Laser Engineered Net Shaping Advances Additive Manufacturing and Repair
Welding Journal, January 2007
(Reprinted with permission)
Click Here to download a PDF version of this article.
Also online at http://files.aws.org/wj/2007/01/wj200701/wj0107-44.pdf
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The LENS® technology
is transitioning into a
process accepted by a
growing number of
commercial, aerospace,
and Department of
Defense customers
BY ROBERT P. MUDGE AND
NICHOLAS R. WALD
ROBERT P. MUDGE and NICHOLAS R. WALD are with RPM & Associates, Inc.,
Rapid City, S.Dak.
LENS® is a Registered Trademark of Sandia National Laboratories.
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The focus of this article is to
present an overview of the
Laser Engineered Net Shaping
(LENS®) process, which
includes the summary of several
successful applications that cover a
variety of repairs and freeform projects.
Pertinent operating data and technology
capabilities and limitations are presented
to give a much better understanding of
what projects might be considered for this
technology.
The Beginning
Initial development of the LENS®
technology was a team effort between
Sandia National Laboratories, Albuquerque,
N.Mex., and Pratt & Whitney.
Follow-up development work was sponsored
by a Cooperative Research and Development
Agreement (CRADA). Members
of this group included Allied Signal,
Inc., Lockheed Martin Corp., Eastman
Kodak Co., 3M Co., Hasbro, Inc., Laser
Fare, MTS Systems Corp., Teleflex, Inc.,
Wyman-Gordon Co., Optomec Design
Co., Ford Motor Co., Los Alamos National
Labs, and NASA. The purpose of
this group was to share in the LENS® research
and development and to promote
the advancement of the technology and
speed its commercialization.
In 1997, the LENS® technology was
licensed to Optomec, Inc., Albuquerque,
N.Mex. Since that licensing, Optomec has
been the sole manufacturer and distributor
of LENS® systems. Presently there
are more than 20 systems operating. Most
of these are the smaller L750 systems with
a 12 × 12 × 12-in. work envelope and are
located at universities or large company
R&D departments. There are six of the
new larger L850R systems in service,
which have a 36 × 60 × 36-in. work envelope.
The earlier systems were coupled
with a 1-kW Nd:Yag laser, while most of
the more recent systems are coupled with
the new fiber lasers ranging from 1 to 3
kW.
Some of the more visible users are Anniston
Army Depot with one L750 and one
L850R that are being used to repair worn
turbine engine components on the M1
tanks. Currently, the only companies that
are actively seeking outside contracts are
ICE Prototyping & Fabrication, which operates
a L750 system coupled with a 1-kW
Nd:Yag laser and RPM & Associates, Inc.,
which operates a L850R system coupled
with a 3-kW IPG fiber laser.
Disruptive Additive Process
LENS® may be characterized as a
“disruptive additive process” that may be
utilized for a variety of repairs and
freeform fabrications. Disruptive, not in
the negative sense of the word, but disruptive
in the fact that this technology
challenges one to think outside of the box
because of the unique capabilities it possesses.
No other additive process combines
excellent material properties with
near-net-shape, direct-from-CAD, part
building and repair quite like this process.
Applications include the repair of worn
components, performing near-net-shape
freeform builds directly form CAD files,
and the cladding of materials.
How Does It Work?
Fig. 1 — Basic layout and flow paths for a
typical LENS® system.
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Figure 1 shows the typical process layout.
The deposition substrate or “target”
is aligned to the desired start point of the
deposit. The powder feeder(s) feed the
powder delivery nozzle assembly, which
creates a powder stream that converges
at the point of the deposit. Next, the laser
provides a focused beam that is delivered
to the point of deposit. The focused laser
beam melts the surface of the target and
generates a small molten pool of base material.
Powder that is being delivered to
this same spot is absorbed into the melt
pool, thus generating a deposit that may
range from 0.005 to 0.040 in. thick and
0.040 to 0.160 in. wide. Motion control for
the deposit may be programmed manually
or may be generated from CAD files
that are processed by the system’s software.
Deposits are typically made in a controlled
argon atmosphere containing less
than 10 ppm oxygen. Some cladding work
may be performed utilizing a shielding gas
system similar to the gas metal arc welding
process.
All LENS® deposits are metallurgically
bonded and exhibit heat-affected
zone (HAZ) and dilution zones ranging
from 0.005 to 0.025 in. thick. Low heat
input and minimal distortion are consistent
deposit characteristics. Due to the
small melt pool and high travel speeds, the
deposits cool very fast (up to 10,000°C/s),
which generates very fine grain structures
that may be one order of magnitude
smaller in size than comparable wrought
products. Mechanical properties and the
quality of the deposits are typically better
than castings and approach properties of
wrought products. In some cases, like titanium,
the deposits may actually exceed
typical handbook values.
