Cracking Sensitivity And Evaluation Method Of Laser Cladding Ni-based Alloy Coatings

Aiming at the high crack sensitivity of Ni based alloy laser cladding,the crack initiation and propagation of Ni based laser cladding layer are studied systematically.By micro structure,elemental composition and phase analysis,as well as fracture observation,the structure factor of cracking mechanism was investigated;the mechanical factors of crack formation are studied by means of finite element simulation.Crack susceptibility testing method for laser cladding technology was newly-designed,to standardize cracking susceptibility of laser cladding material by quantitative indicators.With preheating and gas protection,Ni-based cladding layer has good forming,and the cladding layer is smooth,flat and low ripple;The Ni based cladding layer with pneumatic powder feeding is relatively flat,but the cladding layer is less formed,and there is no melting powder on the surface.The number of cracks and the total length of the crack were obviously decreased with the preheating,gas protection and pneumatic powder feeding.The number of cracks and the total length of the crack were obviously decreased with the preheating,gas protection and pneumatic powder feeding.The influence of substrate thickness on the crack sensitivity of Ni based cladding layer was the most significant,with the decrease of the thickness of the substrate,the restraint degree of the cladding layer decreases,and the crack sensitivity decreases rapidly.Most of the cracks of Ni-based laser cladding layer originates from the near surface area of the cladding layer and near the fusion line zone,and expand opposite throughout the cladding layer,ultimately end in the form of interwoven.Very small amount of micro-cracks along crystal or crystalline form of wear,providing a source of macro crack.The structure factor of Ni-based cladding cracks is analyzed.Ductile matrix phase such as?-?Fe,Ni?and other hard phase like carbide and boride are formed in the cladding layer.And Ni-based cladding layer contains carbon,boron,iron,silicon,tungsten,chromium,nickel and other elements,ensuring high hardness and good wear and corrosion resistance of the cladding layer.In this paper,the finite element analysis software SYSWELD is used to establish a better heat source model,which can conform to the experimental results.Thus the distribution and variation of temperature field and stress field can be obtained during laser cladding process.It is found that laser cladding specimens'residual stress is concentrated on the cladding layer and it is mainly manifested by longitudinal tensile stress and transverse tensile stress,the longitudinal tensile stress is the main stress factor of the crack,the size of longitudinal tensile stress and transverse tensile stress determines the direction of crack propagation;Matrix thickness is the main influence factor of residual stress in cladding layer.According to the idea of Longitudinal Varestraint Test,together with the characteristics of laser cladding process,a brand-new crack test Free Sheet Deformation Test was designed.By evaluating the critical deformation?_cr,crack sensitivity is assessed by quantitative indicators.Theoretical analysis shows that the matrix deformation of thin plate is proportional to the force of contraction during laser cladding process;the size of shrinkage stress can be determined by measuring the deformation of the substrate under different cladding parameters.Experimental results show that critical deformation?_cr of Ni-based alloy was approximately7.22mm.If the sheet deformation rate is bigger than?_cr,then it can be judged that laser cladding layer is cracking tendency under the process parameters,and deformation between the larger and larger,the crack sensitivity higher,and the greater the deformation gap,the higher the crack sensitivity.