XXXVII CONGRESSO AIAS

XXX International Conference on
Surface Modification Technologies (SMT30)
29TH JUNE - 1ST JULY, 2016, MILAN, ITALY
Fiber Laser Remelting and Surface Hardening of a Nickel-based Superalloy
G. Daurelio*a, A. Angelastroa, S. L. Campanellia, L.A.C. De Filippisa, A. S. Pugliesea, A.D.
Ludovicoa
a
Politecnico of Bari, Department of Mechanics, Mathematics and Management, Viale Japigia 182, 70126, Bari,
Italy
_________________________________________________________________________________________________________________________________
Abstract
In some specific industrial applications, employing Nickel based super-alloy components, a surface enhancement of product
tribological properties is required. In this work, a recasting surface treatment (Soft Hardening) that would produce a slight
increase in micro-hardness, in a localized and pre-defined zone, thus enhancing the mechanical properties of the surface, was
performed. A Diode Laser, Fiber Active, Ytterbium doped, λ 1070 nm, with power variable in the range 400W -4 kW and an
integrating segmented mirror, which produces an integrated laser beam (in a knife blade or rectangle), as well as a laser
intensity homogenization and which was of interest to a depth of fusion, in the range 0.2 to 1.0 mm, were used. Different
laser working parameters were tested. Remelted tracks, having a width 8 mm, were characterized in terms of surface visual
inspection, morphology, metallographic examination as well as longitudinal and transverse micro-hardness. Moreover, a
comparison was performed between properties of the remelted material and the base alloy. Laser Efficiency was also
evaluated both in terms of Specific Molten Volume [mm3/s] and in Dau Unit [mm3/kJ], giving some interesting
considerations vs laser power and working speed.
Keywords: Ni based SuperAlloys; Laser Heat Treating; Laser Surface Hardening; Laser Surface Re-Melting; HPDL; Active Fiber Laser
Diode; Material Processing; Laser Surface Treatment; Selective Laser Surface Hardening; Laser Transformation Hardening; Laser Surface
Absorption.
_________________________________________________________________________________________________________________________________
Nomenclature
ρ density
Tm melting temperature
R electrical resistivity
Cp specific heat
K thermal conductivity
ΔL average thermal expansion
Th heating temperature
λ laser wavelength
Tq quenching temperature
HB Brinnel hardness
HV Vickers micro-hardness
A% laser surface absorption
Dt thermal diffusivity
α(T) temperature coefficient
of electrical resistivity
1. Introduction
Ni based superalloys have been developed over the past 60 years from a simple Ni-Cr matrix to the present
multi element and phase systems, having a fully austenitic face centered cubic (fcc) structure which maintains a
superior tensile, fatigue and creep properties at high temperature to a body centered cubic (bcc) alloy. One of the
major applications of Ni superalloys is gas turbine engines. They comprise over 50% of the weight of advanced
_____________________________
* Corresponding author. Tel.: +39 080 596 2841; fax: +39 080 596 2788.
E-mail address: [email protected]
aircraft engines and include wrought and cast turbine blades and powder metallurgy (P/M) route turbine discs as
attested by Meetham G.W. (1981).
One of the most important goals of engine design is increasing turbine entry temperature (TET): the
temperature of the hot gases entering the turbine arrangement (Reed R.C., 2006). This implies that the resistance
against the environmental attack, i.e. high temperature, under a severe mechanical force is the priority challenge
and indeed Ni based superalloys are used in the hottest (operating at temperatures up to 1050 °C) as well as the
highest tensile pressure of the gas turbine engine component.
Yilbas et al. (1997) investigated laser surface modification of metals, and they showed that it improves the
surface properties such as resistance to corrosion and wear. Laser surface hardening results in new phases
because of the high heating and cooling rates, which in turn leads to conditions that produces considerable grain
refining.
