ANALYSING THE CHALLENGE OF ALUMINIUM TO COPPER FSW

I. Galvão 1,*, D. Verdera 2, D. Gesto 2, A. Loureiro 1 D. M. Rodrigues 1

1 CEMUC, Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis

Santos, 3030-788 Coimbra, Portugal 2 AIMEN, Relva 27A Torneiros, 36410 Porriño, Spain

* e-mail - ivan.galvao@dem.uc.pt, tel. + (351) 239 790 700, fax. + (351) 239 790 701

ABSTRACT

Over the last 3 years, significant efforts have been made by CEMUC and AIMEN research

groups in order to improve the global understanding of aluminium to copper friction stir

welding phenomena. Morphological, metallographic and structural analyses of AA 5083-

H111 and AA6082-T6 to copper-DHP friction stir butt and lap welds, produced under different

welding conditions, were conducted. The welding conditions under study included the relative

positioning of the plates, the process parameters, the tool geometry and the type of Al-Cu

pairs to be joined, namely, AA5083/copper-DHP or AA6082/copper-DHP. The influence of

the welding conditions on the metallurgical phenomena taking place during friction stir

welding, the intermetallic phases formation mechanisms and their relation with welds

morphology and final strength was analysed.

Keywords: Friction stir welding, AA 5083/copper-DHP, AA 6082/copper-DHP, Welding

conditions, Intermetallic phases

1 - INTRODUCTION

Friction Stir Welding (FSW) is an innovative solid-state joining technology, which has great

potential for joining materials with high chemical affinity and completely different physical and

mechanical properties, such as aluminium and copper [1]. The production of

aluminium/copper (Al/Cu) hybrid systems would enable the development of new engineering

solutions combining copper´s improved mechanical, thermal and electrical properties with

aluminium’s low specific weight and cost. However, although some experiments in FSW of

aluminium to copper have already been reported, sound joining of these metals was not

achieved yet and several issues still require extensive research [2,3]. In fact, the different

physical and mechanical properties of the base materials, as well as its chemical affinity

make mandatory the optimization of the welding parameters in order to provide adequate

metal flow around the tool and, simultaneously, prevent the formation of a large amount of

brittle aluminium/copper intermetallic compounds.

Over the last 3 years, significant efforts have been made by CEMUC and AIMEN

research groups in order to improve the global understanding of aluminium to copper FSW

phenomena [4-6]. Morphological, metallographic and structural analyses of AA 5083-H111

and AA6082-T6 to copper-DHP friction stir butt and lap welds, produced under different

welding conditions, were conducted. Important conclusions in which concerns the influence

of welding conditions on material flow mechanisms, on the formation/distribution of brittle

intermetallic phases during welding and its consequences in the structure and morphology of

the welds were reached. The main findings from these researches, as well as an analysis of

the global challenge in Al/Cu FSW, based on extensive bibliographical analysis, will be

summarized and discussed in this paper.

The welding conditions to be analysed include the relative positioning of the plates, the

process parameters, the tool geometry and the type of Al-Cu pairs to be joined, namely,

AA5083/copper-DHP or AA6082/copper-DHP. The influence of the welding conditions on the

metallurgical phenomena taking place during FSW, the intermetallic phases formation

mechanisms and their relation with welds morphology and final strength will be discussed.

2 - EXPERIMENTAL PROCEDURE

2.1 - MATERIALS AND WELDING PROCESS

Butt Welding

One millimetre-thick plates of oxygen-free copper with high phosphorous content (copper-

DHP, R240) and 5083-H111 aluminium alloy (AA 5083-H111) were friction stir butt welded in

an ESAB LEGIO FSW 3U apparatus. The welds were carried out using tools with two

different shoulder geometries (3º conical and scrolled), which are illustrated in Figure 1, and

varying welding parameters (traverse and rotation speeds). Despite the different geometries,

both tools were composed of a 3 mm-diameter pin and a 14 mm-diameter shoulder.

Table 1 displays the full set of welding conditions considered to produce the butt joints.

