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Microstructural evolution and characteristics of bonding zone in multilayer laser cladding of Fe-based coating
Journal of Materials Processing Tech. 263 (2019) 50–58
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Microstructural evolution and characteristics of bonding zone in multilayer laser cladding of Fe-based coating K.Y. Luoa, X. Xua, Z. Zhaoa,b, S.S. Zhaob,c, Z.G. Chengb, J.Z. Lua,
T
⁎
a
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212000, China Jiangsu ZKDG Laser Technology Co, Ltd, Zhenjiang 212000, China c Laboratory of All-Solid-State Source, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multilayer laser cladding Fe-based coating Bonding zone Microstructural evolution Element distribution Phase transformation Microhardness
Multilayer coatings were prepared on H13k steel substrate by laser cladding, and Fe-based powders mixed with different Mo and Ni contents were used. The microstructure change between layers in the multilayer deposit process was analyzed. The effects of Mo and Ni contents on the microstructure, phase transformation, element distribution, and microhardness were also systematically investigated. Results showed that dendrite matrix and interdendrite netlike eutectic structure were found in Fe-based cladding layers. Furthermore, the bonding zone was mainly composed of coarse dendrites because of remelting and reheating induced by secondary energy input. Due to the heat accumulation, relatively large dendrites in the bonding zone and thick bonding zone could be formed in the latter layers rather than in the early layers. The phases of sample 1 and sample 3 at room temperature are martensite, residual austenite, residual δ ferrite, M2B and metal carbide (M23C6, and M7C3). The phases of sample 2 at room temperature are austenite, residual δ ferrite, M2B and metallic carbide (M23C6, and M7C3) because more Ni content leads to the change of solidification mode. With increasing Ni content up to 8 wt %, the microhardness of laser cladding layer decreased from 600 HV0.5 to 400 HV0.5 but was distributed homogenously from the interior region to the bonding zone.
1. Introduction Laser cladding (LC) is a new surface modification technology, and produces a surface strengthening layer on the metal substrate by rapid melting and freezing of the substrate and the coating when laser beam with high power density irradiates into the metal surface. The LC layer displays excellent wear resistance, corrosion resistance, and antioxidation properties. LC has achieved significant progress since it was developed by D. S. Gnanamuth in the United States in 1974. Since the early 1990s, LC technology has been extensively used in all fields of industrial production. Fe-based coating is widely used in various aspects in manufacturing industries because of its low cost, high-abrasive resistance, and high hardness. A series of different elements or reinforced particles can be introduced into a Fe-based metal system to cause the improvement in the properties of Fe-based coating. Ramiro et al. (2018) proved that the crack free Fe-based coating by laser metal deposition could be produced without preheating the base material. Wang et al. (2014) added different kinds of Ni powder (Ni-based powder and pure Ni powder) into Fe-based coatings and reported that the number of microdefects in the
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coating increased upon the addition of pure Ni powder but not Ni-based powders. The crack tendency can be reduced by adding Ni-based powder. Chen et al. (2012) cladded a Fe-based cladding coating on the surface of pure titanium substrate, showing that the coating hardness can reach 800 HV0.2 higher (four to five times) than that of the substrate. This finding could be due to the formation of hard particles, such as Fe2Ti, Fe2B, Fe3Si, and Ti2Ni. Fu et al. (2015) investigated the composition and formation mechanism of microstructures in Fe-based coatings and concluded that Fe-based cladding coating mainly contains dendrites and eutectic. At high solidification rates, dendrites are formed first, followed by the formation of eutectic by the unsolidified metallic liquid that nucleated among dendrites. The addition of La2O3 can refine crystals in the coating. Wang et al. (2018) added V8C7 particles into Febased coating system and got the pore-free coating, the microhardness and wear resistance of coating was improved. Gao et al. (2016) found three types of microstructures (planar, columnar dendrites, and equiaxed dendrites) under different solidification rates in different regions. The addition of Ti reduced the number of dendrite structures and increased the number of equiaxed grains, thereby improving the microstructure. However, the hardness decreased because the size of the
Corresponding author at: Xuefu Road 301, Jingkou District, Zhenjiang 212013, China. E-mail addresses: [email protected] (K.Y. Luo), [email protected] (J.Z. Lu).
https://doi.org/10.1016/j.jmatprotec.2018.08.005 Received 3 April 2018; Received in revised form 10 July 2018; Accepted 4 August 2018 Available online 07 August 2018 0924-0136/ © 2018 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 263 (2019) 50–58
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crystal phases increased. Wang et al. (2010) demonstrated the formation of carbides based on the change of Mo content in the cladding layer. When the amount of Fe–Mo (wt.%) increased from 0% to 15%, the volume fraction of carbide gradually increased, whereas the primary carbide size decreased. This finding could be mainly attributed to the smaller size of (Ti/Mo) C hard particle than that of TiC, and that Mo can play fine grain role in the Fe–TieMoeC system. Yang et al. (2007) concluded that Ni affected the microstructure of austenite phase in the FeeCreNi system and provided the opportunity for the formation of interdendritic regions with high Cr content. The microhardness decreased, but the electrochemical corrosion resistance improved with increasing Ni content. Lu et al. (2018) provided a 3-times laser scanning strategy to get the crack free Fe-based coating by laser cladding. Multilayer cladding is widely used in some cases, such as roller repair and piston repair. Scholars have explored the coating performance. Bykovskiy et al. (2015) investigated composition and hardness distribution in bilayer coating with different second-layer track direction (at angles of 0°, 45°, and 90° than in the first case). The elemental composition and equal microhardness were similar between the two cases, but the microhardness distribution along depth direction was homogeneous at an angle of 0° in the direction of the second cladding layer. Xu et al. (2006) discussed the effect of cladding method on crack sensitivity. Constant chemical composition and gradient chemical composition during laser multilayer cladding process were used in this study. The crack sensitivity when getting gradient material distribution is lower than that of constant chemical composition distribution because of the gradual distribution of WC particles. Studies have focused on the change of microstructure and properties of cladding layers with the different element content and parameters. The properties of cladding coating applied with different cladding methods have been extensively studied. However, few scholars have investigated the bonding zone between cladding layers; as such, the properties of the bonding zone should be studied deeply. This study aims to investigate the microstructure revolution, phase solidification and bonding characteristics in multilayer coating. The strategy of reciprocated deposition, namely, multilayer LC, was applied considering the material properties of the elements added. The microstructural revolution and element distribution in the cladding layer and in the bonding zone were investigated. The hardness distribution in the coating layer was also examined.
