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Fiber laser cladding of nickel-based alloy on cast iron
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Fiber laser cladding of nickel-based alloy on cast iron ˜ b , J. Penide a , F. Lusquinos ˜ a , F. Quintero a , F. Arias-González a,∗ , J. del Val a , R. Comesana a a a A. Riveiro , M. Boutinguiza , J. Pou a b
Applied Physics Dpt., University of Vigo, EEI, Lagoas-Marcosende, Vigo E-36310, Spain Materials Engineering, Applied Mechanics and Construction Dpt., University of Vigo, EEI, Lagoas-Marcosende, Vigo E-36310, Spain
a r t i c l e
i n f o
Article history: Received 23 June 2015 Received in revised form 2 October 2015 Accepted 3 November 2015 Available online xxx Keywords: Laser cladding Laser alloying Cast iron Nickel-based alloy Microstructure Hardness
a b s t r a c t Gray cast iron is a ferrous alloy characterized by a carbon-rich phase in form of lamellar graphite in an iron matrix while ductile cast iron presents a carbon-rich phase in form of spheroidal graphite. Graphite presents a higher laser beam absorption than iron matrix and its morphology has also a strong influence on thermal conductivity of the material. The laser cladding process of cast iron is complicated by its heterogeneous microstructure which generates non-homogeneous thermal fields. In this research work, a comparison between different types of cast iron substrates (with different graphite morphology) has been carried out to analyze its impact on the process results. A fiber laser was used to generate a NiCrBSi coating over flat substrates of gray cast iron (EN-GJL-250) and nodular cast iron (EN-GJS-400-15). The relationship between processing parameters (laser irradiance and scanning speed) and geometry of a single laser track was examined. Moreover, microstructure and composition were studied by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). The hardness and elastic modulus were analyzed by means of micro- and nanoindentation. A hardfacing coating was generated by fiber laser cladding. Suitable processing parameters to generate the Ni-based alloy coating were determined. For the same processing parameters, gray cast iron samples present higher dilution than cast iron samples. The elastic modulus is similar for the coating and the substrate, while the Ni-based coating obtained presents a significantly superior hardness than cast iron. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Laser cladding is a technique that is gaining popularity in the industry for commercial applications, encouraged by the development of reliable and robust industrial high power lasers. It is a manufacturing process to generate a dense and metallurgical bonded coating over a substrate. Laser beam is used as heat source to generate a melt pool in a substrate, in which the precursor material is fed [1]. The relative movement between the beam and the workpiece makes possible to generate a layer with a thickness ranged from microns to millimeters [2]. On new components, laser cladding can be applied to improve the surface properties; on old components, it can be applied to restore worn or damaged surfaces in order to reuse them. Cast iron is a group of ferrous alloys which contains, at least, 2% carbon and 1–3% silicon [3]. The most common types of cast irons are gray cast iron, which exhibits a carbon-rich phase
∗ Corresponding author. E-mail address: [email protected] (F. Arias-González).
composed of lamellar graphite, and ductile cast iron, which presents a carbon-rich phase composed of spheroidal graphite. In 1700s, the development of cast iron as an engineering material made the industrial revolution possible. Nowadays, it is seen as a brittle, weak and dirty material that is not proper for applications that require high strength in comparison to other alloys. In some applications, it is being substituted by more expensive metallic materials like steel, aluminum-alloys, titanium-alloys, etc. However, the good castability and machinability of cast iron are an opportunity for significant cost savings in manufacturing those parts which are not the more likely to break. Moreover, its high damping capacity is interesting to reduce noise and vibrations. So, cast iron alloys are still being widely used in industry to manufacture brake discs and drums, bearings, gears, engine components, machine tool structural parts, components in rock crushers, etc. There exists a high demand in industry to improve the performance and durability of all the components. In order to increase the life of cast iron parts, it has been proposed to enhance its surface properties like its hardness, wear resistance and corrosion resistance [4–6] or to restore worn or damaged components for reusing them [7,8]. It can be possible to do both using the here
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proposed technique, laser cladding, and the same precursor material, a nickel-based alloy. Nickel-based alloys have an outstanding wear and corrosion resistance, even at elevated temperatures and it is a recommended filler alloy to weld cast iron. Therefore, the performance and durability of cast iron parts can be improved by the generation of a coating of this alloy on new parts. It can be used also to rebuild old components and increase their life. Among Ni-based alloys, NiCrBSi presents particularly good performance [9]. In this alloy, chromium improves the high temperature corrosion and oxidation resistance; it also increases the hardness. Boron and silicon decrease the melting point of this material. There is an intense research on the generation of NiCrBSi coatings by different thermal spraying techniques like flame spraying (FS) [10], atmospheric plasma spraying (APS) [11] or high velocity oxygen fuel (HVOF) [12]; and also it has been widely studied the laser cladding of NiCrBSi over carbon steel [13–15], or pure Ti and Ti6Al4V [16,17]. It was possible to find only one research work of laser cladding of NiCrBSi over cast iron using a CO2 laser source [6], but mainly focused on studying the wear behavior of the coatings. Laser cladding on cast iron is difficult because of its heterogeneous composition and it is not very frequently reported on literature as it has been pointed out [4]. Graphite presents higher laser beam absorption than iron matrix. Additionally, the graphite morphology has a strong influence on thermal conductivity of this material. The thermal conductivity of the iron matrix is considerably lower than the thermal conductivity of graphite and the thermal conductivity of gray iron is superior to that of ductile cast iron. As a result, non-homogeneous thermal fields are generated in the laser process and the type of cast iron may strongly influence on the results. Moreover, metals present a higher absorption coefficient for 1 m laser radiation (Nd:YAG, diode, disk, or fiber laser) than for 10 m (CO2 laser). Diode, disk or fiber lasers present a greater wall-plug efficiency than CO2 lasers and they can be delivered using optical fiber. All these features make high power diodes, disk or fiber lasers the most interesting choices for industrial cladding applications. So, there is a lack of research on the processing parameters to generate Ni-based coatings over cast iron by means of a nearinfrared laser. Particularly, it may be of interest a comparison between different types of cast iron substrates (with different graphite morphology) to analyze its impact on the process results. The objective of this work is to generate a NiCrBSi coating over cast iron by means of fiber laser cladding over flat substrates of gray cast iron (EN-GJL-250) and ductile cast iron (EN-GJS-400-15), also known as nodular cast iron. The relationship between processing parameters (laser irradiance and scanning speed) and geometry of a single laser track were examined. Moreover, microstructure and composition were studied by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). The hardness and elastic modulus were analyzed by means of micro- and nanoindentation.
