NiCu alloy is a crucial material used in the aerospace industry and petrochemical and other fields because of its excellent corrosion resistance, thermal conductivity, and ductility. However, problems of low hardness and poor wear resistance need to be solved to extend its service life and expand its scope of application. Currently, laser-directed energy deposition (L-DED), with a Gaussian or near-flat-top laser heat source, is applied to fabricate metal-ceramic composites to improve their performance, and the ceramic particles employed include tungsten carbide (WC), titanium carbide (TiC), and titanium nitride (TiN). However, proven drawbacks have been reported using these heat sources; these include easily emerging cracks, higher temperature gradient, greater residual stress, and an uneven distribution of WC particles. Meanwhile, laser oscillation provides a new technological approach for L-DED. Some research has offered evidences that employing this advanced technology in L-DED can mitigate the above-mentioned defects. In this study, NiCu alloy and NiCu/30%WC composite materials are fabricated by L-DED with a circular oscillating laser beam to improve the hardness and wear resistance of NiCu alloy and reveal the strengthening mechanism. This work can provide technical and data references for the preparation of new wear-resistant composite materials.
The experimental materials are NiCu alloy powder and spherical WC particles (Fig.1), with average particle sizes of 45-106 μm and 53-109 μm, respectively. A circular oscillating laser beam is chosen as the heating source to conduct the experiment, and the process parameters are given in Table 2. The laser beam shape is converted into a circular ring by a pair of scanning galvanometers. Upon irradiation with the high-energy laser, the substrate is partially melted, resulting in the formation of a molten pool on the surface. Compared with a Gaussian laser, forced convection occurs in the molten pool as a result of the stirring effect of the circular oscillating laser beam (Fig. 2). The phase composition, residual stress, microstructure, microhardness, and friction and wear of NiCu alloy and NiCu/30%WC composite materials are ascertained using an X-ray diffractometer (XRD), an X-ray stress tester, a scanning electron microscope (SEM), a microhardness tester, and a pin disk wear testing machine, respectively (Fig. 3). The impacts of the circular oscillating laser and WC particles on the phase composition and microstructure of NiCu alloy are investigated, along with their effects on the strengthening mechanism of its microhardness and friction and wear properties.
The experimental results demonstrate that the NiCu/30%WC composite comprises NiCu eutectic, WC, W2C, and M23C6 phases (Fig. 4). By adding WC particles, the residual stress on the surface of the sample transforms from (44.5±12.0) MPa to (-212.4±19.0) MPa, the diffraction peak of the NiCu eutectic phase significantly shifts leftward, and the peak width increases. The reason for these changes is a combined effect of multiple factors of WC particles hindering grain growth and promoting grain refinement, lattice distortion, and dislocations.
In addition, the stirring effect of the circular oscillating laser promotes convection in the molten pool, facilitates solute diffusion, decreases the temperature gradient, disrupts the columnar crystal structure, and provides more nucleation sites for equiaxed crystal growth. As a result, there is a uniform distribution of WC particles within the deposition layer (Fig. 5), and more equiaxed crystals are generated in the NiCu alloy (Fig. 6). Moreover, WC particles can also hinder grain growth, promoting the formation of more equiaxed crystals and cellular crystal around them (Figs. 6 and 8). Compared with the grain size (14.02 μm) of the NiCu sample, the average grain size of the NiCu/30%WC sample is 7.71 μm, a reduction of 45% (Fig. 7). Furthermore, partially decomposed WC particles and newly formed carbides are distributed in the structure (Figs. 9 and 10), encouraging grain refinement and improving the performance of the NiCu alloy.
Moreover, the average microhardness value of the NiCu/30%WC deposition layer is 418 HV, which is 19% higher than that (351 HV) of the NiCu alloy (Fig. 11). The improvement in microhardness is a consequence of the joint action of fine grain strengthening and dispersion strengthening of WC particles and carbide. The results of friction and wear experiments indicate that the addition of WC-reinforcing particles greatly enhances the wear resistance of the deposition layer (Fig. 12). This is due to the ability of the unfused WC particles to hinder the wear of the NiCu alloy substrate by grinding balls, as well as the dispersion strengthening effect resulting from the formation of WC particles and carbides. This change in strengthening mechanism also leads to a conversion from adhesive wear, which occurs in the pure NiCu alloy, to abrasive wear (Figs. 13 and 14). As a result, the width of the wear scar slightly increases but the depth significantly decreases, and the mass loss is reduced by 56%.
The NiCu alloy and NiCu/30%WC composite is manufactured by L-DED using a circular oscillation laser. The results suggest that the stirring effect of the circular oscillating laser in the molten pool can effectively promote melt flow and enhance thermal convection, thereby reducing the temperature gradient, remelting columnar crystals to generate more fine equiaxed crystals, and refining grains to enhance performance. In addition, the WC particles and the generated reinforcing phases are also evenly distributed in the deposited layer under stirring, achieving dispersion strengthening. In summary, the properties of NiCu alloy are significantly improved through the combined effects of fine grain strengthening and dispersion strengthening.