Parameters for the low side are laser
power of 400–500 W with a 1-mm spot
size, deposition rates less than 1 in.3/h and
powder utilization rates less than 20%. Parameters
for the high side are laser power
of 2500–3000 W with a 3–4 mm spot size,
deposition rates up to 14 in.3/h, and powder
utilization rates up to 80%.
Stainless steels (304, 316, 410, 420, 17-4PH),
tool steels (H13), nickel alloys (617,
625, 718), cobalt alloys (#6 Stellite, #21
Stellite), titanium alloys (Ti-6-4, Ti-6-2-4-2),
and a variety of hardfacing or cladding
alloys are some of the materials that are
successfully being deposited utilizing this
process. Aluminum and copper alloys are
very difficult to deposit due to their reflective
properties. Research work is also
being performed on tantalum, tungsten,
rhenium, and molybdenum alloys. Functionally
graded deposits are also being investigated.
This is where two noncompatible
materials, A and B, are joined by gradually
changing the deposit chemistry, one
layer at a time, from alloy A to alloy B.
Flexibility is a key ingredient guiding
this technology. LENS® systems are typically
coupled with lamp-pumped Nd:Yag
lasers or more recently the new fiber
lasers. Both lasers have wavelengths that
are ~1 micron long. The optical absorption
of these laser beams is much higher
for the Nd:Yag and fiber laser beams than
that of the CO2 laser beam, whose wavelength
is 10 microns. Having a higher absorption
percentage relates to lower overall
energy required to perform a comparable
laser deposit. Typically, the Nd:Yag
and fiber lasers require only one-half the
wattage of a CO2 laser to achieve the same
deposition rates. The Nd:Yag and fiber
laser beams may also be delivered using
fiber optics where the CO2 beam must be
delivered via reflective mirrors. This
means the component being processed
must be manipulated and moved under
the stationary CO2 beam. This may still
be the case for the Nd:Yag and fiber lasers,
but their delivery fibers also have the ability
to be manipulated as part of the motion
control system. This flexibility opens
up many more potential applications.
The final considerations for the laser
power sources are the floor space required
and overall wall plug efficiencies
to deliver power to the workpiece. The
CO2 laser requires the most floor space
and the most energy. The lamp pumped
Nd:Yag requires less floor space and less
energy, while the fiber laser requires the
least floor space and the least energy, but
arguably it provides the highest quality
laser beam.
System Improvements
Supported by South Dakota’s Congressional
delegates, a team which included
the South Dakota School of Mines
& Technology (SDSM&T), AeroMet,
Inc., and RPM & Associates, Inc. (RPM),
was able to secure FY2002 funding and a
contract with the Army Research Laboratory.
RPM’s portion of the funds was
applied to the purchase of a new Optomec
850R LENS® system, which included an
IPG 3-kW fiber laser. This system was delivered
in March 2003 and was the first
large system manufactured by Optomec.
It was also the first system to incorporate
an IPG 3-kW fiber laser and the first to
use G&M codes instead of the standard
DMC codes for the motion controls.
These firsts made start up a challenge.
RPM and Optomec staffs worked diligently
together to transition the laboratory-grade
machine into the reliable industrial
hardened machine that is operating
today at RPM. Many of the hardware
and software changes that were made to
the original RPM system have been incorporated
into the new L850R systems currently
manufactured by Optomec.
To complement the original system
setup, the RPM staff designed and fabricated
a laser delivery head system that is
very user friendly and capable of delivering
the full 3 kW of laser power. When
compared to the original delivery head assembly,
the RPM head has additional
cooling capacity and the ability to easily
adjust the focusing lens, which adjusts
final delivery spot size. The RPM design
is also capable of using a four-nozzle delivery
system or a concentric cone (coaxial)
delivery system. Converting from the
four-nozzle delivery to the coaxial delivery
takes only a few minutes. This single
upgrade is proving to be the most valuable
upgrade to date.
Strength of the Technology
The high quality of the deposits is the
backbone or strength of this technology.
It is the reason this technology is being
evaluated by the medical industry, aerospace
industry, and Department of Defense,
as well as commercial industries
that include electric power generation,
oil/gas, chemical processing, and mining.
The versatility and flexibility of the
process is evident in applications where
a variety of materials are deposited on
several different geometries at a wide
range of deposition rates. The realized
cost, time, and material savings due to the
utilization of this technology is impressive
and certainly worthy of additional
evaluations.