2. Materials and methods The laser cladding with side setup was selected to generate the coatings (see Fig. 1(a)). In this setup, laser beam is employed as heat source to generate the molten pool in a substrate. A material in form of particles is fed into the molten pool. The relative movement between the beam and the workpiece makes possible to generate a clad layer. The coatings were generated on flat substrates of cast iron with dimensions of 50 mm × 50 mm × 15 mm. Two different types of cast iron were employed: gray cast iron (ENGJL-250) and nodular cast iron (EN-GJS-400-15). Fig. 2(a) and (b)
Fig. 1. (a) Outline of the laser cladding with lateral particle injection experimental set-up; and (b) sketch representing geometrical parameters of single laser clad track: height (h), depth (d), width (w) and clad angles (˛1 and ˛2 ).
shows the characteristic microstructure of each type of cast iron; the corresponding chemical compositions provided by the supplier are detailed in Table 1. The experiments were done using a high power fiber laser (IPG YLR 3000) to generate the molten pool and a CNC table was employed to move the substrate with regard to the laser head. Commercial NiCrBSi alloy powder, Ni401 from Sandvik Osprey Ltd., with a particle size between 125 m and 150 m was used as precursor material for the coatings. The powder was carried by argon and laterally injected in the molten pool by a convergent nozzle. Morphology and size of the particles can be seen in Fig. 2(c) and the chemical composition provided by the supplier is detailed in Table 1. The processing parameters for single clad tracks are shown in Table 2. 12 samples were generated in total, 6 samples for ductile cast iron (from a1 to a6) and 6 samples for gray cast iron (from b1 to b6), varying power (300 W and 500 W) and scanning speed (1 mm/s, 2 mm/s and 5 mm/s). Laser beam was focused employing a lens with a diameter of 50 mm and a focal length of 200 mm. The working distance, from the substrate to the lens, was longer than the focal length to obtain a spot diameter of 1.7 mm; this value was kept constant for all the experiments. The mean irradiance was 132 W/mm2 or 220 W/mm2 for a delivered laser power of 300 W or 500 W, respectively. Mass flow of NiCrBSi alloy particles is fixed at a rate of 8.5 g/min. Samples of single laser cladding tracks were cut, embedded in resin and polished to observe the cross-section. The geometrical parameters represented on Fig. 1(b) were measured on the cross-section of the samples: height (h), depth (d), width (w) and clad angles (˛1 and ˛2 ). This parameters were also employed to calculate the width-to-height aspect ratio (wh), a ratio between width and height of the clad [1]; and the mean clad angle (˛m ), the average between both clad angles (˛1 and ˛2 ). By studying
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Table 1 Chemical composition (wt.% ± SD) of the experimental materials, NiCrBSi particles, gray cast iron (gray CI) and ductile cast iron (ductile CI). Element
Ductile CI
Gray CI
NiCrBSi
Fe C Si Cu Mn P Mg S Cr Ni B
Bal. 3.37 ± 0.02 2.84 ± 0.02 0.41 ± 0.01 0.23 ± 0.01 0.053 ± 0.001 0.037 ± 0.001 0.023 ± 0.001 0.016 ± 0.001 0.010 ± 0.001 –
Bal. 2.93 ± 0.03 2.27 ± 0.04 0.92 ± 0.01 0.48 ± 0.01 0.082 ± 0.001 – 0.041 ± 0.001 0.12 ± 0.01 0.033 ± 0.001 –
3.13 ± 0.01 0.36 ± 0.01 2.94 ± 0.01 – – – – – 10.1 ± 0.1 Bal. 1.97 ± 0.01
Table 2 Laser processing parameters: delivered laser power (P), laser irradiance (I) scanning speed (v), mass flow of NiCrBSi particles (f) and spot diameter (d).
Fig. 2. SEM micrographs showing characteristic microstructure of two type of cast iron substrates used in laser cladding experiments: (a) ductile cast iron graphite (in black) in form of spheres; (b) gray cast iron with graphite (in black) in form of flakes; and (c) SEM micrograph showing particle size and morphology of NiCrBSi particles employed in this work.
these samples, the processing parameters that maximize the aspect ratio and the clad angle are selected to generate a complete coating. These coatings were created on flat substrates by overlapping single clad tracks. Immediately after the finishing of the process, the temperature of the substrate was measured employing an infrared pyrometer Cole-Pharmer 39800-32. The properties of the coatings were characterized by different techniques. Again, the samples were cut, embedded in resin and polished to study the cross-section. The analysis of the morphology was done by optical microscopy. The microstructure was studied by means of scanning electron microscopy (SEM, Philips XL30) employing a backscattering
electron detection mode. The qualitative elemental composition was determined via Energy-dispersive X-ray spectroscopy (EDS, EDAX PV9760 coupled to the SEM). X-ray diffraction analysis was carried out by means of a PANalytical X’Pert Pro X-ray diffrac˚ over tometer, using monochromated Cu-K␣ radiation ( = 1.54 A) the 15–100◦ 2 range with step size of 0.02◦ . Diffractograms were obtained directly from the top surface of the complete coating. The surface to be analyzed by this technique must be flat because a rough sample would give spurious results. Therefore, before the Xray diffraction test, the top surfaces of the samples were polished with a series of abrasive SiC papers. The polishing process was done to obtain a smooth surface with a roughness below 1 m and keeping a coating thicker than 0.5 mm. The XRD analysis was performed to compare the crystal structure of the coatings with the cast iron substrates and the NiCrBSi particles. Finally, mechanical properties of the coating were determined. Vickers microhardness was measured using microindentation (microindenter Shimadzu HMV-G), applying a load of 10 kp during a dwell time of 10 s. Young’s modulus was examined via nanoindentation (nanoindenter NanoXP) using a Berkovich indenter tip, with a continuous stiffness measurement technique and penetrations between 100 and 1000 nm. 3. Results and discussion In order to achieve a coating generated by laser cladding is necessary to study the geometry of single laser cladding tracks. The cross-section of single tracks was observed by optical microscopy to create a processing map for ductile cast iron substrate (Fig. 3(a)) and gray cast iron substrate (Fig. 3(b)). Geometrical parameters
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Fig. 3. Optical micrograph of the cross-section of single clad tracks generated: (a) ductile cast iron substrate and (b) gray cast iron.
