jmbm-2017-0020

J Mech Behav Mater 2017; aop
S. Gnanasekaran*, G. Padmanaban, V. Balasubramanian, Hemant Kumar and Shaju K. Albert
Optimizing the laser parameters to attain
maximum hardness in nickel based hardfacing
surfaces
https://doi.org/10.1515/jmbm-2017-0020
Abstract: In this investigation, an attempt has been
made to optimize the laser hardfacing (LH) parameters
such as power, powder feed rate (PFR), travel speed and
defocusing distance to maximize hardness of Ni-based
hardfacing surfaces. Statistical tools such as the design
of experiments (DoE), analysis of variance (ANOVA) are
used to develop the empirical relationship to predict the
hardness of the deposits at the 95% confidence level.
Response graphs and contour plots are constructed
using response surface methodology (RSM) concept.
From this investigation, it is found that the maximum
hardness of 820.48 HV could be achieved for the deposit
made using a power of 1314 W, PFR of 9 g/min, a travel
speed of 366 mm/min, and a defocusing distance of
32 mm.
Keywords: analysis of variance; austenitic stainless steel;
design of experiment; hardness; laser hardfacing; optimization; response surface methodology.
1 Introduction
Surface modification of traditional materials (steels, aluminum alloys, titanium alloys, etc.) by laser hardfacing
(LH) is attracting more and more due to its momentous
growth in industrial applications. Laser surface modification is carried out by intruding the laser energy on
the substrate with the addition of desired peripheral
ingredients (powder). This results in high-performance
*Corresponding author: S. Gnanasekaran, Centre for Materials
Joining and Research (CEMAJOR), Department of Manufacturing
Engineering, Annamalai University, Annamalainagar-608002,
Tamil Nadu, India, e-mail: gnanas[email protected]
G. Padmanaban and V. Balasubramanian: Centre for Materials
Joining and Research (CEMAJOR), Department of Manufacturing
Engineering, Annamalai University, Annamalainagar-608002,
Tamil Nadu, India
Hemant Kumar and Shaju K. Albert: Materials Technology
Division, Indira Gandhi Centre for Atomic Research (IGCAR),
Kalpakkam-603102, Tamil Nadu, India
exteriors (high in wear, oxidation resistance, corrosion
and fatigue) on a low-priced low-alloy bulk material with
reduced metal cost and rare alloy elements. The laser
energy can also be effortlessly tuned to obtain the anticipated surface properties [1–3]. An extensive demand
for materials with improved properties in terms of their
hardness and their resistance to wear, corrosion, and
oxidation has been the driving force for the development
of various surface hardfacing techniques and materials.
Recently, LH has been explored for the deposition of less
diluted and fusion-bonded thick and thin metallic coatings on a wide variety of metallic substrate materials
with a low heat input. Austenitic stainless steels (AISI
316 LN) are widely used in nuclear reactors owing to their
excellent corrosion resistance. However, their low hardness leads to poor surface properties in terms of wear
and fatigue resistant which confine many of their industrial ­applications [4, 5].
Abundant studies have shown superior characteristics of hardfacing alloys deposited by laser surfacing
compared to other conventional surfacing techniques
[5, 6]. The LH technique has the benefit of depositing a
precise thickness of the material on a selected area of the
substrate. Cobalt- and nickel-based intermetallic alloys
have been developed as an excellent corrosion- and wearresistant material over a wide range of temperatures and
environments [7]. Nickel-based alloys are strengthened by
the presence of large volume fraction of hard intermetallic laves phase in a much softer solid-solution phase or a
eutectic phase mixture [8]. Therefore, tribology is seldom
applied in bulk form, but rather are applied as coatings
[9, 10]. Cobalt-base alloys have been extensively studied
as overlays by laser cladding technique [11] as well as
thermal spray technique [12].
Hemmati et al. [13] reported that dilution of the
hardfaced deposit is influenced by laser power. Percentage of dilution is 5, 10, 15, 25, 30 and 35%, respectively.
It is discovered that for Fe content rises, borides start to
alteration in morphology and slowly reduce. Also, significantly lower amounts of Ni-Si-B eutectic phase’s form
because of more Fe content. Abolition of the strengthening precipitates let’s to lower the hardness of the
deposits (800–500 HV). Ming et al [14] investigated the
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2 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
Table 1: Chemical composition (wt%) of substrate material (AISI 316LN).