Typical Repair Applications
Fig. 2 — Low-wattage Ti-6Al-4V repair and
microstructure.
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Titanium Components
If you are operating any type of mechanical
equipment with moving parts, repairing
or rebuilding worn metal components
is part of everyday life. Repairing of
worn components typically saves dollars
vs. purchasing new parts. Also, when a
worn part is rebuilt, the potential exists to
repair that component in such a manner
that it will have a longer wear life than a
new part. These concepts are not new, but
the use of the LENS® process to repair
components previously considered nonrepairable
is new.
Low-wattage repair of titanium components
covers many potential aerospace
and Department of Defense applications
as well as various commercial projects.
Figure 2 shows a typical low-wattage (less
than 500 W) repair of a simulated defect
Fig. 3 — Low-wattage repair of Ti-6Al-4V
bearing housing.
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in this Ti-6Al-4V plate. Multiple layers,
each 0.005 in. thick, were applied to
achieve the full depth of deposit required
for this repair. Note the typical columnar
grain growth in the deposit and the very
small HAZ in the surrounding base material.
Minimal distortion is experienced
with this type of repair, which may be used
on several aircraft structural components
such as wing spars or bulk heads. Several
gas turbine engine components as well as
land-based turbine blades are potential
candidates for this low-wattage repair. Repairs
of this type typically involve critical
components and require substantial qualifications
of the repair process before the
process can be implemented. Working
through the qualification process is a challenge
to say the least, but the potential
savings in time, materials, and dollars is
so great, it is worth the effort.
Another application employing a low-wattage
repair is shown in Fig. 3. This is
a Ti-6Al-4V bearing housing from a gas
turbine engine. The bearing seating area
was worn to an out-of-tolerance condition,
and the housing was considered
scrap. The process was utilized to build
up the worn area, which was followed by
final machining to print tolerances. This
housing was successfully repaired, with no
measurable distortion, and has completed
an evaluation run in a test engine. The repair
costs are about 50% of new pricing
plus it saves all of the materials that would
be required to manufacture a new housing.
Delivery for the repaired housing is a
few days compared to several weeks for a
new housing.
Fig. 4 — Repair of Inconel® 718 compressor
seal.
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Gas Turbine
Figure 4 illustrates a very finesse repair
of a gas turbine Inconel® 718 compressor
seal. When the labyrinth seal diameter
wears 0.008 in., the engine loses
power and the seal is considered scrap.
This test seal was repaired using Inconel®
718 at a cost of about 45% of the new. The
repair deposit caused minor shrinkage of
two inside diameters, which was identified
during the inspection process. These
diameters were machined to print tolerances,
and the repaired seal was accepted
by the customer. Additional seals are currently
in the repair cycle and will be evaluated
early in 2007.
Drive Shaft Repairs
The LENS® deposits are metallurgically
bonded to the substrate; not mechanically
bonded like spray or chroming
processes. The top-half of Fig. 5 shows a
high-speed (8800 rpm) 4340 drive shaft
that has been repaired using a conventional
spray process. Note the severe
spalling in the repaired tapered area of
the shaft. The user tolerated this recurring
problem due to the high cost of a new
replacement shaft, and because the
chance of a successful repair of the shaft
using conventional welding processes was
very slim.
The LENS® process was tried. Several
of these shafts have been successfully repaired
over the last two years using 420
stainless steel. The 420 repair is harder
(RC 50) and has more corrosion resistance
than the original base material or the
spray repair materials. The repair charge
for this shaft is a one-time cost, which is
less than 50% of the cost of a new part.
Bearing, seal, and coupler surfaces on
shafts typically considered nonrepairable
by conventional welding techniques are
considered great candidates for buildup
and repair utilizing this process. Materials
such as 4340, 4130, and PH grade stainless
steels have been successfully repaired.
High-speed shafts, up to 12,800 rpm, high
power, up to 3500 hp, and high precision
shafts with tolerances measured in 0.0005
in. have been successfully repaired.
Another application was the repair of
the bearing and seal surfaces of a large
25,000-lb dragline swing shaft. The 4340
shaft was 18 in. in diameter and 16.5 ft
long with a gear on one end and a spline
cut on the other end. Approximately 20
lb of deposit was required to repair the
shaft. The cost of this repair was a fraction
of a new one with a turnaround repair
time of four to six weeks.
Fig. 5 — Comparison of thermal spray repaired
(top) shaft and LENS® repaired
shaft.