Table 3 Geometrical parameters measured on the cross-section of the samples. Sample
w (mm)
h (mm)
d (mm)
˛1 (◦ )
a1 a2 a3 a4 a5 a6 b1 b2 b3 b4 b5 b6
2.0 1.8 1.7 1.7 1.0 0.9 2.1 2.2 1.7 1.5 1.1 1.3
2.7 1.9 1.0 2.4 1.9 0.9 1.9 1.2 0.6 2.4 1.5 0.6
0.5 0.4 0.4 0.2 0.2 0.2 0.4 0.6 0.8 0.2 0.2 0.3
48 72 88 32 46 57 50 67 119 30 43 111
˛2 (◦ ) 45 74 92 41 40 68 49 94 108 36 54 92
wh
˛m (◦ )
0.74 0.94 1.70 0.71 0.53 1.00 1.10 1.83 2.83 0.63 0.73 2.17
46.5 73 90 36.5 43 62.5 49.5 80.5 113.5 33 48.5 101.5
Note: see Fig. 1(b) to identify the measured parameters: width (w), height (h), depth (d), clad angles (˛1 and ˛2 ); and calculated parameters: width-to-height aspect ratio (wh) and mean clad angle (˛m ). The uncertainty of the measurements is ±0.1 mm for width, height and depth, and ±1◦ for clad angles.
measured on the cross-section of the samples are presented on Table 3. The selection of the best combination of processing parameters is possible by an inspection of the geometry of the single clad tracks. To obtain coatings without inter-run porosity between overlapped
tracks, the clad angle (˛m ) must be greater than 90◦ [1]. Clad angles of samples processed with the same parameters present in general higher values in gray cast iron substrate than in ductile cast iron, except in the case of a5/b5. It can be appreciated also a direct correlation between the laser irradiance and the clad angle, and between the scanning speed and the clad angle. The greatest clad angles are obtained at the highest speed and laser irradiance. In ductile cast iron substrate (Fig. 3(a)), the highest clad angle has been obtained in the sample a3 (Table 3), generated with a delivered laser irradiance of 220 W/mm2 and a scanning speed of 5 mm/s (Table 2). Other combinations of parameters generate clad tracks with a clad angle lower than 90◦ . In gray cast iron substrate (Fig. 3(b)), samples b3 and b6 present a clad angle higher than 90◦ (Table 3) and they could be appropriate to fulfill the objectives of this work. However, the highest clad angle for gray cast iron substrate has been obtained in the sample b3 (Table 3), which has been generated using the same processing parameters as sample a3 (Table 2); so they were the selected parameters to create complete coatings. The selected conditions for the generation of a complete coating have been a delivered laser irradiance of 220 W/mm2 and a scanning speed of 5 mm/s. Attending to the width of the single clad tracks, 1.7 mm; the selected distance between subsequent clads was 1.5 mm to get overlapped clad tracks. Using the selected
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Fig. 4. Optical micrograph of the cross-section of the complete coating generated over cast iron substrates: (a) ductile cast iron and (b) gray cast iron.
parameter values, it has been possible to generate complete coatings by overlapping of single tracks on both types of cast iron. The cross-section of the complete coatings was studied by optical microscopy and the corresponding micrographs are shown in Fig. 4. A Ni-based alloy coating was obtained on both substrates, see Fig. 4; but with the presence of some micropores. Porosity observed is also present on single clad tracks (see Fig. 3) and it not only appears between subsequent tracks; so, it is not “inter-run porosity”. In this case, it is a consequence of trapped gas in the coating after the cooling of the molten pool [18]. This gas can be argon used to carry and inject the particles, or it can be volatile compounds formed by chemical reactions of the involved elements in the process. The gas cannot leak out of the molten pool in a short time if the distance between the bubble position and the external surface is too large. Fig. 5 shows SEM micrographs exhibiting the interface between the Ni-based coating and the substrate, ductile cast iron (a) and gray cast iron (b). For both types of cast iron, the substrate keeps its original microstructure. In the case of ductile cast iron (Fig. 5(a)) is possible to appreciate the characteristic spheres of graphite, in black; and for ductile cast iron (Fig. 5(b)), the graphite appears in form of flakes, also in black. The coating presents a dendritic microstructure that can be seen in Fig. 5. Its composition was studied by EDS (see Fig. 6(a) and (b)). Qualitatively, it is similar for both types of substrates and it presents nickel, iron, chromium, carbon, and silicon as main elements. The melting of part of the substrate produces the dilution of the injected particles; so, the composition of the coatings is an alloy formed by part of the substrate material and the particles of NiCrBSi. Chemical composition has been determined via EDS for single clad tracks (Table 4) and for complete coatings (Table 5). However, the provided semi-quantitative data is not fully reliable because EDS is not accurate to quantify low atomic number elements such as boron or carbon. In all the measurements, carbon content should be much lower attending the chemical composition of the NiCrBSi particles and cast iron substrates; and boron is not detected at all.
Fig. 5. SEM micrograph showing the interface between the Ni-based coating and the substrate: (a) ductile cast iron and (b) gray cast iron.
However, the ratio between %Ni+%Cr (relative weight content of Ni and Cr) and %Ni+%Cr+%Fe (relative weight content of Ni, Cr and Fe) measured by EDS can be accurate enough and this ratio can be employed to determine the dilution of the samples. Dilution can be defined as the percentage of total weight of the surface layer contributed by melting of the substrate. Eq. (1) was applied to calculate the dilution for each sample. Weight content of Ni and Cr in the substrate material was considered insignificant in comparison to the content of the particles to simplify the equation. The uncertainty in the calculation of the dilution was calculated by assuming an uncertainty of ±2% in the quantification of the weight content by EDS.
Dilution (%wt) = 1 −
%Ni+%Cr %Ni+%Cr+%Fe sample %Ni+%Cr %Ni+%Cr+%Fe NiCrBSi
(1)
The dilution determined for each sample is presented in Fig. 6(c). Gray cast iron samples generally present higher dilution than the equivalent samples in ductile cast iron. It is observed an increase in the dilution values for the complete coatings respect to samples a3 and b3, which have been processed using with the same values of laser irradiance and scanning speed. The higher dilution of the coatings is a result of a growth of the molten pool dimensions produced by a higher temperature of the substrate. The increase in the temperature of the substrate when a single clad track is generated is not significant; however, when a complete coating is created, the much larger irradiation time promotes an increase of the temperature of the substrate up to 350 ± 20 ◦ C. The substrates employed are relatively small and the heat conduction from the melt pool is high enough to rise their temperature.
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Fig. 6. EDS spectrum of NiCrBSi coating: (a) ductile cast iron and (b) gray cast iron. (c) Dilution calculated for single clad tracks and complete coatings.
Table 4 Semi-quantitative chemical composition (wt.%) of single clad tracks determined by EDS.