In the present study, the laser surface treatment of Ni-based superalloy was carried out. Superalloy properties
and applications was studied by Nageswara (2011), Matthew D. (2002), Daurelio G. (2013, 2015). The aim of
the research was to obtain superficial remelted tracks, from 5 to 8 mm wide, on a Ni based Superalloy for
industrial interest. Said surface remelting (SLM - Selective Laser Melting or Re-Melting), had to be obtained by
a laser beam, a typical TEM00 modal structure or integrated laser beam (a knife blade or rectangle shape), and
should be of interest to reach a thickness (depth of fusion) in the range from 0.2 to 0.8 mm at most. At the same
time, an increment of about 20% to 25% of the surface HV microhardness had to be reached. Considerable
research studies were carried out to examine laser surface hardening, such as Grum J. (2007, 2013), Kennedy E.
et al. (2004), Bachmann F. (2003) and Li Lin (2000). A great interest was devoted to the use of a laser source by
radiation in the near IR (λ 1060-1070 nm), in order to carry the laser beam easily via an optical fiber and to use it
without difficult even at considerable distances (even tens of meters) from the same source, with very small
losses of laser energy.
In these experimental conditions, it is also possible to produce a laser heating of a Nickel Superalloy, reaching
a surface temperature between 800 and 900 °C, rising the surface micro-hardness, therefore improving the
tribological properties and the overall mechanical characteristics, as stated by Grum J. (2013).
2. Experimental Setup
2.1. Experimental Material
Superalloys represent a category of very wide metal alloys, designed to retain a good mechanical strength and
corrosion resistance even at temperatures around 1200 °C, very useful in aeronautics and/or aerospace. The most
widely used are the Ni based Superalloys, hardened by a secondary phase of formula Ni 3Al or Ni3Ti, present in
the form of coherent precipitates, with face-centered cubic crystal structure (fcc) and containing solid solution of
heavy metals and refractory ones (in particular W, Mo, but also Re, Ta or Nb) and Co, hardened also them by
solid solution and also through the precipitation of metallic carbides.
Some of the most known Ni based Superalloys that are also examined by Nageswara (2011), Matthew D.
(2002), Daurelio G. (2013, 2015), are: REX-78, K-42B, Inconel 617, FSX-414, Udimet – 700, IN – 718, CMSX2, In MA-6000E, Renè 41, Renè 65, Renè 77, Renè 80, Renè 100, Renè N5, Renè N6.
The process described in this work, which investigates the laser remelting of Ni-based Superalloys with a Ni
percentage between 40% and 60% and a Cr percentage between 10% to 18%, can be realized, in the used process
conditions, due to the thermal diffusivity and melting temperature values of Superalloys having these
percentages of Ni and Cr.
Thermo-physical properties are very similar to those of 300-series Stainless Steels (304, 316, 321, etc.) as
reported by Daurelio G. (1982, 2013, 2015), Nageswara (2011) and Matthew D. (2002). However, there are
some significant differences in thermal conductivity and diffusivity parameters, average thermal expansion and
melting temperature range Vs increasing of Ni % content from 40 % to 60 %, according to its thermo-physical
properties.
Table 1 provides the superalloy chemical composition, while Table 2 shows some main thermo-physical
parameters of interest in this research work.
It is important to specify that A% is referred to a temperature value of 20°C and to a wavelength of 10.6 μm
(CO2 laser); these measures are obtained for each value of R, and provided by Eq. 1, given by Daurelio G. et al.
(1982) and Arata-Miyamoto (1978):
A% 1.12 R
(1)
It is clear that:
there is a variation of R according to a variation of Ni content;
operating at equal Ni content, a temperature rise from room temperature, provides an increase of R with its α
(T) temperature coefficient of electrical resistivity ; therefore A% also grows;
the above cited formula of A% is valid for λ ≥ 2 μm; therefore it is not valid for λ 1.060-1.070 μm, as in this
case. However, the calculation of A% at λ 10.6 μm (CO2 laser) can provide some helpful indication in the
comprehension of the laser – material interaction.
Table 1. Ni-based Superalloy chemical composition.
Components
Composition percentage
C
Cr
Mo
Al
Ti
Co
B
Zr
Ni
≤ 0,07
14,6
4,2
4,3
3,3
15
0,015-0,016
0,04
58
Table 2. Thermo-physical parameters.
