AGING BEHAVIOUR OF LASER PROCESSED INCONEL 718
Rahul Chamola
Assistant Professor, Department of Mechanical Engineering,
Dev Bhoomi Group of Institutions, Chakrata Road Navgaon, Manduwala,
Dehradun-248007, Uttarakhand, India
Vijay Singh Chauhan
Assistant Professor, Department of Mechanical Engineering,
Dev Bhoomi Group of Institutions, Chakrata Road Navgaon, Manduwala,
Dehradun-248007, Uttarakhand, India
Ajay Singh Tomar
Research Scholar, Department of Mechanical Engineering,
Indian Institute of Technology, Roorkee, Uttarakhand, India
Sunil Kumar Tiwari
Assistant Professor, Department of Mechanical Engineering,
Dev Bhoomi Group of Institutions, Chakrata Road Navgaon, Manduwala,
Dehradun-248007, Uttarakhand, India
Abstract
Inconel 718 is the most promising nickel based superalloy
commercially useful in nuclear reactors heavy machines and
other high temperature application because of its good
corrosion resistance mechanical properties and high
temperature withstand capacity up to 650⁰C. A single crystal or
solidified crystal is employed for high temperature application
because of its better creep resistance and thermo mechanical
fatigue behavior in the absence of grain boundaries.
The manufacturing cost of single crystal is one the major
problems. Hence an alternative solution of the problem was
resolved by the development of laser processed inconel 718 by
the deposition on mild steel substrate. An intense laser heating
of the powder feed is followed by the cooling process to a
dendritic microstructure in layer wise growth of the material.
Substrate is exposed to further heating during multilayer
growth of material upto 28mm in length and 15 mm in
thickness.
Precipitation of inter metallic phase during heat treatment
improves creep resistance in inconel718. Prior to this, a
solutionizing treatment is also necessary at 955⁰C for 1 hr.
Thus, a microstructure comparison of the as-prepared sample
was done with respect to the samples heat treated at 718⁰C for
8hr (HT-2) and another at 718⁰C for 8hr and holding it at 621⁰C
for 18 hr (HT-1), as commercial practice.
Keywords: inconel 718, optical structure of 718, XRD
analysis, Laser deposition process or laser metal deposition
(LMD)
1. Introduction
Superalloys have different characteristics than that of other
materials. Some of the superalloys consist of multiple elements,
may be more than 14. These materials are directly or indirectly
related to civilizations. Inconel 718 superalloy was inaugurated
by International Nickel Corporation in 1950s and it was found
most successful super alloy for aero engines gas turbine blades
due to its high creep resistance oxidation resistance thermal
fatigue resistance and corrosive resistance. Inconel 718
withstands at an excessively high temperature due to solid
solution strengthening and precipitation strengthening [1, 3].
Nickel based, Iron-nickel based and Cobalt based superalloys
are the widely used materials for high temperature applications
[2, 4]. Nickel based alloys are employed for the temperature of
1024º to 1371 °C. Solid solution strengthened alloys,
Precipitation (age) hardened alloys and Oxide dispersion
strengthened (ODS) alloys are the different types of Ni-base
superalloy [5]. Nickel Iron superalloys have Face Cubic Center
(FCC) configuration, less expensive than that of Ni-base
superalloys. Cobalt based materials hold Hexagonal Closed
Packed (HCP) configuration and it has high thermal fatigue
resistance in compared to Ni-base alloy [6].
Modern superalloys consist of different microstructure phases.
Some of them are gamma phase (γ′), gamma double prime
(γ′′) and delta phase (δ) [7]. CAD/CAM software can produce
three-dimensional parts using selective laser melting or direct
metal deposition [8]. This technique is being used by various
industries for manufacturing repair layer to layer net like
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structure [9]. The main purpose of using this technique is lead
time and minimization of material waste. Plasma metal
deposition has umpteen benefits over LMD but sound surface
finishing is obtained with laser processing that's why laser
technique is used for costly material parts [10]. Laser powder
melting technique or laser net shaping technique are the
modified forms of LMD process but basic principle is same in
all the technologies [11]. A numerical control system or
computer aided design system is used to produce the
component layer by layer [12].
The numerous researches have been conducted on laser
processing or laser metal deposition (LMD). Marucco [13] has
measured lattice contraction after aging at temperature range of
450°C to 600°C in 20Cr-25Ni steel, sanicro 71 and Inconel 690
with the composition of 0.024, 0.40 and 0.038% respectively.
Nath et al. have conducted a study to find out the aging effects
on different nickel based alloys such as Nimonic 80A, a
wrought nickel based precipitation hardened superalloy used
in steam turbines. They used X ray diffractrometry to reduce
aging at 450-600°C [14].
