In the plasma-gas metal arc welding (GMAW) hybrid welding of aluminum alloy, the weld penetration under different plasma welding currents was compared. When the plasma welding current was lower than 150 A, the hybrid welding penetration was greater than the sum of the single plasma welding and the single GMAW penetration. When the plasma welding current reached 130 A, with the increase of the plasma welding current, the advantage of the weld penetration of the hybrid welding was gradually weakened compared with that of the single welding. The above change mechanism is revealed from the perspectives of heat source energy distribution and molten pool stress. The results show that when the plasma welding current is lower than 110 A, the area of the high-temperature region in the arc zone of GMAW in the hybrid welding increases compared with that in the single GMAW. The trend of arc energy transfer to the penetration direction increases under the action of arc pressure, arc shear force, electromagnetic force, and droplet impact. With the increase of plasma welding current, the high-temperature area of the GMAW arc first increases and then decreases. When the plasma current reaches 130 A, the effect of arc pressure, arc shear force, electromagnetic force, and droplet impact on increasing the penetration begins to weaken.
Research Article
Effect of plasma welding current on heat source penetration ability of plasma-GMAW hybrid welding
https://doi.org/10.21203/rs.3.rs-1634367/v1
This work is licensed under a CC BY 4.0 License
You are reading this latest preprint version
penetration ability
hybrid welding
pool behavior
Plasma-GMAW
gas metal arc welding (GMAW) is widely used in aluminum alloy welding because of its easy automation and high production efficiency [1, 2]. When welding thick-plated high-strength aluminum alloys using GMAW process, the weld requires multilayer welding because the GMAW arc energy is not concentrated. However, high heat input will lead to a series of welding problems, such as coarse grain, crack and so on, which will reduce the mechanical properties of welded joints [3]. In order to solve the above problems, the high energy beam was combined with GMAW to increase the penetration and reduce the welding heat input. The most common is laser - GMAW hybrid welding and plasma - GMAW hybrid welding. For laser-GMAW hybrid welding, aluminum alloy has a strong reflection on laser, and plasma absorbs laser energy, which reduces laser efficiency. Moreover, due to the high welding speed, pores are easily formed [4]. Plasma-GMAW hybrid welding can effectively avoid the above problems.
According to the relative position of the heat source space, plasma-GMAW hybrid welding is divided into coaxial and paraxial. In 1972, Essers et al. [5] in Philips Research Laboratories firstly introduced the coaxial plasma-GMAW process. The wire is surrounded by double arcs and the number of melting coating increases [6–8]. However, due to the plasma arc surrounding the GMAW arc, the plasma arc compression effect decreases. Nonetheless, given the coaxial welding torch structure and the coupling effects between the plasma and GMAW arc [9, 10], the arc divergence has the effect of reducing the speed of the droplets arriving at the weld pool [11, 12]. Divergence of plasma arc and reduction of droplet momentum are unfavorable to weld penetration [13, 14]. Therefore, coaxial plasma-GMAW has little advantage for welding thick plates. Paraxial plasma-GMAW hybrid welding is different from coaxial welding. The welding process is stable and can give full play to the advantages of plasma and GMAW arc. The paraxial plasma-GMAW hybrid welding has better stability than single plasma welding and better penetration ability than single GMAW [15, 16]. Compared with single GMAW, plasma-GMAW hybrid welding can refine grains and improve the mechanical properties of welded joints [17, 18]. For the penetration ability of plasma-GMAW, Sun et al. established a hybrid heat source model and considered that increasing the plasma power appropriately can increase the weld penetration [19]. Wei et al. considered that the weld penetration increased with the increase of plasma arc current (50-120A) [20]. Although the penetration ability of plasma-GMAW has been studied, the mechanism of welding parameters affecting the penetration ability of heat source is lack in-depth discussion.
In this research, the influence of plasma welding current on the penetration ability of plasma-GMAW hybrid arc is studied. The penetration forces between the hybrid heat source and the single heat source under different welding parameters are compared. The mechanism is discussed by analyzing the arc energy distribution and the stress of the molten pool.
