| Various high-strength,high-hardness and high-plasticity metal materials have been widely used in the fields of aviation,aerospace,military industry,etc.,and the requirement on machining precision in practical applications is higher and higher;however,machinability of these metal materials tends to be poor,and these metals belong to typical difficult-to-machine materials.Atmospheric pressure cold plasma jet can improve cooling and lubricating conditions of the cutting area,thereby reducing cutting force,and improving tool life and surface quality.However,treatment efficiency of the plasma jet on wettability of the cutting interface is low;in addition,influence mechanisms of the plasma jet on tool-workpiece materials in the cutting area have not been revealed yet.Therefore,it is difficult to determine applicable ranges of workpiece materials and cutting parameters when the plasma jet is applied to assisted machining processes,and the popularization and application of the plasma jet assisted machining are therefore restrained;meanwhile,coupling effects of the plasma jet and different cooling media on cutting process,tool life and surface quality are difficult to predict.According to the aforementioned problems,on the basis of analyzing generation principle and methods of atmospheric pressure cold plasma jet,this thesis designs a new type of cold plasma jet generation device that can realize high efficiency and low damage hydrophilization for various metals;then,influence mechanism and law of the cold plasma jet on cooling and fracture mechanical properties of the cutting interfaces are revealed through surface modification,cooling,tensile and single particle scratch experiments;finally,TC4 titanium alloy,Inconel718 nickel-based superalloy and pure iron are selected as typical difficult-to-machine materials to carry out micro machining experiments under the combined action of cold plasma jet and minimum quantity lubrication(MQL)cooling medium.The main research work of the thesis can be summarized as follows:(1)In order to rapidly improve the wettability of metal surfaces in the cutting area,a bubbling device is used to wet the working gas;then the wet water-containing gas is mixed with dry gas,and an atmospheric pressure bare electrode discharge water-mixing cold plasma jet with high treatment efficiency,good uniformity and stability is developed.Experimental results indicate that mixing water into the working gas can effectively improve the treatment efficiency of the plasma jet;when water content of the working gas is 0.045%,superhydrophilic modification of various metal surfaces can be realized within 10 seconds.Compared with the plasma jet without mixing water in the working gas,the required time is shortened by more than 90%,surface structure is not obviously damaged;and the plasma jet can be applied in the cutting area for rapidly improving the permeability of cooling media.(2)In order to study the influence mechanism of the cold plasma jet on the cooling condition of the cutting area,the water-mixing cold plasma jet is used to treat TC4 titanium alloy,pure iron and other metal surfaces,and the effect and mechanism of the water-mixing cold plasma jet on surface wettability,surface morphology,chemical composition and heat transfer efficiency are investigated.The results indicate that the water-mixing cold plasma jet can rapidly modify various metal surfaces to be superhydrophilic by changing surface chemical composition and improving surface energy.When the modified superhydrophilic surface is placed in air,wettability of the modified surface quickly recovers to the original state within 6 hours;when the modified superhydrophilic surface is placed in water,the superhydrophilicity can be maintained for long time.The hydrophilization effect of the plasma jet on metal surfaces can promote cooling media to wet the cutting area,effectively improve cooling efficiency of cooling media on surfaces at high temperatures,thereby improving the cooling condition of the cutting area.(3)In order to study the influence mechanism and law of the cold plasma jet on fracture mechanical properties of the cutting interfaces,tensile tests and single particle scratch experiments are carried out in air,nitrogen and cold plasma jet atmospheres.The results indicate that the water-mixing plasma jet can generate a deformation layer with a thickness of 1 ?m?2 ?m on TC4 titanium alloy surface whose lattice type is close-packed hexagonal.There are many substructures in the deformation layer,such as dislocation entanglements,dislocation cells and sub-grains;the movement of dislocations is hindered by the substructures;and cracks are more easily nucleated,thus reducing surface plasticity and fracture toughness of the material.For 0.1 mm thick TC4 titanium alloy and pure titanium tensile samples,when the tensile experiments are conducted under the cold plasma jet atmosphere,elongations at break are obviously reduced,and the fracture occurs within the region treated by the cold plasma jet.With the increase of the sample thickness,the effect of water-mixing cold plasma jet on the stretching process gradually becomes less obvious.In addition,the water-mixing cold plasma jet can promote brittle fracture on the surface of TC4 titanium alloy during the single particle scratching process,and cracks are obviously more on the scratches obtained under plasma jet atmosphere compared with common scratches.However,for pure iron,pure molybdenum,pure nickel and other metal materials whose lattice type is body-centered cubic lattice or face-centered cubic lattice,after cold plasma jet treatment,no significant deformation layer is observed on material surfaces.Therefore,under the water-mixing cold plasma jet treatment,the surface plasticity and fracture toughness of the material do not change greatly,the tensile samples fractures occur at random positions within the gauge lengths;and surface morphology obtained by single grain scratches has no significant difference compared with that obtained by ordinary scratches.(4)In order to study the influence of the cold plasma jet on micro and precision machining of difficult-to-machine metal materials,micro-milling experiments of TC4 titanium alloy,Inconel 718 nickel-based superalloy and pure iron are carried out under five different cooling and lubricating conditions:air,nitrogen,water-mixing cold plasma jet,minimum quantity lubrication(MQL)cooling medium,water-mixing cold plasma jet+MQL cooling medium.The influences of the cold plasma jet on cutting force,surface quality and tool wear are investigated.The results indicate that the plasma jet can effectively adjust micro and precision machining process of the difficult-to-machine metal materials.For TC4 titanium alloy,the water-mixing cold plasma jet can significantly reduce cutting force,alleviate surface tool marks and improve surface quality by reducing surface plasticity and fracture toughness.With the increase of cutting depth,the effect of only applying cold plasma jet on cutting process of TC4 titanium alloy becomes less obvious;however,when MQL cooling medium exists in the cutting area,hydrophilization effect of the plasma jet can promote the cooling medium to wet the cutting area,so that surface quality is further improved,and tool wear is alleviated.When cutting depth is 30 ?m,compared with dry and MQL assisted micro-milling,surface roughness Ra obtained by water-mixing cold plasma jet+MQL assisted micro-milling can be reduced by 50.3%and 30.2%,respectively;and tool cutting distance can be prolonged by more than 5 times compared with dry micro-milling.For Inconel 718 nickel-based superalloy and pure iron,since the cold plasma jet has no obvious effect on their mechanical properties,when only plasma jet is induced into the cutting area,its effect is limited.However,as plasma jet can effectively promote the cooling medium to enter the cutting area,the composite effect of the plasma jet and the MQL cooling medium can obviously improve machinability of the materials,and the obtained surface quality is significantly improved compared with that of dry and MQL assisted micro-milling;compared with dry micro-milling,tool cutting distance can be prolonged by more than two times. |
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