direction, and the powder was sent into the melt pool instantaneously. When one cladding path was finished, the laser beam without energy output and side powder nozzle moved to the start point of the following layer and traveled in the same direction along the path to deposit the following layer. In width direction, five to seven cladding tracks should be deposited to obtain a wide cladding surface. In thickness direction, 10 layers should be deposited for observation and further tests. After the deposition, the samples were cooled under the condition of air cooling. For each sample, the processing parameters did not change. The powder was dried to remove moisture before cladding, uniformly mixed, and placed in the powder feeder. The substrate surface was polished and cleaned by acetone. An IPG 4000 W ytterbium fiber laser system was used in this experiment. The beam spot was round, and the energy presented a nearly Gaussian distribution. A method of positive defocus distance was used to obtain an adjustable focal length. Thus, the interaction between laser beam and powder can be improved through side feeding. After optimization, the laser power was 1.7 kW, and the diameter of laser spot was 5 mm. The overlapping rate was 60%, and the speed in laser scanning was 5 mm/s. Pure shield argon was served as shielding gas and its flow rate was 30 L/min. The feeding speed of powder was 10–15 g/min. Three kinds of powders were employed for LC experiments. After cladding, the cladding samples were sectioned by line cutting, mounted by abrasive paper, and polished by grinder for metallographic examination. Chloroauric acid (HCl: HNO3 = 3: 1) was used as metallographic etchant. As shown in Fig. 1b, the bottom of the layer was on the XeOeY plane, the precipitation direction was along the Z-axis, and the observation area was selected from the YeOeZ plane (longitudinal cross section). The macroscopic morphology and the observation area of the experimental sample can be seen in Fig. 2. The morphology and the microstructure were observed using microscope and scanning electron microscopy (SEM) (JSM-6480, JEOL, Japan) with region elemental analysis using energy-dispersive X-ray spectroscopy (EDS) (Naron System Six, Thermo Electron Corporation, USA). Microhardness (HV0.5) was obtained by Vickers hardness tester with a 0.5 kgf loading and 15 s dwell time.
2. Experimental procedures
In this research, 10 layers coating was deposited. Planar and cellular crystals only exist between substrate and coating, but not between layers. Fig. 3a shows the optical micrograph of fifth to seventh layers in sample 1, and two types of microstructures are distinguished in the coating. Optical microscopic image of bonding zone between layers (Fig. 3b) showed that a white-bright “band” which mainly contained with incorporated equiaxial dendrites could be found. The columnar dendrites occurred within short distance from the bonding zone (Fig. 3b). In the interior region of each layer, small and refined equiaxial dendrites could be found (Fig. 3c). But steering dendrites also occurred at the top of early layers (Fig. 4). Fully-grown columnar dendrites were also easier to be found in the early layers than those in the latter layers (Fig. 4). Two main parameters (the growth rate R and the temperature gradient G) are introduced to describe the microstructural evolvement. According to the research by Kou (2003), the change of G/R ratio might
3. Results and discussion 3.1. Microstructure analysis
2.1. Experimental materials Three different samples (X431, X431 + 8%Ni45, X431 + 1.5%Mo) using three kinds of powders were built for LC experiment. The Febased alloy was X431 stainless steel alloy powder manufactured by Höganäs Corporation. Ltd. H13 K was the substrate, and the Ni-based alloy material Ni45 and the Molybdenum powder were used. The particle size of the powder was within an range of 45–106 μm, and the element content of samples are shown in Table 1. 2.2. Experimental process The deposition approach is shown in Fig. 1a. After LC process beginning, the laser beam and powder nozzle traveled along the length Table 1 Main chemical composition of three samples (wt%). Sample
Materials
Fe
Ni
Cr
C
Si
B
Mo
Mn
1 2 3
431 431 + 8%Ni45 431 + 1.5%Mo
Balance Balance Balance
2.36 7.97 2.32
18.1 17.7 17.8
0.17 0.16 0.17
0.90 1.07 0.89
0.60 0.75 0.59
0.05 0.05 1.55
0.17 0.16 0.17
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Fig. 1. Schematic diagram of (a) the multi-layer laser cladding process and (b) observation strategy.
Fig. 2. Macroscopic morphology and observation area of experimental sample.
lead to the transformation of solidification mode, and the G·R value dominates the size of solidification structure (Fig. 5). When laser beam scans through the surface of substrate, a molten pool will emerge where the laser beam irradiates. Cladding powder is sent into the molten pool by protective airflow and then the molten pool will be solidified when the laser beam removes from the molten region. As the G/R ratio decreases, the type of solidified structure transfers from planar grain to cellular grain and then columnar dendrites and equiaxed dendrites. Hemmati et al. (2011) investigated the microstructural distribution in single-track single-layer coating. Planar crystal, cellular crystal, columnar dendrite, equiaxial dendrite and steering dendrite were formed
Fig. 4. Typical microstructure at the second layer in Sample 1.
orderly from the bottom of layer to the top of layer. Some columnar dendrites growing along the scanning direction (steering dendrites) occurred at the top region of deposit layer because the direction of G changed, due to the result of contacting air and the movement of laser beam. But for the multi-layer coating deposition process, the grain
Fig. 3. Typical optical micrograph of sample 1. (a) Typical micrograph of fifth to seventh layers, (b) the bonding zone A, and (c) the interior region B. 52
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temperature of the molten pool increased with increasing the of cladding layer because of heat accumulation during LC process. The steering dendrites which were observed at the upper part of early layers were not remelted because the size of remolten pool is small. For the latter layers, almost all the steering dendrites are remelted by the next layer deposition process because of the large size of remolten pool. It could be concluded that, with the multilayer cladding process going on, the heat accumulation increases which leads to different G/R ratio distribution between different layers. The morphology and scale of structures between layers are different because of above reasons. The microstructure among three samples are similar with a little different in the size and scale of grains (Fig. 6). It can be concluded that increasing little amount of Ni and Mo in the coating will not affect the microstructural features but the equiaxial dendrite growth orientation in the interior region is more obvious and the grain distribution is more uniform. Research by Zhao et al. (2005) showed that surface tension of molten pool determines its temperature field and flow field. Addition of Ni into Fe-based alloy will obviously change its surface tension of molten pool. Thus it can be inferred that the change of growth orientation and grain distribution is probably due to the effect of additive alloy elements on the molten pool flow and the direction of G.
