Trends in Joining Dissimilar Metals by Welding

Trends in Joining Dissimilar Metals by Welding

Paul Kaha, Madan Shrestha Jukka Martikainen

Laboratory of Welding Technology and Laser processing, Lappeenranta University of Technology,

Lappeenranta, FI-53851, Finland

a Corresponding author’s e-mail: [email protected]

Keywords: Dissimilar materials, Weldability, Intermetallic compounds (IMC), Fusion welding, Low- dilution welding, Non-fusion welding

Abstract. The welding of dissimilar materials finds a wide variety of applications in the fields of

industrial construction and manufacturing, where the characteristic features of the different materials

are optimized for the desired application to result in cost effectiveness and value addition. Non-fusion

welding methods such as solid state welding and high energy beam welding are more popular for

welding dissimilar metal combinations, due to fewer complications, than fusion welding, which melts

the base metal and forms brittle intermetallic compounds (IMCs) that may lead to failure. Various

factors have to be considered when assessing the feasibility of welding dissimilar metals and

producing a sound weld joint. This paper presents a broad classification of the most commonly used

welding processes for dissimilar materials, discusses some of the commonly used welding processes

with examples of some common material combinations, critical factors for good welding, and

practical difficulties arising from the physical and chemical properties of materials. From the

findings, it can be inferred that continuous improvement and research is still required in the field of

dissimilar metal welding, particularly in the light of increasing demand for tailored material for

modern engineering and industrial applications.

Introduction

The need for joining dissimilar metals arises from the complex functionality of many modern

industrial applications. As manufacturers focus on reducing production and operational costs, search

for enhanced mechanical and thermal properties, and lightweight solutions for sectors like the

shipping, aviation, and automobile industries, multiple material combinations are increasingly being

used for many products [1]. An emerging field of joining dissimilar metals is transportation, where

multi-material solutions consisting of steel, aluminum, magnesium, and composites are replacing

monolithic steel structures, thus reducing the weight of vehicles and improving fuel efficiency [2].

Fig. 1 presents methods commonly used for the purpose of joining dissimilar metals. Low dilution

and non-fusion joining methods are generally used for high production and special application joining

in which there is minimum alloying between the dissimilar materials. Dissimilar welds encountered

in the power and process industries are more often done by fusion welding [3]. In the case of fusion

welding of dissimilar materials, alloying between the base metals and filler metal is a major

consideration that has to be taken into account. The weld metal formed can exhibit entirely different

characteristics from one or both of the base metals. The main factors that contribute to the failure of

joints between dissimilar metals by arc welding are alloying problems (formation of the brittle phase

and limited mutual solubility), improper joint design, great differences in the melting temperature or

the coefficient of thermal expansion (CTE) of the materials involved, thermal conductivity

differences, and corrosion problems including galvanic corrosion, oxidation, hydrogen-induced

cracking, and sensitization [4]. Conflicts may arise when the optimum heat control of the metals

differs, and compromises are thus required. In light of the complexity of the process and the

compromises required, dissimilar metal welding (DMW) requires more careful study than

conventional, similar-metal welding procedures [3]. This paper gives a brief overview of the joining

Applied Mechanics and Materials Vol. 440 (2014) pp 269-276Online available since 2013/Oct/31 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.440.269

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Fig. 1 Welding methods most commonly used for dissimilar metals [3,4].

methods of dissimilar materials, essential considerations for the feasibility of dissimilar joining, and

problems associated with dissimilar metal welding.

Joining Methods for Dissimilar Metals

Several techniques exist for the joining of dissimilar materials. The main methods used in industrial

applications and forming the topic of research are as follows:

Fusion Welding. Various conventional fusion welding methods such as shielded metal arc welding

(SMAW), gas metal arc welding (GMAW), submerged arc welding (SAW), flux cored arc welding

(FCAW), and gas tungsten arc welding (GTAW) can be used for the welding of dissimilar metal

combinations, as shown in Figure 1. However, there are many difficulties associated with the fusion

welding of dissimilar metals [3]. These difficulties arise from problems associated with metallurgical

incompatibility, e.g. the formation of brittle phases, segregation of high and low melting phases due to

chemical mismatch, and possibly large residual stresses from physical mismatch. Fusion welding is

one of the most commonly used methods for joining metals and despite the difficulties, continuous

efforts are being made to apply fusion welding to the welding of dissimilar metal combinations [5].

Of the various fusion welding techniques for dissimilar materials, cold metal transfer (CMT), a

modified GMAW has found successful applications for combinations of steel and aluminum alloys.

