28 Industrial Laser Solutions NOVEMBER/DECEMBER 2016 www.industrial-lasers.com
welded successfully, and this is particularly true for high-reflectivity metals.
Early work on this topic was performed
for the automobile industry by welding aluminum alloys >1mm thick to steel for vehi-
cle light weighting [ 3]. Welding copper—
the most challenging material of all—using
high-brightness infrared fiber lasers in conjunction with a beam manipulation technique known as wobbling is now commercially accepted, as is welding very thick
copper [ 4] using very high-average-power
multimode fiber lasers.
The beam delivery system and laser controller employed for these trials was a conventional laser marking setup, with the only
difference being the YLR 150/1500 AC SM
fiber laser. A standard 100mm-focal-length
collimating optic and 163mm f-theta scan
lens were used—this combination gives a
calculated spot size of 23µm. For most of
the trials reported here, 227W CW power
as measured at the workpiece was used.
25 × 75mm-long foil samples were overlapped by 25mm and a single-line stitch
weld with seven 2mm-long stitches was
made across 22mm of the sample width.
This sample configuration generally
resembles the relevant ASTM standards.
An extensive series of trials were performed with different combinations of metals (see http://bit.ly/2eKXMl7 for details of
the results). In most instances, establishing
the partially optimized procedure for each
combination consisted of:
1. Fixing average power at 98% of max-
imum from this laser in CW mode
(227W) and changing weld speed
incrementally to identify stable keyhole
2. If average power created excess spatter
for a particular material combination,
average power was reduced to the
point at which no spatter occurred
3. If the phenomenon of humping was
observed (which occurs during high-speed welding of stainless steel), average power was again reduced to the
point at which humping was reduced
to an acceptable level; and
4. By adjusting weld speed, combinations
of speed and power were identified that
in each case produced partial penetration welds that appeared to produce the
best weld quality.
5. No cover gas was used in any of
Weld strength testing
The most widely used and straightfor-
ward technique for assessing the strength
of welded foils is to produce a simple lap-weld joint between two sheets of material.
Strain hardening during testing and the difficulty of measuring small-weld fracture
surface areas limits the accuracy of weld-strength figures.
We therefore measure final load-to-
failure of a standard 1in.-wide sample as
a practical semi-quantitative technique
to assess and compare weld strengths
between these different metals and metal
combinations. An Instron 3366 was used
to test all of the different material combinations to failure, and after lap shear
testing, the weld surfaces and the weld
fracture surfaces were examined.
The width of the failure zones at the mate-
rial interfaces were ~60µm on most of the
combinations, and all were below 100µm.
The surprising feature of these results
is the high weld speeds in every case.
Clearly, when above the fluence thresh-
old for absorption, the physical properties
of aluminum allow a very efficient keyhole
welding process. FIGURE 2 shows cop-
per well distributed in the aluminum weld.
FIGURE 3 shows a sawtooth weld shape,
another technique that may be used to
improve weld strength and controllability.
Follow-up tests included using a lon-
ger-focal-length, 254mm scan lens to
weld 100µm stainless steel foils. With
this lens, it was shown that high-quality
lap welds could be produced between
two 100µm-thick 302 stainless steel foils
at 8mm above and below the focal of the
lens simply by adjusting weld speed with
a resultant increase in weld width and
strength. With this focus system, the weld
process is therefore highly insensitive to
focus position and if an assembly requires
welds at different levels, automation
requirements might be greatly simplified.
FIGURE 2. The upper surface of an
FIGURE 3. A sawtooth-shaped aluminum-
The surprising feature of these results is the
high weld speeds in every case. Clearly, when
above the fluence threshold for absorption,
the physical properties of aluminum allow
a very efficient keyhole welding process.