Microstructure and Performance Analysis of FeAlCuCrNiNbx High Entropy Alloy Overlay Welding Layers

Literature Overview

This paper by Su Yunhai, Liang Xuewei, Deng Yue, and Liu Yunqi from Shenyang University of Technology, published in the Transactions of the China Welding Society in 2020 (Vol. 41, No. 4, pp. 38-43), investigates the effect of niobium content on the microstructure and properties of FeAlCuCrNiNbx high entropy alloy overlay welding layers deposited on carbon steel substrates. The work was supported by the National Key R&D Program of China (2017YFB1103603) and the Liaoning Provincial Science and Technology Department Joint Fund (20180510030). The study systematically varies the Nb molar ratio from 0.4 to 1.0 and evaluates microhardness, wear resistance, and electrochemical corrosion performance through a combination of metallographic analysis, X-ray diffraction, Vickers hardness testing, pin-on-disk wear testing, and potentiodynamic polarization measurements.

Core Technical Findings

The overlay layers exhibit a BCC solid solution matrix based on the Fe-Cr phase, with a small quantity of MC-type eutectic carbides precipitating in the interdendritic regions. The microstructure presents a typical dendritic morphology, consisting of gray dendrites (DR) and white interdendritic structures (ID). This dendritic pattern is characteristic of rapid solidification conditions inherent to GMAW overlay welding, where the cooling rate at the weld surface is significantly higher than in bulk castings.

Effect of Nb Content on Hardness and Wear Resistance

The addition of Nb in appropriate proportions substantially improves both microhardness and wear resistance. The maximum hardness value reaches 602 HV at a Nb molar ratio of 0.8, and the minimum wear loss of 0.30 g is also achieved at this composition. The mechanism is attributed to solid solution strengthening by Nb atoms substituting into the BCC lattice, combined with the precipitation of fine Nb-rich MC carbides that act as effective obstacles to dislocation motion. At lower Nb ratios (0.4 and 0.6), the carbide volume fraction is insufficient to provide optimal reinforcement, while at the highest ratio (1.0), the matrix may become slightly coarser due to reduced cooling rate associated with higher thermal conductivity of the Nb-rich alloy.

Corrosion Performance

Electrochemical polarization tests reveal that adding Nb reduces the self-corrosion current density and slows the corrosion rate. All tested compositions outperform 304 stainless steel in corrosion resistance, with the best performance observed at Nb molar ratio of 1.0. The enhanced corrosion resistance is likely due to the formation of a more stable and dense passive film containing Nb oxide species, which inhibit anodic dissolution. This finding is particularly significant for applications in aggressive chemical environments where high entropy alloy overlays can replace more expensive nickel-based or cobalt-based overlays.

Engineering Practice Implications

From an engineering perspective, this research provides a practical route for surface hardening and corrosion protection of carbon steel components through GMAW overlay welding of high entropy alloys. The optimal Nb ratio of 0.8 for wear resistance and 1.0 for corrosion resistance suggests that the composition must be selected based on the dominant service requirement. For components subjected to both wear and corrosion, a compromise composition near Nb = 0.9 may be warranted. The GMAW process used here is readily available in industrial settings, and the wire feed rates, shielding gas compositions, and travel speeds can be optimized based on standard overlay welding practice. However, engineers should be aware that the dendritic microstructure may require post-weld heat treatment to homogenize the composition and reduce segregation-induced brittleness in thick overlay deposits.

Study Insights and Reflections

This work demonstrates that high entropy alloys, once considered purely academic curiosities, can be practically deposited through conventional welding processes. The finding that modest Nb additions yield dramatic improvements in hardness and corrosion resistance is encouraging for industrial adoption. However, the study does not address long-term thermal stability or oxidation resistance at elevated temperatures, which would be critical for applications in power generation or petrochemical equipment. Future work should also examine the effect of multi-pass welding on the interpass temperature and its influence on grain coarsening and carbide morphology. The combination of high entropy alloy chemistry with overlay welding technology represents a promising frontier for surface engineering in the steel industry.