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Effect of electro spark deposition process parameters on WC-Co coating on H13 steel
Accepted Manuscript Effect of electro spark deposition process parameters on WC-Co coating on H13 steel
M. Salmaliyan, F. Malek Ghaeni, M. Ebrahimnia PII: DOI: Reference:
S0257-8972(17)30385-7 doi: 10.1016/j.surfcoat.2017.04.040 SCT 22283
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
11 June 2016 11 January 2017 17 April 2017
Please cite this article as: M. Salmaliyan, F. Malek Ghaeni, M. Ebrahimnia , Effect of electro spark deposition process parameters on WC-Co coating on H13 steel. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.04.040
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of Electro Spark Deposition process parameters on WC-Co coating on H13 steel *
,
,
CR
IP
T
a ,b,c Department of Materials Science and Engineering, Tarbiat Modares University, P.O. Box 14115-111, Tehran, Iran
ABSTRACT
US
H13 steel is the primary choice for the material of Aluminum Die casting molds. The life of these tools are usually enhanced by the application of WC-Co coating using Electro Spark
AN
Deposition(ESD) process. The objective of this study is to establish the effect of ESD process
M
parameters including voltage, duty cycle, and frequency on the characteristics of WC-Co coating on H13 steel. Design of experiment technique was used and coating thickness, coating hardness,
ED
and occurrence of defects in the coatings were studied. The results indicated that coating cracking
PT
at low spark energy is different from that at high spark energy. Moreover, it was found that by increasing spark energy, coating hardness and thickness increases. It is also shown that process
CE
parameters have interactive effects on the coating features and there is an optimum condition for
AC
achieving a sound coating. The maximum coating hardness was obtained at high spark energies.
Keywords:
H13 steel, electro spark deposition, WC-Co coating, Design of experiment
*
Corresponding author. Tel.: +98 21828844388; fax: +98 2182884390. E-mail addresses: [email protected] (M. Samaliyan), [email protected]
ACCEPTED MANUSCRIPT 1. INTRODUCTION Electro-spark deposition (ESD) is a microwelding fusion process that uses rapid electrical power discharges to accomplish metal transfer from an electrode to a contacting substrate[1][2]. Electrospark deposition has many applications in rebuilding and coating conductive materials. Because of its very low heat input, it is ideal for rebuilding metals which are susceptible to heat affected
IP
T
zone (HAZ) cracking(like H13 die casting molds)[3].
CR
Electro-spark deposition process is a surface treatment technology to meet the special demands, in which pulsed micro-arc generated between the electrode and the substrate, ionizes the air to
US
form high-temperature and high-stress areas in which alloying occurs. It features a highly intensive heat input and a minute heat-affected zone, as well as a high-melting-point composite
AN
coating, which is generated by a momentary temperature as high as ten thousand degrees[4][5].
M
Die casting process has now developed into a very sophisticated manufacturing process that plays an important role in providing products for industries. For examplein automotive industry, about
ED
85-90% of aluminumcomponents areproduced by die casting. Furthermore,the need for high
PT
volume, more accurate dimension, and lower production cost, superior surface finishing and improved mechanical properties of castings is the driving force for the application of new coating
CE
technologies to die casting tools. On the other hand critical areas of die surfaces can be damaged
AC
by melt and thermal fatigue[6][7]. In these cases, by applying protective or rebuilding coatings, life time of die casting molds can be increased. The objective of the current study isinvestigation of the effect of electrical parameterson metallurgical and mechanical properties of WC-Co coating on H13 steel.
2. Material and Method Heat treated hot work tool steel, H13 hadthe following chemical composition (wt %):C 0.37,Cr 5.3, Mo 1.3 , Si 1.08, Mn 0.38 , Nb 0.003 , V 0.83 , W 0.02 ,Fe balanced. The electrode (WC –
ACCEPTED MANUSCRIPT Co)had the following composition(wt %): WC95.7, Co3.7, V0.3, S0.1 and Al0.2 in the form of a round pillar 3.5 mm diameter and 100 mm lenght. Electro spark deposition of WC–Co on H13 steel was accomplished using an ESD machine developed at Tarbiat Modares University. The H13 samples were prepared by wire cutting in the form of round bars 15 mm in diameter and 5 mm height. The bar heads were polished and
IP
T
cleaned for coating trials.
CR
ESD deposition was performed using a hand held gun with a co-axis argon shield gas with a flow rate of 10 L/min.First, a number of tests were performed to establish a suitable range of ESD
US
process and then design of experiment (DOE) was drawn. DOE method that was used in this paper was full factorial method.
AN
Designparameters for DOE were: voltages, frequency and duty cycle. Other parameters including
M
capacity, electrode turning speed, moving speed, gas flow rate and time of deposition were kept constant. The values of variable parameters and constant parameters along with their positions
ED
are shown in Table 1 and 2 and Fig.1 respectively.
