Abstract
In this study, the WC reinforced Fe-based composite coatings with various CeO2 contents were fabricated on the H13 hot-working die steel by laser cladding. The high-temperature wear resistance of composite coatings was analysed by friction-abrasion testing machine, white light interferometer, and scanning electron microscope (SEM). The microstructure and microhardness were investigated by SEM and Vickers hardness tester. The influence mechanism of CeO2 on microstructure and wear resistance was explained. The results showed that appropriate CeO2 could effectively refine the microstructure of coatings, and affect the crystal morphology. The microhardness of composite coatings was about 4–5 times that of the die steel substrate. Affected by microstructure and microhardness, the Fe-based composite coatings with CeO2 exhibited a higher high-temperature wear resistance than the substrate.
Introduction
Hot stamping is a process that uses the principle of metal thermoplastic forming to heat the material to the recrystallization temperature for processing, and then rapidly cooled and quenched to form a mechanical part [1-3]. The formed parts have the advantages of high bending strength, lightweight and high complexity, and are widely applied in the automotive field. The application of hot stamping technology has greatly improved the rigidity and strength of the car body's overall structure [4-6]. H13 hot-working die steel, owing to the advantages of high hot strength, toughness and hardenability, have been widely applied in casting dies, extrusion dies, and Hot stamping dies [7,8]. However, due to the complex and harsh working conditions of hot stamping, the die is subjected to alternating loads of cold and heat and severe friction, causing wear failure to become the main failure form of hot stamping dies. Therefore, improving the wear resistance of die steel is the key to prolonging the service life of the die [9-11]. Researchers improve the wear resistance of H13 by various means, such as plasma nitriding, thermal spraying and jet deposition [12-14]. Laser cladding has the advantages of low dilution rate, small deformation, small heat-affected area, good metallurgical bonding, and has become a research hotspot in mould surface modification [15-19].
Metal-ceramic composite powders, which combine the toughness of metal matrix with high hardness of ceramic particles, and the prepared coatings have the advantages of high hardness and wear resistance, has been widely used in laser cladding [20,21]. However, the difference in the physical and chemical properties between the metal matrix and the ceramic particles will cause the temperature gradient of the molten pool to be larger, which will lead to defects such as holes and cracks in the cladding layer [22]. Some studies have shown that rare earth oxides can reduce crack sensitivity and inhibit defects such as holes and cracks. Liu et al. [23] studied the effect of CeO2 on TiC/Ti2Ni enhanced Ti-based composite coatings, and CeO2 effectively inhibited the formation of ‘herringbone’ crack, reduced the crack sensitivity, and improved the surface quality of layers. Li et al. [24] found that the addition of CeO2 can reduce the porosity of the composite coatings, refine the grain size, and improve the microhardness of the coatings. In addition, some scholars have found that rare earth oxides can also refine the microstructure and improve wear resistance of the coatings. In the study of Cai et al. [25], a defect-free TiC/Fe-based composite coating was prepared on Cr12MoV by laser cladding. The microhardness and wear resistance of the coatings were significantly improved after adding an appropriate amount of rare earth elements. Wang et al. [26] prepared Ni-WC-CeO2 coatings on Ti6Al4V, and the coatings had an excellent metallurgical combination with the substrate. Rare earth oxides significantly improved the microhardness and wear resistance of the coatings.
In summary, many scholars have carried out research on the influence of rare earth oxides on the structure and performance of metal-ceramic composite coatings, but it is mainly reflected in the use of Ni-based and Ti-based as the main components of the powders. The high cost limits its engineering applications, and most of them are coatings performance studies at room temperature. The influence of CeO2 on high-temperature wear resistance of Fe-based composite coatings was rarely studied. Consequently, in the present study, WC reinforced Fe-based composite coatings were fabricated on H13 hot-working die steel by laser cladding. The microstructure, microhardness and high-temperature wear resistance were investigated of coatings with various CeO2 content. And the influence mechanism of CeO2 on microstructure and wear resistance was explained. The research results have theoretical significance and engineering value for improving the wear resistance and prolonging the useful life of hot-working dies.
Materials and methods
Materials and laser cladding parameters
Composition of the cladding powders (wt-%).
Composition of the cladding powders (wt-%).
Process parameters of laser cladding.
