Evaluation of the mechanical properties of WC-FeAl composite coating fabricated by laser cladding method

Journal Pre-proof Evaluation of the mechanical properties of WC-FeAl composite coating fabricated by laser cladding method

Alireza Mostajeran, Reza Shoja-Razavi, Morteza Hadi, Mohammad Erfanmanesh, Masoud Barekat, Mohammad Savaghebi Firouzabadi PII:

S0263-4368(19)30965-5

DOI:

https://doi.org/10.1016/j.ijrmhm.2020.105199

Reference:

RMHM 105199

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date:

3 December 2019

Accepted date:

9 January 2020

Please cite this article as: A. Mostajeran, R. Shoja-Razavi, M. Hadi, et al., Evaluation of the mechanical properties of WC-FeAl composite coating fabricated by laser cladding method, International Journal of Refractory Metals and Hard Materials(2020), https://doi.org/10.1016/j.ijrmhm.2020.105199

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© 2020 Published by Elsevier.

Journal Pre-proof

Evaluation of the mechanical properties of WC-FeAl composite coating fabricated by laser cladding method a

b

c

a

Alireza Mostajeran , Reza Shoja-Razavi , Morteza Hadi , Mohammad Erfanmanesh , a Masoud Barekat , Mohammad Savaghebi Firouzabadib a

Faculty of Materials & Manufacturing Technology, Malek-Ashtar University of Technology, Iran

c

Metallurgy and Materials Engineering Department, Golpayegan University of Technology, Iran, P.o Box: 87717-65651

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b

Department of Materials Engineering, Malek-Ashtar University of Technology, Iran

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Abstract

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WC-Co coating, which is a subcategory of Tungsten Carbide-based coatings, is prominent among a variety of industries. However, because of its expense, poisoning, and low corrosion resistance of Cobalt in acidic

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environments, alternative compositions have been designed. One of these alternatives is the Iron Aluminide intermetallic compound which can replace Cobalt. This study investigates laser cladding of WC-FeAl powder

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on a 321 Stainless-Steel substrate. WC-FeAl powders were synthesized by mechanical alloying of initial Aluminum and Iron powders, milled for 20 hours, followed by an hour of annealing at 800 degrees Celsius.

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Then, the annealed particles were mechanically alloyed with WC powders for 50 hours. The result of the X-ray diffraction (XRD) analysis showed that no brittle and destructive phase was formed during synthesis. Subsequently, powders were coated on the stainless-steel substrate by laser cladding method. Effect of the

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main parameters of the laser cladding, including laser power, laser probe velocity, and powder spray rate, on the coating properties, such as porosity, geometry, thickness and, dilution were studied. Results indicate that

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with a higher power of the laser, the penetration depth and the width of the coating increased. Besides, with a higher velocity of the laser probe, dilution and penetration depth decreased. Furthermore, the Higher rate of powder spray led to a thicker coating. The optimum parameters of different samples were 250W power, 4mm/s probe velocity, and 400 mg/s powder spray rate. Evaluation of the mechanical properties indicated that the 1600 Vickers hardness, 5.7 MPa.m1/2 fracture toughness, and 355 GPa Young’s modulus were obtained. Besides, The evaluation of the mechanical properties of the coating showed that the hardness, fracture toughness, and elasticity modulus are 1600V, 5.7 MPa.m1/2, and 355 GPa respectively. Obtained results revealed that in comparison with the WC-FeAl composite coating with 500ppm additional Boron and WC-Co coating both fabricated by thermal spray coating, for the WC-FeAl coating studied in this investigation, respectively the hardness is 1.16 and 1.21 times higher and the fracture toughness is 2.5 and 2.8 times higher. As well, Young's modulus of the coating was 1.56 times higher than the WC-Co coating made by the laser cladding method.

Journal Pre-proof Keywords: WC-FeAl, Laser Cladding, Agglomeration, Mechanical properties

Introduction The Iron-aluminide intermetallic compound is a suitable choice for high-temperature applications. This compound, due to the formation of a layer of Aluminum oxide on the surface, has a very good oxidation resistance in many environments. Besides, high specific strength, high sulfidation corrosion resistance, low cost, and low density are other remarkable properties of this composition. During abrasion, which loads are in the form of pressures on the surface, hardness, strength, and work hardening amounts are vital. As a consequence, intermetallic compounds, and their composites with a hard-matrix are major

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options for wear applications [1-4].

