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Vol. 8. Issue 5.
Pages 3950-3958 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3950-3958 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.003
Open Access
Effect of pulse frequency on arc behavior and droplet transfer of 2198 Al–Li alloy by ultrahigh-frequency pulse AC CMT welding
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Dianlong Wang, Chaofeng Wu, Yingchao Suo, Liwei Wang
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wangliwei110127@163.com

Corresponding author.
, Zhimin Liang
School of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P R China
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Tables (2)
Table 1. Process parameters of UHF-ACCMT welding.
Table 2. Composition of base metals and wire (wt.%).
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Abstract

In this study, 2198 Al–Li alloy, a low density and high-performance material for aerospace equipment, was welded using ultrahigh-frequency pulse alternating current cold metal transfer (UHF-ACCMT) welding technique. Influence of different ultrahigh-frequencies on arc behavior and droplet transfer were experimentally studied and theoretically analyzed. The results show that the UHF-ACCMT arc exhibited a good stability. During one cycle of arc burning, trends of arc length and diameter of UHF-ACCMT were similar to that of CMT arc. The average arc length of UHF-ACCMT decreased as the pulse current frequency increased from 20 to 60 kHz. An increase in the current frequency decreased the electrical conductivity, leading to a decrease in distance for sustaining arc from wire tip to base metal. The effect of average arc diameter with current frequency was inversely proportional to that of average arc length. Furthermore, the droplet diameter decreased whereas droplet length increased with an increase in the pulse current frequencies from 20 to 50 kHz. With regards to the pinch effect, an increase in the pulse current frequency enhanced the pinch force, which could be attributed to an increasing current density flowing through droplet per unit time. From 60 to 80 kHz, diameter of the arc and length of the droplet were maintained within an optimum range.

Keywords:
AC
CMT
Ultrahigh-frequency pulse current
2198 Al–Li alloy
Arc behavior
Droplet transfer
Full Text
1Introduction

Al-Li alloys are good structural materials for aerospace applications due to their low density, high strength, excellent performance at low temperatures and superior resistance for stress corrosion. In comparison with the second generation of Al–Li alloys, the third generation of Al–Li alloys have reduced Li content and increased Cu, Zr, Mn and Zn elements, leading to an enhanced strength, hardness, and lower anisotropy [1,2]. However, welding of Al alloys are considered to be difficult through conventional arc welding [3]. Al–Li alloys have difficulties during welding due to thermal cracking and porosity. Careful surface preparation, use of suitable filler materials and optimization of welding parameters were effective to eliminate porosity and cracking. Therefore, choosing suitable welding parameters (particularly low heat input) is critical to improve quality and properties of Al–Li weld joints [4–6].

The popular methods for welding Al–Li alloys include arc welding, laser welding, friction stir welding, etc [4–7]. Laser welding has a lower heat input that can reduce the quantity of pores and cracks during welding. Friction stir welding (FSW) has advantages related to the absence of radiation, sparking, weld cracks, porosity and shrinkage defects. Furthermore, it does not need beveling while welding the middle regions in thick plates. Therefore, FSW is thus often used in the welding of high strength Al alloys for aircraft. However, welding of Al–Li alloys has suffered from limitations such as high reflection, serious vaporization due to low boiling point of alloying elements, and wide focal spot during laser welding. As a result, FSW can only be applied to a relatively simple, long and straight seam as well as with the presence of the key hole at the end of a seam [8–11].

Lowering the heat input and cleaning oxide-film are effective ways to speed up the escape of pores in the welding of Al–Li alloys [12,13]. Therefore, direct current (DC) cold metal transfer (CMT) to weld Al alloys has been widely used [14,15]. Consequently, reduction of heat input and generation of metal compounds during the welding process can reduce the weld cracks [16]. It has also been found that there has been very little control over the solidification structure of the weld pool under conditions of low heat input and free growth of crystal grains [17,18]. As a result, mixed structures of coarse equiaxed dendrites and fine equiaxed non-dendrite can decrease the strength of the weld joints. With the advantage of cleaning oxide-film, AC CMT with positive polarity associated with electrode positive (EP), EP-CMT-phase, and negative polarity associated with electrode negative (EN), EN-CMT-phase, has been proposed to effectively control the crystallization process of the weld pool, refine grains, and further improve the structural properties of the joint [19,20].