Fig. 5. Effect of G and R on the morphology and scale of solidification structures (Kou, 2003).
growth orientation is more complex than that of single-layer coating because of heat accumulation and complicated temperature gradient direction. When latter cladding layer is prepared on the surface of previous layer, the top region of previous layer will be remelted and resolidification. Li et al. (2015) reported the high G/R ratio at the bonding interface in Fe-based coating. The G/R ratio at the bottom of molten pool is higher than that at the top, and solidification starts from the bottom region. Because of heat accumulation, planar and cellular grains do not exist at the bottom of molten pool where columnar dendrites are formed (Fig. 3b). Columnar dendrites grow along the direction of G because of the high cooling rate. With the movement of solidliquid interface, the G/R ratio keeps decreasing. Research by Kurz et al. (2001) showed that a columnar to equiaxed transition (CET) would occur in the interior region of layer when the G/R ratio is lower than some critical value. In the upper part of previous layer where is under the remolten region, this region will be reheated by the remolten region. Dendrites in the reheating region have a change to grow and finally to create a white-bright “band” known as bonding zone. Due to the limited heat accumulation and the rapid solidification rate in the early layers, the grain growth is suppressed. The size of dendrite in the reheating zone is small and a thin bonding zone could be found (Fig. 4). With the increase of cladding layer, the heat accumulation of cladding coatings also increases. The heat accumulation reduces the high temperature gradient. Thus, the G/R ratios in the reheating region and at the bottom of remolten pool in the latter layer are lower than those in the early layers, and the solidification rate is low. More columnar dendrites transfer into equiaxial dendrites. High heat accumulation provides a larger scale of reheating zone. So more coarse dendrites and thicker bonding zones between latter layers could be observed. Bontha et al. (2009) and Hofman (2009) proved, in the multilayer laser cladding with constant laser parameters, the scale and
3.2. Component and phase composition analysis Solidification phases of stainless steel depend strongly on its solidification mode. Solidification process can be inferred by the ternary phase diagram. Because of the high cooling rate during the LC process, solidification will not start from the liquidus line but below it. L → γ and L → δ will occur when liquid cools through the T0 line which is regarded as the start of solidification. The phase diagram adding T0 line is shown in Fig. 7. T0 line defines whether the diffusionless transformation occurs or not. T0 curve is below the liquidus line but above the solidus line in the equilibrium phase diagram according to the research data from David et al. (1987). The compositions of three samples are shown in phase diagram with T0 lines. The primary solidus phase depends on whether the undercooled melt crosses the T0 line of γ, δ or both of them. The amount of δ and γ is controlled by the different T0 temperatures of δ and γ. As shown in Fig. 7, temperature of δ, γ for sample 1 is almost the same and it can be concluded that when the undercooled liquid cools down below the T0 lines, similar amount of both phases will be formed simultaneously. As the solidification goes on, most δ translates into γ as the primary solidification phase. For the sample 2 with more Ni, the undercooled liquid crosses T0 line of γ firstly and forms γ as the primary phase directly. The Ni and Cr amount of sample 3 is similar with that of sample 1, so as the similar solidification mode for sample 3 but with a little more δ could be determined. The δ in sample 1 and 3 cannot translate into γ completely mainly because of high solidification rate in the laser cladding process. As the temperature goes down further, martensite transformation may occur. Martensite transformation temperature (Ms) determines whether martensite exists at room temperature. Ms temperatures for different coatings are calculated using Eq. (1) and are shown in
Fig. 6. Typical microstructure at the bonding zones between fifth and sixth layer in three samples. (a) Sample 1 (b) sample 2, and (c) sample 3. 53
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Table 2.
Ms (°C ) = 550 − 350 C− 40Mn − 20Cr − 10Mo − 17Ni − 8 W− 35 V − 10Cu + 15Co + 30Al
(1)
According to the results in Table 2, martensite could be found in sample 1 and sample 3. The connection between element composition of coatings and their equilibrium phases at room temperature can be determined by Schaeffler diagram. To find the position of 3 samples in the Schaeffler diagram, chromium equivalent (Creq) and Nickel equivalent (Nieq) should be calculated by the reasonable Schaeffler equivalent formulas as shown blow and the calculation results are shown in Table 3.
Table 2 Ms temperature of laser cladded coatings. Material
Ms (℃)
1 2 3
X431 X431 + 8%Ni45 X431 + 1.5%Mo
31.08 −53.3 22.76
(2)
Ni eq = Ni + 30 C+ 0.5Mn
(3)
Kim and Peng (2000) used finite element method to predict the cooling rate around the melting pool center, and the highest cooling rate can reach 2850 k⋅s−1. During the LC processing, the cooling rate was extremely high and can reach 103–106 k⋅s−1, which is much higher than that of traditional welding technologies. David et al. (1987) took the cooling rate into account to modify Schaeffler diagram. In this research, simple calculation (Eq. (4)) investigated by Jenny and O’Brien (2001) was used to estimate the cooling rate roughly. The calculation was made using only the 3D case which is appropriate for the higher cooling rates.
Fig. 7. Position of three samples’ composition in ternary phase diagram during partitionless solidification (David et al., 1987).
Sample
Creq = Cr + Mo + 1.5Si + 0.5Nb
R = ∂T ∂t = [2πk (T − Ti )2] (Qη)
where k is the thermal conductivity of material, T is the temperature where the cooling rate is calculated, Ti is the interval temperature, Q is the heat input per unit length and η is the laser efficiency. For the calculation, a laser efficiency factor of 0.2 was used. T was defined as the liquidus temperature of 2462 K for all alloys. And the Ti was the room temperature, 298 K. The thermal conductivity k for alloys in high temperature was around 25 W ⋅m−1⋅k −1. The result of cooling rate was around 0.01 × 106 k⋅s−1. The cooling rate at the remolten region can be also inferred by the SDAS theoretical system. Joly and Mehrabian (1974) demonstrated the relationship between cooling rate and dendrite arm spacing. Cáceres et al. (2002) studied how the cooling rate effects the secondary dendrite arm spacing (SDAS) and provided the
Table 3 Cr-equivalent and Ni-equivalent of laser cladded coatings. Sample
Material
Cr-eq
Ni-eq
Cr-eq/Ni-eq
1 2 3
X431 X431 + 8%Ni45 X431 + 1.5%Mo
19.5 19.4 20.7
7.5 12.9 7.5
2.6 1.5 2.8
(4)
Fig. 8. The position of solidification phases at room temperature on modified Schaeffler diagram (David et al., 1987). 54
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Fig. 9. XRD spectrum analyses of cladding coatings. (a) Sample 1 (b) sample 2, and (c) sample 3.
Fig. 10. Typical SEM images and EDS analysis results of corresponding elements of region A and region B in three cladding layers. (a) Sample 1, (b) sample 2, and (c) sample 3. •
calculated method. The SDAS at the bottom of remolten region was approximately 7 μm. The equation (Eq. 5) which is used to calculate SDAS is as follows:
SDAS = A (T )−n
(5)
•
Where T is the cooling rate, A and n are constant values which depends on the different alloy system. As demonstrated (Liu et al., 2016), for Fe 55
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Fig. 11. Typical SEM image (a) and the corresponding element distributions (b, c, d, e) around the bonding zone in sample 1.
