TECHNOLOGY’S NEW HORIZON —
Laser Cladding and Cladding Services
high power direct diode [HPDDL] laser and its unique beam make for
a highly efficient tool to use in cladding operations. Laser cladding
is performed by melting a pre-placed powder onto a substrate to
ensure a bond with minimal dilution, nominal melting and a small
heat affected zone. The laser used in the experiment was a 4000
watt Direct Diode laser system (HPDDL) mounted on a Panasonic VR-16
robot. The pre-placed powders chosen for this experiment are ANVAL
410, 156 and C22. 410 and C22 were selected for their superior corrosion
resistance. 156 is a general-purpose cobalt hard facing material.
The cladding substrate was ASTM 1018 steel. The dilution of the
coatings was analyzed through the use of a Scanning Electron Microscope
[SEM]. Through analysis it was discovered that dilution is kept
to a minimum, in the range of 0 to .02%. The corrosion resistance
and wear resistance was also measured for the appropriate samples.
This process is highly advantageous in comparison with competing
coating methods such as plasma spraying, arc welding, and other
laser sources. The rewards being lower porosity reduced post-machining,
optimum edge detail.
As tools for
use in industrial applications, HPDDL, also known as semiconductor
lasers, are becoming more prevalent.1,2,3 Diode laser technology
has been used for a number of years in compact disks, laser printers
and laser pointers. Their low cost, high efficiency, and compact
design make them an attractive technology in the industrial manufacturing
environment. The electrical to optical conversion efficiency of
the HPDDL is as high as 55%.
The light emitted at the facet of the laser diode is highly divergent
and astigmatic. To make this usable, a lenslet array is close
coupled to a two dimensional array of laser diodes. Since the
other axis, referred to as the "slow axis," is not collimated
and is left to diverge, the final focusing lens will produce a
concentrated line of light, which is very useful for large area
applications such as cladding. This beam is very uniform, having
a nearly tophat intensity profile along the long axis with a guassian
profile perpendicular to the line along the short axis. The HPDDL
used in this feasibility study employs 4 stacks of 20 bars, which
are brought to a line by a single macro lens [Figure 1]. With
dimensions of approximately 12.5 mm X <1 mm with a 125 mm focal
length lens. With different macro lenses this laser can achieve
power densities greater than 200 kW/cm2.
1 – Focus Configuration of Line Source HPLLD
An ideal application
for the HPDDL is large surface area laser cladding. As shown in
Figure 1 the line of laser light along the short axis is moved
perpendicular to the long axis. The biggest benefit of HPDDL laser
cladding is that the unique line source allows the user to produce
clads with a controllable width without scanning. CO2 and Nd:
YAG lasers have a smaller spot; thus the laser must be scanned
over the cladded area. The wavelength of the HPDDL is 808 nm,
compared with 1.06 _ m for a Nd: YAG laser and 10.7 _ m of the
CO2 laser. The shorter wavelength of the HPDDL allows for higher
absorption into the material being cladded, therefore a higher
process speed can be achieved. Both CO2 and Nd: YAG lasers often
require binders when using pre-placed powders. The use of binders
often leads to porosity due to the evaporation of volatiles during
the cladding pass4. The HPDDL system does not necessitate the
use of binders to hold the powder together before a cladding pass.
Another advantage of the HPDDL is that the thermal input can be
precisely controlled thus yielding minimal dilution and a small
heat affected zone.
During a laser cladding
process dilution is expected to be minimized. In cladding operations
dilution is often defined as the amount of intermixing of the
clad and substrate. Dilution is measured by visual analysis or
through a SEM elemental line scan. Visual analysis allows the
user to get a quick estimate of the dilution of the clad, however
this method of measurement is not very accurate. Through visual
analysis dilution is defined as the distance the clad layer extends
below the substrate. SEM analysis is a true, accurate measure
of the dilution, or intermixing of the clad and substrate. Laser
alloying is a process that is often grouped with laser cladding
operations. Laser cladding and alloying are traditionally distinguished
by the relative amounts of the consumable material added and substrate
melted. Generally the two categories are arbitrarily separated
by their relative amount of dilution, laser alloying being classified
as having greater than 10% dilution, laser cladding having less
than 10% dilution4. In laser alloying it is generally desired
to mix portions of the coating with the substrate to produce an
alloyed layer, thus a high dilution and high intermixing is expected.
