Steel and aluminium were long thought impossible to join
together permanently by thermal means. Here Gerd
Trommer explains how Fronius partnered with steelmaker
Voestalpine to develop a sheet made of both steel and
aluminium, joined by Fronius’s thermal joining arc process,
CMT (Cold Metal Transfer)

When comparing steel and
aluminium as structural
materials, weight, strength,
rigidity and workability should be taken into
account as well as resource availability and
costs. Particularly in lightweight
construction, it is often desirable to select an
optimum combination of both materials:
solid steel for heavy-duty constructional
zones, combined with lightweight
aluminium for less highly stressed areas.

Until recently arc-joining was not feasible
for such joins, despite its many technical and
economic merits.

Lightweight constructions deliver benefits
in automotive manufacturing. Bodywork
accounts for around 40% of a vehicle’s
weight. If the transport sector’s CO2
emissions are to be reduced, weight savings
in auto body construction are a place to
start. Reducing weight by 100kg can result
in 700g/100km fewer CO2 emissions.

Materials-joining technology plays a key
role in lightweight construction, particularly
when joining steel to aluminium. Fronius
partnered with Austrian steelmaker
Voestalpine to address this challenge,
developing a high-formability hybrid sheet
made of both steel and aluminium, joined by
Fronius’ patented thermal joining arc process
CMT (Cold Metal Transfer).

The difficulty in joining steel and
aluminium together permanently by thermal
means stems from their widely different
melting points and, in particular, the
formation of intermetallic phases (IMP).The
greater the amount of heat inputted into the
seam, the thicker the IMP and the poorer the
mechanical and engineering properties of the
joint. Further challenges result from the
widely differing thermal expansion
coefficients and the abrupt change in the
electrochemical potential. In the light of
these physical factors, metallurgists and
welding experts have defined the following
prerequisites for joining steel to aluminium:
The process should generate the lowest
possible thermal input.

The surface of the steel sheet must be
coated with a zinc layer.

The thickness of the IMP must be
minimised.

Joining process
Three main factors that influence the formation
of a flawless join are the two work pieces to be
joined, the joining process and the filler metal.

The aluminium sheets are made from
materials of the AW5xxx or 6xxx series. The
special gas-shielded arc process CMT fulfils
the crucial requirements of the joiningprocess:
low thermal input and good
controllability. When steel is joined to
aluminium, the filler metal and the
aluminium wet the galvanised steel sheet and
the filler metal fuses with the aluminium. On
the steel side, a brazed join is obtained,
which the aluminium is then welded against.

For the hybrid sheets, it has been found best
to use welding systems which braze-weld ‘in
sync’ with the CMT process on both sides.

The third main factor is the filler metal.

Fronius has a patent for the special
aluminium alloy used here in braze-welding.

Steel-aluminium hybrid sheet
To explore the product’s suitability for actual
practice, both partner companies and external
institutions performed extensive testing on
braze-welded steel-aluminium hybrid sheets
using the materials AW5182-H111 and DX54D,
and filler metal Z200 made of AlSi3Mn1.

The following tests were carried out:
Tensile strength, corrosion test, deformability
behaviour and crash-test.

A key result of the tensile test was that
final rupture of the sample occurs in the
aluminium sheet, with the braze-welded
joint surviving the tests intact. The steelaluminium
hybrid sheet also survived the
salt spray test, with neither crevice/stress
corrosion nor intercrystalline corrosion
occurring after 300h exposure. The only
slight traces of corrosion were on the surface.

The deformability behaviour of the sheets
plays a critical role in autobody construction.

The steel-aluminium hybrid sheets were
proven suitable for reproducible sheet
metalworking.

When they are designed from the outset
as safety-critical components, hybrid sheets
can systematically absorb accident impact
energy. On the hybrid impact absorber, the
energy absorption takes place almost entirely
in the aluminium part of the component.

Both the braze-welded seam and the steel
part of the component are undamaged. In
contrast to this, a steel-only component
absorbing the same amount of energy
sustains more serious damage.

Fronius and Voestalpine have developed a
volume-production-ready fabrication process
for steel-aluminium hybrid sheets. Extensive
testing has proven their practical suitability
and additional construction and application
benefits in certain cases.