Effect of positive/negative electrode ratio on cold metal transfer welding of 6061 aluminum alloy
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Variable polarity cold metal transfer (VPCMT) is a newly developed welding process and has drawn extensive interests because of its potential in further reducing the heat input, offering greater gap bridging ability, and improving the deposition rate. Using 6061 aluminum alloy as an example, this paper systematically investigated the influences of the key parameter in VPCMT, i.e., “positive/negative electrodes ratio (EP/EN Balance),” on weld formation, microstructure, and mechanical properties. It was found that, with the increase of the EP/EN Balance value, the duty cycle of the negative phase was reduced but the peak current and maximum temperature of the weld pool were both increased resulting in a higher heat input. The positive/negative electrode ratio had little impact on the grain misorientation angle in the fusion zone. Hardness of the joint was higher with larger positive duty cycles because of the precipitate phase and small grain size. An increased negative duty cycle resulted in lower heat input leading to insufficient melting of the material and deterioration of the mechanical strength of the welds.
KeywordsVariable polarity cold metal transfer Positive/negative electrode ratio Aluminum welding process Mechanical properties Grain size
In response to the low carbon emission target rectifying the climate change challenge, light weight structures are desired in aerospace, transportation, and many other industrial applications, where welded high strength Al structures are preferred due to its low density, high specific strength, and high structural integrity . Cold metal transfer (CMT) has been widely applied for welding aluminum alloys due to its low heat input and excellent stability in terms of process control. Pickin and Young  found that the droplet transfer during CMT is rather different from other arc welding methods. The direction of the wire movement would be reversed after contacting the welding pool. Key parameters affecting the microstructure, mechanical properties, and the welding quality include welding speed, peak current, short-circuiting time, and background time etc. which have been studied extensively in the past decade. For example, Heng Zhang et al.  used different welding speeds to study the microstructures of deposited AZ31 magnesium alloy clad. Large number of pores were observed in the fusion zone at a high welding speed. With the decrease of welding speed, the HAZ became more susceptible to liquation crackings. Wang et al.  found that increased short-circuit current resulted in larger grain size. Cao et al.  obtained a good brazed joint between aluminum and galvanized steel by carefully controlling the wire feed speed.
A new droplet transfer mode for CMT welding was recently developed by manipulating the ratio of positive/negative polarity, i.e., the so-called variable polarity cold metal transfer (VPCMT) advance (C + A) process , which offers greater gap bridging ability, even lower heat input and higher deposition rate. The positive/negative polarity would change in the short-circuit phase. Guojin Li et al.  were able to use the same parameters for different gap width and successfully achieved satisfactory mechanical strength in all weld joins which demonstrated an excellent gap bridging ability through this VPCMT. Cong et al.  found that variable polarity cold metal transfer could reduce the porosity of weld seams with appropriate combination of parameter. Chen Zhang et al.  investigated the wire arc additive manufacturing of Al-6Mg alloy using variable polarity cold metal transfer. The variable polarity cold metal transfer had lower heat input and better deposition rate ability. During the welding process, there is a unique parameter called “EP/EN Balance” representing the electrode positive/negative ratio. Peng Wang et al.  used different electrode positive/negative ratio to weld Mg/Al dissimilar welded joints and found that, with the increase of EN-CMT-cycles, the energy input was lower and the strength of joints was higher. However, the influences of the electrode positive/negative ratio, i.e., “EP/EN balance,” on the welding process and weld quality are still unclear and systematic study is therefore needed for the interest of both industry and academia.
This paper is to provide a further study on the influences of the “EP/EN Balance” parameter on welding process in terms of the weld formation, mechanical properties of the joints, and welding defects control. It is hoped that this study could provide further understanding of the variable polarity CMT and potentially be used as a technical guidance for people who want to manipulate the CMT welding process and achieve a better welding quality.
2 Experimental procedure
The testing system for the welding current/voltage acquisition consisted of an oscilloscope (DSOX4024A) and a current probe (N2780B). The frequency of droplet transfer could be extracted from the current waveform. The high-speed CCD camera was a Phantom VEO 640S (resolution: 1280 × 600) which was able to record the dynamic behavior of the arc and molten droplets. The interval time between every two successive photos was about 0.2 ms. In addition, an FLIR A615 IR thermal imaging system was used to acquire the temperature signals. The infrared images of the molten pool were collected in real time by the IR thermal imaging system that were used to investigate the relationship between electrode positive/negative ratio and the temperature of the welding pool.
Chemical composition of 6061 aluminum alloy and ER4043 filler metal
Welding parameters for the experiment
Welding speed (mm/s)
Arc length correction (%)
3 Results and discussion
3.1 The current waveforms
The relationship between EP/EN Balance and positive/negative cycles
Electrode positive/negative ratio
3.2 Weld bead formation
3.3 Arc behavior and droplet size
The droplet behavior prior contacting the weld pool in short-circuiting transfer was observed by the high-speed video. The droplet would detach from the wire tip by retracting the filler wire. The adjustment of positive/negative electrode ratio had little impact on the droplet size. Maoai Chen et al.  found that the droplet size increases with an increase in the boost current. Due to the same peak current, the droplet size during positive cycle remained unchanged. The peak current of negative cycle increased when the EP/EN Balance increased. However, the difference in current was very small so that the droplet size was nearly the same when the polarity switched to negative phase. Guojin Li et al.  have calculated the droplet size of different polarity and obtained a relationship between different polarities, that is: Vcp = 1.024Vc, Vcn = 1.65Vc, where Vc is the volume for CMT mode, Vcn is the volume for negative phase, and Vcp is the volume of droplet for positive phase. The droplet size during negative cycle was larger than the size during positive phase. This was mainly because that the positive ions in the arc had a greater impact on the droplet, which revealed that more heat input was used to melt the wire for the negative cycle to improve the deposition rate.
3.3.1 Effect on infrared imaging
3.4 Grain size and crystal orientation
EBSD analysis was carried out at the center of the fusion zone and HAZ of transverse cross-sections for two samples. The majority of the fusion zone contained equiaxed grains. The formation of equiaxed grains was mainly because of the arc swinging and arc pulsation. There are seldom any columnar grains because of the use of cold metal transfer arc mode with periodic changes of polarity.
3.5 Mechanical properties
The variety of positive/negative electrode ratio would affect the current waveform. The peak value of the negative current became lower with the decrease of the positive/negative electrode ratio, while the peak current of positive cycle kept unchanged. The time of droplet transfer for positive cycle was longer due to the lower plasma drag force.
The arc length for negative cycle was longer with the increase of the EP/EN Balance value. The maximum temperature of welding pool increased with a larger proportion of positive cycles according to the infrared images which would affect the weld formation.
The grain size was larger with increased positive duty cycles because of the higher heat input. The HAZ exhibited a smaller grain size but a larger misorientation angle comparing with the fusion zone.
By increasing the positive duty cycles, the microhardness in both HAZ and fusion zone were increased correspondingly. The tensile strength of the welded joints increased first but then slightly decreased when the positive duty cycle exceeded certain level. Some holes on the fracture surface could easily be formed with the fewer positive cycle due to the lower heat input.
This research was supported by the Natural Science Foundation of China (51605276, 51905333), Shanghai Sailing Program (19YF1418100), Shanghai Science and Technology Committee Innovation Grant (17JC1400600, 17JC1400601), Karamay Science and Technology Major Project (2018ZD002B), Aid for Xinjiang Science and Technology Project (2019E0235), and Shanghai local colleges and universities capacity building special plan project (19030501300).
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