Microstructure Characteristics and Elevated-Temperature Wear Mechanism of FeCoCrNiAl High-Entropy Alloy Prepared by Laser Cladding

Abstract

This paper investigated the FeCoCrNiAl high-entropy alloy on H13 steel, prepared using laser cladding, to improve the elevated-temperature wear resistance of the alloy. The results revealed that FCC and BCC phases, in terms of the coating, produced a large dislocation density. The coating exhibited a columnar and equiaxed crystal microstructure. With the comprehensive effects of fine-grain strengthening, solid solution strengthening, and dislocation strengthening, the average hardness of the coating (500 HV_0.1) was improved by 150% compared with that of H13 steel (200 HV_0.1). The wear experiments were conducted at 623 K, 723 K, and 823 K. Compared with H13 steel, the wear volume of the coating decreased by 59.20%, 70.79%, and 78.20% under different temperatures. The wear forms impacting the coating were mainly abrasive wear and oxidation wear. However, H13 steel presented adhesive wear and fatigue wear, in addition to abrasive wear and oxidation wear.

1. Introduction

Regarded as a mold steel with good resistance to high temperatures and impacts, H13 steel is often applied in the production of die casting molds [1,2,3]. According to the statistics, 70% of mold failures are wear failures, which not only causes the waste of resources, but also increases the cost of remanufacturing. However, wear failure mostly starts on the surface of the material; therefore, improving the mechanical performance of the surface of the mold is key to research in this area [4,5]. Nowadays, surface modification technologies mainly include thermal spraying [6], anodic oxidation [7], physical vapor deposition (PVD) [8,9], chemical vapor deposition (CVD) [10], and laser cladding [11,12,13]. Among them, laser cladding has advantages of a fast-cooling speed, a lower dilution rate, and stronger metallurgical bonding [14,15]. Therefore, the wear resistance of H13 steel can be significantly improved by laser cladding technology.

Laser cladding materials used on H13 steel are mainly traditional alloys, such as Fe alloys, Ni alloys, and Co alloys [16,17,18,19,20]. Traditional alloys face limitations in regard to needing to improve their elevated-temperature wear and oxidation resistance; so in recent years, research interest in traditional alloys has been gradually lessening.

Because of its advantages of high hardness, good elevated-temperature wear resistance, and strong corrosion resistance, metal–ceramic composite coatings have been widely studied. The most common ceramic strengthening phases involve TiB, TiC, and WC particles. Wang [21] fabricated TiB₂/Cu composite coatings by adding TiC directly, and the yield limit of the material increased from 175 Mpa to 422 Mpa. Tran [22] also prepared a TiB₂/Cu composite coating through in situ synthesis. They found that the uniform distribution of TiB₂ particles resulted in the coating having high microhardness (650 HV_0.5). Zhao [23] studied the influence of TiC content on the properties of a TiC/Ni composite coating. As a result, the coating with 30%TiC had the highest hardness and wear resistance. A Stellite-6/WC composite coating was achieved with a mixture of Co-coated WC (WC-12Co) particles and Stellite-6 powder [24]. The addition of WC-12Co particles reduced the decomposition of WC, efficiently improved the wear resistance of the coating.

However, because of the large thermal expansion coefficient of ceramic particles, there is a limitation in terms of improving the elevated-temperature wear resistance and oxidation resistance of such alloys. It is difficult to meet the high-temperature service requirements of hot-work die steel with metal–ceramic composite coatings. It is crucial to explore a coating material that not only improves the surface hardness and plasticity, but also reinforces the elevated-temperature wear resistance and thermal stability of the material.

High-entropy alloys (HEAs) have lots of advantages, for example, high strength, high hardness, and good thermal stability, which significantly improves the combination properties of metals. However, at present, the research on laser cladding HEA alloys on H13 steel has mainly focused on improving the microstructure characteristics and room-temperature friction and wear. Shu [25] studied the influence of amorphous content on a CoCrBFeNiSi HEA and discovered that the higher the amorphous content, the stronger the wear resistance and corrosion resistance of the coating. Li [26] found that the addition of Nb made an Al_0.5CoCrFeNi alloy form a hypereutectic structure, with strong hardness and wear resistance.