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Tungsten Carbide, according to its excellent hardness, low friction coefficient, good thermal conductivity, and high oxidation resistance, is an important material in industries. This compound is mostly used in the

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production of mining facilities, cutting tools, and drilling tools, due to its high fracture toughness, hardness and wear resistance. However, one of the drawbacks of this composition is its high brittleness,

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so that Cobalt has been added to the mixture as a matrix to compensate the brittleness. Even so, a reduction in the corrosion resistance and oxidation resistance of the composite, in a mediocre temperature,

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is the disadvantage of the composite. Replacement of the Cobalt with Iron-aluminide leads to a better

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wear and oxidation resistance of the composite, in comparison with the WC-Co [5-7]. Foroshima et al. compared WC-FeAl composite with WC-Co composite and concluded that the mechanical properties of the WC-FeAl, such as hardness and fracture toughness, are better than the WC-

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Co composite; the WC-FeAl composite can be used in applications in which the control on the

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microstructure is important. They found out that the grain growth does not occur in WC-FeAl and the hardness of the composite varies in 1100

to 2100

range. Furthermore, the fracture toughness of

the composite is between 8 to 15 MPa. Also, Mesbah et al. studied the differences between the two composites and indicated that the wear fraction of the Iron-aluminide is 33% less than cobalt [7,8]. Howerver, Not so many investigations have been done on the coating of the WC-FeAl composite. Mehrbani et al. studied the thermal spray of the WC-FeAl and the WC-Co composites. They reported that the WC-FeAl coating has a higher hardness in comparison with the WC-Co coating, due it the higher hardness of the Iron-aluminide than Cobalt [9]. if it can be concluded that the WC-FeAl composite has higher oxidation and wear resistance in comparison with WC-Co composite. Given that no investigation has been done on the laser cladding of the WC-FeAl composite, and according to the better adhesion of the coating to the substrate and the fewer defects in the coatings made by this method [10], this study aims to survey the cladding of the WC-FeAl powders, by laser, on a 321 stainless-steel substrate. the

Journal Pre-proof optimum parameters of the single-pass laser cladding, including laser power, laser probe velocity, and powder spray rate were investigated. Then coatings with different amounts of overlapping, in a multi-pass form, were investigated, followed by mechanical tests, such as hardness and fracture toughness, on the coating with the optimum overlapping. Materials and Methods Initial powders of Iron (99.0% purity, 150µm average particle size), Aluminum (99.0% purity, 100µm average particle size), and Boron (99.0% purity, 60µm average particle size) were mixed by mechanical alloying method, in order to obtain Fe - 40% at. Al – 500ppm B composition. Milling was conducted for

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20 hours, with a rotation speed of 300rpm. The ball to powder ratio was chosen to be 10:1. In order to prevent the oxidation of the elements, milling was done in an Argon atmosphere, with 1% wt. ethanol

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addition. A planetary ball-mill machine, “FRITSCH” model, with 10mm-diameter steel balls and a steel

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vial, was used. Resulted powders were annealed for an hour at 800 degrees of Celsius, with a continuous current of Argon gas. Then, the resulted powders were mixed with the Tungsten Carbide (99.9% purity,

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less than 10µm particle size) particles, so that the outcome composition was WC-11.5% wt. FeAl. Milling was employed for 50hours, with a 100rpm rotation speed and a ball to powder ratio of 8:1, in an Argon

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atmosphere inside a Tungsten Carbide vial. Also, the balls used for milling were made of Tungsten Carbide with a diameter of 10mm. Resulted powders were heated at 100 degrees of Celsius for 120min to

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completely dry, followed by spraying of a solution, consisted of 2 wt.% PVA and the rest distilled water, on the powders in order to increase fluidity and particle size. Then, the resulted solution was injected into

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a spray dryer system, at 140 degrees of Celsius with a 900rpm rotation speed. Obtained powders were graded in a grain size of 50 to 100µm.

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Subsequently, derived powders were coated on a 321 stainless-steel substrate by laser cladding method. For this process, an Nd: YAG system with a 400W rated power was used. Argon gas was used as the shielding gas and the transporter of the powders, respectively with a current velocity of 20 and 15 Liter/min. The parameters used for the single-pass laser cladding are shown in table 1. Dilution is an important parameter in the formed coatings, defined by equation number 1 [11].