Parameter
Values
ρ (at Troom) [g/cm3]
7.7-8.03
Tm [°C]
1450-1510, 1370 –1400, 1204-1400
R [µΩ·cm]
55, 70, 34
Cp [J/kg°C]
460-500 (a 20°C), 477 (da 50 a 100°C)
K [W/m·°C]
16-25 (a 20°C), 42.7 (a 100°C)
ΔL [m/°C]
10 · 10 -6, 16 – 17 · 10 -6
HB
335 (quenched in oil, tempered at 425 °C)
Th (recommended) [°C]
788
Tq (recommended) [°C]
1204 (in controlled atmosphere, for 5-15 min)
A%
8.31% (at T room) for R=55 µΩ·cm
6.53 % (at T room) for R=34 µΩ·cm
Figure 1 shows the trend of reflectivity, that is the complement to one of absorption, Vs λ for some metals,
among which also Ni. It is evident that, with the λ of 1.060 – 1.070 μm, A% much higher than is possible with a
λ of 10.6 μm, typical of a CO2 laser, should expect.
Fig. 1. Laser absorption vs λ for some materials [11-14].
The thermal diffusivity is a parameter that indicates the speed with which, in the non-stationary thermal
condition, the heat diffuses through the material itself. The thermal diffusivity is therefore an index of thermal
inertia of the structure. Once known the data on the thermal conductivity, specific heat and density, one can
calculate the thermal diffusivity. Thermal diffusivity varies with T: therefore increases versus T, as it can be seen
below in the Eq. 2 and in Table 3.
Dt
K
Cp
Table 3. Indicative values of thermal diffusivity of Nickel superalloys in function of temperature.
(2)
Temperature T [°C]
Thermal diffusivity Dt
[cm2/s]
20
100
0.06
0.11
Although it would be very useful and interesting calculate the D t values around 1000 °C, unfortunately, in the
scientific as well as in technical literature it is not possible to find the K, ρ and Cp values at temperatures higher
than 100 °C.
For comparison, in Table 4 are showed the values, at T=20 °C , of K, Dt and A% (at λ 10.6 μm), for stainless
steel AISI 304, 316, 430 and some Nickel Superalloys.
Table 4. Comparison of K, Dt and A% values of some steels and some Nickel Superalloys.
Material
K [W/m·°C]
Dt [cm2/s]
AISI 304
15
0.034
9.4
AISI 316
16
0.037
9-9.5
AISI 430
25
0.058
7.8-8
Nickel Superalloy (40 to 60% Ni, 10 to18%
Cr)
16-25
0.05-0.06
6.5-9.3
A%
2.2. Experimental Planning
Due to the particular material to be tested and the aim to achieve, the choice of the type of laser with which to
perfom the experimentation, as well as the surface treatment technique to be used and other related problems
were studied and discussed. It was better to use a CO2 laser or a diode laser or a fiber laser? A laser beam with a
Gaussian Transverse Electromagnetic Mode (TEM 00) or an integrated laser beam shaped as a knife blade or a
rectangle? In focused or defocused condition? Other possibilities were to use a close Gaussian Beam (TEM 01 or
TEM 10), a top-hat beam, a polarized or a random polarized beam, a rastered beam with a scanning strategy like
dithering, rotation, square spiral or zig-zag. In the case of an integrated beam, it would have been more
convenient to move along the short side or the long one of the rectangular or knife blade spot shape? And, in this
case, to get the integrated beam, it would be appropriate to use an integrating segmented mirror or an integrating
segmented ZnSe lens? Furthermore, which of the power density value would be appropriate to use to make the
remelting possible? It was better to proceed with a constant laser power or a constant surface temperature, the
melting temperature of the particular Ni based Superalloy treated? Another condition to be taken into
consideration concerned the realization of the treatment on the surface as it is or coated with an absorbent
material. If it was decided for the use of an absorber, which would have been better to use: colloidal graphite,
Zinc Phosphate, Manganese Phosphate, simple black acrylic painting? It would have been better to use a
covering gas, like N2, Ar, He, or realize the treatment in open air?
Fig. 4. Segmented mirror used a); knife blade shape spot obtained b); integration scheme of the used mirror c); space power distribution of
beam d).
Finally, in the light of suggestions, studies and experiments reported in literature, it was decided to work with
a fiber laser (λ 1.070 μm), using a constant power (1 and 2 kW), and an integrating segmented mirror to obtain a
knife blade shape spot (Fig. 4), as showed by G. Daurelio et al. (2015). The beam was moved perpendicular to
the long side. No coating was used to avoid the possible dispersion of the material coating in the molten
superalloy. The power density to be used must be in the range 100-300 W/mm2, as shown in Figure 5, according
to the previous studies investigated by Bachmann F. (2003), Li Lin (2000), Charschan J. (1972) and Duley W.W.