Nevertheless, there are only a few reports on the elevated-temperature friction and wear properties of HEA coatings, especially featuring comparative analysis of the wear resistance of HEA coatings and H13 steel for different experimental temperatures. Therefore, in this study, an FeCoCrNiAl HEA was fabricated on H13 steel using laser cladding, and the microstructure characteristics and microhardness were studied, in particular the elevated-temperature wear mechanism at different experimental temperatures was systematically analyzed.

2. Materials and Methods

2.1. Materials and Laser Experimental Parameters

H13 steel was used as the substrate material.

Table 1. Chemical composition of H13 steel.

Element	C	Si	Mn	Cr	Mo	V	S	P	Fe
wt.%	0.32–0.45	0.80–1.20	0.20–0.50	4.75–5.50	1.10–1.75	0.80–1.20	≤0.03	≤0.03	Bal.

illustrates the chemical composition of H13 steel. The size of the substrate was 60 mm × 50 mm × 5 mm. Firstly, the surface of the substrate was grinded using 400^# sandpaper and polished.

The metal powders used in this study were purchased from Xingtai Xinnai Metal Materials Co., Ltd. (Xingtai, China). The particle size and purity of the powder were 50–80 μm and 99.0–99.5%, respectively. The powders were measured, according to the amounts detailed in

Table 2. Element content of FeCoCrNiAl powder.

High-Entropy Alloy	Fe	Co	Cr	Ni	Al
wt.%	22.12	23.34	20.60	23.25	10.69

, and were fully mixed in a ball mill for two hours.

The laser cladding equipment used was a DL-2000 cross-flow CO₂ laser (Shenyang Continental Laser Complete Equipment Co., Ltd., Shenyang, China). The process parameters are shown in

Table 3. The process parameters of laser cladding.

Laser Power	Scanning Speed	Overlap Rate	Spot Diameter	Shielding Gas	Shielding Gas Flow
1200 W	240 mm/min	30%	3 mm	argon	5 L/h

.

2.2. Microstructure and Hardness Test

The XRD results were obtained from a TD-3500 X-ray diffractometer (Dandong Tongda Technology Co., Ltd., Dandong, China). The equipment parameters were as follows: Ni-filtered radiation, equipped with Cu Ka source; the voltage was 40 kV; the current was 30 mA; the acquisition angle was 20°~90°; the step width was 0.068; and the sampling time was 1 s. Before the experiment, the sample was cut into small pieces, with the length and width both being 10 mm and a thickness of 5 mm.

A Zeiss Sigma 300 field emission scanning electron microscope (Zeiss, Oberkochen, Germany) was used to assess the microstructure characteristics and the wear morphology. An X-MAX50 energy dispersive spectrometer (Oxford, United Kingdom) was used to analyze the element distribution in the coating. Aqua regia (HNO₃/HCl = 1:3) was selected as the corrosion solution and the corrosion time was 30 s.

The microhardness was tested using a HXD-1000TMC/LCD Vickers (Wuxi Metes Precision Technology Co., Ltd., Wuxi, China) meter, with a load of 0.98 N for 15 s. Furthermore, the microhardness was tested three times at 0.1 mm intervals, along a cross-section of the sample, and the arithmetic mean value was taken.