)1( In this equation, ‘D’ is the dilution amount, ‘b’ is the penetration depth of the coating, and ‘h’ is the coating thickness. After specifying the optimum sample of the single-pass mode, multi-pass coatings with 30, 50, and 70 percent of overlapping were made.

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P(W)

V(mm/s)

F(mg/s)

Specimen

P(W)

V(mm/s)

F(mg/s)

WF 1

300

2

300

WF 13

200

2

300

WF 2

300

2

400

WF 14

200

2

400

WF 3

300

3

300

WF 15

200

3

300

WF 4

300

3

400

WF 16

200

3

400

WF 5

300

4

300

WF 17

200

4

300

WF 6

300

4

400

WF 18

200

4

400

WF 7

250

2

300

WF 19

150

2

300

WF 8

250

2

400

WF 20

150

2

400

WF 9

250

3

300

WF 21

150

3

300

WF 10

250

3

400

WF 22

150

3

400

WF 11

250

4

300

WF 23

150

4

300

WF 12

250

4

400

WF 24

150

4

400

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Specimen

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Table 1: Parameters of the single-pass laser cladding

In order to investigate the constituent phases of the powders and the coatings, X-ray Diffraction (XRD)

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analysis (BRUKER, Cu Kα radiation, λ=0.1541nm) was used. The time of each step, with the magnitude of 0.05 degrees, was 1second. The test was conducted in the range of 10 to 90 degrees. Besides, for understanding the morphology changes of the powders and the coatings, Field Emission Scanning

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Electron Microscopy (FESEM, MIRA3 Tescan) was used. To evaluate the micro-hardness of the coated

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samples, a 400g load was used, based on the ASTM E384-99 standard. Nop indentation method was applied for examination of the elasticity modulus of the coatings, based on the equation number 2 [11]. A Leitz micro-hardness tester, with 400gF load (3.924 N), was employed for 15 seconds.

)2( In the equation ‘E’ is Young’s modulus (GPa), ‘Hk ’ is the Nop micro-hardness resulted from the indenter, ??′/??′is the ratio of the small diameter to the big one after elastic recovery, b/a is the certain ratio of the Nop indenter’s geometry (0.14), and the ‘α’ is the constant amount of 0.45. To determine the fracture toughness of the optimum coating, 3, 5, and 10 Kg loadings were applied for 15 seconds. Evans model (equation 3) was used for calculation of the fracture toughness [12].

Journal Pre-proof (

⁄

)

(

)

For

)3(

In this equation, KIC is the fracture toughness (MPa.m1/2 ), ‘P’ is the applied force by the indenter, ‘a’ is half the diameter of the indenter’s effect (m), and ‘c’ is the length of the crack, from the center of the notch (m). this equation is usable while the ratio of c to a be in the range of 0.6 to 4.5.

Results and Discussion Figure 1 shows the X-ray diffraction (XRD) analysis pattern of the Iron-aluminide powders, after

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annealing. As shown, beside the FeAl intermetallic compound, Fe 3 AlC0.5 compound is formed, which is because of the presence of ethanol in the mixture during mechanical alloying. The mentioned phase is

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hard and brittle, in which Carbon atoms are at interstitial positions [1,13].

Figure 1: X-ray diffraction pattern of the Iron-aluminide powders, after annealing at 800 degrees of Celsius .

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Morphology of the Iron-aluminide powders, before and after annealing at 800 degrees of Celsius, is displayed as figure 2. Size of the particles is mostly in the range of 3µm to 7µm; it does not change with the heat treatment and the average particle size is approximately the same. Annealing process only has led to the formation of the Iron-aluminide intermetallic phase. Figure 3 shows the XRD patterns of the Tungsten Carbide and WC-FeAl powders. In figure 3-b, peaks are dedicated only to Tungsten Carbide and Iron-aluminide phases, therefore no AlFe3 C0.5 peak is observed in the pattern. This can be due to the small amount of the phase which could not be identified.

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Figure 2: SEM images of the Iron-aluminide powders a) before and b) after annealing at 800 degrees of Celsius .

Figure 3: XRD patterns of the a) pure Tungsten Carbide and b) WC-FeAl powders.

