Analysis of weld defects in similar and dissimilar resistance seam welding

Analysis of weld defects in similar and dissimilar
resistance seam welding of aluminium, zinc and
galvanised steel
T. Mira-Aguiar, I. Galvão, C. Leitão and D. M. Rodrigues*
In the present investigation, a microstructural characterisation of welds performed by resistance seam
welding was carried out, with special focus on weld defect analysis. In order to perform a comparative
weldability analysis, the welds were performed using similar welding procedures. Similar welds in
aluminium (5754-H22) and zinc (Zintek) alloys, as well as dissimilar welds between galvanised steel
and zinc, were studied. The defective aluminium welds were found to be characterised by important
grain growth inside an inhomogeneous nugget and by the presence of important voids and cracks.
The zinc welds showed a well defined nugget, but with porosities and some cracks. In the dissimilar
steel–zinc welds, important macroscopic defects were observed. Microstructural analysis evidenced
the occurrence of melting at the zinc side of the welds; meanwhile, no microstructural modifications
could be observed for the steel side. Defect formation, as well as weld morphologies, was related to
the variation in welding parameters.
Keywords: Resistance seam welding, Microstructural analyses, Liquation cracking, Solidification cracking, Porosities
Introduction
Resistance seam welding (RSEW) is a joining process
very similar to resistance spot welding (RSW), but
enabling the fabrication of continuous lap welding
between long plates. In fact, in RSEW, the welding
electrodes are motor driven wheels rather than stationary bars.1 Despite this, the main operative principles of
the RSEW process are very similar to that of spot
welding, i.e. the welding results from the transmission of
electric current through the plates to be welded, which
are overlapped between two electrodes. The electric
current promotes heat generation, by Joule’s effect, and
localised melting of the base materials. The weld results
from the solidification, under the pressure exerted by the
electrodes, of the overlapped contacting surfaces.2 As in
any fusion welding process, some weld defects may be
formed during both RSEW and RSW, which will
influence the morphological and mechanical properties
of the welded joints. Thickness reduction in the joining
zone,3 voids,4 porosities5 and cracks6 are the most
usually reported defects.
The welds produced by RSEW always display a
thickness decrease, relative to the base materials, due to
the pressure exerted by the electrodes over it. The higher
the pressure applied, the greater the thickness reduction.
Although the electrodes pressure is pointed as the most
common cause for this defect, other variables can also
have an indirect influence on it, such as the nature of the
CEMUC, Department of Mechanical Engineering, University of Coimbra,
Rua Luiı́s Reis Santos, Coimbra 3030-788, Portugal
*Corresponding author, email [email protected]
Ñ 2015 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 14 January 2015; accepted 29 March 2015
DOI 10.1179/1362171815Y.0000000031
base materials and the heat generated. Actually, the
thickness decrease can also be related to base materials
shrinkage upon solidification. Under similar welding
conditions (electrodes pressure and diameter), the softer
the base material, the higher the thickness decrease.
