Journal of Manufacturing Processes 56 (2020) 106–115

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Journal of Manufacturing Processes

journal homepage: www.elsevier.com/locate/manpro

Cladding Inconel 625 on cast iron via bypass coupling micro-plasma arc T
welding
Jiankang Huanga,*, Shien Liua, Shurong Yub,*, Liang Anc, Xiaoquan Yua, Ding Fana,
Fuqian Yangd,*
a
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metal, Lanzhou University of Technology, Lanzhou 730050, China
b
School of Mechanical and Electrical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
c
School of Bailie Mechanical Engineering, Lanzhou City University, Lanzhou 730070,China
d
Materials Program, Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA

A R T I C LE I N FO A B S T R A C T

Keywords: There is great interest to deposit a layer of Inconel 625 nickel-base super-alloy on the surface of metallic sub-
Bypass coupling micro-plasma arc strate to increase the corrosion resistance for the services in harsh environment. In this work, we use the bypass-
Inconel 625 coupled micro-plasma arc welding to form a layer of Inconel 625 on the plate of QT-400 nodular cast iron and
QT-400 nodular cast iron study the effect of the bypass current on the topology of the Inconel 625 layer. There exists spatial distribution of
Cladding
dendrites in the cladding layer along the direction normal to the bonding interface between the cladding layer
and the plate of QT-400 nodular cast iron. The fusion zone near the interface between the cladding layer and the
plate of QT-400 nodular cast iron has the largest Vickers hardness of 630 HV.

1. Introduction examine the local mechanical behavior of the layer of Inconel 625-
Cr3C2 cermet formed by laser cladding, and found that the indentation
Inconel 625 is a nickel-chromium alloy, which possesses high modulus and hardness of the layer of the Inconel 625-Cr3C2 cermet are
strength, high corrosion resistance [1,2] and wear-resistance at high larger than Inconel 600. Verdi et al. [14] determined the contact
temperature [3,4]. The unique properties of Inconel 625 have made it modulus and indentation hardness of the laser-cladded Inconel 625 on
attractive to the applications in aerospace, including engine exhaust steel by nanoindentation. Feng et al. [15] found the segregation of Mo
systems, fuel and hydraulic line tubing, etc. However, the high cost of and Nb and the formation of fine microstructure after the laser cladding
Inconel 625 has limited its use as structural materials in engineering of Inconel 625 and the increase of the resistance to abrasive wear of the
applications. Instead, surface coating and/or cladding of Inconel 625 laser-cladded Inconel 625. Fesharaki et al. [16] compared the micro-
have been used to improve structural durability and performance of structures of the Inconel 625 coatings formed by laser cladding and TIG
engineering structures. (tungsten inert gas) cladding, respectively, and observed that the laser
There are a variety of techniques available for the surface coating cladding led to the formation of finer microstructure in the coatings.
and/or cladding of Inconel 625, including pulsed gas tungsten arc Using gas tungsten arc welding, Wang et al. [17] constructed an Inconel
welding [5,6], laser cladding [7,8], cold metal transfer welding [9], gas 625 “wall” and examined the spatial distribution of microstructure and
metal arc welding [10], etc. Using laser cladding, Abioye et al. [11] mechanical behavior. They observed the transition for directional
formed a layer (cladding beads) of Inconel 625 on 304 stainless steels, dendrites near the bottom layer to equiaxed microstructure near the top
and studied the corrosion resistance of the cladding beads. Verdi et al. layer. It needs to be pointed out that the high cost and high require-
[12] investigated the evolution of the microstructure of the Inconel ments for workpiece assembly for laser cladding have limited its ap-
625, which was laser-cladded on ferritic and stainless steels, at high plications in industry. The poor weldability of QT-400 nodular cast
temperatures in air, and suggested that the substrate (ferritic and associated with the high content of carbon makes it very difficult to use
stainless steels) has no observable effects on the evolution of micro- traditional arc welding techniques to clad Inconel 625 of high quality
structure and indentation response of the Inconel 625 heat-treated at on QT-400 nodular cast iron in contrast to the arc-cladding of Inconel
high temperatures. Chang et al. [13] used indentation technique to 625 on stainless steels. Thus, it is of practical importance to develop a