Table 5 Semi-quantitative chemical composition (wt.%) of complete coatings determined by EDS.
Sample
Ni
Cr
Fe
Si
C
Sample
Ni
Cr
Fe
Si
C
a1 a2 a3 a4 a5 a6 b1 b2 b3 b4 b5 b6
77.6 64.4 62.7 77.7 77.6 73.1 73.3 56.2 46.6 77.1 76.0 56.1
9.8 8.7 8.2 10.3 9.4 9.2 9.4 7.3 6.2 10.3 9.8 7.4
5.4 18.5 21.8 4.7 5.0 10.3 10.3 29.3 40.0 4.1 6.6 28.8
3.5 3.1 3.2 3.3 3.1 3.5 3.4 3.1 3.1 3.7 3.6 3.4
3.7 5.3 4.1 4.0 4.9 3.9 3.6 4.1 4.1 4.8 4.0 4.3
Coating over ductile CI Coating over gray CI
46.8 35.4
6.7 4.7
38.4 52.3
3.1 2.9
5.0 4.7
The analysis by X-ray diffraction is performed to compare the coating and the initial materials (ductile cast iron, gray cast iron and NiCrBSi particles). These comparatives are presented in Fig. 7. The X-ray diffractograms of the substrates are similar for both types of cast iron; they are represented by a red line in Fig. 7. The crystalline phase detected in the substrates is ferrite, BCC iron (␣-Fe; JCPDSICDD ref. 00-006-0696).
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Fig. 7. Comparative of X-ray diffractograms for coatings generated: (a) ductile cast iron and (b) gray cast iron.
Fig. 8. (a) Sketch of the test by micro- and nanoindentation to study the hardness and elastic modulus of the coating. The indentations were performed from the substrate to the coating. The original surface of the substrate is taken as zero reference point. (b) Young’s modulus measured by nanoindentation. (c) Hardness measured by microindentation.
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X-ray diffraction pattern for NiCrBSi particles is represented by a green line in Fig. 7. The main crystalline phase of the NiCrBSi particles is FCC nickel (␥-Ni; JCPDS-ICDD ref. 00-004-0850). XRD of NiCrBSi particles presents other peaks which cannot be reliably identified because of their low relative intensity and the overlapping of one with others. X-ray diffractograms of NiCrBSi particles or coatings can be found in other works [19–21]. The X-ray diffraction patterns showed in those articles are analogous to the ones obtained in the present work for NiCrBSi particles. The main phases are identified as ␥-Ni, (Cr,Fe)7 C3 , CrB and Ni3 B. However, X-ray diffraction pattern of the coatings show some differences with regard to the one corresponding to NiCrBSi particles. The peaks identified as ␥-Ni for NiCrBSi particles (green line) are shifted an average of 1.1◦ to the left in the coating (black line). The left shift of peak position indicates an increase of lattice parameters as it was reported by Qiang et al. [20]. The peaks are also wider, which can be a consequence of lower crystallinity. The peaks that correspond to (Cr,Fe)7 C3 , CrB and Ni3 B are not observed in the X-ray diffractogram of the coating. As it was reported [14,15], variations in the thermal conditions during solidification can modify the type of phases in the NiCrBSi coating generated by laser cladding. Nevertheless, in those cited works the dilution of the coating was found to be between 5% and 15% while in the present work, it is between 40% and 55%. Iron (atomic number 26) and nickel (atomic number 28) present high affinity and they form a NiFe alloy. The crystalline phase of the coatings is an FCC austenitic matrix based on both: iron and nickel (␥-NiFe; JCPDS-ICDD ref. 00-023-0297). FCC NiFe pattern is very similar to the FCC nickel, but the peaks on the FCC NiFe are shifted to the left with regard to FCC nickel. The wider peaks can be a consequence of a heterogeneous distribution of iron and nickel in the matrix. Different concentrations of nickel and iron produce slight variations on the lattice parameters of the unit cells and distortions in the crystal structure, which produce an increase in the width of the peaks. The Young’s modulus and the hardness have been studied by nanoindentation and by microindentation, respectively. The indentations were performed from the substrate to the coating (Fig. 8(a)). The original surface of the substrate is taken as reference, as zero point. The Young’s modulus values measured are represented in Fig. 8(b). The elastic modulus is similar for both substrates and the coating: 230 ± 30 GPa. The elastic modulus values measured in the coating are consistent with those reported in other works for NiCrBSi coatings generated by laser cladding [22] or by thermal spraying combined with laser remelting [23]. The results of the hardness measurements are represented in Fig. 8(c). The coatings present higher hardness than the substrate. The increase is significant; in ductile cast iron, from 180 ± 20 HV (substrate) grows to 550 ± 40 HV (coating); and in gray cast iron, from 270 ± 40 HV (substrate) grows to 470 ± 40 HV (coating). Analogous values of hardness have been reported in laser cladding coatings of a similar NiCrBSi alloy [13]. 4. Conclusions A hardfacing coating has been generated by fiber laser cladding. Suitable processing parameters to generate the Ni-based alloy coating were presented. For the same processing parameters, gray cast iron samples present higher dilution than cast iron samples. Moreover, the complete coating presents more dilution than single track because the much larger laser irradiation time produces a significant increase of the temperature of the substrate. The elemental composition of the coating was characterized and it presents Ni, Cr, Fe, B, Si and C. The fraction of elements reveals the dilution between the substrate and the injected particles, resulting in a coating formed by a mixture between part of the substrate and the injected particles. The main crystalline phase present in
the coatings is identified as FCC austenitic matrix based on nickel and iron. The elastic modulus is similar for the coating and the substrate, while the Ni-based coating obtained presents a significantly superior hardness than cast iron. Acknowledgments The authors wish to thank the technical staff from CACTI (University of Vigo) for their technical assistance and Fundiciones Rey S.L. for providing the substrate materials. This work was partially supported by EU and Government of Spain (FEDER-Innterconecta CLADRING), FPU program FPU13/02944 grant, Xunta de Galicia (CN2012/292, POS-A/ 2013/161) and the University of Vigo (Research Grant F. AriasGonzález). References [1] E. Toyserkani, A. Khajepour, S.F. Corbin, Laser Cladding, 2005. ˜ ˜ a. Riveiro, F. Quintero, J. Pou, Fibre laser [2] F. Lusquinos, R. Comesana, micro-cladding of Co-based alloys on stainless steel, Surf. Coat. Technol. 203 (2009) 1933–1940, http://dx.doi.org/10.1016/j.surfcoat.2009.01.020. [3] J.R. 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Fiber laser cladding of nickel-based alloy on cast iron ˜ b , J. Penide a , F. Lusquinos ˜ a , F. Quintero a , F. Arias-González a,∗ , J. del Val a , R. Comesana a a a A. Riveiro , M. Boutinguiza , J. Pou a b
Applied Physics Dpt., University of Vigo, EEI, Lagoas-Marcosende, Vigo E-36310, Spain Materials Engineering, Applied Mechanics and Construction Dpt., University of Vigo, EEI, Lagoas-Marcosende, Vigo E-36310, Spain
a r t i c l e
i n f o
Article history: Received 23 June 2015 Received in revised form 2 October 2015 Accepted 3 November 2015 Available online xxx Keywords: Laser cladding Laser alloying Cast iron Nickel-based alloy Microstructure Hardness
