Figure 1 A cladding process Alabama Laser calls Alabama
Laser Hot Wire, or ALHW, combines a solid-state laser with a
wire heated by a gas metal arc welding power source (the laser
head is out of the frame).
Two processes sometimes are better than one
Pushing the envelope with hybrid cladding, welding
By Tim Heston, Senior Editor
or years some cutting-edge fabrication technology has come out of the southern Appalachian
foothills. In the 1990s you could travel about 10
miles south of Interstate 20, down a two-lane road
through farms and woodland, and pull into the parking
lot in front of an unadorned facade just outside Munford, Ala. Walk inside, and you’d see a 6-k W CO2 laser
cutting at 197 feet per minute. This was part of a system
that laid the groundwork for a major initiative in laser
blanking. And this was more than a decade ago.
On the front burner now are hybrid technologies. Several years ago the company started work on a project involving hybrid laser-arc keyhole welding. Most recently it
has delved into what it calls Alabama Laser Hot Wire, or
ALHW, a cladding process that combines a preheated wire
with a solid-state laser, offering minimal dilution with the
base metal and high deposition rates (see Figure 1).
The company’s business itself is a bit of a hybrid. It’s
part job shop, part laser equipment supplier, part R&D
center. And that’s just Alabama Laser, which is a division
of Alabama Specialty Products Inc. (ASPI).
ASPI CEO Don Johnson started Metal Samples Co. in
1980, in a small storefront, machining specimens for corrosion and metal testing. Metal Samples Co. is now an
ASPI division, along with Alabama Laser; Alabama Research and Development, which produces medical lab
equipment, among other things; and Urbanus of Alabama, which manufactures metal furniture. ASPI operates a machining and fabrication job shop with a full
range of lasers (obviously), punch presses, waterjet cutting systems, press brakes, and machining centers. Altogether, the divisions employ about 250 and take up
600,000 square feet of production and testing space.
Thanks to its diversity, the organization was spared
the worst during the recession. Although its job shop
business declined, other areas have sustained, including
its specialty laser work. According to sources, Alabama
Laser often acts as a last resort, a place where people go
when nothing else has worked. Unusual and challenging
jobs are its bread and butter, and this includes its work in
hybrid laser welding and cladding.
F
full penetration at 100 IPM in 0.375-in. plate.
In such hybrid applications, the laser provides the
penetration, while GMAW mitigates the fit-up issues typically associated with the laser beam welding on its own.
GMAW’s filler metal also adds mass to the weld pool,
which allows slow cooling and helps to mitigate solidification cracking.
High speed and resistance to cracking are benefits
enough for some to consider the hybrid process, even if
joint prep isn’t an issue, according to Wayne Penn, Alabama Laser president. In a fillet weld, for instance, a laser
alone tends to produce a bead with undercuts, which are
crack-susceptible stress concentration points in the bead
profile. GMAW and the associated filler metal help produce a smooth radius corner, wetting the sides and reducing stress that could lead to cracking.
Laser Cladding
In hybrid keyhole welding, the keyhole acts as a venturi
during the weld and swirls the molten and filler metal
into a fine grain structure—desirable attributes for welding. Unfortunately, this is exactly what you don’t want in
a cladding application. So when Alabama Laser engineers
ramped up efforts in cladding process development,
they had to take a fundamentally different approach, and
much of it hinged on some inherent limitations in existing cladding technologies.
Thermal spray cladding lacks strength because it produces a mechanical bond, not a metallurgical bond.
Cladding with GMAW does produce that metallurgical
bond, but there are drawbacks. To attain the required
corrosion resistance, clad layers need to be chemically
pure, and to get that a GMAW process must deposit
many layers of clad metal, because the dilution zone between the base and clad metal is relatively large.
Hot-wire gas tungsten arc welding (GTAW with wire
feed) also can clad a metallurgical bond by preheating the
wire before feeding it under the tungsten torch. This produces a thinner dilution zone, so the clad may not have to
be as thick. But the deposition rate is relatively slow, and
the dilution zone still is much thicker than the one a laser
can create. A conventional weld process clad dilution zone
is measured in fractions of a millimeter or even in millimeters; a laser process’s dilution zone is measured in microns.
This is exactly what makes powder laser cladding so
attractive. A mature though not widely adopted technology, this process uses a laser to melt powder and create a clad with a thin, strong metallurgical bond (or
dilution zone) with the base metal on the order of 100
microns (see Figures 2 and 3). This makes for a near
chemically pure clad that is ideal for corrosion resistance.
Such dilution properties for powder laser cladding “are
herculean,” Penn said.
Less clad metal not only means less material cost, but
also improved material qualities, such as ductility, be-
cause the process can be highly controlled. As Penn ex-
plained, “What if you had a clad that was a few
thousandths thick of chrome-moly material? What if the
material flexes? It cracks, and if it’s thick material, other
issues might crop up. So you tune in the parameters for
the thickness, and minimize the heat. We have made duc-
tile deposits as thin as 0.050 inch and as thick as 1 inch in
materials that would generally be considered unweldable.
To our knowledge, that’s something you can do only with
a high-energy-density beam.” In addition, unique mate-
rial properties can be achieved with custom metal matrix
composites using the rapid melting and quenching prop-
erties of laser processing, he said.
Figure 2 This powder laser cladding process delivers powder
via a coaxial nozzle that surrounds the laser beam.
Hybrid Welding
Joint prep kills efficiency when it comes to full-penetra-tion, thick-plate welding. Traditional arc welding in such
metal requires beveled V joints, and grinding or flame
cutting foot upon foot for weld prep takes serious man-hours. A butt-joint geometry simply isn’t an option with
such thick material—until you add a laser. Hybrid laser-arc welding can indeed handle butt joints and so doesn’t require that arduous joint prep. And then there’s the
speed benefit. Hybrid laser keyhole welding can achieve
Figure 3 A large rod is clad at Alabama Laser with powder
using a laser.