Figure 4-a is the scanning electron microscope (SEM) images of the initial Tungsten Carbide powders. As shown, powders are in an irregular shape and mostly are below 10µm. Also, figure 4-b depicts the SEM image of the WC-FeAl powders and the Energy-dispersive X-ray spectroscopy (EDS) analysis of them at two different points. It can be seen that the Iron-aluminide and Tungsten Carbide particles have become smaller and their size is in the 0.9µm to the 1.7µm range. The bright phase is dedicated to the Tungsten Carbide, while the darker one is the Iron-aluminide; this is due to the difference of the densities of the Tungsten Carbide (15.6 g/cm3) and Iron-aluminide (5.56 g/cm3), which leads to a brighter look of the phase with higher density [14]. Furthermore, the results of the elemental mapping from different areas are shown in table 2.

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Figure 4: SEM images of the a) Tungsten Carbide and b) WC-FeAl powders.

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Table 2: Results of the EDS analysis of the demonstrated areas in figure 4-b

Fe (Wt%)

Al (Wt%)

W (Wt%)

A

52.67

36.59

10.74

B

4.18

2.16

93.66

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Point

As mentioned earlier, WC-FeAl powders tend to become smaller during milling and the morphology of

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them remain non-uniform and angular; so that, they have no fluidity. therefore, they have to get granulated to be suitable for laser cladding. Figure 5 shows the dried particles of the composite, by the spray drying method that increases the fluidity and particle size. As shown, the agglomerated particles have a semi-spherical shape, which in comparison with the non-agglomerated form (figure 4-b) have a more uniform morphology, leading to a higher fluidity. Figure 6 shows the optical microscope images of the cross-section of the WC-FeAl coated samples, respectively with the 300, 250, 200, and 150 Watt laser powers. In the figure, from left to right, laser probe velocity changes from 2mm/s to 4mm/s and from up to down powder spray rate increases from 300 to 400 mg/s.

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Figure 5: SEM image of the granulated WC-FeAl powders.

Figure 6: Optical microscope images of the single-pass coatings of the WC-FeAl, at 300, 250, 200, and 150 Watt powers.

Journal Pre-proof In a general view, it can be observed that with any of the process parameters, single-pass coatings have good adhesion to the substrates and have been formed well. In table 3, the average of the three measurements for the geometric properties and the porosity percentage of each coating sample is shown. As can be seen in figure 6 and table 3 generally, at high laser powers, high powder spray rates, and low laser probe velocities, broad and thick coatings were made. In contrast, a narrow and thin coating is obtainable with low powder spray rate, low laser power, and high laser probe velocity. Table 3: Geometric properties and porosity percentage of all single-pass coatings, with different process parameters

325 300 220 204 216 193 225 217 157 145 123 112 138 130 122 114 110 99 108 111 88 98 -

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22.8 24.2 16.3 21.6 17.0 17.2 21.1 20.2 25.9 13.2 20.5 21.1 20.8 25.9 14.8 15.2 17.6 13.4 14.1 10.9 18.6 17.5 -

D (%) ± 1

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b (µm) ± 5

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2163 2180 2133 2024 2019 2148 2061 2101 1996 2121 1964 2043 1772 1800 1719 1866 1680 1937 1733 1740 1700 1706 -

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145 173 100 124 134 125 163 188 156 165 152 164 116 122 98 103 98 108 56 77 72 76 -

θ (°) ± 2

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h (µm) ± 2 w (µm) ± 15 WF 1 WF 2 WF 3 WF 4 WF 5 WF 6 WF 7 WF 8 WF 9 WF 10 WF 11 WF 12 WF 13 WF 14 WF 15 WF 16 WF 17 WF 18 WF 19 WF 20 WF 21 WF 22 WF 23 WF 24

% porosity

Geometric characteristics

sample

69 63 68 62 61 60 58 53 50 46 44 40 54 51 55 52 52 47 65 58 55 56 -

)± 0.1( 4.4 2.1 4.7 2.1 4.0 8.9 1.8 2.0 6.3 5.5 1.8 1.1 7.4 5.1 3.4 5.6 0.8 4.3 1.5 1.0 3.9 3.2 -

In figure (7-a) the effect of the laser power on the width of the single-pass coating is depicted. At a 300W laser power, because of the high energy input to the substrate, melt puddle size increases, followed by the increase of the coating width and dilution. According to the figure, at the 2mm/s laser probe velocity, high energy input has led to a wider coating. In addition, with a constant laser probe velocity, by increasing the laser power, resulting in a higher amount of energy input to the substrate, melt puddle size increases and wider coating forms.

