cold metal transfer (cmt) technology - a review · fronius of austria in 2004[7], after many years...

Cold metal transfer (CMT) technology - A review

S. Balamurugan1, R. Ranjith2

1Assistant Professor, Department of Mechanical Engineering, Sri Krishna

College of Engineering & Technology, Coimbatore

[email protected]

2Assistant Professor, Department of Mechanical Engineering, SNS

College of Technology, Coimbatore

Abstract Cold Metal Transfer technology is one of the updated welding

technologies for joining dissimilar and similar materials with low heat

input. This low heat input during welding ensures no-spatter welding

process and improved weld bead aesthetics with controlled metal

deposition. In this article a review has been done on microstructure

and other weld characteristics for Aluminium alloy 6061.

Keywords: CMT, dissimilar welding.

1. Introduction

Fronius of Austria in 2004[7], after many years of research and development

have unveiled new arc welding based on modified MIG welding process called cold

metal transfer welding. CMT is a type of MIG welding process, but novelty is that

droplet transfer occurs by new mechanical droplet cutting method. In CMT; droplet

transfer process, when electrode wire tip brought in contact with the molten pool, a

high short circuit current flows and this control of short circuit is performed

dropping the welding current and retracting the wire which stimulate the

detachment of droplet. To retracting the wire the servomotor of the ‘robacter drive’

which is under digital process control will reverse the welding torch. During metal

transfer, the current drops to near-zero without any spatter generation. As the

droplet is cut and metal transfer completed, the CMT hot process ensues, involving

the arc being re-ignited, the wire being fed forward once more, and the set welding

current reflowing.

2. Literature Review

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 2185-2196ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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The steel has been the major material of the automotive industry over a long

period of time, thereafter usage of steel with the iron decreasing year by year due to

focus in low strength-to-weight ratio of iron and steel and also need to enhance

mechanical properties with other materials [1-2]. Automobile industries are trying

to make their vehicle higher efficient one by reducing weight of its body. This can do

by adding some aluminum parts with steels structures. While joining aluminum

with steel by fusion welding is one of major problem due to formation of brittle

intermetallic compounds which can weaken the mechanical properties of the welded

joints [3]. M. Kreimeyer et al [4] shown that when joining aluminum to steel if the

compound layer is less than 10m thick, the welding joint can be mechanically

behave well. Furthermore, the author also suggests that the existence of zinc

coating can increase the fusion metal wettability to steel. Hermans [5] suggests

fusion welding is one of the methods to solve the dissimilar metal joining problem

because of their high efficiency. Hence, a fusion welding method with low heat input

and high efficiency may give a result to aware the aluminum use in automobile.

During welding a very common incidence in gas metal arc welding is spatters,

which are the droplets of molten material that generated at or near the welding arc

which create problems to the welder. A current advancement in welding technology

is the cold metal transfer (CMT) process which is enhanced to welding aluminum

and dissimilar joint due to the no-spatter in welding progression and very low

thermal input nearly zero. [6] The CMT process is a modified metal inert gas

welding process, the principles of this welding process is that the motion of the wire

integrated into welding process and also overall control of the process. During the

short circuit the droplet will be detached due to assistance of wire motion which

ensures the transfer of metal into the welding pool without any aid of the

electromagnetic force. This leads to low spatter and significantly decreases the heat

input. [7] Droplet transfer process is similar to MIG/MAG welding, however when

the electrode wire tip made in to contact with the molten pool, the CMT cold process

engages the servomotor of the ‘robacter drive’ makes welding torch to reversed by

digital process control. This makes the wire to retract and helps to droplet cutting

and with the welding current reducing to near-zero. This near-zero state ensures

metal transfer without any spatter generation. As the droplet are cut and metal

transfer completed, the CMT hot process ensures, the arc being re-ignited and wire

being fed forward once more, and the set welding current reflowing. The CMT

processing cycle varies depending on suitable selection of welding characteristics.

Wire feed/retract operations are executed on average 63 times (max 70 times) on

every second, during the hot and cold processes being alternately repeated. The

CMT process thus provides the world’s first wire operation system to be

incorporated in process control, being performed in this case by digital control.

3. Research Objectives

International Journal of Pure and Applied Mathematics Special Issue

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To examine the importance of cold metal transfer process and its contribution

towards welding industries.

To examine the various control parameter in CMT process for no-spatter

welding and improved weld bead aesthetics.

To examine the microstructure and other weld characteristics of CMT.

4. Research Methodology

To achieve the objectives of the study, review has been done on

microstructure and other weld characteristics for Aluminium alloy 6061. The CMT

process is a novel welding technology that can be employed to handle welding tasks

formerly viewed as tremendously difficult or impossible. This welding process is

highly recommended for robot applications and any automatic applications. For any

common purpose base metals and wires can be handled, also new product

development are possible.