It should also be noted that laser alloying requires convection
and laser cladding does not. In many laser alloying processes
the cooling rate is often monitored to ensure intermixing and
the formation of unique metallurgical compounds. Ultrafast quench
rates of the order of 1011 Ks-1 are often required4 as well as
a high solubility of the clad material in the parent material.
Laser alloying experiments were not conducted in this study, however
throughout the experimentation there was an expectation that at
a low process speed some alloying of the powder and substrate
may occur. This was not true for the HPDDL process because laser
alloying requires very high quench rates and a keyhole as seen
in Nd:YAG and CO2 lasers.
The denser microstructure
and better bonding of laser clads allows for enhanced corrosion
and wear resistance with a single pass. Laser cladding is a viable
alternative to plasma spraying and TIG or MIG processes. The clad
material deposit does not intermix with the substrate in many
applications; therefore the dense, uniform microstructure of the
clad layer allows for enhanced single pass corrosion or wear resistance
in a HPDDL clad. It is difficult to produce a clad with a TIG,
MIG or plasma spray system without having less than 5% dilution,
therefore as many as 15 overlapping passes are required to obtain
an undiluted clad layer5. Conventional arc welding processes generally
impart a significant amount of heat into the part resulting in
a large heat affected zone and distortion. Post-weld treatment
can improve the properties of the joint, but can also lead to
distortion of the component6. The surface finish of overlapping
passes produced with the HPDDL are relatively flat, however a
TIG cladding process often results in distinct ridges and valleys,
which lead to cracking when bent7. In addition, the arc welding
processes often are also responsible for the losses of alloying
elements8. A direct comparison of a laser clad layer with an arc-welded
layer indicates that the HPDDL clad has significant grain refinement,
which in some cases lead to an increased wear resistance9. The
HPDDL also surpasses flame spray technology, since flame spray
produces a more porous coating with limited adhesion10.
Laser cladding also
has several advantages over plasma cladding processes. The substrate
of laser clads are free of the micro-cracks and pores typical
with the plasma clad process. Other advantages of the HPDDL over
plasma processes include the uniformity of the HPDDL coating,
the manual requirement of plasma processes, cracks and pores in
a plasma clad. The sharp boundary of the plasma clad layer with
the substrate also often leads to pores and cracking9. The interface
between the clad and substrate of a HPDDL clad is smooth with
Multiple pass samples were prepared which demonstrated uniform
cladding thickness [Figure 3]. Recent research has been performed
on 100% overlapping clad passes that indicate that this significantly
increase the cladded surface properties10. Corrosion testing indicated
that the overlapping passes could withstand prolonged salt spray
exposure. Surface roughness and uniformity of the clad are two
important properties that are influenced by overlapping clads4.
Overlapping passes result in a decrease in surface roughness and
are typically dense and well bonded.
of the clad material alone will not determine the properties of
the clad on the substrate. The solubility of the clad, which determines
the amount of intermixing of the clad and substrate, i.e. dilution,
is important. The resulting microstructure of the clad, the dilution
layer and heat-affected zone are all important areas in determining
the quality of the clad. Finally, solubility and wetting issues
that lead to pits and pores. All of the above influence the wear
and corrosion resistance of the clad.
Wear and corrosion resistant powders were selected for the experimentation.
The corrosion resistant powders include C22 is a NiCrMo alloy
in the Hastealloy C family, and 410 is a basic stainless T410
material. The nominal composition of each alloy is listed in Table
2. The substrate used was 1018 steel, which was selected because
it is a commonly used and inexpensive material.
The 156 material is
a cobalt based hardfacing alloy used for increased wear resistance.
The composition of this alloy consists mainly of cobalt, however
Cr is also largely alloyed in this material [Table 2].
2: Nominal compositions of the clad materials.
/ Performance Evaluation
The powder was pre-placed to a thickness of .050"
on a 1018 steel substrate. The thickness and width of the cladding
pass changes with modifications in processing speed. As the processing
speed increases the clad track has an increasingly gaussian profile
due to the surface tension of the melt. However, a decrease in
speed results in a flatter, wider clad with high visual dilution
[Figure 2]. Overlapping passes wet together to form a relatively
flat profile regardless of processing speed .
