Загрузил Алексей Постаногов

Cold metal transfer (CMT) technology - An overview

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Defence Technology 14 (2018) 28e44
Contents lists available at ScienceDirect
Defence Technology
journal homepage: www.elsevier.com/locate/dt
Cold metal transfer (CMT) technology - An overview
S. Selvi a, *, A. Vishvaksenan a, E. Rajasekar b
a
b
Mechanical Engineering, Institute of Road and Transport Technology, Erode, 638 316, Tamilnadu, India
Automobile Engineering, Institute of Road and Transport Technology, Erode, 638 316, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2017
Received in revised form
26 July 2017
Accepted 18 August 2017
Available online 1 September 2017
Cold Metal Transfer technology has revolutionized the welding of dissimilar metals and thicker materials
by producing improved weld bead aesthetics with controlled metal deposition and low heat-input. In
this study, the process, weld combinations, laser-CMT hybrid welding and applications of CMT welding
are critically reviewed. Microstructure and other weld characteristics have been discussed at length for
various base metal combinations. Particularly, the welding of aluminium and steel with better results has
been possible with CMT Welding. The results reviewed in this article indicate that the CMT-Laser hybrid
welding is more preferable to Laser or Laser hybrid welding. CMT welding has found applications in
automobile industries, defence sectors and power plants as a method of additive manufacturing.
© 2018 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:
Cold metal transfer [CMT]Welding
Laser-CMT welding
Additive manufacturing
Composite joint
Metal inert gas[MIG]
Metal active gas[MAG]
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Cold metal transfer process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
CMT welding of similar metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.1.
Inconel 718 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.
Aluminium 7075 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.
Aluminium AA6061 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4.
Galvanized sheet steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5.
Galvannealed steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.6.
Aluminium 5083-H116 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.7.
Aluminium AA7A52 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.8.
AA2219-T851 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
CMT welding of dissimilar metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.1.
Zinc coated steel (Q235) and wrought aluminium (6061) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.
Magnesium AZ31 and aluminium 1060 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3.
Magnesium AZ31 band and 6061 Al alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.
Hot dip galvanized steel and aluminium 1060 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.5.
Aluminium (AA6061) and low carbon steel alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.6.
Magnesium AZ31 and hot dipped galvanized mild steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.7.
Aluminium A6061-T6 and titanium Tie6Ale4V alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.8.
Aluminium AA6061-T6 to galvanized steel alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.9.
Magnesium alloy AZ31B and pure copper T2 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.10.
Pure titanium TA2 to magnesium alloy AZ31B alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.11.
5083-H111 and 6082-T651 aluminium alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
* Corresponding author.
E-mail address: selvimech@yahoo.com (S. Selvi).
Peer review under responsibility of China Ordnance Society.
http://dx.doi.org/10.1016/j.dt.2017.08.002
2214-9147/© 2018 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
S. Selvi et al. / Defence Technology 14 (2018) 28e44
5.
6.
7.
8.
9.
29
4.12.
Titanium TA2 to pure copper T2 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.13.
Hot-dip galvanized steel sheet and aluminium 5052 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.14.
Aluminium 5A06 with pure Ni N6 plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.15.
5182-O and 6082-T4 aluminium alloy sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.16.
Titanium AMS4911L with 316L stainless steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.17.
A6061-T6 aluminium alloy to dual phase 800 steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.18.
AC 170 PX aluminium alloy and ST06 Z galvanized steel sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.19.
304 stainless steel and 5A06 aluminium alloy sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Comparison of CMT and metal inert gas welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Laser-CMT arc hybrid welding of metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.1.
Laser-CMT arc hybrid welding of T2 copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.2.
Laser-CMT arc hybrid welding of AA6061 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.3.
Laser-CMT arc hybrid welding of S420 MC D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Effect of base metal and CMT weld treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7.1.
Post weld heat treatment (PWHT) of CMT weld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7.2.
Welding steel sheets treated by nitro-oxidation using CMT process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Alternative applications of cold metal transfer process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.1.
Low-dilution cladding of INCONEL 718 superalloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.2.
Cladding of Al 6061 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.3.
Cold metal transfer deposited AZ31 magnesium alloy clad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.4.
Al-Si-Mn alloy coating on a commercially pure Al plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.5.
Wetting of galvanized steel by Al 4043 alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.6.
Wetting of Mg AZ61 alloy/galvanized steel in cold metal transfer process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.7.
Additive manufacturing of Al-6.3% Cu alloy by CMT process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.8.
Compositeecomposite joints reinforced with cold metal transfer welded pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.9.
Crack repair welding of steam turbine cases by CMT brazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
Cold Metal Transfer welding is a modified MIG welding process
based on short-circuiting transfer process developed by Fronius of
Austria in 2004. This process differs from MIG/MAG welding process only by the type of mechanical droplet cutting method not
previously encountered [1]. During welding, temperature variations in welds and parent metals have important effects on material
characteristics, residual stresses as well as on dimensional and
shape accuracy of welded products [2]. Cold Metal Transfer provides controlled method of material deposition and low thermal
input by incorporating an innovative wire feed system coupled
with high-speed digital control [3]. The wire feed rate and the cycle
arcing phase are controlled to realise sufficient energy to melt both
the base material and a globule of filler wire [4]. There are two main
features of the CMT process: one is at the point of short circuit with
low current corresponding to a low heat input, another is the short
circuit occurrence in a stable controlled manner.