The frequency of pulse current can significantly refine the solidified microstructure of Al and Ti alloys [21,22]. In a certain range the higher the frequency of pulse current is, the smaller the grains are. In alternating polarity arc welding coupled with high-frequency pulse current, reasonable frequency of pulse current is likely to generate a positive effect on the solidification structure and crystallization process of the weld pool, thereby improving the mechanical properties of the welded joint [23–26]. The effect of pulse current on the microstructure of weld is a consequence of the arc shape and arc force on the flow of molten pool. The highest frequency of coupled pulse current is currently 10 kHz for Al–Li alloy welding [27]. Therefore, the AC CMT coupled with ultrahigh frequency current from 20 to 80 kHz is employed in the current study to weld 2198 Al–Li alloy. Using a combination of test measurements and related plasma physics models, the effects of pulse current frequency on arc length, diameter and droplet transfer are studied. This work fundamentally ensures a better stability of arc and droplet transfer to improve quality of Al–Li alloy welding.

2Experimental procedures

AC CMT has welding currents in electrode positive (EP) phase and electrode negative (EN) phase, as opposed to conventional welding methods. There are significant differences between EP and EN phases. In the EP phase, cations hit the molten pool and surface of the work piece, which can quickly clean the oxide film on the surface of the work pieces. In the EN phase, the arc is relatively constricted, resulting in a higher heat production and fast melting of wire. Compared with EN phase, the arc in EP phase is more divergent and a higher heat input is transferred to the work pieces, which is undesirable and affects the quality of the welded joint [28–30]. Therefore, in the current study, phase was selected to couple with ultrahigh-frequency pulse current to improve the arc characteristics. Two relatively independent power systems were connected in parallel, and then linked to the same welding gun. There was only one arc in the welding supported by UHF-ACCMT, as shown in Fig. 1. The current I of AC CMT was 100 A. The base current Ib of UHF-ACCMT was 85 A, the peak current Ip of UHF-ACCMT was 30 A, duty cycle δ was 50%, and frequencies were 20, 30, 40, 50, 60, 70 and 80 kHz. Therefore, the total current of UHF-ACCMT is given by:

It = Ib + Ip × δ = I

Fig. 1.

The experimental installation of AC CMT advanced supply coupled with ultrahigh-frequency power supply.

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These specific parameters are listed in Table 1. High speed camera of type FASTCAM-u512 with a photo-frequency of 1000 fps was used to photo arc formation and droplet transfer. American Agilent oscilloscope (Type: 54622D, bandwidth: 100 MHz) was used to acquire current waveforms.

Table 1.

Process parameters of UHF-ACCMT welding.

Welding current Ib/A  Pulse current Ip/A  Welding rate v/(cm·min−1Gas flow rate q/(L·min−1EP duration (ms)  EN duration (ms) 
85  30  70  20  130  80 

In the Photoshop software, the “Levels” command in the “Create New Fill or Adjustment Layer” was used to set the black, gray and white parameters of the captured high-speed camera image to process the gray-scale of the arc boundary region. The black, gray and white parameters were set to 189, 0.62, and 199, respectively. In the Adobe Acrobat software, the “Measurement Tools” in the “Tools” was used to measure the length and diameter of arc, the processed scale is 1:1. The same methods were used to measure the length and diameter of droplet.

2198-T8 Al–Li alloy plates (135 mm × 95 mm × 2 mm) and Al–Si wire ER4043 with a diameter of 1.2 mm were used. The ER4043 wire can provide a large number of Al–Si eutectics during welding [31]. Eutectics had good filling capacity for reducing heat cracks [32]. In order to eliminate macros-pores effectively, the oxide film on both sides of the butt (at least 0.15 mm thickness) was cleaned using wire brush and scraper before the welding process. The grease and water stains were removed with acetone [33]. Chemical composition of 2198 Al–Li alloy and Al–Si wire ER4043 are listed in Table 2. Flat welding, using guide arc plate and crater arc plate and shielding gas of pure argon, were used in this study.

Table 2.

Composition of base metals and wire (wt.%).

