Fachzeitschrift_OeGS_03_04_2019

Mechanical properties and microstructural study of homogeneous
and heterogeneous laser welds in α, β, and
α + β titanium alloys
• L. Weiss, J. Zollinger, P. Sallamand, E. Cicala, A. Mathieu,
E. Fleury
Heterogeneous welding has been investigated for three different
couples of titanium alloys: α/α + β, α/β, and α + β/β.
Plates of 100 × 60 mm and 1.6 or 1.8 mm thick were welded
with a Yb:YAG laser. Tensile tests show that the resistance
of the heterogeneous welded specimens was generally
controlled by those of the weakest material except for
the α + β/β where the ultimate tensile strength was approximately
equal to the average value of both materials. In
every case, the elongation of the welded sample was found
to be smaller than that of the base metals. The rupture generally
took place outside the weld metal and was found to
be most of the time located in the alloy having the lowest
mechanical properties. Nevertheless, a few large-size porosities
detected by tomography in the α + β/β couple could
explain why rupture for these samples occurred in the weld
bead. For each couple, the porosities were situated at the
board between the heat-affected zone and the molten
zone. EBSD maps and EDX enabled the observation of different
microstructures, which could be correlated to the
heterogeneous composition and diffusion into the melted
bath. When the stable microstructure of one of the couple
alloys is the β phase, the molten zone of the bead consists
of an alternative disposition of thin layers made of large
equiaxed β grains and nano-martensite α′. That is explained
by the weak diffusion of the alloying elements.
Liquid interlayer formation during friction stir spot welding
of aluminum/copper
• A. Regensburg, F. Petzoldt, T. Benss. J.P. Bergmann
The fabrication of dissimilar aluminum/copper joints for
electrical application raises the challenges for conventional
joining technologies. Within the solid-state processes, friction
stir welding (FSW) provides numerous advantages to
realize different joint configurations, especially by minimizing
the heat input and hence the formation of brittle intermetallic
phases. However, the joints also have to provide a
high contact interface with firm bonding in order to provide
a minimal contact resistance. Therefore, joints of 1 mm EN-
CW004A and EN AW1050A with a controlled melt layer formation
were produced by friction stir spot welding (FSSW).
By using a pinless tool and the positioning of copper as the
upper joining partner, local melt formation at the interface
with a eutectic composition was promoted without significant
intermixing, resulting in wetting of the aluminum and
a contact area increase. The rotational speed was varied
between 1800–2400 rpm, in which range samples with up
to 300-μm-thick melt layers were produced. The wetting
effect at the interface shows a positive influence on the
shear strength with ductile failure behavior even at high
layer thickness. The microstructural composition at the interface
showed a eutectic composition for small layer thickness
and an inhomogeneous composition with hypo- and
hypereutectic solidification structures for higher thickness
values. However, the formation of intermetallic compounds
other than CuAl2 was mostly inhibited by the short process
times and high cooling rate.
Welding design methodology for optimization of phase
balance in duplex stainless steels during autogenous
arc welding under Ar–N 2
atmosphere
• A. Rokanopoulou, P. Skarvelis, G. D. Papadimitriou
This study deals with the selection of appropriate welding
parameters during autogenous arc welding of duplex stainless
steels in order to achieve an optimum phase balance of
austenite and ferrite in the as-welded microstructure. Specimens
of duplex stainless steel 2205 with dimensions
(40 × 40 × 10) mm 3 were welded using autogenous arc welding
under 95% Ar + 5% vol. N 2
atmosphere. The weld pool
temperature was measured by non-contact infrared temperature
measurement, the weld bead dimensions were determined
using scanning electron micrographs, and the final
nitrogen concentration was evaluated by optical emission
spectroscopy. The kinetics of nitrogen absorption and desorption
in molten duplex stainless steel was discussed and
all the relevant variables were presented. The effect of welding
current and speed on the final nitrogen concentration
was also discussed. Finally, based on this analysis, a method
was set up which can be used to optimize the phase balance
by using predictive methods of the Ferrite Number, such as
the Welding Research Council (WRC)-92 diagram.
Numerical investigations on the thermal efficiency in
laser-assisted plasma arc welding
• S. Jäckel, M. Trautmann, M. Hertel, U. Füssel, D. Hipp,
A. Mahrle, E. Beyer
Numerical investigations on the thermal efficiency in laserassisted
plasma arc welding (LAPAW) have been carried out
by the combination of a magneto-hydrodynamic (MHD) arc
model and a smoothed-particle-hydrodynamics (SPH) model
of the weld pool. The comparison of the calculated weld
seam cross-sections gained from numerical simulation as
well as experimental examinations shows a good agreement.
By the use of the weld pool model, the sensitivity of
different influencing variables was investigated. The analysis
clearly reveals the major influence of the central heat flux
density on the penetration profile and on the thermal efficiency
of the process. The higher the heat flux of the laser
beam and the higher the constriction of the heat flux profile
of the arc, the higher the thermal efficiency of the process.
SCHWEISS- und PRÜFTECHNIK 03-04/2019 67