a b s t r a c t Gray cast iron is a ferrous alloy characterized by a carbon-rich phase in form of lamellar graphite in an iron matrix while ductile cast iron presents a carbon-rich phase in form of spheroidal graphite. Graphite presents a higher laser beam absorption than iron matrix and its morphology has also a strong influence on thermal conductivity of the material. The laser cladding process of cast iron is complicated by its heterogeneous microstructure which generates non-homogeneous thermal fields. In this research work, a comparison between different types of cast iron substrates (with different graphite morphology) has been carried out to analyze its impact on the process results. A fiber laser was used to generate a NiCrBSi coating over flat substrates of gray cast iron (EN-GJL-250) and nodular cast iron (EN-GJS-400-15). The relationship between processing parameters (laser irradiance and scanning speed) and geometry of a single laser track was examined. Moreover, microstructure and composition were studied by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). The hardness and elastic modulus were analyzed by means of micro- and nanoindentation. A hardfacing coating was generated by fiber laser cladding. Suitable processing parameters to generate the Ni-based alloy coating were determined. For the same processing parameters, gray cast iron samples present higher dilution than cast iron samples. The elastic modulus is similar for the coating and the substrate, while the Ni-based coating obtained presents a significantly superior hardness than cast iron. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Laser cladding is a technique that is gaining popularity in the industry for commercial applications, encouraged by the development of reliable and robust industrial high power lasers. It is a manufacturing process to generate a dense and metallurgical bonded coating over a substrate. Laser beam is used as heat source to generate a melt pool in a substrate, in which the precursor material is fed [1]. The relative movement between the beam and the workpiece makes possible to generate a layer with a thickness ranged from microns to millimeters [2]. On new components, laser cladding can be applied to improve the surface properties; on old components, it can be applied to restore worn or damaged surfaces in order to reuse them. Cast iron is a group of ferrous alloys which contains, at least, 2% carbon and 1–3% silicon [3]. The most common types of cast irons are gray cast iron, which exhibits a carbon-rich phase
∗ Corresponding author. E-mail address: [email protected] (F. Arias-González).
composed of lamellar graphite, and ductile cast iron, which presents a carbon-rich phase composed of spheroidal graphite. In 1700s, the development of cast iron as an engineering material made the industrial revolution possible. Nowadays, it is seen as a brittle, weak and dirty material that is not proper for applications that require high strength in comparison to other alloys. In some applications, it is being substituted by more expensive metallic materials like steel, aluminum-alloys, titanium-alloys, etc. However, the good castability and machinability of cast iron are an opportunity for significant cost savings in manufacturing those parts which are not the more likely to break. Moreover, its high damping capacity is interesting to reduce noise and vibrations. So, cast iron alloys are still being widely used in industry to manufacture brake discs and drums, bearings, gears, engine components, machine tool structural parts, components in rock crushers, etc. There exists a high demand in industry to improve the performance and durability of all the components. In order to increase the life of cast iron parts, it has been proposed to enhance its surface properties like its hardness, wear resistance and corrosion resistance [4–6] or to restore worn or damaged components for reusing them [7,8]. It can be possible to do both using the here
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proposed technique, laser cladding, and the same precursor material, a nickel-based alloy. Nickel-based alloys have an outstanding wear and corrosion resistance, even at elevated temperatures and it is a recommended filler alloy to weld cast iron. Therefore, the performance and durability of cast iron parts can be improved by the generation of a coating of this alloy on new parts. It can be used also to rebuild old components and increase their life. Among Ni-based alloys, NiCrBSi presents particularly good performance [9]. In this alloy, chromium improves the high temperature corrosion and oxidation resistance; it also increases the hardness. Boron and silicon decrease the melting point of this material. There is an intense research on the generation of NiCrBSi coatings by different thermal spraying techniques like flame spraying (FS) [10], atmospheric plasma spraying (APS) [11] or high velocity oxygen fuel (HVOF) [12]; and also it has been widely studied the laser cladding of NiCrBSi over carbon steel [13–15], or pure Ti and Ti6Al4V [16,17]. It was possible to find only one research work of laser cladding of NiCrBSi over cast iron using a CO2 laser source [6], but mainly focused on studying the wear behavior of the coatings. Laser cladding on cast iron is difficult because of its heterogeneous composition and it is not very frequently reported on literature as it has been pointed out [4]. Graphite presents higher laser beam absorption than iron matrix. Additionally, the graphite morphology has a strong influence on thermal conductivity of this material. The thermal conductivity of the iron matrix is considerably lower than the thermal conductivity of graphite and the thermal conductivity of gray iron is superior to that of ductile cast iron. As a result, non-homogeneous thermal fields are generated in the laser process and the type of cast iron may strongly influence on the results. Moreover, metals present a higher absorption coefficient for 1 m laser radiation (Nd:YAG, diode, disk, or fiber laser) than for 10 m (CO2 laser). Diode, disk or fiber lasers present a greater wall-plug efficiency than CO2 lasers and they can be delivered using optical fiber. All these features make high power diodes, disk or fiber lasers the most interesting choices for industrial cladding applications. So, there is a lack of research on the processing parameters to generate Ni-based coatings over cast iron by means of a nearinfrared laser. Particularly, it may be of interest a comparison between different types of cast iron substrates (with different graphite morphology) to analyze its impact on the process results. The objective of this work is to generate a NiCrBSi coating over cast iron by means of fiber laser cladding over flat substrates of gray cast iron (EN-GJL-250) and ductile cast iron (EN-GJS-400-15), also known as nodular cast iron. The relationship between processing parameters (laser irradiance and scanning speed) and geometry of a single laser track were examined. Moreover, microstructure and composition were studied by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). The hardness and elastic modulus were analyzed by means of micro- and nanoindentation.