Kah et al. introduced the short-circuiting transfer process
named “mechanically assisted droplet deposition” which is applied
in controlling short circuit by retracting the wire from shortcircuiting [5]. Schierl reported that the droplet detachment mode
of CMT process is without the aid of the electromagnetic force
compared to the conventional MIG process, so the spatter can
decrease [6]. Pickin and Young previously reported the basic
operating principles of the process [3]. Feng et al. pointed that the
CMT process is especially suitable for welding thin aluminium alloy
sheets due to the low heat input and the slight deformation [7].
Additional studies by Zhang et al. and Cao et al. concentrated on the
application of the process in dissimilar alloys joining owing to the
low heat input, which restrains the formation of brittle intermetallic compounds [8,9].
nia Meco et al. using
A graphical interface was developed by So
interpolation and neural network method to help the user select
the appropriate CMT welding parameters for the desired application, by a graphical visualization of the welding profiles, which
leads to time, material and cost saving [10]. Amin S. Azar produced
a heat source model to simulate the effect of periodic and recurrent
arcing and metal deposition phenomena in the cold metal transfer
type of welding. This model will facilitate studying of weld pool
behavior and resultant mechanical properties [11]. Neutron imaging and Quantitative infrared analysis are some of the recent nondestructive tests performed on the CMT weld specimen [12,13]. A
required model for simulating the characteristic cooperation between wire feeding and heat input was published by Fengyuan Shu
et al. [14].
2. Cold metal transfer process
In the CMT process, when the electrode wire tip makes contact
with the molten pool, the servomotor of the ‘robacter drive’
welding torch is reversed by digital process control. This causes the
wire to retract promoting droplet transfer which is depicted in
Fig. 1. During metal transfer, the current drops to near-zero and
thereby any spatter generation is avoided. As soon as the metal
transfer is completed, the arc is re-ignited and the wire is fed forward once more with set welding current reflowing [1].
A typical CMT welding electrical signal cycle can be defined as
the period required to deposit a droplet of molten electrode into the
weld pool. The analysis of current and voltage waveform is essential
to study the energy distribution of different phases in droplet
transfer process [15]. The cycle is divided into three phases as
follows:
(i) The peak current phase: This is a constant arc voltage corresponding to a high pulse of current causing the ignition of
the welding arc easily and then heats the wire electrode to
form droplet.
30
S. Selvi et al. / Defence Technology 14 (2018) 28e44
(ii) The background current phase: The phase corresponds to a
lower current. The current is decreased to prevent the
globular transfer of the little liquid droplet formed on the
wire tip. This phase continues until short circuiting occurs.
(iii) The short-circuiting phase: In this phase, the arc voltage is
brought to zero. At the same time, the return signal is provided to the wire feeder which gives the wire a back-drawing
force. This phase assists in the liquid fracture and transfer of
material into the welding pool [7].
The complex waveform of the welding current in the CMT
process and the ‘back feeding’ of the filler wire that mechanically
forces the metal transfer make it difficult to understand the relation
between welding parameters, metal transfer and heat transfer as
shown in Fig. 2, which is studied by Mezrag et al. for Al 4043-S235
weld joint [16]. With perfect arc length management and high edge
coupling tolerances, the CMT process is sure to be involved in
various industrial applications in future as a solution to overcome
the drawbacks of current welding practices [17].
3. CMT welding of similar metals
CMT has accomplished the efficient welding of many similar
aluminium alloys. The similar metal welds prepared using CMT for
variety of alloys are discussed below.
3.1. Inconel 718 alloy
The microstructural analysis has demonstrated no lack of fusion
proving the weld quality as good. The Heat Affected Zone (HAZ)
presented in Fig. 3, is small in size (0.5 mm) when compared to the
same produced by classical MIG welding. The size and geometry of
crystallites in the weld zone, i.e. large dendrites, are similar to those
obtained in classic MIG process. After performing EDS chemical
analysis, no significant variation has been detected in the
Fig. 2. Current and Voltage waveforms of CMT process.
homogeneity of the weld bead. The residual stresses are found to be
minimum. This work by Benoit et al. demonstrates that the CMT
welding is fully suitable for the welding of Inconel 718 [18].
3.2. Aluminium 7075 alloy
The joints were prepared without spatter, cracks and having
very low porosity. The joints exhibited minimum micro-hardness in
the Weld Zone (WZ) depicted in Fig. 4, and slight hardness decrease
in HAZ compared to the Base Metal (BM). The comparison of
microhardness between WZ and HAZ could be observed in Fig. 5.
The joint had mechanical property coefficients of 77%, 60% and 69%
for yield strength, ultimate tensile strength and elongation
respectively. The CMT welding performed by Elrefaey was found to
produce joints with mechanical characteristics better than the
Fig. 1. High-speed images of droplet transfer.