⁎
Corresponding authors.
E-mail addresses: sr2810@163.com (J. Huang), yushur1991@163.com (S. Yu), fyang2@uky.edu (F. Yang).

https://doi.org/10.1016/j.jmapro.2020.03.058
Received 11 January 2020; Received in revised form 15 March 2020; Accepted 29 March 2020
1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
J. Huang, et al. Journal of Manufacturing Processes 56 (2020) 106–115

Table 1
Chemical compositions of Inconel 625 wire and QT-400 nodular cast iron.
Material Element

C Si Mn P S Fe Mg Cr Ni Mo Nb

QT400 3.4 3.2 0.2 0.06 0.01 － 0.04 － － － －

Inconel 625 0.012 0.04 < 0.01 0.03 0.05 0.40 － 22.65 64.24 8.73 3.5

Fig. 1. (a) Schematic of the bypass-coupled micro-plasma arc processing, and (b) optical image of the deposition of Inconel 625 on a substrate by the bypass-coupled
micro-plasma arc processing.

“new” technique for the cladding of Inconel 625 of on QT-400 nodular Realizing that the surface coating of Inconel 625 can improve the
cast iron. structural durability and performance of engineering structures, we
The bypass coupling arc welding was first proposed by Zhang and explore the feasibility of using the bypass-coupled micro-plasma arc
co-workers [18–20]. This method not only can control the heat input of processing to deposit Inconel 625 on QT-400 cast iron, and study the
the base metal and realize the high efficiency welding by adjusting the morphological variation of the cladding layer with the processing
bypass current but also can ensure the high melting efficiency. Chen parameters. The microstructure and hardness of the cladding layer are
et al. [21] explored the possibility of using GTAW as the main circuit also analyzed.
and welding wire as the bypass. Their results suggested similar me-
chanism for the melting of the welding wire to GMAW and the im-
provement of the deposition rate. Using bypass-coupled micro plasma 2. Experimental details
to control the heat input via the bypass current, Huang et al. [22] de-
monstrated the formation of good welding. Currently, most studies Plates of QT-400 nodular cast iron with dimensions of 100 × 150 ×
have suggested that the bypass-coupled arc welding has great potential 10 mm3 were used for the cladding of Inconel 625. The plates were
in many areas. mechanically ground and cleaned with acetone sequentially to remove
surface debris and contaminants prior to the cladding of Inconel 625.

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Fig. 2. Schematic of experimental setup.

Fig. 3. Geometric dimensions of the cladding beads and the positions for the measurement of temperature.

Table 2 other one passing through the welding wire and an adjustable resistor
Welding parameters. to the positive pole of the welding power source. There is
1
Welding case Iz (A) Ip (A) Feeding rate (cm⋅ min-1)Welding speed(mm⋅ ) Iz = Im + Ip (1)
min-⋅ min−1)

1 50 0.83 70 80 where Iz is the resultant current passing through the tungsten torch, Im
2 50 7.64 70 80 is the current passing through the workpiece, and Ip is the bypass cur-
3 50 13.87 70 80 rent passing through the bypass melting electrode. Under the condition
4 50 17.89 70 80 of the same resultant current, the current passing through the work-
piece, Im, can be adjusted by changing the bypass current, Ip. For the
same value of the current, Im, increasing Ip by reducing the resistance of
Wire of Inconel 625 with 1.2 mm in diameter (ERNICrMo-3, Lough-
the adjustable resistor can increase the resultant current, resulting in
borough, UK) was used in the cladding. Table 1 lists the chemical
the increase of the current for the melting of the filler wire, i.e. the
compositions of the Inconel 625 wire and QT-400 nodular cast iron.
increase of the heat input, and the improvement of the welding power.
The setup for the bypass-coupled micro-plasma arc processing
Fig. 2 shows a schematic of the experimental setup for the cladding
consists of a plasma welding torch and a bypass melting-electrode-
of Inconel 625 on QT-400 nodular cast iron. Two Hall-current sensors
welding torch, as shown schematically in Fig. 1a. Fig. 1b demonstrates
were used for the monitor of electric current and electric voltage. Three
the cladding process on a substrate by the bypass-coupled micro-plasma
K-type thermocouples were used to measure the temperature variations
arc processing. The micro-plasma torch is connected to the negative
at three different locations. The computer was used to control the
pole of a micro-plasma arc welding machine, and both the workpiece
welding process
and the bypass melting torch are connected to the positive pole of the
Fig. 3 depicted schematically the locations, A, B, and C, of the three
machine. There are two independent electric currents passing through
K-type thermocouples during the cladding. The thermocouples were
the torch at the tip of tungsten electrode, one passing through the
placed on the top surface of the substrate with the distances to the
workpiece to the positive pole of the welding power source, and the
central line of the cladding being 10, 20 and 30 mm for A, B and C,

