Study On Evolution Of Microstructure In Deep Undercooled Non-quilibrium Solidified Alloys And Mechanical Properties Of Laser Cladding

Coal mining is usually in harsh environment and harsh conditions.Mining machinery and equipment are in the condition of temperature,humidity and strong corrosion for a long time,facing the problems of high temperature,wear and corrosion.It is an urgent problem to develop a kind of mining machinery alloy material and processing technology with high yield strength,wear resistance,surface high temperature resistance,surface corrosion resistance and excellent laser welding performance.In this paper,the non-equilibrium solidification microstructure of two-and ternary Cu-base alloys was obtained on the basis of Cu₅₅Ni₄₅ alloy by the technology of melting glass purification and cyclic superheating.The controlling factors of dendrite growth in undercooled alloy melt were analyzed with BCT model,and the grain refinement mechanism under different undercooling degree was confirmed.The solidification behavior of Cu-Ni alloy and Cu-Ni-Co alloy was studied by infrared temperature measurement and high speed photography.At the same time,the effects of increasing the content of Co and Cu on the microstructure,characteristic undercooling degree,critical undercooling degree of refinement,microhardness and solidification rate of Cu-Ni alloy are also studied.The alloy cladding layer is prepared on the matrix material by laser cladding technology and processed by strong spinning technology to improve the surface properties of the cladding layer by plastic deformation.Heat treatment was performed to remove large residual stress,and mechanical properties such as hardness,wear resistance and corrosion resistance of the cladding layer were studied.The main research results obtained are as follows:（1）The microstructure of Cu-Ni alloy and Cu-Ni-Co alloy has the same evolution process with the change of undercooling degree,and the transformation process of thinning microstructure appears in both small and large undercooling cases.At low undercooling,the microstructure of the alloy has high strength texture,and the morphology is coarse dendrite to equiaxed grains.At high undercooling,the microstructure of the alloy has twin structure,large Angle grain boundary and recrystallization,resulting in the microstructure thinning consistency,and the morphology is oriented fine dendrite to equiaxed grains.（2）By combining the BCT model and metallographic analysis,the intrinsic factors of microstructure transformation in the alloy were studied.In the case of small undercooling,solute diffusion dominates dendritic growth,but the undercooled melt is limited to grow within a narrow range,resulting in the formation of coarse dendritic morphology in Cu-Ni alloys.As the undercooling increases,the growth of dendrites is gradually controlled by both solute diffusion and thermal diffusion,and the remelting effect of dendrites is also gradually enhanced.The dendrites formed by undercooled melts with lower undercooling during rapid solidification are remelted to form numerous crystal seeds,and the alloy structure is refined.The residual strain energy in the dendrite fragments provides a driving force for the occurrence of recrystallization in the organization,which refines the organization due to recrystallization.（3）During rapid solidification,Cu-Ni and Cu-Ni-Co alloy melts only exhibit one re glow phenomenon.As the undercooling increases,the re radiance approximately increases in a linear form,and a transition process of"small angle plane sharp front smooth arc"occurs at the solidification front of the alloy melt.In addition,the solidification rate of the alloy follows a power function relationship with undercooling.The increase in Cu element content and the addition of Co element have almost no effect on the microstructure and morphology transformation of Cu-Ni alloy.However,with the increase of Cu content,the critical undercooling for refining Cu-Ni alloy with large undercooling will decrease,while the undercooling for refining Cu-Ni alloy with small undercooling has little effect.The addition of Co element enhances the characteristic undercooling of microstructure transformation in Cu-Ni-Co alloy,but has little effect on the critical undercooling.Meanwhile,the addition of Co element can increase the microhardness of the alloy by about 5%,while increasing the Cu element can actually reduce the microhardness of the alloy.In addition,the increase of Cu and Co elements will both reduce the solidification rate of the alloy and weaken its recalescence effect.（4）The cobalt-based alloy coating was obtained by laser cladding technology,and the coating was heat treated by different processes.The microstructure of the sample was analyzed and the influence of heat treatment on mechanical properties was discussed.The Fe-base alloy cladding layer was prepared on the surface of 27Si Mn steel cylinder,and the influence of high speed laser cladding process parameters on the microstructure,hardness and wear resistance of the cladding layer was investigated.（5）Stellite12 coating was prepared on 304steel substrate by laser cladding technology.Through testing and analysis of the microstructure,element distribution,phase and electrochemical corrosion behavior of the coating,and comparative analysis of the corrosion resistance of the coating and 304stainless steel,it was found that cracks occurred when the iron-based amorphous alloy coating was coated on the surface of 304 stainless steel substrate.The crack forming mechanism of the coating was studied by qualitative analysis of elements around the crack,hardness and precipitates at the crack initiation.