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Journal of Materials Processing Tech.

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Heat input, intermetallic compounds and mechanical properties of Al/steelcold metal transfer joints

Jin Yanga,⁎, Anming Hub, Yulong Lic,⁎, Peilei Zhanga, Dulal Chandra Sahad, Zhishui Yua

a School of Materials Engineering, Shanghai University of Engineering Science, Shanghai, 201620, Chinab Beijing Engineering Researching Center of Laser Technology, Beijing, 100124, Chinac Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, Nanchang, 330031, Chinad Centre for Advanced Materials Joining, Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario,N2L 3G1 Canada

A R T I C L E I N F O

Associate Editor: J.E. Kinder

Keywords:Aluminum and steel jointCMT welding/brazingIntermetallic compoundsShear strengthInterplanar mismatch

A B S T R A C T

Cold metal transfer welding/brazing of AA5754 aluminum alloy and Q235 low carbon steel was performed in alap joint configuration using an AlSi12 filler wire. The effect of heat input on intermetallic compounds (IMC)formations and mechanical properties has been investigated. At a low heat input of 157 J/mm, a layer of IMC,with a composition of Al7.2Fe1.8Si was generated at the fusion zone/steel interface. Low interfacial energy andgood interfacial bonding between fusion zone and Al7.2Fe1.8Si led increased joint strength. An additional Fe(Al,Si)3 IMC formed at the interface in conjunction to Al7.2Fe1.8Si at a high heat input of 201 J/mm. The jointstrength was significantly decreased and attributed to a high interfacial energy and poor interfacial bondingbetween Al7.2Fe1.8Si and Fe(Al,Si)3. The results showed that the interfacial energy at interphase boundaries hadremarkable effect on the joint strength.

1. Introduction

Application of multi-materials in the automotive industry is one ofthe effective ways of reducing both fuel consumption and greenhousegas emission. A proper joining of dissimilar materials (for example, Alto steel) by a welding method is difficult to achieve due to large dif-ferences in their physical and chemical properties. In literature, variouswelding technologies have been reported. For example, fusion weldingtechniques, including laser welding and electron beam welding, havebeen used to join Al and steel, where the joining is achieved by themixing and inter-diffusion between molten Al and steel. Because of thelow solubility of Fe in Al, a thick and brittle intermetallic compounds(IMC) is readily generated at the weld interface which can deterioratethe load bearing capacity of the weld, as reported by Torkamany et al.(2010). In addition, the joints may also produce some weld defects suchas shrinkage voids, porosities and solidification cracking, etc. (Dindaet al. (2019)).

To overcome the complexities in fusion welding, solid-state weldingtechniques, such as diffusion bonding, ultrasonic spot welding, frictionstir welding and vaporizing foil actuator welding, have also been ex-plored in the past. The solid-state welds were achieved by the inter-diffusion of Fe and Al atoms under high pressure, low temperature or

their combinations. However, the aforementioned methods either re-quires a longer processing time (e.g., diffusion bonding) or specificworkpiece geometries (e.g., friction stir welding). To overcome theselimitations, a combined welding/brazing technique has been developedfor Al/steel joining. The technique has a dual characteristic as it createsa fusion weld at Al side and a brazed joint at steel side. In the welding/brazing application, laser, arc, and their combinations have usuallybeen used as main heat sources. Investigations by Xia et al. (2018) andYang et al. (2015) showed that welding/brazing technology offers agreat potential for dissimilar joining of Al to steel. One of the advancedarc welding/brazing technologies is cold metal transfer (CMT) welding/brazing, which has been used in Al/steel joining due to its low heatinput, small deformation and free spattering. In the investigation ofCMT welding/brazing of Al to boron steel, Cao et al. (2014) pointed outthat the Al/galvanized steel joints possessed higher tensile strengththan that of other types of coated/uncoated steels (e.g., uncoated, Al-Sicoated, and galvannealed). It was reported that the interfacial layerthickness of CMT Al/steel joint was ranging from 2 to 5 μm. In anotherstudy of CMT welding/brazing of Al to steel by Cao et al. (2013), theyoptimized the welding process parameters by Taguchi method. Theyalso pointed out that the degradation of aluminum heat-affected-zoneand thickness of the IMC was minimized by controlling the heat input

https://doi.org/10.1016/j.jmatprotec.2019.05.004Received 24 October 2018; Received in revised form 2 May 2019; Accepted 6 May 2019

⁎ Corresponding authors.E-mail addresses: jyang@sues.edu.cn (J. Yang), liyulong@ncu.edu.cn (Y. Li).