Figure 2: A comparison of the profiles
of two NiCrMo clads. The clad on the right was produced at a travel
speed of 0.45 m/min, the clad to the left at a process speed of
Experimental Procedure The HPDDL
was used to clad the pre-placed powders onto the substrate. The
line source was passed along the short axis over the powder. The
speeds varied from 0.3 to 0.8 m/min at 4 kW of laser power. The
variance in the speed allowed for clads to be produced with varying
levels of visual dilution. Each powder was cladded with a visual
dilution of 0, 10 and 60%. Two clads were produced for each dilution
One of the two clads was sent for SEM analysis, one of the hardfacing
clads at each dilution level was sent for wear testing and the
corrosion resistant clads were used for corrosion analysis. SEM
analysis was completed on all of the samples to determine the
level of dilution and change in dilution with overlapping passes.
The profile of these samples was a relatively flat surface. Corrosion
testing was done on the stainless steel samples. This test was
performed by immersing the samples in nitric acid for a period
of 24 hours to determine the effect of the acid on the substrate
and clad. Corrosion testing was also completed on the NiCrMo alloy
by immersing the clad and substrate in a phosphoric acid solution.
Wear testing was done on the Cobalt based clad layer. The standard
pin on disk test was done in accordance with ASTM G99 to determine
the resistance to galling of the clad. A water-jet test in accordance
with ASTM D5367-94 was also performed to determine the wear resistance
of the clad. Multiple pass samples were also produced.
126.96.36.199 Visual Examination Visual
measurement of dilution was performed by using the substrate as
a base for all measurements. As the clads were produced with the
HPDDL a portion of each clad was cut off, ground with 180 grit
paper, and etched in 2% Nital to determine the visual dilution.
The portion of the clad that was above the substrate was measured
at the highest point as well as the entire length of the clad
layer. The portion of the clad below the substrate was divided
by the length of the total clad layer to produce a percentage
visual dilution [Figure 4]. The initial dilution measurements
described above are shown in Table 2. The drawback to this method
of measuring dilution is the lack of accuracy in measurements.
However, visual dilution measurements are a straightforward approach
to determining the approximate dilution of a sample while processing.
Figure 4: Visual measurement of dilution
was performed through the equation L2/L1.
Table 2: Dilution as measured by visual
An acid etch was performed on each of the samples
to bring out the microstructure of the clad layer. The etch used
for the 410T stainless material was oxalic acid, while the NiCrMo
and cobalt based alloys were etched electrolytically in a solution
containing equal amounts of CrO3 and potassium permanganate, and
8% sodium hydroxide. The microstructures indicate thorough melting
of the powder. Both the NiCrMo and Cobalt based alloy show a dendritic
microstructure within the clad layer [Figure 5]. Grain growth
is seen in the heat affected zone of the clad, however there is
no evidence of the melting of the substrate. The 410 T SS powder
shows also shows grain growth in the heat affected zone, but the
microstructure of the clad show is primarily martensitic due to
the rapid quench rate of the powder [Figure 6]. The microstructures
present indicate that the dilution of the clad into the substrate
is minimal and that changes in process speed do not reflect changes
Figure 5: Dendritic formation in the Cobalt
based clad layer , also the interface between the clad and substrate
is shown on the left.
Figure 6: Martensitic formation in the
410T Stainless clad layer , also the interface between the clad
and substrate, left.
SEM Analysis A SEM line
trace was used on each of the samples to determine the dilution
of the clad layer as defined by the amount of intermixing of the
clad layer and substrate. Each powder has a reasonable amount
of Chromium; therefore this element was traced in the clad layer
for each powder. Iron was traced in the substrate.
At a high process speed the dilution of the clad into the substrate
is minimized. The cobalt based hardfacing powder was clad at a
speed of 0.7 m/min at a power of 4 kW
At a lower processing speed the dilution is still minimized. A
clad was produced at a speed of 0.40 m/min at 4 kW with the cobalt
based hardfacing powder
The properties of overlapping passes with regard to dilution and
amount of intermixing are similar to those of a single pass. The
same cobalt based powder has minimal dilution and intermixing
at a process speed of 0.7 m/min, 4 kW
Samples produced with the stainless steel and NiCrMo powders produced
similar results with respect to dilution. At all of the process
speeds the dilution was minimal. As overlapping passes are produced
to create a 100% clad surface, no effect on dilution is observed.