CMT is a modified GMAW process having a mechanism where the filler wire is intentionally

retracted immediately after the short circuit and is operated in a very low current having low heat

input compared to conventional fusion welding processes. Fig. 2(a), shows a schematic of welding by

CMT, between aluminum alloy 6061-T6 (2mm thick) and zinc coated low-carbon steel (1.2 mm

thick) with ER 4043 (1.2 mm diameter) as the filler wire and various torch off-set and pre-setting gaps.

Fig. 2(b) shows a scanning electron micrograph (SEM) image of the intermetallic layer (IM) with an

average thickness less than 5 μm with relatively high weld strength [6]. Thus, CMT can produce

acceptable joints between aluminum and special zinc coated steel with suppressed intermetallic

compound (IMC) level and IM thickness controlled well below the acceptable level of 10 µm.

Low Dilution Welding. This method of welding dissimilar metal combinations also falls under

the category of fusion welding. However, it is characterized by relatively little melting of the base

metals into the weld, no addition of filler metal, and a sound weld quality compared to conventional

fusion welding processes. The processes under this category suitable for the welding of dissimilar

metal combinations are electron beam welding, laser beam welding, and pulsed arc welding, as shown

in Fig. 1 [3]. Depending upon mutual solubility between dissimilar materials, we may define group of

compatible dissimilar materials suitable for welding and in some cases fulfilling the pre-requisite

conditions for specific combination. As an example, the suitability of electron beam welding for

various combinations of materials is shown in Fig. 3 [7]. Selection of a suitable welding process is,

thus, to be done based on mutual solubility between the dissimilar metals and fulfillment of necessary

conditions as per requirement for specific combinations which varies from case to case, limited in the

scope of discussion in this article.

270 Advanced Materials & Sports Equipment Design

Fig. 2 CMT welding between aluminum alloy 6061-T6 and zinc coated carbon steel. (a) Schematic

weld configuration (b) SEM image of a typical intermetallic layer [6].

Fig. 3 Suitability of various combinations of materials for electron beam welding [7]

Several welding methods have been tried for creation of joints between steel and aluminum. Sound

joints with good mechanical properties, however, are difficult to achieve by those conventional

welding processes which form brittle Fe-Al intermetallic compound (IMC) layer at the interface of a

steel/aluminum weld [8]. Furthermore, welding aluminum to steel is difficult due to the large

differences in fusion temperatures, thermal conductivities, and mutual solubilities [9,10].

Apart from non-fusion welding, laser welding is considered one of the most applicable welding

processes for welds between aluminum and steel. Laser welding offers many benefits over

conventional joining methods due to its rapid processing, high quality welds, flexibility, controlled

irradiation, and ease of automation. The use of laser welding is favored for joining steel/aluminum

alloy due to its ability to limit the size of the brittle interface by the highly localized energy input of the

welding source, high cooling rates, and short processing time compared to other techniques [9]. To

minimize the thickness layer of the IMC, the laser beam can be irradiated onto the steel and aluminum

alloys simultaneously but melting only the aluminum alloy due to its lower melting point. The

wettability of aluminum alloys for good weld joint can be improved by laser heating steel to a level at

which the steel sheet is not melted [11].

As an example of good tensile strength for dissimilar weld by low dilution welding process, figure

4 shows an enhanced joining method of laser roller, in which the aluminum alloy is melted by

irradiating laser through the steel material side, after which the joint is pressed with a roller [12,13].

Tensile tests on the specimen of the joint reported to have a fracture in the aluminum alloy base metal

side instead of weld zone and a good welded joint was obtained [13]. The wide weld lap generation in

this laser roller welding method accounted for this good welded joint. A limitation of this method is

that the surface between the dissimilar materials is not flat and the material with a high melting point

should be a thin sheet [11]. Despite these limitations, good quality joints between aluminum alloys

and steel can be produced by laser welding with the assistance of roller pressure.