C
0.020
Ni
Cr
Mo
Si
Mn
Cu
Nb
S
P
W
12.55
17.27
2.35
0.29
1.69
0.047
0.02
0.027
0.026
0.03
effect of powder feed rate (PFR) and translation speed
(TS) of a laser cladding with three nickel-based hardfacing powders. Deposits produced at higher PFRs registered a higher hardness than deposits made with lower
PFRs. Higher PFR produces a higher percentage of hard
phases and, the percentage of hard phases decreases
linearly with increasing TS. Zhang et al. [15] attempted
a hardfacing by Colmonoy 6 powder on 316 L austenitic
stainless steel using CO2 laser process. They investigated
the effects of laser power, traveling speed, defocusing
distance, PFR on bead height, bead width, penetration
depth and dilution. They found that preheating is essential for preventing cracking in the LH procedure and
450°C is the suitable preheating temperature. The friction and wear test results showed that the friction coefficient of specimens with laser cladding is lower than that
of specimens without laser cladding, and the wear resistance of specimens has been increased 53 times after laser
cladding, which reveals that laser cladding layer plays
a major role in wear resistance. The microstructures of
laser cladding deposit are composed of Ni-rich eutectic,
boride and carbides.
It is well known that the hardfacing parameters plays
a major role in determining the deposit quality as the
process facts have not been disclosed so far and hence, the
selection of LH process parameters to nickel-based alloys
is very difficult. Response surface methodology (RSM) is
a collection of mathematical and statistical techniques
useful for the modeling and analysis of problems in which
a response of one interest is influenced by several variables and the objectives are to optimize this response [16].
In this investigation, RSM was used to reduce the number
of experiments and optimize the process parameters that
yield the higher hardness.
It is understood that the hardfacing process parameters play a major role in deciding the quality of the
deposits. Very limited investigations have been carried
out to understand the effect of individual LH parameters
on mechanical properties and microstructural characteristics. There is no literature available in optimizing the
LH parameters to attain maximum hardness on nickel
based hardfaced deposit on 316 LN austenitic stainless
steel. Hence, in this study, an attempt has been made to
optimize the important LH parameters to attain maximum
Fe
Bal
hardness in nickel based hardfaced deposits on 316 LN
austenitic stainless steel by RSM.
2 Experimental work
2.1 Identify important LH parameters
The chemical composition of base metal and hardfaced
powder are presented in Tables 1 and 2, respectively. The
important LH parameters were identified and selected
from the literature [15–17]. They are power (P), powder
feed rate (F), travel speed (T), defocusing distance (D).
Table 3 shows the physical properties of Colmonoy – 5
powder used for LH. Laser hardfacing system consists of
solid state disc laser Trumpf TruDisk 4002, model which
emits wavelength of 1030 nm and maximum available
power is 4 kW.
2.2 Feasible working range of LH parameters
Trial runs were carried out using 12 mm-thick 316 LN
austenitic stainless steel plate and nickel based alloy to
find out the feasible working limits of LH parameters.
The optimization process was carried as per the flow
chart (Figure 1). Different combinations of parameters
Table 2: Chemical composition (wt.%) of hardfaced powder
(Colmonoy-5).
C
0.41
Fe
Cr
Si
B
O
3.10
10.44
4.02
2.26
0.03
Ni
Bal
Table 3: Physical properties of hardfaced powder.
Flow rate
(s/50 g)
16
App. density
(g/cm3)
Hardness
(HRC)
Particle size
(mesh) μm
4.50
51–54
±45 to 180
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 3
Figure 1: Flow chart for process optimization.
were used to carry out the trial experiments. This was
done by varying any one of the factors from minimum
to maximum while keeping the other parameters at constant (Table 4). The feasible working limits of the individual parameters were identified by macrostructure
(cross section of the deposits). A deposit which showed,
a smooth appearance without any visible macro level
defects such as crack, pores was chosen as the feasible
working parameter. The chosen levels of the selected
process parameters with their units and notations are
presented in Table 5.
2.3 LH experiments and hardness evaluation
Figure 2 shows the multi-track hardfaced deposit configuration with 50% overlapped used in this investigation.