The wear resistance of specimens was evaluated with a Rtec MFT-50 wear test machine, and the dimension of specimens was 15 × 10 × 12 mm. YG6 balls with a diameter of 6 mm were selected as the counterpart material. The experimental parameters were a load mass of 100 N, a stroke of 4.5 mm, a sliding speed of 18 mm s−1, a friction temperature of 350°C, and a wearing time of 30 min. The weight losses of the specimens were weighed by an electronic balance with an accuracy of 0.1 mg, and all the specimens were washed by absolute ethyl alcohol with an ultrasonic wave before weighing. The section profiles and 3D morphologies of the wear trace were obtained with a white light interferometer. Wear surfaces were observed by SEM.
Microstructure observation and microhardness tests
After laser cladding, a wire-cut electrical discharge machine was used to cut the metallographic specimen perpendicular to the scanning direction, and the dimension of each specimen was 10 × 10 × 12 mm. Then the specimens were ground and polished according to standard procedures. After ultrasonic cleaning with absolute ethanol, all polished sections were chemically etched with a 4% nitric solution. The microstructure of the coatings was observed by means of the FEI QUANTA FEG 250 scanning electron microscopy (SEM). The microhardness distribution in the cross-section of the coatings was obtained using a 402 MVD Vickers hardness tester with a load of 500 g and dwell time of 15 s. Each microhardness value was the average of five measurements.
Results and discussion
High temperature wear properties and mechanisms
Wear resistance comparison of substrate and coatings
After the wear test, the wear loss and wear track profile curve of the H13 substrate and coatings were annotated in Figure 1(a,b). The wear of the H13 substrate was 31.8 mg, which was about 2.94 times the wear of the coating without CeO2. When CeO2 was added, the abrasion of the coating was further reduced. The wear amount image showed a trend of decreasing first and then increasing, and the content of 3% was the minimum wear amount. It can be seen from the section profile of wear track that the wear track width and depth of the H13 substrate were the largest, and the wear track of the coating with 3% CeO2 was the narrowest and shallowest.
(a) The wear weight loss of the H13 substrate and coatings; (b) the section profiles of wear track; (c) coefficient friction curves of the H13 substrate and coatings.
The friction coefficient of the H13 substrate and coatings is shown in Figure 1(c). It can be clearly seen that the friction coefficient of the substrate was the largest, about 0.26, and the average friction coefficient of CeO2 content of 0–4% was about 0.19, 0.17, 0.16, 0.13 and 0.14, respectively. The friction coefficient of the coatings decreases with the increase of CeO2 content, and the friction coefficient achieved the minimum when the content of CeO2 was 3%. However, when the content was 4%, the friction coefficient has rebounded. The results showed that the friction coefficient of H13 substrate and coatings was consistent with its wear amount and wear depth.
The 3D morphology after wear test was observed, as illustrated in Figure 2. The depth and width of the H13 substrate (Figure 2(a)) wear track were about 20 μm and 0.8 mm, respectively, and plastic deformation occurs at the edge of the wear track, but the surface with cladding does not. As can be seen from Figure 2(b–f), as the content of CeO2 increases, the depth and width of the wear scar gradually decrease and achieves the minimum when the content of CeO2 was 3%. However, when CeO2 reaches 4%, the depth and width of the wear scar increase again.
3D morphology: (a) H13 substrate; (b) 0%; (c) 1%; (d) 2%; (e) 3%; (f) 4% CeO2 coatings.
It can be seen from the high-temperature wear test:
The wear weight losses of all specimens are less than that of the H13 substrate, indicating that the wear resistance of the coatings at a certain temperature is higher than that of the H13 hot-working die steel. The content of CeO2 in the cladding powders affects the wear resistance of the coatings. When CeO2 is 3% and Fe901+WC is 97%, the wear resistance of the coating is the best.
To further explore the influence mechanisms of CeO2 on the wear resistance of the H13 substrate and coatings, the worn surface of each specimen was observed by SEM, as shown in Figure 3. The worn surface of the substrate was rough and there were a lot of spalling areas; the micro-cutting effect on the substrate by the grinding ball was obvious. The fragments after peeling were mixed between the surface and the grinding balls as abrasive, which increases the degree of wear. Therefore, the wear mechanisms of the substrate comprised adhesive wear, abrasive wear and fatigue wear.
Worn surface morphology: (a) H13 substrate; (b) 0%; (c) 1%; (d) 2%; (e) 3%; (f) 4% CeO2 coatings.