Applied Mechanics and Materials Vol. 440 271

Non-fusion Joining. Joining processes that do not melt the base metals and are carried out at a

temperature below the melting point of the base metals and without adding filler metal fall under this

category. Solid state welding such as diffusion, explosion, friction, friction stir, and ultrasonic

welding, along with brazing, soldering, adhesive bonding, and mechanical joining are typical

non-fusion joining methods for dissimilar metal combinations. Non-fusion joining techniques can

eliminate some of the problems inherent in fusion welding as the base metals remain in a solid state

without the formation of IMCs [3,5].Fig. 5(a) illustrates the schematic of a commonly used friction

stir weld for dissimilar materials, and Fig. 5(b) shows a macroscopic view of the cross-section of a

dissimilar weld between Al 6013-T4 aluminum alloy and X5CrNi18-10 stainless steel. The

experimental results suggest that the fatigue properties of Al 6013-T4/X5CrNi18-10 stainless steel

joints are approximately 30% lower than those of Al 6013-T6 alloy base metal. However, the

properties are suitable for use in most normal applications [14]. Thus, FSW can produce acceptable

welds between aluminum alloy and steel, with strength comparable to the aluminum alloy base metal.

Fig. 4 Schematic diagram of laser roller dissimilar material welding [11]

Fig. 5 Friction stir weld of Al 6013-T4 to X5CrNi18-10 stainless steel (a) Schematic configuration (b)

Macroscopic overview of the weld cross-section [14]

A motor company has recently developed a robotic technology based on friction stir welding

(FSW) for the continuous welding of steel to aluminum for mass production. The steel/aluminum

sub-frame produced with this technique resulted in a 25 percent (6 kg) lighter weight than that of the

previous model with aluminum and steel joined by bolting, thus resulting in improved fuel efficiency

of the vehicle. The plastic intermixing of the base metals by frictional heat generated in the process

produces an IMC layer (Fe4Al13) as thin as only 1 µm, as shown in Fig. 6. The crucial parameters for

the process are the tool axial pressure and rotation velocity [15]. This robotic FSW technology for

joining steel and aluminum in mass-production is a notable achievement in the automotive welding

sector and has scope for application in other fields as well.

In a study by Patel et al. [16], ultrasonic spot welding (USW) was employed to weld dissimilar

joints of AZ31B-H24 magnesium alloy (2 mm thick, 80 mm by 15 mm) to 5754-O aluminum alloy

(1.5 mm, 80 mm by 15 mm) with a tin interlayer placed in between the faying surfaces. Introducing a


tin interlayer eliminated brittle Al12Mg17 IMCs, which were replaced by a layer of composite like tin

and Mg2Sn structures, as shown in the micro-structural detail in Fig. 7(a) and 7(b). Introducing the tin

interlayer improved the wettability of the Mg and Al alloys and resulted in significant improvements

in joint strength and failure energy. Comparison of the lap shear strength of a Mg/Al joint produced by

various joining techniques like friction stir spot welding, friction stir lap joining, resistance spot

welding, and ultrasonic spot welding (with and without Sn interlayer) is shown in Fig. 7(c). The

strong weld result for USW of Mg and Al alloys with a Sn interlayer, due to elimination of brittle

IMCs, should be investigated further to establish the potential of USW with other dissimilar metals

and to ascertain suitable interlayers for improved joint strength.

Hybrid Welding. In this method, the welding of dissimilar material combinations is done with

more than one welding method. The use of hybrid welding for dissimilar metals has been presented by

Thomy and Vollertsen [17]. Sheets of aluminum alloy (AA6016), in the thickness range of 1 mm,

were joined to zinc coated steel (DC05+ZE) in a butt joint configuration with a filler wire (SG-AlSi

12 – diameter 1.2 mm) by laser-MIG hybrid joining, as illustrated in Fig. 8(a). The process parameters

varied in the study were laser power, MIG arc power, wire feed rate, welding speed, and arc position

relative to the abutting edges. The study showed that for the process parameter

Fig. 6 Metallic bonding of aluminum and steel with intermetallic compound by robotic FSW

technology [15].

Fig. 7 (a) Microstructure detail of USW Mg/Al joint without (b) with Sn interlayer (c) lap shear

strength of Mg/Al joints with different welding techniques [16].

envelope shown in Fig. 8(b), a dissimilar weld with adequate and reproducible joint properties can be

produced, with a sound weld bead, sufficient and regular wetting, a thin intermetallic phase layer of

less than 4 µm, and tensile strength exceeding 180 MPa. Hybrid welding combines the positive

aspects of the processes used and compensates for the shortcomings of each process to produce an

overall positive result.

Low dilution welding, non-fusion joining techniques and hybrid welding are more suitable than

fusion welding for the joining of dissimilar metal combinations due to relatively little (or zero) IMC

formation. However, service conditions, required joint geometry, as well as resource and time

constraints may make particular processes unsuitable [5].


Important Factors Influencing Dissimilar Metal Welds

Fig. 8 Laser MIG hybrid welding of aluminum to steel (a) Schematic diagram (b) Process parameter

envelope [17]

In the case of dissimilar joint produced by fusion welding processes, it has much alloying between

the base metals and filler metal, and the intermetallic compound becomes a major consideration.

