# Effect of Copper Addition on the Phase Composition and Microstructure Evolution of Fe-Based Amorphous Alloy Coatings Prepared by Laser Cladding

Authors: Xiao Mingying; Xiu Qiang; Jiang Fengchun
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Laser clad coating’s susceptibility to cracking is extremely high due to the brittleness of the material itself and the enormous residual stress caused by rapid heating and cooling. In order to relieve the residual stress, a series of Fe-based amorphous-Cu composite coatings were fabricated on 20 steel using laser processing. The effects of copper addition on the phase composition and microstructure evolution of the coatings were then investigated. The as-prepared coatings, which are mainly composed of Fe-based amorphous, Cu, and (Fe, Cr)23 (C, B)6 have a network microstructure. A large amount of spherical-shaped copper metal is present in the coating and is evenly dispersed within the amorphous matrix. The copper in the coating has little effect on the crystallisation of Fe-based amorphous alloys due to the negligible solubility of solid copper and the Fe-based amorphous alloy. The coating average hardness reduces considerably and shows a significant difference, improving the coating stress distribution.


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## Body

## 1. Introduction

Laser cladding is an advanced surface modification technology, which has been widely used to fabricate high-quality coatings with unique functions such as corrosion and wear resistance and antioxidation in recent years [1–6]. However, laser clad coating’s cracking susceptibility is very high, especially when the coating is made of cermet, amorphous alloy, and high entropy alloy. Generally, the coating material’s brittleness at room temperature [7, 8], microdefects [9, 10], and enormous residual stress [11, 12] are considered to be the leading cause of the cracking. Several methods, such as heat treatment [13], ultrasonic vibration aided manufacturing [14], and friction-stirred processing aided manufacturing [15], have been proposed to reduce the residual stress in the coating.Compared with the methods mentioned above, incorporating toughness and plasticity materials into the laser cladded coating is considered a simpler and more effective way to prevent cracking. Shu et al. [16] prepared a five-layer gradient Ni-based WC coating on the surface of medium carbon steel by laser cladding technology. It is found that the microstructure and hardness of the coating show gradient changes, and the coating has no defects such as cracks and pores. Zhao et al. [17] discovered that a small amount of CeO2 added to the cladded coatings can make WC particles smooth, which helps to improve coating toughness and effectively reduce coating cracks. Li et al. [18] studied the effect of La2O3 on the microstructure and properties of laser cladded Ni-based ceramic coating. In the process of laser cladding, rare earth powder can improve the solidification morphology and surface tension gradient of the cladding coating, reduce the coating’s internal stress, and inhibit the generation of cracks in the coating. Zhang and Sun [19] added MoS2 during laser cladding Ni-based composite coating on the surface of TC4 alloy. They found that with increasing MoS2 in the coating, the crack rate first decreased and then increased. When the MoS2 content in the coating is 20%, the crack rate is the lowest.However, these added constituents may react with the elements within the coating materials and change the phase composition of the coating during laser cladding, which will affect its function. Yuan et al. [3] recently successfully prepared self-lubricating antiwear NiCr/TiC-Cu coating by adding copper using laser cladding technology. As a result, a lot of copper metal is present in the coating without any reaction with the other coating elements or oxidation occurring. Inspired by this work, we aimed to fabricate an Fe-based amorphous-Cu composite coating using laser cladding technology. The effect of copper addition on the phase composition, microstructure, and elemental distribution of the laser clad coatings was subsequently investigated via optical microscopy, X-ray diffraction, scanning electron microscopy, and electro-probe microanalysis.