  Cu  Li  Zn  Mn  Mg  Si  Fe  Al 
2198  2.9–3.5  0.8–1.1  ≦0.35  ≦0.5  0.25–0.8  ≦0.08  ≦0.01  Bal. 
ER4043  0.3  –  0.1  0.05  0.05  4.5–6.0  0.8  Bal. 
3Results and discussion3.1The output characteristics of UHF-ACCMT

Al–Li alloy plates of type 2198 were welded by using UHF-ACCMT at different frequencies. Current waveforms of CMT coupled with different frequencies pulse current during welding were recorded by Agilent oscilloscope, as shown in Fig. 2.

Fig. 2.

The output current waveforms of CMT coupled with different frequencies of pulse current. Note: The horizontal axis is 10us/DIV and the vertical axis is 20A/DIV.

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When the AC CMT arc is used as a load (Fig. 2) the oscilloscope shows that the waveform is regular, whereas current spikes appear in waveforms of ultrahigh-frequency current. The instantaneous effect on stability of arc is slight. This could be ascribed for the abrupt change in the current of AC CMT while the arc is initiated or the wire is drawn back. Ultrahigh-frequency pulse current is disturbed by ultrahigh-frequency harmonics, resulting in instantaneous current spikes.

3.2The plasma physics models and the effects of pulse current frequency on the arc behavior3.2.1The plasma physics models [34]

The arc occurs when electric current passes from wire to base metal, which is governed by Ohm’s law

where I is electric current, V is voltage, and R is resistance. Eq. (1) implies that the electric current easily passes from wire to base metal, or arc is easily ignited and sustained for a high voltage or lower resistance between wire and base metal given by
where l is gap between wire and base metal, σ is electrical conductivity, and A is effective cross-sectional area of arc. Eq. (2) also indicates that the arc between wire and base metal can be initiated earlier or easily sustained by decreasing the distance (l), and increasing electrical conductivity (σ) and cross section of the arc (A).

Electron density N in arc increases with ionization produced by Joule heat, as given by

where GJ is Joule heat, and j is electric current density. Eqs. (2) and (3) indicate that electrical conductivity plays an important role in sustaining the arc between wire and base metal. To determine the electrical conductivity, Newton’s second law of a simple harmonic oscillator or the Lorentz model for dynamics of a bound electron was used.
where displacement, acceleration and velocity of an electron are given by x,x¨≡d2x/dt2 and x˙≡v=dx/dt, respectively. e is charge and mass of an electron; E is electric field intensity; γ is rate of collisions per unit time, k is the spring constant with frequency ω0=k/m, or k=mω02.

The term on the left hand side of Eq. (4) stands for inertia of the electron, whereas the first, second and last terms on the right hand side represent electric field acting in the x-direction, a spring-like restoring force due to the binding of the electron to the nucleus, and a friction-type force proportional to velocity of the electron, respectively. In the limit, frequency corresponding to the spring ω0=0indicates an unbound electron in a good conductor. If electric current is of a frequencyω, and expressed asEt=Eeiwt, and electron displacementx(t)=xeiwt, Eq. (4) becomes

when i≡−1, Eq. (5) yields

The corresponding velocity of the electron is

Electrical conductivity of a material is described by Ohm’s law, namely, j=σE. Electric current densityj≡Nev, where Nis the number of electrons per unit volume. It follows from Eq. (7) that

Electrical conductivity (the reciprocal of electrical resistivity) is therefore dependent on frequency of electric current, given by

where ε0 is permittivity of free space, plasma frequency is

To describe the arc or collisionless plasmaω0=γ=0, electrical conductivity is given by Eq. (9) which is pure imaginary:

The above equation indicates that electrical conductivity decreases or electrical resistivity increases with increasing current frequency and with decreasing plasma frequency or electron density [34].

3.2.2The effect of pulse current frequency on arc behavior

Fig. 3 shows the dynamic evolution of arc shape within one pulse period in the EP-CMT-phase with the frequency of 0, 20, 30, 40, 50, 60, 70 and 80 kHz.

Fig. 3.

The dynamic evolution of the arc shape during one pulse period in the EP-CMT-phase coupled with different frequencies.

