Fachzeitschrift_OeGS_09_10_2019

Abstracts aus „Welding in the World“ No. 4/2019
mit freundlicher Genehmigung des IIW
Wire-arc additive manufacturing of a duplex stainless
steel: thermal cycle analysis and microstructure
characterization
• V. A. Hosseini, M. Högström, K. Hurtig, M. Asuncion,
V. Bermejo, L.-E. Stridh, L. Karlsson
The evolution of microstructures with thermal cycles was
studied for wire-arc additive manufacturing of duplex stainless
steel blocks. To produce samples, arc energy of 0.5 kJ/
mm and interlayer temperature of 150 °C were used as low
heat input–low interlayer temperature (LHLT) and arc energy
of 0.8 kJ/mm and interlayer temperature of 250 °C as
high heat input–high interlayer temperature (HHHT). Thermal
cycles were recorded with different thermocouples
attached to the substrate as well as the built layers. The microstructure
was analyzed using optical and scanning electron
microscopy. The results showed that a similar geometry
was produced with 14 layers—4 beads in each layer—
for LHLT and 15 layers—3 beads in each layer—for HHHT.
Although the number of reheating cycles was higher for
LHLT, each layer was reheated for a shorter time at temperatures
above 600 °C, compared with HHHT. A higher austenite
fraction (+ 8%) was achieved for as-deposited LHLT
beads, which experienced faster cooling between 1200 and
800 °C. The austenite fraction of the bulk of additively
manufactured samples, reheated several times, was quite
similar for LHLT and HHHT samples. A higher fraction of secondary
phases was found in the HHHT sample due to longer
reheating at a high temperature. In conclusion, an
acceptable austenite fraction with a low fraction of
secondary phases was obtained in the bulk of wire-arc
additively manufactured duplex stainless steel samples
(35–60%), where higher austenite fractions formed with a
larger number of reheating cycles as well as longer reheating
at high peak temperatures (800–1200 °C).
Weldability of high-strength aluminium alloy EN
AW-7475-T761 sheets for aerospace applications,
using refill friction stir spot welding
• I. Kwee, W. De Waele, K. Faes
Refill friction stir spot welding is a solid-state welding process
for joining lightweight sheets in the overlap configuration
by means of frictional heat and mechanical work. The
objective is to investigate refill friction stir spot welding of
aluminium alloy EN AW-7475-T761, since fusion welding of
such high-strength alloys is problematic due to solidification
and liquation cracking. This alloy is used in highly
stressed structural parts in aerospace applications, because
of its low weight, superior strength, high corrosion resistance
and corrosion fatigue behaviour. The process parameter
optimization and the effect of the parameters on the
weld characteristics were examined. The dwell time, the
plunge depth and the rotational speed were varied according
to a multi-level factorial design. An increase of the
dwell time resulted in a larger stir zone area and smaller
amount of defects. The width of the central coarse-grained
band became smaller, containing a more refined grain size.
An increase of the rotational speed lead to a smaller stir
zone area, a discontinuous joint line remnant and a larger
width of the central coarse-grained band. An increase of
the plunge depth lead to a larger stir zone area. Welds produced
with a high heat input exhibited a lower average
hardness and a higher hardness drop, compared to welds
with a low high heat input. Analysis of variance has shown
that the dwell time, the plunge depth and their interaction
have a statistically significant effect on the lap shear
strength: a longer dwell time, a higher plunge depth and
their interaction resulted into a higher lap shear strength.
Reduced weld strength due to secondary cell formation
in vibration weld region of microcellular glass fiber
reinforced nylon-6 shells
• T. Guo, A. Edrisy, S. H. Eichhorn, J. Vanderveen, B. Baylis,
H. Colwell
Microcellular plastic parts reduce weight and increase
dimensional stability, but a significant decrease in weld
strength is observed when the weld region reaches the cell
area. In this study, 30 wt.% glass fiber reinforced nylon-6
shells with weight reductions of 0%, 7%, and 10% were
fabricated by microcellular injection molding with nitrogen
gas followed by vibration welding. Although the weld depth
of the vibration weld was much less than the thickness of
the cell-free surface layer, microstructural analysis of fracture
surfaces by scanning electron microscopy and optical
microscopy confirmed the presence of cells at the weld
region that lowered burst pressures by 17% and 22% for
shells with weight reductions of 7% and 10%, respectively,
when compared with the burst pressure of 1.16 MPa for
solid shells. The irregular sizes and elongated shapes of these
cells suggest that they were generated in the molten polymer
by secondary nucleation of residual nitrogen gas
during the vibration welding process. This assumption is
corroborated by the fact that no cells are formed in the
weld area of solid shells with 0% weight reduction and is
consistent with recently reported similar findings for glass
fiber reinforced polypropylene.
SCHWEISS- UND PRÜFTECHNIK 09-10/2019 187