Abstract
A novel molten wire tungsten inert gas welding process was presented. In this process, a welding arc is used to melt the workpiece, and a melting arc is used to melt the feeding wire. Metal transfer is separated from the melting of the wire and the workpiece, and the arcs are stable no matter the droplet transfer mode is spraying or globular. The spatter rate in the process is close to zero. Mechanical properties of low carbon steel joint bonded with this process are superior to those bonded with the metal inert gas welding process.
Keywords
Introduction
Tungsten inert gas (TIG) welding is a widely used fusion welding process, in which an arc is established between a non-consumable tungsten electrode and a workpiece. Filler metals can be used if necessary. The TIG process is stable and spatter free, so the quality of a TIG welded joint is superior to those manufactured by most of other fusion welding processes. The TIG process can be applied in joining almost any kinds of metals. The major disadvantages of the TIG process are low penetration and low deposition rate [1, 2]. So it is very uneconomical to weld components of large section thickness with the TIG process. Limited by its low penetration and low efficiency, the TIG process is mainly used to weld thin sheets and tubes.
With the fast development of the manufacturing industry, there is a strong demand to improve productivity continuously to remain competitive. The demand for joining processes with high speed and high efficiency promote fast development of such processes as metal inert/active gas (MIG/MAG) welding and submerged arc welding (SAW). Many MIG/MAG and SAW processes with high efficiency like twin-wire MIG/MAG [3] and three-wire SAW [4] have been developed. But MIG/MAG and SAW processes are not comparable with the TIG process in process stability, all-position suitability and weld quality. So a lot of efforts are spent on developing TIG processes with high efficiency and high speed, like hot wire TIG, activated TIG (A-TIG), key hole TIG (K-TIG), and hybrid TIG-MIG process.
In a hot wire TIG process [5], a separate power source is connected to the automatically fed wire, and the resistance heat produced is used to heat the wire before it reaches the molten pool. The heated wire melts faster than a cold one, and the welding efficiency and welding speed increase. The hot wire TIG process has been applied successfully in joining carbon steels, low alloy steels, stainless steels, nickel alloys and titanium alloys. In an A-TIG process, activating fluxes is used to increase the penetration of the TIG process, so welding efficiency and welding speed of it can be higher [6]. When a high current is used in a TIG process, a key-hole can be made in the molten pool, just like that in the plasma key hole welding process. This process is called K-TIG [7]. Owing to its high current, welding efficiency and welding speed of K-TIG are high. In a hybrid TIG-MIG process [8], a TIG torch is combined with a MIG torch to weld the base metal. The MIG torch is electrode positive, and the TIG torch is electrode negative. The hybrid TIG-MIG process appears to be more stable than the traditional MIG process and spatter free.
Though so many TIG processes with high efficiency and high speed have been realised, the efforts are going on. In this paper, a novel molten wire TIG (MWTIG) process is presented.
Materials and methods
Setup of the MWTIG process
The schematic diagram of the MWTIG process is shown in Figure 1. A TIG torch was connected to the cathode of a DC power source, and the workpiece was connected to the anode. The DC power source was equipped with a high frequency generator. A welding current adjuster was put between the workpiece and the power source. A MIG torch was connected to the anode through a melting current adjuster. A certain filler wire was fed forward by a wire feeder through the MIG torch. The MIG torch and the TIG torch were fixed together with an angle of about 75° between their axes. The MIG torch and the TIG torch were arranged in a plane perpendicular to the welding direction. The angle between the MIG torch and the workpiece was about 30°, and the angle between the TIG torch and the workpiece was about 75°. Extension of the wire was 15 mm. The setup of the MWTIG equipment is shown in Figure 2.
Schematic diagram of the MWTIG process: (1) power source, (2) high frequency generator, (3) TIG torch, (4) MIG torch, (5) wire feeder, (6) melting current adjuster, (7) welding current adjuster, (8) workpiece. Setup of the MWTIG equipment.
When the welding process started, a high frequency pulse was sent out from the high frequency generator, and a welding arc was ignited between the tungsten electrode of the TIG torch and the workpiece. In the meantime, the wire was fed forward by the wire feeder, and a melting arc was ignited between the wire tip and the tungsten electrode. The workpiece was melted by the welding arc to form the molten pool, and the wire was melted by the melting arc. Droplets grew up on the wire tip, driven by electromagnetic force, arc plasma force, gas flow force and gravity force, detached from the wire tip and went into the molten pool, and became part of it, as shown in Figure 3. The droplets were created in the melting arc between the wire and the negative tungsten electrode, but, instead of falling onto the negative electrode, they went out of the melting arc and entered the molten pool. So they did not disturb the stability of the melting arc. The melting arc was very stable, no matter the droplets were transferred as spray or in a globular form. Before the droplets entered the molten pool, only a small part of the welding arc was interfered by them. The TIG welding arc was very stable too.
Principle of the MWTIG process.