Fig. 12. Typical SEM image (a) and the corresponding element distributions (b, c, d, e) around the bonding zone in sample 2.
alloy, the values of A and n are 100 and 0.35, respectively. Thus, the cooling rate when the molten pool solidified reached 0.2 × 103 k⋅s−1, which was similar with the value calculated by the simple calculation (Eq. (4)). The modified Schaeffler diagram at the cooling rate in this research is shown in Fig. 8, and the phase composition of coating can be inferred by this diagram. It can be concluded that the phases in sample 1 and 3 are martensite, austenite and δ ferrite, while the only phase in sample 2 is austenite. To prove the correct of theoretical deduction, X-ray diffraction results of laser cladding coating are given in Fig. 9. The phases in sample 1 are α-Fe, γ-Fe, M2B, M23C6, and M7C3. The phases in sample 3 are similar with those in sample 1 but with less γ-Fe because more Mo will promote the formation of ferrite and suppress the formation of residual austenite. The phases in sample 2 are γ-Fe, M2B, M23C6 and M7C3. Results of analysis show that addition of Ni and Mo may affect the solidification mode of coatings. SEM was used to show the scanning election micrographs and characterize the densification and chemistry of coatings. EDS mappings showed element distribution in the specific area, which included the
bonding zone and both-sides nearly interior region. It can be seen from Fig. 10 that the bonding zone of coating is mainly composed of dendrites and interdendritic netlike eutectic structures (INES). The dendrites nucleated from the liquid at the beginning, and then the remaining liquid transferred into INES through the eutectic reaction. Then the dendrite matrix in the bonding zone will grow during the reheating process. Figs. 11–13 show the corresponding element distribution (Fe, Ni, Cr, Mo and C) among three samples. Element content of INES and dendrite matrix in the bonding zone in three types of layers was detected as shown in Fig. 10. According to the EDS mapping results and pointelement-content analysis, the content of Cr, Ni and C is relatively high in the INES. In the coating of sample 3 which added 1.5% Mo, Mo only existed in INES. Discussed with the XRD results together, it can be determined that almost of M23C6 and M7C3 exists in the INES. Some δ ferrite also can be found in eutectic structures because adding Mo can promote the formation of ferrite. The Fe content in the dendrite matrix is highest which can be indicated that the dendrites are mainly composed of martensite, austenite and δ ferrite. 56
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Fig. 13. Typical SEM image (a) and the corresponding element distributions (b, c, d, e, f) around the bonding zone in sample 2.
4. Conclusion The aim of this study was to analyze the microstructure and properties of bonding zone in LC layer using a multilayer deposition approach. The morphology, microstructure, phase transformation, element distribution, and microhardness of the coatings with different element compositions were analyzed. The conclusions from this research are as follows. (1) In multilayer cladding process, the remolten region and the reheat region at the upper part of previous layer could be produced by the next layer deposit process. The white-bright band which is known as bonding zone between layers could be found. The bonding zone is mainly composed of coarse dendrites because the reheating energy input provides the motivation for dendrite growth. (2) When using the constant laser process parameters, the size of remolten region and reheat region will increase with the increase of layers because of the heat accumulation. The larger dendrites and thicker bonding zones could be formed in the latter layers than in the early layers. Steering dendrites could be found at the upper part of early layers but not latter layers because of the change of reheat region scale. (3) The phases of sample 1 and sample 3 at room temperature are martensite, residual austenite, residual δ ferrite, M2B and metal carbide (M23C6, and M7C3). The phases of sample 2 at room temperature are austenite, residual δ ferrite, M2B and metallic carbide (M23C6, and M7C3) because more Ni content leads to the change of solidification mode. A little different content of M2B, M23C6, and M7C3 among three samples occurred because of different content of Ni and Mo which can affect the formation of metal boride and metal carbide. (4) Dendrites and INES can be found in the bonding zone. The dendrite structures are mainly composed of austenite, martensite and δ ferrite. Almost of M23C6 and M7C3 exists in the INES. Addition of Mo can promote the formation of δ ferrite in the INES. (5) Along the vertical direction of bonding zone, the microhardness in samples 1 and 3 decreased slightly from the interior region to the bonding zone. The reason was probably because the content of INES, which included more Cr in the bonding zone, was lower than that in the interior region. However, in sample 2, which includes more Ni, the microhardness transformation was not obvious because the amount of INES was similar to that of dendrites. Ni addition can obviously decrease the coating microhardness.
Fig. 14. Surface microhardness curves along the vertical direction of the boundary of three samples.
3.3. Microhardness analysis Fig. 14 shows that the average hardness (approximately 400 HV0.5) in sample 2 is obviously lower than that (600–700 HV0.5) in samples 1 and 3 because of more Ni contents in sample 2. The microhardness distribution along the vertical direction of the boundary among the four different layers was roughly homogenous with a slight change. In samples 1 and 3, a decreasing trend of hardness was observed toward the interior region to the bonding zone. This finding could be due to the low scale of eutectic structures which included high Cr amount in the bonding zone. In sample 2, the microhardness difference between the interior region and bonding zone cannot be distinguished probably because the number of dendrites and INES was almost the same in the bonding zone, and the grain size was also similar in both microstructures. When the Ni content in the coating was less, the average microhardness was high but with obvious difference in microhardness between the bonding zone and interior region. As Ni increased, the average microhardness decreases with homogenous hardness distribution.