Plasma-GMAW hybrid welding system consists of plasma welding power supply (Fronius MagicWave3000), GMAW power supply (Fronius TPS4000), SUPER-MIG welding torch, and KUKA robot. The experimental setup including data acquisition system is schematically illustrated in Fig. 1. The cross-section of the molten pool along the welding direction was observed by a “sandwich” method. SUPER-MIG welding torch includes a plasma torch perpendicular to the working plane and a GMAW torch at an angle of 16° with the plasma torch. The distance between plasma and GMAW torch keeps 8 mm, and the distance from the nozzle to the workpiece keeps 6mm.
Bead weld with various parameters was performed on the flat position by plasma-GMAW process. 7075 aluminum alloys with thickness of 10 mm and ER5356 with diameter of 1.6 mm were employed as base metal and welding wire respectively. The orifice and GMAW shielding gases were surrounded by the overall shielding gas. Purity argon (99.9%) was used as the three gases, with gas flow rates of 2.3, 23, and 40 L min− 1. Thoriated tungsten with 2-mm inner shrinkage was used as the electrode for the plasma arc. GMAW current was 200 A. Tungsten electrode as the cathode in plasma arc welding. The welding speed was controlled at 400 mm·min− 1.
| Test | Plasma welding current(I/A) | GMAW current(I/A) | Welding speed (mm min− 1) |
|---|---|---|---|
| 1 | 0 | 200 | 400 |
| 2 | 90 | 200 | 400 |
| 3 | 110 | 200 | 400 |
| 4 | 130 | 200 | 400 |
| 5 | 150 | 200 | 400 |
| 6 | 170 | 200 | 400 |
| 7 | 190 | 200 | 400 |
3.1. Weld penetration statistics
In order to ensure the reliability of the experimental results, each group of experimental parameters in Table 1 was welded three times, and the average penetration is counted. The results are shown in Fig. 2. The GMAW current is 200 A and remains unchanged. When the plasma welding current is lower than 150 A, the hybrid welding penetration is greater than the value added by the single plasma welding and the single GMAW penetration. The single GMAW weld penetration is greater than the single plasma weld penetration. When the plasma welding current is 110 A, the plasma weld penetration is only 0.067 times that of the GMAW weld penetration. When the plasma welding current is greater than 110 A, the advantages of hybrid welding are gradually weakened compared with that of single welding. Under the same conditions, the weld depth depends on the penetration ability of the heat source. It can be seen from the above results that the plasma welding current has a significant indigenous effect on the penetration of the plasma-MIG hybrid heat source, but the relationship between them is not a simple linear increase. With the increase of plasma current, the penetration depth of single plasma welding increases gradually, while that of hybrid welding increases first and then decreases. Therefore, it is inferred that when the plasma current increases to a certain value in the hybrid welding, the penetration ability of the GMAW arc may be reduced.
3.2 Mechanism of plasma welding current affecting hybrid arc penetration
3.2.1 Energy distribution of hybrid welding heat source
The energy distribution of welding arc has a great influence on its penetration ability. The change of plasma welding current will change the coupling state of hybrid arc, thus changing the energy distribution of hybrid arc. In the spectral diagnosis of arc plasma, the arc characteristic image is connected with the emission coefficient according to the change trend of the gray value, and then the arc temperature is calculated by the standard temperature method [21–23]. This method can establish a qualitative relationship between the brightness of the arc characteristic image and the arc temperature. Under general conditions, the higher the brightness of the character arc image is, the higher the arc temperature is. The hybrid arc images at different plasma welding currents were collected by a high-speed camera with Ar 794.8 nm ± 1.5 nm narrow-band filter. The obtained images were pseudo-colored according to the gray value of the image, as shown in Fig. 3. With the increase of plasma welding current, the plasma arc temperature in hybrid welding increases gradually. When the plasma welding current is lower than 110 A, there is almost no difference between the plasma arc temperature in hybrid welding and the single plasma arc at the same current. When the plasma welding current is 90 A, the high-temperature area of GMAW arc in hybrid welding is larger than that of single GMAW. When the plasma welding current increases to 110 A, the high-temperature area of GMAW in hybrid welding increases further. In the plasma-GMAW hybrid welding, due to the opposite direction of the current flowing through the plasma arc and the GMAW arc, the GMAW arc will be a repulsive force from the plasma arc in the welding direction. When the GMAW arc deviates from the wire axis under the action of repulsive force, the magnetic field generated by the electric wire will act on the GMAW arc to make it return to the wire axis as far as possible. At this time, the GMAW arc will