The base metal composed of fully elongated austenitic
grains. The LH deposits were made as per the conditions
dictated by the design matrix (Table 6) at random order
so as to avoid the noise creeping output response. The
substrate was preheated to 400°C to relieve the internal
stresses and also to reduce the cooling rate to avoid the
formation of cracks after deposition [15]. The average
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4 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
Table 4: Macrostructure analysis for fixing the working range of laser hardfacing.
S.No
Process parameters
Parameters range
1
Power (P)
2
3
4
Powder feed rate (F)
Travel speed (T)
Defocusing distance (D)
Macrograph
Name of the defect
Reason for defect
P > 1900 W
Crack, dilution
Higher heat input
P < 1100 W
Pores and escaping
of powder
Insufficient heat
input
F > 11 g/min
Cracks
Insufficient
specific energy
input
F < 3 g/min
High depth of
penetration and
dilution
Higher specific
energy input
S > 500 mm/min
Cracks
Low heat input
S < 300 mm/min
High dilution
Higher heat input
D > 37 mm
Poor bonding
Low energy
density per unit
D < 17 mm
Pores
Higher energy
density per unit
deposited thickness was about 0.8–2 mm of the stainless
steel. An automatic disk LH machine was employed to
conduct the experiments. Few of the fabricated deposits
are displayed in Figure 3. After hardfacing, the deposit
was cut into small samples for the metallography
and hardness study. A Vickers microhardness testing
machine (Make: SHIMADZU, Japan; Model: HMV − 2T)
was employed for measuring the hardness across the
hardfaced deposit cross section with a load of 0.5 kg and
dwell time of 15 s.
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 5
Table 5: LH process parameters and their working range.
S. no
Factor
1
2
3
4
Unit
Power
Powder feed rate
Travel speed
Defocusing distance
Notation
W
g/min
mm/min
mm
Levels
P
F
T
D
−2
−1
0
1
2
1100
3
300
17
1300
5
350
22
1500
7
400
27
1700
9
450
32
1900
11
500
37
Figure 2: Single layer hardfacing.
Table 6: Design matrix and experimental results.
Expt. no. 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Coded value P F T D
−1 1 −1 1 −1 1 −1 1 −1 1 −1 1 −1 1 −1 1 −2 2 0 0 0 0 0 0 0 0 0 0 0 0 −1 −1 1 1 −1 −1 1 1 −1 −1 1 1 −1 −1 1 1 0 0 −2 2 0 0 0 0 0 0 0 0 0 0 −1 −1 −1 −1 1 1 1 1 −1 −1 −1 −1 1 1 1 1 0 0 0 0 −2 2 0 0 0 0 0 0 0 0 −1 −1 −1 −1 −1 −1 −1 −1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 −2 2 0 0 0 0 0 0 Actual value Hardness of
the deposit
P F (g/ T (mm/ D
(HV)
(W) min)
min) (mm)
1300 1700 1300 1700 1300 1700 1300 1700 1300 1700 1300 1700 1300 1700 1300 1700 1100 1900 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 5 5 9 9 5 5 9 9 5 5 9 9 5 5 9 9 7 7 3 11 7 7 7 7 7 7 7 7 7 7 350 350 350 350 450 450 450 450 350 350 350 350 450 450 450 450 400 400 400 400 300 500 400 400 400 400 400 400 400 400 22 22 22 22 22 22 22 22 32 32 32 32 32 32 32 32 27 27 27 27 27 27 17 37 27 27 27 27 27 27 573
475
778
603
475
574
794
703
743
568
820
602
545
581
680
648
727
551
487
770
799
721
575
681
769
766
769
770
766
766
Figure 3: Laser hardfaced deposits.
3 D
eveloping an empirical
relationship
Hardness of deposit is a function of the LH parameters
such as power (P), powder feed rate (F), travel speed (T),
defocusing distance (D), and it can be expressed as,
Hardness of deposit = f (P, F, T, D) (1)
Second-order polynomial (regression) equation used
to denote the response surface Y is given by,
Y = b0 + ∑ bi x i + ∑ bi xi2 + ∑ bijxixj
(2)
selected polynomial could be expressed as,
H = b0 + b1 (P) + b2 (F) + b3 (T) + b4 (D) + b12 (PF) + b13 (PT)
+ b14 (PD) + b23 (FT) + b24 (FD) + b11 (P ) + b22 (F ) + b33 (T )
(3)
+ b44 (D2 ) HV
2
2
2
where, b0 is the mean value of response and b1, b2, b3 … b44
are linear interactions and square terms of factors. The
value of co-efficient was calculated using Design Expert
7 software at 95% confidence level. The significance of the
each co-efficient was calculated from t-test and p-values.
