Joining of aluminum alloys to galvanized mild steel by the pulsed DE-GMAW with the alternation of droplet transfer

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Abstract

In this study, the welding-brazing process of pulsed double electrode gas metal arc welding (pulsed DE-GMAW) provides a potential method to join dissimilar metals (i.e., joining of aluminum alloys to galvanized mild steel). In order to evaluate the stability of welding process, a mathematical model of the alternation of droplet transfer for the pulsed DE-GMAW is established based on the spring-mass-damper model. The droplet alternative transfer is a free flight with short circuiting transfer mode, which is analyzed and detected during the welding process with double pulsed current. Meanwhile, the pulsed DE-GMAW technology is found to be feasible to join the aluminum alloys to the galvanized mild steel. The variable weldability process of aluminum-galvanized steel could be obtained with a wire by optimizing welding parameters. Microstructures of the intermetallic reaction layer and bonding mechanism of optimized weld-brazed specimens with aluminum ER4043 and ER5356 wires are analyzed and conducted by SEM and tensile test.

Introduction

There exits urgent demand of welding with high efficiency, high quality and low consumption in the welding technology due to pollution and environmental issues [1]. As an important manufacturing forming technology in the automotive industry, the great challenge of welding technology is to reduce vehicle weight by using a light aluminum alloy with excellent corrosion resistance for fabricating the car body [2], [3]. Nevertheless, steel is still the automakers’ choice for material, since it offers good formability, weldability and low overall component production costs. Moreover, the use of mild steel in automotive body applications has created a comprehensively technical base over the past century. The presence of zinc coating on these advanced steel surfaces sharply improves the resistance of its corrosion which leads to widespread application of Zn coated steel in the automotive industry [4]. Hence, the hybrid structures of the aluminum alloy and steel have been verified to improve the fuel efficiency and reduce the air emissions by reducing the weight [5], [6], [7]. Therefore, it has become an attractive research field to join aluminum alloy and steel together in recent years.

However, the joining of aluminum to steel is a great challenge because of the large differences in thermo-physical properties between the two materials, such as the large electrochemical difference of 1.22 volts, subsidiary precipitates created by solidification, different thermal properties, dissimilar thermal expansion, heat capacity and thermal conductivity, lattice transformation, large difference between the melting points (660 °C for Al alloy and 1497 °C for steel) and nearly zero solid solubility of iron in aluminum. Furthermore, the brittle Al-Fe intermetallic compounds (IMC), such as Fe2Al5, FeAl3 and FeAl [8], [9], were formed at elevated temperatures in the reaction layers between solid iron and molten aluminum, and the joint of aluminum-steel lose the strength in the bond areas due to the formation of the brittle IMC [10]. In order to join aluminum alloy and steel, various mechanical, solid state, fusion, and brazing joining processes have been investigated.

Previous studies have shown that many welding methods and weld joint types are used to produce the aluminum–steel welds, such as the tungsten inert gas (TIG) welding [11], Metal inert gas (MIG) welding [12], friction welding [13], friction stir welding [14], and laser brazing/welding [15], [16]. However, the key point of the development of new technique to join the aluminum alloys is the way to control the size and quantity of the Al/Fe intermetallic layer by controlling the heat sink with some new techniques, such as hybrid welding (i.e., TIG-MIG welding, laser-MIG welding [17]), arc-assisted laser welding [18], and minimize the brittleness of the intermetallic compound layer.

In order to reduce the welding heat input in common arc welding, many researchers have improved the arc welding technique to join the aluminum-to-steel. For instance, Su et al. [19] studied the Al–steel lap joints via direct-current pulsed gas metal arc welding (DPG) and alternate-current double-pulse gas metal arc welding (ADG). It is found that the thickness of intermetallic compound layer (which is composed of Fe2Al5, FeAl3 and Fe3Al) and the diffusion of Fe element from steel to weld seam are dependent on the amount of welding heat input. Hyoung et al. [20] evaluated the characteristics of welds by joining dissimilar alloys, steel SPRC440 and aluminum alloy 6K21. The joint was obtained by means of AC pulse MIG welding, which alternated between direct current electrode positive (DCEP) and direct current electrode negative (DCEN) based on the EN ratio. Zhang et al. [21] examined the arc characteristics and the metal transfer of the cold metal transfer (CMT) process and used it to join the aluminum and zinc-coated steel with a lap geometry by a welding–brazing method; and the microstructure of the joint and the tensile strength of the joint of aluminum-to-steel was investigated. In another study, the feasibility of applying the CMT process to weld various aluminum alloys, e.g., AA6061, AA7075 and AA5183, with galvanized mild steel for automotive applications was concerned by Cao et al. [22], who further optimized the design-of-experiment of the process windows for CMT joining process, and analyzed the microstructures and bonding mechanism of the majorized weld-brazed specimens. As a result, the heat input could be well controlled by the welding current, and the wetting of the aluminum-to-steel was also improved by dropletlet transfer with short circuiting in metal inert gas welding. Thus, the IMC formation and thickness enabled the optimization of the joint strength upon the premise of better weld seam shaping effectively.