Journal of Materials Processing Tech. 272 (2019) 40–46

Available online 06 May 20190924-0136/ © 2019 Elsevier B.V. All rights reserved.

T

http://www.sciencedirect.com/science/journal/09240136https://www.elsevier.com/locate/jmatprotechttps://doi.org/10.1016/j.jmatprotec.2019.05.004https://doi.org/10.1016/j.jmatprotec.2019.05.004mailto:jyang@sues.edu.cnmailto:liyulong@ncu.edu.cnhttps://doi.org/10.1016/j.jmatprotec.2019.05.004http://crossmark.crossref.org/dialog/?doi=10.1016/j.jmatprotec.2019.05.004&domain=pdf

within 100–200 J/mm.Although it is well-established relationship between heat input, in-

terfacial IMC thickness and joint mechanical properties in laser Al/steelwelds as presented by Pardal et al. (2014) and Wang et al. (2017), thisrelationship is still not conclusive in CMT Al/steel joining. Therefore,this paper aims to study the influence of heat input on interfacial IMCand joint strength for CMT Al/steel joints.

2. Experimental

2.1. Materials

A 2.0mm thick AA5754 aluminum alloy and a 1.8mm thick Q235low carbon steel sheets were used. An Al-Si based alloy (ER4043) with adiameter of 1.2mm was selected as a filler metal. The chemical com-positions of the materials are given in Table 1. In order to preventoxidation, a brazing flux QJ201 was used.

2.2. CMT welding/brazing process

The sheets were machined as rectangular strips of 100mm×150mm. An integrated ABB six-axis robot with a Fronius arc weldingsystem (CMT 4000 Advanced) was used for welding/brazing applica-tion. Prior to the welding/brazing, the sheets were cleaned withacetone to remove oil and debris. The aluminum sheet was placed ontop of the steel sheet in a lap joint configuration with an overlap dis-tance of 15mm. The schematic diagram of the joint configuration isillustrated in Fig. 1. Preliminary experiments were conducted to opti-mize the process parameters (Table 2). The heat input was calculated byusing Eq. (1)

=K IUWs (1)

where, I, U and Ws were welding current, welding voltage, and weldingspeed, respectively.

2.3. Microstructure characterization

The samples were prepared using standard grinding and polishingmethod followed by etching with Keller’s reagent to reveal the micro-structure. The microstructure characterization was carried out by op-tical microscope (OM, model: Olympus 4XCJZ) and scanning electronmicroscope (SEM, model: Hitachi S3400-N) with EDAX Genesis energy-

dispersive X-ray spectrometry (EDS). The X-ray diffraction (XRD) ana-lysis was performed on a Panalytical X’Pert X-ray diffractometer (Cu-Kα, Holland).

2.4. Mechanical testing

Rectangular tensile-shear coupons were cut by a wire saw from thejoints with the dimension of 20mm×160mm. The joint tensile-sheartesting was performed at a cross-head speed of 1.0mm/s. The micro-hardness measurements were carried out using an automated compu-terized hardness tester with a 25 g load and 15 s dwell time.

3. Results

3.1. Metallography

Fig. 2 shows weld appearance and cross sections of CMT Al/steeljoints with different heat inputs. An acceptable surface appearancewithout spattering and undercut was achieved. An uneven weld toewith rough surface was observed when heat input was less than 157 J/mm due to the poor wettability of the molten filler metal. The wett-ability was gradually improved with more stable and uniform surfaceappearance at a higher heat input. To validate the statement, quanti-tative analysis was performed. The molten filler metal’s wetting-spreading ability was evaluated by measuring the wetting angle andbrazed width (Fig. 2). As the heat input increased from 111 J/mm to260 J/mm, the wetting angle gradually decreased from 102 ± 9° to66 ± 7°, while the brazed size progressively became wider from3.8 ± 0.2mm to 10.0 ± 0.7mm. The decreased wetting angle andincreased brazed width is an indication of improved wetting andspreading ability of molten filler metal.

