Study on MIG-TIG hybrid brazing of galvanised thin sheet

To increase productivity when joining galvanised sheet metal, metal inert gas (MIG) and tungsten inert gas (TIG) arc brazing are combined in a hybrid process. This coupling of two arc processes increases brazing speed and reduces weld reinforcement and the tendency to spatter. Because the arcs are 6 mm apart, they influence each other. Therefore, the blowing action is a major challenge in hybrid arc brazing. A current modulation and the optimal relative position of the arcs to each other are compulsory conditions. Selected brazing parameters are tested in order to verify the hybrid arc brazing. Overlap joints are brazed on galvanised sheets. The mechanical-technological properties of the brazed joints are determined. It has been shown that hybrid brazing can achieve significantly higher brazing speed with the same or better brazing quality compared to standard arc brazing. For the realisation of the hybrid arc brazing process, software-controlled welding machines are used, which will replace conventional power sources in the future.


Introduction
Single processes like arc or laser beam brazing quickly reach their limits in terms of welding speed, deposition rate, and penetration. Therefore, hybrid processes were developed. These are combinations of two (or more) joining processes, which operate simultaneously in the same weld pool. In the recent years, the importance of such processes has increased. On one hand, the utilisation of the positive characteristics, as well as the elimination and compensation of the respective negative characteristics of the single joining processes, is the reason for this developement. On the other hand, new, high-strength, and partly coated materials require innovative welding and brazing processes, which are optimised with regard to production times, production, and investment costs. The requirements for such processes are electronic and software-controlled machines, which will replace conventional power sources in the future [1][2][3]. These machines are also necessary for the realisation of the hybrid arc brazing process.
Hybrid brazing combines energetically decoupled arcs with non-consumable (TIG) and consumable electrodes (MIG). The two arcs are 6 mm apart. Therefore, the process takes place in a common process zone. As mentioned, the blowing action is the main problem. It is caused by the electromagnetic fields. Consequently, the arcs influence each other. As a result, the arcs are not stable, have spatter output or irregular behaviour. According to Meng et Al. [4], the ratio between TIG and MIG welding currents and the correct distance between the electrodes can stabilise the welding process.
Kanemaru et al. authors [5,6] present studies in TIG-MIG/MAG process. In these studies, the process consists of a leading TIG arc that heats and opens a gap in the workpiece to allow deep penetration, while the MIG/MAG process applies the material in a highly efficient manner to complete the process. The distance between TIG and MIG processes is 4 mm. They came to the following conclusions when investigating the current balance between TIG and MIG arcs: The TIG current has to be higher than the MIG current so that the MIG arc remains stable when using pure argon [5]. Furthermore, when considering the torch angle, it was found in [6] that there is no remarkable effect on the stability of the process so that it can be expected that this method has a high tolerance for the torch angle.
In Rose Alifah Ellyana Roslan et al. [7], TIG current was varied with a constant MIG current. Here, the TIG current is always selected to be lower than the MIG current (cf. [5]). With an increase of the TIG current, the droplet release frequency of the MIG process was increased in [7], with a simultaneous reduction of the droplet diameter. It is assumed that this can have a positive effect on the weld quality. Wu et al. [8], Han et al. [9], and Mishima et al. [10] developed numerical simulations of the TIG-MIG/MAG hybrid welding process so as to analyse the thermal behaviour of the welded joints. The numerical simulation showed that there is the possibility of optimising the concentration of heat generated by the electric arcs by adjusting the angle between TIG and MIG torches.
Meng et al. [4] state that TIG-MIG/MAG hybrid welding, relative to conventional processes, can double the performance because it increases the penetration of the weld, offers greater control over the heat-affected zone, and increases the speed of welding, which, together, characterise increased productivity.
However, no specific study was found to optimise the MIG-TIG hybrid process for brazing with a trailing TIG arc. Due to the different positioning of the arcs, the leading process is always mentioned first, followed by the trailing process. Therefore, the aim in the present work is to counteract the blowing action by appropriate current modulation and the optimal position of the arcs. In addition, potential problems of MIG-TIG hybrid brazing of galvanised sheets, such as too-high heat input, spattering, and the settling of zinc vapour in the torch nozzle, must be avoided.
MIG-AC pulsed arc [11,12] welding offers the advantage of lower heat input [13]. The process sequence is similar to the MIG pulsed arc. In the pulse current phase, the current intensity increases, and there is an increase in current density as well as a radial constriction of the arc by the magnetic field (pinch effect). The pinch force (Lorentz force) constricts the molten electrode end and leads to drop detachment. If the shielding gas is chosen correctly [14,15] and the process parameters are set, one drop per pulse is detached almost without spattering [16,17]. The difference to the standard pulsed arc is that a polarity change occurs twice in the base current phase. This reduces the heat input into the base material and increases the deposition rate, while slightly reducing the penetration [18]. Aluminium sheets with a thickness of less than 1 mm can be welded as overlap and I-shaped butt joints without any problems [19,20].