S. Selvi et al. / Defence Technology 14 (2018) 28e44
conventional MIG and TIG processes and comparable to FSW and
LBW processes [19].
3.3. Aluminium AA6061 alloy
When Pavan kumar et al., welded thin aluminium alloy sheets
using filler, which is of same composition as of base metal, the weld
exhibited 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. The microstructures for different weld parameters seen
in Fig. 6 revealed fine recrystallization at the joints. A uniform
distribution of grains and its size in weld HAZ and base metal was
distinctly visible [20].
3.4. Galvanized sheet steel
Joints of galvanized steel, made with electrode wire CuSi3 were
31
subjected to microscopic metallographic examination displayed in
Fig. 7, which included both the weld zone and the base material.
Macro- and microscopic metallographic examination by Magda
et al. confirmed the high quality of the brazed joints, showing both
the existence of a copper diffusion area and the undamaged zinc
layer in areas adjacent to the weld [21].
3.5. Galvannealed steel
Low and high heat input conditions had a tendency to less
porosity formation in weld bead, whereas the medium heat input
conditions were the most susceptible to porosity formation. Solidification started early in low heat input conditions, resulting in
small porosities near the weld root which are avoided in high heat
input conditions [22]. Fig. 8 depicts the different porosity formation
mechanisms. Ahsan et al. developed optimized welding conditions
to reduce porosity for two heat range inputs, one at low heat input
ranging from 200 to 250 J/mm and the other at high heat inputs,
starting from 350 J/mm and rising up to 550 J/mm [23].
3.6. Aluminium 5083-H116 alloy
Jair Carlos Dutra et al. used two different wire electrodes Al 5183
and Al 5087. Weld using Al 5087 electrode showed better mechanical performance in tensile tests. The micro-hardness was
similar in both the WZ and HAZ. Practically, both wire-electrodes
showed the same toughness. Crack Tip Opening Displacement
Toughness test results indicate that the applied combinations of
base and feed material yield good cracking resistance characteristics. Fig. 9 displays SEM images, which illustrate higher incidence of
pores with the Al 5183 wire electrode [24].
Fig. 3. Weld zone of Inconel 718 alloy.
3.7. Aluminium AA7A52 alloy
Feng et al. found that the intergranular segregation, which gave
birth to the coarse grain boundary between the weld passes as
depicted in Fig. 10, were indicated to exhibit inferior mechanical
performances. Tri-axial stress distribution in the fusion zone was
indicative of tendency to tensile failure under service conditions
[25]. The softened zone was much wider inside the base plates than
close to the flat surfaces. The strip-shaped quenched zone was
obviously narrower than the averaging zone internal plates [26].
3.8. AA2219-T851 alloy
Fig. 4. Weld zone of Al 7075 alloy.
A narrow finger-shaped geometry was observed by Cong Baoqiang et al. using the conventional CMT process. There are a large
number of gas pores in the lower and upper parts of welds. Fig. 11
shows the weld microstructure in longitudinal direction using
conventional CMT welding. The porosity was reduced effectively
with the help of CMT welding [27].
4. CMT welding of dissimilar metals
CMT welding is also widely employed for welding of dissimilar
metals such as aluminium and steel. The novel dissimilar welds
prepared using CMT are discussed below.
4.1. Zinc coated steel (Q235) and wrought aluminium (6061)
Fig. 5. Microhardness profile at WZ/HAZ interface.
The intermetallic layers formed at the interface between zinc
coated steel and wrought aluminium are predominantly FeAl3
phase. Zhang et al. found that CMT increases the strength of the
dissimilar metal lap joint by decreasing the thickness of the brittle
intermetallic compound at the interface between aluminium and
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S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 6. Microstructures of AA6061 alloy as seen from Optical Microscopy.
steel. The tooth like structure as displayed in Fig. 12, predominantly
formed during solidification is mainly controlled by the diffusion of
Fe and Al atoms at the interface between molten aluminium and
solid steel [28].
The CMT welding of Q235 with Al6061-T6 by Cao et al. produced strength equal to CMT welding of Al6061-T6 with Al6061-T6.
The joint strength was found to depend on the thickness of the
intermetallic layer shown in Fig. 13, and softening of the Aluminium
heat affected zone [9].
4.2. Magnesium AZ31 and aluminium 1060 alloy
Fig. 7. Brazed weld zone.
Wang et al. observed no weld defects as low heat input and
addition of Si to the weld effectively inhibit the creation of brittle
intermetallic compounds, which is checked using X-ray diffraction.
Fig. 14 is the fusion zone near the Mg substrate, which dictates the
strength of the joint and its microstructure. Four continuous layers
consisting of solid solution layer, eutectic structure layer, Mg17Al12
Fig. 8. Porosity formation mechanisms at different heat inputs.
Fig. 9. SEM micrographs of fractures in weld joints using 5087 and 5183 filler wires.
S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 10. Microstructure of molten zone.
33
zone of Al side, and Cu based solid solution was generated in weld
zone, while Cu2Mg and AleCueMg ternary eutectic structure was
formed in the fusion zone of Mg side. The bonding strength of the
joint was 34.7 MPa. Fig. 15 presents the fracture morphology. The
fracture occurred at the fusion zone of Mg side where the value of
micro-hardness was the highest due to large amount of Cu2Mg
intermetallic compound [30].