With reference to the testing conditions, the nomenclature adopted in the text to classify the

welds identifies the tool (C or S), the rotational and welding speeds used and the material

positioned at the advancing side of the tool. Thus, the C1000_16-Cu acronym identifies a

weld produced with the conical tool, with rotational and welding speeds of 1000 rev.min-1 and

160 mm.min-1, respectively, and with the copper plate positioned at the advancing side of the

tool.

(a) (b)

Figure 1 - Schematic representation of the conical (a) and scrolled (b) shouldered tools.

Table 1 – Welding parameters used to carry out the butt welds.

Weld Tool Geometry Rotational Speed

(rev.min-1)

Traverse Speed

(mm.min-1)

Adv. Side Material

C750_16-Cu

Conical Shoulder

750 160 Copper-DHP

C1000_16-Cu 1000 160 Copper-DHP

C750_16-Al 750 160 AA 5083-H111

S750_16-Cu Scrolled Shoulder 750 160 Copper-DHP

Lap Welding

Dissimilar friction stir lap welds of copper-DHP (R240) with two different aluminium alloys,

the heat treatable AA 6082-T6 and the non-heat treatable AA 5083-H111 alloys, were

produced in a MTS I-Stir PDS equipment. As illustrated in Figure 2, the 1 mm-thick copper-

DHP plate was placed at the top of the joints and the 6 mm-thick plates of both aluminium

alloys at the bottom. The welds were carried out with a 10 mm-diameter conical tool with an

8º shoulder cavity and a 3 mm-diameter cylindrical probe. In order to study the effect of

aluminium alloy type on welding results, all welds were produced with the same welding

parameters, namely, a rotational speed of 600 rev.min-1 and a traverse speed of 50 mm.min-

1. The nomenclature adopted in the text for labelling the different lap welds was selected in

order to identify the only variable welding condition, i.e., the aluminium alloy. So, the copper-

DHP/AA 5083-H111 and copper-DHP/AA 6082-T6 lap welds are identified by the acronyms

L5 and L6, respectively.

Figure 2 - Schematic representation of the Al/Cu friction stir lap joints.

2.2 - EQUIPMENTS, TECHNIQUES AND METHODS

After welding, a qualitative and quantitative macroscopic inspection of the weld surfaces was

performed by means of visual inspection and image data acquisition, using ARAMIS optical

analysis equipment, respectively. Transverse cross-sectioning of the welds was performed

for metallographic analysis. The samples were prepared according to standard

metallographic practice and etched in order to enable the identification of the different

materials in the weld. Metallographic analysis was performed using optical microscopy, in a

ZEISS HD 100 equipment. Scanning electron microscopy (SEM) and micro X-ray diffraction

were performed in the cross-section and on the surface of some selected welds, using a

PHILIPS XL30 SE microscope and a PANalytical X´Pert PRO micro-diffractometer,

respectively.

3 - RESULTS AND DISCUSSION

3.1 - BUTT WELDING

Influence of Base Materials Positioning on Welds Structure and Morphology

Pictures of the surface, optical macrographs of the transverse cross-sections and

micrographs of selected cross-section areas of the C750_16-Al, C750_16-Cu and

C1000_16-Cu welds are illustrated in Figures 3 to 5, respectively. From the photographs it

can be observed that all weld surfaces are formed by a shiny layer of irregularly distributed

material (Figures 3.a, 4.a and 5.a). In fact, the pictures show that zones of strong material

accumulation and regions with significant material absence compose the top layers of all

welds, which points to strong irregularity in the material deposition process. However, despite

these similarities, important differences can be observed in the surface of the welds

produced with reverse base materials positioning. Effectively, contrary to that registered for

the C750_16-Cu and C1000_16-Cu welds (Figures 4.a and 5.a), which were carried out with

copper plate positioned at the advancing side of the tool, strong material expulsion was

observed for the surface of the C750_16-Al weld. In fact, as observed in Figure 3.a,

positioning the aluminium plate at the advancing side resulted in the production of very thin

welds with massive aluminium flash.

