Formation Mechanism of Stray Grain in Laser Remelting Zone of DD6 Nickel-Based Single Crystal Superalloy
Objective Nickel-based single-crystal superalloys are widely used in the manufacture of single-crystal turbine blades in the combustion chambers of aircraft engines owing to their excellent high-temperature mechanical properties.However,these components often suffer from severe damage,such as edge erosion,cracking,and pitting,owing to the harsh operating conditions,including high temperatures and pressures,requiring repairs to extend their service life.Laser additive manufacturing technology has garnered significant attention for repairing nickel-based single-crystal superalloys owing to its unique advantages,including controllable heat input,the ability to fabricate complex structures,and reparability.However,the appearance of grain defects during the repair process may lead to serious failure phenomena,such as crack propagation and component fracture,during component operation,thereby posing potential risks to the safety and reliability of aircraft engines.Therefore,this study selected a DD6 second-generation nickel-based single-crystal superalloy as the substrate material and used lasers with powers of 1200 and 1500 W to re-melt the substrate,revealing the mechanisms of stray grain formation at the fusion line,top of the molten pool,and at the intersection of the dendrites,thereby providing a theoretical basis and technical support for the laser additive repair of nickel-based single-crystal superalloys.Methods This study employed a DD6 nickel-based single-crystal high-temperature alloy prepared via directional solidification as an experimental substrate material.Subsequently,a laser cladding additive manufacturing device equipped with a 2 kW German Rofin fiber-coupled semiconductor laser was utilized.The process parameters included laser powers of 1200 and 1500 W,a scanning rate of 3 mm/s,and a spot diameter of 4 mm for the single-track remelting experiments on the substrate(001)crystal surface.To prevent sample oxidation during remelting,argon gas was used as a protective gas at a flow rate of 10 L/min.The dendritic morphologies of the substrate and remelted region were observed using an OLYMPUS DSX510 optical microscope.Simultaneously,scanning electron microscopy and X-ray energy-dispersive spectroscopy were employed for detailed analysis of the molten pool morphology and elemental distribution.Electron backscatter diffraction(EBSD)analysis was performed to further investigate the crystallographic properties of the specimens.The sample tilt angle was set to 70°,with a scan step size of 3 μm,using nickel as the calibration phase.HKL Channel 5 post-processing software was employed for texture and orientation deviation analysis of the samples.Results and Discussions The results show that,after laser remelting,the molten pool could be divided into four regions([001],[100],[010],and[010])based on the growth direction of the grains(Fig.3),and the primary dendrite spacing increased with increasing laser power.From the crystallographic texture characteristics of the molten pool(Fig.5),it is evident that carbides mainly occurred at the fusion line,top of the molten pool,and intersection of the turning dendrites.Carbides at the fusion line occurred owing to the local collapse of the rough growth interface,causing a deviation in the crystal growth direction.This also increased the diffusion rate of the solute atoms and enhanced the degree of undercooling at the solidification interface,providing heterogeneous nucleation sites for grain nucleation,thereby inducing carbide formation.The carbides at the top of the molten pool were caused by the columnar-to-equiaxed transition,where the numerical simulation showed that the temperature gradient gradually decreased and the solidification rate increased from the bottom to the top of the molten pool(Fig.11),promoting the columnar-to-equiaxed transition.Carbides at the intersection of turning dendrites occur because of collisions resulting from the lower temperature gradient at this location compared with adjacent areas and changes in the direction of the temperature gradient.The thermal stress numerical simulation results(Fig.12)indicate that a low laser power input effectively increases the temperature gradient and reduces the residual stress level,which is beneficial for inhibiting carbide formation in single-crystal repairs.Conclusions This article utilized two distinct laser powers,1200 and 1500 W,to conduct single-track laser remelting on DD6 nickel-based single-crystal superalloys,in conjunction with numerical simulations,to investigate the mechanisms behind stray grain formation in various regions within the molten pool.The principal findings are as follows:1)Stray grains predominantly emerged in three areas:the fusion line of the molten pool,top of the molten pool,and convergence point of diverging dendrites.2)At a laser power of 1500 W,there was an increased quantity and larger size of the stray grains.No stray grains were observed at the smooth interface of the fusion line,whereas carbides formed at rough interfaces could lead to the collapse of the solid-liquid interface,causing a deviation in the dendritic growth direction and inducing stray grain formation.3)The development of stray grains at the apex of the molten pool was closely related to the temperature gradient of the molten pool and movement rate of the solid-liquid solidification interface.The minimal temperature gradient at the summit of the molten pool enhanced the likelihood of a columnar-to-equiaxed transition,fostering the generation of stray grains.Furthermore,it was discovered that a reduced laser power could increase the temperature gradient and suppress the formation of stray grains.4)The genesis of stray grains at the confluence of diverging dendrites was attributed to collisions caused by the temperature gradient at the dendrite junction,which was lower than that in the surrounding areas,coupled with a shift in the direction of the temperature gradient.
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