Pengyong Lu

In order to prepare coating with good friction and wear resistance at elevated temperature on the surface of hot-working tool steel, by using a CO₂ laser, FeCrNiTiZr high-entropy alloy coating with different laser scanning speeds (360, 480 and 600 mm/min, respectively) was successfully fabricated by using laser cladding technology on the surface of H13 steel in this paper. Phase constitutions, microhardness, microstructure, and wear characteristics of FeCrNiTiZr coatings under different laser scanning speeds were analyzed. It was determined that 480 mm/min was the optimal laser scanning speed. The results showed that the coating at the scanning speed of 480 mm/min consists of a BCC phase with significant lattice distortion and high dislocation density; the crystal structure is cellular crystal and dendrite crystal. The coating demonstrates the highest microhardness (842 HV_0.2), which is 4.2 times that of the substrate (200 HV_0.2). Its average friction coefficients at room temperature and 823 K are approximately one-seventh and one-third of the substrate’s, respectively, and its wear volume is reduced by about 98% and 81% under these conditions. Compared to the substrate, the coating underwent slight abrasive wear, adhesive wear, and oxidative wear at both room temperature and 823 K. In contrast, the substrate underwent severe abrasive wear, adhesive wear, oxidative wear, and even fatigue wear.

To improve the wear and corrosion resistance, Al-Ni coating was prepared on Mg alloy by laser cladding, and the influence of the laser scanning speed on the microstructure, wear and corrosion resistance of the coatings was systematically analyzed. The results showed that the coatings with different scanning speeds were composed of Al₃Ni₂, Mg₁₇Al₁₂ and Mg₂Al₃ phases. The coatings presented fine needle-like grains. Under different scanning speeds, the microhardness of the coatings was 3.3–4.8 times that of the substrate, and the wear volume of the coatings was decreased by 40.08–51.38%. The coating with a laser scanning speed of 600 mm/min had the highest hardness, the best wear and corrosion resistance.

In order to meet the mechanical property of the die steel, this study used laser cladding to prepare a high-entropy alloy coating on Cr12MoV. A finite element method using a double ellipsoidal heat source model is proposed to simulate the evolution of the temperature field in laser cladding. The simulation results showed that with the increase in the power, the peak temperature of the molten pool increased from 2005.5 °C to 2357.4 °C, and the depth of the molten pool increased from 1.60 mm to 2.04 mm. The coating with the laser power of 1600 W had a good macroscopic quality and high lattice distortion (2.43 × 10⁻²). Due to the increase in laser energy density, the size of equiaxed crystals gradually increased from 1400 W to 1700 W. Under the comprehensive effect of the solution and fine grain strengthening, the coating with the power of 1600 W had a higher average microhardness (600 HV), which is 150% higher than that of the substrate. The experiment results further confirmed the accuracy of the simulation.

Al-Si/7.5 wt.%Y₂O₃ coatings were prepared on Mg alloy with laser cladding to enhance the wear and corrosion resistance of substrate. The influence of laser power on the microstructure and properties of the coating were discussed. The results uncovered that the coatings consisted primarily of Mg₂Si, Mg₁₇Al₁₂, Mg₂Al₃, Al₄MgY, and α-Mg phases. Through calculation, it was observed that the crystal size decreased with the decrease in the laser power. Y₂O₃ gave the coating a better strengthening effect due to the fine-grain strengthening and hard-phase strengthening. The average hardness of the coating with laser power of 1100 W achieved 312 HV, which was approximately 4.2 times that of the substrate. The wear volume of the coating was 22.2% that of the substrate. Compared with Mg alloy, the self-corrosion potential of the coating increased by 1.09 V, and the self-corrosion current density decreased by three orders of magnitude.

Cu–Cr–Zr alloys reinforced in situ with TiC_x nanoparticles were prepared via combustion synthesis and electromagnetic stirring casting. The microstructure of TiC_x/Cu-Cr-Zr composites with various contents was analyzed. The microhardness and Brinell hardness of the composites were determined; the average volumetric abrasive wear rate and worn surface of the composites were investigated; and the electrical, thermal conductivity and thermal expansion coefficients of the materials were discussed. The results indicated that the addition of TiC_x particles transformed the Cu–Cr–Zr matrix alloy microstructure from a dendritic to an equiaxed crystal, and the grain size was significantly refined as the amount added was increased. The composites with high TiCx content possessed higher hardness and abrasive wear resistance. The addition of TiC_x particles reduced the electrical and thermal conductivity and thermal expansion coefficients of the materials.

Cr12MoV has been widely used in the manufacture of stamping and drawing dies. In the present study, an attempt was made to improve the mechanical properties of Cr12MoV by laser cladding Ni60 alloy reinforced by WC. X-ray diffraction (XRD), scanning electron microscopy (SEM), a microhardness tester, and a friction and wear test prototype were used to analyze the macroscopic morphology, microstructure, and mechanical properties of the coating. The results showed that the coating mainly was composed of Cr-Fe-Ni, γ-(Fe, Ni), Cr₂₃C₆, Cr₇C₃, and W₂C phases. The cladding layer presented the dendritic eutectic structure enriched Cr, Fe, and Ni. Zigzag-shaped dendrites with thicknesses of 50~80 μm of the bonding zone ensured the strong metallurgical bonding. Due to solid solution strengthening, dispersion hardening, and grain refinement, the hardness of the coating reached 745 HV, which was 3.5 times that of the substrate. The wear volume of the coating was 14 × 10⁻³ mm³, which was 48% lower than that of the substrate (27 × 10⁻³ mm³). The coating had the abrasive wear; however, the substrate had the adhesive wear besides the abrasive wear.