Under the condition of high temperature, a black adhesion film was found on the surface of the specimens (see Figure 3(b–d)). Figure 4 is the mapping scanning results of the worn surface of the coating with 2% CeO2. The results indicated that the black film was mainly composed of C element and O element. On the one hand, adhesive wear occurs during the friction process, and the C element in the grinding ball was transferred. On the other hand, oxidation wear occurred at high temperatures, and an oxidation film was formed. The aggregation of W elements also appeared in the mapping scan, because the unmelted WC particles were distributed in the coating. It is worth noting that the distribution characteristics of Ce elements are not found in the mapping scan results. The addition of CeO2 effectively refines the crystal grains and improves the wear resistance, but because CeO2 addition is small, and it tends to be distributed at the grain boundaries, coupled with the existence of oxide film on the wear surface. Therefore, the distribution characteristics of Ce elements in element mapping are not obvious. The coating without CeO2 increases the tendency of the coating to crack due to the uneven distribution of WC particles. The oxide film can reduce the contact area between the specimens and the counterpart and play a protective role, but if the bonding strength between the oxide film and surface is poor, it will cause part of the oxide layer to fall off and increase the degree of surface wear.
Mapping scanning results of the worn surface of the coating with 2% CeO2.
It can be seen from Figure 3(b–e): With the increase of CeO2, the peeling phenomenon was gradually reduced. The worn surface of the coating with 3% CeO2 was smooth, and the grooves were very shallow, mainly due to the increased bonding strength of the oxide film and the coating. During the friction process, the oxide film was removed uniformly, which plays a protective role in the coatings and significantly improves the wear resistance. Figure 3(f) shows that when the content of CeO2 is 4%, the depth of the furrow on the coating surface increases and the wear resistance decreases slightly.
The microstructure is an essential factor in determining the wear resistance of materials. Hence, the crystal structure of the coatings was analysed from a microscopic point of view, revealing its wear resistance mechanism, and providing support for engineering applications.
Effect of CeO2 on the microstructure of coatings
Figure 5 SEM image shows the microstructure of the upper, middle and bottom of the coatings with different CeO2 content.
SEM microstructure of the coatings: (a1–c1) 0%; (a2–c2) 1%; (a3–c3) 2%; (a4–c4) 3%; (a5–c5) 4%CeO2.
Through the horizontal comparison of the coatings microstructure image, the upper (Figure 5(a1–a5)), middle (Figure 5(b1–b5)), and bottom (Figure 5(c1–c5)) of the cladding layers shows different microstructures. At the beginning of the solidification of the molten pool, the temperature gradient G at the bottom of the molten pool and the substrate is substantial, and the crystallization rate R approaches zero, so G/R approaches infinity. At this time, the nucleation rate is much faster than the growth rate, and the microstructure appears as planar crystals. At the solid–liquid interface, the actual temperature of the liquid phase gradually decreases before crystallization, and the degree of supercooling increases, so the solidification rate R increases and the G/R ratio decreases. Because of the large heat dissipation rate perpendicular to the substrate, the microstructure changes from planar crystals to columnar crystals distributed along the normal of the bonding line. Due to the continuous heat dissipation of the molten pool, the temperature of the solid–liquid interface is the highest, which is a negative temperature gradient. Under this condition, the crystal grows in a dendritic manner. The temperature gradient G decreases, the solidification rate increases, and the G/R ratio decreases. At this time, the microstructure is dendrite and cell crystal. At the upper of the cladding layer, due to the heat dissipation of the surface, the solidification speed R reaches the peak, the grain growth rate is much faster than the nucleation speed, and the impurities at the top float to become the core of the heterogeneous nucleation, so the microstructure appears as uniform equiaxed crystals and fine dendrites.
Through the longitudinal comparison of the microstructure of the coatings, the CeO2 content has a great impact on the microstructure of the coatings.
When the coating without CeO2, small cracks appear in the microstructure and the dendrite spacing is larger, as shown in Figure 5 (a1–c1). The 1% and 2% CeO2 coatings have dendrites and cell crystals in the middle, respectively. The microstructure begins to become denser under the action of CeO2, the dendrite spacing decreases, and no cracks were found, as shown in Figure 5(a2–c3). It indicated that the addition of rare earth oxides refines the microstructure and reduces the cracking tendency of the coatings.
As shown in Figure 5(a4–c4), in the coating with 3% CeO2, it has the most apparent effect on the refinement of the microstructure, and the grain size was further reduced. Compared with other coatings, the area with plane crystals and columnar crystals was reduced. The interface where dendrites appear obviously shifted down, indicating that the presence of CeO2 makes the negative temperature gradient appear earlier in the solidification process, which is more conducive to the formation of dendrites.
As illustrated in Figure 5(a5–c5), when the content of CeO2 is 4%, the microstructure becomes coarse again, and the grain spacing increases, accompanied by defects such as pores and cracks. It may be that agglomeration occurs with the increase of CeO2 addition, which weakens the effect of CeO2 to refine the microstructure.