2.3. Elevated Friction and Wear Test

The elevated-temperature friction and wear test of the coating and the H13 steel was performed using MGW-02 wear testers (Jinan Yihua Tribology Testing Technology Co., Ltd., Jinan, China), and the experimental temperatures used were 623 K, 723 K, and 823 K. The grinding ball was a ZrO₂ ceramic ball, with a diameter of 6.5 mm. The parameters adopted in the experiment were a load of 20 N, a frequency of 5 HZ, a test time of 30 min, and a wear distance of 3 mm. The calculation formulas for the wear volume are illustrated in Formulas (1) and (2).

$$V=({\displaystyle \frac{1}{2}}\theta R^2-{\displaystyle \frac{1}{2}}\sin {\theta }_1{R}^2)\cdot L$$

(1)

$${\theta }_1=\arcsin {\displaystyle \frac{L_1}{2R}}$$

(2)

where θ is the degree of the arc; R is the radius of the grinding ball; θ₁ is the angle; L is the wear distance; V is the wear volume; and L₁ is the wear scar width.

3. Results

3.1. Phase Analysis of the Coating

Figure 1 shows the XRD result of the FeCoCrNiAl coating. The distinctive high-entropy effect of the liquid alloy results in a mixing entropy that exceeds the critical formation entropy required for the formation of intermetallic compounds, thereby preventing their formation [27]. Furthermore, the high-cooling speed produced by laser cladding also restrains the precipitation of intermetallic compounds. Therefore, through the analysis, it can be seen that the phases in the coating consist of BCC (Fe–Ni) and FCC (Fe–Cr), without intermetallic compounds.

Utilizing Bragg’s law Equations (3) and (4), the lattice constants (a) for Fe–Cr at 2θ = 43.966° and Fe–Ni at 2θ = 43.123° have been calculated and are presented in

Table 4. The a, ε, ξ, b, and ρ of Fe–Cr and Fe–Ni phases in the coating.

Phases	a (Å)	ε	ξ	b (Å)	ρ (cm−2)
Fe–Cr	2.910	1.118 × 10−2	0.648	2.518	9.534 × 1015
Fe–Ni	3.630	2.562 × 10−3	0.533	3.144	4.146 × 1015

. The lattice constants of the Fe–Cr (2.91 Å) and Fe–Ni (3.630 Å) phases in the coating exceed those of their standard counterparts, with values of 2.876 Å for Fe–Cr and 3.621 Å for Fe–Ni [28], which indicates that Fe–Cr and Fe–Ni produce lattice distortion in laser cladding.

$$2d\sin \theta =n\lambda$$

(3)

$$d={\displaystyle \frac{a}{\sqrt{h^2+k^2+l^2}}}$$

(4)

Here, d, n, λ, θ, and a are the crystal face distance, the diffraction series (n = 1), the wavelength of the X-ray (λ = 1.54056 Å), the diffraction angle, and the actual lattice constant, respectively.

The expression of lattice distortion (ε) for cubic crystals is as follows [29]:

$$\epsilon ={\displaystyle \frac{\left|\alpha -{\alpha }_0\right|}{{\alpha }_0}}$$

(5)

In the formula, α denotes the measured lattice constant, while α₀ signifies the theoretical lattice constant. The calculated lattice distortion for the Fe–Cr and Fe–Ni phases is detailed in Table 4. As illustrated, the lattice distortion of the coating is higher than that of the AlCoCrFeNi high-entropy alloy (7.0 × 10⁻⁴) [30]. This is mainly due to the addition of the Al element, which has a larger atomic radius. Furthermore, the rapid solidification characteristic of laser cladding also causes lattice distortion.

The dislocation density within the coating is ascertained using the Williamson–Hall method, based on the results of the X-ray diffraction analysis [31]. The dislocation density (ρ) is expressed as follows:

$$\rho =14.4{\displaystyle \frac{{\xi }^2}{b^2}}$$

(6)

In the above formula, ξ is the microscopic strain of the coating and b is the absolute value of the Bergdahl vector.

In addition, ξ and b are obtained by the expression Formula (7) and (8) [31]:

$${\displaystyle \frac{\beta \cos {\theta }_{hkl}}{\lambda }}=\xi {\displaystyle \frac{2\sin {\theta }_{hkl}}{\lambda }}+{\displaystyle \frac{1}{d}}$$

(7)

$$b=\sqrt{3}\alpha /2$$

(8)

Here, β, d, and θ are the half-height width of the diffraction peak, the grain size and the diffraction angle, respectively; the values of β, d, and θ are achieved through the measurement results of the XRD analysis (Figure 1). Moreover, λ is the wavelength of the X-ray (1.54056 Å) and α is the actual lattice constant.