Comparing the cross-section macrographs of the welds (Figures 3.b, 4.b and 5.b), in

which the pin and shoulder influence areas are indicated by vertical lines, strong influence of

base materials positioning on nuggets morphology can also be observed. From Figure 3.b it

can be observed that, for the C750_16-Al weld, a top layer of copper, which was dragged

from the retreating to the advancing side of the tool, expulsed the softer aluminium from the

shoulder influence zone, giving rise to the strong thinning and massive flash formation

observed in Figure 3.a. From the micrograph registered in the nugget of this weld (Figure

3.c), it can be concluded that, although some copper particles are dispersed over the

aluminium matrix, no base materials mixing took place during welding. On the other hand,

strong base materials interaction took place during welding with copper plate positioned at

the advancing side of the tool, as is shown in Figures 4.b and 5.b. In these figures it can be

observed aluminium and copper layers at the top and bottom of the nugget, respectively, as

well as important base materials mixing zones (at dark), which extend from the pin influence

area to the advancing side of the welds, can be observed in Figures 4.b and 5.b. The BSE

micrographs registered in these regions, which are illustrated in Figures 4.c and 5.c, show

the formation of complex mixing structures with a tumultuous fluid-like morphology.

Figure 3 – Surface photograph (a), cross-section macrograph (b) and optical micrograph

registered in the nugget of the C750_16-Al weld [4].

Figure 4 – Surface photograph (a), cross-section macrograph (b) and SEM micrograph

registered in the nugget of the C750_16-Cu weld [5].

Figure 5 – Surface photograph (a), cross-section macrograph (b) and SEM micrograph

registered in the nugget of the C1000_16-Cu weld [5].

Influence of Welding Parameters on Welds Structure and Morphology

Although important base materials interaction have been observed for both welds produced

with the copper plate at the advancing side of the tool, significant differences in the

morphology of these mixing zones were identified, depending on the welding parameters

used to produce the welds.

Comparing the cross-sections presented in Figures 4.b and 5.b, it can be observed

that the dark mixing zone of the weld produced with higher rotational speed (C1000_16-Cu)

and, consequently, with higher heat input, is much larger than the same zone of the weld

produced with lower tool rotation (C750_16-Cu). Furthermore, from the BSE micrographs

illustrated in Figures 4.c and 5.c, it can be observed that the mixing zone of the C1000_16-

Cu weld is also much more homogenous. Effectively, whereas aluminium and copper

intercalated with aluminium-rich and copper-rich mixed lamellae were identified in the mixing

zone of the C750_16-Cu weld, an almost homogeneous mixture, exclusively composed of

copper and copper-rich mixed structures, was identified for the weld produced with higher

rotation. So, it can be concluded that increasing the rotational speed resulted in the formation

of mixed material zones with increased dimension and homogeneity.

The results of the XRD analysis performed in the nugget of the C750_16-Cu and

C1000_16-Cu welds are shown in Figure 6. For the C750_16-Cu weld (Figure 6.a), zones

with base materials composition, mixing regions with significant amounts of f.c.c. Cu, f.c.c.

Al, Cu9Al4 and CuAl2 and mixing areas only composed of f.c.c. Cu and Cu9Al4 were identified,

which is in accordance with the heterogeneous morphology of the mixing structure, already

depicted in Figure 4.c. On the other hand, for the C1000_16-Cu weld (Figure 6.b), only f.c.c.

Cu and Cu9Al4 were detected in the homogeneous base materials mixing region. This way, it

can be concluded that the phase composition of the nugget, similarly to its morphology,

evolves with heat input, since increasing the tool rotational speed increased the intermetallic

content and the homogeneity of the nugget. The evolution of the mixing regions’ morphology

and phase content with the welding parameters is exhaustively analysed and supported in

Galvão et al (2011) [5], in which important relations between the physical properties of both

intermetallic phases (CuAl2 and Cu9Al4), their formation mechanisms and the metallurgical

and material flow phenomena taking place during Al/Cu FSW are established.

(a)

(b)

Figure 6 - Results of the XRD analysis carried out in the nugget of the C750_16-Cu (a) and

C1000_16-Cu (b) welds [5].