Experimental setup
The experimental setup is used to analyse the hybrid process. A real-time measurement and control system from National Instruments (cRIO-9067, National Instruments Corporation, Austin, TX, USA) is used to measure the arc voltage (U S ) and the welding current (I S ). By means of a high-speed (HS) camera (type Os 7-V3-S3, Imaging Solutions GmbH, Eningen, Germany), synchronous recordings of the process are realised. Filters and a backlight lamp are used. The following welding machines are used from company MER-KLE (MERKLE Schweißanlagen-Technik GmbH, Kötz, Germany): -TIG: LogiTIG 300 AC/DC and -MIG: HighPULSE AC 354 DW The software ProDok from MERKLE is used for the current modulation. Hybrid brazing is investigated with regard to the joining properties. In detail, visual, surface crack, and hardness tests are carried out, and the joint strength is determined by tensile tests.

Position of the arcs in relation to each other
In the first step, an arc distance of 30 mm is chosen. The angle in relation to the vertical axis of the TIG torch is − 20° and of the MIG torch + 20° (Fig. 1, left). Preliminary tests have shown that the torch position basically works. The arcs do not influence each other. For the MIG process, a pulsed arc characteristic is set. Alternating current (AC) is used for the TIG process. The evaluation of the current/voltage curves and the HS-camera recordings show that the droplet detachment occurs at every current peak. If all conditions are equal, a change of the TIG torch angle to 0° (Fig. 1, right) has no influence on the process. Figure 2 shows the current/ voltage curves for 100 ms of the MIG-TIG hybrid process with different torch angles. As previously described in [6], the small influence of the torch angle on the process stability can be confirmed. To reduce the arc distance, the MIG torch angle is set to 35° to the vertical axis.
At a distance of up to 15 mm, no blowing action can be observed. With an MIG pulsed arc and TIG AC, distances of up to 10 mm mean that the TIG arc is extinguished after crossing zero from the negative current phase. With pulsed DC for the TIG arc and the same settings, the MIG wire electrode is melting as desired. That means that one droplet detaches per current peak.
Based on current/voltage investigations and HS-camera recordings, the angle was set to 0° (TIG arc) and + 35° (MIG arc) to the vertical axis. A minimum arc distance of 6 mm is achievable. The position of the torches is shown in the designed MIG-TIG hybrid torch (Fig. 3). The TIG process can be seen on the left and the MIG process on the right side. The processes are electrically isolated against each other (shown in blue), and a common protective gas nozzle is constructed (shown in purple). The single shielding gas nozzle (shown in orange) is maintained for the TIG process for better protection of the electrode. Due to the blowing action and the increased process energy caused by the additional TIG arc compared to the single process, the standard characteristic curves can only be used in a limited range.