A MIG torch can be combined with a TIG torch to make welds in different ways. The first combination has been made by Lu and Kou [9]. In their works, a TIG torch has been used as the cathode, replacing the workpiece, to measure the power input due to the combined action of arc radiation/convection and filler metal droplets. The workpiece has not been connected to neither anode nor cathode, so it is heated only by arc radiation/convection and filler metal droplets. This setup is similar to the so-called twin-wire indirect arc welding process [10], in which the TIG torch is replaced by another MIG torch. The indirect arc welding process has a high melting rate and a low penetration depth of the weld joint. It is very suitable for surface cladding, in which the dilution ratio of the cladding layer is very critical. And its virtue becomes its disadvantage when penetration is a major concern. However, in the MWTIG process, a welding arc is employed to melt the base metal, and the penetration is larger compared with the indirect arc process.
Another way to combine a TIG torch and a MIG torch can be found in the hybrid TIG-MIG process [8], in which the TIG torch is connected to the cathode of a TIG power source, and the MIG torch is connected to the anode of a MIG power source. The anode of the TIG power source and the cathode of the MIG power source are connected together and connected to the workpiece. The TIG arc can make the MIG arc more stable, but the interference of droplets on the MIG arc cannot be avoided. However, neither arcs are interfered by the droplets in the MWTIG process and are more stable.
Experiments
Composition of Q235 steel (wt-%).
Mechanical properties of Q235 steel.
Chemical composition of ER50-6 wire (wt-%).
Process parameters of MWTIG.
Process parameters of TIG and MIG processes.
The welded specimens were sectioned, polished and etched with a 4% nitric acid alcohol solution. Microstructures were examined with scanning electron microscopy. Mechanical properties of the joints were tested.
Results and discussion
Influence of arc voltage and melting current on weld appearance
The influence of arc voltage on weld appearance is shown in Figure 4. When the arc voltage was low, like 10 V, the melting arc was unstable, and the wire did not melt properly, so the welding process could not be carried on (Figure 4(a)). When the arc voltage was 15 V, both of the melting arc and the welding arc were stable, and the base metal melted to form a molten pool. Droplets were transferred into the molten pool and solidified together with the melted base metal to form the weld bead. However, appearance of the weld bead suggested that the heat input was insufficient, as shown in Figure 4(b). The weld bead was narrow, high and not straight. When the arc voltage was 20 V, the weld bead was sound, as shown in Figure 4(c). Increasing the arc voltage further to 25 V (Figure 4(d)), appearance of the weld bead suggested that the heat input was a little too high if no changes were made to welding speed or welding current.
Influence of arc voltage on weld appearance when melting current is 150 A: (a) arc voltage is 10 V, (b) arc voltage is 15 V, (c) arc voltage is 20 V and (d) arc voltage is 25 V.
The influence of melting current on weld appearance is shown in Figure 5. When the melting current was 130 A, less filler metal melted, and the weld was narrow (Figure 5(a)). Though the weld could be produced, wetting on the base metal was not good. When the melting current was 150 A (Figure 4(c)), 170 A (Figure 5(b)), or even 190 A (Figure 5(c)), a proper weld could be made.
Influence of melting current on weld appearance when arc voltage is 20 V: (a) melting current is 130 A, (b) melting current is 170 A and (c) melting current is 190 A.
Spatter rate of MWTIG process
Figure 6 shows the spatter rate of the MWTIG process and the TIG and MIG processes. It can be found that the spatter rate in the MWTIG process is much lower than that in the MIG process, a little higher than that in the TIG process. The MIG welding current and the MWTIG melting current used in this investigation were 165 and 170 A, respectively, in the range below the transition current for a Φ 1.2 mm low carbon steel wire [11], and the metal transfer mode in both the processes were globular mode. The difference of the spatter rate between the MIG process and the MWTIG process is due to different behaviours of the droplets in these two processes.
Spatter rate of MWTIG, TIG and MIG processes.
Spatters originate from metal droplets that form at the electrode tip but do not transfer axially to the weld pool owing to the presence of non-axial or explosive forces [11]. In the MIG process with a globular metal transfer mode, droplets bigger than the diameter of the electrode wire are transferred from the wire tip to the molten pool through the arc with low frequency, usually several tens of droplets per second, causing not only large variations in the arc length and arc voltage, but also great distortion of the flow of the arc plasma [12]. Spatter is severe in this metal transfer mode due to the unstable arc. With the increasing of welding current, metal transfer changes into the spray mode. Droplets with a size comparable to the diameter of the electrode wire or even smaller are transferred with high frequency, usually a hundred droplets per second, causing less variations of arc length and arc voltage, and the flow of the arc plasma is more stable. The arc is stable and the spatter is lower in this metal transfer mode. So, most of the MIG processes are carried out in the spray mode to achieve high weld quality. However, the spray metal transfer mode can only be realised with welding current higher than a certain criterion, which is called the transition current. High welding current brings high heat input into the joint, which has been trying to be avoided for many years.