Influence of Tool Geometry on Welds Structure and Morphology

Base materials positioning and welding parameters revealed to have strong influence on the

structure and morphology of the welds produced with a traditional conical tool. A detailed

analysis of the influence of shoulder geometry on the morphology and structure of dissimilar

Al/Cu welds was performed by comparing the results discussed above with the welding

results obtained by using a scrolled shoulder tool.

Surface pictures, an optical macrograph of the transverse cross-section and a SEM

micrograph registered in a selected cross-section zone of the S750_16-Cu weld are

illustrated in Figure 7. Comparing Figures 4.a and 7.a, in which surface pictures of welds

produced with the same welding parameters, but using different shoulder geometries

(C750_16-Cu and S750_16-Cu), are illustrated, it can be concluded that the weld produced

with the scrolled shoulder displays much smoother surface than the weld produced with the

traditional conical tool. In fact, as opposed to the quite irregular surface of the C750_16-Cu

weld, the surface of the S750_16-Cu weld displays fine and regularly distributed arc shaped

striations, with characteristics similar to those observed in Al/Al or Cu/Cu friction stir welds

[7,8].

Similarly to that observed for the C750_16-Cu weld (Figure 4.b), the transverse cross-

section illustrated in Figure 7.b also shows aluminium and copper at the top and the bottom

of the S750_16-Cu weld’s nugget, respectively. However, despite this similarity, important

differences are also observed by comparing both nuggets morphology. Effectively, unlike that

observed for the C750_16-Cu weld, a well-defined tongue of grey material going upwards

through the advancing side of the S750_16-Cu weld is observed in Figure 7.b. This tongue

is embedded in a copper matrix and presents a quite homogeneous morphology, in which no

lamellae of intercalated materials are discernible. In fact, the BSE micrograph acquired in the

tongue, which is illustrated in Figure 7.c, shows a homogenous aluminium-rich material

matrix, in which some copper-rich particles are dispersed. The different material flow

mechanisms promoted by the conical and scrolled shouldered tools, which are deeply

analysed in Galvão et al (2010) [4], are on the basis of the dissimilar morphologies presented

by the mixing regions of the C750_16-Cu and S750_16-Cu welds.

Figure 7 – Surface photograph (a), cross-section macrograph (b) and SEM micrograph

registered in the nugget of the S750_16-Cu weld [6].

The results of the XRD analysis performed in the mixing zone of the S750_16-Cu weld

are shown in Figure 8. From the diffractogram, it can be observed that, contrary to the highly

heterogeneous phase content of the mixing region of the C750_16-Cu weld, the material

tongue formed in the nugget of this weld is mostly composed of CuAl2, which agrees well

with the homogeneous morphology of this structure (Figures 7.b and c). In fact, only an

almost negligible quantity of copper (f.c.c. Cu) and small amounts of aluminium (f.c.c. Al) and

Cu9Al4 were detected in this zone. Comparing the C and S (750_16-Cu) welds, it can be

concluded that, although both welds have been done using the same welding parameters,

both the morphology and the phase content of the nuggets are completely different.

Effectively, whereas the scrolled tool promoted the formation of a mixed region almost

exclusively composed of CuAl2, the conical tool gave rise to a highly heterogeneous mixture

in the nugget, with lower intermetallic content than that of the scrolled weld. The

dissimilarities observed in the mixing structures’ phase content are addressed, in detail, in

Galvão et al (2012) [6]. Relations between the material flow mechanisms induced by the

conical and scrolled geometries and the relative amounts of each base material present in

the mixing volumes are discussed by the authors.

Figure 8 - Results of the XRD analysis carried out in the nugget of the S750_16-Cu weld [6].