Using the Formulas (6) to (8), the values of ξ, b, and ρ for the Fe–Cr and Fe–Ni phases were calculated and are listed in Table 4. It is evident that the dislocation density of the coating exceeds that of the alloy subjected to work hardening (10¹¹–10¹² cm⁻²). The reason for this is that the laser beam acts on the metal surface in a very short time, generating atmospheric pressure of up to 10⁵, which results in the metal producing strong plastic deformation and forming high-density dislocation.

3.2. Microstructure Analysis of the Coating

The microstructure of the FeCoCrNiAl coating is illustrated in Figure 2. Figure 2a shows the cross-section morphology of the coating. It is apparent that the microstructure of the coating is dense and uniform. The dilution rate can be calculated by (h/(h + H)), the result of the calculation of the dilution rate of the substrate is 27.7%. Although the dilution rate can be increased due to the significant amount of the Fe element in the substrate, the dilution rate is not significantly high, which ensures the high-entropy properties of the coating. Furthermore, as illustrated in Figure 2a, the gradient distribution for Fe, Cr, Co, Ni, and Al are shown, and as a result interdiffusion at the bonding zone is produced, which means that the bonding strength between the coating and substrate has been improved.

The solidification structure characteristics are related to G (the temperature gradient) and R (the solidification rate) [32]. At the bottom of the molten pool, due to the heat dissipation effect of the substrate, the G/R are higher than the other regions in the pool, thus the solidified structure exhibits planar crystal characteristics, as illustrated in the zone indicated by the dashed line in Figure 2b. As the solid–liquid interface moves toward the coating surface, the G/R gradually decrease, and the solidified structure transforms into cellular crystals (Figure 2b). Then, some of the cellular crystals act as the nucleus core, and rapid growth occurs at the vertical solid–liquid interface and forms columnar crystals, without having a preferred growth mode (Figure 2c). Meanwhile, due to the constitutional supercooling in the other regions, the directionality of the heat dissipation is lost, and impurities floating on the surface of the liquid phase provide nucleation sites for the formation of new phases, allowing the crystals to grow freely and, finally, forming equiaxed crystals (Figure 2d).

The map scanning and point scanning images of the magnified equiaxed crystals are illustrated in Figure 3 and Figure 4, respectively. As seen in Figure 3 and Figure 4, it can be found that the atomic contents of all the elements at the grain boundary are consistent and there is no element segregation.

3.3. Microhardness Analysis of the Coating

The microhardness distribution of the FeCoCrNiAl coating is presented in Figure 5. As exhibited, compared with the substrate (200 HV_0.1), the average microhardness of the HEA coating is about 500 HV_0.1, which is an improvement of 150%. The main reason is as follows: Firstly, the Al element, with the larger atomic radius in the high-entropy alloy, produces lattice distortion (Table 4), which increases the potential barrier needed by lattice sliding and leads to solution strengthening. Secondly, the fine equiaxed grains in the coating produces a high grain boundary density, which inhibits the movement of the dislocation and results in fine-grain strengthening. Finally, the coating generates a large dislocation density (Table 4), which hinders the movement of dislocations as they interact with each other, thereby strengthening the coating. In addition, the fine-grain strengthening caused by the fine equiaxed grains during the laser cladding process also improves the hardness of the coating.

Furthermore, it can also be found that the surface of the FeCoCrNiAl coating has a lower microhardness than the substrate. That is caused by the fact that the surface of the coating is directly affected by the laser, which leads to the elements on the surface being burned and, as such, produces a loose structure and a decrease in microhardness. Due to the rapid heating and cooling effect of the laser, the nucleation rate of the subsurface layer of the coating increases, the grain size becomes finer, and the microstructure becomes denser, which results in a higher microhardness. This result is consistent with the research in the literature [33].