Modification of MIG and TIG arc processes
To create the MIG characteristic curve for hybrid brazing, CuSi3 wire with a diameter of 1.0 mm and 99.99% Ar (l1) shielding gas are used. The chemical composition of the filler metal is displayed in Table 1.
For the MIG process, the wire feed speed is varied for short, pulsed, and AC pulsed arcs. All MIG arc types are considered as single and hybrid processes. In hybrid brazing with an MIG short arc (v w = 3.6 m·min −1 ), the short-circuit frequency is reduced from 28 to 7 Hz (compared to the single process). The additional energy input from the TIG arc increases the droplet size and the tendency to spatter. Therefore, it is necessary to change the current after dissolution of the short circuit bar of 45 A lowered to 22 A. Furthermore, the voltage threshold for short circuit detection (U r ) has been reduced from 13 to 11 V. A short circuit frequency of 14 Hz is set. Hybrid brazing with a short arc is subject to spatter in the material transition. The use of the short arc in hybrid brazing is not suitable.  For hybrid brazing with a pulsed arc, the standard characteristic can be used. Figure 4 shows the current/voltage curve of hybrid brazing for a wire feed speed of 2.9 m·min −1 with non-pulsed TIG-DC. The diagram shows that, at the beginning of the pulse current phase (black graph), the voltage of the TIG process (red graph) drops. At the same time, the TIG current (grey graph) follows the pulse current. This means that the TIG arc passes through to the MIG process (Fig. 5). This does not affect the process stability. This phenomenon occurs with all examined TIG current types and MIG pulsed arcs.
The difference of the standard pulsed arc and the MIG-AC pulsed arc is that a part of the basic current phase has negative polarity. Table 2 shows the changed parameters for the MIG-AC pulse process compared to the standard operating points. Figure 6 shows the current of the MIG-AC pulse process with pulsed TIG-DC. The negatively polarised part of the basic current phase is 10 ms. The time portion of the negative phase varies depending on the wire feed speed and the associated pulse frequency (see Table 2). A polarity reversal with a wire feed speed up to 4.5 m·min −1 is not possible because of the short time of the base current phase (≈ 3 ms). Table 3 shows the brazing power (P b ) and the energy per unit length (E b ) of the single process in comparison to the hybrid process. It can be seen that the energy per unit length is lower for hybrid brazing (Eq. 1, [21]) despite higher brazing power. The reason is the increased brazing speed (v b ) compared to the single processes. The standard MIG pulse process has the lowest energy per unit length. However, the heat input (Q b ) (Eq. 3, based on [22]) is smaller in the MIG-AC pulse process due to the lower thermal efficiency (η th ) [13], as the negative component increases the deposition rate with a slight decrease in penetration [19].

