PROCESS BREAKS CONVENTIONAL
LASER WELDING RULES
JACK GABZDYL and DANIEL CAPOSTAGNO
Industrial use of nanosecond lasers for applica- tions such as marking, engraving, cutting, and even micromachining is well known—however, the use of these sources for welding and joining is rel- atively new [ 1]. Most applications of high-peak- power, short-pulsed lasers tend to be related to material removal, so their use as a beam source for
joining is perhaps counterintuitive. But for those in the know,
the versatility afforded by master oscillator power amplifier
(MOPA)-based nanosecond fiber sources give unparalleled
flexibility in terms of control of the output characteristics.
These lasers can be used, for example, with high-peak-power
nanosecond pulsed output with tunable pulse duration and
high frequency-modulated quasi-continuous-wave (QCW)
modes, as well as operated as a more conventional continuous-wave (CW) laser.
On paper, one could quite rightly question the capability of
these lasers for welding. Compared to CW and QC W sources,
they have very low average powers (typically <100W) and
pulse energies of just 1mJ, which is 1000X less than typical
diode-pumped solid-state (DPSS) and QCW pulses. This does
currently restrict the use of pulsed fiber lasers to relatively thin
sections up to 0.5mm, but recent results show that penetration depths of up to 1mm are possible. Operating with pulse
durations often in the 100–500ns range, with peak powers
of up to 10k W and pulse repetition frequencies of >50kHz,
these lasers are significantly differentiated from conventional
QCW sources. This ability to tailor the input energy characteristics to the application at hand is key to their use as a
tool for joining.
In general, when bonding thin material, there is a requirement for reliable joining processes that avoid over-penetration,
distortion, and warping. This is addressed by conventional
keyhole welding processes using CW lasers, but overcoming material thresholds requires relatively high powers (
usually >200W). The challenge is even greater for joining bright
metals, and even more so when the materials are dissimilar.
The joining of dissimilar metals has long
been a challenge for welding and design
engineers who have often been told that
it’s difficult or that it can’t be done. Every
welding handbook contains a table showing the allowable com-
binations of materials that can be welded—and those that can’t.
As any metallurgist will tell you, it is the combination of physical
properties and metallurgical incompatibility that governs weld-
ability. However, there has always been a lot of interest in this area
because there are a lot of potential applications that require such
joints, particularly bright metals to steels and aluminum alloys.
Over the years, this has prompted a significant research focus
in this area—innovation has been incremental, so it’s still work
in progress. SPI Lasers’ nanosecond welding process (patent
pending) has provided a differentiated mechanism that has to
date proven itself to be extremely capable in joining a wide vari-
ety of dissimilar metals (FIGURE 1).
Fusion welding of dissimilar metals creates problems asso-
ciated with the formation of brittle intermetallic phases. These
can form planes of weakness, making the joints susceptible
to fracture. These intermetallics typically manifest themselves
in the interface of the weld pool and form based on time and
temperature. Heat input is a key factor in their formation.
The use of nanosecond lasers for joining is unlike conven-
tional welding in that it does not generate a large melt pool,
so opportunity for the formation of these unwanted interme-
tallic phases is inhibited. The spot size is very small (often in
the 30µm range), so to make spot welds or linear welds, tech-
niques such as spiraling and beam wobbling are used. The spot
size can be easily changed by appropriate selection of beam
FIGURE 1. Examples
of various metals
welded to stainless