PT
Table 1: ESD process variable parameters through the experiments. Variables
and
their
Upper limit
100
180
B: Duty cycle (%)
1
4
C: Frequency (Hz)
100
620
CE
lower limit
AC
designation A: Voltages (v)
ACCEPTED MANUSCRIPT
Table 2: ESD constant parametersthrough the experiments parameters
value 600
Electrode rotation
1500
T
Capacitance ( f)
IP
speed(rpm)
Clockwise
Time of deposition
6
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Rotation orientation
AN
(min)
M
As can be seen, total number of experiment tests arenightheen(sixteen samples at the cubic
ED
corners and three samples at the cubic center). The spark energy shown in table 3 is calculated by using thefollowing equations:
CE
PT
(1)
Where ton, Vm and Im are pulse duration, pulse voltage and pulse current, respectively. Duty
AC
cycle is the ratio of spark on times to the total pulse time (on time + off time). Spark on timein a unit of time(d)is related to duty cycle and can be obtained by multiplying the frequency(f) by pulse duration (
):
(2)
Finally total spark energy
) is calculated as:
ACCEPTED MANUSCRIPT
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IP
T
(3)
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CE
PT
ED
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Fig. 1. Schematic of the design of experiments for ESD coating parameters
ACCEPTED MANUSCRIPT Table 3:Sets of process conditions for the test specimens Specimen
Voltage
Duty cycle
Measured
Frequency
Total
Spark on
No.
(V)
(level)
current
(HZ)
spark
time
energy
(μs)
(A)
36
2
36
100
15
336
US
1
2.1
100
2
180
1
3.15
100
3
100
4
4.5
4
180
4
6.4
100
39
336
5
100
1
0.96
620
0.3
5.6
6
180
1
1.5
620
1
5.6
7
100
4
3.5
620
11
52
8
180
4
4
620
22
52
9
140
3.4
410
6
30
ED
CR
PT
2.5
IP
0.8
AN
100
M
1
T
( )
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For observing coating microstructure of ESD layer and its association with base metal structure, Optical Microscopy (OM)was used.For this purpose, the samples were subjected to the standard
AC
metallographic preparation procedure starting with grinding on SiC grit papers (up to 2000), followed by polishing in alumina particle suspension (3 and 2.5 and 1 lm size). Finally, the specimens were etched in Nital etchant for 10s. Vickers micro-hardness measurement was carried out using a Bohler micro-hardness machine. The load was set at 25 gr with 10 s dwell time.The average of three hardness measurements in coating region were taken into account.Finally, amountof porosity were measured by Imagej software. Further details in measurement of crack and porosity are described in result section.
ACCEPTED MANUSCRIPT 3. Result and discussion The results obtained including coating thickness(as measured by metallography), cracks and porosities, and hardness of the coating are shown in Table 4. Table 4: Test results
) 4
0.27
2
16
1.5
3
18
0.5
4
27
5
20
6
15
7
17
700 800
0.28
750
0.35
0.65
1100
3
0.1
640
1.5
1.2
750
1.8
0.6
1000
29
1.9
1.2
1280
19
1.1
0.4
730
ED
M
US
0.25
AN
20
PT
CE
9
Hardness VHN
1
8
Porosity
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(
Crack
T
Thickness
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specimens
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3.1 Effect of Electrical parameters on coating thickness
Thickness values that are shown in table 4 was measured by metallugrahy method.The most effective parameters on thickness were extracted from Pareto chart. As shown in Fig.2 voltage (A) and Duty cycle (B) have the highest effect on thickness. Furthermore, voltage and duty cycle (AB) have positiveinteractive effect on coating thickness.
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T
ACCEPTED MANUSCRIPT
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Fig. 2. Pareto diagram for coating thickness response
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Figure 3&4 show the relationship of voltage and duty cycle on the thickness. It can be seen that
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figures are center points of the tests.
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coating thickness increases with increasing both voltage and Duty cycle. Red points in following
Fig. 3. Relation between thickness and voltage (B parameter=2.5 %, C parameter =410 HZ)
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T
ACCEPTED MANUSCRIPT
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Fig. 4. Relation between thickness and duty cycle (A parameter = 140 v, C parameter = 410HZ) Response of above observations(Fig. 3 & 4)can be related to total spark energy. By
M
increasingduty cycle or voltage, spark energy increases and leads to an increase in coating
ED
thickness. The peak coating thickness were obtained at maximum totalsparkenergy , see
PT
tables3&4(for example: 39 related to sample 4 and 22 related tosample 8). Further more, the results indicate that the duty cycle at this range of variables has more effect on
CE
coating thickness than voltage. According to Eqs.1, this observation can be related to ton. In other
AC
word, in this range of electrical parameters, effect of increase of ton on thickness is more than effect of increase of voltage. As mentioned earlier it is observed that voltage and duty cycle have a strong interaction. This interaction means, effect of one parameter on the output response depends on the value of the other parameter. As shown in Fig.5,deposition behavior and its dependence on electrical parameters (e.g. A or B) at various levels is completely different. In Fig. 5, red line indicates relation of thickness and voltage at constant duty cycle 4 and black line indicates relation of thickness and voltage at constant duty cycle 1. In general,by increasing spark energy, coating
ACCEPTED MANUSCRIPT thickness increases. But in this investigation (as shown in Fig. 5) it was observed that, this principle does not work for all range of parameters. In other word,it was observed, to achieve a high thickness, there is an optimum parameter level. When voltage is on the top/down level (180/100 v), thickness increases only if duty cycle is at top/down level too.This observation at top level for both voltage and duty cycle, can be related to spark energy too. It means, by increasing
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duty cycle and voltage, spark energy increases and that leads to a thickness increase.
