Study On Microstructure And Properties Of Laser Cladding Ni-based Alloy Coatings Under The Alternating Current And Steady Magnetic Field

Laser cladding technology was widely used in aerospace,equipment manufacturing,petrochemical and other fields.Although the laser cladding coating could significantly improve the wear and corrosion resistance of the workpiece,there were many problems in the coating,such as cracks,pore and surface ripple.These defects would shorten the life of the cladding workpiece and brought serious safety risks.Therefore,the improvement of coating quality was of great significance to the development of laser cladding technology.In this paper,the surface morphology,cross-section size,solidification structure,crack number and microhardness of laser cladding coatings under different electromagnetic fields were investigated in detail.Pure iron was used as substrate and Ni60 self-fluxing alloy powder as cladding material.A single alternating electric field,a single steady magnetic field and an electromagnetic composite field were used as auxiliary forms,respectively.Alternating current was applied to the molten pool during laser cladding.The effect of current intensity on the macro-morphology,microstructure and properties of laser cladding Ni-based alloy coating was studied.The results showed that the fluctuation degree of coating surface increased with the increase of alternating current.When the alternating current was 600 A and 1200 A,the cross-section width of the coating did not change much.When the current reached 1800 A and 2100 A,the coating width increased obviously.Alternating current did not change the phase composition of the coating(?-Ni,Cr7C3,CrB and Ni3B).Alternating current could improve the solid solubility of elements in matrix phase y-Ni,and promoted the transformation of grain structure from columnar to equiaxed at the top and bottom of the coating,and the refining effect of grain structure at the bottom of the coating was particularly remarkable.When the current intensity was 2100 A,the number of cracks on the coating surface was the least,and the average microhardness of the coating increased by about 4.1%.The parameters of the steady magnetic field with the strength of 0.1 T,0.15 T and 0.2 T were set respectively.The effects of steady magnetic field on the macro-morphology,microstructure and properties of laser cladding Ni-based alloy coatings were studied.The results showed that the fluctuation degree of coating surface was alleviated with the addition of magnetic field,and the increase of coating width was not obvious.Only when the magnetic field intensity was 0.2 T,the coating width increased obviously.The steady magnetic field did not change the phase composition of the coating,but it could refine the grain structure of the coating.At the same time,the hard phase CrB also exhibited a "cluster" aggregation effect,and the growth direction of the "cluster" structure was gradually parallel to the direction of the magnetic field with the increase of the magnetic field intensity.The maximum magnetic field intensity was 0.2 T,and the number of cracks was the least with the magnetic field parameter.The average microhardness of the coating was increased by about 8.6%.On the basis of a single alternating current and a single magnetic field,the alternating current and steady magnetic field were combined to further study the effect of electro-magnetic composite field on the structure and properties of the coating.The results showed that the electro-magnetic composite field maked the coating surface smooth and the cross-section width increased.The electro-magnetic composite field did not change the phase composition of the coating,and the grain refinement increased with the increase of electro-magnetic parameters.Although the optimum parameters of a single current and a single magnetic field were 2100 A and 0.2 T,the optimum parameters of an electro-magnetic composite field were not a simple combination with the two optimum parameters,but were related to the electromagnetic force formed in the composite field.The increase of electromagnetic force could promote the refinement of solidification structure,solution of elements,reduction of surface cracks and improvement of microhardness.When the electro-magnetic parameter was 0.2 T+1200 A,the maximum increase of average microhardness of the coating could reach 13.6%.However,the increase of electr-magnetic parameters was not conducive to the decrease of surface cracks and the increase of microhardness.By comparing the effects of three external fields(a single alternating current,a single steady magnetic field and an electro-magnetic composite field)on the microstructure and properties of laser cladding Ni-based coatings,it was found that the effect of electro-magnetic composite field on solidification process was much higher than that of two single external fields,and a high quality laser cladding coating could be obtained by using smaller electro-magnetic composite parameters.It has an important practical significance in actual production.