Study On Surface Fractal Behavior Of TiAl Alloys Corroding And Electrochemical Machining

TiAl alloy is widely used in aerospace,automobile manufacturing and other fields because of its low density,high strength,high hardness,high temperature resistance,corrosion resistance and creep resistance.At the same time,the alloy has low room temperature plasticity and low fracture toughness,which makes it difficult to machine.Electrochemical machining?ECM?has the advantage of high material removal rate and tool wear-free.It is the most suitable processing method for TiAl alloy at present.In this paper,the electrochemical corrosion characteristics and ECM of Ti-48Al-2Cr-2Nb alloy were studied.In order to obtain the optimal ECM technology of the TiAl alloy,the fractal theory was used to study the corrosion mechanism and the surface quality of ECM.Firstly,the electrochemical characteristics of the TiAl alloy in electrolyte solutions with different concentrations were studied.Then,the mapping relationship between surface morphology and interface behavior of alloy electrodes and their dynamic evolution process are explored in depth and systematically.Finally,orthogonal experiment was used to reveal the influence of ECM process parameters on the fractal structure and surface roughness of the machined surface.Specific research contents and main conclusions are as follows:?1?The electrochemical characteristics of Ti-48Al-2Cr-2Nb alloy in four NaNO₃ solutions?5%,7%,9%and 11%?were studied by means of EIS,linear scanning anodic polarization curve,corrosion surface observation and corrosion products analysis.The results show that the decomposition voltage of the alloy in 11%NaNO₃ solution is the lowest,the corrosion rate is the fastest,and the residual products on the corrosion surface are the least.This concentration is suitable for ECM of TiAl alloy.The electrolyte has the strongest electrochemical activity when the temperature of the electrolyte is about 40?;Both acidic and alkaline environments can accelerate the corrosion of TiAl alloys,especially the corrosion sensitivity to acidity is higher.?2?Box-counting dimension,multifractal spectrum and lancurarity were applied to characterize quantitatively surface heterogeneity of TiAl alloy electrode,and electrochemical impedance spectroscopy was used to research electrochemical behavior of TiAl alloy electrode interface.it is found that the characteristic parameter B of multifractal spectrum and integral lancunarity can well reflect the fractal structure of the surface morphology of the electrode,and have a good functional relationship with the interface behavior of the electrolytic interface,which is more evident in the high-power super-depth digital microscope photos.?3?Dynamic evolution process of corroded surface and dynamic characteristics of interface behavior.The chronoamperometric method was used to track the dynamic formation of corrosion surface of TiAl alloy during 18V anodic dissolution.Multifractal analysis was used to analyze the dynamic change of electrode surface morphology,and impedance spectroscopy was used to track the dynamic interface behavior of the electrode.It is found that in the initial stage of CA,the discrete resistance decreased rapidly,the discrete capacitance increased continuously,the characteristic parameters W increased and?f decreased,which indicated that in the initial stage of CA corrosion,the corrosion products accumulated continuously and the anisotropy of alloy surface became more and more complex;after 16 seconds,the electrochemical parameters and multifractal parameters were relatively stable,and the complexity of electrode interface behavior and surface texture tended to be stable.?4?In order to obtain high surface quality,the electrochemical processing parameters of TiAl alloy were optimized by orthogonal experiment.Based on the surface roughness,fractal box dimension,multifractal and integral missing as main evaluation indexes,and the duty cycle,feed speed,processing voltage and electrolytic hydraulic pressure of pulse power supply as four main factors,the influence of four main factors on the surface quality is analyzed.The experimental results show tha the optimum process parameters are as follows:The duty cycle is80%,the feed speed is 0.8mm/min,the processing voltage is 27V and the electrolytic hydraulic pressure 0.4Mpa.