In the same way, the higher the heat input, the higher the
thickness decrease due to softening. If the heat input is
excessive, it can also lead to material projections out of
the weld joint, which, in addition to the decrease in the
weld thickness, also promotes poor surface finishing.7
Other typical resistance seam weld defects are voids,
porosities and cracks located in the fusion zone (FZ)
and/or heat affected zone (HAZ). The formation of
voids and porosities can be related to three factors:
projections of material, shrinkage and gaseous
inclusions.8 Material projections, already referred
above, are caused by an inadequate choice of electrodes
and/or welding parameters, and can occur either at the
electrode/plate contact interface or at the plate/plate
interface. In the former case, surface quality and
electrodes’ life can be affected, and in the second case,
the mechanical strength of the welds is adversely
affected. Since the electric current needed is lower in
steel welding than in aluminium welding, projections are
more likely to occur in aluminium welding.9
The gaseous inclusions are gas bubbles trapped inside
the molten material, which, after solidification, give rise
to porosities inside the FZ. The gaseous inclusions may
result from hydrogen inclusions, volatilisation of alloying elements or other sources. The hydrogen inclusions
are a common cause of porosities in welds, whenever this
element is highly soluble in the base material. Since the
hydrogen is present in the atmosphere and also in
the sheets and electrodes, when they are wet, special care
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should be taken in order to avoid hydrogen inclusions.10
The gaseous inclusions due to the volatilisation of
elements, on the other hand, are restricted to the base
materials that contain volatile alloy elements. During
welding, due to the high heat input in the FZ, these
elements may evaporate, leading to the appearance of
gaseous inclusions. Other sources of gas inclusions can
be the burning of lubricants and organic elements when
the plate surfaces are not properly cleaned. Finally, the
porosities associated with solidification shrinkage are
located in the zones of the weld that are the last to
solidify. These shrinkage porosities are smaller than that
resulting from gaseous inclusions. However, these two
phenomena may occur simultaneously combining each
other and giving rise to bigger pores.7,11
The presence of cracks in resistance seam welds results
from some well known weld cracking mechanisms12 such
as hot cracking. Hot cracking may occur in two different
ways: solidification and liquation. While the first one
occurs in the FZ, the second one is characteristic of the
HAZ.13 As the name indicates, solidification cracking is
due to base material shrinkage during the liquid to solid
transition. Hot cracking in the HAZ, which occurs
during heating, also results from the segregation of low
melting point phases to the grain boundaries and
thermal shrinkage stresses.8,14
In the literature, it is possible to find a large number of
works addressing similar RSW of a large range of base
materials such as aluminium,15 magnesium,16 steel17 and
titanium,18 and dissimilar such as steel/steel,19 steel/Al,20
Al/Mg21 and steel/Mg.22 However, in which concerns to
RSEW, the information available is much scarcer. In the
present work, the main defects in similar welds of
aluminium (5754-H22) and zinc (Zintek) alloys, and
dissimilar galvanised steel–zinc welds, produced by
RSEW, are identified and its formation correlated with
base material characteristics and welding parameters.
Experimental
Similar welds between 1 mm thick plates of aluminium
(5754-H22) and zinc (Zintek) alloys, as well as dissimilar
welds between 0.85 mm galvanised steel plates and 1 mm
thick zinc plates, were performed in the current work.
The base materials’ nominal chemical compositions are
shown in Table 1. The welding parameters used are
shown in Table 2. As illustrated in Fig. 1, the upslope,
Weld defects in RSEW of Al, Zn and galvanised steel
downslope and maintenance parameters shown in
Table 2 define the periodicity of the current, as well as
the heating and cooling rates in each current cycle.
Transverse and longitudinal sections of the welds were
prepared according to standard metallographic practice
and etched with modified Poulton’s (aluminium), zinc
etchant (zinc) and nital (galvanised steel) in order to
enable the identification of the different weld zones. The
reagent compositions are shown in Table 3. Metallographic analysis was performed using an optical microscope Leica DM 4000 M LED. Scanning electron
microscopy/energy dispersive X-ray spectroscopy
(SEM/EDS) was performed in the cross-section of one of
the welds using a Philips XL30 SE.
Figure 2 schematises the welding current pulses for the
three base metals. In Fig. 2a, where the aluminium
parameters are shown, it can be seen that the Al2 and
Al3 welds were performed with the same power, as
opposed to the Al1 weld, which was produced using
higher power values. However, the figure also shows that
the Al1 weld was performed using longer current pulsation periods than those used in Al3 and Al2 welding,
resulting in a lower heat input frequency. Figure 2b
illustrates the welding parameters for the similar Zn and
the dissimilar FeZn welds. Through the figure, it can be
inferred that the welding parameters used were similar,
being only used a slightly lower power value in the FeZn
welding. Moreover, comparing these two welds with the
Al2 weld, it can be concluded that the only difference is
the fact that the latest joint was performed with a higher
power value. In any case, the power used was adapted in
order to obtain welds without any macroscopic
discontinuity.