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As shown in Fig. 3, arc length becomes shorter whereas the arc diameter becomes bigger in the case of coupling ultrahigh-frequency current. In comparison with CMT arc (namely coupled with 0 kHz pulse current), the arc length of UHF-ACCMT decreases with the increase of current frequency from 20 to 60 kHz. From 60 to 80 kHz, the arc length slightly increases but the change is small. Meanwhile, the arc diameter of UHF-ACCMT increases with the increase of current frequency from 20 to 60 kHz. From 60 to 80 kHz, the arc diameter slightly decreases but the change is small.

After processing the gray-scale in the arc boundary region by Photoshop software, arc length L and diameter D were measured by Adobe Acrobat software, as shown in Fig. 4. Measured arc length, diameter, and current waveforms as a functions of time are shown in Fig. 5. The average arc length and diameter at different frequencies were obtained by averaging arc length and diameter within one arcing cycle, as shown in Fig. 6.

Fig. 4.

Schematic illustration on the measured arc length and diameter.

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Fig. 5.

Curves of arc length and diameter and current waveform at different frequencies.

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Fig. 6.

Average arc length and diameter at different frequencies.

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(1) The change in arc length and diameter within one arc cycle

As shown in Fig. 5, the changing trend in arc length and diameter of UHF-ACCMT is similar to that of CMT arc. In the arc initiation phase, the arc rapidly increases to the highest point due to a wire withdrawal. The arc length and diameter increases rapidly until the peak current phase is reached. Arc length then begins to show a downward trend due to the feeding of wire. Moreover, the arc diameter displayed signs of fluctuation within a small range. At the end of peak current, the arc length and diameter decreased. There has been a gradual decrease in the arc length with a decreasing rate of 0.2 mm/ms, meanwhile the arc diameter was maintained in a smaller range from 13 to 15 mm.

(2) The change of average arc length and diameter at different frequencies

Evolution of the arc shape at different frequencies is shown in Fig. 3. The arc shape of UHF-ACCMT was bell-shaped, which is similar to traditional arc. Arc length becomes shorter whereas the arc diameter becomes bigger in the case of coupling ultrahigh-frequency current [35]. As shown in Fig. 6, in comparison with CMT arc (namely coupled with 0 kHz pulse current), average arc length of UHF-ACCMT decreases with the increase of current frequency from 20 to 60 kHz. Eq. (11) shows that an increase in current frequency decreases the electrical conductivity. Therefore, increased electrical resistivity decreases the distance for sustaining arc from wire tip to base metal. i.e., arc length becomes short. For instance, the arc length was the shortest at a pulse current frequency of 60 kHz. As the frequency increased to 70 kHz, the electrical conductivity decreased, meanwhile, there was a continuous increase in electrical current density per unit time (see Eq. (11)), leading to an increase in Joule heat. Since the frequency of pulse current was quite high, argon was highly ionized by Joule heat (see Eq. (3)). Plasma frequency increases with the number of electrons per unit volume (see Eq. (10)). The quantum of increase in ωp2 and pulse current frequencyω was equal, also, the electrical conductivity was maintained in a small range when the pulse current frequency changed from 60 to 80 kHz. Therefore, the average arc length slightly increases but the change is small during this frequency range (60–80 kHz), as shown in Fig. 6. Contrarily, the arc diameter displayed an opposing trend (i.e. initial decrease followed by an increase) as compared to the arc diameter over a similar frequency range, as shown in Fig. 6. It can be considered that the arc volume is hardly affected by the ultrahigh-frequency current.

3.3The effects of pulse current frequency on droplet transfer

Fig. 7 shows the dynamic evolution of droplets and transfer during one pulse period in the EP-CMT-phase coupled with the frequency of 0 kHz and 50 kHz respectively. Note: The time interval between two pictures is 1 ms. The dynamic evolutions of droplets and their transfer coupled with pulse current frequencies of 20, 30, 40, 60, 70 and 80 kHz were similar.

Fig. 7.

The dynamic evolution of droplets forming and transfer during one pulse period in the EP-CMT-phase coupled with frequencies of 0 kHz and 50 kHz.

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Based on photos for different frequencies of pulse current, periods of droplet transfer at different frequencies pulse current are shown in Fig. 8. The droplets length and diameter within one droplet cycle at different frequencies were measured using Adobe Acrobat software. The variations of measured droplet length and diameter with respect to time at frequencies of 20, 30, 40, 50, 60, 70 and 80 kHz are shown in Fig. 9.