Fig. 3 shows the cross-section of the joints with different heat in-puts. A layer of IMC was observed at the interface between fusion zoneand steel. The measured thickness of the reaction layers with variedheat inputs were shown in Fig. 2. The increase of reaction layerthickness with heat input was related to the formation and growth ofIMC, i.e., time- and temperature- controlled diffusion process. Similarresults were reported by Li et al. (2018) for dissimilar Al/steel jointsmade by laser welding/brazing.

It is clear that only one reaction layer was observed in Fig. 3 (a–c);while, some fine dark features were generated within the reaction layerfrom the steel side as shown in Fig. 3(d–e). It was indicated that a newphase was formed at the fusion zone/steel interface, which was con-firmed by SEM analysis and discussed in a later section. Since the phaseconstituents were identical with only minor differences when heat input

Table 1Chemical compositions of the materials in wt.%.

Materials Mg Zn Ti Si Cr Mn Ti Fe S C Al

Q235 steel – – – 0.22 – 0.60 – Bal. 0.02 0.18 0.455754 Al 2.6-3.6 0.2 0.15 0.4 0.3 0.5 0.15 0.4 – – Bal.ER4043 0.1 0.01 – 4.5-6.0 – – – 0.17 – – Bal.

Fig. 1. Schematic diagram of the joint configuration of CMT welding/brazing.

Table 2The optimized welding parameters used for welding/brazing.

Welding parameters Optimizedrange

Values

Current (I) [A] 67-69 69 68 67 69 68Voltage (U) [V] 10.9-11.8 11.3 11.8 11.7 11.2 11.4Peed (Ws) [mm s−1] 3-7 7 6 5 4 3Wire feed speed (Wf) [m

min−1]4 4

Calculated heat input (K) [Jmm−1]

– 111 129 157 201 260

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was less than 157 J/mm (single-phase reaction layer) or larger than201 J/mm (dual-phase reaction layer), the discussion on microstructurewas focused in the joints obtained at heat inputs 157 J/mm and 201 J/mm, denoting as low and high heat input, respectively.

Fig. 4(a) represents the SEM image of typical IMC layer with lowheat input (157 J/mm). A single-phase reaction layer composed ofserrated IMC was formed at the fusion zone/steel interface. Based onEDS analysis, the IMC layer (P1) composed of 71.5 at. % Al, 9.2 at. % Siand 19.3 at. % Fe (Table 3). According to the Fe-Al-Si ternary phasediagram, the IMC layer was identified as Al7.2Fe1.8Si. However, at highheat input (201 J/mm), a dual-phase reaction layer was observed at thefusion zone/steel interface (Fig. 4(b)). Adjacent to the fusion zone, ascallop-like IMC was obvious, which had the chemical composition of73.2 Al at. %, 17.8 Fe at. % and 9.0 Si at. % (Table 3). Thus, the IMCwas identified as Al7.2Fe1.8Si. Adjacent to the steel side, a newly formedneedle-like IMC with a composition of 71.6 Al at. %, 23.2 Fe at. % and5.6 Si at. % (P3) was observed. The possible phase of the IMC wasidentified as Fe(Al,Si)3. This kind of dual-phase IMCs layer, consisted ofAl7.2Fe1.8Si and Fe(Al,Si)3, was also reported by Song et al. (2009) intungsten inert gas welded Al/steel joints.

3.2. Mechanical properties

Fig. 5 shows microhardness profile across the fusion zone/steel in-terface produced at different heat inputs. The hardness values of the

fusion zone and steel substrate were about 66 HV and 144 HV, respec-tively. In all cases, the hardness at the fusion zone/steel interfacesharply increased over 350 HV. The increase of the microhardness wasattributed to the hard and brittle nature of Fe-Al and Fe-Al-Si IMCs.

