The process heats metal above its melting temperature without vapor formation. Power densities range from 104 to 105 W/cm2 and depend
on the thermal conductivity of the metal; carbon and stainless are easier to weld with this
technique than is aluminum, for example.
While heat conduction welding presents a
highly aesthetic weld seam—positioned perpendicular ( 90 degrees) to the laser beam,
though there is some angular flexibility with a
compromise to penetration depth—the e;ciency of the process is somewhat poor. When
the process uses a solid-state laser producing
1-µm light, 68 percent of the energy reflects o;
the irradiated area of the workpiece, resulting
in a low coupling e;ciency that restricts the
penetration depth and weld speed. With a CO2
laser, coupling is even poorer with 88 percent
of the light reflected o; the irradiated area,
making heat conduction welding with a CO2 laser impractical.
Despite some limitations, heat conduction
welding is still hugely popular among manufacturers, particularly in highly visible applications
where a rounded edge is a requirement. Think
of all the stainless steel appliances in your kitchen, or peek into the kitchen of a restaurant and
have a look at all the stainless steel surfaces.
Look closely and you might see grinding marks
and inconsistent radii from all the manual refinishing that goes into dealing with issues created
by conventional welding.
Look at those same parts produced with heat
conduction welding with a laser, and you’ll notice that those problems go away. This really
drives home the ever-increasing interest in laser welding, particularly in environments where
lots of rework is the norm.
With the same laser source and beam delivery
The Laser Leads the Way
system, it’s possible to manipulate the beam
density and focus position to weld with the
second technique. Deep-penetration welding,
or keyhole welding, uses an approximately 0
focus position; that is, the focus is at or near the
material surface, creating a high energy density
at the workpiece. While coupling e;ciency
is relatively low in heat conduction welding,
with keyhole welding it’s quite high with both
solid-state and CO2 laser sources; the coupling
e;ciency is 10 and 15 percent, respectively.
The process heats the workpiece above the
temperature at which vapor formation occurs
and forms a vapor capillary through the abla-
tion pressure of the outflowing metal vapor,
creating the “keyhole” that gives the process
its name. Power densities are from 105 to 106 W/
cm2, with the penetration depth dependent on
the formation of the keyhole.
This welding technique o;ers high weld
speeds, a narrow heat-a;ected zone, and sub-
stantial weld penetration depth. Because of the
low energy transfer and large penetration depth,
deep-penetration welding is much more suitable
for thick materials or when the weld preparation
is surface-to-surface or surface-to-edge. The
ideal seam preparation is an edge-to-edge butt
joint, though the keyhole process usually can
work well for a variety of joint configurations.
For choosing between heat conduction weld-
ing and deep-penetration welding, the applica-
tion drives the selection. Though a heat conduc-
tion weld arguably looks better, the simple fact
of the matter is that deep-penetration welding
generally will be the cheaper of the two op-
tions, thanks mostly to its high welding speed.
That said, you still can benefit from the speed
and low energy transfer of deep-penetration
welding while still getting that nice, consistently
rounded edge unique to heat conduction weld-
ing. Simply go over the already keyhole-welded
seam with the beam in a higher focus position.
The laser continues to solve more and more
manufacturing problems, and process variables
such as beam diameter and manipulation continue to have a meaningful impact. From cutting
and welding to adding material layers or removing them, advancements in laser technology are
sure to be a key component of success in the
Fourth Industrial Revolution.
Brett Thompson is a sales engineer with TRUMPF Inc.,
111 Hyde Road, Farmington, CT 06032, 860-255-6000,
fiber ( 100 μm)
fiber (400 μm)
Focused beam Ø
Focused beam Ø
The latest technology gives us an idea of how truly vast the laser’s potential is for metal fabrication
and beyond. Consider ultrashort-pulse-duration lasers. To give an idea of scale, light travels at
186,000 miles per second. In one second, light can travel the circumference of the Earth 7. 5
times. In a picosecond, light travels only 300 µm! If the duration of absorption of the material
being processed is smaller than that of the electron-phonon interaction time, cold ablation
occurs; the metal isn’t heated or melted but completely dissociated.
Cold ablation has applications in metal and in various other materials, including glass. In
most cases, glass is processed with a scribe and break process, where force snaps the material
at the scribe lines, or surface ablation using ultraviolet (UV) lasers.
Why UV lasers? It has to do with absorption. Under normal conditions, a photon of infrared
light (~1 µm) is not absorbed by transparent material. Those of us who have tried to cut transparent materials or coatings on a disk or fiber laser are all too aware of this. That’s why glass
processors have used UV lasers, but they also can take an alternative approach: nonlinear light
absorption via lasers with ultrashort pulse durations.
Again, under linear absorption conditions, transparent material does not absorb photons.
But in nonlinear light absorption, several photons are absorbed simultaneously, combining
their energy and allowing an IR (such as a disk or fiber) to do the work of a UV laser.
This is achieved by reaching those ultrashort pulse durations. They combine energy with
ablation performed not by thermal processes but via direct dissociation of the material. This
cold ablation allows for much more precise processing of materials. This process, combined
with optics that create an elongated beam profile, allows the laser to achieve extremely high
cutting speeds in transparent material.
» Figure 2
Beam diameter can be controlled to some extent by
shifting the position of the focusing lens.
» Figure 3
The delivery fiber diameter determines the beam