Journal Pre-proof According to the accomplished investigations, penetration depth and dilution are two important parameters for laser cladding and are essential for coating’s adhesion to the substrate; on the other hand, high amount of these parameters are considered harmful for the coating [15-21]. A higher laser power, at a constant powder spray rate and laser probe velocity, leads to a higher penetration depth, which is because of the energy input increase and thus more energy absorption by the substrate; this can be observed in figure 7-b. when the laser probe velocity increases, penetration depth decreases, due to the

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less laser exposure to the surface.

Figure 7: Diagrams for a) changes of the coating width with the laser probe velocity, at a constant 300mg/s powder spray rate, at different laser powers; b) changes of the penetration depth with the laser power, at a constant 400mg/s powder spray rate, at different laser probe velocities; c) changes of the dilution wit h the laser probe velocity at a constant 250w laser power, at different powder spray rates and d) changes of the coating thickness with the laser probe velocity, at a constant 250w laser power, at different powder spray rates .

Journal Pre-proof The powder spray rate affects the penetration depth, coating thickness, and dilution. As figure 7-c shows, at a constant laser probe velocity, with a higher powder spray rate, because of the more melting of the powder, coating thickness increases. Dilution depends on the penetration depth and the coating thickness; changing the laser parameters affect these properties. The powder spray rate and the laser probe velocity is effective on the amount of dilution, which is demonstrated as figure 7-d. Based on this figure, at a constant rate of powder spray, higher laser probe velocity leads to a fewer penetration depth and dilution, which is because of the less laser exposure to the surface. As shown in figure 6, all samples have a certain amount of porosity. In laser cladding method, one of the

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important parameters is the number of porosities that form because of the high tendency of the elements to oxidation and formation of the gases during process. Besides, the amount of powder input and the

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synthesis method of the powders are effective on porosity amount. During the laser cladding of the Tungsten Carbide based powders, partial melting of the Tungsten Carbide or the extra heat input leads to

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the decomposition of the Tungsten Carbide to the Tungsten and Carbon elements. Decomposed Carbon reacts with the Oxygen of the air and produces Carbon Dioxide and Carbon Monoxide. Due to the time

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shortage for the escape of the gases from the coating melt puddle during laser cladding process, they

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remain inside the coating and make the porosities. Increase of the laser power leads to a prolongs the solidification and decreases the porosity percentage; although it increases the decarburization of the 25].

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Tungsten Carbide compound and leads to the less amount of the Carbon atom inside the phase [11, 22-

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Based on the mentioned contents and the optical microscopy images of the cross-section of the singlepass coatings, there is no crack in any of them; therefore, in order to determine the optimum coating,

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other parameters such as less porosity and dilution, coating width, and the torsion angle should be considered. According, the WF 12 coating was chosen as the best coating, which (table 3) had 2 percent of porosity and the least dilution in comparison with other samples. Therefore, the parameters of the mentioned coating, which are 250w laser power, laser probe velocity of 4mm/s, and 400mg/s powder spray rate are the optimum parameters. Subsequently, these parameters were used for the formation of the multi-pass coatings, with different overlap percentages, which the results are shown as table 4. In a 30% overlapping, due to the separation of the passes from each other, a gap occurs between passes and causes ups and downs on the surface; this makes the surface treatment, including machining, essential to obtain a smooth surface. As shown in figure 4, with increasing the overlapping up to 50%, the height increases; however, many cracks are observable in this sample and does not have an appropriate geometric appearance. Besides, no uniformity and proper thickness are achieved. Though, for the sample with 70% of overlapping, which is shown in figure 8, the distance between coating passes is not

Journal Pre-proof observable. Besides, the appropriate appearance of the coating, in comparison with the coatings with 30% and 50% of the overlapping is achieved. According to table 4, for the samples with 30%, 50% and 70% of the overlapping, the thickness of the coating increases with the overlapping amount. Also, in comparison with the optimized single-pass coated sample, in the form of overlapping, dilution decreases. This is due to the partial absorption of energy by the last pass of coating and decrease of the energy input to the substrate, therefore the dilution of the coating in comparison with the single-pass coating mode decreases; this is consistent with the results of the previous investigations about coatings made by laser cladding [26,27].