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Fig. 4. Temporal evolution of temperatures at the positions of A, B, and C for different bypass currents (Iz = 50 A).

current of 40 mA.
The Vickers hardness of the cladding beads, the material in the fu-
sion zone and the base metal were measured on a Vickers hardness
tester (HVS-1000) at a maximum load of 9.8 N. The dwell time was 15
s.

3. Results and discussion

Fig. 4 shows the temporal evolution of the temperatures at the po-

sitions of A, B, and C for different bypass currents with Iz = 50 A. It is
evident that the temperatures at all the three positions increase first
with the increase of the cladding time, reach maximum, and then de-
crease with the increase of the cladding time for all the bypass currents
used in this work. During the cladding, the temperature at the position
of A is the first to reach the maximum, which is the largest one, and the
Fig. 5. Variation of maximum temperatures at positions of A, B and C with temperature at the position of C is the last to reach the maximum,
bypass current.
which is the smallest one. Such behavior is associated with the heat
input from the micro-plasma torch, the heat release to the environment,
respectively. Table 2 lists the welding parameters used in the cladding which are dependent on the welding speed, the bypass current and the
of Inconel 625. distance between micro-plasma torch and temperature test positions.
Multiple samples were obtained from the cladding beads by the It is known that it takes time for heat (thermal energy) to propagate
wiresaw cutting. The surface of the samples were mechanically ground through materials. According to the theory of thermal conduction, the
and polished to obtain mirror-like surface. The polished surface was thermal diffusivity is proportional to the thermal conductivity of the
etched in a solution consisting of 10 g FeCl3, 10 ml HCl, 10 ml HNO3 material and inversely proportional to the specific heat capacity and
and 10 ml deionized water. Optical imaging was performed over the density of the material. The time for a spatial position with a distance,
etched surfaces on a metallographic microscope (Olympus GX51), and x, to the heat source to reach a temperature is a monotonic increasing
the optical images were used to determine the geometrical character- function of the ratio of the square of the distance to the thermal con-
istics, H (height) and W (width) of the cladding beads, as shown in ductivity. That is to say, increasing the spatial distance to the heat
Fig. 3. The results reported in this work are average values of three source increase the time for the heat to propagate to the designed po-
different cross sections of the same cladding bead. sition, which supports the results shown in Fig. 4. The differences in the
The chemical composition of the cladding beads was analyzed on a maximum temperature at three different positions can be attributed to
scanning electron microscope (SEM) (JSM-6701F) with an energy dis- the loss of the heat, the distance to the arc torch and the motion of the
persive spectrometer (EDX). The X-ray diffraction (XRD) analysis of the base metal during the cladding. The larger the distance to the heat
structures of the cladding beads was conducted on a Ragiku Smart Lab source (the arc torch), the more is the heat loss. Note that the heat input
X-ray diffractometer, using Cu Ka radiation at a voltage of 40 kV and a is a function of Im and Ip.

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Table 3
Typical topologies and cross-sections of the cladding beads of Inconel 625 formed with different bypass currents at a feeding rate of 70 cm/min for the filler
wire.
Bypass current Ip (A) Topology of cladding layer Cross section

0.83

7.64

13.87

17.89

Fig. 7. (a) A layer of the cladding beads on the surface of a plate of QT-400
nodular cast iron, and (b) optical image of the cross-section of the layer of the
cladding beads (welding speed: 80 mm/min, feeding speed: 70 cm/min, Iz = 50
A, Ip = 7.64 A).