Results and discussion
Microstructural analysis of similar aluminium
welds
Results of the longitudinal cross-section analysis of the
Al1 weld are illustrated in Fig. 3. Specifically, a macrograph of the whole section is displayed in Fig. 3a, and
magnified pictures of the lap interface are shown in
Fig. 3b and c. From Fig. 3a, it can be observed that the
longitudinal section of this weld has an unsymmetrical
morphology, i.e. one of the weld surfaces is undulated,
whereas the other is smooth. The surface undulations
seem to be periodic, which can be associated with the
Table 1 Chemical compositions of base materials
Material
Si
Fe
Cu
Mn
Mg
Zn
Ti
Cr
Al
Aluminium
Zinc
Galvanised Steel
0.4
...
0.015
0.4
...
Rest
0.1
0.080, 1
...
0.5
...
0.5
2.6, 3.6
...
...
0.2
Rest
...
0.15
0.060, 0.20
0.001
0.3
...
...
Rest
0 0,015
...
C
P
...
...
0.11
...
...
0.017
Table 2 Welding parameters
Material
Sample (number)
Upslope/s
Downslope/s
Maintenance time/s
Power/kVA
Aluminium
Al1
Al2
Al3
Zn
FeZn
8
0
1
0
0
3.5
3.2
3.5
3.5
3.5
0.1
0.1
1.5
0.1
0.1
10.0
8.5
8.5
7.0
6.5
Zinc
Steel–zinc
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1 Scheme of welding parameters
Table 3 Etchant compositions: modified Poulton’s, zinc
etchant and nital
Etchant
Composition
Poulton’s
12 mL HCl; 6 mL HNO3;
1 mL HF; 1 mL H2O
50 mL Poulton’s etchant;
25 mL HNO3; 25 mL H2O;
12 g chromic acid
10 g NaOH; 100 mL H2O
49 mL ethyl alcohol; 1 mL HNO3
Modified Poulton’s
Zinc etchant
Nital
periodicity of the current shown in Fig. 2. The discontinuous black line at the middle of the joint (Fig. 3b
and c), suggesting non-continuous joining across the
weld, can also be associated with the periodicity of the
current, i.e. when a current peak was reached, the heat
was sufficient to promote the joining. No joining
occurred during the up- and downslope of the current.
It should also be noted that the appearance of surface
undulations exclusively in one of the weld sides may be
related to differences in electrodes surface quality, i.e.
one of the electrodes had higher wear and/or soiling than
the other.
A transverse cross-section macrograph of the Al1
weld (Fig. 4a) as well as micrographs of specific crosssection zones, namely, base metal (Fig. 4b), HAZ
(Fig. 4c and e) and FZ (Fig. 4d), are now shown in
Fig. 4. Comparing Fig. 4b and c, it can be concluded
that grain growth relative to the base material occurred
in the HAZ. Moreover, some evidence of melting is
Weld defects in RSEW of Al, Zn and galvanised steel
discernible in the FZ micrograph shown in Fig. 4d.
Actually, although well defined dendritic structures are
not clearly observed, the extensive porosity and cracking
noticed in this zone points to melting during welding.
However, as illustrated in the picture, the solidification
indicative structures are restricted to a very small region
of the FZ, i.e. the welded materials interface, suggesting
a very localised volume of aluminium being melted
during welding. The analysis of the transverse crosssection also reveals the presence of cracks following the
contours of HAZ grain boundaries (Fig. 4e). This crack
morphology in the HAZ is characteristic of liquation
cracking mechanisms. Liquation cracking results from
the segregation to the grain boundaries of low melting
point elements/compounds, being very sensitive to base
alloy chemistry.13,14,23–25 Actually, the effect of Si, Cu,
Mg and Mg2Si content on aluminium alloys hot cracking sensitivity is well documented in the literature
through crack sensitivity curves,12,13 which show that
the susceptibility to hot cracking increases, with the
increase in alloys content, until a maximum is reached,
and then falls down to relatively low levels. In the case of
Al–Mg binary26 system (5xxx series of alloys), the hot
cracking sensitivity reaches the maximum for an Mg
content of 1.2%, becoming very small for Mg contents
higher than 5%. According to Table 1, the Mg content of
the AA 5754 varies between 2.6 and 3.6%, which is still
inside the high cracking sensitivity range, explaining the
occurrence of the liquation cracking during RSEW.
