Cold metal transfer(CMT)technology-An overview

S.Selvi,A.Vishvksenn,E.Rjsekr

aMechanical Engineering,Institute of Road and Transport Technology,Erode,638 316,Tamilnadu,India

bAutomobile Engineering,Institute of Road and Transport Technology,Erode,638 316,Tamilnadu,India

Contents

1. Introduction.......................................................................................................................29

2. Cold metal transfer process..........................................................................................................29

3. 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

4. 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 Ti-6Al-4V 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

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

5. Comparison of CMT and metal inert gas welding.......................................................................................38

6. 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

7. 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

8. 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. Composite-composite joints reinforced with cold metal transfer welded pins........................................................42

8.9. Crack repair welding of steam turbine cases by CMT brazing......................................................................42

9. Conclusions.......................................................................................................................43

.........................................................................................................................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 inwelds 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 short circuiting[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].

A graphical interface was developed by S′onia Meco et al.using 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.

(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 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 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 CuSi3were 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].

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

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)

The intermetallic layers formed at the interface between zinc coated steel and wrought aluminium are predominantly FeAl3phase.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 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

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,Mg17Al12layer and Mg2Al3layer are observed.The micro-hardness in the fusion zone near Mg side is about 230-240 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,Cu9Al4were present in the fusion zone of Al side,and Cu based solid solution was generated in weld zone,while Cu2Mg and Al-Cu-Mg 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,Mg2Al3and Mg17Al12distributed 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 Polarity CMT welding(VPCMT)by Peng Wang et al.The Mg-Al IMC layers were formed in the weld interface,near the AZ31B side of the welded joints and consisted of three intermediate layers:Mg2Al3layer,Mg17Al12layer,and Mg17Al12+a-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 Fe2Al5and FeAl3phase.Fig.21 showsa SEM image of Steel-Weld metalinterface.The thickness of the intermetallic compound layer was controlled under 5μm guaranteeing the joint strength.The tensile strength arrived at was 83 MPa[8].

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 Ti-6Al-4V alloys

The IMCs at the brazing interface shown in Fig.24 mainly composed of two layers:the continuous layer which consisted of Ti3Al and TiAl close to the solid Ti alloy,and the discontinuous serration shaped TiAl3layer 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 5-8μm thick FeAl3intermetallic.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+Al6Cu4Mg5+a-Mg)layer.However,the intermediate brazing zone observed in Fig.26(b)consisted of two IMC(Mg2Cu+MgCu2+Al6Cu4Mg5and Mg17Al12+Al6Cu4Mg5+a-Mg)layers.When the thickness of the brazing interface layers between the Mg weld metal and the Cu base were in the range of 80-350 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 Mg-Ti joint,satisfied weld appearance and higher tensile load of 2.10 kN was obtained.For Ti-Mg joint,a tensile load of 1.83 kN was detected.The brazing interface developed by Cao et al.was mainly composed of Ti3Al,Mg17Al12and Mg0.97Zn0.03intermetallic.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.

4.11.5083-H111 and 6082-T651 aluminium alloys

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 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 sheet-bottom Ti sheet)reached 197.5 N/mm while the tensile shear strength of the Joint II(top Ti sheet-bottom 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 welding-brazing butt joint of Titanium TA2 to pure Copper T2 alloy byCao et al.,the thickness of the IMCs layer was not uniformed:117-129 μm in middle groove surface and 80-100μm 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].

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 Fe2Al5layer 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].

4.14.Aluminium 5A06 with pure Ni N6 plates

The weld joint prepared by Liu et al.can be divided into four parts:the nickel base metal;Ni3Al,Ni0.9Al1.1and Ni2Al3IMC layer;columnar NiAl3layer;and Al-Si 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 NiAl3IMC layer[46].

4.15.5182-O and 6082-T4 aluminium alloy sheets

In the 5182 sheets,the HAZ microstructure showed fine precipitates of second phase and coarsening of the Mg2Al3precipitates in the aluminium matrix.In general,it was found by Ahmed Elrefaey and Nigel G.Ross that CMT welding of 5182-6082 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].

4.16.Titanium AMS4911L with 316L stainless steel

Gon?alo 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

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 Fe2Al5phases 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-FeAl3and g-Fe2Al5phases 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 0.6μm.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 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

The tests conducted by Mateusz Grzybicki and Jerzy Jakubowski 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

Jan Frostevarg et al.compared the hybrid weld with a close-to production setup for low and medium wire deposition rates.Fig.43 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

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 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].

8.Alternative applications of cold metal transfer process

8.1.Low-dilution cladding of INCONEL 718 superalloy

Microstructural study of INCONEL 718 superalloyclad 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 hight emperature 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].

8.5.Wetting of galvanized steel by Al 4043 alloys

Yanlin Zhou and Qiaoli Lin performed the wetting of galvanized steel by 4043 Al-Si 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].

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.Composite-composite 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 CFRP-CFRP 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 CFRP-CFRP 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

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 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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