This article takes a deeper look at two metal transfer techniques used in Wire + Arc Additive Manufacturing (WAAM) – Cold Metal Transfer (CMT) and Super Active Wire Process (SAW-P). Both these methods are modifications of short-circuit gas metal arc welding. This article provides a primer on short-circuit metal transfer and discusses the advantages of CMT and SAW-P additive manufacturing.
Fig 1: basic explanation of Wire Arc Additive Manufacturing. Click here to read the introduction to WAAM
To start with a definition: WAAM is a modification of the wire arc welding process applied for additive manufacturing. Additive manufacturing (AM), also known as 3D printing, uses computer-aided design to build objects layer by layer. This contrasts with traditional manufacturing, which cuts, drills, and grinds away unwanted excess from a solid piece of material, often metal [ASTM standard]. WAAM is a GMAW-based process with a robotic arm and positioner integrated with a welding power source, together with a wire feeding system that can deposit metal in additive layers to form the desired geometry and degree of freedom.
The mechanism of wire deposition on the printing surface is called the ‘metal transfer mode’, and is determined by welding process parameters. The deposited bead geometry (width, height, penetration depth, heat affected zone (HAZ) and surface finish) depends on the metal transfer mode. There are 3 broad modes of metal transfer – globular, pulse and short-circuit. The metal transfer mode is determined by the current and voltage waveform, where each transfer mode operates in a different V-I range. Short-circuit transfer is used for both CMT and SAW-P processes.
Figure 2: The current and voltage waveforms during short-circuit welding
Amongst the different transfer modes, short-circuit transfer has the lowest heat input due to the relatively low current and voltage. In this mode, metal is transferred from the wire to the liquid melt pool by a series of electrical short-circuits. The wire is continuously fed through the welding torch and touches the melt pool, causing the arc to extinguish and short-circuit. A current passes through the wire in this short-circuit condition and detaches the drop of metal from the wire onto the melt pool and reignites the arc. This process is repeated 50-200 times a second during metal deposition. The current and voltage waveforms during short-circuit welding are shown in Fig 2.
In the last 20 years, modifications to the traditional short-circuit transfer have been designed to improve the quality of the weld and efficiency of production. These modifications are made to the wire feeding system and V-I waveforms to aid droplet detachment and deposit spatter-free welds with a good surface finish.
Two such modified short-circuit processes are discussed below – Cold Metal Transfer (CMT) and Super Active Wire Process. SAWP is used for WAAM operation at RAMLAB using Panasonic welding robot systems.
Cold Metal Transfer (waveform in figure 3) gets its name from the process happening at a lower temperature than conventional short-circuit GMAW. The reduction of the welding heat input reduces distortion, and minimizes the residual stresses in the work piece. The electrode wire is also retracted by the wire feeder in the robot arm at every short-circuit when the current drops to near-zero, which reduces undesirable weld spatter. The presence of lower heat makes CMT ideal for welding thin joints and reducing the risk of burn-through (holes caused by high heat input). This process also allows for the joining of dissimilar metals like Aluminum to steel due to the low heat input. [3]
In SAWP, the welding system first detects a short-circuit and subsequently reverses the direction of wire feeding to aid droplet transfer and reduce spatter. The system also dynamically adjusts the set wire feed speed based on a feedback control system to maintain a stable arc and deposit defect free welds. SAWP offers a wider range of bead geometries due to its ability to maintain stable short-circuit transfer at a higher amperage than the critical globular to short-circuit current. SAWP also allows for higher travel speeds without affecting bead geometry, which results in higher production rates.
The SAW-P Waveforms can be found in Figure 4. The data is recorded by MaxQ (Figure 5)
Figure 5: MaxQ is used for data recording
CMT and SAW-P both are innovations that were developed to take advantage of the low heat input of short-circuit welding with a modified waveform to increase efficiency and reduce spatter. Both methods reduce distortion of the part and can be used to weld dissimilar metals to manufacture functionally graded components. SAW-P, however, can maintain a stable arc at a higher range of amperages compared to CMT. SAW-P also allows for higher travel speeds, which can increase the production rate as a consequence.
RAMLAB’s WAAM robots are equipped with a Panasonic power source that can enable SAW-P welding. It is possible to integrate MaxQ with a Fronius power source as well.
[1] Kim, Y. S., & Eagar, T. W. (1993). Analysis of metal transfer in gas metal arc welding. WELDING JOURNAL-NEW YORK-, 72, 269-s. https://eagar.mit.edu/publications/Eagar117.pdf
[3] S. Selvi, A. Vishvaksenan, E. Rajasekar. Cold metal transfer (CMT) technology – An overview, Defence Technology, Volume 14, Issue 1, 2018. ISSN 2214-9147. https://doi.org/10.1016/j.dt.2017.08.00
[4] Cong, B., Ding, J., & Williams, S. (2014). Effect of arc mode in cold metal transfer process on porosity of additively manufactured Al-6.3%Cu alloy. The International Journal of Advanced Manufacturing Technology, 76(9–12), 1593–1606. https://doi.org/10.1007/s00170-014-6346-x
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