23 / 48
November 2015
AFRICAN FUSION
21
Low stress no distortion welding
ponent trials as shown in Figure 13. The weld paths to be
welded comprised four long straight welds that join the upper
and lower pressing for this product. These welds are actually
stitchwelds in production, but for the purposes of this trial the
weld lengths were extended tomagnify the distortion effects.
The current production product is known to suffer high
levels of distortion and production operators report that once
the main beam has been welded and then removed from the
weld fixture, the final resultant distortion is such that it could
not be placed back into the same fixture once cooled. The
upper and lower pressing panels for the bumper beam are
bothmade from2.0mm steel to XF350material specification.
The weld pathwas straight for each of the four welds used
for the trial; however these are located in an area of significant
form in the pressings and, therefore, would test the robustness
of the seal arrangement to conformto thepart profile. A robotic
program was developed to carry out each weld immediately
Figure 12: A comparison of the measured out of plane distortion on
the centreline of the closing plate for the top-hat section.
Figure 11: A point cloud comparison to laser scanned results of
distortion measurements for the welded top-hat section.
Figure 13: Images of the trial bumper beam sub-assembly showing
the weld pattern used for the trials.
after each other, as would be done in a production situation,
switching the coolant onwhen the arc startedbut also keeping
coolant on until the cooling head had completely traversed
over the weld stop location so as to cool the full length of
the weld run. The coolant was switched off whilst the robot
traversed at speed to the next position to start welding, this
was controlled via the robot programwith the coolant control
panel being interfaced with the robot controller.
During set-up of the robot program, trials were conducted
on the beam using the standard GMAW process, where a fixed
block was used to observe the movement of the part under
welding. To do this, the part was welded in a fixture that only
had end restraint on the lower panel.
When welded using conventional GMAW processes, the
beam was observed to distort towards the fixed block at the
side of the part by up to 6.0mmat a certain stage duringweld-
ing and away from the block by 5.0 mm at others, illustrating
the magnitude of and distortion problems associated with
welding such slender parts. This in fact meant that, at certain
stages of the program, the weld torch was significantly off the
weld seam, at times burning into the wall of the pressings and
at other timesmissing the part altogether as the part distorted
towards and away from the torch.
To ensure such problems did not happen in the trials the
fixture was built to restrict the movement of the part during
thewelding process and to ensure that weldingwould always
remain on the path of the weld seam. In order to maintain a
consistent weld joint fit-up gap and position, the upper and
lower panels were tacked together by hand using small tacks.
The weld paths were developed in the robot program and
the sealing position of the cooling systemoptimised to ensure
it followed the weld path whilst maintaining a good seal to
Figure 14: The LSND welding head in position on the axle component during
testing of the coolant sealing and the weld path.