With ER4043 as filler metal, the CMT weld of AZ31B magnesium and 6061 aluminium alloy developed by Shang Jing et al. had
uniform micro-hardness in both the sides of the substrate, about
540 MPa in Mg side and 350 MPa in Al side as seen in Fig. 16. The
highest value of micro-hardness was 2380 MPa in the fusion zone
of Mg side. The micro-hardness in the weld from Mg side to Al side
showed a decreasing trend with reduction of intermetallic compounds. The joint with low bonding strength was brittle fractured
in the intermetallic compound layer of the fusion zone of Mg side.
Intermetallic compounds of Mg2Si, Mg2Al3 and Mg17Al12 distributed continuously in the fusion zone presented in Fig. 17, are
responsible for the fracture [31].
In the presence of Al-5%Si as filler metal, maximum tensile
strength of 360 N/mm was achieved by Madhavan et al. Increase in
tensile strength was attributed to minima tensile stress and finer
precipitates. Improved pitting corrosion resistance was observed
due to the formation of Mg2Si and Al6Mn in the interfacial layer. The
micrograph of the entire weld section can be observed in Fig. 18
[32].
Magnesium AZ31B and Al 6061 alloy was welded with Variable
Fig. 11. Weld microstructure in longitudinal direction.
layer and Mg2Al3 layer are observed. The micro-hardness in the
fusion zone near Mg side is about 230e240 HM higher than the
weld metal 120 HM and the Mg substrate 60 HM [29].
4.3. Magnesium AZ31 band and 6061 Al alloy
When Jing Shang et al. used pure Cu as filler metal, the intermetallic compounds AlCu, CuAl2, Cu9Al4 were present in the fusion
Fig. 12. Interface between weld metal and steel.
Fig. 13. Microstructure of brazing interface.
Fig. 14. Micrograph of fusion zone.
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S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 15. Fracture morphology.
Fig. 17. Fusion zone.
Polarity CMT welding (VPCMT) by Peng Wang et al. The MgeAl
IMC layers were formed in the weld interface, near the AZ31B side
of the welded joints and consisted of three intermediate layers:
Mg2Al3 layer, Mg17Al12 layer, and Mg17Al12þa-Mg solid solution
eutectic layer (very thin) as shown in Fig. 19. With decreasing EP/EN
ratio from 4:1 to 1:4, the thickness of the whole IMCs layer was
gradually increasing and the tensile strength increased significantly. All samples were fractured in the hard brittle IMCs layer
[33].
Welding of Mg AZ31B and Al 6061-T6 sheets yielded significant
amount of Mg rich intermetallic compounds displayed in Fig. 20,
which degraded the weld strength at the side of Mg alloy base
metal. Both Madhavan et al. and Cao et al. obtained similar results
for the above weld [34,35].
4.4. Hot dip galvanized steel and aluminium 1060 alloy
Zhang et al. noted that the compound layer at the interface
between steel and weld metal mainly consisted of Fe2Al5 and FeAl3
phase. Fig. 21 shows a SEM image of Steel-Weld metal interface. The
thickness of the intermetallic compound layer was controlled under 5 mm guaranteeing the joint strength. The tensile strength
arrived at was 83 MPa [8].
Fig. 16. Distribution of microhardness of the joint.
4.5. Aluminium (AA6061) and low carbon steel alloy
Higher shear strength and fusion line failure were recorded by
Jian Lin et al. when zinc coating was provided to the steel sheet.
Otherwise, it led to lower shear strength and interface failure. The
maximum principle stress and deformation energy were proposed
as the criteria for the interface failure and plastic strain was proposed as a criterion for the fusion line failure. Fig. 22 displays the
interface between steel and aluminium [36].
4.6. Magnesium AZ31 and hot dipped galvanized mild steel
Cao et al. reported that the zinc coating on the surface of the
steel is essential for a sound weld. Fig. 23 shows the brazing
interface, which consists of Al, Zn, Mg intermetallic compounds and
oxides (i.e., MgFeAlO4, Fe2O3, and Mg2Zn11) and a magnesium solid
solution. Aluminium in the welding wire magnesium AZ61 enhances the wettability of an Mg-rich weld metal on Zn-coated steel
sheet [37].
4.7. Aluminium A6061-T6 and titanium Tie6Ale4V alloys
The IMCs at the brazing interface shown in Fig. 24 mainly
Fig. 18. Weld cross-section.
S. Selvi et al. / Defence Technology 14 (2018) 28e44
35
Fig. 21. Steel-Weld metal interface.
Fig. 19. Weld interface.
composed of two layers: the continuous layer which consisted of
Ti3Al and TiAl close to the solid Ti alloy, and the discontinuous
serration shaped TiAl3 layer next to the weld metal. Cao et al.
observed fractures at the welding/brazing interface and weld
metal, and at the Al HAZ with most joints fractured in the latter
mode. The tensile strength of the joint is high up to 194 N/mm [38].