The joints were all fractured at the fusion zone/steel interface aftertensile-shear testing. Thus, the joint strength was characterized by shearstrength using the following equations:

τ=F/A (1)

A=Wb*L (2)

where, τ was the joint shear strength, F was the joint fracture load, Awas the interface bonding area, Wb was brazed width (shown in Fig. 2),L was the specimen width (20mm). The plot of measured joint shearstrength and interfacial layer thickness as a function of heat input areshown in Fig. 6. The joint shear strength first slightly decreased fromabout 65MPa to 54MPa when heat input increased from 111 J/mm to157 J/mm, and then it dropped to around 28MPa as heat input ex-ceeded 201 J/mm. On the other hand, a nearly linear correlation be-tween interfacial layer thickness and heat input was identified. Thus,the remarkable decrease in the joint strength was little correlated withthe interfacial layer’s thickness variation. Besides, at low heat input, thefracture location was in the interface between fusion zone andAl7.2Fe1.8Si as shown in Fig. 6. At a high heat input, the fracture loca-tion changed to the Al7.2Fe1.8Si /Fe(Al,Si)3 interface. Hence, it implies a

Fig. 2. Macrostructure, wetting angle, brazed width, thickness of reaction layer of the CMT Al/steel joints with different heat inputs.

Fig. 3. Optical microscopy images of the fusion zone/steel interface with different heat inputs: (a) 111 J/mm, (b) 129 J/mm, (c) 157 J/mm, (d) 201 J/mm, and (e)260 J/mm.

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strong correlation between the joint shear strength and joint failurelocation which will be discussed in the later section.

3.3. Fractography

Fig. 7 shows the fracture surface of the joint at a low heat input. Atthe fusion zone side, the typical cast structure with lots of dendrites wasobserved (Fig. 7(a)). The average chemical composition of the fracturesurface was 90.1 Al at. %, 1.1 Fe at. % and 8.8 Si at. % (Table 3). Thus,it was identified as α-Al and Al-Si eutectic. At the steel side, a smoothplane with some terrace-like structures was obvious. Based on the EDSanalysis, the IMC at the fracture surface was identified as Al7.2Fe1.8Si(Table 3). According to the morphology of the fracture surfaces, itsuggested that a brittle fracture propagated along the fusion zone/Al7.2Fe1.8Si interface, which was further confirmed by XRD analysis asshown in Fig. 7(b) and Fig. 7(d).

Fig. 8 presents the fracture surface of the joint at a high heat input.At the fusion zone side, a smooth plane with some step-like structureswas observed in Fig. 8(a). It was identified as Al7.2Fe1.8Si (Table 3). Atthe steel side, a relatively smooth surface consisting of Fe(Al,Si)3 phasewas confirmed (Table 3). Therefore, it was concluded that a brittlefailure occurred at the interphase boundary between Al7.2Fe1.8Si and Fe(Al,Si)3. The phase constitutions at the fracture surface were also con-firmed by XRD analysis as shown in Fig. 8(b) and Fig. 8(d).

4. Discussion

It has been identified that the phase constituents at the fusion zone/steel interface in CMT Al/steel joints vary with the heat input, viz.,Al7.2Fe1.8Si with low heat input and Al7.2Fe1.8Si+ Fe(Al,Si)3 with highheat input. The formation mechanism of phase constituents can beunderstood from the thermodynamics analysis. It is well-known thatGibbs free energy GΔ is a criterion for chemical reaction to occur. At

= − +GΔ 142770.0 50.8TFe(Al,Si)30 (5)

The plot of calculated GΔ as a function of temperature ranging from700 to 1200 K is shown in Fig. 9. It is found that

and the appearance of a new interface, i.e., Al7.2Fe1.8Si/Fe(Al,Si)3, islikely to be responsible for the remarkable decrease of joint strength.This statement can be supported by the fractography in Figs. 7 and 8. Ata low heat input, the joint fracture occurs at the fusion zone/Al7.2Fe1.8Siinterface, while it occurs at the Al7.2Fe1.8Si/Fe(Al,Si)3 interface at ahigh heat input. It infers that the bonding strength at the Al7.2Fe1.8Si/fusion zone interface is higher than that of Al7.2Fe1.8Si/Fe(Al,Si)3 in-terface.

It is well-established that interphase bonding strength stronglycorrelates to the interplanar mismatch at the interphase boundary. Forexample, in the investigation of resistance spot welding of Mg alloy toZn-coated steel, Liu et al. (2011) demonstrated a nanocale transitionalFe2Al5 layer significantly improved the joint strength, without which itwas difficult to join owing to original sharp interface and high inter-facial mismatch. By using the edge-to-edge matching model proposedby Zhang and Kelly (2005), Nasiri (2013) revealed that interplanarmismatch determined interfacial energy at the interphase boundarywhich in turn affected interphase metallurgical bonding strength inlaser Mg/steel joints with Ni, Zn and Sn transitional layers. The abovetransitional layers play the key role of the interplanar mismatch re-duction.