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Acknowledgments
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This work was supported by the National Key R&D Program of China (No. 2017YFB1103603), the National Natural Science Foundation of China (grant number 51775250), the Jiangsu Provincial Science and Technology Projects (grant numbers BE2016148, and BE2017142), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References Bontha, S., Klingbeil, N., Kobryn, P., Fraser, H., 2009. Effects of process variables and size-scale on solidification microstructure in beam-based fabrication of bulky 3D structures. J. Mater. Sci. Eng. A 513, 311–318. Bykovskiy, D.P., Petrovskiy, V.N., Murzakov, M.A., Mironov, V.D., Dzhumaev, P.S., Polski, V.I., 2015. Formation of multilayer claddings using high-power fiber laser radiation. J. Phys. Procedia 71, 227–231. Cáceres, C.H., Davidson, C.J., Griffiths, J.R., Newtona, C.L., 2002. Effects of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy. Mater. Sci. Eng. A. 325, 344–355. Chen, J.M., Guo, C., Zhou, J.S., 2012. Microstructure and tribological properties of laser cladding Fe-based coating on pure Ti substrate. J. Trans. Nonf. Metal. Soc. China 22, 2171–2178. David, S.A., Vitek, J.M., Hebble, T.L., 1987. Effect of rapid solidification on stainless steel weld metal microstructures and its implications on the Schaeffler diagram. J. Korean Weld. Join. Soc. 66, 289. Fu, Z.K., Ding, H.H., Wang, W.J., Liu, Q.Y., Guo, J., Zhu, M.H., 2015. Investigation on microstructure and wear characteristic of laser cladding Fe-based alloy on wheel/rail materials. Int. J. Wearable Device 330-331, 592–599. Gao, W.Y., Zhang, Z.Y., Zhao, S.S., Wang, Y.B., Chen, H., Lin, X.C., 2016. Effect of a small addition of Ti on the Fe-based coating by laser cladding. Surf. Coat. Technol. 291, 423–429. Hemmati, I., Ocelı´k, V., Hosson, J.T.M.D., 2011. Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings. J. Mater. Sci. 46, 3405–3414. Hofman, J., 2009. Development of an Observation and Control System for Industrial Laser Cladding. PhD Thesis. University of Twente, The Netherlands, pp. 121–126.
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Microstructural evolution and characteristics of bonding zone in multilayer laser cladding of Fe-based coating K.Y. Luoa, X. Xua, Z. Zhaoa,b, S.S. Zhaob,c, Z.G. Chengb, J.Z. Lua,
T
⁎
a
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212000, China Jiangsu ZKDG Laser Technology Co, Ltd, Zhenjiang 212000, China c Laboratory of All-Solid-State Source, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multilayer laser cladding Fe-based coating Bonding zone Microstructural evolution Element distribution Phase transformation Microhardness
Multilayer coatings were prepared on H13k steel substrate by laser cladding, and Fe-based powders mixed with different Mo and Ni contents were used. The microstructure change between layers in the multilayer deposit process was analyzed. The effects of Mo and Ni contents on the microstructure, phase transformation, element distribution, and microhardness were also systematically investigated. Results showed that dendrite matrix and interdendrite netlike eutectic structure were found in Fe-based cladding layers. Furthermore, the bonding zone was mainly composed of coarse dendrites because of remelting and reheating induced by secondary energy input. Due to the heat accumulation, relatively large dendrites in the bonding zone and thick bonding zone could be formed in the latter layers rather than in the early layers. The phases of sample 1 and sample 3 at room temperature are martensite, residual austenite, residual δ ferrite, M2B and metal carbide (M23C6, and M7C3). The phases of sample 2 at room temperature are austenite, residual δ ferrite, M2B and metallic carbide (M23C6, and M7C3) because more Ni content leads to the change of solidification mode. With increasing Ni content up to 8 wt %, the microhardness of laser cladding layer decreased from 600 HV0.5 to 400 HV0.5 but was distributed homogenously from the interior region to the bonding zone.
1. Introduction Laser cladding (LC) is a new surface modification technology, and produces a surface strengthening layer on the metal substrate by rapid melting and freezing of the substrate and the coating when laser beam with high power density irradiates into the metal surface. The LC layer displays excellent wear resistance, corrosion resistance, and antioxidation properties. LC has achieved significant progress since it was developed by D. S. Gnanamuth in the United States in 1974. Since the early 1990s, LC technology has been extensively used in all fields of industrial production. Fe-based coating is widely used in various aspects in manufacturing industries because of its low cost, high-abrasive resistance, and high hardness. A series of different elements or reinforced particles can be introduced into a Fe-based metal system to cause the improvement in the properties of Fe-based coating. Ramiro et al. (2018) proved that the crack free Fe-based coating by laser metal deposition could be produced without preheating the base material. Wang et al. (2014) added different kinds of Ni powder (Ni-based powder and pure Ni powder) into Fe-based coatings and reported that the number of microdefects in the
⁎
coating increased upon the addition of pure Ni powder but not Ni-based powders. The crack tendency can be reduced by adding Ni-based powder. Chen et al. (2012) cladded a Fe-based cladding coating on the surface of pure titanium substrate, showing that the coating hardness can reach 800 HV0.2 higher (four to five times) than that of the substrate. This finding could be due to the formation of hard particles, such as Fe2Ti, Fe2B, Fe3Si, and Ti2Ni. Fu et al. (2015) investigated the composition and formation mechanism of microstructures in Fe-based coatings and concluded that Fe-based cladding coating mainly contains dendrites and eutectic. At high solidification rates, dendrites are formed first, followed by the formation of eutectic by the unsolidified metallic liquid that nucleated among dendrites. The addition of La2O3 can refine crystals in the coating. Wang et al. (2018) added V8C7 particles into Febased coating system and got the pore-free coating, the microhardness and wear resistance of coating was improved. Gao et al. (2016) found three types of microstructures (planar, columnar dendrites, and equiaxed dendrites) under different solidification rates in different regions. The addition of Ti reduced the number of dendrite structures and increased the number of equiaxed grains, thereby improving the microstructure. However, the hardness decreased because the size of the
Corresponding author at: Xuefu Road 301, Jingkou District, Zhenjiang 212013, China. E-mail addresses: [email protected] (K.Y. Luo), [email protected] (J.Z. Lu).
https://doi.org/10.1016/j.jmatprotec.2018.08.005 Received 3 April 2018; Received in revised form 10 July 2018; Accepted 4 August 2018 Available online 07 August 2018 0924-0136/ © 2018 Elsevier B.V. All rights reserved.
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crystal phases increased. Wang et al. (2010) demonstrated the formation of carbides based on the change of Mo content in the cladding layer. When the amount of Fe–Mo (wt.%) increased from 0% to 15%, the volume fraction of carbide gradually increased, whereas the primary carbide size decreased. This finding could be mainly attributed to the smaller size of (Ti/Mo) C hard particle than that of TiC, and that Mo can play fine grain role in the Fe–TieMoeC system. Yang et al. (2007) concluded that Ni affected the microstructure of austenite phase in the FeeCreNi system and provided the opportunity for the formation of interdendritic regions with high Cr content. The microhardness decreased, but the electrochemical corrosion resistance improved with increasing Ni content. Lu et al. (2018) provided a 3-times laser scanning strategy to get the crack free Fe-based coating by laser cladding. Multilayer cladding is widely used in some cases, such as roller repair and piston repair. Scholars have explored the coating performance. Bykovskiy et al. (2015) investigated composition and hardness distribution in bilayer coating with different second-layer track direction (at angles of 0°, 45°, and 90° than in the first case). The elemental composition and equal microhardness were similar between the two cases, but the microhardness distribution along depth direction was homogeneous at an angle of 0° in the direction of the second cladding layer. Xu et al. (2006) discussed the effect of cladding method on crack sensitivity. Constant chemical composition and gradient chemical composition during laser multilayer cladding process were used in this study. The crack sensitivity when getting gradient material distribution is lower than that of constant chemical composition distribution because of the gradual distribution of WC particles. Studies have focused on the change of microstructure and properties of cladding layers with the different element content and parameters. The properties of cladding coating applied with different cladding methods have been extensively studied. However, few scholars have investigated the bonding zone between cladding layers; as such, the properties of the bonding zone should be studied deeply. This study aims to investigate the microstructure revolution, phase solidification and bonding characteristics in multilayer coating. The strategy of reciprocated deposition, namely, multilayer LC, was applied considering the material properties of the elements added. The microstructural revolution and element distribution in the cladding layer and in the bonding zone were investigated. The hardness distribution in the coating layer was also examined.
