| Aeroengine,as the pillars of a great power,is known as the "pearl in the crown of industry".It plays a great role in promoting the development of national economy and science and technology and is an essential strategic guarantee for national security and great power status.Single-crystal(SX)superalloy high-pressure turbine blades are the core power components of an engine.Due to long-term service in an extremely high temperature,high pressure,and high-stress environment,SX blades will be damaged and lead to failure.The high replacement cost significantly improves the repair demand of SX blades.However,the laser repair technology of SX blades has been monopolized and blocked by famous international engine companies for a long time.This technology is also one of the "stranglehold" technologies that need to be solved urgently in the field of aeroengine and gas turbine in the Sci-Tech Innovation 2030 Agenda in China.This thesis focuses on the problems such as the difficulty in continuous epitaxial growth control of SX,thermal cracks,and stray grain(SG)defects in the deposited zone,which seriously affect the high-temperature performance during laser melting deposition(LMD)for SX blades.The LMD characteristics of CMSX-10 alloys,the mechanism of hot cracking and SGs formation were systematically studied using experiments and theoretical calculations.A magnetic field coupled LMD method was proposed to control defects,which provided a theoretical basis for the efficient green repair of single crystal superalloyIn order to solve the difficulty of continuous epitaxial growth of SX,the LMD characteristics of SX were studied.As the height of the deposited zone increases,the temperature gradient decreases and the solidification rate increases.From the bottom to the top of the SX deposited zone,planar crystal,cellular crystal,columnar dendritic crystal,and equiaxed crystal structures are formed successively.The analysis of epitaxial growth characteristics shows that the columnar crystal region composed of planar crystals,cellular crystals,and columnar dendrites is a single crystal structure,confirming the feasibility of LMD for repairing SX superalloys.The influence of process parameters on the epitaxial growth was obtained by orthogonal experiment.With increases laser power and decreases scanning speed,the SX epitaxial growth height increases while the epitaxial growth rate decreases.With the increase of feeding rate,the epitaxial growth height increases first and then reduces,whereas the epitaxial growth rate drops significantly.Based on the corrected criterion of columnar to equiaxed transition(CET),the microstructure selection maps of CMSX-10 SX superalloy were obtained by calculation.The intermittent deposition strategy and dynamic deposition strategy were proposed.Compared with the traditional continuous deposition strategy,the epitaxial growth height is increased from 1.7 mm to 3.8 mm and 4 mm,and the epitaxial growth rate is increased from 40% to 90% and92% for the intermittent deposition strategy and dynamic deposition strategy.In order to solve the problems of hot cracks and SGs which seriously affect the high-temperature performance in the deposition area,the mechanism of hot cracks formation was studied.Hot cracks occur at high-angle grain boundaries and especially at low-angle grain boundaries in the CMSX-10 SX deposited zone.The minimum grain boundary angle is 6.9°.Hot cracking is caused by a stable liquid film,stress concentration,and Re-rich precipitates.The stability of the liquid film depends on dendrite coalescence undercooling,which is related to the misorientation angle.The minimum grain boundary angle for CMSX-10 alloy to form a stable liquid film was calculated to be 4.2°,which is the critical angle for developing hot cracks in the alloy.The dendrite coalescence undercooling at a low-angle grain boundary(misorientation angle 6.9°)is 178 K,which is far higher than the vulnerable temperature interval of38 K for hot cracking within a single dendrite.Stress concentration provides the driving force for crack initiation and propagation.Re-rich precipitates promote crack initiation by a pinning effect on the liquid feed.The mechanism of SGs formation in the SX deposited zone was further studied,and the relationship between the Marangoni convection in the molten pool and the SGs at the intersections of turning dendrites was established.The formation mechanism of SGs on the fusion interface between deposit and substrate was clarified.As interdendritic precipitates and eutectic phases near the fusion interface remelted and caused the solid-liquid interface to collapse during the melting process,the temperature gradient direction variates.Cells deviate from the original growth direction and form SGs.Based on the criterion of minimum growth rate,the relationship between the molten pool morphology and the SGs at the intersection of turning dendrites was obtained.The promoting effect of the undulant molten pool morphology on the SGs was illustrated.Combined with the calculation of the physical properties of SX and the variation of laser energy density,the mechanism of Marangoni convection promoting the molten pool geometry from semicircular to undulant was revealed.Finally,the relationship between Marangoni convection and SGs formation at the intersection of turning dendrites was established.Based on the mechanisms of hot cracks and SGs,an external magnetic fieldassisted laser melting deposition method was proposed to control the defects in the deposit.The results show that the transverse and longitudinal steady magnetic fields promote the transformation of the molten pool morphology from undulant to semicircular.The Marangoni convection in the pool is significantly inhibited,and the inhibition effect becomes stronger with the increase of magnetic field intensity.In addition,cells transform to columnar dendrites by applying a steady magnetic field in the deposited zone.The mechanism of steady magnetic field restraining molten pool flow was revealed: Marangoni convection carries the conductive liquid phase to cut the magnetic induction line motion and generates the induced current.The induced current and magnetic field further interact to form Lorentz force opposite to the motion direction of the initial liquid phase and finally suppresses the Marangoni convection in the molten pool.The gradient magnetic field induced by the gradient solute at the solid-liquid interface further promotes the solute enrichment and leads to the cell/columnar dendrites transformation.Finally,an entire SX structure with a height of more than 5 mm without hot cracks and SGs was obtained by multi-layers deposition applying a longitudinal steady magnetic field. |
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