Liquation cracking can be avoided through a careful
selection of the alloy composition and/or process parameters. The faster the welding speed, the faster the
cooling rate and the less time the weld will be in the hot
cracking temperature range.
It is also important to stress that the thickness
reduction in the nugget of the Al1 welds did not exceed
8% of both plates thickness.
Figure 5 illustrates the longitudinal cross-section of
the Al2 weld as well as two transverse cross-sections
macrographs (Fig. 5b and c) corresponding to two
distinct sections in Fig. 5a. Micrographs of specific
transverse cross-section regions (Fig. 5d and e) are also
shown in the figure. As opposed to that registered for the
Al1 weld, macrocavities were found to be present in the
nugget of the Al2 weld, as a result of material
a similar welding of aluminium; b similar welding of zinc and dissimilar welding of galvanised steel–zinc
2 Scheme of the welding current pulses
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Weld defects in RSEW of Al, Zn and galvanised steel
a longitudinal section; b welded zone; c non-welded zone
3 Microstructural analysis of Al1 weld
a transversal section; b BM; c grain growth in HAZ; d pores and cracking in FZ; e liquation cracking in HAZ
4 Al1 weld microstructure
projections, being periodically distributed along its
longitudinal section, as shown in Fig. 5a. Like the
periodicity of the Al1 weld surface undulations, the
periodic distribution of Al2 weld cavities may also be
related to the welding current periodicity, i.e. each
section of cavities corresponds to a current peak. Furthermore, it should be noted that, similar to the previous
phenomenon, cavities formation was also found to occur
exclusively in one of the weld sides (Fig. 5a), which, as
referred above, results from differences in electrodes
surface quality.
Important differences can be noticed by comparing
the transverse cross-section macrographs of the Al2 weld
(Fig. 5b and c). Actually, while no solidification
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Weld defects in RSEW of Al, Zn and galvanised steel
a longitudinal section; b, c transverse sections at locations ‘non-fusion section’ and ‘fusion section’ in a respectively;
d magnification of grain growth in b; e magnification of dendrites in c
5 Al2 weld microstructure
structures were observed in the section illustrated in
Fig. 5b, being only possible to notice important grain
growth as well as a discontinuous dark line coincident
with the initial interface of both plates (Fig. 5d ), dendritic structures were identified in the cross-section
shown in Fig. 5c. These structures, as well as the presence of some scattered solidification defects (porosities),
are clearly discernible in the micrograph illustrated in
Fig. 5e. However, it should be noted that the dendritic
structures were found to be present only in restricted
areas of the cross-section. Actually, as illustrated in
Fig. 5c, a heterogeneous microstructure composed of FZ
and HAZ characteristic structures was observed
throughout the entire thickness of the transverse section.
Unlike the FZ, the HAZ is characterised by a coarse
grain structure resulting from the high temperatures
experienced during the process.
Longitudinal and transverse cross-sections macrographs of the Al3 weld are illustrated in Fig. 6a and b
respectively. Micrographs registered in specific regions
of the transverse section (Fig. 6c–e) are also shown in the
figure. From Fig. 6a, it is possible to observe large
number of cavities and porosities almost homogeneously
distributed over the longitudinal section of the weld.
In the same way, in the transverse cross-section shown in
Fig. 6b, melting indicative microstructures can also be
seen throughout the nugget, enabling to conclude that
some of the larger cavities may result from material
projections. On the other hand, the smaller pores visible
in the central zone may be associated with material
shrinkage upon solidification, to magnesium and zinc
gaseous inclusions (volatile elements) and to the high
hydrogen solubility of the aluminium alloy.11,27
Besides the cavities and pores, this weld also displayed
solidification cracks in the molten material areas
(Fig. 6e) and important grain growth in the HAZ, which
can be noticed by comparing Fig. 6c and d. Similar to
that registered for the Al1 weld, liquation cracking was
found to occur in some regions of the HAZ (Fig. 6e).