## 2. Materials and Experimentation

This study used as-received Fe-based amorphous alloy powder and copper powder as the raw materials to fabricate the composite coatings. The Fe-based amorphous alloy powder’s chemical composition was Fe41Co7Cr15Mo14C15B6Y2 (at. %), and the powder particle size ranged between 30–50 μm. The purity of the copper powder was more than 99.9%, and the particle sizes were within the range of 30–50 μm. The homogeneously mixed powders were prepared by ball milling with different mass fractions of copper powders (5wt%, 10wt%, 20wt%, and 40wt%) with the following parameters: speed: 320 r/min; ball diameter: 6 mm; time: 1 hour; temperature: 25°C.As described in Figure1, a cladding system was developed to prepare the coatings, which consisted of a 6 kW fibre output semiconductor laser (RFL-A6000D), powder feeding system, and a CNC machining centre. The laser deposition parameters employed in the deposition were laser power: 3000 W; spot diameter: 3 mm; scanning speed: 3 m/min; powder feeding rate: 16 g/min. Nitrogen with a purity of 99.99% was used as the powder carrier gas and was consumed at 5 L/min, and argon with a purity of 99.995% was used as the shielding gas and was consumed at a constant 15 L/min during the laser cladding. A 100 mm (length) × 55 mm (width) × 11 mm (wall thickness) #20 steel plate was selected as the substrate, which was thoroughly cleaned in an ethanol bath before use.Figure 1
(a) A schematic of the laser cladding system; (b) images of the laser cladding equipment.Laser cladded specimens were cut into 10 mm × 10 mm squares using an EDM machine. The cross-sections of the cladding layers were polished and then etched via aqua regia. X-ray diffraction (XRD, Rigaku D/Max-2500 PC, Japan), with Cu-Kα radiation, was used to identify the coating phases. An optical microscope (OM, Axioscope 5, Germany) was used to observe the cross-sectional surfaces of the coatings. A scanning electron microscope (FESEM, JEOL-7800F, Japan) was used to observe the microstructure, and the local element composition and element mapping were analysed by an electron probe microanalyser (EPMA, JXA-8530F, Japan). A Vickers microhardness tester (VMT, Hv-1000A, China) was used to measure the microhardness value on the surfaces of the polished samples following ASTM E384-16. The applied load was 0.2 kg for 15 s.