be subjected to two forces in the opposite direction at the same time, so that the GMAW arc is compressed in the welding direction and the arc energy is more concentrated than the single GMAW arc. The advantages of hybrid arc in energy distribution are reflected. From Fig. 3, when the plasma welding current reaches 130 A, the cathode region of GMAW extends to the plasma arc. In hybrid welding, the GMAW current is constant. When the plasma current reaches a certain value, the electrons natural path effect begins to dominate the arc trajectory [24], and the GMAW arc will shift to the plasma arc during the base current period, as shown in Fig. 4. Due to the thermal inertia of the welding arc, the GMAW arc will also bias to the plasma arc in the period from t0 (The time when GMAW current reaches the pulse peak is t0) to t0 + 0.5 ms, so that the arc energy will diverge. When it reaches t0 + 1ms, the GMAW arc shape is similar to that when the plasma welding current is 90A, and is subject to certain compression in the welding direction. At the same time, when the plasma welding current reaches 130 A, the plasma arc temperature of the hybrid welding is obviously higher than that of the single plasma arc with the same current from t0 to t0 + 0.5 ms. Therefore, the reason why the penetration advantage of the hybrid heat source begins to decrease or even disappear after the plasma welding current reaches 130 A cannot be considered only from the perspective of the energy distribution of the heat source.
3.2.2 Weld pool behavior of hybrid welding
To obtain a larger welding penetration, more energy is needed to be transferred to the bottom of the molten pool. In the molten pool behavior, the flow direction of molten metal and the impact of droplets on the molten pool will affect the transmission of arc energy to the bottom of the molten pool. Since the plasma arc temperature in the hybrid welding is higher than that in the single plasma after the plasma welding current reaches 130 A, the advantages of the hybrid welding begin to decrease compared with that when the plasma welding current is 110 A, so the GMAW area in the hybrid pool should be the focus of consideration. The spatial relative position of plasma arc and GMAW arc makes the influence of plasma arc on GMAW mainly reflected in the welding direction. Figure 5 shows the weld pool cross-section of plasma-GMAW hybrid welding along the welding direction under different plasma welding currents. The greater the angle α between the solid-liquid interface line and the X-axis (welding direction) in the GMAW area, indicating that the arc energy can be transferred to the Z-axis (penetration direction). As shown in the figure, when the plasma welding current is 90 A, the angle α between the solid-liquid interface line and the X-axis in the GMAW area is greater than that of the single GMAW and the plasma current is 150 A.
The molten pool is mainly driven by the following special driving forces: arc pressure, arc shear force, electromagnetic force, capillary pressure, Marangoni force, gravity, buoyance, and droplet impact. For plasma-GMAW hybrid welding, with the change of plasma welding current, the capillary pressure, gravity, and buoyance of the GMAW pool changed little and had little effect on welding penetration.
Marangoni force is caused by the surface tension gradient of the pool. Surface tension can be expressed as:
Where T is the temperature of the metal melt, xi is the component mass content of the melt, and σ0 is the surface enthalpy of the metal material; s is the surface entropy, σ is the surface tension of the melt. Since the surface entropy is always greater than zero, the temperature coefficient of surface tension is always less than zero, that is, the higher the surface temperature is, the smaller the corresponding surface tension is. The temperature at the center of the weld pool is higher than that at the edge of the pool, and the resulting surface tension gradient causes the surface metal to flow from the center to the edge. Compared with the single GMAW welding and the paraxial plasma-GMAW hybrid welding, the heating of the specimen by the plasma arc is equivalent to the increase of the pre-welding temperature of the GMAW area. The increase in the specimen temperature will reduce the temperature gradient of the weld pool surface, thereby reducing the surface tension gradient of the pool. The trend of the pool surface metal from the center to the edge will be weakened to a certain extent. However, the plasma arc is close to the GMAW arc, and the welding speed is 400 mm·min− 1. The influence of plasma arc on the temperature gradient in the GMAW region may be small. The weld width can reflect the influence of Marangoni force on the flow of the pool. Figure 6 shows the weld section under different plasma welding currents. When the GMAW current is 200 A, the weld width of the hybrid welding does not decrease compared with that of the single GMAW welding, indicating that only when the plasma welding current changes, the influence of Marangoni force on the hybrid welding penetration is limited.