Important thickness reduction after welding can be
noticed by comparing the thickness of the transverse
cross-sections of samples Al2 (Fig. 5b and c) and Al3
(Fig. 6b) with the initial thickness of base material
plates. However, unlike in the Al1 weld, the thickness
reduction noticed in the Al2 and Al3 welds cannot be
exclusively associated with the pressure exerted by the
electrodes. Actually, it can also be related to the occurrence of material projections, which conducted to the
formation of very irregular weld surfaces and non-uniform thickness reduction across the weld length.
Comparing these three aluminium welds, it can be
concluded that the Al1 weld was performed under a
lower heat input condition than Al2 and Al3 welds.
According to the welding parameters analysis, previously described in the experimental procedure, this is
due to the longer current pulsation period used in Al1
welding. The use of low heat input conditions enabled to
reduce the formation of defects such as pores, voids and
solidification cracking, but was not sufficient to avoid
liquation cracking in the HAZ. The joining between the
plates was also not continuous.
Microstructural analysis of similar zinc welds
Longitudinal (Fig. 7a) and transverse (Fig. 7b) crosssections macrographs of the Zn weld are displayed in
Fig. 7. Micrographs registered in specific regions of the
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Weld defects in RSEW of Al, Zn and galvanised steel
a longitudinal section; b transversal section; c BM; d HAZ; e liquation and solidification cracking
6 Al3 weld microstructure
transverse section (Fig. 7c–h) are also illustrated in the
figure. The longitudinal section macrograph shows a
homogeneous weld morphology, all along the weld
length, as well as a small overlap between successive
weld nuggets, which is naturally related to the periodicity of the current (Fig. 7a). Unlike to that observed for
the aluminium welds, no remnants of the plates interface
can be observed in this picture, which proves that there
was continuous melting all across the interface.
The three characteristic weld zones (BM, HAZ and
FZ) can be easily distinguished in the transverse crosssection of the Zn weld (Fig. 7b). From the BM micrograph shown in Fig. 7c, it is possible to conclude that the
BM consisted of elongated grains, aligned with the
rolling direction. Important grain growth in the HAZ
can be noticed by comparing BM and HAZ microstructures in Fig. 7c and d. However, some cracks can
also be observed in the HAZ (Fig. 7e), which can be
associated with liquation cracking mechanisms, i.e. with
the segregation of low melting point elements or
compounds to the grain boundaries. In turn, the FZ,
which corresponds to the whole nugget area, can be seen
at the centre of Fig. 7b. The microstructure illustrated in
Fig. 7f corresponds to the outer part of the nugget,
showing the formation of columnar grains, aligned with
the cooling direction of the weld. The dimension and
distribution of the columnar grains around the nugget
are uniform, enabling to conclude that heat dissipation
during weld cooling was uniform. Also in the FZ,
dendrites (Fig. 7g) and equiaxial grains can be observed,
the latter ones being found at the nugget/HAZ interface.
As illustrated in Fig. 7h, the centre of the nugget contains several pores, which, due to their small size and
localisation, can be associated with material shrinkage
upon solidification. The formation of columnar grains is
naturally associated to the steep thermal gradient at the
weld/base material boundary. However, the cooling rate,
which is very high near the periphery of the nugget,
decreases towards the centre of the weld, where it
reaches minimum values. Formation of equiaxed grains
takes place in this zone of the weld where the thermal
gradients become insignificant.7,28
Depending on the application to which this material
will be subjected, a greater overlap between successive
weld nuggets may be required, in order to confer a better
mechanical resistance to the weld. Additionally, the
formation of shrinkage porosities can be reduced
through a controlled increase in electrodes pressure.