2. Materials and methods The laser cladding with side setup was selected to generate the coatings (see Fig. 1(a)). In this setup, laser beam is employed as heat source to generate the molten pool in a substrate. A material in form of particles is fed into the molten pool. The relative movement between the beam and the workpiece makes possible to generate a clad layer. The coatings were generated on flat substrates of cast iron with dimensions of 50 mm × 50 mm × 15 mm. Two different types of cast iron were employed: gray cast iron (ENGJL-250) and nodular cast iron (EN-GJS-400-15). Fig. 2(a) and (b)
Fig. 1. (a) Outline of the laser cladding with lateral particle injection experimental set-up; and (b) sketch representing geometrical parameters of single laser clad track: height (h), depth (d), width (w) and clad angles (˛1 and ˛2 ).
shows the characteristic microstructure of each type of cast iron; the corresponding chemical compositions provided by the supplier are detailed in Table 1. The experiments were done using a high power fiber laser (IPG YLR 3000) to generate the molten pool and a CNC table was employed to move the substrate with regard to the laser head. Commercial NiCrBSi alloy powder, Ni401 from Sandvik Osprey Ltd., with a particle size between 125 m and 150 m was used as precursor material for the coatings. The powder was carried by argon and laterally injected in the molten pool by a convergent nozzle. Morphology and size of the particles can be seen in Fig. 2(c) and the chemical composition provided by the supplier is detailed in Table 1. The processing parameters for single clad tracks are shown in Table 2. 12 samples were generated in total, 6 samples for ductile cast iron (from a1 to a6) and 6 samples for gray cast iron (from b1 to b6), varying power (300 W and 500 W) and scanning speed (1 mm/s, 2 mm/s and 5 mm/s). Laser beam was focused employing a lens with a diameter of 50 mm and a focal length of 200 mm. The working distance, from the substrate to the lens, was longer than the focal length to obtain a spot diameter of 1.7 mm; this value was kept constant for all the experiments. The mean irradiance was 132 W/mm2 or 220 W/mm2 for a delivered laser power of 300 W or 500 W, respectively. Mass flow of NiCrBSi alloy particles is fixed at a rate of 8.5 g/min. Samples of single laser cladding tracks were cut, embedded in resin and polished to observe the cross-section. The geometrical parameters represented on Fig. 1(b) were measured on the cross-section of the samples: height (h), depth (d), width (w) and clad angles (˛1 and ˛2 ). This parameters were also employed to calculate the width-to-height aspect ratio (wh), a ratio between width and height of the clad [1]; and the mean clad angle (˛m ), the average between both clad angles (˛1 and ˛2 ). By studying
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Table 1 Chemical composition (wt.% ± SD) of the experimental materials, NiCrBSi particles, gray cast iron (gray CI) and ductile cast iron (ductile CI). Element
Ductile CI
Gray CI
NiCrBSi
Fe C Si Cu Mn P Mg S Cr Ni B
Bal. 3.37 ± 0.02 2.84 ± 0.02 0.41 ± 0.01 0.23 ± 0.01 0.053 ± 0.001 0.037 ± 0.001 0.023 ± 0.001 0.016 ± 0.001 0.010 ± 0.001 –
Bal. 2.93 ± 0.03 2.27 ± 0.04 0.92 ± 0.01 0.48 ± 0.01 0.082 ± 0.001 – 0.041 ± 0.001 0.12 ± 0.01 0.033 ± 0.001 –
3.13 ± 0.01 0.36 ± 0.01 2.94 ± 0.01 – – – – – 10.1 ± 0.1 Bal. 1.97 ± 0.01
Table 2 Laser processing parameters: delivered laser power (P), laser irradiance (I) scanning speed (v), mass flow of NiCrBSi particles (f) and spot diameter (d).
Fig. 2. SEM micrographs showing characteristic microstructure of two type of cast iron substrates used in laser cladding experiments: (a) ductile cast iron graphite (in black) in form of spheres; (b) gray cast iron with graphite (in black) in form of flakes; and (c) SEM micrograph showing particle size and morphology of NiCrBSi particles employed in this work.
these samples, the processing parameters that maximize the aspect ratio and the clad angle are selected to generate a complete coating. These coatings were created on flat substrates by overlapping single clad tracks. Immediately after the finishing of the process, the temperature of the substrate was measured employing an infrared pyrometer Cole-Pharmer 39800-32. The properties of the coatings were characterized by different techniques. Again, the samples were cut, embedded in resin and polished to study the cross-section. The analysis of the morphology was done by optical microscopy. The microstructure was studied by means of scanning electron microscopy (SEM, Philips XL30) employing a backscattering
electron detection mode. The qualitative elemental composition was determined via Energy-dispersive X-ray spectroscopy (EDS, EDAX PV9760 coupled to the SEM). X-ray diffraction analysis was carried out by means of a PANalytical X’Pert Pro X-ray diffrac˚ over tometer, using monochromated Cu-K␣ radiation ( = 1.54 A) the 15–100◦ 2 range with step size of 0.02◦ . Diffractograms were obtained directly from the top surface of the complete coating. The surface to be analyzed by this technique must be flat because a rough sample would give spurious results. Therefore, before the Xray diffraction test, the top surfaces of the samples were polished with a series of abrasive SiC papers. The polishing process was done to obtain a smooth surface with a roughness below 1 m and keeping a coating thicker than 0.5 mm. The XRD analysis was performed to compare the crystal structure of the coatings with the cast iron substrates and the NiCrBSi particles. Finally, mechanical properties of the coating were determined. Vickers microhardness was measured using microindentation (microindenter Shimadzu HMV-G), applying a load of 10 kp during a dwell time of 10 s. Young’s modulus was examined via nanoindentation (nanoindenter NanoXP) using a Berkovich indenter tip, with a continuous stiffness measurement technique and penetrations between 100 and 1000 nm. 3. Results and discussion In order to achieve a coating generated by laser cladding is necessary to study the geometry of single laser cladding tracks. The cross-section of single tracks was observed by optical microscopy to create a processing map for ductile cast iron substrate (Fig. 3(a)) and gray cast iron substrate (Fig. 3(b)). Geometrical parameters
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Fig. 3. Optical micrograph of the cross-section of single clad tracks generated: (a) ductile cast iron substrate and (b) gray cast iron.