2. Material and Methods
2.1 Dimension of Inconel Tensile Specimen
2.2 Composition of Inconel 718
The powder had a composition of 53.5 Nickel, 19 Chromium,
18 Iron, 5 Nidium, 3 Molybdenum, 1 Titanium and 0.5
Aluminum in weight % shown in Table 1. Laser deposition
parameters (laser power: 750 watts, scanning speed: 6.25
mm/s; powder feed rate: 0.2 g/s) was kept constant for both
deposits. Helium is used to prevent oxidation and to provide
shielding.
Table 1:- Chemical Composition in Weight
Percent
2.3 Heat Treatment
In this study the aging effect of Inconel 718 is analyzed on three
different specimens at different working conditions. First study
has been conducted at no aging condition and other two
specimens are at under the heat treatment HT-1 and HT-2 aged
at different conditions. Conditions for heat treatment HT-1 and
HT-2 are as follows-
Table 2:- Process Parameters for HT-1 and HT-
2
HT-1 955ºC for 1hr; air cool to
room temperature
718ºC for 8hr; furnace cool
to 621ºC
621ºC for 18hr; air cool to
room temperature
HT-2 955ºC for 1hr; air cool to
room temperature
718ºC for 8hr; air cool to
room temperature
The metallographic analyses were conducted before and after
the aging heat treatment. The samples were polished on emery
paper in the descending order of coarseness i.e. 100, 200, 900,
1200 and 1500 grit Sic paper. Waterless kalling solution(CuCl2,
HCl and C2H5OH) has been used during the experiments for
better visibility of microstructure through metallurgical digital
microscope (Lieca).Precipitate phase analysis is conducted
using X-Ray Diffraction machine (XRD) and thereafter the
graph is analyzed using X-pert high score software.
Furthermore, the specimen is grinded and polished for the
hardness testing using Vickers hardness testing machine 10kg
of load for 10 sec.
3. Results and Discussion
Ni Cr Fe Nb Mo Ti Al
53.5 19 18.5 4.5 3 1 0.5
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Page 190 of 193
Fig 1 (a, b, c):- Optical Microstructure of as Received at 10x,
20x and 50x
The microstructure shows that laser pattern is bidirectional. Fig
c shows the zig zag pattern during the deposition at an angle of
+45⁰ and -45⁰. Fig 2 the optical microstructure of single stage
aging shows that there is no grain boundaries parent in the
microstructure. XRD shows precipitate Fe0.64Ni0.36 and
Cr.19Fe.70Ni.11 present in the matrix.
Fig 2 (a, b):- Optical Microstructure of Single Stage Aging at
50x, 100x
Optical microstructure for two stage aging shows dendritic
growth with lower magnification than that of single stage aging.
Hardness has been improved with long time period.
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Page 191 of 193
Fig 3:- Optical Microstructure of Single Stage Aging at 50x,
100x
XRD analysis the phase obtained for HT-1 and HT-2. In single
stage aging hardness is low due to presence of orthorhombic
crystal structure of Mo1.24Ni0.76. Taking about the average
hardness of the specimen recorded as 260.2 VHN. Hardness
goes up with heat treatment of HT-2 and HT-1 397.8VHN and
345 VHN respectively. Single stage aging (HT-2) shows
increase in hardness by 34% due to precipitate formation while
two stage aging (HT-1) shows the less hardness due to
formation of acicular phase.
3.1 Tensile Test Result
As indicated in the table, two stage aging specimen has higher
stress values than that of single stage aging. The tensile
properties influence by the crystallographic texture and the
distribution of the precipitates.
Table 3:- Results of Tensile Tests of Age Hardened Samples
Compared with as-Received Sample of Inconel 718
4. Conclusion
Laser processing has bidirectional optical microstructure.
Inconel 718 single crystal is deposited on the polycrystalline
stainless steel substrate. It has been observed that single stage
has more precipitate than two stage aging. XRD analysis shows
that two stage aging results in less hardness due to acicular
phase while single stage aging shows high hardness due to
absence of delta phase. Mechanical properties improve with the
application of heat treatment and experiments indicate that the
strength of two stage aging sample is higher than the single
stage but the reduction in ductility.
4.1 Future Scope of Work
Sample show high corrosive resistance even after exposed to
the atmospheric condition. Corrosion studies should be carried
out in detail for future research and development. Inconel718
usually applicable for high temperature conditions, at such
condition creep failure are dominating in nature therefore study
of creep behavior should be done.
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Hardness
500
400
300
200
100
0
as received single two
sample stage(HT2) stage(HT1)
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Page 192 of 193
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