Thus, the dissimilar weld joint produced by the fusion welding process can behave entirely differently

from one or both of the parent base metals. The main factors that influence the properties of dissimilar

welded joints and any variations in these factors that may lead to failure (cracking) are discussed

below:

Weld Metal. The composition and properties of the weld metal are the most important considerations

to be taken into account in dissimilar metal joining by fusion welding, and they are dependent on the

compositions of the base metals, filler metal (if used), and the relative dilution of these metals, as well

as their mutual solubility. The solidification characteristics of dissimilar welds are also influenced by

the relative dilutions and compositional gradients near each base metal. The solidification

characteristic is important with respect to hot cracking of the joint during solidification [18].

In fusion welding of dissimilar metals, it is important to assess the phase diagram of the metals

involved to find if mutual solubility exists. Combinations with mutual solubility can be joined

successfully and filler metal soluble with both dissimilar base metals can be added if the two base

metals are not mutually soluble. The microstructural composition of the IMC of the resultant weld

metal depending upon mutual solubility between the dissimilar metals determines the properties of

the joint, for example, crack sensitivity, ductility, and corrosion [18].

Dilution. In the fusion welding of dissimilar metals, the final composition and microstructure of

the weld zone is strongly determined by the weld composition and its dilution level. Thus, careful

control of the welding parameters and the corresponding dilution level is needed to ensure a suitable

microstructure and properties for the required application. In the fusion welding of dissimilar metals,

the volumetric filler metal feed rate and arc power are the primary welding variables that control

dilution. An increase in the filler metal feed rate causes a dilution decrease, whereas an increase in the

arc power results in an increase in the dilution level. The filler metal, when used to join dissimilar

metals, should alloy with the base metals to produce weld metal with a continuous, ductile matrix

phase, and it should also be able to accept dilution (alloying) by base metals without producing a

crack sensitive microstructure. A good weld between dissimilar metals should be as strong as the

weaker of the two base metals i.e. a weld metal with tensile strength and ductility of the weaker base

metal [19].

Melting Temperatures. The melting temperature is also one of the major factors to be considered

when welding dissimilar metals, especially in the case of fusion welding, where a considerable

amount of heat is involved and at the same heat input, one of the metals is melted long before the other.

Large differences between the melting temperatures of the base metals or the filler metal can cause a

rupture of the metal with a lower melting temperature. The solidification and contraction of the metal


with a higher melting temperature induces stresses in the metal with a lower melting temperature

since it is weak and partially solidified. Buttering of the filler metal with an intermediate melting

temperature can be done to solve the problem of differences in the melting temperatures of the base

metals. Provision of a layer of transition by buttering between the base metals can help the problem of

highly different coefficients of thermal expansion that must endure the cycling temperature in service

and provide a barrier layer to reduce the migration of undesirable elements from the base metal to the

weld metal at elevated temperature conditions [18].

Thermal Conductivity. For the welding of dissimilar metals with a considerable difference in

thermal conductivities, the heat source should provide difference of heat for proper heat balance, as

the rapid conduction of heat from the molten weld pool to the base metal with greater thermal

conductivity takes away heat energy and affects the energy input required to locally melt the base

metal. Therefore, for heat balance, the heat source is usually directed at the metal with higher thermal

conductivity. Preheating the base metal with higher thermal conductivity can control heat loss to the

base metal and reduce the cooling rate of the weld metal and the heat affected zone (HAZ) [18].

Coefficient of Thermal Expansion. A great difference in the coefficient of thermal expansion

between dissimilar metals induces tensile stress in one of the metals and compressive stress in the

other during the cooling process. The weld metal under tensile stress can undergo hot cracking during

welding or cold cracking in service if the stresses are not relieved. This is critical in cases where joints

have to operate in elevated temperatures at cyclic modes [18].

Problems Encountered in Welding Dissimilar Materials

Major factors to be taken into consideration before selecting a suitable welding method for joining

dissimilar materials include aspects such as the composition of the dissimilar base materials and filler

material (if used), service conditions, and the geometry of the joint. The service conditions may make

particular processes unsuitable: for example, soldering and adhesive bonding are not applicable for

high temperature applications, and mechanical joints are not suitable for leak-proof joints. The

geometry of the joint makes some welding methods difficult to apply, e.g. friction welding. The main

causes of difficulties and complexities in the welding of dissimilar metal joints, are:

1) Compositional gradients and microstructural incompatibility between the dissimilar base

metals which lead to a large variation in chemical, physical, and mechanical properties in the joint,

resulting in the formation of brittle IMC.