(1982).
Fig. 5. Power density Vs interaction time for different laser processes [19-20].
2.3. Experimental equipment
Experiments were carried out using a 3 axis machine equipped with a 4 kW fiber laser, the Ytterbium Laser
System YLS 4000 by IPG Photonics, and a laser head with a cooled integrating segmented mirror by Kugler,
with a focal length of 250 mm, placed at 200 mm – mirror-sample distance, to obtain a knife blade shape spot
(Fig. 6). The roughness of obtained tracks was measured with a roughness tester, the Surtronic 10 by Rank
Precision Industries. Later on it, in order to perform a metallographic investigation, all samples were sectioned
with a metallographic cutting machine, cleaned in an ultrasonic washing machine, incorporated in a
thermosetting resin, polished and chemical etched (KELLER etching reagent). Finally, the cross sections of the
samples were subjected to macro analysis, by means a Nikon stereo microscope, with a magnification from 5x to
63x and to micro-hardness measurements by means of a REMET HX1000 TM micro-hardness tester.
Fig. 6. Laser source a); laser head equipped with the cooled integrating segmented mirror b).
3. Results and discussions
For the experiments, performed on 5.5 mm thick plates, the following parameters were adopted:
Spot dimensions: 1x8 mm2;
Laser power: 1 and 2 kW;
Working distance: 200 mm (distance between the segmented mirror axis and the treated surface);
Laser speed: 1 to 25 mm/s;
Shielding gas: N2;
Gas pressure: 2 bar;
Gas flow rate: 62 Nl /min;
Sample dimensions: 30x80x5.5 mm.
3.1. Surface aspect of the molten traces
Fig. 7. Surface aspects of the molten tracks.
As it can be noted in Fig. 7, the tracks appeared covered with a surface oxide, polished in some cases, matt in
other ones. It is clear that they were oxides of Cr. In fact the molten track, coming out from the cone of shielding
N2, was still hot enough, compared to ambient temperature, so the Cr still had the possibility to react with the
atmospheric O2. This problem can be avoided adding a trailing shield complementary to the nozzle, already
existing, for covering the gas, in order to extend both the zone to be protected and the protection time. An
important phenomenon to note is the so-called “surface rippling”, induced by surface tension gradients during
the laser surface melting. As reported by Antony and Cline (1977), and after by Kim and Sim (1997 and 2005),
during laser surface melting, temperature gradients on the melt surface between the laser beam impact point and
the intersection line of the solid-liquid interface with the surface, generate surface tension gradients that sweep
liquid away from beam impact point. The resulting flow of liquid creates a depression of the surface under the
action of the laser beam, pushing and rippling the surface of the liquid elsewhere. As the beam crosses other
areas of the surface, this distortion of the liquid surface is frozen in, creating a roughened ripple surface. If the
laser beam sweep velocity exceeds a certain critical speed, the liquid does not have sufficient time to form
ripples, so that the rippling from surface tension gradients can be avoided. This effect can be seen by examining
the tracks numbered from 1 to 12, performed with an increase of the working speed.
3.2. Macro analysis of cross sections
After the usual cross-sections of the tracks, the specimens were embedded in a thermosetting resin, polished
and chemical etched by the Keller solution (2 ml HF, 3 ml HCl, 5 ml HNO3, 190 ml distilled H2O). This
solution has been shown to be suitable for the macro visualization of the treated area, as well as for the measures
of heights of molten pool, but not useful for the resolution of the microstructure.
Fig. 8. Structure of base material at 50x on the left ( 1st and 2nd ) – at 25 X ( 3rd ), at 63 x on the right.
Fig. 9. Cross-sections of molten tracks.
Table 5. Process and energetic parameters employed in the experiments and dimensions of molten tracks.