The value of “Probe > F” is less than 0.05, indicates that
model terms are significant. In this case P, F, T, D, PF, PT,
FD, TD, P2, F2 and D2 are the significant terms. The final
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6 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
Table 7: ANOVA test results.
Source
Model
P
F
T
D
PF
PT
PD
FT
FD
TD
P2
F2
T2
D2
Residual
Lack of fit
Pure error
Cor total
Std. deviation R2
Adj. R2
Sum of squares (SS)
3.334E + 005
41417.04
1.226E + 005
1751.04
7245.38
12265.56
23485.56
1580.06
6201.56
14220.56
10251.56
33380.36
36063.57
2480.86
48696.50
3195.42
2608.08
587.33
3.366E + 005
14.60
0.9905
0.9816
Degree of freedom
Mean square F-value
p-Value (Prob > F)
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5 29 23817.52 41417.04 1.226E + 005 1751.04 7245.38 12265.56 23485.56 1580.06 6201.56 14220.56 10251.56 33380.36 36063.57 2480.86 48696.50 213.03 260.81 117.47 27.17
47.65
129.73
4.76
8.46
10.09
32.46
1.09
3.00
15.47
11.07
45.15
51.57
1.94
51.89
1.51
Pred. R2
Press
Mean
CV%
Adeq. precision <0.0001 <0.0001 <0.0001 0.0118 <0.0001 <0.0001 <0.0001 0.0157 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0039 <0.0001 0.1959 0.9529 15868.32 671.10 2.17 34.478 Whether
significant or not
Significant
Not significant
empirical relationship was built using only these co-­ expression with the significant terms. The value of adj.
efficient and the established final empirical relationship of R2 = 0.981 is also high and indicates the high significance
laser hardfaced deposit of Colmonoy-5 alloy is given below. of the model.
The pred. R2 value is 0.822 which means that the
Hardness of the deposit (H) = [772.50 − 41.92 (P) + 69.17 (F)
model could explain 82.2% of the variability in prediction.
− 13.25 (T) + 17.67 (D) − 23.62 (PF) + 42.38 (PT) − 7.75 (PD) This is in reasonable agreement with the adj. R2 of 0.952.
The value of the coefficient of variation as low as 2.17,
+ 12.88 (FT) − 29.25 (FD) − 24.75 (TD) − 38.17 (P2 )
which indicates that the deviation between experimental
(4)
− 40.79 (F2 ) − 7.92 (T2 ) − 40.92 (D2 )] HV
and predicted values are low. Adequate measures of the
The adequacy of the above relation is tested by analysis of variance (ANOVA). The ANOVA test results are
given in Table 7 at the desired confidence level of 95%.
The relationship may be considered to be adequate. If
the calculated value of the F ratio of the developed relationship does not exceed the tabulated value of F-ratio
for an anticipated level of confidence, the model is found
to be adequate. The Fisher’s F-test with a very low probability value demonstrates a very high significance of
the regression model. The goodness of fit of the model
is fitted by the determination coefficient (R2). The coefficient of determination was calculated to be 0.99 in
response which implies that 99% of the experimental
values confirm the compatibility with data as predicted
by the model. The R2 value should always be between 0
and 1. A model is statistically good the R2 value should
be close to 1.0. Then adjusted R2 value reconstructs the
Figure 4: Correlation graph.
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 7
Table 8: Confirmation test results for the developed empirical relationship.
S. no
01
02
03
Power
(W)
Powder feed
rate (g/min)
Travel speed
(mm/min)
Defocusing
distance (mm)
Actual
hardness (HV)
Predicted
hardness (HV)
Error (%)
1400
1600
1800
4
6
10
325
375
450
20
24
35
585
658
635
570
625
643
−2.6
−5.2
1.2
signal to noise ratio, a ratio greater than 4 is desirable.
During this investigation, the ratio is 34.47, which indicates an adequate signal. This model can be used to navigate the design space. The correlation graph shown in the
Figure 4, it shows predicted and actual hardness of laser
hardfaced deposit, it could indicate the deviation between
the actual and predicted hardness is low. Table 8 presents
the actual and predicted value of hardness.