Fig. 5. Interaction ofvoltage and duty cycle at constant frequency (C parameter = 410 HZ)
AC
CE
3.2 Effect of electrical parameters on porosity
Voids fractions were calculated from the proportion of the porosity area per coating surface area. Thesemeasurements have been done by Imagej software. In Fig.6 and Equation 4 show how voids fraction were measured by image j software.
CR
IP
T
ACCEPTED MANUSCRIPT
(4)
M
AN
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Fig. 6: showing coating area vs porosities area
ED
PT
The most effective parameters on coating porosity are shown in Fig.7. As can be seen from it,
CE
voltage (A) & duty cycle (B) have a negative effect on porosity. That means, by increase of A &
AC
B parameters,amount ofporosity increases.
CR
IP
T
ACCEPTED MANUSCRIPT
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Fig.7:Pareto diagram for coating porosityresponse In Fig.8, 9 relationship of voltage and duty cycle along with porosity are shown. Porositiesare
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dominant by increase of voltage and duty cycle.
Fig.8: Relation between porosity and voltage (B paraneter = 2.5%, C parameter = 410 HZ)
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T
ACCEPTED MANUSCRIPT
Fig.9: Relation between porosity and duty cycle (A= 140V, C=410 HZ)
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Responses of porosity observations (Fig.8&9) may be related to droplet size and spark energy. It
ED
seems that,with increasing duty cycle and voltage (spark energy)droplet size increases that leads
PT
to air/gas entrapment to occure between droplets. This hypothesis is strengthened by the fact that porosity decreases with increasing frequency. However some pervious studies also have reported
CE
similar trends[8][9]. It is also suggested that with increasing spark energy, due to increased evaporation of substrate or coating material, porosity can increase[10].The best parameter for
AC
achievement of minimum porosity was found to be: duty cycle: 1, voltage: 100 and frequency: 620, corresponding to sample 5. Fig.10 shows the optical image of coating sections with two different frequencies. Fig10-a shows low thickness but moderate quality and Fig15-b shows high thickness but a low quality. In other word,with increase of duty cycle and voltage, thickness increases at expense of increasing porosity.
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T
ACCEPTED MANUSCRIPT
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Fig.10:Optical microscopy images of WC – Co coatings on H13 steel a) low spark energy-sample
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3.3 Effect of electrical parameters on crack
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5 b) high spark energy – sample 8 (BM: base metal – CM: coating metal)
The cracks in coatings were classified in two categories: lamination cracks and transverse cracks.
M
Lamination cracks were those cracks which were parallel to the coating layer. These lamination
ED
defects can cause total spalling of the coating and total loss of integrity. Thus, they may be considered as having a higher risk. Transvers cracks were those extending perpendicular to the
PT
layer , i.e were through the coating thickness. Some typical cracks of each kind are shown in
CE
Fig.11. Thje cracks were given a severity score from 1 to 10 as shown in Fig 11. After calculation of crack severity scores for all cracks in each specimen, these values were divided by
AC
coating area investigated.
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CE
PT
ED
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AN
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T
ACCEPTED MANUSCRIPT
Fig.11: Cracks severity score from 1 to 10 that identified by OM in the WC – Co coatings on H13 steel
ACCEPTED MANUSCRIPT
Fig 12-a shows poor coating quality with longitudinal cracking and Fig12–b shows nearly good
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coating qualitywith transvers cracking.
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Fig.12:Optical microscopy images of WC – Co coating section and H13 steel a) poor coating
4
PT
ED
quality (too low spark energy)–sample 1– b) good coating quality (proper spark energy)– sample
According to this observation (Fig. 12), nature of coating cracks at low energy is completely
CE
different compared to high energy. In other word,as can be seen in Fig.12 at low spark energy,
AC
cracks occurred parallel with coating surface (lamination), but at average or high spark energy, cracks occurred in transverse direction. Thus, it is quite plausible to think of lamination cracks as lack of fusion defects in fusion welding as at a low spark energy, heat may not be sufficient for melting the substrate or the interlayer fusion. On the other hand, at a high spark energy, energy for good melting is sufficient but difference in expansion coefficient between coating and substrate can lead to transvers cracking.In fact, in addition to expansion coefficient effect, with increasing of spark energy, coating thickness
ACCEPTED MANUSCRIPT increases. The increase in the coating thickness can lead to residual stress build up, which eventually may lead to transverse cracking. 3.4 Effect of electrical parameters on coating hardness
T
The most effective parameters on coating hardness are shown in Fig.13. Similar to coating
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thickness response, voltage (A) & duty cycle (B) have a positive effect on hardness.