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The process was also used to repair a
pressurized air (PA) fan shaft for a coalfired
electric power plant. The PA fan impeller
failed and damaged the drive shaft,
which is typically not an inventoried spare
part. With the PA fan out of service, the
plant capacity was reduced from 500 MW
down to 250 MW. The first choice was, to
perform a temporary spray repair to the
PA shaft and get the fan back into service
as quickly as possible, about one week.
Then order a new shaft and once it was
delivered, schedule another downtime to
replace the spray-repaired shaft with the
new shaft.
The second choice was to perform a
permanent LENS® repair to the damaged
shaft and put it back into service as quickly
as possible. The second option was selected
and the shaft was repaired and delivered
back to the plant within 6 days. Full
production of the plant was achieved
within a similar time frame as the spray
repair option, but it was not necessary to
purchase a new shaft or to schedule another
outage. This repaired shaft has been
operating for more than two years and the
cost savings realized by employing the
process are significant.
Drive Coupler
Fig. 6 — Successfully repaired atomizer
drive coupler gear.
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Figure 6 shows a typical drive coupler
from a rotary atomizer used in flue gas
desulfurization systems. This gear assembly
has two OD surfaces and a thrust face
that require rebuilding back to a tolerance
of a new part. As shown in the figure, the
thrust face is being repaired after the diameter
repairs were complete. Upgraded
material, 420 stainless steel, was applied
to the worn surfaces in an effort to improve
the wear life
of the rebuilt component
as compared
to a new
component.
A similar repair
application was the
repair of a spindle
from a 4-in. Lucas
horizontal boring
mill. The bearings
had seized on the
spindle due to lack
of lubrication. The
standard method of repair would be to
premachine the damaged bearing seats,
chrome that surface, and grind to final dimension.
The LENS® process was used
for a permanent metallurgically bonded repair.
The repair was successful with no
measurable shrinkage on the ID of the
spindle assembly.
Typical Freeform Applications
The process may be used to deposit
freeforms of near-net-shape metal components
that are nearly 100% dense with
mechanical properties comparable to
wrought materials directly from processed
CAD files. Freeforms may be thin wall
(minimum 0.060 in. thick) or solid to any
thickness. In any event, it is necessary to
overbuild or add some “protect” material
that is typically machined off to achieve
the final desired component. This capability
may be employed to make the complete
part or to add special features to a
simplified casting or forging.
To make a complete component, a
CAD file of the desired part must be provided.
This CAD file is then modified and
processed by the system’s software, which
generates the tool path for the laser. A
target plate is required as a base to start
the build. The target plate may be incorporated
into the final desired part or may
simply be removed when the freeform
build is complete. Thermal treatments of
the completed freeform builds may be required
depending on the specific application.
Typically, a portable CMM with a
scanning laser system is used to scan the
freeform build so the actual build geometry
can be compared to the original CAD
file. In this manner, any out-of-tolerance
areas may be identified and tool path
modifications implemented.
Special freeform features may be
added to simplified fabrications, castings,
or forgings. The goal here is to reduce the
overall materials required to make the finished
part. For example, consider the wall
of a forging is 3 in. thick because protruding
attachments that are an integral to the
final design, which only has a wall thickness
of 1.5 in. Then consider if the forging
thickness is reduced from 3 to 2 in.
and the special attachment features were
applied using the LENS® technology,
there would be substantial savings in the
amount of materials required as well as
savings in machining time required to remove
1 in. of excess material. This is a simple
example of thinking outside of the box
and is a little “disruptive” to existing
thought processes.
Fig. 7 —
Freeforms of 316 stainless steel at various parameters.
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Freeform builds may be performed
using low-wattage/small spot size parameters
or high-wattage/large spot size parameters.
Figure 7 shows the same 316
stainless steel freeform build deposited at
different parameters. The 1-mm spot size
deposit took 190 min to build at 0.40 lb/h
and 19% powder utilization. The 2-mm
spot size deposit took 80 min to build at
0.94 lb/h and 31% powder utilization. The
3-mm spot size deposit took 49 min to
build at 1.67 lb/h and 45% powder utilization.
The 4-mm spot size deposit took 36
min to build at 2.22 lb/h and 54% powder
utilization. Note there is a visual difference
in the surface finish of the builds.
The larger the spot size, the higher the
laser power, the shorter the build time,
the rougher the surface finish. When
building a relatively small part that requires
finish machining, these differences
may not be important while other larger
builds that are not finish machined, the
as-deposited surface finish may be very
critical. Mechanical properties of these
deposits are currently under evaluation.