On the other hand, hot cracking can only be avoided
through a careful control of alloy composition.13
Microstructural analysis of dissimilar steel – zinc
welds
Figure 8 illustrates a longitudinal cross-section, before
etching, and a transverse cross-section, after etching, of
the FeZn weld. In the longitudinal cross-section of
Fig. 8a, the interface of both base materials can be
clearly observed, with a high density of large cavities and
pores distributed along it. However, these defects are not
visible in the transverse cross-section of Fig. 8b, where a
strong thickness reduction in the zinc side of the weld
can be observed, resulting from the sticking of this
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Weld defects in RSEW of Al, Zn and galvanised steel
a longitudinal section; b transversal section; c BM; d grain growth in HAZ; e liquation cracking in HAZ; f columnar grains;
g dendrites; h pores
7 Zn weld microstructure
material to the electrode. The SEM detail of the transverse cross-section macrograph shown in Fig. 8c clearly
illustrates the formation of microstructural solidification
features only at the zinc side of the weld nugget. Effectively, no microstructural changes, relative to the initial
base material structure, were registered in the steel side,
as can be observed in Fig. 8d and e.
Comparing to the similar zinc welds, the welding
parameters used to perform the dissimilar welds only differ
in the power used, which was lower (Fig. 2b). However,
according to Liu et al.,29 the differences in electrical resistivity between the steel and the zinc base materials had a
strong influence on heat distribution inside the weld area.
Owing to the higher resistivity of the steel, the heat
generation by Joule effect was higher at the steel side of the
weld. In this way, comparing to what happens in similar
zinc welding, it can be inferred that the steel side of the
dissimilar FeZn joint worked as an additional heat source
to the zinc, which, as referred above, was melted during
welding. Actually, as the melting temperature of the steel is
much higher than that of the zinc alloy,30,31 fusion exclusively occurred at the zinc side of the weld. Moreover, Liu
et al.,29 in a study on dissimilar RSW of steel and magnesium, concluded that the welding resulted from the
solidification of the zinc coating of the steel plates, which
was found to melt at the sheets’ interface. Since in current
study one of the welded materials was zinc, joining by a
similar mechanism can be assumed.
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Weld defects in RSEW of Al, Zn and galvanised steel
a longitudinal section before etching; b transversal section after etching; c SEM image of transversal section; d, e magnifications
of galvanised steel microstructure
8 FeZn weld microstructure
Conclusions
In the current work, similar welds in zinc and aluminium
and a dissimilar weld in steel–zinc were analysed.
The microscopic inspection of the welds revealed the
presence of voids, porosities and cracks, for all the
similar welds, independently of the base material.
Meanwhile, the presence of large voids was associated
with the projection of material from the weld seam
during joining, the presence of porosities was associated
with gas inclusions, alloy element volatilisation and
solidification shrinkage. Solidification and liquation
cracking mechanisms were also found to be responsible
for large crack formation inside the FZ and HAZ respectively. For the aluminium alloy, a close relation
between weld current pulsation and weld morphology
was found, i.e. longer current pulsation periods enabled
to reduce the incidence of high heat input related defects,
such as material projections and solidification cracking.
For the dissimilar steel–zinc welds a large influence of
the asymmetry in heat generation and dissipation on
weld morphology was found. Meanwhile, no fusion, or
even microstructural changes, was reported for the steel
side of the weld; the occurrence of fusion and voids was
reported in the zinc side. Actually, large quantities of
zinc were removed from the weld seam due to excessive
heat input from the steel side of the joint to the zinc side.
Acknowledgements
This research work is sponsored by national funds from the
Portuguese Foundation for Science and Technology (FCT)
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via project no. PEst-C/EME/UI0285/2013 and by FEDER
funds through the program COMPETE – Programa
Operacional Factores de Com-petitividade, under project
no.
CENTRO-07-0224-FEDER-002001(MT4MOBI).
One of the authors, C. Leitão, is supported by the Portuguese Foundation for Science and Technology through
SFRH/BPD/93685/2013 fellowship. All supports are
gratefully acknowledged.
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