Table 3 Geometrical parameters measured on the cross-section of the samples. Sample
w (mm)
h (mm)
d (mm)
˛1 (◦ )
a1 a2 a3 a4 a5 a6 b1 b2 b3 b4 b5 b6
2.0 1.8 1.7 1.7 1.0 0.9 2.1 2.2 1.7 1.5 1.1 1.3
2.7 1.9 1.0 2.4 1.9 0.9 1.9 1.2 0.6 2.4 1.5 0.6
0.5 0.4 0.4 0.2 0.2 0.2 0.4 0.6 0.8 0.2 0.2 0.3
48 72 88 32 46 57 50 67 119 30 43 111
˛2 (◦ ) 45 74 92 41 40 68 49 94 108 36 54 92
wh
˛m (◦ )
0.74 0.94 1.70 0.71 0.53 1.00 1.10 1.83 2.83 0.63 0.73 2.17
46.5 73 90 36.5 43 62.5 49.5 80.5 113.5 33 48.5 101.5
Note: see Fig. 1(b) to identify the measured parameters: width (w), height (h), depth (d), clad angles (˛1 and ˛2 ); and calculated parameters: width-to-height aspect ratio (wh) and mean clad angle (˛m ). The uncertainty of the measurements is ±0.1 mm for width, height and depth, and ±1◦ for clad angles.
measured on the cross-section of the samples are presented on Table 3. The selection of the best combination of processing parameters is possible by an inspection of the geometry of the single clad tracks. To obtain coatings without inter-run porosity between overlapped
tracks, the clad angle (˛m ) must be greater than 90◦ [1]. Clad angles of samples processed with the same parameters present in general higher values in gray cast iron substrate than in ductile cast iron, except in the case of a5/b5. It can be appreciated also a direct correlation between the laser irradiance and the clad angle, and between the scanning speed and the clad angle. The greatest clad angles are obtained at the highest speed and laser irradiance. In ductile cast iron substrate (Fig. 3(a)), the highest clad angle has been obtained in the sample a3 (Table 3), generated with a delivered laser irradiance of 220 W/mm2 and a scanning speed of 5 mm/s (Table 2). Other combinations of parameters generate clad tracks with a clad angle lower than 90◦ . In gray cast iron substrate (Fig. 3(b)), samples b3 and b6 present a clad angle higher than 90◦ (Table 3) and they could be appropriate to fulfill the objectives of this work. However, the highest clad angle for gray cast iron substrate has been obtained in the sample b3 (Table 3), which has been generated using the same processing parameters as sample a3 (Table 2); so they were the selected parameters to create complete coatings. The selected conditions for the generation of a complete coating have been a delivered laser irradiance of 220 W/mm2 and a scanning speed of 5 mm/s. Attending to the width of the single clad tracks, 1.7 mm; the selected distance between subsequent clads was 1.5 mm to get overlapped clad tracks. Using the selected
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Fig. 4. Optical micrograph of the cross-section of the complete coating generated over cast iron substrates: (a) ductile cast iron and (b) gray cast iron.
parameter values, it has been possible to generate complete coatings by overlapping of single tracks on both types of cast iron. The cross-section of the complete coatings was studied by optical microscopy and the corresponding micrographs are shown in Fig. 4. A Ni-based alloy coating was obtained on both substrates, see Fig. 4; but with the presence of some micropores. Porosity observed is also present on single clad tracks (see Fig. 3) and it not only appears between subsequent tracks; so, it is not “inter-run porosity”. In this case, it is a consequence of trapped gas in the coating after the cooling of the molten pool [18]. This gas can be argon used to carry and inject the particles, or it can be volatile compounds formed by chemical reactions of the involved elements in the process. The gas cannot leak out of the molten pool in a short time if the distance between the bubble position and the external surface is too large. Fig. 5 shows SEM micrographs exhibiting the interface between the Ni-based coating and the substrate, ductile cast iron (a) and gray cast iron (b). For both types of cast iron, the substrate keeps its original microstructure. In the case of ductile cast iron (Fig. 5(a)) is possible to appreciate the characteristic spheres of graphite, in black; and for ductile cast iron (Fig. 5(b)), the graphite appears in form of flakes, also in black. The coating presents a dendritic microstructure that can be seen in Fig. 5. Its composition was studied by EDS (see Fig. 6(a) and (b)). Qualitatively, it is similar for both types of substrates and it presents nickel, iron, chromium, carbon, and silicon as main elements. The melting of part of the substrate produces the dilution of the injected particles; so, the composition of the coatings is an alloy formed by part of the substrate material and the particles of NiCrBSi. Chemical composition has been determined via EDS for single clad tracks (Table 4) and for complete coatings (Table 5). However, the provided semi-quantitative data is not fully reliable because EDS is not accurate to quantify low atomic number elements such as boron or carbon. In all the measurements, carbon content should be much lower attending the chemical composition of the NiCrBSi particles and cast iron substrates; and boron is not detected at all.
Fig. 5. SEM micrograph showing the interface between the Ni-based coating and the substrate: (a) ductile cast iron and (b) gray cast iron.
However, the ratio between %Ni+%Cr (relative weight content of Ni and Cr) and %Ni+%Cr+%Fe (relative weight content of Ni, Cr and Fe) measured by EDS can be accurate enough and this ratio can be employed to determine the dilution of the samples. Dilution can be defined as the percentage of total weight of the surface layer contributed by melting of the substrate. Eq. (1) was applied to calculate the dilution for each sample. Weight content of Ni and Cr in the substrate material was considered insignificant in comparison to the content of the particles to simplify the equation. The uncertainty in the calculation of the dilution was calculated by assuming an uncertainty of ±2% in the quantification of the weight content by EDS.
Dilution (%wt) = 1 −
%Ni+%Cr %Ni+%Cr+%Fe sample %Ni+%Cr %Ni+%Cr+%Fe NiCrBSi
(1)
The dilution determined for each sample is presented in Fig. 6(c). Gray cast iron samples generally present higher dilution than the equivalent samples in ductile cast iron. It is observed an increase in the dilution values for the complete coatings respect to samples a3 and b3, which have been processed using with the same values of laser irradiance and scanning speed. The higher dilution of the coatings is a result of a growth of the molten pool dimensions produced by a higher temperature of the substrate. The increase in the temperature of the substrate when a single clad track is generated is not significant; however, when a complete coating is created, the much larger irradiation time promotes an increase of the temperature of the substrate up to 350 ± 20 ◦ C. The substrates employed are relatively small and the heat conduction from the melt pool is high enough to rise their temperature.
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Fig. 6. EDS spectrum of NiCrBSi coating: (a) ductile cast iron and (b) gray cast iron. (c) Dilution calculated for single clad tracks and complete coatings.
Table 4 Semi-quantitative chemical composition (wt.%) of single clad tracks determined by EDS.
