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17 AluReport 01.201316 AluReport 01.2013
INNOVATION
Joining Innovations
DELTASPOT®
RESISTANCE SPOT WELDING OF HIGH-STRENGTH
AMAG TOPFORM® UHS SHEETS
High-strength
AlZnMg(Cu) alloys
of the 7xxx series
are well-established
materials for use in products for
aircraft and sports applications.
!e automotive industry has also
realized the lightweight potential
of this alloy class, which has a
tensile strength of up to 700 MPa,
and is evaluating whether to use it
in car bodies of all-aluminium de-
sign, of multi-material design, or
in light-metal doors and closures.
Having developed a high level of
expertise in high-strength and
super-high-strength aluminium
alloys over the decades, AMAG is
aiming to make a major contribu-
tion towards developing alloys and
processes to ensure that the 7xxx
family of alloys with its light-
weight potential is applied in the
automotive industry, too.
!e processes must be adjusted to
match the material conditions so
as to enable designers to use alloys
of this family without hesitation.
!e high strength of these alloys
goes hand in hand with lower
formability at room temperature
compared to 5xxx- or 6xxx-series
alloys. It is not possible to employ
conventional processes and com-
positions for surface treatment
of sheets for structural and skin
panel applications. Usually, it will
also be necessary to take measures
to prevent corrosion and to deve-
lop new solutions for joining.
AMAG is doing research on all
these issues jointly with partners
and has already reported on
results of the development work
and on practical approaches. For
example, warm forming of 7xxx
alloys at approximately 200 °C is
considered a production route
especially suited for formed parts
[1]. Starting from the super-high-
strength T6 temper, the sheet is
formed and largely retains its high
strength even after forming.
�is article is particularly
intended to address “metallur-
gical joining” by resistance spot
welding.
So far, alloys of the naturally hard
5xxx and heat-treatable 6xxx fami-
lies have been used for structural
components and hang-on parts in
car bodies. Until recently, primarily
laser welding, flow-drill screw-
ing, MIG welding, solid punch
and self-piercing riveting, and
adhesive bonding processes had
been used for series production in
an all-aluminium design. Adhesive
bonding is also preferably used in
multi-material concepts to achieve
electrical isolation, e.g. from steel.
Although attractive in terms of
manufacturing, the aluminium
spot welding process has posed
some challenges with respect
to process stability and quality,
relating to less resistance heating
as a result of the high conductivity
of aluminum alloys, and varying
surface conditions of the alumini-
um components as a function of
the oxide layer or surface treat-
ment. In particular, the tendency
of the copper electrodes to rapidly
pick up and stick in the areas of
contact with aluminium resulted
in a varying quality of the weld
nuggets and surface spatter, and it
was almost impossible to use spot
welding in large-scale production
in a cost-efficient manner. Mo-
reover, as is well known, there is a
high risk of cracking in medium-
strength and super-high-strength
aluminium alloys, which can only
be controlled by specially adjusted
electrode-force-versus-time curve
control and installing the related
equipment.
Now resistance spot welding has
joined the rank of large-scale
aluminium joining technologies
already mentioned: DeltaSpot® by
Fronius is used at Audi in the TT
Roadster and TT Coupé models, at
Hyundai in the Equus and Genesis
models, and at Porsche in the
Panamera model [2, 3]. Tesla Mo-
tors also uses the state-of-the-art
DeltaSpot® resistance spot welding
process in its Limousine Model S
to produce a large number of two-
and three-layered joints between
aluminium sheets, sections and
castings [4]. Electrical resistance
spot welding is used in conven-
tional steel shells for automotive
applications to a very large extent
and is characterized by level joints,
without the need for any joining
aids as are standard in metal-clinch
fastening or self-piercing riveting.
When using rivets or bolts in alu-
minium sheet-metal construction,
we must address issues such as gal-
vanic corrosion and recyclability,
which should be considered when
taking stock of the total cost.
All of the large-scale applications
mentioned above, however, relate
to established AlMg and AlMgSi
wrought alloys and AlSiMg die-
casting alloys [4].
!e companies AMAG and Froni-
us, in collaboration with the deve-
lopment partner Leichtmetallkom-
petenzzentrum Ranshofen (LKR),
are aiming to make the Deltaspot®
spot welding process applicable
to the super-high-strength alloys
of the AlZnMgCu family. !e-
se alloys are considered to have
limited weldability if conventional
fusion welding processes such as
MIG or TIG are used because the
large solidification interval and the
copper content typically lead to
high susceptibility to cracking and
substantial decrease of strength in
the weld.
DeltaSpot® Resistance Spot
Welding Process
!e most obvious feature to
overcome the typical problems
associated with resistance welding
is the spooling process tape used
in Fronius‘ Deltaspot® (Fig. 1),
which offers several advantages:
Every spot is welded with what
is effectively an unused electrode
because, contrary to conventional
processes, typical deposits from
component surfaces do not occur
on the electrode cap but on the
process tape that moves forward
after every spot weld, thus ensu-
ring exceptionally high reproduci-
bility and process reliability, with
virtually no surface spatter.
