Abstract:
Both
dip and pulsed modes of metal transfer were investigated to establish which method
produced the better bead geometry and penetration on 2024 and 2014 aluminium alloys
using 2319 filler wire. The MIG electrode configuration used was electrode positive,
which
provides cathodic cleaning for aluminium alloys. Some initial work was also
carried out
using 6013 with 2319 as the consumable.
A second
part of the project involved the investigation of different combinations of
shielding gas, and during both this and the previous task it was necessary to adjust the
welding parameters to achieve acceptable welds. Initially, visual inspection and
measurement of bead
geometry were used for analysing the welds, but
macrographs/micrographs, radiographs, hardness profiles and tensile
tests were carried
out to look more
closely at the welds, once acceptable bead geometry and penetration
was obtained. The microstructure was
investigated to ascertain the extent of any porosity
and chemical
changes that might affect the mechanical performance of the joint by
depletion of alloying elements in the HAZ.
Pulse mode metal transfer
improved the weld properties in comparison with the use of
dip mode and was therefore used for all the work following the first section. During the
work
relating to shielding gas composition it was found that a 30% helium balance
argon shielding gas produced the best results. It was observed during the speed trials that
porosity increased with speeds in the region of 1000mm/min and also below 650mm/min.
Pulse
frequency and pulse time seemed to affect the arc performance and also influenced
the levels of
porosity. The need to thoroughly clean the joint area was also seen to be
paramount in reducing porosity to a minimum.
Hardness
profiles indicated a dip in hardness just inside the HZ adjacent to the fusion
line, with all tensile specimens failing at this point when the beads were left on. The
tensile results carried out on 1.6mm 2024 at Cranfield
using 30% helium in the shielding
gas produced UTS strengths equivalent to 85% compared to parent metal strength, while
those
performed by Shorts produced UTS results up to 89%. This indicates an increase in
strength over previous published results of 100%. Ibbotson cites two sets of TIG welds
having UTS values of 191 and 194 MPa equivalent to 42% compared to parent metal
strength while the welds produced at Cranfield using 30% helium in the shielding gas
were 402
l\/IPa, equivalent to 86%. The tensile results for the 2mm 2014 aluminium
alloy were even better, producing UTS results equivalent to 94% compared to the parent
metal. With the 2014
alloys Ibbotson cites two sets of welds in both TIG and MIG and
here UTS results were 221 and 243 MPa, equivalent to 55% compared to parent metal
i
ABSTRACT MIG
Welding Aluminium alloys
strength, while similar welds produced at Cranfield were 395MPa equivalent to 94%.
The
fractographs for this showed the start of a shear fracture indicating that the strength
was
nearing that of the parent material. Against these results even the thick section
]2.7mm 7150 welds
having a UTS of 347MPa equivalent to 58% compared to parent
metal
strength look quite promising.
The time trials
relating to the natural age hardening process on 2024 welds confirmed
that
they achieve their maximum strength after 30 days, increasing by some 15% from
half an hour after
welding to 30 days.
The use of thick section materials in the form of 12mm 2024 and 12.7mm 7150 became a
requirement during the latter stages of the project with the investigation into the use of a
number of
commercially available filler wires dominating the research. Post weld heat
treatment was seen to
improve the strength of the welded joints when employing several
different filler wires, but when
incorporating a dual post weld heat treatment with 7150
using 1.6 5180 filler wire a marked improvement was observed.
Arc
monitoring techniques were used in the form of Arcwatch to log the arc voltage and
welding current values, and a number of statistical evaluations were conducted during the
project using the Arcwatch data. Geometrical data relating to weld bead dimensions was
collected at numerous
stages throughout the research and employed to build a joint
prediction statistical model. This provides the end user with a means of process
parameter selection and a visualisation of the influence of parameter variation on weld
bead
geometry.
It has
clearly been demonstrated that it is possible to produce viable welds using the MIG
welding process with electrode positive, with comparatively low heat inputs when using
a number of different
high strength aluminium alloys and different composition filler
W1I`CS.
This
piece of research has involved the welding of high strength aluminium alloys that
have, in the past been classified as difficult to weld and with the 7xxx series, regarded
unweldable. The results illustrate a
major advancement in the welding of high. strength
aluminium
alloys, represent a considerable improvement in the strengths obtained in
weldments
using these alloys and has encouraged the U aerospace industry to pursue
further research in this field.