PT
Fig.13: Pareto diagram for coating hardness response
CE
Relationship of voltage and duty cycle along with hardness areshownin Fig.14, 15. As it can be
AC
seen, hardness is dominant by increasing of voltage and duty cycle.
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IP
T
ACCEPTED MANUSCRIPT
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ED
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AN
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Fig.14: Relation between hardness and voltage(B= 2.5%, C parameter= 410 HZ)
Fig.15: Relation between hardness and duty cycle (V= 140 V, C parameter= 410V) Response of these observations can be related to spark energy too. In fact, with increasing of spark energy, coating adhesion and aggregation dominate and will lead to coating hardness increases.The best electrical parameter for obtaining of a good hardness were at duty cycle: 4, voltage:180 and frequency:100 or 620, these process conditions belong to samples 4 and 8.
ACCEPTED MANUSCRIPT 4. Summary Electro spark deposition of WC-Co coatings on H13 tool steel was investigated and in summary the following findings were established:
For achievingproper coating properties, there is an optimum value for electrical
With increase of spark energy,thickness and hardness are increased, but on the other hand
IP
T
parameters of ESD process.
CR
the risk of tranverse type of cracking in the coating is increased.
Coating cracks at low spark energy is different from coating cracking at high spark
US
energy.In this research it is shown that, at low spark energycracks are more lack of fusion
AN
or lamination types of cracking. But,at high spark energy, cracks are more likely transverse.
With increase of coating density and uniformity at high spark energy, the coating
CE
PT
Effect of duty cycle on thickness is more pronounced than the effect of voltage.
AC
ED
hardness also increases.
M
ACCEPTED MANUSCRIPT Reference J. Gould, “Application of electro-spark deposition as a joining technology,” Weld. J., vol. 90, no. 10, pp. 191–197, 2011.
[2]
M. Ebrahimnia, F. M. Ghaini, and H. R. Shahverdi, “Hot cracking in pulsed laser processing of a nickel based superalloy built up by electrospark deposition,” Sci. Technol. Weld. Join., vol. 19, no. 1, pp. 25–29, 2014.
[3]
M. Ebrahimnia, F. M. Ghaini, Y. J. Xie, and H. Shahverdi, “Microstructural characteristics of the built up layer of a precipitation hardened nickel based superalloy by electrospark deposition,” Surf. Coatings Technol., vol. 258, pp. 515–523, 2014.
[4]
W. Ruijun, Q. Yiyu, and L. Jun, “Interface behavior study of WC92–Co8 coating produced by electrospark deposition,” Appl. Surf. Sci., vol. 240, no. 1–4, pp. 42–47, 2005.
[5]
M. Ebrahimnia, F. M. Ghaini, Y. J. Xie, and H. R. Shahverdi, “Developing new microstructure through laser melting of electrospark layer of precipitation hardened nickel based superalloy,” Sci. Technol. Weld. Join., p. 1362171815Y–0000000090, 2015.
[6]
E. Lugscheider, K. Bobzin, T. Hornig, and M. Maes, “Increasing the Lifetime of Aluminium and Magnesium Pressure Die Casting Moulds By Arc Ion Plating Pvd Coatings,” Proc. 6th Int. Tool. Conf. Karlstad, 2002.6th Int. Tool. Conf., pp. 979–990, 2002.
[7]
S. K. Tang, “The Process Fundamentals and Parameters of Electro-Spark Deposition,” 2009.
[8]
K. R. C. S. Raju, N. H. Faisal, D. S. Rao, S. V Joshi, and G. Sundararajan, “Electro-spark coatings for enhanced performance of twist drills,” Surf. Coatings Technol., vol. 202, no. 9, pp. 1636–1644, 2008.
[9]
J. Durocher and N. L. Richards, “Evaluation of the Low Heat Input Process for Weld Repair of Nickel-Base Superalloys,” J. Mater. Eng. Perform., vol. 20, no. 7, pp. 1294– 1303, 2010.
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[1]
[10] A. D. Thamer, M. H. Hafiz, and B. S. Mahdi, “Mechanism of building-up deposited layer during electro-spark deposition,” J. Surf. Eng. Mater. Adv. Technol., vol. 2, no. 04, p. 258, 2012.