Height

properties (the

Depth of

(µm) ± penetration

mean)

(µm) ± 3

porosity

(%) ± 2

(%) ±

overlapping

179

50%

196

70%

220

of height

Crack

0.2

compared with single track ± 0.1

Percents of dilution decrease compared with single track ± 0.1

30

2.9

No

8.5

24.4

82

34

3.8

Yes

16.3

15.7

53

23

2.3

No

25.4

42.3

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12

73

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Sample WF

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30%

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Amounts of

Percents increase

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2

Dilution

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Table 4: Geometric properties of the WF 12 coating with different amounts of overlapping

Figure 8: Optical microscope images of the WF12 coating sample with 70 percent of overlapping.

Journal Pre-proof Figure 9 demonstrates the XRD pattern of the coating of the WF-12 sample, with 70 percent of

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overlapping. As Can be seen, the FeAl, W2 C, Fe3 W3 C, and WC phases are detected in the XRD graph.

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Figure 9: XRD pattern of the WF12 coating sample with 70 percent of overlapping.

Presence of W2 C phase, as a constituent phase of the coating, shows the decarburization of and

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decomposition of the Tungsten Carbide. According to the previous investigations, the heat made by the laser causes the matrix to melt, which a proportion of the Tungsten Carbide dissolves in it, therefore the

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molten phase becomes Carbon-rich and Tungsten-rich. Then, a part of the dissolved Carbon oxidizes and departs from the melt in the form of CO and CO 2 gases; this leads to the formation porosities inside the

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coating. Besides, near the interface of the coating and the substrate, Carbon amount decreases and Tungsten remains rich, so the W2 C phase, or η (Fe3 W3 C, CO3 W3 C) phase, during solidification forms

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[6,28]. The reason for the formation of the Fe 3 W3 C phase is that increaseing in the concentration of the Iron, due to mixing of the coating and the substrate. It can be concoluted that, with the laser power

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increase or the probe velocity decrease, the concentration of Iron along with the interface significantly increases, which leads to the increase of the amount of the Fe3 W3 C phase and decrease of the WC [29]. Figure 10 shows a cross-section of the coating, perpendicular to the angle of the laser probe movement; part a is a image of the interface and b is the microstructure image of the coating. Homogenous distribution of the Iron-aluminide among the particles of the Tungsten Carbide is observable; this is because of the milling that had been conducted in order to obtain a WC-FeAl composite. This distribution, and the optimum parameters used for the laser cladding, resulted to a uniform coating, less porosity and denser coating in comparison with coated achieved by thermal spray [10].

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Figure 10: microstructure of cross -section of the coating in the electron backscatter diffraction mode, from a) close to the interface, b) top of the coating, and c) a higher magnification from the middle of the coating .

A dark background with bright particles is visible in figure 10. Energy-dispersive X-ray spectroscopy (EDS) results are shown in table 5. In the electron backscatter diffraction images, heavier atoms with more atomic number, diffract more electrons, which means look brighter [12, 28]. Mean while, according to the results of the XRD, it can be said that the phase with the dark look (point A in figure 10-c) is dedicated to the

Iron-aluminide phase, the phase of the gray particles (point B in the

figure 10-c and the gray particles that are along the interface in figure 10-b) is Fe3 W3 C, and the phase with a bright look (point C in figure 10-c)

is the Tungsten Carbide. The previous investigations

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Iron-aluminide significantly increases the

room temperature plasticity of the WC-FeAl composite; furthermore, improves the grain boundary coherency and leads to less growth of the Tungsten Carbide particles [14]. Particle size of the Tungsten Carbide particles is approximately 2 to 4µm, showing the effect of Boron addition on the particle growth Table 5: Results of spot EDAX elemental analysis obtained from different regions showed in Fig.10-c

Fe (Wt%)

Al (Wt%)

W (Wt%)

A

62.23

20.35

17.42

B

44.94

2.36

52.7

C

20.74

0.17

78.79

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The results of the micro-hardness test, of the specimen with optimum condition are demonstrated as table 6. The numbers are each average of three times of testing, which indicates that the hardness at the top of

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the coating is almost 6.4 times higher than the substrate.

substrate

Vikers Hardness

250

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Point

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Table 6: the results of the micro-hardness of the coating and the substrate (400 grf for 15s) Interface

Top Clad

910

1600

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Table 7 shows the hardness of the different phases of the coating [1, 31 – 33]. according to table 5, the hardness of the sample decreases near the interface of the coating and the substrate; this is due to the

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diffusion of the Iron, from the substrate to the coating zone leading to the formation of the Fe 3 W3 C phase