5. Results and Discussion

Peng Wang et al [8] have taken 2 mm thick 6061-T6 aluminium alloy sheets

(150 mm × 50 mm) with a constant travel speed of 10 mm/s. The Al-Si alloy wire

ER4043 with a diameter of 1.2 mm was selected. For all the trials, pure argon was

adopted as the shielding gas with a flow rate of 15 l/min, and the contract tube-to-

work piece distance was kept to 15 mm. From this research work, the effects of

characteristic parameters on the energy input characteristic, metal transfer

behavior, weld geometry, and microstructure of deposited weld metal have

investigated. During welding, CMT welding parameters were controlled by the

remote control unit (RCU5000i). The various parameters are I boost (A), t I boost

(ms), I sc wait (A), vd sc wait (m/min), and I sc2 (A) were studied. Five main

characteristic parameters were performed for different wire feeding speed of 3.7,

4.9, and 6.2 m/min. Fig.2 shows the effects of I boost on weld geometry of CMT

welded joints. Since the greater part of the energy input of boost phase directly used

to heat the work-piece, Awp increased more quickly than Awr..

Jie Pang et al [10] have investigated CMT with addition of pulses (CMT+P)

process is a new CMT welding method with Aluminium alloy 6061-T6. The CMT+P

transfer mode is a combination of a projected transfer mode with one droplet per

pulse and a short circuit transfer mode during the cold metal transfer period. The

results indicate that the current and voltage waveforms of the CMT+P welding

process were quite different from those of the traditional CMT process. A greater

penetration and contact angle of the weld bead can be obtained by increasing the

pulse number. The actual current and voltage waveforms for a weld cycle of 4 pulses

and 2 CMT short circuits are shown in Fig. 2. Before the first pulse begins, there is

a short time (circle 1) with a current higher than that of the pulse base time phase.


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Fig. 2. The actual current and voltage waveforms for a weld cycle of 4 pulses and 2

CMT short circuits

The high current provides a high heat input for the arc ignition. The most

notable difference is during the peak time of the CMT period. The peak time for the

first CMT period (circle 2) following the pulse period has the same current as the

pulse base time phase. A small current pulse step (circle 3) appears at the end of

each S/C phase time throughout the weld cycle. This current pulse step results in an

increase in the heat input for the short circuit phase. The high current for the initial

arcing time (circle 1) preheats the wire so that the arc is stable during the pulse

period. Circle 2 shows that the same current is maintained during the pulse base

time to guarantee that the short circuit transfer process with low heat input is

smooth. The current pulse steps (circle 3) ensure that the short circuit transfer

process provides a buffer between the high current of the pulse time and the low

current of the S/C phase time. The CMT+P process is a stable welding process with

no spattering for all the welding parameters tested in this study.

Hai Yang Lei et al [11] have investigated in three welding modes namely

Standard, Pulsed and CMT on AA6061-T6 with 1 mm thick to identify the

advantages of CMT and further investigated by four CMT spot welding modes:

direct welding (DW) mode, plug welding (PW) mode, direct welding on a chill block

(DWB) mode and plug welding on a chill block (PWB) mode. CMT arc mode

produced welds having the fewest welding defects, such as gas pore and partial

tearing, leading to welds having the best mechanical properties amongst those

welding arc modes further it is clear that the application of the direct welding with


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block (DWB) mode achieved the largest weld nugget diameter at the faying

interface and welding defects such as partial tearing were eliminated, therefore, the

best strength and toughness of the four welding modes could be achieved at the

welding time of 0.9 s.

R. Ahmad et al [12] have used 6061 aluminium alloy with a thickness of 10

mm and investigated the effect of a post-weld heat treatment (PWHT) on the

mechanical and microstructure properties of an AA6061 sample welded using the

gas metal arc welding (GMAW) cold metal transfer (CMT) method. Fig. 3a shows

the size and spacing between grains formed at the top surfaces of the CMT welded

region. The gap between the grains was pinpointing of the ductility of the welded

joint. In the welded joint, the grains are huge, and the space between grains was

high compared with the heat-treated specimens, in whom the grain size was

constant and fairly small and the grains were located close to each other, as shown

in Fig. 3b. As is clear from the mechanical tests, the strength of the alloy after

PWHT was normally enhanced, but the region of the HAZ was still the weakest

spot, and the material by and large failed at that point.

Fig. 3. SEM fractographs of the top surfaces of tensile tested specimens; (a) as-

welded; (b) heat treated.

Li Guojin et al [13] have investigated the bead formation, microhardness,

shear strength, fracture characteristic, and forming mechanism of 6061 aluminum

alloy joints at different gap widths. Table 1 shows the weld bead appearances of the

joints at different gap widths and the corresponding wire offset. In order to optimize

the weld seam, the wire should be closer to the weld seam with increasing gap

width to ensure that the molten metal can be spread evenly. It was found that weld

bead formation is quite different at different gap widths. Because the wire feeding

speed is the same, the volume of molten metal is the same while the gap width is

different. For a narrow gap, the molten metal is enough to fill the gap, so that the

welding seam is wide. For a wider gap, large amounts of filler metals are required

to fill it. As a result, the width of the welding seam is small, and the weld seam is


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not very uniform, and exhibits slight ripples. This is mainly because of the uneven

heat distribution caused by the accumulation of heat input.

