Study On Surface Modification Of 7Cr13Mo Steel By Ultrasonic Assisted Powder Mixing EDM

Ultrasonic assisted mixed powder EDM(US-PMEDM)is a kind of compound machining technology which adds axial ultrasonic vibration to workpiece on the basis of mixed powder EDM to improve machining efficiency.In this paper,the polarity effect of EDM was tested at first,then the effects of different kinds and concentrations of mixed powder on the machining properties were investigated.finally,the surface modification effects of EDM and ultrasonic assisted mixed powder EDM under different parameters were compared and studied.The main research contents and conclusions are as follows:(1)According to the test requirements,an ultrasonic vibrator(ultrasonic transducer and horn)clamping device with fine tuning function is designed.at the same time,in order to avoid the interference of the ultrasonic vibration system and the electric circuit of the powder mixing EDM system,The workpiece fixture with insulation function is designed and machined.the fixture is fixed on the end face of the ultrasonic horn by thread connection.(2)The effect of polarity effect on the surface modification of 7Cr13 Mo steel during ordinary EDM was discussed.Based on the comprehensive analysis of the surface micromorphology,roughness,material removal rate,nano-hardness and cross section morphology of the modified workpiece,it is concluded that the modification effect of positive polarity processing on 7Cr13 Mo material is better.(3)The effects of silicon carbide and titanium carbide powder on the properties of surface modified layer under different concentration conditions were studied by setting five concentration gradients.When the powder concentration increases gradually,the modification effect of the two kinds of mixed powder shows about the same changing trend,the surface roughness value decreases at first and then increases,the material removal rate and nano-hardness increase at first and then decrease,and the surface micro-morphology becomes worse.At the same time,the surface is attached to tiny particles.The modification effect of titanium carbide powder is better than that of silicon carbide powder under the same conditions,and all the indexes reach the best when the concentration is 10g/L.(4)On the basis of the previous experiments,a four-factor and three-level orthogonal test was designed,and the surface modification tests were carried out under the two methods of ordinary EDM and ultrasonic-assisted powder mixing EDM,respectively.Keeping the current unchanged and increasing the pulse width,the diameter of the etched pit becomes larger and the surface micro-crack defects increase.under the condition of low current and large pulse width,this phenomenon is the most obvious,and the machined surface presents the shape of cracking and the surface quality is very poor.When the current is high and the pulse width is narrow,the surface quality is the best and there are almost no crack defects.at the same time,the surface roughness of US-PMEDM processed samples is reduced,and the material removal rate and nano-hardness are greatly improved.The performance parameters are better than those of EDM under the same processing conditions.The single factor test shows that the surface roughness,material removal rate and nano-hardness increase to a certain extent when the peak current is increased.When the pulse width is increased,the surface roughness and material removal rate increase,but the surface nano-hardness decreases.(5)By using the comprehensive balance method,the optimum parameter combination is obtained: the peak current is 9A,The pulse width is 15?s,the gap voltage is 60 V,the processing time is 10 min,the surface nano-hardness increases to 11.895 GPa,which is about twice as high as that of the matrix.Compared with EDM,the material removal rate is increased by 285%,the surface roughness is lower than that of EDM,and the surface modified layer is continuous and uniform in thickness,and the compactness is better.