Corrosion Testing Corrosion testing was performed on the stainless
steel samples by immersing the clad and substrate in nitric acid
for a period of twenty-four hours to determine the effect of the
acid on the substrate and clad. Corrosion testing was also completed
on the NiCrMo alloy by immersing the clad and substrate in a pure
phosphoric acid solution for twenty-four hours.
The corrosion analysis indicates that a great deal of corrosion
occurs for each of the 410 Stainless Steel samples. However, this
analysis is also somewhat misleading. The entire clad and substrate
was immersed in the acidic solution, the majority of the corrosion
occurred in the substrate. In most industrial applications only
the clad would be exposed to the corrosive media. A visual of
analysis of the clad and substrate of the single pass stainless
steel samples before and after the corrosion testing indicates
that the clad is virtually unaffected, however the substrate has
dissolved in the acid [Figure 11]. Overlapping passes produced
similar results with the majority of the material loss being in
the substrate [Figure 12]. The 410 Stainless Steel clads also
have a visible change in color after immersion in the nitric acid.
This is an indication that the passive Cr2O3 layer has been removed
thereby increasing the corrosion rate.
Figure 11: A visual comparison of the 410
Stainless Steel clad produced at 4kW, 0.65 m/min before and after
immersion in nitric acid indicates that most of the material loss
is in the substrate
Figure 12: Overlapping passes produced
at 0.65 m/min, 4 kW show the majority of material loss being in
The C22 alloy was fairly resistant to the phosphoric
acid. The clad layer is unaffected in this acidic solution, however
pitting can be observed in the substrate. A change in the color
of the substrate is also observed, this indicates the beginning
of a loss of the normally present thin film of iron oxide in the
Pin on Disk To produce consistent values for relative
wear resistance, a standard pin-on-disk wear testing machine was
used in accordance to ASTM standard G99. The data indicates that
with a slower processing speed the wear resistance will slightly
increase, or the percent mass loss will decrease [Figure 13].
The 0.40 m/min observed a slightly lower mass loss than that of
the samples produced at faster speeds. The decrease in mass loss
with a decrease in speed is due, in part, to the denser microstructure
produced at a slower speed. The overlapping passes also have a
slightly lower loss of material than the single pass samples.
The decrease in mass loss is not significant enough to draw reasonable
conclusions. However, this may be a slight indication that the
overlapping passes have superior properties than single pass samples
due to increased surface roughness and the denser microstructure.
It is also visible from figure 13 that the mass loss of all of
the clad layers is significantly less than that of the substrate.
Ablation Testing The further wear testing of the clad layer was
performed using a water-jet and scanning the 100 grit garnet fluid
over the top of the clad layer and substrate at a speed of 2.54
m/min [Figure 10]. The pressure of the water-jet was at 344 MPa,
the stand off of the jet was 0.0127 m. The change in thickness
from the original clad profile was measured and recorded. The
percent material loss was determined by the equation:
(tcontrol sample – tablated sample)/ tcontrol sample 
The results were recorded and a direct comparison can be made
between the material loss in the clad layer and the material loss
in the substrate [Figure 14]. It was found that the 1018 steel
generally experienced a greater loss of material than the Cobalt
based alloy. However, the 0.75 m/min clad was subjected to a higher
degree of mass loss than the base material.
The profile of the clad and substrate was examined and compared
to a control specimen from the same cladding pass that received
no treatment. Visual examination indicates that there is a substantially
greater loss of material in the substrate than in the clad layer
Figure 14: Measured loss of material in
the substrate and clad. The bottom picture shows a comparison
of the profiles of the Cobalt based clad layer produced at 0.7
m/min at 4 kW. The picture to the left is untreated, the clad
on the right has been water-jet wear tested.
Results and Discussion
Through analysis it was discovered that the clads
produced with the HPDDL performed well throughout various tests.
