Bikas et al., 2016). Visser et al. (2015) used a direct-write method named laser-induced forward transfer (LIFT) by drop-based deposition. Chao et al. (2013) and Yi et al. (2016) fabricated 3D complex components by metal micro-droplet 3
deposition manufacture (MDDM) method, which was considered as a novel effective and low cost method for fabricating metal parts. Cao et al. (2006) fabricated 3D parts made of pure aluminum by depositing molten aluminum droplets, layer-by-layer, via computer information transferred through the Internet in a real-time mode. These
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studies were classified into the fused deposition modeling (FDM) process, which was one of the AM techniques.
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Another AM technique is the wire-arc additive manufacturing (WAAM), which is a high-deposition rate process and may be used for manufacturing large-scale AM parts
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(Ding et al., 2015). WAAM process uses an electric arc, either the gas metal arc
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(GMA) or the gas tungsten arc (GTA), as a heat source and wire as feedstock. In
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addition to gravity, arc force plays a most important role in the deposition process of
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metal droplet. The arc force is composed of the electromagnetic force, the plasma
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flow force, the evaporation recoil force and the charged particle impact force. It is precisely because of the arc force, the metal droplet transfer process in WAAM is
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complex. Therefore, WAAM process is different from FDM process. But, due to its
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high deposition rate, environmental friendliness and cost-competitiveness, the WAAM process becomes more attractive (Ding et al., 2016). The stability of GTA was advantageous enough to be used as heat source to produce components in WAAM.
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The thermal effect of GTA was shown by the details of the molten pool behavior and temperature field, which were important to microstructure (Wang et al., 2016). To increase the interpass temperature can facilitate phase transformation by extending the high temperature period and produce the desired microstructure (Ma et al., 2015). So 4
the GTA-WAAM process is capable of producing full density alloyed components with tensile properties that are comparable to powder metallurgy methods (Shen et al., 2016). The components produced by this method are reported to be efficient and less expensive than components produced by laser-based powder AM processes
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(Bermingham et al., 2015). In additions, it is important to succeed in obtaining repeatable and reproducible
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material with acceptable quality. The electric arc and metal droplet are important factors that affect the quality of the fabricated component in GMA-WAAM process. It
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is effective to improve the quality of manufacturing by sensing and analyzing the
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information of arc and droplet in GMA-WAAM process (Xiong et al., 2016). These
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informations include droplet size and mean value of droplet detachment frequency in
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GMA-WAAM process (Anzehaee et al., 2011). High-speed photography technology
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synchronized with the arc current and arc voltage signals is a classical experimental method to aid the understanding of the arc and droplet transfer behavior (Scotti et al.,
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2014). The phenomenological explanations based on arc physics are given to justify
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the main governing factors, such as arc force and wire feed rate, for the particular metal transfer characteristics (Yang et al., 2016). Generally, with the increase of arc current, the droplet diameter and mass decrease, while the plasma drag force and the
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gravity acting on the droplet decrease (Li et al., 2015). The transfer frequency of droplet increases with augment of wire feed rate or decrease of inductance correction value (Sun et al., 2015). In the former researches, the studies about the pulses effect on arc and droplet 5
transfer in WAAM process were insufficient. More studies focusd on numerical simulation of heat transfer in WAAM process (Foteinopoulos et al., 2018). But there was not enough experiment method and result to support the technological research. In this paper, an application of pulsed arc in WAAM process of aluminum alloys was
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proposed and the focus was the effect of pulsed arc on the metal droplet transfer behaviors. We proposed the experiment method and calculation method to analyze in
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section 2 and 3. Arc current, voltage and acoustic emission (AE) signals detected in
manufacturing were used as information source to identify droplet transfer. The
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feature information detected in experiment was used as variables to develop
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computational models and to analyze the droplet characteristics. Computational
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methods and models about droplet size, deposition rate and arc force were proposed to
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aid understanding of the arc pulse effect and predict manufacturing efficiency in
WAAM process.
