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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 - [email protected], 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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