Brazed joints on galvanised sheet metal
Hybrid brazing is used to produce overlap joints of galvanised sheet metal (DX51D + Z; s = 1 mm). The chemical composition of the base material is displayed in Table 4. The width of the two sheets is 150 mm, with an overlap of 20 mm and a seam length of 400 mm. The TIG arc is trailing and  serves to reduce the seam reinforcement, which is caused by a high brazing speed. The MIG arc ignites first, then the TIG arc. The maximum available brazing speed is determined iteratively. The aim is a brazed seam with the smallest possible seam reinforcement without undercuts. MIG pulse and MIG-AC pulse with TIG-DC arc are used. On one hand, if TIG current is too high, the zinc layer on the bottom of the sheet burns and the base material is melted. On the other hand, if the current in the TIG process is too low, it has no effect on the external brazed seam geometry. It turns out that the best brazing results are achieved with a ratio of total brazing power to the TIG power of about 50%. Table 5 shows the braze joint surface at the left side for the single process (MIG-AC pulsed arc) and at the right side for the hybrid process (MIG-AC pulsed with TIG-DC arc). Furthermore, the cross-sections with the associated parameters are shown. Increased spatter can be seen on the sample, produced with the single process. A possible explanation is the evaporation of zinc, which impairs the stability of the arc process. In contrast, with hybrid brazing, there are almost no spatters with the used parameter settings.
As described, the energy per unit length of the MIG single process (2.04 kJ•cm −1 ) is higher than the hybrid brazing (1.66 kJ•cm −1 ). This can also be seen on the bottom of the seam; more zinc has burned off. The cross-sectional area of the brazing seam is reduced by around 60% due to the higher brazing speed that has been increased by more than 150%. In addition, the height of the brazed seam has been reduced  with the trailing TIG arc, caused by the wetting angle, which dropped from 80 to 20°. Hybrid brazing with MIG pulse and TIG-DC arcs is also process reliable. In comparison to hybrid brazing with an MIG-AC pulsed arc, there are more micro spatters but significantly less than with the MIG single process.
The hardness tests based on [23] are carried out to check whether embrittlement of the liquid metal has occurred. The so-called liquid metal embrittlement, which takes place during the brazing process, may intensify the corrosion. The brazing filler material infiltrates the galvanised steel along the grain boundaries and causes an embrittlement. When looking at the results, it quickly becomes clear that similar high hardness values are achieved in all brazed samples with a single process. It is uniform hardness across the crosssection of the brazed joint. The galvanised thin sheet had hardness of 146 HV1 < for the brazing joints with a single process. Any differences can be explained by measurement inaccuracies. Figure 7 shows an example of the hardness curve of a hybrid brazed sample and the distribution of the measuring points. The measuring points are taken in 2 series, whereby the distance between the measuring points is 0.5 mm. The measuring point is always in the centre of the sheet cross-section. When looking at the results (Fig. 7, above) it can be seen that the hardness increases selectively (170 HV1 <) in places where the heat accumulates in the sheet. As a result, these parts are slightly embrittled. However, the EDS mapping excludes the possibility of liquid metal embrittlement by copper (Fig. 8, right).
The tensile specimens were taken from the test specimen in accordance with DIN 1900 [24]. The specimen geometry was deviated from the standard to determine the actual strength of the brazed seam. For this purpose, the radii start directly after the fillet weld in order to determine the strength of the brazed seam. Despite the modification of the tensile specimens' geometry, the selected specimens showed that all brazed overlap joints, independent of the process parameters with a single or hybrid process, failed in the base material. The base material has a tensile strength of 385 N·mm −2 .

Conclusion and outlook
In this paper, the hybrid brazing technology, consisting of TIG and MIG arc brazing processes, was investigated using current modulation and brazing galvanised sheet metal. By using the latest welding machine technology, a current modulation of the MIG process is possible and that is why an arc distance of 6 mm could be achieved. Extensive brazing tests on the innovative MIG-TIG hybrid process show that the process has a very high economic potential compared to the MIG single process. The most important advantages are summarised below: 1) In hybrid brazing of a galvanised thin sheet, with a trailing TIG arc and an MIG-AC pulse process, the brazing speed is increased by over 150% compared to MIG brazing. Specifically, this means an increase in brazing speed from 30 to 77.5 cm•min −1 . 2) Despite the second heat source (TIG arc), the heat input in the base material with E b = 1.66 kJ•cm −1 is lower than the standard MIG process (cf. E b = 2.04 kJ•cm −1 ).

Fig. 7
Results of the hardness test with microhardness measurement point distribution 3) In addition, spatter formation is reduced, and the seam reinforcement is significantly reduced from 80 to 20°.
The mechanical-technological properties show that hybrid brazing can be used to produce seams that meet the requirements. With the creation of characteristic curves for hybrid brazing, the basis for a user-friendly operation of the hybrid process has been created.
Furthermore, this process coupling can also be used for welding; for example, joining of corrosion-resistant steel (t = 2 mm). The TIG arc is leading in order to increase the penetration. The trailing MIG process provides the consumable electrode. The process combination leads to a doubling of the welding speed and a decrease of the linear energy in comparison to the standard process.
Funding Open Access funding enabled and organized by Projekt DEAL. This article was written as part of the "Central Innovation Program" as a cooperative project between the University of Applied Sciences Zwickau and MERKLE Schweißanlagen-Technik GmbH under the funding code ZF4154809FH9 and the title "MIG-TIG hybrid process-Innovative hybrid arc process for welding and brazing."

Competing interests The authors declare no competing interests.
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