It can be seen from Fig. 3 that the arc shape of GMAW will change with the increase of plasma current, and the change of arc shape will directly affect the arc pressure on the weld pool. Figure 7 shows the arc pressure (Fa) of the GMAW pool under different plasma welding currents. The smaller the angle θ between arc pressure and Z-axis is, the greater the component force (Fa´) in the penetration direction is. For the single GMAW, the arc basically maintains the same axis as the welding wire. When the plasma welding current is 90 A, the GMAW arc is repelled by the plasma arc in the welding direction, the angle θ decreases, and the component force of the arc pressure in the penetration direction increases compared with the single GMAW. The increase of arc pressure in the penetration direction is conducive to the generation of weld pool depression and better transfer of arc energy to the bottom of the pool. When the plasma welding current is 150A, the GMAW arc is biased towards the plasma arc from t0 to t0 + 0.5ms. At this time, the angle θ between the arc pressure of the weld pool and the Z-axis increases, and the component force of the arc pressure in the penetration direction decreases. When the time reaches t0 + 1ms, the GMAW arc is repelled by the plasma arc in the welding direction, the angle θ decreases, and the component force of the arc pressure in the penetration direction increases. The arc shear force acts on the surface of the molten pool, which drives the metal flow from the center of the weld pool to the edge of the weld pool. Therefore, the smaller the arc shear force is, the more favorable the welding penetration is. For arc shear force, the larger the area of the contact area between the arc and the test plate is, the greater the arc shear force is. From Fig. 3, when the plasma welding current is 90 A, the GMAW arc is compressed in the welding direction. In hybrid welding, the arc shear force of the weld pool in the GMAW area in the welding direction is less than that of the single GMAW. When the plasma welding current is 150 A, the cathode area of the GMAW arc becomes larger in the period from t0 to t0 + 0.5 ms and is compressed in the welding direction at t0 + 1 ms. Therefore, when the plasma welding current is 150 A, the arc shear force of the GMAW pool in the welding direction is greater than that of the GMAW pool when the plasma welding current is 90 A.
In hybrid welding, the GMAW pool is also affected by the electromagnetic force (Fe) from the plasma arc and the current passing through the plasma weld pool. As shown in Fig. 8, the current in the GMAW pool will produce electromagnetic force (Fe1) in the penetration direction of the molten metal. Moreover, because the current direction in the GMAW pool is opposite to that in the plasma arc and the current direction in the plasma weld pool, the molten metal in the GMAW area is also subject to repulsion from the plasma area, and the GMAW molten pool near the plasma arc side is subject to the downward electromagnetic force (Fe2). And far away from the plasma side by upward electromagnetic force (Fe3), the molten pool metal downward flow is conducive to energy downward transfer and increases the penetration. Since the front side of the GMAW pool is close to the plasma arc, the electromagnetic force Fe2 from the plasma area is greater than that of Fe3. For the electromagnetic force Fe1 in the GMAW pool itself, the size of the arc cathode area determines the electromagnetic force. When the plasma current is 90–110 A, the GMAW arc is compressed in the welding direction, and Fe1 is increased compared with the single GMAW welding. When the plasma welding current increases to 130 A, the GMAW arc expands to the plasma arc in the period from t0 to t0 + 0.5 ms, and Fe1 decreases compared with the plasma welding current of 90 A.