C with the bypass current. The maximum temperatures increase first

with the increase of the bypass current, reach maximum, and then
decrease with the increase of the bypass current under the condition
that the total current is maintained constant. Such behavior can be
attributed to the dependence of the heat input on the bypass current.
There are two sources contributing to the heat input; one is the arc heat
and the other is Joule heat. Increasing the bypass current leads to the
decrease of the current passing through the workpiece, resulting in the
decrease of Joule heat. However, there is the contraction of the arc,
leading to the increase of the arc heat. When the increase of the arc heat
Fig. 6. Dependence of the geometrical dimensions of the cladding beads on is larger than the decrease of Joule heat, the maximum temperature
bypass current; (a) height and width, and (b) aspect ratio of W/H. increases with the increase of the bypass current. Otherwise, the max-
imum temperature decreases with the increase of the bypass current, as
shown in Fig. 5.
From Fig. 4, the maximum temperatures were obtained at all three
Table 3 shows typical topologies and cross-sections of the cladding
positions of A, B and C for different cladding conditions. Fig. 5 shows
beads of Inconel 625 formed with different bypass currents at a feeding
the variation of the maximum temperatures at the positions of A, B and

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Fig. 8. Optical micrographs: (a) the cross-section of a layer of the cladding beads showing four positions of I, II, III and IV for the analyses of microstructures, and (b)
the microstructures at the four positions of I, II, III and IV.

rate of 70 cm/min for the filler wire. It is evident that continuous the bypass current, while the width of the formed cladding beads de-
cladding beads were formed for all the welding conditions. However, creases with the increase of the bypass current. This trend suggests that
the cladding beads exhibit different cross-sections, which reveal the decreasing the bypass current enhances the nominal wetting between
effect of the bypass current on the deposition of molten droplets and the the Inconel 625 and the QT-400 nodular cast iron during the cladding,
cladding. The largest cladding bead was formed at the bypass current of which improves the spreading of the cladding beads and the bonding
7.64 A, which likely is associated with the heat input and the heat between the cladding beads and the substrate. Note that a small bypass
transfer during the cladding. current led to an increase in the dilution rate. For example, the cladding
To examine the effect of the bypass current on the dilution during with low dilution rate was present at a bypass current of 7.64 A.
the cladding, a yellow line representing the boundary of the fusion zone Pinkerton and Li [23] suggested that the quality of a cladding is
was drawn on the cross-section images in Table 3. It is evident that dependent on the aspect ratio. That is to say, the aspect ratio of clad-
increasing the bypass current decreased the size of the fusion zone and ding likely is an important parameter used to assess the quality for
the dilution ratio. In general, increasing the dilution ratio reduces the overlapping deposition. Fig. 6(b) shows the variation of the aspect ratio
cladding efficiency, while decreasing the dilution ratio can cause weak of W/H with the bypass current. The aspect ratio of W/H decreases with
bonding between the cladding layer and the base metal. the increase of the bypass current. Such behavior is due to the size
From the optical images of the etched cross sections, such as those in dependence of the welding droplet on the heat input used for the
Table 3, we determined the geometrical characteristics, H (height) and melting of the filler wire and the dependence of the contact angle on the
W (width) of the cladding beads. Fig. 6(a) shows the variations of the temperature of substrate. For the same feeding speed and welding
height and width of the formed cladding beads with the bypass current. speed, the size of the welding droplet decreases with the decrease of the
The height of the formed cladding beads increases with the increase of current passing through the torch, i.e. increasing the bypass current

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Fig. 9. (a) SEM image of fusion zone; EDS mapping of fusion zone in a cladding bead: (b) Fe, (c) Ni, (d) Cr, (e) Mn, (f) Nb.

