Short CommunicationThe CMT short-circuiting metal transfer process and its use in thin aluminium sheets welding
Introduction
In order to protect the environment, using light material is becoming more and more interesting in the automobile industry [1], [2], [3], [4]. Because aluminium alloy is light and strong and can be recycled, new types of car produced from thin aluminium alloys are under rapid development and some products are already on the market. Then the welding of the thin aluminium is the key problem to accelerate the use of the aluminium alloy and guarantee the property of the cars made of aluminium alloy. Welding of thin aluminium sheets have many problems, such as burning through and distortion. Conventional MIG welding usually used in aluminium welding, but when joining thin aluminium sheets, the lack of control over penetration often limits its use in the field. Short-circuiting metal transfer is a suitable method to join thin aluminium sheets because of its low heat input characteristics [5]. But the excessive spatter during the welding process also gives the producer a big problem. A recent development in welding technology is the cold metal transfer (CMT) process which is ideally suited to welding aluminium owing to the no-spatter welding process and low thermal input.
The cold metal transfer process as we called CMT is a modified MIG welding process which was invented by Fronius company. The principal innovation is that the motions of the wire have been integrated into the welding process and into the overall control of the process. Every time the short circuit occurs, the digital process-control both interrupts the power supply and controls the retraction of the wire. The wire retraction motion assists droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the aid of the electromagnetic force. Then the heat input and spatter can be decreased greatly [6].
The major aim of this article is to examine the waveform of welding current and voltage and the metal transfer of the CMT process. At the same time, bead-on-plate welding on the pure aluminium was done to evaluate this process fitness for thin aluminium sheets welding.
Section snippets
Experimental set-up and procedure
Through-the-arc sensing of the welding current and arc voltage and high-speed imaging of the droplet transfer are combined with each other to capture the pulse waveform and the metal transfer process during CMT welding process. Fig. 1 illustrates the schematic of the welding experimental set-up. It consists of four systems: (1) the industrial PC as a central control computer, (2) the welding system including CMT power source and its accessories, (3) the sensing system for the welding current
The waveform features of the process
A typical CMT welding electrical signal cycle is shown in Fig. 2. A cycle can be defined as the period required to deposit a droplet of molten electrode into the weld pool. From the figure, we can divide the cycle into three phases:
- (i)
The peak current phase. This is represented by a constant arc voltage corresponding to an high pulse of current. The high pulse current make the ignition of the welding arc easily and then heats the wire electrode to form droplet.
- (ii)
The background current phase. The
Conclusion
A current and voltage waveforms sensing system and a metal transfer visual sensing system were constructed to examine the process characteristics of the CMT. The results show that the metal transfer process is very stable based on the special wave control features and a assistant back-drawing force. Bead-on-plate tests were done on the thin pure aluminium sheets, a good appearance can be obtained owing to the no-spatter welding process. Meanwhile, the low heat input of CMT can not only decrease
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