In GMAW welding, the droplet carrying energy into the molten pool will also affect the weld penetration. After the droplet is separated from the wire, it will be accelerated by the arc plasma flow. The arc pressure acting on the droplet surface is expressed as:
$${P}_{r}={\int }_{R}^{r}d{P}_{r}=\frac{{I}^{2}}{\pi {R}^{4}}\left({R}^{2}-{r}^{2}\right)$$
2
Where R is the radius of the GMAW arc column, and r is the distance from any point to the center of the arc column. In the plasma-GMAW hybrid welding, because the droplet flows through the current before it falls off from the wire, the droplet is subject to the repulsion force of the plasma arc in addition to gravity, plasma flow force, and surface tension. According to Biot-Savart law, the greater the plasma welding current is, the greater the repulsion force of the droplet is, and the farther the droplet deviates from the arc center (r is greater). Figure 9 shows the position of the droplet in the arc space under different plasma welding currents in hybrid welding. When the droplet falls off from the welding wire, the GMAW current is at the base value stage. Compared with the single GMAW welding, when the plasma welding current is 90 A, the GMAW arc in the hybrid welding is compressed, and the droplets of the two are basically located in the arc center. Therefore, the arc pressure on the droplets in the hybrid welding is higher than that in the single GMAW welding, and the impact on the weld pool is thus increased. When the plasma welding current is 150 A, the arc radius of GMAW in the hybrid welding increases, and the distance between the droplet and the arc center increases, so the arc pressure on the droplet decreases.
In plasma GMAW hybrid welding of aluminum alloy, when the welding current of GMAW is200A, with the increase of plasma welding current (90-190A), the advantages of plasma-GMAW hybrid welding in the depth of penetration compared with single welding are increased first and then decreased. When the plasma welding current is 110 A, the advantage of composite welding is the largest compared with that of single welding. The mechanism is discussed by using the energy distribution of the heat source and the force of the weld pool. The following conclusions were drawn.
-
With the increase of plasma welding current, the arc burning path of GMAW in hybrid welding changes, and the area of the GMAW high-temperature zone increases first and then decreases. When the plasma welding current is lower than 110 A, the GMAW arc is compressed in the welding direction, the arc energy is more concentrated and the penetration force increases. When the plasma welding current is greater than 130 A, the GMAW arc is close to the plasma arc, the GMAW arc divergence, and the penetration force decrease.
-
The change of plasma welding current changes the GMAW arc morphology in the hybrid welding, thus changing the force of the weld pool and affecting the weld penetration. When the plasma welding current is lower than 110 A, the GMAW arc is compressed, the arc pressure in the deep direction of the pool increases, the arc shear force decreases, the electromagnetic force increases, the droplet impact increases, and the penetration ability increases. When the plasma welding current is greater than 130 A, the GMAW arc is biased towards the plasma arc. The arc pressure in the penetration direction decreases, the arc shear force increases, the electromagnetic force decreases, the droplet impact decreases, and the penetration ability begins to decrease.
Funding: This work was financially supported by the National Natural Science Foundation of China (grant no. 51665044). The authors also received financial support from the Science and Technology Programs of Inner Mongolia (grant no. 2020GG0313), and the Natural Science Foundation of Inner Mongolia (grant no. 2019LH05017, 2021SHZR2300).
Conflict of interest: No conflict of interest exists in this paper, and all the authors listed have approved the manuscript that is enclosed.
Availability of data and material: All data generated or analyzed during this study are included in this published article.
Code availability: The codes used or analyzed during the current study are available from the corresponding author on reasonable request.
Ethics approval: Not applicable.
Consent to participate: Written informed consent was obtained from all authors.
Consent for publication: The authors agreed with the submission of the manuscript to the journal.
Author contributions: Yongquan Han, and Jiao Han contributed to the conception of the study; Jiao Han and Haitao Hong performed the experiment; Jiao Han contributed significantly to the analysis and manuscript preparation; Jiao Han, Yongquan Han performed the data analyses and wrote the manuscript; Jiao Han, Yongquan Han, Zhenbang Sun helped perform the analysis with constructive discussions.
