Ultrasonic And Plasma Oxidation Assisted Grinding Of Titanium Alloys

Titanium alloys is widely used in aerospace,chemical,marine and other industries because of its high specific strength,wide working temperature and good corrosion resistance.But at the same time,due to its active chemical properties,lower thermal conductivity and elastic modulus,titanium alloys has some difficulties in machining,such as tool adhesion,tool wear,high processing temperature and so on.In most industrial applications,titanium alloys are first formed by precision casting,and then machined by turning,milling or grinding to trim geometry and remove excess material.But it shows bad machinability in turning,milling and other cutting technologies,result in poor machining efficiency,accuracy,and high processing cost.The grinding process has the characteristics of high wheel speed and small amount material removal of single abrasive particles,which can avoid the machining difficulties of titanium alloys and improve the processing efficiency and quality.However,the traditional surface grinding can not solve the difficulties in titanium alloys processing perfectly.There are still some problems such as high grinding temperature,poor machining quality and serious wear of grinding wheel.Therefore,it is necessary to develop a hybrid grinding method which can effectively reduce the grinding force and improve the machining efficiency and accuracy.This paper investigated existing hybrid grinding methods,such as ultrasonic assisted grinding and ultrasonic electrochemical grinding,combined with their respective characteristics and shortcomings,propose the ultrasonic and plasma oxidation assisted grinding process,which can improve the grindablity of titanium alloys.In this paper,the material removal mechanism as well as grinding experiment is studied.Firstly,an ultrasonic plasma oxidation device was built up to oxidize the Ti-6Al-4V alloy.It was found that both the ultrasonic vibration and the increasing of plasma voltage could increase the thickness of oxide layer on the surface of Ti-6Al-4V workpiece.At 10 V ultrasonic plasma oxidation,the maximum oxide layer on Ti-6Al-4V workpiece can up to 18 nm and its surface microhardness decreased by about 21 % in nano indentation test.In scratching test,the oxidized Ti-6Al-4V workpieces showed smaller tangential force and deeper scratching grooves under the same normal force.And the chips from oxidized Ti-6Al-4V workpiece were softer and brittler.The bottom of scratching grooves were smoother as well.These results showed that ultrasonic plasma oxidation can improve the grindablity of Ti-6Al-4V,but it is also found that these effects are related to the thickness of oxide layer.When the thickness of oxide layer does not match with the scratching depth,the effects will gradually weaken.On the other hand,the device of ultrasonic plasma oxidation assisted grinding was built up,and the surface grinding and hole drilling experiments of Ti-6Al-4V workpiece was carried out under different processing parameters.The results show that the ultrasonic plasma oxidation assisted grinding of titanium alloys can combine the advantages of ultrasonic vibration in promoting plasma oxidation and improving grinding efficiency and workpiece surface quality,let Ti-6Al-4V workpiece to be soft,brittle and easy to be remove.Compared with conventional grinding,the average normal and tangential grinding forces in surface grinding is reduced by 60 % and 70 % respectively,and the peak grinding force in hole drilling is also reduced by 60 %.With the increasing of plasma voltage,the material removal rate was improved and the grinding wheel wear was reduced.Ultrasonic vibration and plasma oxidation can also reduce the surface roughness of the workpiece.However,when the plasma voltage increases to a certain extent,the surface roughness does not decrease but rise due to serious discharge and ablation.