ACCEPTED MANUSCRIPT Highlights
Electro spark deposition of WC-Co coatings on H13 tool steel was investigated and Highlights the following findings were established: For achievingproper coating properties, there is an optimum value for electrical
T
With increase of spark energy,thickness and hardness are increased, but on the other hand
CR
IP
parameters of ESD process.
the risk of tranverse type of cracking in the coating is increased. Coating cracks at low spark energy is different from coating cracking at high spark
US
AN
energy.In this research it is shown that, at low spark energycracks are more lack of fusion or lamination types of cracking. But,at high spark energy, cracks are more likely
With increase of coating density and uniformity at high spark energy, the coating
ED
M
transverse.
hardness also increases.
CE
PT
Effect of duty cycle on thickness is more pronounced than the effect of voltage.
AC
M. Salmaliyan, F. Malek Ghaeni, M. Ebrahimnia PII: DOI: Reference:
S0257-8972(17)30385-7 doi: 10.1016/j.surfcoat.2017.04.040 SCT 22283
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
11 June 2016 11 January 2017 17 April 2017
Please cite this article as: M. Salmaliyan, F. Malek Ghaeni, M. Ebrahimnia , Effect of electro spark deposition process parameters on WC-Co coating on H13 steel. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.04.040
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of Electro Spark Deposition process parameters on WC-Co coating on H13 steel *
,
,
CR
IP
T
a ,b,c Department of Materials Science and Engineering, Tarbiat Modares University, P.O. Box 14115-111, Tehran, Iran
ABSTRACT
US
H13 steel is the primary choice for the material of Aluminum Die casting molds. The life of these tools are usually enhanced by the application of WC-Co coating using Electro Spark
AN
Deposition(ESD) process. The objective of this study is to establish the effect of ESD process
M
parameters including voltage, duty cycle, and frequency on the characteristics of WC-Co coating on H13 steel. Design of experiment technique was used and coating thickness, coating hardness,
ED
and occurrence of defects in the coatings were studied. The results indicated that coating cracking
PT
at low spark energy is different from that at high spark energy. Moreover, it was found that by increasing spark energy, coating hardness and thickness increases. It is also shown that process
CE
parameters have interactive effects on the coating features and there is an optimum condition for
AC
achieving a sound coating. The maximum coating hardness was obtained at high spark energies.
Keywords:
H13 steel, electro spark deposition, WC-Co coating, Design of experiment
*
Corresponding author. Tel.: +98 21828844388; fax: +98 2182884390. E-mail addresses: [email protected] (M. Samaliyan), [email protected]
ACCEPTED MANUSCRIPT 1. INTRODUCTION Electro-spark deposition (ESD) is a microwelding fusion process that uses rapid electrical power discharges to accomplish metal transfer from an electrode to a contacting substrate[1][2]. Electrospark deposition has many applications in rebuilding and coating conductive materials. Because of its very low heat input, it is ideal for rebuilding metals which are susceptible to heat affected
IP
T
zone (HAZ) cracking(like H13 die casting molds)[3].
CR
Electro-spark deposition process is a surface treatment technology to meet the special demands, in which pulsed micro-arc generated between the electrode and the substrate, ionizes the air to
US
form high-temperature and high-stress areas in which alloying occurs. It features a highly intensive heat input and a minute heat-affected zone, as well as a high-melting-point composite
AN
coating, which is generated by a momentary temperature as high as ten thousand degrees[4][5].
M
Die casting process has now developed into a very sophisticated manufacturing process that plays an important role in providing products for industries. For examplein automotive industry, about
ED
85-90% of aluminumcomponents areproduced by die casting. Furthermore,the need for high
PT
volume, more accurate dimension, and lower production cost, superior surface finishing and improved mechanical properties of castings is the driving force for the application of new coating
CE
technologies to die casting tools. On the other hand critical areas of die surfaces can be damaged
AC
by melt and thermal fatigue[6][7]. In these cases, by applying protective or rebuilding coatings, life time of die casting molds can be increased. The objective of the current study isinvestigation of the effect of electrical parameterson metallurgical and mechanical properties of WC-Co coating on H13 steel.
2. Material and Method Heat treated hot work tool steel, H13 hadthe following chemical composition (wt %):C 0.37,Cr 5.3, Mo 1.3 , Si 1.08, Mn 0.38 , Nb 0.003 , V 0.83 , W 0.02 ,Fe balanced. The electrode (WC –
ACCEPTED MANUSCRIPT Co)had the following composition(wt %): WC95.7, Co3.7, V0.3, S0.1 and Al0.2 in the form of a round pillar 3.5 mm diameter and 100 mm lenght. Electro spark deposition of WC–Co on H13 steel was accomplished using an ESD machine developed at Tarbiat Modares University. The H13 samples were prepared by wire cutting in the form of round bars 15 mm in diameter and 5 mm height. The bar heads were polished and
IP
T
cleaned for coating trials.
CR
ESD deposition was performed using a hand held gun with a co-axis argon shield gas with a flow rate of 10 L/min.First, a number of tests were performed to establish a suitable range of ESD
US
process and then design of experiment (DOE) was drawn. DOE method that was used in this paper was full factorial method.