!e metal process tape can be va-
ried as to material, thickness and
coating to adjust the thermal input
in such a manner as to ensure that
the requirements with respect to
nugget formation and load-bea-
ring capacity of the joint are met
even if amperages are comparably
low or the components greatly
vary in thickness. One set of
INNOVATION
19 AluReport 01.201318 AluReport 01.2013
INNOVATION
electrodes and process tapes will
be enough to weld 5000 to 10000
spots without interruption, which
cannot be taken for granted when
conventional technologies are
used, neither for aluminium nor
for galvanized steel. Cap cutting of
electrodes is no longer necessary.
Additional features of the Del-
taSpot® system, which are also
particularly advantageous for
aluminium spot welding, include
the servo-electric main drive
that provides for soft touch, fast
follow-up and force control during
welding, thus preventing crack
and pore formation. Low-ripple
direct current ensures fast control
response and high energy input,
with the welding time remaining
the same – another plus towards
stable, compact welding process
windows.
Spot Weld Characteristics
Several series of tensile shear
specimens and peel test samples
were produced on clean uncoa-
ted flat test sheets of a thickness
of 2 mm using DeltaSpot® and
evaluated to determine the basic
characteristics of spot welds on
AMAG TopForm® UHS sheets of
the AA7075-T6 type. !e welding
parameters were varied until op-
timum results were obtained. !e
spot diameters achieved exceeded
the minimum values of 7.8 mm
as required for a wall thickness
of 2 mm and the tensile shearing
loads achieved in the static tensile
test were in a range from 7–9
kN. Directly comparable data
from literature are unknown; as a
rough estimate, the tensile shear
strengths achieved for brushed
medium-strength AA6082-T6
sheets of a thickness of 2 mm
are just above 10 kN when the
electrode caps are new, however,
with ultimate tensile shear loads
falling below 7 kN after only 700
spot welds [5]. DeltaSpot® delivers
a uniform spot weld quality over a
considerably longer tool life.
Additionally, dissimilar joints
between the AA7075 T6 sheets
of a thickness of 2 mm as used
before and AA6016-T4 sheets of
a thickness of 1.5 mm were produ-
ced in analog test series [Fig. 2],
from which we can draw conclu-
sions about aluminium combina-
tions in which AA7075 is used for
local reinforcement of customary
6000-series body sheets and flush
joints are required.
In addition to conducting ten-
sile shear tests and chisel tests,
microsections were prepared to
examine the criteria relating to
pores, shrinkage and cracks. No
limiting irregularities were found.
Investigations of the combina-
tion of an AA7075-T6 with an
AA6016-T4 alloy yielded spot
diameters in a range from 6.8 to
8.5 mm (required minimum: 6.75
mm) and tensile shear loads in a
range from 3.5 to approximately
6 kN, depending on the process
parameters, the preferred fracture
mode mostly being nugget pull-
out. !e welding range covered
is perfectly suitable for industrial
use [6].
AMAG TopForm UHS for-
mally corresponds to the
high-strength standard 7075
alloy and has been specially
optimized for warm forming of
high-strength components. It
has good formability at mode-
rate temperatures and a low
tendency to thermally overage.
Forming is performed in the
high-strength as-delivered
temper at a typical temperature
of about 200 °C and at forming
rates comparable to those of
conventional cold forming. Even
without any additional heat
treatment being performed after
forming, the specific strength
in the component ranks top
among all structural metals.
Hence, target applications
primarily include high-strength
components, e.g. to ensure the
structural integrity of survival
cells in automotive structures in
the event of a crash.
AMAG TopForm® UHS
BIBLIOGRAPHY:
[1] “Hochfeste Aluminiumbleche der 7xxx-Serie für den Automobil-Leichtbau” AluReport 03.2012
www.amag.at
[2] A. Kreuzwieser: Einsatz der Delta-Spot Technologie beim Audi TT, 3rd International Conference –
Fügetechnische Herausforderungen im modernen Automobilbau, Sattledt, Oct. 10–11, 2012
www.fronius.com
[3] “Innovation im Widerstands-Punktschweißen bewährt sich bei Anwendern” ALUMINIUM Vol. 87,
12-2011, Giesel Verlag
[4] D. Reckhorn: “Weld Challenges at the production of the Tesla Model S”, 3rd International Conference
- Fügetechnische Herausforderungen im modernen Automobilbau, Sattledt, Oct. 10–11, 2012
www.fronius.com
[5] L. Dorn: Resistance Welding, TALAT Lecture 4500, EAA - European Aluminium Association
www.alueurope.eu/talat (1994) p.19
[6] R. Gradinger, N. Sotirov, G. Rettenbacher, D. Uffelmann, C. Melzer, C. Pangerl, P. Dörner, S. Minichs-
hofer, A. Becirovic “Untersuchungen zur Schweißeignung von höherfesten AA-7075 Blechen mittels
DeltaSpot®-Widerstandspunktschweißverfahren”, Proceedings of the 7th Ranshofener Leichtmetalltage in
2012, www.lkr.at
Outlook
We are currently conducting addi-
tional studies on DeltaSpot® spot
welding of AA7075-T6 sheets to
be able to go into other aspects,
e.g., the influence of anti-corrosi-
on coatings on weldability and the
resulting corrosion behavior. We
will report on it in AluReport. Fig. 2: Transverse section of a spot-weld nug-
get with superimposed microhardness values
(top 1.5 mm AA6016, bottom 2 mm AA7075)
INNOVATION
Fig. 1: Process tape at DeltaSpot® resis-
tance spot welding process