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2 Experiment details
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section 4. As a result, this study provides a basis for the application of pulsed arc in
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The experiments were performed on an automatic arc processing system, as shown in Fig.1, and the characteristics of components in experiment system were provided in Table 1. In this system, wire metallic materials were melted and accumulated by
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GMA heat source. The filler wire was fed to the torch tip coaxially. The filler wire tip was melted to form metal droplets by the arc discharge caused between the wire tip and base plate. As the arc moved along the targeted path, accumulated metal droplets were deposited and solidified. A programmable 3-axis linear stage system was used to 6
move the workpiece during processing while the arc torch was kept stationary. Filler wire of aluminum alloy (ER-4043), which diameter was 1.2mm, was used in the experiments. Pure Ar was used as shielding gases and the gas flow rate was kept at 15 L/min. 6061 aluminum alloy with 8mm thickness was used as base plates in
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experiment. In view of the arc stability in electrode positive (EP) region, the direct-current electrode positive (DCEP) was used to govern the metal droplet
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dynamics. The value of arc current is the main factor determining the critical condition of projected transfer mode. Usually, the arc current needs to be larger and
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the arc voltage is matched with the arc current. Table 2 shows the processing
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parameters of pulsed arc in experiment. The arc current (I) was set as ranged from
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164A to 200A, the acr voltage (U) ranged from 22.6V to 27.0V and the pulse
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frequency ranged from 240Hz to 300Hz. The combination of these parameters can
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achieve projected transfer mode of metal droplet in pulsed arc. The non-pulsed arc needed a larger arc current than pulsed arc to realize the projected transfer mode. We
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try to make pulsed arc and non-pulsed arc work in similar projected transfer mode.
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Therefore, the matching of arc current and arc voltage of the non-pulsed arc in Table 3 was greater than that of the pulsed arc as shown in Table 2. The dynamic waveforms of arc current and arc voltage in processing were recorded
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in real-time. Hall sensor was used to measure the arc current and voltage signals. Piezo-electric sensor was mounted on the surface of base plate, which was used to measure the AE signals in processing. A CCD camera was used to record the high-speed video (HSV) of manufacturing at 1000 fps. Multiple neutral-density filters 7
were used to capture the arc behavior and droplet transfer during welding. The use of a band-pass filter with a 625~650 nm range helped in achieving the desired object sharpness and separation. The arc current, voltage and AE signals were processed by preamplifier and signal conditioner, and then they were synchronized and transferred
according to arc current and voltage signals, and it was expressed as
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P(t ) U (t ) I(t )
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to computer by data acquisition unit. The arc power (P) was calclulated by computer
(1).
The arc power was used to indicate the combined effects of the arc current and
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voltage in this paper.
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3.1 Projected transfer mode of metal droplet
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3 Results and calculation
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The metal droplet transfer mode is gonverned by the matching of the arc current
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and voltage. As the arc current exceeds a certain critical value, the arc root homogenously covers the metal droplet and results in its rapid detachment by smaller
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droplet size. Fig.2 shows the process of droplets detachment, which is called projected
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transfer mode (PTM) of metal droplet. As long as the arc current exceeds the critical value and matches an appropriate voltage, the PTM will occur both in pulsed arc and in non-pulsed arc. The images in Fig.2a recorded the real process of droplet
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transferred in PTM. It takes only about 8 ms to finish the transfer of droplet. Of course, the droplet transfer cycle is different in different process conditions, and the impact of the pulse can not be ignored. Especially, the cycle of droplet transfer depends on the pulse frequency in pulsed arc mode. Fig.3 shows the image of the 8
metal droplet and filler wire, where h is the travel distance of metal droplet transferring to base plate, which is equal to the initial height of the droplet. Owing to the continuous melting of the solid portion of the filler wire, the metal droplets are provided by filler wire and transferred to base plate to realize deposition modelling.
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Because of the driving effect from the arc force and the gravity of droplet, the metal droplet induces impact force to base plate during transferring process. The AE energy
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is excitated by the impact force and releases AE signals. The droplet dynamics
governed by arc pulses are included in the information of the arc power signals and
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the AE signals.
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Fig.4a shows the waveforms of acquired voltage and current signals as pulsed arc,
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which consist of the same phase of voltage and current pulses. The average current of
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pulsed arc is 164A and the average voltage is 22.8V. The waveforms of arc voltage and current for non-pulsed arc are shown in Fig.4b, the arc current is set to 220A and
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the arc voltage is set to 25.8V. Compared with Fig4a, the output current and voltage of
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the non-pulsed arc is smoother. Thus, the impact of pulses on the metal droplet transition is included in the characteristics of signals. In order to make use of the common effect of current and voltage to describe the characteristics of the
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manufacturing process, the arc power signal is used as the source of information.