In the MWTIG process, the wire is melted by the melting arc, which is established between the wire and the tungsten electrode tip. The droplet grows on the tip of the wire. After detaching from the wire tip, the droplet flies out of the melting arc and goes into the molten pool. The distance between the tip of the wire and the tip of the tungsten electrode (and so the arc voltage) is affected only a little by the growth of the droplets, not influenced by the droplets after detaching. So even in the globular metal transfer mode, the diameter of the droplet being large, the melting arc is still very stable. The distance between the tungsten electrode tip and the substrate is not influenced by metal transfer, and the welding arc is as stable as a pure TIG arc. The metal transfer is separated from the melting of the wire and the workpiece, and has little influence on the stability of the arc. So the spatter is very low in both the globular and the spray metal transfer modes.
No filler metal was used for the TIG process in this investigation, and no metal transfer took place in the TIG process. It was spatter free.
Microstructures of MWTIG weld
Microstructures of the MWTIG weld are shown in Figure 7(a). Typical microstructures in a low carbon steel weld, ferrite and pearlite, are found in the MWMIG weld. Compared with the microstructures in the TIG weld (Figure 7(b)) and the MIG weld (Figure 7(c)), the microstructures of the MWTIG weld consist of less pearlite and more ferrite. The microstructure of the TIG weld is finer. In this study, almost identical welding currents and arc voltages were adopted for the MIG and TIG processes, and the welding velocity in the TIG process was lower than that in the MIG process. So the heat input in the TIG process was higher. The welding velocity in the MWTIG process was identical to that in the MIG process, and the total current (welding current plus melting current) was higher than that in the MIG process. The nominal heat input of the MWTIG process was higher than that of the MIG process. High heat input slowed down the cooling rate of the MWTIG welded specimen, so more proeutectoid ferrite and less pearlite precipitated after solidification of the weld.
Microstructures of the MWTIG weld, TIG weld and MIG weld: (a) MWTIG weld, (b) TIG weld and (c) MIG weld.
Mechanical properties of MWTIG joint
Figure 8 shows the hardness of the MWTIG joint compared with those of the TIG joint and the MIG joint. Hardness of the weld in the MWTIG joint is higher than that of the base metal. However, the hardness of the weld in the MWTIG joint is lower than that of the weld in the TIG joint (Figure 8(a)) and that of the weld in the MIG joint (Figure 8(b)). This corresponds to the microstructures of less pearlite and more ferrite in the MWTIG weld.
Hardness of the MWTIG joint vs. the TIG joint and the MIG joint: (a) the MWTIG joint vs. the TIG joint and (b) the MWTIG joint vs. the MIG joint.
Figure 9 shows tensile strength and elongation of the MWTIG joint compared with those of the TIG joint, the MIG joint and the base metal. The tensile strength of the MWTIG joint, similar to the TIG joint, is a little higher than that of the base metal. The tensile strength of the MIG joint is the lowest among the three joints, a little lower than that of the base metal, with much higher data scatter. Elongation of the MWTIG joint is almost the same as those of the other two joints and is lower than that of the base metal. This is caused by the uneven hardness distribution of hardness in the joint after welding. Uneven hardness distribution results in uneven plastic deformation in the joint. The hardness of the welds is higher than that of the HAZs in all joints (Figure 8(a,b)). When the specimens were stretched, plastic deformation appeared in the soft HAZs and base metals, but not in the welds. All the fracture of the tensile specimens appeared in the HAZs, as shown in Figure 9(c). So the nominal elongation of the three joints is lower than the base metal, which can be stretched evenly.
Tensile strength and elongation of the MWTIG joint compared with those of the TIG joint, the MIG joint and the base metal: (a) tensile strength, (b) elongation and (c) MWTIG specimens after tensile strength test.
Figure 10 shows the impact energy of the MWTIG weld and HAZ, compared with those of the TIG and the MIG welds and HAZs. Impact toughness of the MWTIG weld is a little higher than those of the TIG and the MIG welds. Impact toughness of all the HAZs is the same.
Impact energy of the MWTIG weld and HAZ, compared those of the TIG and the MIG welds and HAZs.
Conclusion
A novel MWTIG welding process was presented in this paper and the performance of the process were compared with the TIG welding and MIG welding processes. The conclusions are as follows:
The MWTIG welding process is stable no matter the droplet transfer mode is spraying or globular. Spatter rate in the MWTIG welding process is much lower than that in the MIG welding process, closer to zero. Hardness of the low carbon steel weld joined by the MWTIG welding process is lower than those joined by both the TIG welding and MIG welding processes. Tensile strength of the low carbon steel joint joined by the MWTIG welding process is higher than that joined by the MIG welding process, closer to that joined by the TIG welding process. Impact toughness of the low carbon steel weld joined by the MWTIG welding process is higher than those joined by both the TIG welding and MIG welding processes. Impact toughness of the low carbon steel HAZ joined by the MWTIG welding process is the same as those joined by both the TIG welding and MIG welding processes. Elongation of the low carbon steel joint joined by the MWTIG welding process is the same as those joined by both the TIG welding and MIG welding processes.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.