direction, and the powder was sent into the melt pool instantaneously. When one cladding path was finished, the laser beam without energy output and side powder nozzle moved to the start point of the following layer and traveled in the same direction along the path to deposit the following layer. In width direction, five to seven cladding tracks should be deposited to obtain a wide cladding surface. In thickness direction, 10 layers should be deposited for observation and further tests. After the deposition, the samples were cooled under the condition of air cooling. For each sample, the processing parameters did not change. The powder was dried to remove moisture before cladding, uniformly mixed, and placed in the powder feeder. The substrate surface was polished and cleaned by acetone. An IPG 4000 W ytterbium fiber laser system was used in this experiment. The beam spot was round, and the energy presented a nearly Gaussian distribution. A method of positive defocus distance was used to obtain an adjustable focal length. Thus, the interaction between laser beam and powder can be improved through side feeding. After optimization, the laser power was 1.7 kW, and the diameter of laser spot was 5 mm. The overlapping rate was 60%, and the speed in laser scanning was 5 mm/s. Pure shield argon was served as shielding gas and its flow rate was 30 L/min. The feeding speed of powder was 10–15 g/min. Three kinds of powders were employed for LC experiments. After cladding, the cladding samples were sectioned by line cutting, mounted by abrasive paper, and polished by grinder for metallographic examination. Chloroauric acid (HCl: HNO3 = 3: 1) was used as metallographic etchant. As shown in Fig. 1b, the bottom of the layer was on the XeOeY plane, the precipitation direction was along the Z-axis, and the observation area was selected from the YeOeZ plane (longitudinal cross section). The macroscopic morphology and the observation area of the experimental sample can be seen in Fig. 2. The morphology and the microstructure were observed using microscope and scanning electron microscopy (SEM) (JSM-6480, JEOL, Japan) with region elemental analysis using energy-dispersive X-ray spectroscopy (EDS) (Naron System Six, Thermo Electron Corporation, USA). Microhardness (HV0.5) was obtained by Vickers hardness tester with a 0.5 kgf loading and 15 s dwell time.
2. Experimental procedures
In this research, 10 layers coating was deposited. Planar and cellular crystals only exist between substrate and coating, but not between layers. Fig. 3a shows the optical micrograph of fifth to seventh layers in sample 1, and two types of microstructures are distinguished in the coating. Optical microscopic image of bonding zone between layers (Fig. 3b) showed that a white-bright “band” which mainly contained with incorporated equiaxial dendrites could be found. The columnar dendrites occurred within short distance from the bonding zone (Fig. 3b). In the interior region of each layer, small and refined equiaxial dendrites could be found (Fig. 3c). But steering dendrites also occurred at the top of early layers (Fig. 4). Fully-grown columnar dendrites were also easier to be found in the early layers than those in the latter layers (Fig. 4). Two main parameters (the growth rate R and the temperature gradient G) are introduced to describe the microstructural evolvement. According to the research by Kou (2003), the change of G/R ratio might
3. Results and discussion 3.1. Microstructure analysis
2.1. Experimental materials Three different samples (X431, X431 + 8%Ni45, X431 + 1.5%Mo) using three kinds of powders were built for LC experiment. The Febased alloy was X431 stainless steel alloy powder manufactured by Höganäs Corporation. Ltd. H13 K was the substrate, and the Ni-based alloy material Ni45 and the Molybdenum powder were used. The particle size of the powder was within an range of 45–106 μm, and the element content of samples are shown in Table 1. 2.2. Experimental process The deposition approach is shown in Fig. 1a. After LC process beginning, the laser beam and powder nozzle traveled along the length Table 1 Main chemical composition of three samples (wt%). Sample
Materials
Fe
Ni
Cr
C
Si
B
Mo
Mn
1 2 3
431 431 + 8%Ni45 431 + 1.5%Mo
Balance Balance Balance
2.36 7.97 2.32
18.1 17.7 17.8
0.17 0.16 0.17
0.90 1.07 0.89
0.60 0.75 0.59
0.05 0.05 1.55
0.17 0.16 0.17
51
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Fig. 1. Schematic diagram of (a) the multi-layer laser cladding process and (b) observation strategy.
Fig. 2. Macroscopic morphology and observation area of experimental sample.
lead to the transformation of solidification mode, and the G·R value dominates the size of solidification structure (Fig. 5). When laser beam scans through the surface of substrate, a molten pool will emerge where the laser beam irradiates. Cladding powder is sent into the molten pool by protective airflow and then the molten pool will be solidified when the laser beam removes from the molten region. As the G/R ratio decreases, the type of solidified structure transfers from planar grain to cellular grain and then columnar dendrites and equiaxed dendrites. Hemmati et al. (2011) investigated the microstructural distribution in single-track single-layer coating. Planar crystal, cellular crystal, columnar dendrite, equiaxial dendrite and steering dendrite were formed
Fig. 4. Typical microstructure at the second layer in Sample 1.
orderly from the bottom of layer to the top of layer. Some columnar dendrites growing along the scanning direction (steering dendrites) occurred at the top region of deposit layer because the direction of G changed, due to the result of contacting air and the movement of laser beam. But for the multi-layer coating deposition process, the grain
Fig. 3. Typical optical micrograph of sample 1. (a) Typical micrograph of fifth to seventh layers, (b) the bonding zone A, and (c) the interior region B. 52
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temperature of the molten pool increased with increasing the of cladding layer because of heat accumulation during LC process. The steering dendrites which were observed at the upper part of early layers were not remelted because the size of remolten pool is small. For the latter layers, almost all the steering dendrites are remelted by the next layer deposition process because of the large size of remolten pool. It could be concluded that, with the multilayer cladding process going on, the heat accumulation increases which leads to different G/R ratio distribution between different layers. The morphology and scale of structures between layers are different because of above reasons. The microstructure among three samples are similar with a little different in the size and scale of grains (Fig. 6). It can be concluded that increasing little amount of Ni and Mo in the coating will not affect the microstructural features but the equiaxial dendrite growth orientation in the interior region is more obvious and the grain distribution is more uniform. Research by Zhao et al. (2005) showed that surface tension of molten pool determines its temperature field and flow field. Addition of Ni into Fe-based alloy will obviously change its surface tension of molten pool. Thus it can be inferred that the change of growth orientation and grain distribution is probably due to the effect of additive alloy elements on the molten pool flow and the direction of G.