Sample
Molten
Process
Specific
Track Track
Power
Thermal Interaction
Power Speed
Area
Efficiency – Molten
width height
Density
Input
time
[kW] [mm/s]
(measured)
Dau
Volume
[mm] [mm]
[W/mm2]
[J/mm]
[10-1 s]
2
3
3
[mm ]
[mm /kJ] [mm /s]
1
1
1
5.83
0.30
1.47
125.0
1.5
1.5
1000
1.00
2
1
1.33
7.57
0.84
4.99
125.0
6.6
6.6
751.9
0.75
3
1
1.5
7.66
0.70
5.13
125.0
7.7
7.7
666.7
0.67
4
1
1.8
8.03
0.72
5.10
125.0
9.2
9.2
555.6
0.55
5
1
2
7.79
0.76
4.65
125.0
9.3
9.3
500
0.50
6
1
2.7
7.63
0.66
3.82
125.0
10.3
10.3
370.4
0.37
7
1
3
7.61
0.62
3.57
125.0
10.7
10.7
333.3
0.33
8
2
6
8.23
0.62
3.94
250.0
11.8
23.6
333.3
0.17
9
2
9
7.91
0.43
2.97
250.0
13.4
26.7
222.2
0.11
10
2
12
7.64
0.25
2.32
250.0
13.9
27.8
166.7
0.08
11
2
20
6.90
0.23
1.51
250.0
15.1
30.2
100
0.05
12
2
25
6.78
0.19
1.30
250.0
16.3
32.5
80
0.04
From Tab. 5, it can be noted that the width of the molten tracks remained almost constant varying the process
parameters; it was expected, because of the use of an integrated laser spot, of size 8x1 mm2, in which the
distribution of energy is equalized along the transversal direction to the laser scanning speed. Similar trend were
found for track height and molten area (Fig. 10), which decreased with increasing of the working speed.
3.3. Efficiency of the process
Among the different parameters reported in Tab. 5, there are also two relating to the calculation of process
efficiency. Some researchers prefer to express efficiency in terms of specific molten volume, in mm3/s (molten
area multiplied by process speed). This parameter, however, does not take into account the laser power used. In
this regard, the process efficiency can be express with a unit of measurement called Dau [mm3/kJ], as examined
by Daurelio G. (2011). It is defined as the specific molten volume divided by the incident laser power. This unit
of measurement, born to be applied to the laser welding, now is assumed in all laser processes, such as welding,
cutting, drilling, and all the surface heat treatment (hardening, alloying, coating, tempering, remelting). In this
work, it is evident the increase in efficiency with increasing speed (Fig. 9 and Tab. 5), even if the treated area
decreases. This trend is largely dependent on the prevalence of speed compared to the molten area of the cross
section; in fact, even if the molten area decreases, the efficiency grows when the speed increases. Tab. 5 and
plots of Fig. 10 show that the values of the track height and molten area obtained with 2 kW are practically lower
than those obtained with 1kW; this is due to much higher values of speed and to a consequent and substantial
reduction of both the values of the interaction time and thermal input. All the above considerations are true
except for the Test n.1, where, due to a very low speed (1 mm/s), there has been an excessive thermal input and
interaction time; so there has been an overheating of the material, with an excessive surface rippling, as
underlined by Anthony T. R. et al. (1977) and Kim and Sim (1997 and 2005), a partial vaporization, associated
with a very low molten height and molten width of the track, and the lowest efficiency value, only 1.5 Dau.
From all the above, it is evident that, given the width and height of the molten area to achieve, it need to choose
the optimal operating parameters.
a)
c)
b)
d)
Fig. 10. Influence of laser power and speed on process efficiency, track height, track width and molten area.
3.4. Surface Roughness measurements
Roughness of obtained tracks was measured by the Surtronic 10 by Rank Precision Industries roughness
tester. Measurements were carried out by setting the measuring range to 0.1-40 µm, the evaluation length to 5
mm, the cut-off to 0.8 mm, the exploration speed to 2 mm/s.
Samples 6 and 8 have been sanded, before the laser treatment, with a slight mechanical surface peeling,
resulting in the removal of oxide present; therefore, the surface roughness Ra of the base material is slightly
lower than that which is measured on the base material as supplied; in fact, roughness of the sample 6 shows
values very similar to those found on the base material. Contrariwise, on sample 8, the Ra measured transversely
assumes values similar to those of the base material, while the Ra measured longitudinally to the melt, assumes
slightly higher values than those the base material.
Roughness of all tracks (except for samples 3, 6 and 8) falls in the range between 1.7 up to 3.2 µm.
For samples 3,4,5,6,7 and 11, it was not possible carry out measurements of Ra, in the direction transverse to
the molten track, since the measurable trace width was approximately equal to or slightly more than the length of
the running exploration (5mm) of the instrument. Samples 9 and 10 showed values of Ra similar to those of the
base material. Sample 3 showed in the molten track the highest value of Ra compared to the other samples; this
is probably due to the already higher value of roughness of the base material on sample 3.