4.8. Aluminium AA6061-T6 to galvanized steel alloy
It was found by Cao et al. that a sound joint could be obtained if
the wire feeder speed is properly controlled. The brazing interface
between the Al weld metal and galvanized mild steel was found to
consist of about 5e8 mm thick FeAl3 intermetallic. The microstructure of the fusion zone is displayed in Fig. 25. In addition, the
material stacking sequence affected the strength of CMT spot plug
welded joints. The strength of spot plug welded AA6061 joints was
found to be lower than that of Al AA6061-to-galvanized mild steel
joint [39].
4.9. Magnesium alloy AZ31B and pure copper T2 alloy
The weld toe-brazing zone and weld root-brazing zone depicted
in Fig. 26(a) and (c) respectively were similar and consisted of only
one IMC (Mg17Al12 þ Al6Cu4Mg5 þ a-Mg) layer. However, the intermediate brazing zone observed in Fig. 26(b) consisted of two
Fig. 22. Steel aluminium interface.
IMC (Mg2Cu þ MgCu2 þ Al6Cu4Mg5 and Mg17Al12 þ Al6Cu4Mg5 þ aMg) layers. When the thickness of the brazing interface layers between the Mg weld metal and the Cu base were in the range of
80e350 mm, the lapped joint can reach higher strength of 172.5 N/
mm. It was concluded by Cao et al. that, in this range the thickness
of intermetallic brazing interface layers has no obvious effect on the
tensile shear strength of the lapped joint [40].
4.10. Pure titanium TA2 to magnesium alloy AZ31B alloy
For MgeTi joint, satisfied weld appearance and higher tensile
Fig. 20. Magnified view of weld cross section and corresponding schematic phase distribution.
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S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 23. Weld metal near brazing interface.
Fig. 25. AA6061-T6/Steel fusion zone.
and AlCu2Ti were present in titanium-weld interface. The microstructures of Ti-weld interface and Cu-weld interface are depicted
in Fig. 30. The tensile shear strength of the Joint I (top Cu
sheetebottom Ti sheet) reached 197.5 N/mm while the tensile
shear strength of the Joint II (top Ti sheetebottom Cu sheet) can
reach 205.8 N/mm. The Joint II and I had a comparable strength to
CMT lap welded Cu-T2 to Cu-T2 with a tensile strength of 194 N/
mm. The joints all fractured in the Cu HAZ with plastic fracture
mode [43].
In CMT weldingebrazing butt joint of Titanium TA2 to pure
Copper T2 alloy by Cao et al., the thickness of the IMCs layer was not
uniformed: 117e129 mm in middle groove surface and 80e100 mm
in root groove surface, are presented in Fig. 31 and 32 respectively.
The IMCs layers at the brazing interface mainly consisted of Ti2Cu,
TiCu and AlCu2Ti respectively from the Ti base metal to the weld
metal. Tensile loads of 5.10 kN were reached, and fracture occurred
at Cu HAZ [44].
Fig. 24. Brazing interface between fusion joint and Ti alloy matrix.
load of 2.10 kN was obtained. For TieMg joint, a tensile load of
1.83 kN was detected. The brazing interface developed by Cao et al.
was mainly composed of Ti3Al, Mg17Al12 and Mg0.97Zn0.03 intermetallic. Elements Al and Zn in the Mg base metal and Mg wire are
crucial to join successfully Mg and Ti base metals [41]. The weld
interface of Mg/Ti joint and Ti/Mg joint are displayed in Fig. 27 and
28 respectively.
4.13. Hot-dip galvanized steel sheet and aluminium 5052 alloy
Minjung Kang and Cheolhee Kim concluded that the Si
composition of the filler metal primarily influence the thickness of
the IMC layer. Using AlSi (Al 4043 and Al 4047) filler wire, the
growth of the trapezoidal Fe2Al5 layer into the steel base materials
was restricted, and a nearly flat interface between the IMC layer and
steel was observed. The specimens were fractured at the HAZ of the
Al 5052 alloy. Fig. 33 shows the IMC thickness variation from the
root [45].
4.11. 5083-H111 and 6082-T651 aluminium alloys
4.14. Aluminium 5A06 with pure Ni N6 plates
The micro-hardness of the welded joints was similar to characteristic hardness traverse across weldments, hardness drops
were slightly close to the base metal. The weld joints and base
metal had adequate tensile strength values. Fig. 29 shows the
Macrograph and SEM photos of the fracture surfaces of the Fatigue
specimen. It was noted by Beytullah Gungor et al. that the CMT
welding results were closer to FSW, and had higher yield strength
values than any other welding methods [42].
4.12. Titanium TA2 to pure copper T2 alloy
Cao et al. obtained satisfactory lap welded joints with desired
welding appearance and good wettability and spreadability of filler
metal on the surface of both alloys. A layer of IMCs, i.e. Ti2Cu, TiCu
The weld joint prepared by Liu et al. can be divided into four
parts: the nickel base metal; Ni3Al, Ni0.9Al1.1 and Ni2Al3 IMC layer;
columnar NiAl3 layer; and AleSi solid solution weld, as formed
sequentially from the nickel side to the aluminium side. With an
increase in welding velocity, the thickness of the IMC layer first
decreased and then grew. This is depicted in Fig. 34. The greatest
shear strength obtained was 42 MPa. The joint strength continued
to decrease as the IMC layer thickened. The fractures were mainly
located in the NiAl and NiAl3 IMC layer [46].