3.4. Elevated-Temperature Wear Resistance Analysis of the Coating

To explore the elevated-temperature wear resistance of the FeCoCrNiAl coating, elevated-temperature friction and wear tests on the coating and the H13 steel were carried out at 623 K, 723 K, and 823 K. The wear scars on the HEA coating and H13 die steel are illustrated in Figure 6. As Figure 6 demonstrates, under different temperatures, the wear scars on the coating exhibit a large smooth region, a shallow groove parallel to the sliding direction, and debris (Figure 6a–c). Due to the spalling of the oxide at high temperatures, the debris in terms of the wear scar at the test temperature of 823 K increases significantly (Figure 6c). The element contents of the debris in terms of the wear scar at the temperature of 823 K were examined by EDS and the results are displayed in Figure 6g. As shown, it is conjectured that the debris on the coating mainly consists of Fe₂O₃, followed by the oxides of Al and Cr. It is apparent that the wear forms of the coating are mainly abrasive wear and oxidation wear.

However, H13 steel exhibits severe plastic deformation, deep grooves, cracks, and spalling because of its low hardness and strength (Figure 6d–f). In addition, with the increase in the experimental temperature, severe softening of the H13 steel occurs, which accelerates the degree of plasticity deformation and expands the cracks, and finally causes the peeling of the oxide layer (Figure 6f). It can be seen in Figure 6h that the oxygen content of the wear debris is greater than 50%, indicating that H13 steel also undergoes oxidative wear. Moreover, from the wear characteristics illustrated in Figure 6d–f, it can be found that the wear forms of H13 steel that exist are abrasive wear, adhesive wear, and fatigue wear, in addition to oxidation wear.

In summary, it is clearly that the coating displays better wear resistance after experiencing elevated-temperature wear. As illustrated in Figure 7, compared with H13 steel, the wear volume of the HEA coating was reduced by 59.20%, 70.79%, and 78.20% at 623 K, 723 K, and 823 K, respectively. It is also found that the wear volume of the coating remains basically unchanged with the increase in temperature; however, the wear volume of H13 steel significantly increases.

The excellent elevated-temperature wear resistance and stability of the FeCoCrNiAl coating is related to its unique phase composition and element ratio. A variety of elements are integrated into the crystal structure and the diffusion of different potential energy leads to slow diffusion kinetics, which results in the high-entropy alloy having good elevated-temperature resistance. At the same time, the Al element, with a large atomic radius, in the coating not only promotes the formation of the BCC phase, which has high hardness, but also refines the grains. Grain refinement increases the grain boundary density and presents difficulties in regard to dislocation movement, thereby the coating is strengthened.

In addition, during the elevated-temperature wear test, the Al and Cr elements, which are present in the coating, will produce an oxide film on the coating surface and this oxide film reduces the surface roughness, while preventing the coating from continuing to oxidize, thereby further improving the wear resistance of the FeCoCrNiAl coating.

4. Conclusions

An FeCoCrNiAl high-entropy alloy was fabricated on H13 die steel, via laser cladding, in this study. The microstructure and elevated-temperature wear mechanism of the coating were analyzed. The conclusions are as follows:

(1) The coating consists of a BCC and FCC solid solution, and the lattice distortion occurs in regard to the phase structure, resulting in a large dislocation density. The solidified structure of the bonding zone is mainly made up of planar crystals. The microstructure of the coating is comprised of columnar and equiaxed crystals;

(2) The microhardness of the coating (500 HV_0.1) is 150% higher than that of the substrate (200 HV_0.1), due to fine-crystal strengthening, solid solution strengthening, and dislocation strengthening;

(3) The wear form of H13 steel is mainly composed of abrasive wear, adhesive wear, fatigue wear, and oxidation wear, while the wear mechanism of the coating is abrasive wear and oxidation wear. Compared with H13 steel, the wear volume of the coating was reduced by 59.20%, 70.79%, and 78.20%.