Fig.5a shows the synchronous waveforms of the arc power signals and the AE signals acquired in manufacturing process. The AE events caused by the impact of the 9
metal droplet transfer are clearly visible, which are called droplet events, as shown in Fig.5b. With the increase of the arc energy output at t1, the arc continues to burn and the filler wire is melted to form the droplet. When the arc energy output reaches the maximum value, the droplet is transferred into the molten pool under the affection of
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the pulsed arc force and gravity at t2. Subsequently, the arc energy output decreases continuously at the trailing edge of the pulse, but the arc continues to burn at t3.
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Obviously, the occurrence time of these droplets is corresponding to the peak duration
of the arc power pulse signal. This means that the metal droplet transfer occurs when
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the maximum energy of the pulsed arc is output. Also, an arc power pulse corresponds
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to a droplet transfer, which is called the one pulse one droplet mode. So, the process
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of metal droplet transfer is closely related to the pulse effect of the arc in the projected
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transfer mode.
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For non-pulsed arc, the process of metal droplet transfer is different. Fig.6a shows the correspondence between arc power signals and AE signals as non-pulsed arc. It
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can be seen that the evolution of arc energy output is completely different from the arc
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power signals in Fig.5a and Fig.6a. The output of the arc energy under the affection of pulse is pulsating, but the energy output of the non-pulsed arc is smoother. Because the waveform of the arc power signal is smooth, the metal droplet transfer in
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non-pulsed arc mode can not be identified directly by the waveform of arc power signal. But the droplet events in AE signals are very distinct as shown in Fig.6b, which reveals the information of droplet transfer in time domain. The characteristics such as transfer frequency and duration of metal droplet transfer in non-pulsed arc 10
mode can be distinguished by the information of droplet events. Thus, AE signals can be used as an information source to identify the metal droplet transfer. Due to the different energy characteristics of the non-pulsed arc output, the process of metal droplet transfer may show completely different characteristics. The arc energy output
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reaches the maximum value at t1, after which, the arc continues to burn and the filler wire gradually melts to form the droplet. When the droplet grows to a certain volume,
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the droplet is transferred into the molten pool under the affection of the non-pulsed arc force and gravity at t2. Subsequently, the arc continues to burn at t3. It can be seen
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from the above analysis that the droplet transfer is closely related to the arc force,
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whether it is a pulsed arc or a non-pulsed arc. In the following sections, the
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characteristic information of the droplet is calculated and analyzed according to the
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signals characteristics detected in experiment.
3.2 Calculation method
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In previous works and even in recent works, the dynamic characteristics of metal
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transfer were computed numerically (Choi et al., 1998; Tipi et al., 2015) and tested experimentally (Norrish et al., 2014). The droplet transfer process in projected transfer mode was illustrated clearly in these researches, which provided a good
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fundation for following research. Under the effect of surface tension, the metal droplet travels to base plate in the form of approximate spherical element after the detachment from wire tip. Thus, the metal droplet is approximated to be a sphere in the following calculation, which volume (VR) is calculated by 11
VR
4 R3 3
(2),
where R is the radius of the metal droplet. Assuming n droplets are transferred in a manufacturing cycle, the total volume (VT) of these droplets is
VT n VR
(3).
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The total mass of these droplets is
mT VT
(4),
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where mT is the total mass of droplets transferred from wire to base plate and ρ is the
density of droplet. The density of 4043 aluminum alloy wire in the molten state is
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2.38 g/cm3, which was used in the calculation. Assuming the mass of base plate is m0,
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the total mass of base plate and droplets transferred to base plate is mt, which can be
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measured separately by balance before and after WAAM processing. The mass of
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droplets transferred from wire to base plate is
(5).
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mT mt m0
Accordingly, the radius of droplet is calculated by Eq. (6). 1
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3 mt m0 3 R 4 n
(6)
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Where, n is the number of droplets transferred in a manufacturing cycle, which can be identified from the AE signals acquired in manufacturing process. mt and m0 can be
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obtained in experiment. Vf is the additive volume per second, which is called deposition rate and is calculated by V f VR f
(7).
Where, f is the transfer frequency of the metal droplet, which can be calculated from AE signals or arc power signals. According to Eq.(2), Eq.(6) and Eq.(7), the Vf is 12
Vf
f mt m0 n
(8).