Table 1 Welding bead appearance at different gap widths

Weld bead appearance Welding gap width and wire offset

Gap width 1 mm

Wire offset 2.5 mm

Gap width 2 mm

Wire offset 2 mm

Gap width 3 mm

Wire offset 1 mm

(a) Gap width 1 mm (b) Gap width 2 mm

(c) Gap width 3 mm

Fig. 4. Fracture morphology of joint


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Figure 4 shows the scanning pattern of the fracture surface; the fracture

mode is ductile fracture. A large number of dimples exist on the fracture surface,

the dimples are small, and their distribution is relatively uniform. Porosity in

fractured surfaces is low regardless of the size of the gap width. The existence of

pores causes stress concentration and reduces the strength of the joint. The weld

formation was good when the joints were welded at different gap widths by the

same parameters (except wire offset). The penetration decreased with increasing

gap width, and the shear strength decreased as the gap increased.

N. Pavan Kumar et al [14] have investigated the effect of welding current and

welding speed, depth of penetration; weld pool width, reinforcement height,

dilution, weld bead contact angle parameters during cold metal transfer (CMT)

process of 2mm thin Aluminium alloy 6061 sheet of thickness. The welding speed is

varied from 6.6 mm/s to 10 mm/s and welding current is varied from 50 A to 70 A

while maintaining voltage constant. From the Table 4, for higher currents, imaging

scars/surface modifications are visible on the rear side which contradicts the

observations for lower currents. As can be seen, ripple profile appeared in these

specimens in Table 2. Moreover, with the increase of welding speed, ripple profile

gradually became clearer. Lower welding speed leads to more heat input and better

fluidity of weld pool. Consequently, the fluid weld pool could smooth the ripple

profile, narrows the distortions. When welding speed was set as 10 mm/s, less heat

input coupled with higher cooling rate denied the free movement of weld pool, which

exhibits ripples as seen from Table 2. Pulsed-CMT yields a stable and spatter-free

welding of aluminium AA6061 alloy. Remarkable weld is obtained when current is

maintained in the range of 60-70 A and the welding speed maintained at 8-10mm/s.

For welding aluminium alloy thin sheets using a filler which is of same composition

as of base metal i.e., AA6061 exhibits a quasi-binary composition. This composition

is potentially less susceptible to solidification cracking, controlled fusion line,

narrower heat affected zone (HAZ) and reduced intermetallic phase area.

Table 2 CMT welded specimens (10x Magnification) at different input process

parameters

Welding

Current

(A)

Welding

Speed

(mm/min)

Weld Bead Appearance

Top side Bottom side

50 400

No Penetration


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50 500

No Penetration

50 600

No Penetration

60 400

60 500

60 600

70 400

70 500

70 600

Akhil Garg et al [16] have used an adaptive control scheme is employed for

joining Aluminium 6061 alloy sheets by Cold Metal Transfer (CMT) process. The

performance analysis for the proposed adaptive control scheme (Model Reference

Adaptive Controller) and the conventional PID controller are compared. MRAC is

implemented to maintain the welding current at desired range during melting and

electrode wire short circuiting.


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Fig. 5. Response of PID controller.

Fig. 6. Response of the MRAC.

From the observed results, the PID controller is unable to adapt to the

disturbance while detecting a short circuit. Fig. 5 shows initially, the welding

current increases to a value of 50A. After being subjected to a disturbance (short

circuit phase), the welding current reduces which thereby decreasing the bead

width and DOP. The response of the implemented MRAC is shown in Fig. 6. It may

be clearly noted that, the current decreases when a short circuit is detected, and the

electrode feed retracts. The MRAC ensures that the current is maintained at 50 A

with uniform bead width and DOP. Once a short circuit is detected, the current

decreases and the molten droplet fall, and the electrode retracts. Once the arc is

formed, the electrode inches forward and the welding current is increased and is to

be maintained at the desired set point.

6. Conclusion

The CMT welding process, weld parameter combinations and applications of

the Cold Metal Transfer welding detailed by various authors are discussed. The

main conclusions of this study are:


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During welding process the wire retraction during the short circuiting phase

plays a significant role, as it leads to avoidance of spatter creation and also

produces better weld bead aesthetics.

The CMT with Laser hybrid welding process produces welds with improved

mechanical properties and aesthetics than the Laser welding and Laser-MIG hybrid

welding.

7. Limitations and Future Research

The Cold Metal Transfer Welding is one of the latest welding

technologies used for variety of applications such as cladding, additive

manufacturing, composite joint pin fabrication, and crack repair welding.

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cold metal transfer (cmt) technology - a review · fronius of austria in 2004[7], after many years...

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