In order to understand the differences in surface finishing between the welds performed

with conical and scrolled tools, the weld crowns phase content of the C750_16-Cu and

S750_16-Cu welds was analysed, as illustrated in Figure 9. The diffractogram in Figure 9.a

shows that high amounts of the intermetallic phase CuAl2 are distributed over the C750_16-

Cu weld. Small amounts of f.c.c. Al, f.c.c. Cu and Cu9Al4 were also detected. On the other

hand, as illustrated in Figure 9.b, large amounts of aluminium, some copper and very small

amount of intermetallic phases were identified at the surface of the S750_16-Cu weld. So, it

can be concluded that the surface of the S750_16-Cu weld (Figure 7.a) is mainly composed

of aluminium, and for that reason, displays morphology very similar to that of similar

aluminium welds. As opposed to this, for the C750_16-Cu weld, the top layer consists of an

intermetallic-rich mixture with strong non-metallic characteristics, which is irregularly

distributed over the weld surface during tool traverse motion, compromising negatively weld

surface finishing. According to Galvão et al (2012) [6], the different flow mechanisms induced

by the scrolled and conical tools promoted different distributions of the intermetallic-rich

material in the weld structure.

Figure 9 - Results of the XRD analysis carried out on the surface of the C750_16-Cu (a) and

S750_16-Cu (b) welds [6].

3.2 - LAP WELDING

Influence of Aluminium Alloy on Welds Structure and Morphology

Although the influence of technical conditions, such as welding parameters and tool

geometry on dissimilar lap welding results has already been addressed by other authors [9-

11], the influence of the base materials intrinsic properties on Al/Cu friction stir weldability

has never been explored. So, keeping all welding conditions constant, two different

aluminium alloys (heat or non-heat treatable alloys) were friction stir lap welded to copper.

Images of the surfaces, cross-section macrographs and micrographs registered in

some selected cross-section areas of the L5 and L6 welds are illustrated in Figures 10 and

11, respectively. Significant differences in surface finishing can be observed by comparing

the weld surface photographs (Figures 10.a and 11.a). In fact, whereas L5 weld presents a

very smooth surface composed of regular and well-defined striations, similar to those

obtained in similar copper FSW [8], signs of significant tool submerging and formation of

massive flash are observed at the surface of the L6 weld. It is important to stress that,

although both welds have been performed under the same welding conditions, the L6 weld

presents, on surface, defects usually associated to excessive heat input during welding [12].

Comparing the cross-section macrographs of both welds (Figures 10.b and 11.b), in

which pin and shoulder influence zones are indicated by vertical lines, important differences

in the structure and morphology of the TMAZs can also be observed. In Figure 10.b, which

displays the cross-section of the L5 weld, it can be observed that the interaction zone of this

weld is restricted to the pin influence zone, where a very fine recrystallized grain structure is

discernible for copper. Very small evidence of material dragged by the shoulder is also

observed at the top for this weld (Figure 10.c), indicating that the shoulder influence zone

was restricted to the top surface of the copper plate. The totally inefficient mixing, between

the aluminium and copper, registered for the L5 weld, promoted the formation of large

defects throughout the interface of both materials layers (Figure 10.b).

The cross-section macrograph of the L6 weld is illustrated in Figure 11.b. From the

picture, it can be observed that the TMAZ of this weld is significantly larger than that of the

L5 weld. The picture also shows the presence of a well-defined shoulder influenced zone,

which, as illustrated by the deformed copper grains shown in Figure 11.c, extends

throughout the copper plate’s thickness. So, comparing L5 and L6 welds, it can be concluded

that a larger amount of material was dragged by the shoulder for the last one. In good

agreement with this, as illustrated in Figure 11.d, strong base materials interaction took

place during L6 welding, resulting in the formation of mixing structures with morphology

similar to those observed in butt Al/Cu FSW (Figures 4 and 5). In fact, a complex mixing

structure composed of copper and aluminium intercalated with lamellae of material

morphologically different of both base materials, which, according to that observed in butt

joining, have intermetallic-rich phase composition, is discernible in the picture. However, in

spite of more efficient base materials mixing than in L5 welding, which points to a stronger

interaction between the shoulder and pin stirred volumes, some defects were also observed

in the nugget of this weld, specifically, in the mixing structures (Figure 11.d). It is important

to stress that these defects, besides presenting different morphology, are significantly

smaller than those observed for the L5 weld. In fact, besides totally efficient base materials

mixing may have not been achieved in some regions, which should result in the appearance

of small defects, the strong brittleness of new Al/Cu phases formed during welding should

also have some influence on material flow and defects formation. According to some authors,

cracking incidence in intermetallic-rich zones is one of the main causes for the premature

failure of dissimilar Al/Cu lap joints [9,10]. This way, it can be concluded that stronger base

materials mixing during dissimilar Al/Cu FSW does not necessarily mean sound joining.

