Repairing Mechanism of Laser Cladding in Nickel-base Single Crystal Superalloys
The surface defect repair of the nickel-base single crystal superalloy blades is the key to the extension of the life,and the repair mechanism associated with this is also a hot research issue in the area of materials physics and chemistry.At present,the most effective repair method is laser cladding technique.A success repair requires that the crystals oritations in the cladding region is consistent with that in the substrate.The previous investigations have shown that laser parameters,grain boundary,and matrix substrate orientation may have effects on crystall properties,but the interaction between them and the study on the effects of crystall properties are limited,especially for the study of China self-developed nickel-base single crystal superalloy materials.This article mainly focuses on the existing problem,using the laser remelting and laser cladding technique,studied the solidification cracking formation mechanism and epitaxial growth repair mechanism of the DD series of the second generation of nickel-base superalloy blades.In view of the selection of various repair parameters in the process of laser cladding,the laser cladding experiment was used to carry out in-depth systematic study.The aim of the laser cladding research is to explore the influence mechanism of various repair parameters on the operation of laser epitaxial repair and,thereby,to find out the optimal repair window conditions by combining the final experiment results.The effect of different grain boundary angle value on solidification cracking was studied by using laser melting to find the critical grain boundary angle value of domestic nickel-base single crystal superalloy.The details of this study and the main results are summarized below:(1)The critical angle of grain boundary numerical range and cracking formation ability of nickel-based single crystal superalloy DD6 is investigated.It is found that the solidification cracking is not easy to appear when the grain boundary angle is smaller than the critical grain boundary angle.Once the grain boundary angleexceeds the the critical value,the solidification cracking appears.The critical grain boundary angle of the single crystal DD6 is 16°,as determined by the laser remelting experiment and theoretical model calculation.The mechanism of solidification cracking is that,with the increase of the value of the grain boundary angle,the liquid membrane tends to reach a lower temperature zone,which eventually induces the occurrence of solidification cracking.(2)The effects of laser power,scanning velocity,each layer repair height and overlap rate on the formation properties of single crystal during laser cladding is studied.Laser power and scanning velocity exhibited significant effect on the formation of stray grains.With the increase of laser power and scanning velocity,CET capacity of the laser cladding epitaxial growth microstructure was stronger,and the probability of stray grains increased.The each layer repair height value not only plays a major role in the elimination of the stray grains of the laser epitaxial growth microstucture,but also dramatically influences the repair efficiency.If the each layer repair height value is too large,the strongest stray grains region of the laser epitaxial growth orgnizations will retain,which can destroy the monocrystalline properties of the epitaxial growth microstructure.Furthmore,overlap rate of multi-channel multi-layer laser cladding repairing process shows a great impact on shrinkage pore produce ability.With the increase of the overlap rate,the shrinkage pore produce ability in the microstructure of the epitaxial growth was decreased first and then increased.(3)The effect of different substrate orientaiton on the formation ability of stray grains in the epitaxial growth microstructure is studied.It is found that when the repair operation is carried out on the substrate(001)/,it is easy to produce stray grains in the crystal boundary /.The stray grain is mainly grown from the grain boundary to the  crystal region,and a large number of mixed crystals regions are formed in the  crystalline region,which seriously destroys the single crystal properties of the epitaxial organizations.When the substrate(101)/[10 ?] is performed the repair operation, crystal region disappears in the epitaxial growth microstructure,grain boundary / also disappears,so the formation ability of stray grain is reduced greatly.(4)The influence of different substrate orientation on the length of each layer cladding step is analyzed.The results revealed that,comparing with substrate(001)/,substrate(101)/[10 ?] can be used in a larger length of each layer cladding step.This is because that the value of each layer repair height is decided by the size of the previous repair layer / of the grain boundary location in the laser pool.The each layer repair height should be lower than the position of grain boundary / in the laser pool to reduce the formation ability of stray grains.In addition,the rotation operation carried on the substrate  axis will eliminate the / grain boundary in the laser pool,and,under this circumstance,the biggest each layer repair height value could be applied to the repair operation.