Aiming at the low heat input and highly efficient welding technical method, Prof. Zhang put forward a new bypass arc coupled method of double electrodes welding (DE-GMAW). In this process, a bypass gas tungsten arc welding (GTAW) torch was added into the traditional gas metal arc welding (GMAW) system with the bypass part of the current through the base material. In this way, the heat input to the base material was reduced, and by adjusting and controlling the bypass current, the heat input to the workpiece could be well controlled. The desirably stable metal transfer could also be obtained when the heat input was low. In addition, the bypass arc benefited to improve the efficiency of the welding and the control of the welding thermal process as well as the metal transfer [23], [24], [25]. Moreover, in our previous studies, the DE-GMAW was improved by pulsed current, and the Pulsed DE-GMAW welding technique was obtained. Then, the pulsed DE-GMAW aluminum-steel welding process was concerned, especially for the arc length control. Nevertheless, the stability of the droplet free-short circuiting with the alternation of transfer would not be guaranteed [26], [27]. Furthermore, it was found that the pulsed DE-GMAW could join the aluminum to steel by the brazing welding [28], [29].

Based on the former theoretical and application researches, to meet the requirements of dissimilar aluminum and steel joining better (lower heat input and better weld formation with aluminum wetting), a pulsed current waveform is introduced to the novel DE-GMAW system in this paper. For the novel pulsed DE-GMAW brazing, the main and bypass arc currents are both pulsed, and a free-short circuiting droplet with the alternation of transfer is realized, based on the analysis of a mathematical model about the droplet with the alternation of transfer by the double pulse current waveform in the pulsed DE-GMAW. Then, the microstructures and bonding mechanism of optimized weld-brazed specimens with aluminum ER4043 and ER5356 wires are further investigated. Finally, mechanical testing of the aluminum alloys-to-galvanized mild steel welded joint with two kinds of wires was conducted.

Section snippets

Principle of the pulsed DE-GMAW

The pulsed DE-GMAW is developed based on the DE-GMAW. The basic principle of the pulsed DE-GMAW is shown in Fig. 1.

As is depicted by Fig. 1, the total average current Itotal, which is divided into two parts: the bypass average current Ibp and the base metal current Ibm, flows through the wire. The basic relationship is expressed as below:Itotal=Ibm+Ibp

Similar to the description in Fig. 2, in order to reduce the heat input to work-piece and increase the wetting of aluminum alloy, the current Ibm

Modeling of the pulsed DE-GMAW process

To begin with, aimed at formulating the characteristics of the pulsed DE-GMAW process, a mathematical model of the pulled DE-GMAW process is established with spring-mass-damper model [30], [31] about droplet transfer in this section. Then, the process of aluminum-steel pulsed DE-GMAW is simulated, and the double pulse currrent waveform is discovered. Finally, the free-short circuiting droplet with the alternation of transfer is achieved in the pulsed DE-GMAW, which could joint the aluminum

Experiment of the alternation of droplet transfer

A digital control system by xPC [34] is developed to control the wire feed speed, see Fig. 7, and the synchronization of the main and bypass current waveforms, other system parameters, and experimental welding torch is depicted by Fig. 8.

In order to verify that the welding wire melting rate of base metal can be controlled by the bypass current, the experiments are carried out with the unchanged welding feeder speed. It should be mentioned that, with the 1.2 mm ER 5356 wire in the process, the

Aluminum-to-steel welding-brazing lap joint

Based on the feasibility of the pulsed DE-GMAW with the alternation of droplet transfer joining the aluminum-to-galvanized mild steel, the ER 4043 and ER 5356 wires are selected in this study to weld the aluminum alloy and the galvanized steel sheet, and the 5052 aluminum alloy sheets are lap-welded to the galvanized steel work-pieces coated with Zn layer of 100 g/m2 (The base material is the Q235 mild steel), while the aluminum sheet on the top and the galvanized steel at the bottom. The

Conclusions

According to the study on the pulsed DE-GMAW with the alternation of droplet transfer welded aluminum alloy to galvanized steel, the following results can be summarized.

  • (1)

    Based on the droplet spring-mass-damper model, it is feasible to control the bypass pulsed current parameters so as to realize the pulsed DE-GMAW with the alternation of droplet transfer.

  • (2)

    It is feasible to obtain a novel joining of the aluminum alloys to galvanized steel by the pulsed DE-GMAW with free flight with short

Acknowledgement

This research is supported by National Nature Science Foundation of China (Grant No. 51205179 and No. 61365011) and 973 Plan preliminary research projects (2014CB660810).

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