The microstructure of the coatings shown that an appropriate amount of CeO2 can effectively refine the grains. According to the analysis, the mechanism of CeO2 was mainly derived from two aspects (see Figure 6): First of all, under the action of high temperature, CeO2 in the molten pool decomposes into oxygen atoms and Ce atoms. According to the minimum principle of system free energy in thermodynamics, Ce atoms have a large atomic radius and tend to accumulate at the grain boundaries, which has a dragging effect on the movement of the grain boundaries, which limits the growth of the crystal grains and refines the crystal grains. What's more, Ce atom, which has active chemical properties, the oxide generated by reaction with other elements, can be used as the core of the heterogeneous nucleus. The unmelted CeO2 in the molten pool can also be used as a nucleation core during solidification, which increases the nucleation rate and refines the microstructure of the coatings.
The schematic diagram for the action mechanism of CeO2.
The microhardness has an essential influence on the wear resistance. In order to explore the wear resistance of the coatings, the microhardness test of the specimens was carried out. Figure 7(a) presents the microhardness distribution curve from the coating surface to the substrate at different CeO2 contents. The average microhardness of the coating without CeO2 is 848HV0.5, which is significantly higher than that of the H13 substrate. Due to aggregates of WC particles were founded in the microstructure of the coating without CeO2, the fluctuation of the microhardness is large, as illustrated in Figure 7(b).
(a) Microhardness distribution of the coatings; (b) The WC particles of the coating with 0% CeO2.
The coating's microhardness increases with the increase of CeO2 content, which shows that CeO2 plays a role in promoting hardness and uniform distribution. The increase in hardness was mainly attributed to two aspects. Firstly, the addition of CeO2 refines the microstructure, and the decrease in the secondary dendrite arm spacing increases the hardness. Besides, CeO2 promotes the dissolution and uniform distribution of WC particles in the molten pool.
We found that the microhardness achieves the maximum when the content of CeO2 is 3%, the average value is 997 HV0.5. It is worth noting that when the content of CeO2 continues to increase, the hardness of the cladding layer decreases instead. The reason is that excessive CeO2 reduces its refining effect on the microstructure, and the presence of inclusions and pores formed by CeO2 and other components reduces the density of the coatings.
It can be seen that the microhardness of the coatings is much higher than that of H13 die steel, and the hardness is the highest when the CeO2 content is 3wt.%, which is consistent with the aforementioned wear resistance results.
The mechanism of their influence on the wear resistance of the coatings can be obtained through the analysis of the microstructure and microhardness. The addition of CeO2 promotes the refinement and uniform distribution of WC particles, increasing the hardness of the coatings. The WC particles embedded in the metal matrix can reduce the micro-cutting effect of the YG6 ball on the coatings. The depth and width of the wear track gradually decrease with the increase of CeO2 content. CeO2 refines the microstructure, reduces the crack sensitivity, and increases the hardness of the coatings; thus, the grooves of wear surface become shallow. When the CeO2 content is 4%, the wear resistance decreases slightly due to the decrease of the coating hardness. Consequently, the reasonable amount of CeO2 can reduce the plowing effect of the counterparts on the coatings and improve the wear resistance of the coatings. The excessive CeO2 will hinder the improvement of the wear resistance of the coatings.
In this study, the effect of CeO2 on the high-temperature wear resistance, microstructure and microhardness of WC reinforced Fe-based composite coatings was investigated. Some conclusions can be drawn as follows:
CeO2 was found effective enhancing the high-temperature wear resistance of Fe-based composite coatings. Oxide film was formed on the surface of the coating under high-temperature conditions. The wear surface of the coating with 3% CeO2 content was smoothest, where the wear mechanism comprised slight abrasive wear and oxidative wear. The addition of CeO2 reduces defects such as cracks and pores. Ce atoms gathered along the grain boundaries inhibit crystal growth. Unmelted CeO2 can be used as the core of heterogeneous nucleation. CeO2 effectively promotes the refinement of the microstructure of the coatings. Compared with the H13 substrate, the microhardness of the Fe-based composite coatings with 1–4% CeO2 content increased by 4–5 times, and the uniform distribution of CeO2 to WC particles reduced the fluctuation of the microhardness.
Footnotes
Acknowledgement
This work was supported by the science and technology development project of Yantai high-tech zone: Research on key technologies of 3D printing of automobile mould.
Disclosure statement
No potential conflict of interest was reported by the author(s).