Normally, the matching planes at interphase boundaries are theclose-packed or nearly close-packed planes. In the present study, themetallurgical bonding strength at the interphase boundary is semi-quantitatively evaluated by comparing the interplanar mismatch at theinterphase boundaries. As HCP phase, Al7.2Fe1.8Si has the lattice con-stant of aAl7.2Fe1.8Si = 12.406 Å and cAl7.2Fe1.8Si = 26.236 Å. Fe(Al,Si)3has a monoclinic crystal structure with the lattice constant ofaFe(Al,Si)3 = 15.489 Å, bFe(Al,Si)3= 8.083 Å, cFe(Al,Si)3 = 12.476 Å,β=107.7°. As the main constituent phase in fusion zone, α-Al is a FCCphase with the lattice constant of aα-Al =4.050 Å. The close-packedplanes of FCC and HCP are displayed in Table 4. The close-packedplanes of monoclinic Fe(Al,Si)3 are identified by looking at the powderX-ray diffraction intensities, which are {332}, {025} and {620}. The

interplanar spacing d along (hkl) planes is expressed using the equationas proposed by Cullity (2001):

⎜ ⎟= ⎛

⎝+ + − ⎞

⎠d βh

ak β

bl

chl β

a c1 1

sinsin 2 cos

2 2

2

Fe(Al,Si)32

2 2

Fe(Al,Si)32

2

Fe(Al,Si)32

Fe(Al,Si)3 Fe(Al,Si)3

(6)

where, h, k, l are miler indices of a crystallographic plane, d is the in-terplanar spacing along the plane (hkl).

Tables 5 and 6 summarize the interplanar mismatch of all possiblematching planes at the interphase boundaries between α-Al andAl7.2Fe1.8Si as well as Al7.2Fe1.8Si and Fe(Al,Si)3. The interplanar mis-match at the interphase boundary between α-Al and Al7.2Fe1.8Si rangesfrom 62.3% to 82.2%, while it is up from 411.7 % to 539.9% at the Fe(Al,Si)3/Al7.2Fe1.8Si interface. Both the interplanar mismatches arelarger than 10%, indicating the formation of noncoherent interfaces.According to the Hooke’s law:s

=−

E Gv

ε f y1

( )ε 02 2

where, Eε, G, ν, ε0, and f(y) are free (strain) energy density, shearmodulus, Poisson's ratio of the reaction product, lattice mismatch, andthe unit step function, respectively. The equation suggests that the free(strain) energy is always positive and proportional to the lattice mis-match. The increase of free (strain) energy will increase the total in-terfacial energy as stated by Fredriksson and Åkerlind (2012). Thus, ahigh interplanar mismatch leads to a high interfacial energy, which inturn results in a poor interfacial bonding. Hence, at macroscale, α-Al/Al7.2Fe1.8Si interfaces exhibit much higher bonding strength than thatof Fe(Al,Si)3/Al7.2Fe1.8Si interfaces.

Fig. 9. Calculated Gibbs formation energy for Fe(Al,Si)3 and Al7.2Fe1.8Si IMCs.

Table 4The close-packed planes and their interplanar spacings for HCP and FCC crystalstructures.

Crystal structure Close-packed plane Interplanar spacing

HCP (aH, cH)* {0002} c /2H{101̄1}

+

aH cHcH aH

3

4 2 3 2

{101̄0} a3 /2HFCC (aF)* {111} aF3 3

{110} aF2 2{100} aF

* Lattice parameters.

Table 5Calculated interplanar spacing mismatch at the interphase boundary between α-Al and Al7.2Fe1.8Si.

