On the Prediction of Hot Tearing in Al-to-Steel
Welding by Friction Melt Bonding
N. JIMENEZ-MENA, P.J. JACQUES, J.M. DREZET, and A. SIMAR
Aluminum alloy AA6061 was welded to dual-phase steel 980 (DP980) by the friction melt
bonding (FMB) process. Hot tears have been suppressed by controlling the thermomechanical
cycle. In particular, the welding speed and the thermal conductivity of the backing plate have
been optimized. A ﬁnite-element thermomechanical model coupled with the Rappaz–
Drezet–Gremaud (RDG) criterion has been used to explain these experimental observations.
The hot tear susceptibility has been reduced with large thermal gradients and with the formation
of a cellular microstructure. Both eﬀects are favored by a backing plate made of a material with
high thermal conductivity, such as copper.
Ó The Minerals, Metals & Materials Society and ASM International 2018
tight regulations regarding the greenhouse gas
emissions generated by the transportation convinced the
industry to reduce the weight of vehicles’ structures. An
eﬃcient combination of the properties of aluminum and
advanced high-strength steels (AHSS) is regarded as a
promising solution to reduce the weight of cars.
However, the dissimilar welding of aluminum and
steel is challenging due to the metallurgical incompat-
ibility of the two materials.
At ﬁrst, the challenges are
to treat the large diﬀerences in melting temperatures and
thermal expansion coeﬃcients of both materials. In
addition, the formation of a reacting brittle intermetallic
layer (IML) results in poor mechanical properties of the
Most of the joining techniques lead to the
formation of this brittle intermetallic layer (IML). This
is the case for most welding techniques such as friction
According to Tanaka et al.,
ness of the IML determines the toughness of the joint.
There is a signiﬁcant increase in fracture toughness for
IML thicknesses below 1 lm.
Friction melt bonding (FMB) has recently been
developed to join sheets of dissimilar materials in a
This process is adapted to
weld materials showing large diﬀerences in melting
temperature (i.e., aluminum and steel). In this process,
sketched in Figure 1(a), the steel plate is heated up by a
rotating cylindrical tool pressed against its upper sur-
face. The generated heat is not large enough to melt
steel, but it locally melts the aluminum in contact with
its bottom surface. The tight contact between the molten
aluminum and the steel surface leads to some reactivity
and the formation of the IML. No protective atmo-
sphere is needed since the molten Al is conﬁned within
the assembly and is not in contact with the atmosphere.
In previous studies, van der Rest et al.
observed hot tears in the re-solidiﬁed aluminum
after FMB. Such hot tears were located in the molten
pool close to the aluminum–steel interface. This location
corresponds to the last re-solidiﬁed aluminum. van der
Rest et al.
highlighted the inﬂuence of the aluminum
composition on the formation of hot tears when welded
to ultra-low-carbon (ULC) steel. They observed that the
commercially pure aluminum alloy AA1050 was free of
hot tears, while age-hardenable AA2024 led to hot
tearing. Cruciﬁx et al.
considered the thermal cycles
when welding AA2024 to ULC steel. They observed that
the number of hot tears increased as the welding speed
increased. They suggested that the size of the molten
pool might be used as a criterion to predict the
formation of hot tears. They concluded that a larger
molten pool may reduce the hot tear formation.
The Rappaz–Drezet–Gremaud (RDG) criterion
provides a physically based explanation of the hot
tearing phenomenon. It states that hot tears nucleate in
the intergranular liquid during solidiﬁcation. This
occurs in a cavitation-like process if a critical drop of
pressure is reached. They identiﬁed both the permeabil-
ity of the ‘‘solid plus liquid’’ mixture and the thermo-
mechanical cycle during solidiﬁcation to be responsible
for this drop of pressure. With this as criterion, it is
possible to assess the inﬂuence of alloy composition and
N. JIMENEZ-MENA, P.J. JACQUES, and A. SIMAR are with
the iMMC-IMAP, Universite
catholique de Louvain, 1348, Louvain-
la-Neuve, Belgium. Contact e-mail: firstname.lastname@example.org J.M.
DREZET is with the E
cole polytechnique fe
rale de Lausanne, IMX,
1015, Lausanne, Switzerland.
Manuscript submitted July 19, 2017.
Article published online April 13, 2018
2692—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A