Microstructure And Properties Of ZrNiAlCu Amorphous Composite Coatings On Zr-702 Surface By Laser Cladding

Industrial pure zirconium Zr-702 has a very small atomic thermal neutron absorption cross section and is one of the preferred materials in nuclear industry.However,with the development of international nuclear power technology to high burnup and long fuel cycle,in order to improve the burnup of fuel elements,reduce the cost of nuclear fuel cycle and reduce the accident of radioactive material leakage,higher requirements are put forward for the corrosion resistance and mechanical properties of fuel element materials.In order to improve the service performance of Zr-702,ZrNiAlCu amorphous composite coating and ZrNiAlCu+x%TiC(x=1,3,5,7,9)amorphous composite coating were prepared on the surface of Zr-702 by laser cladding technology.The effects of laser power and scanning speed on the microstructure and amorphous formation mechanism of the coating were studied,and then the effects of laser power and scanning speed on the properties of the coating were studied,Finally,on this basis,the effects of adding different content of TiC on the microstructure and properties of ZrNiAlCu coating were studied.The main research contents and conclusions are as follows:(1)The effects of process parameters(laser power and scanning speed)on the microstructure of zrnialcu coating were studied.The results show that the coatings prepared with different process parameters are composed of amorphous phase and crystalline phase.The crystalline phase is mainly Al₂Zr₃,Ni₁₀Zr₇,Cu Zr₂ and Zr phase.With the increase of laser power,the intensity of crystal diffraction peak gradually increases,and the amorphous content first increases and then decreases.With the increase of scanning speed,the amorphous content gradually decreases,and the intensity of crystal diffraction peak gradually decreases.The amorphous phase mainly appears in the surface layer and the middle region of the coating,and the grain structure is mainly massive grain,dendritic grain,snowflake grain and flake grain.With the increase of scanning speed,the grain size decreases gradually.(2)The mechanism of amorphous formation process was studied.It was found that the microstructure of the ZrNiAlCu coating was mainly affected by the shape control factor G/R during the laser cladding process.The farther from the bottom of the coating,the smaller the G/R.At the bottom of the coating,the G/R is large,and the microscopic morphology is a flat crystal growing in the direction of heat dissipation.In the middle area of the coating,the G/R is continuously smaller,which leads to the formation of an amorphous phase in the coating,due to the convective agitation of the molten pool flow field at the top of the coating and the non-uniform nucleation of unmelted impurities,the microstructure of the coating has undergone great changes.Combination area of amorphous phase,dendritic and snowflake-like dendrites.(3)The effects of process parameters(laser power,scanning speed)on the properties of ZrNiAlCu coatings were studied.The results showed that with the increase of laser power,the average microhardness of the coating increased first and then decreased,the corrosion resistance first increased and then decreased,and the corrosion resistance increased first and then decreased.With the increase of scanning speed,the average microhardness of the coating decreased gradually.When the laser power is 1500 W and the scanning speed is 5 mm/s,the average microhardness of the coating is the highest,which is 775.6 HV_0.2,which is about 3.88times of the hardness of the substrate.The wear depth and width of the coating are the smallest,the wear amount(0.2742 mm~3)is the smallest,the wear surface is relatively flat,there are a small amount of wear particles,the wear resistance is the best,and the wear mechanism is abrasive wear.The coating has the smallest self-corrosion current density of 9.228×10^-8 A/cm~2,the largest polarization resistance(1.46724×10~7(?)·cm~2),the largest capacitive arc radius in the Nyquist diagram,and the best corrosion resistance.(4)The effects of adding different TiC contents on the microstructure and properties of ZrNiAlCu coatings were studied.It was found that on the one hand,the addition of TiC reacted with Zr to generate the enhanced phase Zr C in situ,which eliminated the segregation of Zr.On the other hand,TiC decomposed at high temperature,and the mixing of Ti was beneficial to the formation of amorphous.After adding TiC,the amorphous content of the coating increases,and the content of TiC is different,and the phase composition of the coating is different.It can be seen from the microscopic morphology that the amorphous phase appears in the entire coating,and the crystal structure and morphology are mainly lamellar dendrites,snowflake-like dendrites and elongated dendrites.The hardness,wear resistance and corrosion resistance of the ZrNiAlCu coating with TiC are better than those of the substrate.When the TiC content is 7%,the average microhardness of the coating is the highest,which is 949.3 HV_0.2,which is 4.75times the hardness of the matrix,and is higher than that of the coating without TiC.The wear amount of the coating is 0.2568 mm~3,and the friction and wear performance of the coating is better than that of the coating without TiC.