The SEM analysis indicated that regardless of process speed, the
HPDDL clads had minimal dilution. It was also found that overlapping
passes also had minimal levels of dilution. Metallographic analysis
indicated that thorough melting of the clad layer occurred, as
well as rapid quench rates were observed. The stainless steel
and NiCrMo samples were sent for corrosion testing. The immersion
of the stainless steel clad in nitric acid resulted in the dissolving
of the substrate. The NiCrMo clads, when immersed in phosphoric
acid, simply lost their passive layer and experienced some pitting.
Pin on disk wear analysis showed that the wear due to galling
was similar for each of the cladded samples. The abrasive wear
analysis indicated that the clad layer is more resistant to abrasion
than the substrate, as would be expected
it was found that the HPDDL is an effective method of producing
high quality clads with minimal dilution. It was found that the
corrosion and wear properties of HPDDL clads are equal to those
produced with competing methods such as plasma spray, TIG or MIG
deposits. The HPDDL allows the user to produce a single pass clad
with minimal dilution. This can not be accomplished by traditional
arc welding processes, which require multiple passes to achieve
a pure clad layer. The low dilution clads with controllable thickness
are beneficial because the end user can save the expensive of
purchasing excessive amounts of expensive cladding wire and powder.
Laser cladding is highly advantageous over TIG and MIG processes
because the amount of dilution is controllable, it is an automated
process, chemically clean and environmentally friendly. The primary
advantage of the HPDDL in comparison to CO2 and Nd:YAG lasers
is the shorter wavelength and thus higher absorption of the direct
diode laser. Other benefits of the HPDDL over conventional lasers
are the elimination of scanning, controllable dilution and the
elimination of binders with pre-placed powders. The HPDDL is a
highly capable cladding tool that will produce coatings with first-rate
corrosion and wear resistance, low dilution, low porosity, unique
microstructures and aesthetic surface finishes.
- P. Loosen
et. al., SPIE, 2382, 78-87 (1987)
- S. Pflueger
et. al. "Material Processing with high Power diode lasers"
Automotive Laser Applications. 1995 workshop
- S. Pflueger
and F. Kuepper, ESD Technology, April/May 1996
- S. V. Joshi
and G. Sundararajan in N. Dahotre, ed. Lasers in Surface Engineering,
ASM International, Ontario, 1998, pp.121-124, 139-144, 149-153.
- C. L. Horn
et. al. in T. Lyman. Metals Handbook: Welding and Brazing, American
Society for Metals, Metals Park, 1981, pp. 149-161.
- K. C.
Meinert, Jr. and P. Bergan, ICALEO 1999 Proceedings, 87, F49
- T. Heston,
Welding Journal, 79 (7), 46 (2000).
- H. Ocken,
Advanced Materials and Processes, 157 (6), 103 (2000).
- B. Medres,
L. Shepeleva and M. Bamberger in ref. 6, pp. F225-F230.
- R. Hull
et. al. in ref. 6, pp. 41,45-47.
- ASTM Standard
G31-72 (1999) ASTM Subcommittee G01.05.
Crystal M. Cook, Applications Engineer. Ms. Cook graduated from
the University of Missouri – Rolla in 1999 and joined Nuvonyx,
Inc. in January of 2000.
John M. Haake, Vice-President Market Development. Mr. Haake graduated
from the University of Missouri – Rolla in 1988 and worked
at McDonnell Douglas until 1998. Mr. Haake has over 14 patents
in relation to laser diode technology.
Transformation Hardening of Gray Cast Iron
hardening of Class 40 Gray Cast Iron was also performed without
the use of an absorptive coating or inert gas shielding using
a HPDDL. The beam was defocused 4 to 8 mm and moved at various
speeds over the work-piece. The gray cast iron was much more sensitive
than the 4140 due to a very narrow process widow between melting,
producing surface carbides, and desirable hardening. It was determined
that at a 6 mm defocus a case would be produced with a higher
hardness value than the case produced at 8 mm defocus. The 4 mm
defocus produced surface melting and carbides. At a higher degree
of defocusing the case depth would increase at the expense of
hardness. This will occur up to a critical amount of defocusing.
This application will benefit greatly from temperature control,
which is completely within the means of the HPDDL.
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