Yifei Xu

In this study, homogeneous (Ti, Nb)(C, B)/IN625 composite coatings with almost defect-free microstructures were successfully prepared on a 42CrMo steel substrate by coupling ultra-high-speed laser cladding (USLC) with the direct reaction synthesis (DRS) technique to introduce the in-situ exothermic reaction into the cladding materials; these were comparatively analyzed with the pure IN625 coating prepared only by USLC. Our results showed that the interface of the composite coating/substrate was greatly affected by about 670 kJ Joule heat released from the in-situ reaction happening during the cladding process, which was sufficient to remelt the as-deposited materials and significantly increased the coating/substrate interface width to around 24 μm, six times the interface width of pure IN625 coating. Furthermore, the residual stress inside the coating and across the interfacial region was also reduced, alleviating the interface stress mismatch. However, the surface hardness of (Ti, Nb)(C, B)/IN625 composite coating was found to be lower than that of the IN625 coating, and the average wear weight loss was only 10% of that of the IN625 coating, attributable to the in-situ authigenic TiCB, TiC, NbMo₃B₄ and NbMo₂B₂ phases providing load transfer from the hard phases to the IN625 composite matrix to achieve abrasion reduction and wear resistance. It was also found that the formation of nano-equiaxial ultrafine grains in the depth range of 250 nm below the wear surface was facilitated by the coupling of the three fields of plastic rheology-heat-force, which dynamically strengthened the wear surface. Based on these findings, it is suggested to promote the strategy of combing USLC and DRS techniques to achieve an additional ability to enhance the coating microstructure and reduce residual stress, to achieve better tribological performance.

The (Ti, Nb)(C, B)/IN625 composite coating with both homogeneous and defect-free microstructure were successfully prepared by in situ on the surface of 42CrMo steel using the coupling of the ultra-high-speed laser cladding (EHLA in German) technology with the direct reaction synthesis (DRS) technology, and were comparatively analyzed with the IN625 coating prepared by the EHLA. The microstructure of the fused cladding layer was investigated by selected scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and double spherical aberration transmission electron microscopy (DSA-TEM). The residual stress distribution on both sides of the fused cladding interface was characterised by nanoindentation stress test based on the modified O&P method and the G&S energy method. The results show that the interface of (Ti, Nb)(C, B)/IN625 composite coatings is affected by about 670 kJ Joule heat released from the in-situ reaction, and the interfacial width reaches 24 μm, so that it is 6 times higher than that of IN625 coating prepared by EHLA, which effectively reduces the stress gradient in the interfacial region and alleviates the stress mismatch on both sides of the interface. However, the surface hardness of (Ti, Nb)(C, B)/IN625 composite coating is lower than that of the IN625 coating, with a value of about 240 HV0.2, and the average wear weight loss was only 10% of that of the IN625 coating, which is on the one hand attributed to the in-situ authigenic TiCB, TiC, NbMo3B4, and NbMo2B2 phases supporting the (Ti, Nb)(C, B)/IN625 composite coating substrate to achieve the abrasion reduction and wear resistance. On the other hand, it is attributed to the formation of nano-equiaxial ultrafine grains in the depth range of 250 nm below the wear surface area by the coupling of the three fields of plastic rheology-heat-force, which dynamically strengthens the wear surface.

With the aim to improve cladding coating quality and prevent cracking, this paper established an extreme high-speed laser cladding thermo-mechanical coupling simulation model to study the evolution of the temperature field and the residual stress distribution. Process parameters that impacted the macroscopic morphology of single-pass coatings were investigated. Numerical calculations and temperature field simulations were performed based on the process parameter data to validate the effects of the temperature gradient and cooling rate on the coating structure and the residual stress distribution. The results showed that a good coating quality could be achieved using a laser power of 2400 W, a cladding speed of 20 m/min, and a powder feeding rate of 20.32 g/min. The coatings’ cross-sectional morphology corresponded well with the temperature distribution predicted by the numerical modeling of the melt pool. The microstructure of the molten coatings was affected by the temperature gradient and the cooling rate, which varied greatly from the bottom to the middle to the top. Maximum residual stress appeared between the bonding region of the coatings and the substrate, and the coatings themselves had significant residual stress in the form of tensile strains, that were mostly distributed in the direction of the laser cladding.

With the aim to improve cladding coating quality and prevent cracking, this paper established an extreme high-speed laser cladding thermo-mechanical coupling simulation model to study the evolution of the temperature field and residual stress distribution. Process parameters that impacted the macroscopic morphology of single-pass coatings were investigated. Numerical calculations and temperature field simulations were performed based on the process parameter data to validate the effect of the temperature gradient and cooling rate on the coating structure and the residual stress distribution. The results showed that a good coating quality could be achieved using a laser power of 2400 W, a cladding rate of 20 m/min, and a powder feeding rate of 20.32 g/min. The coatings’ cross-sectional morphology corresponded well with the temperature distribution predicted by numerical modeling of the melt pool. The microstructure of the molten coatings is affected by the temperature gradient and cooling rate, which vary greatly from the bottom to the middle to the top. Maximum residual stress appears between the bonding region of the coatings and the substrate, and the coatings themselves have significant residual stress in the form of tensile strains that are mostly distributed in the direction of the laser cladding speed.