## 3. Results and Discussion

The XRD patterns of the composite coatings with copper addition of 5wt%, 10wt%, 20wt%, and 40wt% are described in Figure2(a). As shown, the diffraction patterns have broad peaks in the 2θ region of 40°–50°. There are nine sharp peaks at 37.7°, 41.5°, 43.3°, 44.1°, 48.3°, 50.4°, 70.1°, 80.5°, and 89.9° simultaneously, which are characteristic peaks of (Fe, Cr)23(C, B)6 and copper, respectively. With increasing copper addition, the copper metal phase’s peak intensities increased, while the peak intensities of (Fe, Cr)23(C, B)6 remained unchanged. The cross-sectional OM images of the composite coatings are described in Figure 2(b). It can be seen that the composite coatings are composed of three different phases, namely, a grey-white phase, golden phase, and black phase. The grey-white phase constitutes the main body of the coatings; the golden phase is evenly distributed within the grey-white phase, whereas the black phase is mainly concentrated at the bottom of the coatings in a wavy form.Figure 2
(a) The XRD patterns and (b) the cross-sectional OM images of the coatings with different copper addition percentages (5wt%, 10wt%, 20wt%, and 40wt%).
(a)(b)The different phases of the composite coating are further magnified, as shown in Figure3. In Figure 3(a), no grain boundary is observed within the grey-white phase. A large amount of golden phase is distributed within the grey-white phase matrix in a spherical shape, and no obvious grain boundary is observed in the grey-white phase after further amplification by SEM (Figure 3(b)). In Figure 3(c), the grain boundary can be clearly observed within the black phase. The crystallisation expands outwards to the grey-white phase from the black phase. Further SEM analysis confirmed that this region exhibited a typical columnar growth pattern with spacing several microns in width (Figure 3(d)). Figures 3(e) and 3(f) show the detailed interfacial characteristics, that is, a thin reaction layer with several micron forms within the interface region. The coating and substrate grow alternately, showing a serrated shape.Figure 3
Cross-sectional enlargement images: (a, b) grey-white and golden phases; (c, d) black phase; (e, f) interface between the substrate and coating.
(a)(b)(c)(d)(e)(f)EPMA was used to further analyse the compositional distributions of the different coating phases. As shown in Figure4(a), the grey-white phase has a composition (wt.%) of Fe54Co6Cr12Mo20C2Y2Cu4, consistent with the composition of the Fe-based amorphous alloy powder. Figure 4(b) shows that the black phase is mainly composed of a Fe element. The golden phase is rich in Cu elements (Figure 4(c)). In the grey-white phase and gold phase interface area, the EPMA linear scanning result (see Figure 4(d)) shows that the Cu content gradually decreases from the golden phase to the grey-white phase, while the Fe, Cr, Mo, and Co contents gradually increase. It can be observed that a 3 μm wide composition gradient region exists near the binding interface. Across the black phase, the Fe content initially increases and then decreases, as shown in Figure 5(e). We found that with decreasing Fe content, crystallisation correspondingly disappears. In the coating and substrate interface area, the EPMA linear scanning result (see Figure 4(d)) shows the Fe content gradually increases from the coating to the substrate, while the Fe, Cr, Mo, and Co contents gradually decrease. The composition gradient region is around 30 μm wide, confirming that a metallurgical bond has been formed.Figure 4
EPMA spot scanning result: (a) grey-white phase; (b) black phase; (c) golden phase. The EPMA linear scanning result: (d) interface between grey-white phase and golden phase; (e) across black phase; (f) interface between coating and substrate.
(a)(b)(c)(d)(e)(f)Figure 5
The microhardness of the composite coatings: (a) the microhardness measurement points; (b) the microhardness variation from the upper parts of the coating to the substrate; (c–f) the microhardness variation contour.
(a)(b)(c)(d)(e)(f)Combining the XRD, OM, SEM, and EPMA results, we propose the following conclusion: most of the Fe-based amorphous alloy powder incorporated into Fe-based amorphous alloy during laser cladding and constituted the composite coating’s main structure. A large amount of copper was not oxidised and did not react with other elements, but evolved within the coating in a spherical form under interfacial tension. The diffusion layer between the copper and the Fe-based amorphous alloy phase is thin (3 um), proving that the solubility of both solid copper and Fe-based amorphous alloy is minimal. Moreover, there is no evident crystallisation around the copper metal. Therefore, the addition of copper has little effect on the crystallisation of Fe-based amorphous alloys. However, in the black phase region, crystallisation is clearly evident. According to the chemical composition (Figure4(b)) and location of the black phase, the black phase can be confirmed as originating from the base metal. Under laser beam stirring, the melted base metal enters the molten pool. However, there is insufficient time for full fusion of the base metal and the coating materials, due to the high cooling rate. The chemical composition of the region is similar to that of the base metal, resulting in crystallisation.The microhardness values of the composite coatings with different copper addition percentages are described in Figure5. The microhardness was measured at the same horizontal line with an interval of 75 μm, as shown in Figure 5(a), and three reference points were taken for each horizontal position. Figure 5(b) shows the microhardness variation from the coating’s surface to the substrate. As shown, when the copper addition is less than 20%, the coating microhardness variation trend is consistent. This can be divided into three regions, coating, heat-affected zone, and substrate. The microhardness gradually decreases from the coating’s surface to the substrate. When the copper addition is more than 20%, the coating’s surface hardness is significantly reduced and even lower than that of the substrate. In order to reveal the hardness variation rule of the coating, the hardness contours of the composite coatings were drawn, as shown in Figures 5(c)–5(f). As shown, the microhardness of the composite coatings is highly varied. The average microhardness of the composite coatings was 791.3HV0.2, 784HV0.2, 583.6 HV0.2, and 423.8 HV0.2, with a mean deviation of 22.7 HV0.2, 29.2 HV0.2, 30.3 HV0.2, and 84.1HV0.2. The maximum microhardness was measured as 815.3 HV0.2 in the 5% copper addition coating, whereas the minimum microhardness measured as 205.7 HV0.2 was in the 40% copper addition coating’s surface. With increasing copper metal addition, the coatings’ average microhardness decreased, but the deviation in microhardness increased. This significant deviation indicates that the coatings are not a homogeneous body. The composite coatings are composed of Fe-based amorphous alloy, crystalline phase, and copper metal, as shown in Figure 3. As the main constituent of the composite coatings, Fe-based amorphous alloy possesses high microhardness, leading to high hardness. However, the crystalline phase and the additional copper present in the composite coating are embedded in the coating body, and as a soft phase, copper metal has a lower microhardness. Adding a soft phase to a hard matrix can decrease hardness in composite materials. As seen in Figure 5(f), the hardness of the coating decreased significantly in the area where the copper metal had aggregated, indicating that copper metal plays an important role in reducing the hardness of the coating. It can be calculated by image segmentation that with the copper addition increased from 5 to 10%, the copper present in the coating increased from 2.2 to 5.4%. Under these conditions, the average hardness of the coating is mainly determined by the Fe-based amorphous matrix, and any decrease of coating hardness is not obvious. With copper addition increased to 20%, the copper present in the coating increased to 12.4%. At this point, the Cu effect on the coating’s microhardness intensified, which led to a significant decrease in the hardness of the coating.


## 4. Conclusion

A series of Fe-based amorphous composite coatings with different copper content were synthesised by laser cladding in this study. A large amount of spherical form copper can be incorporated into the coatings under interfacial tension, without oxidation or any reactions with other elements. The diffusion layer between the copper phase and the Fe-based amorphous alloy phase is only 3 um. The addition of copper has little effect on the crystallisation of Fe-based amorphous alloys due to the minimal solid copper and Fe-based amorphous alloy solubility. With increases in supplementary copper, the coating’s average microhardness decreases from 791.3HV0.2 to 423.8 HV0.2, while the microhardness deviation increases from 22.7 HV0.2 to 84.1HV0.2. The composite coatings form a network structure, which is beneficial to improving the coating stress distribution.



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*Source: 3511432-2022-04-25.xml*