Study On Process And Properties Of Ni-Ti-Nb Alloy Coating Prepared By Laser Cladding

In this paper,Ni-Ti-Nb alloy coating was prepared by laser cladding on the surface of TC4 titanium alloy by Nd:YAG solid-state laser.Through the experiment,the influence of laser process parameters on coating performance was investigated,and key problems like the optimization of laser cladding process parameters as well as the design of Ni,Ti,Nb metal powder were thus solved.Moreover,the experiment also leads to the forming of high-quality Ni-Ti-Nb alloy coating,and the microstructure and mechanical properties of it were analyzed.Five process parameters of laser current,frequency,pulse width,defocusing amount and scanning rate were selected.Firstly,the influence of the process on the mechanical properties of the coating was investigated by single factor experiment.Through the investigation,it was found that the way these five process parameters affect the mechanical properties of the coating is by changing the heat input,where the current,frequency and pulse width were proportional to the laser cladding heat input,and were increase proportional to the absolute value of the defocus amount,the scan rate,and the heat input.When the heat input was insufficient to form a molten pool between the coating and the substrate or the molten pool was too shallow,the thickness of the coating might be too thin,and the bonding strength with the substrate thus would not be high enough.Otherwise,the degree of oxidation of the coating increased and the thickness reduced,and the coating properties were thus affected.Based on the results of single factor experiments,the orthogonal parameters of the laser cladding process parameters were optimized by orthogonal test.The orthogonal process results were analyzed by range analysis and variance analysis to obtain the optimal process parameters under the experimental conditions:the laser current was 185 A,with its frequency being 17 Hz,defocus being 15 mm,scan rate being 400 mm/min,and pulse width being 5.5 ms.Under the optimal process parameters,the uniform design of the mixture was used to improve the powder composition,and the regression equation was obtained by regression analysis.Substitute the target component Ni:Ti:Nb=47:44:9(atomic percent)into the regression equation to calculate the corresponding powder ratio Ni:Ti:Nb=52.93:37.22:9.85(atomic percent).It was finally determined that the coating composition was close to the 47Ni-44Ti-9Nb shape memory alloy through the process of laser cladding coating preparation by using optimal process parameters and powder ratio.Under the optimal process parameters and powder ratio,the Ni-Ti-Nb alloy coating was prepared by laser cladding.The hardness of the coating was up to 764.9 HVO.3/5,and the hardness and wear resistance of the coating were higher than that of the substrate.Because of the fast cooling-rate and a large degree of subcooling of the heat-affected zone,the grain formed there was finer than that in the center of the coating.The top of the coating was affected by oxygen oxidation in the air and the performance of the tissue was reduced.By XRD analysis,the coating prepared in this paper mainly contained three phases:NiTi,Ni3Ti0.97Nb0.03 and NiO.