Fig. 5. Effect of G and R on the morphology and scale of solidification structures (Kou, 2003).
growth orientation is more complex than that of single-layer coating because of heat accumulation and complicated temperature gradient direction. When latter cladding layer is prepared on the surface of previous layer, the top region of previous layer will be remelted and resolidification. Li et al. (2015) reported the high G/R ratio at the bonding interface in Fe-based coating. The G/R ratio at the bottom of molten pool is higher than that at the top, and solidification starts from the bottom region. Because of heat accumulation, planar and cellular grains do not exist at the bottom of molten pool where columnar dendrites are formed (Fig. 3b). Columnar dendrites grow along the direction of G because of the high cooling rate. With the movement of solidliquid interface, the G/R ratio keeps decreasing. Research by Kurz et al. (2001) showed that a columnar to equiaxed transition (CET) would occur in the interior region of layer when the G/R ratio is lower than some critical value. In the upper part of previous layer where is under the remolten region, this region will be reheated by the remolten region. Dendrites in the reheating region have a change to grow and finally to create a white-bright “band” known as bonding zone. Due to the limited heat accumulation and the rapid solidification rate in the early layers, the grain growth is suppressed. The size of dendrite in the reheating zone is small and a thin bonding zone could be found (Fig. 4). With the increase of cladding layer, the heat accumulation of cladding coatings also increases. The heat accumulation reduces the high temperature gradient. Thus, the G/R ratios in the reheating region and at the bottom of remolten pool in the latter layer are lower than those in the early layers, and the solidification rate is low. More columnar dendrites transfer into equiaxial dendrites. High heat accumulation provides a larger scale of reheating zone. So more coarse dendrites and thicker bonding zones between latter layers could be observed. Bontha et al. (2009) and Hofman (2009) proved, in the multilayer laser cladding with constant laser parameters, the scale and
3.2. Component and phase composition analysis Solidification phases of stainless steel depend strongly on its solidification mode. Solidification process can be inferred by the ternary phase diagram. Because of the high cooling rate during the LC process, solidification will not start from the liquidus line but below it. L → γ and L → δ will occur when liquid cools through the T0 line which is regarded as the start of solidification. The phase diagram adding T0 line is shown in Fig. 7. T0 line defines whether the diffusionless transformation occurs or not. T0 curve is below the liquidus line but above the solidus line in the equilibrium phase diagram according to the research data from David et al. (1987). The compositions of three samples are shown in phase diagram with T0 lines. The primary solidus phase depends on whether the undercooled melt crosses the T0 line of γ, δ or both of them. The amount of δ and γ is controlled by the different T0 temperatures of δ and γ. As shown in Fig. 7, temperature of δ, γ for sample 1 is almost the same and it can be concluded that when the undercooled liquid cools down below the T0 lines, similar amount of both phases will be formed simultaneously. As the solidification goes on, most δ translates into γ as the primary solidification phase. For the sample 2 with more Ni, the undercooled liquid crosses T0 line of γ firstly and forms γ as the primary phase directly. The Ni and Cr amount of sample 3 is similar with that of sample 1, so as the similar solidification mode for sample 3 but with a little more δ could be determined. The δ in sample 1 and 3 cannot translate into γ completely mainly because of high solidification rate in the laser cladding process. As the temperature goes down further, martensite transformation may occur. Martensite transformation temperature (Ms) determines whether martensite exists at room temperature. Ms temperatures for different coatings are calculated using Eq. (1) and are shown in
Fig. 6. Typical microstructure at the bonding zones between fifth and sixth layer in three samples. (a) Sample 1 (b) sample 2, and (c) sample 3. 53
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Table 2.
Ms (°C ) = 550 − 350 C− 40Mn − 20Cr − 10Mo − 17Ni − 8 W− 35 V − 10Cu + 15Co + 30Al
(1)
According to the results in Table 2, martensite could be found in sample 1 and sample 3. The connection between element composition of coatings and their equilibrium phases at room temperature can be determined by Schaeffler diagram. To find the position of 3 samples in the Schaeffler diagram, chromium equivalent (Creq) and Nickel equivalent (Nieq) should be calculated by the reasonable Schaeffler equivalent formulas as shown blow and the calculation results are shown in Table 3.
Table 2 Ms temperature of laser cladded coatings. Material
Ms (℃)
1 2 3
X431 X431 + 8%Ni45 X431 + 1.5%Mo
31.08 −53.3 22.76
(2)
Ni eq = Ni + 30 C+ 0.5Mn
(3)
Kim and Peng (2000) used finite element method to predict the cooling rate around the melting pool center, and the highest cooling rate can reach 2850 k⋅s−1. During the LC processing, the cooling rate was extremely high and can reach 103–106 k⋅s−1, which is much higher than that of traditional welding technologies. David et al. (1987) took the cooling rate into account to modify Schaeffler diagram. In this research, simple calculation (Eq. (4)) investigated by Jenny and O’Brien (2001) was used to estimate the cooling rate roughly. The calculation was made using only the 3D case which is appropriate for the higher cooling rates.
Fig. 7. Position of three samples’ composition in ternary phase diagram during partitionless solidification (David et al., 1987).
Sample
Creq = Cr + Mo + 1.5Si + 0.5Nb
R = ∂T ∂t = [2πk (T − Ti )2] (Qη)
where k is the thermal conductivity of material, T is the temperature where the cooling rate is calculated, Ti is the interval temperature, Q is the heat input per unit length and η is the laser efficiency. For the calculation, a laser efficiency factor of 0.2 was used. T was defined as the liquidus temperature of 2462 K for all alloys. And the Ti was the room temperature, 298 K. The thermal conductivity k for alloys in high temperature was around 25 W ⋅m−1⋅k −1. The result of cooling rate was around 0.01 × 106 k⋅s−1. The cooling rate at the remolten region can be also inferred by the SDAS theoretical system. Joly and Mehrabian (1974) demonstrated the relationship between cooling rate and dendrite arm spacing. Cáceres et al. (2002) studied how the cooling rate effects the secondary dendrite arm spacing (SDAS) and provided the
Table 3 Cr-equivalent and Ni-equivalent of laser cladded coatings. Sample
Material
Cr-eq
Ni-eq
Cr-eq/Ni-eq
1 2 3
X431 X431 + 8%Ni45 X431 + 1.5%Mo
19.5 19.4 20.7
7.5 12.9 7.5
2.6 1.5 2.8
(4)
Fig. 8. The position of solidification phases at room temperature on modified Schaeffler diagram (David et al., 1987). 54
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Fig. 9. XRD spectrum analyses of cladding coatings. (a) Sample 1 (b) sample 2, and (c) sample 3.