Table 6. Surface roughness Ra of molten tracks.
Test
Ra base mater.
Ra ┴
2.8-3.0-2.7-2.3
Not measurable *
2.1-2.7-1.8-2.32.5-2.4
Not measurable *
4
2.3-1.5-2.1-2.62.7-2.0
Not measurable *
5
1.3-1.8-1.5-1.71.5-1.2
Not measurable *
6
2.2-1.9-2.2-2.02.0-2.1
Not measurable *
7
2.2-1.9-1.7-1.91.7-2.1-2.3-2.51.6-2.0
2.3-2.1
8
2.4-2.1-2.6-1.92.1-2.4-2.9-2.42.1-2.2
2.0-2.5
9
2.4-2.1-2.6-1.92.7-2.7-2.8-2.52.1-2.2
2.7-2.7
10
2.1-2.1-2.2-2.31.9-2.3
Not measurable *
11
* Not quantifiable because the measurable trace
(running length 5mm) of the instrument.
3
Average
values
****
****
****
****
****
2.2
2.4
2.7
****
width
Min
Max
Ra //
3.7-2.6-3.6-2.9****
**** 3.9-3.4
2.6-2.8-3.0-2.4****
**** 2.2-2.0
2.0-2.6-2.8-2.5****
**** 3.2-2.5
1.8-1.8-1.5-1.7****
**** 1.6-1.7
2.3-2.5-2.8-2.1****
**** 2.5-2.0
3.0-2.8-3.1-3.02.4-2.7
1.7
2.5
2.3-2.4-2.0-2.01.8-1.7
2
2.9
2.2-2.6-2.9-3.02.6-2.7
2.5
2.8
2.2-2.3-2.5-2.0****
**** 1.9-2.2
was approximately equal to the exploration
Avera
ge
values
Min
Max
3.4
2.6
3.9
2.5
2
3
2.6
2
3.2
1.7
1.5
1.8
2.4
2
2.8
2.8
2.4
3.1
2
1.7
2.4
2.7
2.2
3
2.2
length
1.9
2.5
3.5. Micro-hardness measurements and Profiles
The average micro-hardness of the molten track was measured at various locations along the transversal
section of sample 5 using a Vickers indenter Remet HX1000 applying a 200 gf load for 15 s. Two measurement
directions were chosen as illustrated in Fig 11.
Figure 11 shows the micro-hardness profiles for sample 5, measured in the transverse cross section,
respectively along the x direction (Fig. 12a) and the z direction (Fig. 12b).
It can be seen that micro-hardness of the base material is about 400HV. The average micro-hardness in the
molten track along x axis is about 472 HV, with a maximum value of 478 HV. Measurements carried out along
the Z axis, have shown a "bell-shaped" trend, with HV values increasing from Z -0.5 mm (HV 454) to Z - 1 mm,
where the maximum value of HV (513) was found, for then decrease from Z -1 to Z -1.5 mm (HV 458).
Hence, the desired goal has been reached with an increasing of about 20% of micro-hardness on the surface
and on the treated depth.
z
x
Fig. 11. Measurement profile directions for micro-hardness
a)
Fig 12. Micro hardness profile for sample 5 along the X-axis (a) and along Z-axis (b).
b)
4. Conclusions
In this work a laser surface remelting treatment on a Nickel-based super-alloy was performed in order to improve
mechanical properties of the surface. For this purpose a fiber laser doped with Ytterbium, emitting a wavelength
of 1.070μm and with a power ranged from 400 W to 4kW was employed. An integrated beam, obtained through
a segmented mirror, allowing homogenization and distribution of the laser power, was used. A knife blade shape
spot, 1 mm x 8mm, at distance of 200 mm between the mirror and the substrate was obtained. Tests, conducted
for two different values of laser power (1kW and 2kW) and with some different values of the working speed
showed a localized and well defined melting for all samples.
Surface roughness Ra values, measured before and after the process, appeared very similar to those of the
base material. Micro-hardness values, measured on the surface and inside the produced molten tracks, confirmed
a remelting and soft hardening of the material, with an increase of about 20%. The laser efficiency of the
process-product was also evaluated as in terms of Specific Melted Volume [mm3/s] as in Dau Unit [mm3/kJ],
giving some interesting evaluations vs laser power and working speed.