AN
Designparameters for DOE were: voltages, frequency and duty cycle. Other parameters including
M
capacity, electrode turning speed, moving speed, gas flow rate and time of deposition were kept constant. The values of variable parameters and constant parameters along with their positions
ED
are shown in Table 1 and 2 and Fig.1 respectively.
PT
Table 1: ESD process variable parameters through the experiments. Variables
and
their
Upper limit
100
180
B: Duty cycle (%)
1
4
C: Frequency (Hz)
100
620
CE
lower limit
AC
designation A: Voltages (v)
ACCEPTED MANUSCRIPT
Table 2: ESD constant parametersthrough the experiments parameters
value 600
Electrode rotation
1500
T
Capacitance ( f)
IP
speed(rpm)
Clockwise
Time of deposition
6
US
CR
Rotation orientation
AN
(min)
M
As can be seen, total number of experiment tests arenightheen(sixteen samples at the cubic
ED
corners and three samples at the cubic center). The spark energy shown in table 3 is calculated by using thefollowing equations:
CE
PT
(1)
Where ton, Vm and Im are pulse duration, pulse voltage and pulse current, respectively. Duty
AC
cycle is the ratio of spark on times to the total pulse time (on time + off time). Spark on timein a unit of time(d)is related to duty cycle and can be obtained by multiplying the frequency(f) by pulse duration (
):
(2)
Finally total spark energy
) is calculated as:
ACCEPTED MANUSCRIPT
AN
US
CR
IP
T
(3)
AC
CE
PT
ED
M
Fig. 1. Schematic of the design of experiments for ESD coating parameters
ACCEPTED MANUSCRIPT Table 3:Sets of process conditions for the test specimens Specimen
Voltage
Duty cycle
Measured
Frequency
Total
Spark on
No.
(V)
(level)
current
(HZ)
spark
time
energy
(μs)
(A)
36
2
36
100
15
336
US
1
2.1
100
2
180
1
3.15
100
3
100
4
4.5
4
180
4
6.4
100
39
336
5
100
1
0.96
620
0.3
5.6
6
180
1
1.5
620
1
5.6
7
100
4
3.5
620
11
52
8
180
4
4
620
22
52
9
140
3.4
410
6
30
ED
CR
PT
2.5
IP
0.8
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For observing coating microstructure of ESD layer and its association with base metal structure, Optical Microscopy (OM)was used.For this purpose, the samples were subjected to the standard
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metallographic preparation procedure starting with grinding on SiC grit papers (up to 2000), followed by polishing in alumina particle suspension (3 and 2.5 and 1 lm size). Finally, the specimens were etched in Nital etchant for 10s. Vickers micro-hardness measurement was carried out using a Bohler micro-hardness machine. The load was set at 25 gr with 10 s dwell time.The average of three hardness measurements in coating region were taken into account.Finally, amountof porosity were measured by Imagej software. Further details in measurement of crack and porosity are described in result section.
ACCEPTED MANUSCRIPT 3. Result and discussion The results obtained including coating thickness(as measured by metallography), cracks and porosities, and hardness of the coating are shown in Table 4. Table 4: Test results
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0.27
2
16
1.5
3
18
0.5
4
27
5
20
6
15
7
17
700 800
0.28
750
0.35
0.65
1100
3
0.1
640
1.5
1.2
750
1.8
0.6
1000
29
1.9
1.2
1280
19
1.1
0.4
730
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0.25
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20
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9
Hardness VHN
1
8
Porosity
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Thickness
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specimens
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3.1 Effect of Electrical parameters on coating thickness
Thickness values that are shown in table 4 was measured by metallugrahy method.The most effective parameters on thickness were extracted from Pareto chart. As shown in Fig.2 voltage (A) and Duty cycle (B) have the highest effect on thickness. Furthermore, voltage and duty cycle (AB) have positiveinteractive effect on coating thickness.
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Fig. 2. Pareto diagram for coating thickness response
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Figure 3&4 show the relationship of voltage and duty cycle on the thickness. It can be seen that
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figures are center points of the tests.