Next, the arc force acting on the droplet is calculated from the perspective of energy conservation. Eq. (9) describes the relationship of energy conservation for a droplet transferred from wire tip to base plate.
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F s Ek E p
(9),
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where Ek is the kinetic energy of the motional droplet and Ep is the potential energy of
the droplet. F is the arc force acting on the droplet during the droplet transferred from wire tip to base plate. s is the travel distance of the motional droplet. This relationship
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1 m v2 m g h 2
(10),
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F s
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is expressed as
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where m is the mass of a droplet and v is the velocity of the droplet moving. h is the initial height of the droplet, which is equal to s. Although F and v in the Eq. (10) are
2 1 mv m g h 2
(11).
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F s
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transient, they are calculated using their mean values in the study, which is
Due to the small mass of a droplet, the potential energy is negligible. So, the Eq. (11)
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is simplified as
F s
2 1 mv 2
(12).
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Where,
v
s t
(13),
and t is the time of droplet moving from tip to substrate, which is not equal to the the cycle of the droplet transfer (T). According to the observation of the droplet transfer 13
process by high-speed video, t is approximately half of T. So, the relationship between t and f is expressed as
t
1 2f
(14).
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According to the above calculation, the average arc force acting on the droplet during the transfer process is
8 F s f 2 R3 3
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(15).
The above equations reveal a feasible experimental and analytical method to the characteristics of metal droplet transfer in GMA-WAAM. Eq.(6) can be used to
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calculate the droplet radius. mt, m0 and n are the variables in this equation, which
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reflects the average value of droplet radius in the whole experiment process. Eq.(8)
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can be used to calculate the deposition efficiency of metal droplet and Eq.(15) can be used to calculate the average arc force driven the droplet transfer. According to the
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calculation results, these equations are applied to analyze the characteristics of droplet
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transfer and regression models are developed in the following section, which closely relates to the manufacturing quality and efficiency in GMA-WAAM.
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4 Discussion
The characteristics of droplet transfer reflect the manufacturing efficiency and
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quality. Transfer frequency (f) is one of the characteristics of metal droplet transfer. The transfer frequency of droplet for pulsed arc can be distinguished from the time domain signals of arc power or AE. However, the transfer frequency of droplet for non-pulsed arc can only be identified from the time domain signals of AE in this study. Fig.7 shows the influence of the arc power to the transfer frequency. The transfer 14
frequency of droplet increases with the increase of arc power not only for non-pulsed arc but also for pulsed arc. Fig.8 shows the output current waveforms of the pulsed-arc and the non-pulsed arc in the similar current setting. Compared with the current of non-pulsed arc, the pulse current of pulsed arc is much larger than the
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critical current value of projected transfer mode. Under our experimental conditions, the critical current of the projected transfer mode is about 140A. It can be seen from
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Fig.8, because of the pulse effect, the current values of the pulsed-arc are much higher
than the critical current, but the current values of the non-pulsed arc are still lower
U
than the critical current. So, the droplet transfer frequency in pulsed arc is much larger
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than that in non-pulsed arc as shown in Fig.7.
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The frequency characteristic of droplet transfer is closely related to the droplet size.
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In general, the droplet transfer frequency is higher, the droplet size is smaller. Fig.9
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shows the influence of the transfer frequency to the radius of droplet. The radius of droplet is calculated by Eq. (6). Because the pulse peak values of pulsed current are
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much greater than the values of non-pulsed current, the pulsed current produces larger
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electromagnetic force and the plasma flow force than the non-pulsed current. These forces act on the metal droplet and prevent the droplet growing up. Therefore, the droplet radius in pulsed arc is smaller than that in non-pulsed arc at the similar droplet
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transfer frequency. Eq. (16) can be used to describe the relationship between the transfer frequency and the droplet radius for non-pulsed arc. The relationship between the transfer frequency and the droplet radius for pulsed arc is described by Eq. (17). The values of R-Square of Eq. (16) and (17) are 0.997 and 0.946, respectively, which 15
indicates a high degree of correlation.
R 0.822 0.002 f 2.596 106 f 2
(16)
R 0.406 9.524 104 f 2.029 106 f 2
(17)
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According to Eq. (8), the deposition rate of manufacturing is calculated, which indicates the manufacturing efficiency. Fig.10 demonstrates that the deposition rate of
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droplet transfer in pulsed arc is higher than that in non-pulsed arc at the similar arc
power. Only to improve the arc power, the non-pulsed arc can achieve a higher
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manufacturing efficiency, but which will undoubtedly increase the heat input and lead
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to a greater thermal deformation. Eq. (18) and (19) are used to describe the influence
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of the arc power to the deposition rate in non-pulsed arc and pulsed arc, respectively.