4.15. 5182-O and 6082-T4 aluminium alloy sheets
In the 5182 sheets, the HAZ microstructure showed fine
S. Selvi et al. / Defence Technology 14 (2018) 28e44
37
Fig. 26. Microstructure of a) Toe-brazing zone b) Root-brazing zone and c) Intermediate zone.
Fig. 29. Fracture surface of fatigue specimens.
Fig. 27. Weld interface of Mg/Ti joint.
4.16. Titanium AMS4911L with 316L stainless steel
Gonçalo Pardal et al. obtained maximum tensile properties at
higher heat input. The IMCs formed are more ductile in nature
when compared to the Fe-Ti IMCs and were mainly located at the
interfaces between the parent metals and the Cu (filler wire). The
maximum hardness measured was 1000 HV0.1. The weld interfaces
are presented in Fig. 36 [48].
4.17. A6061-T6 aluminium alloy to dual phase 800 steel
Fig. 28. Weld interface of Ti/Mg joint.
precipitates of second phase and coarsening of the Mg2Al3 precipitates in the aluminium matrix. In general, it was found by
Ahmed Elrefaey and Nigel G. Ross that CMT welding of 5182e6082
alloy did not show worse mechanical properties compared to 5182/
5182 and 6082/6082 joints. Fig. 35 displays the weld zone images of
5182/6082 weld [47].
The grains in nugget zone near HAZ coarsened and the Mg2Si
phase dissolved, which resulted in Al softening and as a result, the
joint strength was reduced. Madhavan et al. discovered the presence of the Fe3Al and Fe2Al5 phases in the weld nugget from the
XRD and electron microscopy analysis. Thickness of the IM layer
seen in Fig. 37 varied from 1.49 to 3 mm for the P-CMT and CMT
processes respectively. At the interface, fi-FeAl3 and g-Fe2Al5 phases were formed. The CMT and P-CMT welds failed at the Al HAZ.
This failure mode has ductile fracture characteristics with dimples
and voids [49].
4.18. AC 170 PX aluminium alloy and ST06 Z galvanized steel sheets
The thickness of interfacial layer was only 0.6 mm. The tensile
shear strength reached 189 MPa which is 89% of the aluminium
alloy base metal. The spalled needle-like IMCs visible in Fig. 38
were confirmed as Al-Fe-Si ternary intermetallic compounds by
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Fig. 30. Microstructure of a) Ti-weld interface and b) Cu-Weld interface.
Fig. 33. Variation of thickness according to the IMC.
Fig. 31. Middle surface of brazing interface.
Song Niu et al., which had negative effect on tensile strength of the
joint. With increasing welding current, the needle-like IMCs grew
longer and spread further into the weld, reducing the tensile shear
strength of the joint [50].
4.19. 304 stainless steel and 5A06 aluminium alloy sheet
Under the effect of an axial EMF, both the welding arc and the
molten drop were rotated by Lorenz force. The EMF influenced the
growth of the Al/Fe IMC layers during Al/steel welding. Under EMF
application, the diffusion of Fe to the weld was suppressed and the
Si content in the IMC layers increased, which restrained the growth
of brittle Al/Fe IMC phases. Fig. 39 contrasts the difference observed
while applying an axial EMF during CMT process. Yibo Liu et al.
found that the application of the EMF increased the tensile shear
force of the weld joint. At EMF frequencies of 0 Hz and 5 Hz,
stronger joints were obtained, and within crease in coil current, the
joint strength increased even further [51].
5. Comparison of CMT and metal inert gas welding
Fig. 32. Root surface of brazing interface.
The tests conducted by Mateusz Grzybicki and Jerzy Jakubowski
S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 34. Effect of welding velocity on microstructure of AleNi joint a) 9 mm/s, b)11 mm/s, c) 15 mm/s, d)7 mm/s.
Fig. 35. WM/6082 HAZ and WM/5182HAZ.
Fig. 36. Steel/Cu and Cu/Ti weld interface.
39
40
S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 37. a) SEM b) TEM image of IM layer.
shows that compared to traditional MIG variety welding, the CMT
method has several advantages such as low energy, spatter free,
high welding speed. It also enables the welding of thin sheet
metals. However, there is a great danger of incomplete fusion,
especially for lap joints which could be avoided by increasing the
arc length [52]. The CMT welding by Jair Carlos Dutra et al. showed
more stability and the root produced showed a good finish, both on
the surface and back of the joint than the conventional MIG
welding [53].
6. Laser-CMT arc hybrid welding of metals
6.1. Laser-CMT arc hybrid welding of T2 copper
Yulong Chen et al. achieved continuous and regular welds at a
minimum power of 2 kW, which is very much less than the minimum power level of laser welding (about 5 kW). A large amount of
Si-rich precipitates were found in the Fusion Zone (FZ) grains of the
hybrid welds, which caused the FZ of hybrid welds to be harder
than laser welds. Fig. 40 shows the Si-rich precipitates in the FZ of
the hybrid welds. The UTS, YS, and the Elongation of the hybrid
weld with the best performance were up to 227 MPa, 201 MPa, and
21.5%, respectively. The decrease of the porosity was the main
reason for strengthening of hybrid weld [54].
