Mechanism Analysis And Experimental Study Of WEDM In Gas And In Water Mist Medium

Gas and water mistare are used as working medium instead of the traditional liquid medium in wire electrical discharge machining(WEDM).Because gas and water mist medium are conducive to improve the machining quality,it is valued by researchers at home and abroad,and it is one of the research focus on WEDM.It opens up a new process way for WEDM.But compared with the conventional liquid medium,gas and water medium have a big difference in the physical and chemical properties,and the characteristics of discharge process in gas and in water mist is great different with conventional liquid medium.Thus it is very necessary that the discharge mechanism and technological characteristics of WEDM in gas and in water mist medium are systematically studied.The mechanisms of wire electrieal discharge machining in gas and in mist medium were researched more deeply in this thesis.The theory of WEDM in gas which is the conductance and the breakdown of gas medium was detailedly elaborated.The discharging proeess during WEDM in gas begins with Townsend breakdown process and gradually develops to streaming breakdown process.The conductance and breakdown of water mist medium were analyzed systematically on the basis of gas medium.Because water mist medium is composed of continuous gas and tiny liquid drops,the breakdown process of water mist medium has the characteristics of gas medium breakdown.At the same time,electric field distortion is caused within water mist medium because of tiny liquid drops.Consequently.the disruption voltage of water mist is lower,and discharge gap increases significantly.The process of material removal during WEDM in gas and water mist was analyzed on several aspects,which are the formation of discharge channel,the expansion of the interelectrode discharge channel,removal stage of machining debris,the removal of electric corrosion and interelectrode medium deionization.According to the characteristics of gas and mist medium,as well as the differences of WEDM-LS and WEDM-HS,the finishing process characteristics of WEDM in gas and in water medium were separately studied in WEDM-LS and in WEDM-HS.Experimental platforms of WEDM which are needed in the gas and in water mist were built respectively in this two kinds of machine.The cutting speed.surface roughness,straightness and surface micro morphology,surface composition.microhardness and other indicators of the workpiece in WEDM in gas.in water mist.and in liquid(Deionized water,emulsion)were studied systematically.The multi-cutting process of WEDM that gas and water mist medium were applied in the finishing stage was proposed.Compared with multi-cutting in liquid,the experimental results in WEDM-LS and in WEDM-HS showed that the new technique could effectively improve the surface quality(straightness,roughness).The study confirmed the feasibility of multi-cutting process using a variety of media,and provided a new approach to improve the quality of WEDM.Technology of WEDM finishing in water mist was studied experimentally in WEDM-LS and in WEDM-HS.The method of single factor was applied to study the effects of peak current,pulse width,water quantity and the servo feed and other factors on the cutting speed and surface roughness.Because of its processing performance is related to not only the basic electrical parameters,but also with the factors of working medium parameters and other nonelectrical parameters,the process of WEDM finishing in water mist medium is very complex.It is essential that the effects of different factors on the process indicators are better analyzed.On the basis of the central composite experiment.the proved regression models of the surface roughness and the cutting speed of WEDM-HS in water mist were built,and the significant ranks of elements for surface roughness and cutting speed were attained.It provides a more accurate mathematical model for the further WEDM process research.Through theoretical analysis,the spark location detection device for WEDM based on electrode branched current method was developed.Mathematical model is established for the spark location detection.The hardwares of the device are mainly composed of a current sensor,data acquisition card and computer.Development of the software system is built on the virtual instrument software LabVIEW.A special workpiece was designed,and calibration experiment was carried out in WEDM-HS.The online detection of spark locations in WEDM-HS in gas.Emulsion and water mist was realized.,which provides a means to further study the mechanism of WEDM in gas,liquid and water mist medium.On the basis of orthogonal experiment and signal noise ratio analysis,the gray relational analysis method that used the norm gray relational grade to determine the weight was applied.Let the optimization problem of multi objective parameters of WEDM-HS in water mist be transformed into the optimization problem of single targeted gray relational degree.The method conveniently and effectively obtain the weight of each index,So that the obtained the optimized combination of parameters under the requirements of multi technological indexes is more objective and reasonable.