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The values of R-Square of Eq. (18) and (19) are 0.995 and 0.947, respectively, which
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indicates a high degree of correlation. Therefore, these two models can be used for the prediction of the manufacturing efficiency. On the premise of determining the arc
PT
power, the manufacturing efficiency can be evaluated by the deposition efficiency
A
CC E
according to these models.
V f 189.328 134.055 P 12.427 P 2
(18)
V f 104.694 85.351 P 5.821 P 2
(19)
By the above analysis, it is found that the pulse arc is beneficial to improve the
efficiency of additive manufacturing. Also, the production efficiency can be further improved with the increase of arc power. However, the increase of arc power will increase the heat input, and lead to a greater thermal deformation that can not be 16
ignored, even though the heat input of the pulsed arc is smaller than non-pulsed arc at the same conditions. In addition, the arc force acting on the droplet is also affected by the pulsed arc. Fig.11 shows the arc force acting on the metal droplet in pulsed arc and non-pulsed arc, which is calculated by Eq. (15). It is clear that, under the affection
IP T
of pulsed arc, greater arc force will act on the droplets transferred by projected transfer mode. The arc force increases with increasing arc power in both pulsed arc
SC R
and non-pulsed arc. Eq. (20) and (21) are used to describe the arc force acting on the metal droplet in non-pulsed arc and pulsed arc affected by arc power, respectively.
U
From the R-Square values of equations, the degree of correlation is high.
A
F 0.334 0.164 P 0.036 P 2
N
F 0.207 0.021 P 0.010 P 2
(20) (21)
M
According to the preliminary results, we believe that too large arc force acting on
ED
the droplet transfer is likely to cause the collapse of the materials deposition layer, as shown in Fig.11. When the arc force is small, the formation of the deposition layer is
PT
uniform. When the arc force is the largest, the deposition layer shows obvious
CC E
collapse phenomenon. Therefore, the manufacturing efficiency and arc power are the factors that need to be considered. Pulsed arc is more suitable for the wire-arc additive manufacturing of aluminum alloy than the non-pulsed arc. As long as the arc power
A
meets the requirements of manufacturing efficiency, it is necessary to use a smaller pulsed arc power.
As a result, the above sections provide experimental and computational methods to 17
analyze the effect of arc pulse on droplet transfer and deposition efficiency. Computational models about droplet size, deposition rate and arc force were proposed. The experimental results include the required parameter information for the computational models. The calculation results of models were used to develop
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regression models of manufacturing process. The correlation, which is reflected by the value of R-Square, proves the validity of the regression models. This method in
SC R
the study actually provides a new way for the research of WAAM process. 5 Conclusions
U
(1) Pulsed arc can achieve higher droplet transfer frequency than non-pulsed arc.
N
Due to the pulse effect, the size of the droplet in the pulsed arc is smaller than the
A
droplet size in the non-pulsed arc at the similar droplet transfer frequency in projected
M
transfer mode.
ED
(2) According to the deposition rate of droplet transfer, the manufacturing efficiency in pulsed arc is higher than that in non-pulsed arc at the similar arc power.
PT
The regression models of deposition rate (Eq.(18) and Eq.(19)) established in the
CC E
study have a high correlation and can be used to estimate the manufacturing efficiency.
(3) In view of the manufacturing efficiency and heat input controlling, the pulsed
A
arc is more suitable for the wire-arc additive manufacturing of aluminum alloy than non-pulsed arc. Under the affection of pulsed arc, greater arc force will act on the droplet during the transfer process in projected transfer mode, which is not conducive to the deposition forming of aluminum alloy. Moreover, the arc force increases with 18
the increase of the arc power. Therefore, as long as the arc power meets the requirement of manufacturing efficiency, it is necessary to use a smaller pulsed arc power.
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Acknowledgements This work was supported by Basic Science and Frontier Technology Research Project
SC R
of Chongqing Science and Technology Commission of China (Grant No.
cstc2017jcyjBX0065 and No. cstc2015jcyjA60009) and the fund of the State Key
N
U
Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201717).