(a)

(b)

(c)

Figure 10 – Surface photograph (a), cross-section macrograph (b) and optical micrograph

registered in the under shoulder copper grains (c) of the L5 weld.

(a)

(b)

(c) (d)

Figure 11 – Surface photograph (a), cross-section macrograph (b) and optical micrographs

registered in the under shoulder copper grains (c) and in the nugget mixing structures (d) of

the L6 weld.

Since both welds were carried out under the same welding conditions, a strong

influence of aluminium alloys nature on welding results, namely, in structure and morphology

of the welds has to be pointed. Effectively, some studies have already addressed the

markedly different mechanical behaviours of 5083 and 6082 aluminium alloys at high

temperature and strain rates, as well as its relations with the friction stir weldability of both

alloys [12-14]. According to Leitão et al (2012) [14], the 6082 aluminium alloy experiences

strong softening with plastic deformation at increasing temperatures, which is traduced by a

strong decrease of the flow stresses of the material with plastic deformation. On the other

hand, the authors stated that the 5083 alloy presents, at high temperatures, steady flow

stress behaviour. So, as FSW involves plastic deformation at high temperatures and strain

rates, stronger softening of the 6082 alloy is expected during welding. As a result of this,

under approximately the same loading conditions, the extreme thermal softening

experienced by the 6082 alloy led to further submerging of the tool during welding, which

resulted in the strong deepening and massive flash formation observed at the surface of the

L6 weld (Figure 11.a). The higher tool submerging during AA 6082/copper-DHP welding also

resulted in increased amounts of copper and aluminium being dragged by the shoulder and

the pin, respectively, into the shear layer. The strong pin-governed base materials mixing at

the shear layer resulted in the formation of the mixing structures observed in the nugget of

the L6 weld (Figures 11.b and d). As opposed to this, for the L5 weld, the significantly

smaller volume of copper dragged by the tool, at each revolution, as well as the less efficient

deformation promoted by the pin in the aluminium alloy (AA 5083), prevented strong base

materials interaction at the shear layer, which resulted in the formation of sharp and defective

aluminium/copper interfaces in the nugget.

4 - CONCLUSION

In order to improve the global understanding of aluminium to copper FSW phenomena, the

influence of large range of welding conditions on the morphological and structural properties

of dissimilar Al/Cu welds has been studied. Some important conclusions have already been

reached:

Base materials positioning has strong influence on butt welds morphology and

structure. The welds performed with the aluminium placed at the advancing side of

the tool were morphologically very irregular due to the expulsion of the aluminium

from the weld area.

The morphology and the intermetallic phase content of the weld nuggets are strongly

dependent on the welding parameters. Increasing the rotational speed results in the

formation of mixing regions with increased dimension, homogeneity and intermetallic

content.

Shoulder geometry has strong influence on the morphology and intermetallic content

of the welds nugget. Scrolled shoulder promotes the formation of nugget mixing

regions with higher homogeneity and intermetallic content. Shoulder geometry also

strongly determines the surface finishing of the welds, since it has great influence on

the nature of the material deposited on the top of the joints.

The different mechanical behaviours of 5083 and 6082 aluminium alloys, at high

temperature and strain rates, have an important effect on the metallurgical and

material flow phenomena taking place during Al/Cu friction stir lap welding and,

consequently, on the final properties of the welds. Whereas the AA 5083/copper-DHP

welds presented excellent surface finishing, but highly defective nugget, without any

signs of base materials interaction, the AA 6082/copper-DHP welds displayed poor

surface properties, but strong base materials mixing at the nugget.

ACKNOWLEDGEMENTS

The authors are indebted to the Portuguese Foundation for the Science and Technology

(FCT) and European Regional Development Fund (ERDF) for the financial support, and to

company Thyssen Portugal – Aços e Serviços Lda for providing the heat treatments for the

friction stir welding tools.

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