2) Problems associated with welding individual similar base metals.

3) Increased complexity of the joint as a result of adding filler or insert materials.

4) Incompatibility due to great differences in physical properties like melting temperature,

density, thermal conductivity, and the thermal expansion coefficient [5].

Conclusions

The desire to integrate the various beneficial properties of different materials for the purposes of cost

and weight reduction, improved resistance to high temperatures and corrosive environmental

conditions, and better abrasive resistance resulting from a need for longevity in machine components

has created a necessity to join dissimilar materials.

Various joining methods such as welding, brazing, mechanical riveting, and soldering are used for

joining dissimilar materials. Of the various welding methods available, methods that involve the least

amount of melting, with only one of the metals turning into a liquid, minimum intermixing of

dissimilar materials and reduced formation of IMC are to be favored in the welding of dissimilar

materials.

Several variations of low dilution welding, non-fusion welding, and fusion welding are currently

being used for welding of dissimilar materials. Some of these welding methods have relatively few

difficulties in producing an effective, reliable joint. Low dilution, non-fusion welding methods,

brazing, soldering, adhesive bonding and mechanical joining face fewer difficulties than fusion


welding due to elimination of the fusion problem, as the base metal remains in a solid state during

joining. Factors like great differences in the melt temperature, the coefficient of thermal expansion,

thermal conductivity, an incompatible compositional gradient, the formation of brittle intermetallic

phases and improper selection of filler material (if used) pose critical issues for successful joining of

dissimilar materials. Furthermore, even if the weldabilities of the dissimilar materials are known, the

actual weld behavior of the joint is difficult to predict. Use of welding methods involving the

formation of the fewest IMCs, which have the potential to promote brittle fracture, produces a better

joint.

Progress has been made in various aspects of welding of different combinations of dissimilar

metals. However, there is still potential to advance research still further and the use of filler or

interlayer materials is an area that merits additional study.

References

[1] S. Darwish: International Journal of Adhesion & Adhesives, Vol. 24 (2004), p. 347–354

[2] Center for Automotive Research: Automotive Technology - Greener Products, Changing Skills

(Driving Change 2011).

[3] R. Avery: Guidelines for welding dissimilar metals (Nickel Development Institute 1991).

[4] J. R. Davis: Corrosion of Weldments, ASM International, 2006

[5] Z. Sun and R. Karppi: J. of Mat. Proc. Tech. Vol. 59 (1996), p. 257-267

[6] S. Yang, J. Zhang, J. Lian and Y. Lei: Mater. and Des. Vol. 49 (2013), p. 602-612

[7] H. Schultz: Electron beam welding, Abington Publishing, Cambridge, 1993, p. 86-88

[8] M. Roulin, J. Luster, G. Karadeniz and A. Mortensen: Weld. J. Vol. 78 (1999), p. 151–155

[9] W. Lee, M. Schmuecker, U. Mercardo, G. Biallas and S. Jung: Scr. Mater. Vol. 55 (2006), p.

355-358

[10] S. Katayama and H. Fuji: Sci. and Tech. Weld. & Join. Vol. 11 (2006), p. 224–231

[11] S. Katayama: Weld. Int. Vol. 18 (2004), p. 618-625

[12] M. Kutsuna and R. Manoju: Proc. Conf. Japan Welding Society Annual Conference, (2001) p.

92–93.

[13] M. Rathod and M. Kutsuna: Proc. Conf. IIW, Bucharest (2003) IV-814-02.

[14] H. Uzun, C. D. Donne, A. Argagnotto, T. Ghidini and C. Gambaro: Mat. and Des. Vol. 26 (2005),

p. 41-46

[15] Y. Kusuda: Industrial Robot- An Int. J. Vol. 40 (2013), p. 208-212

[16] V. Patel, S. Bhole and D. Chen: Sci. and tech. of weld. & join. Vol. 17 (2012), p. 342-347

[17] C. Thomy and F. Vollertsen: Weld. In the World Vol. 56 (2012), p. 124-132

[18] American Welding Society, in: Dissimilar Metals, volume 4 of Welding Handbook- Metals and

their weldability, (1982), p. 514-547.

[19] ASM International, in: Dilution in Fusion Welding, volume 6A of ASM Handbook, (2011), p.

115-121.


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Trends in Joining Dissimilar Metals by Welding