References
Anthony T. R., H. E. Cline, 1977. Surface rippling induced by surface‐tension gradients during laser surface melting and alloying. J. Appl.
Phys. 48, 3888, dx.doi.org/10.1063/1.324260.
Arata, Miyamoto, 1978. Laser Welding , Technocrat, Vol. 11 , 5.
Bachmann Friedrich, 2003. Industrial Applications of High Power Diode Lasers in Material Processing, Applied Surface Science , 208-209,
pp.125 – 136.
Charschan J., 1972. Lasers in Industry – Western Electric Series.
Daurelio G., L. Cento, C. Esposito, September 1982. Saldatura a Laser CO2 di Acciai al Carbonio e Inossidabili, Monografia CNR
(Consiglio Nazionale delle Ricerche) – Progetto finalizzato LASER di POTENZA, pp. 1 to 166.
Daurelio G., Sept. - Oct. 2011. Il Dau: Una Unità di Misura dell’Efficienza di Processo nel Taglio, Saldatura, Micro-Foratura e Trattamenti
Termici Superficiali a Laser, 3rd Part, Rivista Italiana della Saldatura, Anno LVIII ,n.5 , pp. 659 – 671.
Daurelio G., January 2013. Metals - Steels - Alloys and SuperAlloys - Monografy, pp. 1 to 53 (ResearchGate).
Daurelio G., January 2013. Thermo - Physical and Chemical - Physical Properties of Metals, Steels, Alloys and Super-Alloys.
Daurelio G., April 2015. RENE '77 – SuperAlloy - Main thermo-physical properties et Absorption % of the laser radiation, λ 10.6 μm of a
CO2 laser, pp. 1 to 9.
Daurelio G., A. Angelastro, A. S. Pugliese, June 2015. An Integrating Head for a 1.07 micron Gaussian Laser Beam. Published on
ResearchGate WEB, DOI: 10.13140/RG.2.1.3241.2320.
Duley W.W., 1982. Laser Processing and Analysis of Materials, Plenum Press.
Grum J., 2013. Laser Surface Hardening. Encyclopedia of Tribology, pp 1948-1962.
Grum J., Sept. 2007. Comparison of Different Techniques of Laser Surface Hardening. JAMME - Journal of Achievements in Materials and
Manufacturing Engineering, Vol.24, Issue 1, pp. 17 to 25.
Kennedy E., Byrne G., Collins D.N., 2004. A Review of the use of High Power Diode Lasers in Surface Hardening. Journal of Materials
Processing Technology, 155 – 156, pp. 1855-1860.
Kim Woo-Seung, Sim Bok-Cheol, Nov. 1997. Study of Thermal Behavior and Fluid Flow during Laser Surface Heating of Alloys. Review
Numerical Heat Transfer Applications, 31 (7): 703 to 723.
Li Lin, 2000. The Advanced and Characteristics of High –Power Diode Laser Material Processing. Optics and Lasers in Engineering, 34,
pp.231 – 253.
Luxon J.T., Parker D.E., 1985.Industrial Lasers and their Applications , Prentice–Hall Inc. New Jersey – USA, Book , chap.12 , pp. 222 -228.
Matthew D., Donachie J., Donachie Stephen J., 2002. Superalloys: A Technical Guide, 2nd Edition, by A.S.M. International – The Material
Information Society.
Meetham G.W., 1981. The Development of Gas Turbine Materials 1st ed., Applied Science, London.
Nageswara Rao M., 2011. Materials for Gas Turbines: An Overview. In Book - Advanced in Gas Turbine Technology, by Ernesto Benini
Ed., chapter 13, pp. 293 – 314.
Reed R.C., 2006. The Superalloys, Cambridge University Press, Cambridge.
Sim Bok-Cheol, Kim Woo-Seung, March 2005. Melting and Dynamic-Surface Deformation in Laser Surface Heating. Intern. Journal of
Heat and Mass Transfer, 48 (6):1137 to 1144.
Yilbas B.S., Nickel J., Coban A., Sami M., Shuja S., Aleem A. Laser melting of plasma nitrided Ti-6Al-4V alloy. Wear 1997;212(1):140-9.