- Wu K, Zhan J, Cao X et al (2020) Metal transfer of aluminum alloy double-wire pulsed GMAW with a median waveform[J]. J Mater Process Technol 286:116761
- Wang XW, Huang Y, Zhang YM (2013) Int J Adv Manuf Technology[J] 64(1–4):207–217
- Huang CP, Mao YQ et al (2015) Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of 2060 aluminum lithium alloy[J]. Int J Adv Manuf Technol 81:1419–1431
- Liu F, Xu B, Song K et al (2021) Improvement of penetration ability of heat source for 316 stainless steel welds produced by alternating magnetic field assisted laser-MIG hybrid welding[J]. J Mater Process Technol 299:117329
- Essers WG, Liefkens AC (1972) Plasma-MIG welding developed by Philips. Mach Prod Eng 1(11):632–633
- Ono K, Liu ZJ, Era T, Uezono T, Ueyama T, Tanaka M, Nakata K (2009) Development of a Plasma MIG Welding System for Aluminum, Weld. Int 23(11):805–809
- Bai Y, Gao HM, Qiu L (2010) Droplet transition for plasma-MIG welding on aluminum alloys. Trans Nonferrous Met Soc China 20(12):2234–2239
- Chen S, Song Y, Xiao J et al (2017) Droplet transfer behavior in Pulsed Plasma-MIG welding[J].Welding & joining, (03):12–16
- Ton H (1975) Physical Properties of the Plasma-MIG Welding Arc. J Phys D Appl Phys 8(8):922–933
- Terasaki H, Simpson SW Modelling of the GMAW System in Free Flight and Short Circuiting Transfer,Sci. Technol. Weld. Joining,2005, 10(1), p120–124
- Essers WG, Walter R (1981) Heat Transfer and Penetration Mechanisms with GMA and Plasma-GMA Welding. Weld J 60(2):37–42
- Resende AA, Ferraresi VA, Scotti A, Dutra JC (2011) Influence of Welding Current in Plasma-MIG Weld Process on the Bead Weld Geometry and Wire Fusion Rate. Weld Int 25(12):910–916
- Rokhlin SI, Guu AC (1993) A study of arc force, pool depression and weld penetration during gas tungsten arc welding[J]. Weld J 8(8):S381–S390
- Hong HT, Han YQ, Du MH et al (2016) Investigation on Droplet Momentum in VPPA-GMAW Hybrid Welding of Aluminum Alloys. Int J Adv Manuf Technol 86(5–8):2301–2308
- Han Y, Tong J, Hong H et al (2019) The influence of hybrid arc coupling mechanism on GMAW arc in VPPA-GMAW hybrid welding of aluminum alloys[J]. Int J Adv Manuf Technol 101:1–4
- Tong J, Han Y, Hong H et al (2018) Mechanism of weld formation in variable polarity plasma arc-MIG hybrid welding of high strength aluminum alloy[J]. Trans China Weld Institution 39(5):069–072
- Hong H, Han Y, Yao Q et al (2018) Microstructural Investigation of VPPA–GMAW Welded 7A52 Aluminum Alloys[J]. J Mater Eng Perform 27:5571–5580
- Sun Z, Han Y, Du M et al (2020) Thermal Process Calculation and Analysis in VPPA-MIG Hybrid Welding of Thick High-Strength Aluminum Alloy Plates[J]. Rare Metal Materials and Engineering 49(8):2674–2682
- Sun Z, Han Y, Du M et al (2019) An improved simulation of temperature field in VPPA–GMAW of Al–Cu–Mg alloy[J]. J Mater Process Technol 263:366–373
- Wei B, Pan H, Sun Y et al (2014) Effects of Welding Parameters on Weld Penetration of Paraxial Plasma-MIG Hybrid Welding of 5083 Aluminum Alloy[J]. Hot Working Technology 43(13):162–166
- Farmer A, Haddad GN, Cram LE (1986) Temperature determinations in a free-burning arc. III. Measurements with molten anodes[J]. J phys d Appl phys 19(9):1723
- Hiraoka K (1997) Determining temperature distributions of gas tungsten arc (TIG) plasma by spectroscopic methods[J]. Weld Int 11(9):688
- Hiroshi S, Shinichi T, Kazuhiro N et al (2010) Measurement of dynamical variation in two-dimensional temperature distribution of TIG pulsed-arcs[J]. Trans JWRI 39(2):193–194
- Reis RP, Souza MD, Scotti A (2011) Models to describe plasma jet, arc trajectory and arc blow formation in arc welding. Weld World 55(3–4):24–32