Table 5 Semi-quantitative chemical composition (wt.%) of complete coatings determined by EDS.
Sample
Ni
Cr
Fe
Si
C
Sample
Ni
Cr
Fe
Si
C
a1 a2 a3 a4 a5 a6 b1 b2 b3 b4 b5 b6
77.6 64.4 62.7 77.7 77.6 73.1 73.3 56.2 46.6 77.1 76.0 56.1
9.8 8.7 8.2 10.3 9.4 9.2 9.4 7.3 6.2 10.3 9.8 7.4
5.4 18.5 21.8 4.7 5.0 10.3 10.3 29.3 40.0 4.1 6.6 28.8
3.5 3.1 3.2 3.3 3.1 3.5 3.4 3.1 3.1 3.7 3.6 3.4
3.7 5.3 4.1 4.0 4.9 3.9 3.6 4.1 4.1 4.8 4.0 4.3
Coating over ductile CI Coating over gray CI
46.8 35.4
6.7 4.7
38.4 52.3
3.1 2.9
5.0 4.7
The analysis by X-ray diffraction is performed to compare the coating and the initial materials (ductile cast iron, gray cast iron and NiCrBSi particles). These comparatives are presented in Fig. 7. The X-ray diffractograms of the substrates are similar for both types of cast iron; they are represented by a red line in Fig. 7. The crystalline phase detected in the substrates is ferrite, BCC iron (␣-Fe; JCPDSICDD ref. 00-006-0696).
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Fig. 7. Comparative of X-ray diffractograms for coatings generated: (a) ductile cast iron and (b) gray cast iron.
Fig. 8. (a) Sketch of the test by micro- and nanoindentation to study the hardness and elastic modulus of the coating. The indentations were performed from the substrate to the coating. The original surface of the substrate is taken as zero reference point. (b) Young’s modulus measured by nanoindentation. (c) Hardness measured by microindentation.
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X-ray diffraction pattern for NiCrBSi particles is represented by a green line in Fig. 7. The main crystalline phase of the NiCrBSi particles is FCC nickel (␥-Ni; JCPDS-ICDD ref. 00-004-0850). XRD of NiCrBSi particles presents other peaks which cannot be reliably identified because of their low relative intensity and the overlapping of one with others. X-ray diffractograms of NiCrBSi particles or coatings can be found in other works [19–21]. The X-ray diffraction patterns showed in those articles are analogous to the ones obtained in the present work for NiCrBSi particles. The main phases are identified as ␥-Ni, (Cr,Fe)7 C3 , CrB and Ni3 B. However, X-ray diffraction pattern of the coatings show some differences with regard to the one corresponding to NiCrBSi particles. The peaks identified as ␥-Ni for NiCrBSi particles (green line) are shifted an average of 1.1◦ to the left in the coating (black line). The left shift of peak position indicates an increase of lattice parameters as it was reported by Qiang et al. [20]. The peaks are also wider, which can be a consequence of lower crystallinity. The peaks that correspond to (Cr,Fe)7 C3 , CrB and Ni3 B are not observed in the X-ray diffractogram of the coating. As it was reported [14,15], variations in the thermal conditions during solidification can modify the type of phases in the NiCrBSi coating generated by laser cladding. Nevertheless, in those cited works the dilution of the coating was found to be between 5% and 15% while in the present work, it is between 40% and 55%. Iron (atomic number 26) and nickel (atomic number 28) present high affinity and they form a NiFe alloy. The crystalline phase of the coatings is an FCC austenitic matrix based on both: iron and nickel (␥-NiFe; JCPDS-ICDD ref. 00-023-0297). FCC NiFe pattern is very similar to the FCC nickel, but the peaks on the FCC NiFe are shifted to the left with regard to FCC nickel. The wider peaks can be a consequence of a heterogeneous distribution of iron and nickel in the matrix. Different concentrations of nickel and iron produce slight variations on the lattice parameters of the unit cells and distortions in the crystal structure, which produce an increase in the width of the peaks. The Young’s modulus and the hardness have been studied by nanoindentation and by microindentation, respectively. The indentations were performed from the substrate to the coating (Fig. 8(a)). The original surface of the substrate is taken as reference, as zero point. The Young’s modulus values measured are represented in Fig. 8(b). The elastic modulus is similar for both substrates and the coating: 230 ± 30 GPa. The elastic modulus values measured in the coating are consistent with those reported in other works for NiCrBSi coatings generated by laser cladding [22] or by thermal spraying combined with laser remelting [23]. The results of the hardness measurements are represented in Fig. 8(c). The coatings present higher hardness than the substrate. The increase is significant; in ductile cast iron, from 180 ± 20 HV (substrate) grows to 550 ± 40 HV (coating); and in gray cast iron, from 270 ± 40 HV (substrate) grows to 470 ± 40 HV (coating). Analogous values of hardness have been reported in laser cladding coatings of a similar NiCrBSi alloy [13]. 4. Conclusions A hardfacing coating has been generated by fiber laser cladding. Suitable processing parameters to generate the Ni-based alloy coating were presented. For the same processing parameters, gray cast iron samples present higher dilution than cast iron samples. Moreover, the complete coating presents more dilution than single track because the much larger laser irradiation time produces a significant increase of the temperature of the substrate. The elemental composition of the coating was characterized and it presents Ni, Cr, Fe, B, Si and C. The fraction of elements reveals the dilution between the substrate and the injected particles, resulting in a coating formed by a mixture between part of the substrate and the injected particles. The main crystalline phase present in
the coatings is identified as FCC austenitic matrix based on nickel and iron. The elastic modulus is similar for the coating and the substrate, while the Ni-based coating obtained presents a significantly superior hardness than cast iron. Acknowledgments The authors wish to thank the technical staff from CACTI (University of Vigo) for their technical assistance and Fundiciones Rey S.L. for providing the substrate materials. This work was partially supported by EU and Government of Spain (FEDER-Innterconecta CLADRING), FPU program FPU13/02944 grant, Xunta de Galicia (CN2012/292, POS-A/ 2013/161) and the University of Vigo (Research Grant F. AriasGonzález). References [1] E. Toyserkani, A. Khajepour, S.F. Corbin, Laser Cladding, 2005. ˜ ˜ a. Riveiro, F. Quintero, J. Pou, Fibre laser [2] F. Lusquinos, R. Comesana, micro-cladding of Co-based alloys on stainless steel, Surf. Coat. Technol. 203 (2009) 1933–1940, http://dx.doi.org/10.1016/j.surfcoat.2009.01.020. [3] J.R. 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