Experimental Research On Wire EDM Of Super-elastic Nickel-titanium Shape Memory Alloy

Nitinol,as the most representative shape memory alloy,exhibits excellent shape memory effect（SME）,super elastic（SE）and biocompatibility.In addition,it also has the characteristics of high damping,resistance of wear and corrosion.Therefore,it is widely used in biomedicine,electronic,aerospace,civil engineering and other fields.However,due to the poor thermal conductivity,high ductility,and low Young’s modulus of nickel-titanium memory alloy,the wear of cutter is serious during traditional machining,the machined surface has many burrs,and the hardening of the surface is serious.So the machining quality is difficult to guarantee.Compared with traditional machining methods,wire electrical discharge machining（WEDM）uses discharge energy,generate instantaneous high temperature to remove material on workpiece.This machining process does not produce macroscopic stress.It has the advantages of machining high-strength,high-hardness,high-temperature resistance and other difficult-to-machine materials.Therefore,based on the poor surface quality of super-elastic nickel-titanium memory alloys processed by traditional processing methods,this paper conducts WEDM experiment on Ni_55.9Ti super-elastic nickel-titanium memory alloy,the specific research content is as follows:1)The study on the WEDM machining characteristics of super-elastic nickel-titanium memory alloy was carried out.Through the combination analysis of the physical properties of the material and the basic principle of WEDM,and the comparison of the element content of the processed surface with different parameters,the WEDM machining characteristics of the super-elastic nickel-titanium memory alloy were obtained.On the basis of ensuring the processing efficiency and surface integrity of the workpiece,the plan of WEDM experiment of super-elastic nickel-titanium memory alloy was determined,and the pre-experiment was carried out to select the range of parameters.The results show that the super-elastic nickel-titanium memory alloy has the processing characteristics of easily forming carbon deposits on the WEDM processed surface,which can be controlled by setting reasonable processing parameters.In addition,too large,too small impulse width and too small ratio of impulse interval are not conducive to the processing of workpiece.2)A single factor experiment based on processing efficiency and surface integrity was carried out to explore the influence of impulse width,peak voltage,wire speed and ratio of impulse interval on material removal rate,surface roughness,residual stress on processed surface and surface morphology.And through Taguchi experiment analysis,the material removal rate and surface roughness parameters were optimized,and the optimal processing parameters were obtained.The results show that the peak voltage and impulse width are the biggest influencing factors of material removal rate and surface roughness.Respectively,the wire transport speed has no significant effect on both indexes.3)On the basis of ensuring the processing efficiency and surface integrity,for super-elastic nickel-titanium thin-walled workpieces Processing by WEDM,the deformation recovery offset,processing deformation amount and slit width were used as the characterization methods to explore the super-elastic shape recovery ability of the workpiece,machining deformation and dimensional accuracy regular patterns with each processing parameter.The results show that:the use of WEDM processing methods will weaken the shape recovery of the workpiece,the larger impulse width and peak voltage will increase the deformation recovery offset,processing deformation of the workpiece and slit width;The increasing of wire speed and ratio of pulse interval will improve the shape recovery and processing deformation of the workpiece,but has no significant effect on the slit width.In addition,the influence of processing parameters on residual stress on processed surfaces and processing deformation is consistent.