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coating thickness increases with increasing both voltage and Duty cycle. Red points in following
Fig. 3. Relation between thickness and voltage (B parameter=2.5 %, C parameter =410 HZ)
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Fig. 4. Relation between thickness and duty cycle (A parameter = 140 v, C parameter = 410HZ) Response of above observations(Fig. 3 & 4)can be related to total spark energy. By
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increasingduty cycle or voltage, spark energy increases and leads to an increase in coating
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thickness. The peak coating thickness were obtained at maximum totalsparkenergy , see
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tables3&4(for example: 39 related to sample 4 and 22 related tosample 8). Further more, the results indicate that the duty cycle at this range of variables has more effect on
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coating thickness than voltage. According to Eqs.1, this observation can be related to ton. In other
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word, in this range of electrical parameters, effect of increase of ton on thickness is more than effect of increase of voltage. As mentioned earlier it is observed that voltage and duty cycle have a strong interaction. This interaction means, effect of one parameter on the output response depends on the value of the other parameter. As shown in Fig.5,deposition behavior and its dependence on electrical parameters (e.g. A or B) at various levels is completely different. In Fig. 5, red line indicates relation of thickness and voltage at constant duty cycle 4 and black line indicates relation of thickness and voltage at constant duty cycle 1. In general,by increasing spark energy, coating
ACCEPTED MANUSCRIPT thickness increases. But in this investigation (as shown in Fig. 5) it was observed that, this principle does not work for all range of parameters. In other word,it was observed, to achieve a high thickness, there is an optimum parameter level. When voltage is on the top/down level (180/100 v), thickness increases only if duty cycle is at top/down level too.This observation at top level for both voltage and duty cycle, can be related to spark energy too. It means, by increasing
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duty cycle and voltage, spark energy increases and that leads to a thickness increase.
Fig. 5. Interaction ofvoltage and duty cycle at constant frequency (C parameter = 410 HZ)
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3.2 Effect of electrical parameters on porosity
Voids fractions were calculated from the proportion of the porosity area per coating surface area. Thesemeasurements have been done by Imagej software. In Fig.6 and Equation 4 show how voids fraction were measured by image j software.
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Fig. 6: showing coating area vs porosities area
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The most effective parameters on coating porosity are shown in Fig.7. As can be seen from it,
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voltage (A) & duty cycle (B) have a negative effect on porosity. That means, by increase of A &
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B parameters,amount ofporosity increases.
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Fig.7:Pareto diagram for coating porosityresponse In Fig.8, 9 relationship of voltage and duty cycle along with porosity are shown. Porositiesare
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dominant by increase of voltage and duty cycle.
Fig.8: Relation between porosity and voltage (B paraneter = 2.5%, C parameter = 410 HZ)
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Fig.9: Relation between porosity and duty cycle (A= 140V, C=410 HZ)
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Responses of porosity observations (Fig.8&9) may be related to droplet size and spark energy. It
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to air/gas entrapment to occure between droplets. This hypothesis is strengthened by the fact that porosity decreases with increasing frequency. However some pervious studies also have reported
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similar trends[8][9]. It is also suggested that with increasing spark energy, due to increased evaporation of substrate or coating material, porosity can increase[10].The best parameter for
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achievement of minimum porosity was found to be: duty cycle: 1, voltage: 100 and frequency: 620, corresponding to sample 5. Fig.10 shows the optical image of coating sections with two different frequencies. Fig10-a shows low thickness but moderate quality and Fig15-b shows high thickness but a low quality. In other word,with increase of duty cycle and voltage, thickness increases at expense of increasing porosity.
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Fig.10:Optical microscopy images of WC – Co coatings on H13 steel a) low spark energy-sample
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3.3 Effect of electrical parameters on crack
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5 b) high spark energy – sample 8 (BM: base metal – CM: coating metal)
The cracks in coatings were classified in two categories: lamination cracks and transverse cracks.
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Lamination cracks were those cracks which were parallel to the coating layer. These lamination
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defects can cause total spalling of the coating and total loss of integrity. Thus, they may be considered as having a higher risk. Transvers cracks were those extending perpendicular to the
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layer , i.e were through the coating thickness. Some typical cracks of each kind are shown in
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Fig.11. Thje cracks were given a severity score from 1 to 10 as shown in Fig 11. After calculation of crack severity scores for all cracks in each specimen, these values were divided by
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coating area investigated.
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Fig.11: Cracks severity score from 1 to 10 that identified by OM in the WC – Co coatings on H13 steel
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Fig 12-a shows poor coating quality with longitudinal cracking and Fig12–b shows nearly good
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coating qualitywith transvers cracking.
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Fig.12:Optical microscopy images of WC – Co coating section and H13 steel a) poor coating
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quality (too low spark energy)–sample 1– b) good coating quality (proper spark energy)– sample
According to this observation (Fig. 12), nature of coating cracks at low energy is completely
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different compared to high energy. In other word,as can be seen in Fig.12 at low spark energy,
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cracks occurred parallel with coating surface (lamination), but at average or high spark energy, cracks occurred in transverse direction. Thus, it is quite plausible to think of lamination cracks as lack of fusion defects in fusion welding as at a low spark energy, heat may not be sufficient for melting the substrate or the interlayer fusion. On the other hand, at a high spark energy, energy for good melting is sufficient but difference in expansion coefficient between coating and substrate can lead to transvers cracking.In fact, in addition to expansion coefficient effect, with increasing of spark energy, coating thickness
ACCEPTED MANUSCRIPT increases. The increase in the coating thickness can lead to residual stress build up, which eventually may lead to transverse cracking. 3.4 Effect of electrical parameters on coating hardness
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The most effective parameters on coating hardness are shown in Fig.13. Similar to coating
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thickness response, voltage (A) & duty cycle (B) have a positive effect on hardness.
