MoFe1.5CrTiWAlNbx refractory high-entropy alloy coating fabricated by laser cladding
Introduction
Fe-based tools, with good machinability, are extensively used in modern tool manufacture. Nevertheless, when high-hardness materials are processed, it is difficult to meet the processing and precision requirements, resulting in a short lifetime of tools and an increase in production cost. Therefore, the new cutting tool materials and suitable processing methods have become a key solved the problem.
High-entropy alloys (HEAs) and interstitial alloys based on them with some outstanding microstructure and properties [[1], [2], [3], [4], [5], [6], [7], [8]], is one of the most important discoveries in the field of metallic materials in recent years. Since HEAs could apply in various key areas such as machinery, Metallurgy, aerospace industry, etc. [9,10]. It has attracted great attention of engineers and scientists at home and abroad.
It's well known that the smelting process of bulk high entropy alloy is relatively complex, a bulk HEAs with homogeneous microstructure usually requires repeated remelting [11]. Furthermore, a large amount of noble metal elements contained in HEAs (such as Ta, Zr) greatly increases the cost, which couldn't help promotion and application of HEAs. Therefore, the preparation of HEAs coatings on low cost steel is of great significance to expand the practical application of HEAs. Laser cladding has the characteristics of rapid heating and rapid cool, and the thickness of the prepared coating by the technique can reach the millimeter level, which can meet the requirements of uniform microstructure [12]. At the same time, the surficial modification of the tool by using laser cladding would realize chemical metallurgical bonding between coating and substrate, leading to excellent bonding strength. And the bonding strength is much higher than electrochemical deposition [13,14], thermal spraying [15] and other surficial processing technologies.
By means of high-temperature heating characteristics of laser cladding, high melting point elements could be used in the preparation of high melting point high entropy alloy coating. The FeCrCoNiAlMox [16] high entropy alloy coatings by laser cladding on the 45 steel was found that the hardness and wear resistance of coating were greatly improved compared with the substrate. Zhang [17] fabricated TiZrNbWMo alloy coating with ultra-high temperature resistance by laser cladding plasma spraying. Cai [18] obtained laser cladded Ni-Cr-Co-Ti-V high-entropy alloy coating after laser remelting processing, and found that the wear resistance of the coating was significantly enhanced. Zhang [19] studied the effect of B content on laser cladding FeCrNiCoMnBx coatings. As the B content increases, the boride content increases, which can improve hardness and wear resistance of coatings. Anyway, there are few literatures on laser cladding of refractory high entropy alloy for tool-coating application.
In our previous study, a high-hardness and wear-resistant MoFeCrTiWAlNb coating with dispersed carbide was prepared. And the parameters of laser cladding are further optimized [20,21]. In the present paper, the MoFe1.5CrTiWAlNbx high refractory HEAs coating was prepared on tool steel by laser cladding technology. The effect of Nb on the microstructural evolution and wear resistance of the coating, and the related mechanism is studied in detail.
Section snippets
Experimental procedures
An as-annealed M2 (W6Mo5Cr4V2) tool steel with a size of Φ 50 mm × 10 mm was used as the experimental substrate. Before the experiment, the substrate was polished and cleaned with alcohol to remove dirt and oil. The MoFe1.5CrTiWAlNbx (x = 1.5, 2, 2.5, 3) (labeled as Nb1.5, Nb2, Nb2.5, Nb3) high entropy alloy coatings were designed, and analytically pure metal powders were selected as raw materials to prepare high entropy alloy powder according to the equimolar ratio by electronic balance. Then,
Phase analysis
Fig. 1 shows the XRD patterns of the HEAs coatings and PDF card of Fe, (Nb, Ti) C and Fe2Nb. It is seen from Fig. 1 that The MoFe1.5CrTiWAlNbx high entropy alloy coatings consist of body-centered cubic (BCC), MC and Laves phase. To clarify the existence of BCC phase, it could be confirmed thermodynamically. The calculation formulas for △Smix, △Hmix, δ, Ω, etc. are as follows [[22], [23], [24]]:Where, R is the gas constant, ci and cj represent the mole fractions of element i
Discussion
In the present paper, indeed, MoFe1.5CrTiWAlNbx refractory coating prepared by laser cladding consists of BCC, MC and Laves phase. Traditionally speaking, Laves phase is of brittleness [31], however, our experimental results show that the coating is of excellent match of strength with toughness. The reason is, on one hand, laser cladding is of the character of rapid heating and rapid cooling, the solidification time is very short, there is no enough time for grain to grow, which results in fine
Conclusions
The MoFe1.5CrTiWAlNbx HEAs coatings is successfully prepared on M2 tool steel by laser cladding technology. The coating mainly consisted of BCC, MC and C14-Laves phase. With the increase of Nb, the microhardness gradually increases and reaches the highest value of 910 HV0.2 at Nb3, and wear resistance increases gradually. and the wear mechanism is mainly abrasive wear.
Declaration of competing interest
None.
Acknowledgements
This research work is supported by National Natural Science Foundation of China (NO.51671061) and High-level Innovative Talents Plan of Guizhou Province (No. (2015)4009).
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