4 O
ptimizing LH parameters
The RSM was used to optimize the LH parameters considered in this study. RSM is a collection of a mathematical
and statistical method that are beneficial for designing
experiments, constructing a mathematical model, exploratory for the optimal combination of input parameters
and pressing out the value in graphically [18, 19]. Figure 5
depicts perturbation plot for the response of hardness
of the deposits. This plot offers an outline view of the
response and displays the transformation of hardness,
when each LH parameter moves from the reference point,
with all other parameters held constant as the reference
value. The design of experiment sets the reference point
by default in the middle of the design space. From the
Figure 5: Perturbation graph.
perturbation graph and response surface graphs, it can be
observed that when the hardness increases with increasing PFR, defocusing distance to the certain level and then
decreases. It may be endorsed due to the insufficient
energy or low heat input causes the escaping of powders
and unmelted partials existing in the deposits. Hardness
decreases with increasing laser power, travel speed. It
may be believed that the high heat input will increase the
depth of penetration and dilution of the deposits [15].
To obtain the influencing nature and optimized condition of the process on hardness (H), the surface and contour
plots which are indications of possible independence of
factors have been built up for the proposed empirical relation considering two parameters in the halfway tier, and
two parameters in the X-axis and Y-axis as shown in Figure
6. The contour plots help us to predict the response at any
zone in the design domain [20]. The apex of the response
plot shows the maximum achievable hardness. Characterization involves identifying whether the stationary point is a
minimum or maximum response or a saddle point to classify this; it is more straightforward to analyze it through a
contour plot. Contour plot plays a vital role in the learning
the response surface. It is vibrant from that when the hardness increases with increasing PFR, defocusing distance and
hardness decrease with increasing laser power, travel speed.
To know further about the influencing tendency of
process parameters on hardness, three-dimensional diagrams are plotted for a particular processing condition.
Figure 6A–F as surface and contour plots for each process
parameters. It is clear from Figure 6A that the hardness
falls and increases with an increase of process parameters
such as laser power and PFR. Hardness mainly depends
on dilution and microstructure. Laser power mainly used
for melting the powder and excess heat melts the substrate. Keep on increasing the laser power, high volume of
substrate material melts. The hardness variation in laser
hardfaced sample could affected by the deposit dilution.
As higher dilution means lower hardness, increasing P,
(constant F, T, D) increases dilution and then hardness
decreases. Increasing F (constant P, T, D) diminish dilution so hardness increases because more amount of heat
utilized for melting the powder and small amount heat
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8 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
The valley of response plot gives the maximum hardness. These response contours can help in the prediction
of the response (hardness) in any zone of the experimental
domain [21]. By analyzing the response surface and contour
plots as shown in Figure 6A–F, the maximum achievable
hardness value is found to be 820.48 HV. The corresponding parameters exist maximum hardness value power
of 1314 w, PFR of 9 g/min, a travel speed of 366 mm/min,
melt the substrate material. Increasing the T (constant
P, F, D), powder density per square area (g/mm2) is less
so dilution rate is somewhat increased and hardness is
reduced. Increasing the D (constant P, F, T) diminish dilution so hardness increased. When increasing the defocusing distance, the beam size gets larger so the energy
density per unit of clad pass available becomes less, thus
the penetration depth and dilution decrease.
A
677.5
515
352.5
190
11
1900
9
Power feed rate (g/min)
Hardness (HV)
840
Powder feed rate (g/min)
507.196
Prediction 820.482
7
667.953
507.196
5
3 1100
1300
3
1100
Power (W)
500
595
447.5
300
Travel speed (mm/min)
1700
350
300
1100
1300
1900
Travel speed (mm/min)
742.5
400
587.575
507.196
426.817
890
450
748.332
5
1500
B
500
426.817
1700
7
Hardness (HV)
Hardness (HV)
11
426.817
1300
1500
1700
Power (W)
625.794
1900
722.171
450
625.794
Prediction 820.482
818.549
529.416
350
1500
Power (W)
433.038
300
1100
1300
1500
1700
1900
Power (W)
C
403.25
570
440
310
37
1700
32
27
Defocusing distance (mm)
22
17
1100
1300
1900
1500
Power (W)
Defocusing distance (mm)
489.799
700
Hardness (HV)
Hardness (HV)
37
830
Prediction 820.482
749.445
27
576.348
662.869
22
489.799
17
1100
1300
1500
Power (W)
1700
1900
Figure 6: Response surface graphs and contour plots.