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Fig.13: Pareto diagram for coating hardness response
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Relationship of voltage and duty cycle along with hardness areshownin Fig.14, 15. As it can be
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seen, hardness is dominant by increasing of voltage and duty cycle.
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Fig.14: Relation between hardness and voltage(B= 2.5%, C parameter= 410 HZ)
Fig.15: Relation between hardness and duty cycle (V= 140 V, C parameter= 410V) Response of these observations can be related to spark energy too. In fact, with increasing of spark energy, coating adhesion and aggregation dominate and will lead to coating hardness increases.The best electrical parameter for obtaining of a good hardness were at duty cycle: 4, voltage:180 and frequency:100 or 620, these process conditions belong to samples 4 and 8.
ACCEPTED MANUSCRIPT 4. Summary Electro spark deposition of WC-Co coatings on H13 tool steel was investigated and in summary the following findings were established:
For achievingproper coating properties, there is an optimum value for electrical
With increase of spark energy,thickness and hardness are increased, but on the other hand
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parameters of ESD process.
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the risk of tranverse type of cracking in the coating is increased.
Coating cracks at low spark energy is different from coating cracking at high spark
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energy.In this research it is shown that, at low spark energycracks are more lack of fusion
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or lamination types of cracking. But,at high spark energy, cracks are more likely transverse.
With increase of coating density and uniformity at high spark energy, the coating
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Effect of duty cycle on thickness is more pronounced than the effect of voltage.
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hardness also increases.
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ACCEPTED MANUSCRIPT Reference J. Gould, “Application of electro-spark deposition as a joining technology,” Weld. J., vol. 90, no. 10, pp. 191–197, 2011.
[2]
M. Ebrahimnia, F. M. Ghaini, and H. R. Shahverdi, “Hot cracking in pulsed laser processing of a nickel based superalloy built up by electrospark deposition,” Sci. Technol. Weld. Join., vol. 19, no. 1, pp. 25–29, 2014.
[3]
M. Ebrahimnia, F. M. Ghaini, Y. J. Xie, and H. Shahverdi, “Microstructural characteristics of the built up layer of a precipitation hardened nickel based superalloy by electrospark deposition,” Surf. Coatings Technol., vol. 258, pp. 515–523, 2014.
[4]
W. Ruijun, Q. Yiyu, and L. Jun, “Interface behavior study of WC92–Co8 coating produced by electrospark deposition,” Appl. Surf. Sci., vol. 240, no. 1–4, pp. 42–47, 2005.
[5]
M. Ebrahimnia, F. M. Ghaini, Y. J. Xie, and H. R. Shahverdi, “Developing new microstructure through laser melting of electrospark layer of precipitation hardened nickel based superalloy,” Sci. Technol. Weld. Join., p. 1362171815Y–0000000090, 2015.
[6]
E. Lugscheider, K. Bobzin, T. Hornig, and M. Maes, “Increasing the Lifetime of Aluminium and Magnesium Pressure Die Casting Moulds By Arc Ion Plating Pvd Coatings,” Proc. 6th Int. Tool. Conf. Karlstad, 2002.6th Int. Tool. Conf., pp. 979–990, 2002.
[7]
S. K. Tang, “The Process Fundamentals and Parameters of Electro-Spark Deposition,” 2009.
[8]
K. R. C. S. Raju, N. H. Faisal, D. S. Rao, S. V Joshi, and G. Sundararajan, “Electro-spark coatings for enhanced performance of twist drills,” Surf. Coatings Technol., vol. 202, no. 9, pp. 1636–1644, 2008.
[9]
J. Durocher and N. L. Richards, “Evaluation of the Low Heat Input Process for Weld Repair of Nickel-Base Superalloys,” J. Mater. Eng. Perform., vol. 20, no. 7, pp. 1294– 1303, 2010.
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[1]
[10] A. D. Thamer, M. H. Hafiz, and B. S. Mahdi, “Mechanism of building-up deposited layer during electro-spark deposition,” J. Surf. Eng. Mater. Adv. Technol., vol. 2, no. 04, p. 258, 2012.
ACCEPTED MANUSCRIPT Highlights
Electro spark deposition of WC-Co coatings on H13 tool steel was investigated and Highlights the following findings were established: For achievingproper coating properties, there is an optimum value for electrical
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With increase of spark energy,thickness and hardness are increased, but on the other hand
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parameters of ESD process.
the risk of tranverse type of cracking in the coating is increased. Coating cracks at low spark energy is different from coating cracking at high spark
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energy.In this research it is shown that, at low spark energycracks are more lack of fusion or lamination types of cracking. But,at high spark energy, cracks are more likely
With increase of coating density and uniformity at high spark energy, the coating
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transverse.
hardness also increases.
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Effect of duty cycle on thickness is more pronounced than the effect of voltage.
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