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