A
Reference
M
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ED
frequency in projected spray mode of a gas metal arc welding (GMAW) process. ISA Transactions 50, 409-418.
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Bermingham, M.J., Kent, D., Zhan,H., StJohn, D.H., Dargusch, M.S., 2015.
CC E
Controlling the microstructure and properties of wire arc additive manufactured Ti-6Al-4V with trace boron additions. Acta Materialia 91, 289–303.
A
Bikas, H., Stavropoulos, P., Chryssolouris G., 2016. Additive manufacturing methods and modelling approaches: a critical review. Int J Adv Manuf Technol 83, 389–405. Cao, W.B., Miyamoto, Y., 2006. Freeform fabrication of aluminum parts by direct deposition of molten aluminum. Journal of Materials Processing Technology 173, 19
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Journal of Machine Tools and Manufacture 69, 38-47. Choi, S.K., Yoo, C.D., Kim, Y.S., 1998. The dynamic analysis of metal transfer in
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pulsed current gas metal arc welding. Journal of Physics D Applied Physics 31, 207-215.
U
Ding, J., Colegrove, P., Martina, F., Williams, S., Wiktorowicz, R., Palt, M.R., 2015.
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Development of a laminar flow local shielding device for wire + arc additive
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manufacture. Journal of Materials Processing Technology 226, 99-105.
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Ding, D.H., Pan, Z.X., Cuiuri, D., Li, H.J., Duin, S.V., Larkin, N., 2016. Bead
ED
modelling and implementation of adaptive MAT path in wire and arc additive manufacturing. Robotics and Computer-Integrated Manufacturing 39, 32-42.
PT
Doodman Tipi, A.R., Hosseini sani, S.K., Pariz, N., 2015. Improving the dynamic
CC E
metal transfer model of gas metal arc welding (GMAW) process. International Journal of Advanced Manufacturing Technology 76, 657–668.
A
Foteinopoulos, P., Papacharalampopoulos, A., Stavropoulos P., 2018. On thermal modeling of Additive Manufacturing processes. CIRP Journal of Manufacturing Science and Technology 20, 66–83. Li, K., Wu, Z.S., Liu, C.R., 2015. Measurement and calculation of plasma drag force in arc welding based on high-speed photography technology and particle 20
dynamics. Materials and Design 85, 97-101. Ma, Y., Cuiuri, D., Shen, C., Li, H.J., Pan, Z.X., 2015. Effect of interpass temperature on in-situ alloying and additive manufacturing of titanium aluminides using gas tungsten arc welding. Additive Manufacturing 8, 71-77.
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Norrish, J., Cuiuri D., 2014. The controlled short circuit GMAW process: A tutorial. Journal of Manufacturing Processes 16, 86–92.
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Scotti, A., Ponomarev, V., Lucas, W., 2014. Interchangeable metal transfer phenomenon in GMA welding: Features, mechanisms, classification. Journal of
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Materials Processing Technology 214, 2488-2496.
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Shen, C., Pan, Z.X., Cuiuri, D., Dong, B.S., Li, H.J., 2016. In-depth study of the
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mechanical properties for Fe3Al based iron aluminide fabricated using the
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669, 118-126.
Sun, Z., Lv, Y.H., Xu, B.S., Liu, Y.X., Lin, J.J., Wang, K.B., 2015. Investigation of
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droplet transfer behaviours in cold metal transfer (CMT) process on welding
CC E
Ti-6Al-4V alloy. International Journal of Advanced Manufacturing Technology 80, 2007-2014.
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Tipi, A.R.D., Sani, S.K.H., Pariz, N., 2015. Improving the dynamic metal transfer model of gas metal arc welding (GMAW) process. International Journal of Advanced Manufacturing Technology 76, 657-668. Wang, J.F., Sun, Q.J., Wang, H., Liu, J.P., Feng, J.C., 2016. Effect of location on microstructure and mechanical properties of additive layer manufactured Inconel 21
625 using gas tungsten arc welding. Materials Science and Engineering A 676, 395-405. Visser, C.W., Pohl, R., Sun, C., Römer, G.W., Veld, B.H., Lohse, D., 2015. Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer. Advanced Materials
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27, 4087-4092. Xiong, J., Yin, Z.Q., Zhang, W.H., 2016. Closed-loop control of variable layer width
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Yang, D.Q., He, C.J., Zhang, G.J., 2016. Forming characteristics of thin-wall steel
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Figure captions: Fig.1 Schematic diagram of the experiment system. Fig.2 Schematic diagram of the projected transfer mode of metal droplet. Fig.3
Image of filler wire and metal droplet.