Fig. 10. Typical SEM images and EDS analysis results of corresponding elements of region A and region B in three cladding layers. (a) Sample 1, (b) sample 2, and (c) sample 3. •
calculated method. The SDAS at the bottom of remolten region was approximately 7 μm. The equation (Eq. 5) which is used to calculate SDAS is as follows:
SDAS = A (T )−n
(5)
•
Where T is the cooling rate, A and n are constant values which depends on the different alloy system. As demonstrated (Liu et al., 2016), for Fe 55
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Fig. 11. Typical SEM image (a) and the corresponding element distributions (b, c, d, e) around the bonding zone in sample 1.
Fig. 12. Typical SEM image (a) and the corresponding element distributions (b, c, d, e) around the bonding zone in sample 2.
alloy, the values of A and n are 100 and 0.35, respectively. Thus, the cooling rate when the molten pool solidified reached 0.2 × 103 k⋅s−1, which was similar with the value calculated by the simple calculation (Eq. (4)). The modified Schaeffler diagram at the cooling rate in this research is shown in Fig. 8, and the phase composition of coating can be inferred by this diagram. It can be concluded that the phases in sample 1 and 3 are martensite, austenite and δ ferrite, while the only phase in sample 2 is austenite. To prove the correct of theoretical deduction, X-ray diffraction results of laser cladding coating are given in Fig. 9. The phases in sample 1 are α-Fe, γ-Fe, M2B, M23C6, and M7C3. The phases in sample 3 are similar with those in sample 1 but with less γ-Fe because more Mo will promote the formation of ferrite and suppress the formation of residual austenite. The phases in sample 2 are γ-Fe, M2B, M23C6 and M7C3. Results of analysis show that addition of Ni and Mo may affect the solidification mode of coatings. SEM was used to show the scanning election micrographs and characterize the densification and chemistry of coatings. EDS mappings showed element distribution in the specific area, which included the
bonding zone and both-sides nearly interior region. It can be seen from Fig. 10 that the bonding zone of coating is mainly composed of dendrites and interdendritic netlike eutectic structures (INES). The dendrites nucleated from the liquid at the beginning, and then the remaining liquid transferred into INES through the eutectic reaction. Then the dendrite matrix in the bonding zone will grow during the reheating process. Figs. 11–13 show the corresponding element distribution (Fe, Ni, Cr, Mo and C) among three samples. Element content of INES and dendrite matrix in the bonding zone in three types of layers was detected as shown in Fig. 10. According to the EDS mapping results and pointelement-content analysis, the content of Cr, Ni and C is relatively high in the INES. In the coating of sample 3 which added 1.5% Mo, Mo only existed in INES. Discussed with the XRD results together, it can be determined that almost of M23C6 and M7C3 exists in the INES. Some δ ferrite also can be found in eutectic structures because adding Mo can promote the formation of ferrite. The Fe content in the dendrite matrix is highest which can be indicated that the dendrites are mainly composed of martensite, austenite and δ ferrite. 56
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Fig. 13. Typical SEM image (a) and the corresponding element distributions (b, c, d, e, f) around the bonding zone in sample 2.
4. Conclusion The aim of this study was to analyze the microstructure and properties of bonding zone in LC layer using a multilayer deposition approach. The morphology, microstructure, phase transformation, element distribution, and microhardness of the coatings with different element compositions were analyzed. The conclusions from this research are as follows. (1) In multilayer cladding process, the remolten region and the reheat region at the upper part of previous layer could be produced by the next layer deposit process. The white-bright band which is known as bonding zone between layers could be found. The bonding zone is mainly composed of coarse dendrites because the reheating energy input provides the motivation for dendrite growth. (2) When using the constant laser process parameters, the size of remolten region and reheat region will increase with the increase of layers because of the heat accumulation. The larger dendrites and thicker bonding zones could be formed in the latter layers than in the early layers. Steering dendrites could be found at the upper part of early layers but not latter layers because of the change of reheat region scale. (3) The phases of sample 1 and sample 3 at room temperature are martensite, residual austenite, residual δ ferrite, M2B and metal carbide (M23C6, and M7C3). The phases of sample 2 at room temperature are austenite, residual δ ferrite, M2B and metallic carbide (M23C6, and M7C3) because more Ni content leads to the change of solidification mode. A little different content of M2B, M23C6, and M7C3 among three samples occurred because of different content of Ni and Mo which can affect the formation of metal boride and metal carbide. (4) Dendrites and INES can be found in the bonding zone. The dendrite structures are mainly composed of austenite, martensite and δ ferrite. Almost of M23C6 and M7C3 exists in the INES. Addition of Mo can promote the formation of δ ferrite in the INES. (5) Along the vertical direction of bonding zone, the microhardness in samples 1 and 3 decreased slightly from the interior region to the bonding zone. The reason was probably because the content of INES, which included more Cr in the bonding zone, was lower than that in the interior region. However, in sample 2, which includes more Ni, the microhardness transformation was not obvious because the amount of INES was similar to that of dendrites. Ni addition can obviously decrease the coating microhardness.
Fig. 14. Surface microhardness curves along the vertical direction of the boundary of three samples.
3.3. Microhardness analysis Fig. 14 shows that the average hardness (approximately 400 HV0.5) in sample 2 is obviously lower than that (600–700 HV0.5) in samples 1 and 3 because of more Ni contents in sample 2. The microhardness distribution along the vertical direction of the boundary among the four different layers was roughly homogenous with a slight change. In samples 1 and 3, a decreasing trend of hardness was observed toward the interior region to the bonding zone. This finding could be due to the low scale of eutectic structures which included high Cr amount in the bonding zone. In sample 2, the microhardness difference between the interior region and bonding zone cannot be distinguished probably because the number of dendrites and INES was almost the same in the bonding zone, and the grain size was also similar in both microstructures. When the Ni content in the coating was less, the average microhardness was high but with obvious difference in microhardness between the bonding zone and interior region. As Ni increased, the average microhardness decreases with homogenous hardness distribution.
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Acknowledgments
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