Fig. 8.

The periods of droplet transfer at different frequencies pulse current.

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Fig. 9.

The effect of droplet length and diameter with respect to time at different frequencies.

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Comparison of dynamic evolution of droplet forming and transfer at frequencies of 0 kHz and 50 kHz was shown in Fig. 7. The droplet transfer of AC CMT coupled with 0 kHz and 50 kHz pulse current was stable, which shows that the stability of droplet transfer was not affected by ultrahigh-frequency current. As shown in Fig. 8, it can be found that the time period of droplet transfer decreased with an increase in frequency from 20 to 50 kHz. For example, the time period of droplet transfer was 17 ms for uncoupled ultrahigh-frequency pulse current, and the time period of droplet transfer was 14 ms for the coupled with 50 kHz pulse current. Also, the time period of droplet transfer was maintained at 14 ms from 50 to 80 kHz. These results show that ultrahigh-frequency pulse current speeds up the droplet transfer rate due to an increasing of electromagnetic force, spot force and aerodynamic drag force during a coupled ultrahigh-frequency pulse current. When the pulse current frequency is from 50 to 80 kHz, the arc forces changes to a minimal value, and the period of droplet transfer keeps within a certain range.

As shown in Fig. 9, it can be found that droplet length increased whereas droplet diameter decreased when the pulse current frequencies surged from 20 to 50 kHz. The droplet length and diameter was maintained at a relatively constant value within 50 to 80 kHz. The mechanism could be interpreted as pinch effect, as shown in Fig. 10. Based on Ampere’s Law, an induced magnetic field is around electric current through an electrically conducting cylinder. As a result, Inward Lorentz force or the pinch force therefore compresses the cylinder. Metal droplet of Al–Li alloy is a good electrical conductor. An increase in pulse current frequency enhances the current density flowing through droplet per unit time, which leads to an increase in the pinch force. Therefore, droplet diameter becomes gradually small under enhanced pinch force with an increase in the frequencies. In the case of constant melting rate of wire, the droplet length increased with a decreasing droplet diameter. When pulse current frequency is from 50 to 80 kHz, the current density flowing through a droplet per unit time slightly changes, thereby maintaining a constant value of droplet length and diameter.

Fig. 10.

Schematic representation of pinch effect.

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4Conclusions

  • (1)

    Alternating current (AC) cold metal transfer (CMT) arc coupled with ultrahigh-frequency pulse current resulted in a regular current waveform and stable arc.

  • (2)

    During one arc cycle, changes of arc length and diameter of ultrahigh-frequency (UHF) ACCMT with time are similar to those of conventional CMT arc. In the arc initiation phase, arc length and diameter increased rapidly. At the peak current phase, the arc length shows a downward trend due to the wire feeding and the arc diameter fluctuated within a small range. At the end of peak current, the arc length and diameter demonstrated a slight decrease.

  • (3)

    The average length of UHF-ACCMT arc decreased with an increase in the current frequency from 20 to 60 kHz. When the coupled pulse current frequency is 60 kHz, the average length was the smallest. An increase in the current frequency decreased the electrical conductivity. An increased electrical resistivity decreases the distance for sustaining arc from wire tip to base metal. From 60 to 80 kHz, the average arc length slightly increases because of argon in maximum ionization extent. However, there was an opposite trend for the arc diameter for the same frequency range.

  • (4)

    UHF pulse current increased the droplet transfer rate due to an increase in the electromagnetic force, spot force and aerodynamic drag force. For instance, the period of droplet transfer is shortest (i.e.14 ms) when the coupled pulse current frequency was 50 kHz.

  • (5)

    The droplet diameter decreased with increasing pulse current frequencies from 20 to 50 kHz, in contrast to droplet length behavior. Beyond 50 kHz, the droplet diameter and length were maintained within a certain range. In view of the pinch effect, an increase in pulse current frequency enhanced the pinch force, which compressed the droplet resulting in a smaller diameter.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Grant No. 51875168), Key Research and Development Program of Hebei Province (Grant No. 19250202D), Natural Science Foundation of Hebei Province (Grant No. E2019208089) and Hebei Education Department Foundation (Grant No. QN2018003).

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Journal of Materials Research and Technology

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