Matching planes α-Alinterplanarspacing (Å)

Al7.2Fe1.8Siinterplanarspacing (Å)

Interplanarmismatch (%)

−{111} //{0002}α Al Al7.2Fe1.8Si 2.338 13.118 82.2

−{110} //{0002}α Al Al7.2Fe1.8Si 2.862 13.118 78.2

−{100} //{0002}α Al Al7.2Fe1.8Si 4.049 13.118 69.1

−{111} //{101̄1}α Al Al7.2Fe1.8Si 2.338 10.889 78.5

−{110} //{101̄1}α Al Al7.2Fe1.8Si 2.862 10.889 73.7

−{100} //{101̄1}α Al Al7.2Fe1.8Si 4.049 10.889 62.8

−{111} //{101̄0}α Al Al7.2Fe1.8Si 2.338 10.745 78.2

−{110} //{101̄0}α Al Al7.2Fe1.8Si 2.862 10.745 73.9

−{100} //{101̄0}α Al Al7.2Fe1.8Si 4.049 10.745 62.3

Table 6Calculated interplanar spacing mismatch at the interphase boundary betweenAl7.2Fe1.8Si and Fe(Al,Si)3.

Matching planes Al7.2Fe1.8Siinterplanarspacing (Å)

Fe(Al,Si)3interplanarspacing (Å)

Interplanarmismatch (%)

{0002} //{332}Al7.2Fe1.8Si Fe(Al,Si)3 13.118 2.09 527.7{101̄1} //{332}Al7.2Fe1.8Si Fe(Al,Si)3 10.889 2.09 421.0

{101̄0} //{332}Al7.2Fe1.8Si Fe(Al,Si)3 10.745 2.09 414.1{0002} //{025}Al7.2Fe1.8Si Fe(Al,Si)3 13.118 2.05 539.9{101̄1} //{025}Al7.2Fe1.8Si Fe(Al,Si)3 10.889 2.05 431.2

{101̄0} //{025}Al7.2Fe1.8Si Fe(Al,Si)3 10.745 2.05 424.1{0002} //{620}Al7.2Fe1.8Si Fe(Al,Si)3 13.118 2.10 524.7{101̄1} /{620}Al7.2Fe1.8Si Fe(Al,Si)3 10.889 2.10 418.5

{101̄0} //{620}Al7.2Fe1.8Si Fe(Al,Si)3 10.745 2.10 411.7

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5. Conclusions

1 At a low heat input not higher than 157 J/mm, a single-phase re-action layer, composed of Al7.2Fe1.8Si, is formed at the fusion zone/steel interface. At a high heat input not lower than 201 J/mm, theinterfacial microstructure is changed to a combination of Fe(Al,Si)3and Al7.2Fe1.8Si.

2 The fracture locations vary with the heat inputs: the fusion zone/Al7.2Fe1.8Si interface with low heat input, while interphaseboundary between Al7.2Fe1.8Si and Fe(Al,Si)3 with high heat input.

3 Interfacial lattice mismatch has a significant influence on the jointstrength: relatively low interfacial interplanar mismatch is obtainedat the Al7.2Fe1.8Si/fusion zone interface making a good interfacialbonding and high joint strength (˜54MPa); but it decreases to28MPa due to relatively high interfacial interplanar mismatch andpoor bonding at the Fe(Al,Si)3/Al7.2Fe1.8Si interface.

4 Comparing to the wettability of filler metal, the thickness and phaseconstituent of reaction layer have more pronounced influences onthe joint strength. The heat input should be kept as low as possibleto maintain a thin and single-phase (Al7.2Fe1.8Si) reaction layer atthe fusion zone/steel interface to achieve a good interfacial bondingand a sound joint.

Acknowledgements

Financial supports of the National Natural Science Foundation ofChina (51805315, 51665038 and 51775091), Beijing EngineeringResearching Center of Laser Technology (BG0046-2018-06), theAcademic and Technical Leaders Founding Project of Major Disciplinesof Jiangxi Province (2018) and the Jiangxi Science Fund forDistinguished Young Scholars (2018ACB21015), and the TalentProgram of Shanghai University of Engineering Science(2018RC452018) are gratefully acknowledged. The authors would liketo thank Dr. Denzel Bridges from Department of Mechanical, Aerospaceand Biomedical Engineering, University of Tennessee for valuable

proofreading.

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Heat input, intermetallic compounds and mechanical properties of Al/steel cold metal transfer jointsIntroductionExperimentalMaterialsCMT welding/brazing processMicrostructure characterizationMechanical testing

ResultsMetallographyMechanical propertiesFractography

DiscussionConclusionsAcknowledgementsReferences