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that have less hardness than Tungsten Carbide. This is also shown in table 6 and figure 9-a. Table 7: hardness of different constituent phases of Tungsten Carbide based composite [1, 18-20]

Phase Hardness

WC

W2 C

Fe 3 W3 C

FeAl

2440

3050

1600

460

and Elements Vikers

Reported in previous investigations [9], the micro-hardness of the WC-FeAl and the WC-Co coatings, made by thermal spray method, is respectively 1380 and 1320 Vickers; while the WC-FeAl coating, studied in this investigation, has a micro-hardness respectively 1.16 and 1.21 times higher than the mentioned coatings. This is due to the denser form of the coating, more homogenous dispersion of the Tungsten Carbide particles in the coating, and formation of no layered structure in the laser cladding process. moveover, the elasticity modulus of the coating is 355GPa, while the elasticity modulus of the

Journal Pre-proof WC-Co coating, made by laser cladding method, is 277GPa [6]. Tharefore, The elasticity modulus of the coating is 1.56% higher than the WC-Co coating, which is due to the less elasticity modulus of the Cobalt (209 GPa) in comparison with Iron-aluminide (261 GPa). Furthermore, in order to evaluate the toughness of the coating, a Vickers indenter, in the range of the 3Kg to 10Kg load, has been used; so that a crack is observable in the test with 10Kg loading. Fracture toughness of the WC-FeAl coating with 500ppm additional Boron is 5.7 MPa.m1/2 which in comparison with the coatings made of WC-FeAl

composite, with 500ppm additional Boron (2.2 MPa.m1/2 ) and with WC-Co composite (2 MPa.m1/2 ) both made with thermal spray coating method [9], fracture toughness is 2.5 and 2.8

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times higher because isotropic behavior and homogeneous structure that acquired by lasercladding.

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Conclusion

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1) Milling of the Tungsten Carbide and Iron-aluminide mixture for 50 hours, only leads to a more homogenous mixture with less particle size; so that no new phase forms during the process. Also,

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the agglomeration process increases the fluidity of the WC-FeAl particles and transfigures them

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into a semi-sphere shape, which is suitable for laser cladding process. 2) Laser-cladding samples of WC-FeAl powders, which all were made with a single pass of the laser, were all in an appropriate appearance and had a well metallurgical bonding with the

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substrate. Also, with an increase in the input power of the laser and a decrease in probe velocity, the width of the coating and the penetration depth of it increase. Furthermore, with a constant rate

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of powder spray and laser power, the higher velocity of the probe leads to a less blending and penetration depth. As well, a higher amount of powder spray rate, with sustained laser power and

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probe velocity, increases the thickness of the coating. 3) By examining the coating properties, such as geometric properties, porosity percentage, initial crack amount, thickness, and coatings dilution, the optimum parameters of the laser cladding were evaluated; 250W power, 4mm/s probe velocity, and 400mg/s powder spray rate. Subsequently, by analyzing the overlap samples, the coating with 70 percent of the overlap was chosen to be the optimum coating, which their coating was about 220µm. According to the microstructural analysis of the coating, a homogenous dispersion of the Tungsten Carbide particles among the Iron-aluminide matrix was obtained; so that with the appropriate condition of the laser cladding, a uniform coating was obtained.

4) The evaluation of the mechanical properties of the coating showed that the hardness, fracture toughness, and elasticity modulus are respectively 1600V, 5.7 MPa.m1/2 , and 355 GPa. Obtained

Journal Pre-proof results indicate that, in comparison with the WC-FeAl composite coating with 500ppm additional Boron and WC-Co coating both fabricated by thermal spray coating, for the WC-FeAl coating studied in this investigation, respectively the hardness is 1.16 and 1.21 times higher and the fracture toughness is 2.5 and 2.8 times higher. Besides, elasticity modulus of the WC-FeAl coating made by laser cladding has been 1.56% higher than the WC-Co coating made by the same method.

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Journal Pre-proof Highlights -Milling of tungsten carbide and iron aluminide to achieve WC-FeAl composite powder -Agglomeration process on WC-FeAl composite powders to increase their fluidity and suitability for laser cladding process -Performing laser coating process for the first time for WC-FeAl composite Powder -The effect of laser cladding parameters on geometrical shape of clads was studied

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-Improving the mechanical properties of the WC-FeAl coating of laser cladding in front of the WC-FeAl coating of the thermal spraying method

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