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Fig.4 Waveforms of arc voltage and current as (a) pulsed arc and (b) non-pulsed arc. Fig.5 Correspondence between arc power signals and AE signals as pulsed arc, (a)
SC R
the correspondence from 7.00s to 7.06s and (b) highlight of T1 time interval in (a).
Fig.6 Correspondence between arc power signals and AE signals as non-pulsed arc,
U
(a) the correspondence from 2.40s to 2.48s and (b) highlight of T2 time interval in (a). Influence of the arc power to the transfer frequency.
Fig.8
Illustration of critical current for pulsed-arc and non-pulsed arc.
Fig.9
Influence of the transfer frequency to the radius of droplet.
M
A
N
Fig.7
Comparison of the deposition rate in manufacturing.
Fig.11
Comparison of the arc force acting on the metal droplet in pulsed arc and
A
CC E
PT
non-pulsed arc.
ED
Fig.10
23
IP T SC R
CC E
(b)
PT
ED
M
A
N
U
(a)
Fig.1
Illustration of the experiment system, (a) schematic diagram and (b) the photo
A
of the experimental setup.
24
A
N
U
SC R
IP T
(a)
M
(b)
Fig.2 Process of droplets detachment in projected transfer mode, (a) images of arc
A
CC E
PT
ED
and droplet and (b) schematic diagram of the projected transfer mode.
25
IP T SC R U N
Image of filler wire and metal droplet.
A
CC E
PT
ED
M
A
Fig.3
(a)
26
IP T SC R U
(b)
A
CC E
PT
ED
M
A
N
Fig.4 Waveforms of arc voltage and current as (a) pulsed arc and (b) non-pulsed arc.
(a)
27
IP T SC R U N A M
(b)
ED
Fig.5 Correspondence between arc power signals and AE signals as pulsed arc, (a)
A
CC E
PT
the correspondence from 7.00s to 7.06s and (b) highlight of T1 time interval in (a).
28
M
A
N
U
SC R
IP T
(a)
ED
(b)
Fig.6 Correspondence between arc power signals and AE signals as non-pulsed arc,
A
CC E
PT
(a) the correspondence from 2.40s to 2.48s and (b) highlight of T2 time interval in (a).
29
IP T SC R
Influence of the arc power to the transfer frequency.
CC E
PT
ED
M
A
N
U
Fig.7
Illustration of critical current for pulsed-arc and non-pulsed arc.
A
Fig.8
30
IP T SC R
Influence of the transfer frequency to the radius of droplet.
A
CC E
PT
ED
M
A
N
U
Fig.9
Fig.10
Comparison of the deposition rate in manufacturing.
31
IP T SC R U N
Comparison of the arc force acting on the metal droplet in pulsed arc and
A
Fig.11
A
CC E
PT
ED
M
non-pulsed arc.
32
Characteristics
MIG/MAG power source
DC power system
Filler wire
Coaxial feeding
3-axis linear stage system
Programmable system
Arc torch
Keeping stationary
Hall sensor
Arc current and voltage measurement
Piezo-electric sensor
AE signals measurement
CCD camera
1000 fps frame rate
Band-pass filter
625~650 nm range
U
N A M ED PT CC E A
33
SC R
Components
IP T
Table 1 Components of the experimental setup
Table 2
Experimental parameters of pulsed arc. Current
Voltage
Frequency
Travel velocity
Shield gas flow rate
I/A
U/V
f /Hz
v/mm·s-1
Q/L·min-1
1
164
22.8
240
2
172
23.4
260
3
180
23.8
280
10
15
4
188
24.2
290
5
200
24.8
300
A
CC E
PT
ED
M
A
N
U
SC R
IP T
NO.
34
Table 3
Experimental parameters of non-pulsed arc. Current
Voltage
Frequency
Travel velocity
Shield gas flow rate
I/A
U/V
f /Hz
v/mm·s-1
Q/L·min-1
1
178
22.6
2
186
24.4
3
204
25.4
0
10
15
4
220
25.8
5
230
27.0
A
CC E
PT
ED
M
A
N
U
SC R
IP T
NO.
35