| Compressor blades,spiral micro-tools and other difficult-to-cut components in aerospace engineering,microelectronic machinery and other advanced manufacturing fields are characterized by high material strength and complex processing features.The application of these components exist adverse working conditions with high gradient temperature difference,high frequency load,various corrosive medium,etc,which usually cause fatigue rupture of the samples due to the surface damage.Therefore,extremely high requirement of micro-surface integrity is necessary to meet the demands in service life and reliability.Wire electrical discharge machining(WEDM)has potential to solve above problem due to its obvious advantages such as non-macro cutting force,wide range of processed materials and shape features with high micro surface quality.However,the unstable machining process induced by the abnormal discharge states and the non-uniform discharge point distribution in WEDM seriously limits its application in advanced manufacturing field.Magnetic field assisted technology can improve the stability of discharge spark and material erosion in WEDM,but the micro-surface integrity indicators including recast layer,surface roughness,micro-crack and so on can be damaged by an unsuitable assisted magnetic field when machining different magnetic materials.Meanwhile,the reduction of surface quality defects such as high gradient residual stress and great recast layer accompany by thermal deformation behavior are also required in magnetic field assisted WEDM(MF-WEDM)processing thin-walled workpiece.Aiming at breaking through the application challenges of MF-WEDM in advanced manufacturing field,this paper comprehensively reveals the material erosion mechanism in MF-WEDM of processing different magnetic materials and thin-walled workpieces and improves their micro-surface integrity.The researches details are shown as follows:To reveal the material erosion mechanism in MF-WEDM,the impact mechanism of magnetic field on single pulse discharge channel was studied.the electron beam motion model of single pulse discharge channel under magnetic fields was established and the discharge crack experiments were conducted as confirmatory experiment.The results indicated that the single pulse discharge channel can be confined and its generation time can be reduced under assisted magnetic fields,which improved the discharge energy and stability of single pulse.The impact trend of magnetic fields on the distribution uniformity of continuous pulse discharge points was further studied by analyzing the vibration equation of wire electrode,the force and motion equation of charged debris.It was found that magnetic fields can improve the distribution uniformity of discharge points,and above theoretical analysis was verified by experiments based on the established discharge points observation system.The improvement mechanism of magnetic fields on the surface integrity was investigated by establishing a continuous pulse material erosion model of MF-WEDM based on discharge point distribution.The discharge point distributions under different machining parameters were firstly observed and recorded using the experimental method,and then a discharge point distribution model along workpiece thickness direction was established,which was applied as input for the establishment of the continuous pulses Gaussian heat source model.To improve the computational efficiency,a simplified Gaussian model was established,and its reliability was verified by comparing the stress field and temperature field with previous Gaussian model.Finally,the comparison of discharge point distribution and residual stress in the presence and absence of magnetic field was performed when machining Inconel 718 alloy.The results demonstrated that the validity of the established continuous pulse model was verified,and the workpiece micro surface integrity can be improved with magnetic fields by increasing the distribution uniformity of continuous pulse discharges.Aiming at improving the micro-surface integrity in MF-WEDM processing magnetic and nonmagnetic materials,this paper firstly established a three-dimensional magnetic field redistribution model of different materials using COMSOL software,and then the motion trajectory of material debris in the magnetic fields was analyzed.It can be found that the magnetic field had negative effect on the debris removal in rough machining magnetic materials,while positive effect in finish machining.The impact trend of different discharge parameters and magnetic field intensities on machining quality was explored in WEDM finishing magnetic material of SKD11.The results demonstrated that magnetic field can obviously improve the normal discharge ratio,increase average normal discharge ratio,reduce surface roughness and recast layer thickness.The impact trends of different discharge parameters and magnetic field intensities on the normal discharge ratio,specific energy consumption and surface roughness were also investigated in WEDM processing nonmagnetic material of Inconel 718,and the results proved the improvement of magnetic field on micro-surface integrity.Aiming at improving the micro-surface integrity in MF-WEDM processing thin-walled workpiece,this paper firstly analyzed the impact of workpiece thickness on the thermal deformation in WEDM,and it can be found that there is a significant thermal deformation behavior in WEDM processing thin-walled components.The micro-surface integrity of workpiece can be damaged by the thermal deformation.Based on the analysis of mechanical model of thermal deformation,it was clear that the deformation were induced by the unbalanced internal residual stress.To improve the surface integrity and reduce the thermal deformation in MF-WEDM processing thin-wall components of Inconel 718,the impact trends of different discharge parameters and magnetic field intensities on the thermal deformation,residual stress and recast layer were analyzed.The distortion behavior was also studied in MF-WEDM,and it was found that the magnetic field could reduce the distortion and improve the micro-surface integrity by increasing the distribution uniformity of discharge points.A toolkit based on intelligent optimization algorithm was developed to optimize the relevant multi-objective process parameters for further improving the micro-surface integrity in MF-WEDM processing different types of workpiece.The generalized regression method was firstly used to establish the mathematical regression models of different micro-surface integrity indicators,and the reliability of the above regression models was confirmed through residual error analysis and variance analysis.A modified non-dominant neighborhood immune algorithm(M-NNIA)was proposed to improve the ability of multi-objective optimization.By comparing the optimization results of M-NNIA with that of traditional NNIA algorithm,it was found that this proposed M-NNIA algorithm can not only obtain a better optimal results,but also reduce the iterations number and improve the optimization efficiency.Finally,above optimization algorithms were embedded into the developed toolkit,and verification experiments were carried out in MF-WEDM processing SKD11.It can be obtained that the average relative error of surface roughness,recast layer thickness and material removal rate were all less than 10%,which proved the reliability of the developed toolkit. |
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