causes the decrease of the size of the welding droplet. We can conclude optical microscope. Four different regions of I, II, III and IV, as shown in
that increasing the bypass current results in the decrease of the ratio of Fig. 8a, were selected. Fig. 8b shows the optical micrographs of the
W/H. cladding beads around the four different positions of I, II, III and IV.
Fig. 7(a) shows a layer of the cladding beads formed on the surface Near the surface of the layer of the cladding beads, extensive coaxial
of a plate of QT-400 nodular cast iron under the condition of a welding dendrites (Fig. 8b-I) are present due to the heat transfer along lateral
speed of 80 mm/min and a feeding speed of 70 cm/min for the filler and normal directions. The columnar dendrites are present in the
wire of Inconel 625. The total current and the bypass current were 50A middle of the layer of the cladding beads (Fig. 8b-II), suggesting the
and 7.64 A, respectively. It is evident that a relatively dense layer of presence of large temperature gradient along the thickness direction
Inconel 625 was deposited on the surface of the plate of QT-400 nodular and relatively uniform distribution of temperature along the lateral
cast iron, and there are no lag inclusions, cracks and other defects ob- direction. Random branching of dendrites (Fig. 8b-III) is observed near
servable by optical microscopy. Relatively good bonding was formed the interface between the cladding beads and the plate of QT-400
between the layer of the cladding beads and the plate of QT-400 nod- nodular cast iron. Such behavior can be attributed to the fluctuation of
ular cast iron, as shown in Fig. 7(b). local temperature associated the flow of the molten metal in the molten
The microstructures of the cladding beads were analyzed on an pool. Fig. 8b-IV shows the microstructures around the interface

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Fig. 10. EDS line scans across (a) the interface between the layer of cladding beads and the plate of QT-400 nodular cast iron, and (b) the interface between two
adjacent claddings.

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limited migration rates. There is a fusion layer with a thickness of ∼0.3

mm, in which there are gradients of the concentration of Fe, Ni, Cr, Nb,
and Mn. At the molten state, atoms can migrate at relatively large rates
from the positions of high concentration to those of low concentration
to form the concentration gradients after solidification. There are no
observable differences in the concentrations of Fe, Ni, Cr, Nb, and Mn
between two adjacent claddings, suggesting relatively uniform dis-
tribution of Fe, Ni, Cr, Nb, and Mn in the layer of the cladding beads.
Fig. 11 shows the XRD pattern of the layer of the cladding beads, FZ
(fusion zone) and QT-400 nodular cast iron. For comparison, the XRD
pattern of the filler wire of Inconel 625 is also included in Fig. 11. There
is no significant difference between the XRD pattern of the filler wire
and that of the cladding beads, suggesting that the cladding beads have
the same crystal structures as Inconel 625. There exists γ-Ni phase
confirming the presence of Ni-based superalloys, which can be attrib-
Fig. 11. XRD patterns of a layer of cladding beads and Inconel 625. uted to the filler wire of Inconel 625.
The hardness tests were performed on the cross-section of the
cladding layer in the directions normal and parallel to the interface
between two adjacent claddings. There is a band of coarse coaxial
between the cladding layer and the plate of QT-400 nodular cast iron.
dendrites sandwiched between two fine coaxial dendrites in each in-
Fig. 12 shows the spatial distribution of Vickers hardness over the cross-
dividual cladding bead. The formation of coarse coaxial dendrites is due
section of the cladding layer. It is evident that the Vickers hardness of
to the remelting of the cladding bead formed in previous deposition,
the cladding layer varies in a range of 285 to 250 HV along the direc-
resulting in small temperature gradient and growth rate for the local
tion normal to the interface and is larger than ∼250 HV of the QT-400
growth of dendrites. We can conclude from the recrystallization that a
nodular cast iron. The Vickers hardness in the fusion zone reaches 630
relatively good joint was formed between adjacent claddings for the
HV due to the residual stress associated with the thermal mismatch
bypass current of 7.64 A.
between the QT-400 nodular cast iron and the Inconel 625. According
Fig. 9(a) depicts a SEM image of the fusion zone in a cladding bead.
to Fig. 12, the Vickers hardness of the materials in the cladding layer,
The EDS mapping of the fusion zone was performed. Fig. 10(b)-(f)
which is ∼0.3 mm away from the boding interface, varies in a range of
shows the distribution of elements in the fusion zone. It is evident that
325 to 280 HV. Such a variation can be attributed to the variation of
there are elements of Fe, Ni, Cr, Mn and Nb present in the fusion zone.
local microstructures/dendrites and chemical compositions, as shown
All of the elements distribute relatively uniformly. There are fewer
in Figs. 8 and 9, since the mechanical response of a material is de-
amounts of Ni, Cr, Mn and Nb in the fusion zone on the side of the
pendent on microstructures and chemical compositions. Note that the
workpiece, revealing that the dilution rate of the workpiece is relatively
Vickers hardness of the QT400 nodular cast iron is in a range of
low during the cladding.
250–270 HV.
Fig. 10 shows EDS line scans across the interface between the layer
of the cladding beads and the plate of QT-400 nodular cast iron and the
interface between two adjacent claddings. It is evident that the ele- 4. Conclusions
ments of Fe, Ni, Cr, Nb, and Mn distribute relatively uniformly in the
layer of the cladding beads of Inconel 625. There are no Ni, Cr, Nb, and In this work, we have demonstrated the feasibility of cladding a
Mn present in the plate of QT-400 nodular cast iron, as expected, due to layer of Inconel 625 on the surface of the plates of QT-400 nodular cast