6.2. Laser-CMT arc hybrid welding of AA6061 alloy
Laser-CMT hybrid welding was developed to join 2-mm thick
AA6061 Al alloy by Zhang et al. Acceptable joints without metallurgy defects were obtained. The cross-weld tensile strength of
laser-CMT hybrid welds was up to 223 MPa, 10% higher than that of
the laser-PMIG hybrid weld. Fig. 41 shows the microhardness distributions of transverse joints. The results showed that laser-CMT
hybrid welding could potentially join aluminum alloy thin sheets.
Fig. 42 depicts the equiaxed dendrites in the center of the weld
fusion zone [55].
6.3. Laser-CMT arc hybrid welding of S420 MC D
Fig. 38. Al/STO6 Z steel weld joint.
Jan Frostevarg et al. compared the hybrid weld with a close-toproduction setup for low and medium wire deposition rates. Fig. 43
Fig. 39. Microstructure of the Al/steel lap joint interface (a) without EMF (b) with EMF.
S. Selvi et al. / Defence Technology 14 (2018) 28e44
Fig. 40. Precipitates in FZ of hybrid weld.
41
Fig. 42. Equiaxed dendrites in the centre of the FZ.
displays the High-Speed Image of laser arc hybrid weld pool. They
concluded that the CMT is suitable for laser hybrid welding of
thicker sheets provided the gap is narrow enough to be filled by the
limited wire feed rate. The hybrid weld showed advantages of
higher bead stability, reduced undercut, reduced power supplied,
reduced weld/HAZ width and less sensitive to speed variations [56].
7. Effect of base metal and CMT weld treatments
7.1. Post weld heat treatment (PWHT) of CMT weld
Fig. 43. High Speed Image of Laser Arc hybrid weld pool.
The effect of PWHT on the mechanical and microstructure
properties of welded AA6061 using the CMT GMAW was analyzed
by Ahmad and Bakar. In their investigation, 3.8% increase was
recorded for tensile strength as observed in Fig. 44, hardness
strength was increased by 25.6% and a 21.5% higher elongation was
achieved. The results proved that PWHT was able to enhance the
hardness strength and tensile properties of AA6061 welded joints
using the GMAW CMT method. These were attributed to the fact
that PWHT produces a fine and uniform distribution of precipitates
at the weld joints [57].
7.2. Welding steel sheets treated by nitro-oxidation using CMT
process
The limited heat input and the controlled metal transfer, which
Fig. 41. Microhardness distribution.
are considered as the main advantage of the CMT process, had a
negative impact on weld joint quality. An excessive amount of
porosity was observed, probably due to the high content of nitrogen
and oxygen in the surface layer of the material and the fast cooling
rate of the weld pool. The results show that for steel sheets treated
by nitro-oxidation there was a radical increase in micro hardness
values, up to 47%, in comparison with the values for the same
material without surface treatment. After CMT welding, it was
observed that the microhardness values gradually declined from
the weld metal till the heat affected zone, stabilizing in the base
metal as seen in Fig. 45. The parameters of the CMT process performed by Michalec and Maronek were not suitable for welding
steel sheets treated by nitro-oxidation, due to the high level of
porosity [58].
Fig. 44. Tensile strength of as welded and heat-treated samples.
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S. Selvi et al. / Defence Technology 14 (2018) 28e44
8. Alternative applications of cold metal transfer process
8.5. Wetting of galvanized steel by Al 4043 alloys
8.1. Low-dilution cladding of INCONEL 718 superalloy
Yanlin Zhou and Qiaoli Lin performed the wetting of galvanized
steel by 4043 AleSi alloys under CMT condition and studied the
interfacial structure. The final wettability of this system was mainly
determined by the wettability of Fe by Al (or steel by Al). The residual liquid Zn film after evaporation cannot improve the final
wettability, and also should not be a driving force for spreading but
may be a factor for the contact angle hysteresis. Further, the moving
of the triple line (i.e., spreading) in this system was limited by the
viscosity of the liquid itself [63]. The beneficial role of Zn coating is
the reducing of the heat input, and a thinner intermetallic layer can
be obtained [64]. Qiaoli Lin et al. found that for small wire feed
speed, the Leidenfrost effect was caused by Zn vaporization which
induced the non-wetting and welding splatter. The wettability was
improved using large WFS [65,66].
Microstructural study of INCONEL 718 superalloy clad by Ola and
Doern revealed that the clads were free from porosity and cracking,
and complete bonding of the clads with the substrate was achieved
in all weldments. The outcome of this work showed that the relatively new CMT process, with the choice of suitable welding parameters, is useful for repair build-up of affected areas of worn-out
and service damaged components of gas turbines and other hightemperature equipment that are manufactured from nickel-base
superalloys [59].