Two very similar Ti-6Al-4V freeform
builds are shown in Fig. 8. These two
builds are examples of building thin wall
forms on 0.25-in.-thick flat plates. Typical
applications for this concept would be to
manufacture components with high aspect
ratio features that would require substantial
machining and material waste to manufacture
the finished component. Examples
Fig. 8 — Freeforms of Ti-6Al-4V comparing hatch tool path to
contour-only tool path.
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would be structural components for
aircraft such as bulk heads or spars. In
building the parts shown in Fig. 8, only the
desired wall thickness was changed on the
CAD file to achieve tool paths for each
build. Compare the left-half of the figure
that shows the freeform build using a
hatching tool path to generate a 0.50-in.-thick
wall to the right-half of the figure
that shows the same build using a contouronly
tool path to generate a 0.30-in.-thick
wall. Both builds were performed at 3000
W. The hatch build deposited 9.77 in.3/h
at 59% powder utilization. The contouronly
build deposited 12.25 in.3/h at 78%
powder utilization. The estimated cost for
either of these builds is about $2000. The
cost of a solid titanium block similar in
size was quoted at $3500. The solid block
would require more machining than the
freeform builds to achieve a final form
with 0.200-in.-thick walls. The additional
machining chips are just wasted material.
Fig. 9 — A variety of thin-wall freeforms
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Thin wall freeforms are another application
for this technology. Figure 9 shows
a variety of thin-wall freeforms. Transitions
or ducts, creative hollow shapes, and
hollow stem engine valves are just a few
examples of the wide open potential applications
for this capability. Note the conical
dual wall build with integral internal
connecting ribs shown in the upper-left
corner of Fig. 9. The internal ribs are a
serpentine pattern. These types of
freeforms typically have wall thicknesses
around 0.080 in. and may be deposited
using many of the stainless steels, cobalt
alloys, nickel alloys, or titanium alloys.
| Stellite 6 Cladding |
| Material |
(watts) |
in.3/h |
lb/h |
Efficiency |
| Stellite 6 |
2900 |
7.9 |
2.4 |
55% |
|
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Fig. 11 — Oil field component clad with wear-resistant tungsten carbide alloys.
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Typical Cladding and Composite Manufacturing Applications
Cladding is actually a form of repair
buildup applied to the manufacturing of
new components. The concept of composite
manufacturing has been employed for
many years. Figure 10 illustrates examples
of #6 Stellite® cladding along with typical
production parameters for those deposits.
Stellite products have been available for
many years and have been applied via a variety
of conventional processes. The
LENS® process takes the depositing of
Stellite to the next level. Very consistent
high-quality thin deposits can be made with
little dilution. Overbuilding is kept to a minimum,
which reduces finishing costs.
Hardfacing and cladding using CO2
lasers is highly successful. Combining the
LENS® system with the new fiber lasers
improves on this success. The fiber laser,
with the shorter wavelength laser beam,
can achieve equivalent deposition rates
with approximately 50% of the wattage
required by a CO2 laser. The net result is
similar production rates with less heat and
less stress conveyed into the part being
cladded. The surface finish of the cladding
may be left as-deposited or ground to finish
dimension. Figure 11 shows an oil field
adapter that has been clad with a tungsten
carbide alloy. The main photo shows the
as-deposited state and the inset shows the
final ground product. This same concept
is also being evaluated as a cladding for
boiler tubes in coal-fired power plants.
Future and Conclusions
LENS® is a maturing “disruptive additive
technology” that provides new capabilities
for creative repairs on components
that may have previously been considered
nonrepairable. Its small HAZ and
dilution zones will lead into more cladding
and composite manufacturing applications.
The process also has the ability to
easily change a CAD file for freeform production
revisions vs. remanufacturing
hard tooling. This is not only cost effective,
but will save months of development
time on some projects. This freeform ability
may revolutionize existing manufacturing
processes by employing the concept
of simplifying castings and forgings, and
then applying special features utilizing this
process. We must think outside the box in
all of the above-described scenarios.
Commercial industries are investigating
and successfully employing this technology
in a variety of applications. The
aerospace industry and the Department
of Defense customers are interested in the
potential of the technology and are currently
working on several developmental
projects. However, since this technology
is relatively new and there are no published
handbook data available, there is
much qualification work to be done before
total acceptance is achieved in these
industries.
The next step is to train young engineers
to think outside of the box and to
fully utilize the capabilities of this unique
disruptive additive technology. As this
new line of thinking is employed, the
LENS® process will be ready to deliver
high-quality cost-effective deposits.