Fig. 12. Spatial distribution of Vickers hardness over the cross-section of the cladding layer.

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iron via the bypass-coupled micro-plasma arc welding with the filler [6] Wang JF, Sun QJ, Wang H, Liu JP, Feng JC. Effect of location on microstructure and
wire of Inconel 625 at the welding speed of 80 mm/min and the total mechanical properties of additive layer manufactured Inconel 625 using gas tung-
sten arc welding. Mater Sci Eng A 2016;676:395–405.
current of 50 A. The geometrical dimensions of the cladding beads are [7] Abioye TE, Folkes J, Clare AT. A parametric study of Inconel 625 wire laser de-
dependent on the bypass current. The height of the cladding beads position. J Mater Process Technol 2013;213:2145–51.
increases with the increase of the bypass current, while the width of the [8] Abioye TE, Farayibi PK, Clare AT. A comparative study of inconel 625 laser cladding
by wire and powder feedstock. Mater Manuf Process 2017;32:1653–9.
cladding beads decreases with the increase of the bypass current. [9] Prabu SS, Ramkumar KD, Arivazhagan N. Effect of filler metals on the mechanical
Dendritic structures are formed in the cladding layer, and the properties of Inconel 625 and AISI 904L dissimilar weldments using gas tungsten arc
morphologies of the dendrites vary with the distance to the bonding welding. Mater Sci Eng 2017;263:062072.
[10] Frei J, Alexandrov BT, Rethmeier M. Low heat input gas metal arc welding for
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plate of QT-400 nodular cast iron. There is seldom migration of the 2016;60:459–73.
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protection. J Mater Process Technol 2015;217:232–40.
iron. The Vickers hardness in the fusion zone reaches 630 HV, which is
[12] Verdi D, Garrido M, Múnez C, Poza P. Microscale evaluation of laser cladded
much larger than QT-400 nodular cast iron and Inconel 625. Inconel 625 exposed at high temperature in air. Mater Des 2017;114:326–38.
[13] Chang C, Verdi D, Garrido MA, Ruiz-Hervias J. Micro-scale mechanical character-
Declaration of Competing Interest ization of Inconel cermet coatings deposited by laser cladding. Boletin de la
Sociedad Espanola de Ceramica Y Vidrio 2016;55:136–42.
[14] Verdi D, Garrido M, Múnez C, Poza P. Mechanical properties of Inconel 625 laser
None. cladded coatings: depth sensing indentation analysis. Mater Sci Eng A
2014;598:15–21.
[15] Feng K, Chen Y, Deng P, Li Y, Zhao H, Lu F, et al. Improved high-temperature
Acknowledgement hardness and wear resistance of Inconel 625 coatings fabricated by laser cladding. J
Mater Process Technol 2017;243:82–91.
[16] Naghiyan Fesharaki M, Shoja-Razavi R, Mansouri HA, Jamail H. Evaluation of the
JKH and SRY are grateful for the financial support by the National hot corrosion behavior of Inconel 625 coatings on the Inconel 738 substrate by laser
Natural Science Foundation of China (No. 51665034). and TIG cladding techniques. Opt Laser Technol 2019;111:744–53.
[17] Wang J, Sun Q, Wang H, Liu J, Feng J. Effect of location on microstructure and
mechanical properties of additive layer manufactured Inconel 625 using gas tung-
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