(A) Interaction effect of power and powder feed rate. (B) Interaction effect of power and travel speed. (C) Interaction effect of power and
defocusing distance. (D) Interaction effect of powder feed rate and travel speed. (E) Interaction effect of powder feed rate and defocusing
distance. (F) Interaction effect of travel speed and defocusing distance.
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 9
D
355.219
697.5
456.578
565
432.5
300
500
450
400
350
Travel speed (mm/min)
7
300 3
9
11
Travel speed (mm/min)
Hardness (HV)
Hardness (HV)
500
830
450
557.937
760.654
659.295
400
Prediction
820.482
350
5
Powder feed rate (g/min)
300
3
5
7
11
9
Powder feed rate (g/min)
E
480
300
120
32
27
Defocusing distance (mm)
22
7
17 3
9
11
Defocusing distance (mm)
Hardness (HV)
660
37
Hardness (HV)
37
840
5
Powder feed rate (g/min)
32
Prediction
820.482
722.569
27
600.967
479.364
22
357.762
236.16
17
3
5
7
9
11
Powder feed rate (g/min)
F
660
575
490
32
27
Defocusing distance (mm)
22
17 300
350
400
450
500
Defocusing distance (mm)
Hardness (HV)
604.983
541.214
668.752
745
37
Hardness (HV)
37
830
732.52
32
Prediction
820.482
796.289
27
22
732.52
Travel speed (mm/min)
668.752
604.983
17
300
350
400
450
500
Travel speed (mm/min)
Figure 6 (continued)
and defocusing distance of 32 mm at the valley of response
surface plot and corresponding domain in the contour plot.
Higher F ratio value can give the more significant process
parameter. From the F ratio value, it can be concluded that
the PFR is contributing the major factor to exploit hardness,
followed by power, defocusing distance and travel speed
for the range considered in this investigation. To check the
prediction capabilities of the developed empirical relationship, three more confirmation tests were carried out with
the hardfacing process parameters chosen randomly from
the feasible working range (Table 9). The actual response
was calculated by taking the average of three results. The
developed results reveal that the empirical relationship is
accurate since the variation is ±5%. Table 10 summarize the
experimental values, predicted values and the variation.
Table 11 shows the cross-sectional macrograph, top surface
hardness and indentation image of low, medium and high
hardness laser hardfaced deposits.
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10 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
Table 9: Validation test results for optimization procedure.
S. no
01
02
03
Power
(W)
Powder feed
rate (g/min)
Travel speed
(mm/min)
Defocusing
distance (mm)
Actual
hardness (HV)
Predicted
hardness (HV)
Variation
1386
1414
1329
8.5
9
9
353
437
361
30
25
32
815
809
820
828
821
829
2.7
1.4
1
Table 10: Optimized laser hardfacing parameters.
S. no Main parameters
1
2
3
4
5
Power (W)
Traverse speed (mm/min) Powder feed rate (g/min) Defocusing distance (mm) Preheating temperature (oC) 1300
350
9
32
400
5 M
icrostructure of LH hardfaced
deposits
LH with powder injection was used to produce multitrack deposits with 50% track overlapping. After the LH,
visual inspection of the deposits for both initial compositions revealed a good, continuous appearance without
signs of surface cracks or lack of adhesion, as can be
seen in the micrograph in Figure 7. Cross sections of the
specimens revealed a dendritic structure homogeneously
distributed throughout the deposit with a continuous
interface, of an unrevealed structure, which corresponds
to the dilution zone (Figure 7B). Accordingly, the matrix
may be supposed to be composed of a solid solution
of Ni-γ with a relatively lower iron and silicon content
than the dendrite. Laser-deposited layers in general,
three different microstructures from the bottom to top:
a plane solidification front microstructure with limited
thickness, a transition cellular microstructure, and a
columnar-dendritic microstructure. In addition to that,
Table 11: Cross-sectional macrograph and top surface hardness indentation of laser hardfaced deposit.