8.2. Cladding of Al 6061 alloy
Benoit et al. performed Al 6061 clads using Metal inert gas
(MIG), pulsed MIG, cold metal transfer MIG (CMT) and tungsten
inert gas (TIG) welding. The cladding operations were analysed by
an infrared thermal imaging technique and beads were characterized by X-ray radiography, neutron diffraction and micro-hardness
mapping. The Pulse-Mix CMT process reached higher peak temperature and produced better quality beads than other MIG processes. The level of residual stress present in the HAZ of the CMT
sample was the highest [60].
8.3. Cold metal transfer deposited AZ31 magnesium alloy clad
Cold metal transfer (CMT) with low heat input exhibits a great
potential for magnesium alloys welding as they are susceptible to
grain coarsening, pores and hot cracking during welding. In this
paper, Heng Zhang et al. observed the effect of welding speed on
microstructures of CMT deposited AZ31 magnesium alloy clad. The
results demonstrated that to get a qualified cladding of AZ31
magnesium alloy, pulsed-CMT, with an optimal welding speed of
12 mm/s, was preferred [61].
8.4. Al-Si-Mn alloy coating on a commercially pure Al plate
The CMT process can be used as an energy-efficient technique
for depositing thick coatings and is useful in weld repair of
aluminum alloy components. Rajeev et al. reported that the bead
angle, deposition rate and dilution are nonlinear functions of the
welding speed. Their coating had thickness greater than 2.5 mm
produced in a single pass, which is considerably high compared to
thermal spray processes [62].
8.6. Wetting of Mg AZ61 alloy/galvanized steel in cold metal
transfer process
The dynamic sessile drop method was used by Qiaoli Lin et al. to
investigate the wetting behavior of galvanized steel by molten Mg
AZ61 alloy under cold metal transfer condition. The observed results showed that the wetting behavior was directly determined by
the wire feed speed (or the heat input) [67].
8.7. Additive manufacturing of Al-6.3% Cu alloy by CMT process
Baoqiang Cong presented a paper on the effect of arc mode in
cold metal transfer (CMT) process on the porosity characteristic of
additively manufactured Al-6.3%Cu alloy. Experiments were performed on both single layer deposits and multilayer deposits. The
variants of CMT performed in the experiment were conventional
CMT, CMT pulse (CMT-P), CMT advanced (CMT-ADV) and CMT pulse
advanced (CMT-PADV). CMT-PADV proved to be the most suitable
process for depositing aluminium alloy due to its excellent performance in controlling porosity. The key factors which enabled
this are low heat input, a fine equiaxed grain structure and effective
oxide cleaning of the wire [68].
8.8. Compositeecomposite joints reinforced with cold metal
transfer welded pins
Stelzer et al. performed fatigue tests on both Ti CMT and Steel
CMT pins used to reinforce Carbon Fibre Reinforced Polymer (CFRP)
sheets. Cold metal transfer welded steel pins proved to be an
effective means for reinforcing CFRPeCFRP SLS joints in the
through-the-thickness direction. After failure of the bond line between the two CFRP laps, pins carry the loads and maintain the
joint's stiffness until final failure. CMT welded titanium pins on the
other hand turned out to be less effective in reinforcing CFRPeCFRP
joints. This can be partly ascribed to the lack of a pronounced
ballhead-spike geometry for Ti CMT pins [69].
8.9. Crack repair welding of steam turbine cases by CMT brazing
Fig. 45. Microhardness values trend.
Cold metal transfer welding was investigated by Kota Kadoi
et al. to develop a repair process for cracks in steam turbine cases,
made of Cr-Mo-V cast steel, operated for 188,500 h at 566 C. Silver
and gold brazing filler wires were used as overlaying materials.
CMT brazing using low melting point filler wire generally was
found to decrease the heat input and peak temperature during the
thermal cycle of the process. The creep-fatigue properties of
weldments produced by CMT brazing with BAg-8 were the highest.
Therefore, CMT brazing using low melting point filler wire such as
S. Selvi et al. / Defence Technology 14 (2018) 28e44
BAg-8 is a promising candidate method for repairing steam turbine
cases [70].
9. Conclusions
The process, weld combinations and applications of the Cold
Metal Transfer welding reported by various authors are discussed.
The main conclusions of this study are:
1) The retraction of the wire during the short circuiting phase plays
an important role, as it leads to prevention of spatter generation
and also produces better weld bead aesthetics.
2) The Laser-CMT hybrid welding produces welds with better
mechanical properties and aesthetics than the Laser welding
and Laser-MIG hybrid welding.
3) The Post Weld Heat Treatment (PWHT) caused a positive effect
on the welds prepared by uniformly distributing the fine precipitates, whereas the Nitro-oxidation treatment of the base
metal prior to welding caused an increase in the level of porosity
causing a drastic increase in the microhardness of the weld.
4) The Cold Metal Transfer Welding has a wide variety of applications such as cladding, additive manufacturing, composite
joint pin fabrication, and crack repair welding.
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[2] Talalaev Robert, Veinthal Renno, Laansoo Andres, Sarkans Martins. Cold metal
transfer (CMT) welding of thin sheet metal products. Est J Eng 2012;18:
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