Deposit no LH parameters
01
P – 1300 W
F – 5 g/min
T – 350 mm/min
D – 22 mm
04
P – 1700 W
F – 9 g/min
T – 350 mm/min
D – 22 mm
11
P – 1300 W
F – 9 g/min
T – 350 mm/min
D – 32 mm
Macrograph
Hardness with indentation
Hardness (HV0.5)
475
603
820
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 11
Figure 7: Various microstructures of hardfaced deposits observed by OM.
(A) Low magnification cross-section. (B) Hardfaced deposit-interface. (C) Hardfaced deposit-middle. (D) Hardfaced deposit-top surface.
(E) HAZ. (F) Substrate. (G) Precipitates at top of the deposit. (H) Precipitates at near the interface.
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12 S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel
Figure 8: EDS line scan of the constituent elements across substrate/deposit interface (optimized condition).
the evolution of fourth region which consists of equiaxed
grains at the top (Figure 7D). The microstructure at each
region depends on the cooling rate and temperature gradient. In the laser-deposited Ni-based layer, the main
solidification structure is characterized by columnar
crystals at the bottom and equiaxed crystals on the top
surface. Due to the high-temperature gradient (G), and
lower solidification rate (V), at the solid-liquid interface,
epitaxial growth from the substrate occurs in the bottom
area, showing a typical directional solidification characteristic. In the top area of the melt pool, columnar crystals disappear and equiaxed crystals appeared due to the
local lower G and higher V conditions [22].
The interface appears well defined with a zone that
is apparently free of precipitates (Figure 7E). However,
at greater magnification the existence of a new arborescent phase, whose composition is the same as the dendrites indicating an epitaxial growth. In the overlapped
tracks, higher concentrations of dark precipitates are
observed. This may be due to the remelting of the former
track during LH. This remelting will produce higher segregation of eutectics at the interface between overlapping tracks. The existence of these eutectics is mainly
depending on Cr and C concentrations at the particular
region. Laser deposited Colmonoy 5 coatings consists of
three general components: Cr-rich precipitates such as
CrB, CrC, Ni solid solution dendrites, Ni-B-Si binary and
ternary eutectic phases including NiB, NiSi (Figure 7G
and H) [23].
Figure 8 shows the line scan EDS it gathers the values
of each element (wt.%) as a function of the distance from
the substrate, interface and deposit. As can be seen, substrate consist of iron content is around 70–75%. Near the
interface iron content drastically reduced it is conformed
that dilution is very low, iron content passing from 80%
at the interface to 4% in the deposit. The dilution ensures
a perfect bond between the metal and the clad layer, and
on the other hand the low level of dilution confirms the
quality of the coating that’s improve the hardness of the
deposit. The silicon content is around 4%, the chromium
content around 8% and the nickel content between
Figure 9: Scanning electron micrograph of the deposit and spot EDS (optimized condition).
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S. Gnanasekaran et al.: Optimizing the laser parameters to attain maximum hardness in nickel 13
80 and 85% in the deposit. EDS spectra shows that the
precipitates in the cluster (spot 1) are rich in Cr with
fewer additions of C, Si, Ni and Fe as showed in Figure 9.
The second spot EDS are rich in Ni with definite counts
of Cr, Fe and Si as showed in Figure 9 (spot 2). In addition, the EDS spectra reveal the eutectic interdendritic
constituent containing particles of Ni-rich compounds,
preferably Ni-borides.
6 Conclusions
1. An empirical relationship was developed to predict the hardness of nickel-based layer deposited
on 316LN austenitic stainless-steel substrate with
95% confidence level by incorporating important LH
parameters.
2. A maximum hardness of 829.44 HV could be achieved
in the deposit made using laser power of 1314 W, PFR
of 9 g/min, a travel speed of 366 mm/min, and defocusing distance of 32 mm.
3. Of the four LH parameters, the PFR (based on F value)
is the major influencing factor to predict the hardness followed by power, travel speed and defocusing
distance.
Acknowledgments: The authors are thankful to UGC-DAE
consortium for providing financial assistance to carry out
this investigation (Project No. CSR-KN/CRS-56/2013-14/656
dated 04.09.13). Authors wish to express their sincere
thanks to M/s. Geometrix Laser Solutions Pvt Limited,
Tada for laser hardfacing facility. Authors also express
their sincere thanks to The Director, IGCAR, Kalpakkam
for the base metal supply.
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