A Study On Lubrication-Cooling Mechanisms And Machining Characteristics Of Electrostatic Minimum Quantity Lubrication(EMQL)

Minimum quantity lubrication(MQL)technology is of a novel machining method,in which an environmentally friendly cutting fluid is atomized to micro-scale droplets and dispensed to a cutting area in a high velocity by using compressed air.This benefits the formation of a lubrication film at the friction interface and therefore reduces tool wear.However,the technology tends to produce a high cutting temperature due to a low exchange rate of heat resulted from the convection of compressed air and the evaporation of fluid droplets,which hence limits its application in machining of difficult-to-machine materials such as,stainless steels.In addition,MQL machining process tends to produce a high concentration of fluid droplets floating in the working space,which will cause operators' health and environmental concerns.To address those issues involved in the MQL machining process,this paper proposed and developed a new static electricity-assisted minimum quantity lubrication technology named electrostatic minimum quantity lubrication(EMQL).With EMQL,the introduced static electricity could not only reduce the diameter of fluid droplets and improve droplet distribution and adsorption on target surfaces,but also enhanced the wetting of droplets on friction interfaces,which would improve the lubrication and cooling performance of the atomized cutting fluids,and consequently produced a better machining performance.Furthermore,those electrostatically charged droplets could adsorb onto workpiece surface,cutting tool and machine tool instantly owing to the action of an electrostatic field,which would increase the deposition rate of droplets and hence lowered the quantity of the floating droplets significantly.In this study,a new and feasible technology was developed to solve cooling,lubricating and oil mist concentration issues involved in the MQL machining process,and the main work and results are concluded as follows:(1)Developed an EMQL set-up,by which the charging characteristics of cutting fluids were studied,as well as the charge-to-mass ratio of fluid droplets and the lowest value of the electric field that induces a droplet atomized were investigated.Furthermore,the surface tension,wetting angle and dynamic viscosity of the droplets under EMQL condition were measured,and the physical/chemical properties of EMQL droplets were investiaged as well.Finally,the atomization performance of EMQL at a high ratio of air to liquid was analyzed and the effect of the parameters of EMQL system on the performance was also investigated.The experimental results indicate that EMQL reduced droplet size considerably and increased droplet velocity and uniformity,which hence improved its deposition and wetting rates on target surfaces.(2)The lubricating characteristics of EMQL were investigated using a tribometer,specifically the effects of the main parameters of tribometer and EMQL system on the lubricating characteristics of charged droplets were systematically analyzed.The results suggest that EMQL produced a significantly lower coefficient of friction with a reduced wear loss,in comparison with the conventional MQL.In addition,XPS analysis was used to investigate worn surfaces,and the negative ion radical action mechanism(NIRAM)was employed to reveal the lubrication mechanisms of MQL and EMQL technologies under a boundary lubrication condition.The enhanced tribological performance is attributed to the fact that EMQL could not only improve the wetting and adsorption of droplets on friction interface but also lowered droplet diameter,which would provide more lubricant and oxygen at the interface to promote the formation of a lubricating layer comprising an adsorption film and an oxide layer,following by producing a mixed lubricating action,and consequently improved wear and friction considerably.(3)The equipment of transient-state and steady-state heat transfer for mist droplets was developed to investigate the cooling performance of EMQL,and the effect of EMQL parameters on droplet critical heat flux and coefficient of heat transfer on a heat exchange surface was studied.The results indicate that due to the reduced droplet size,EMQL could significantly promote the vaporization and evaporation of droplets on the heat exchange surface,resulting in a higher interfacial heat-transfer coefficient.Meanwhile,the improved performance in droplet deposition,uniform and wetting could allow them to enter and cover the surface more easily,which would increase the area of heat exchange on the surface and promote their evaporation,therefore resulting in a higher heat transfer coefficient and the significantly enhanced cooling performance.(4)The mist concentration values of PM 10 and PM 2.5 produced from using EMQL in a vertical milling center were measured by use of a gravimetric analysis method,and at the same time,the effect of EMQL processing parameters on the air quality of cutting environment was investigated.The results indicate that when EMQL was used,the utilization and lubrication effects of droplets were increased remarkably as the fluid was changed,which could reduce the friction and temperatures within machining area,and hence reduced droplet evaporation.Meanwhile,an induction electric field would be formed in the cutting space as the charged droplets sprayed to the workpiece surface,cutting tool and machine tool,which allowed those droplets to adsorb onto the surfaces of workpiece and tool more easily,therefore minimized the mist concentration significantly.(5)The machining performance of EMQL for AISI 304 stainless steel,which is one of the difficult-to-machine materials,was systematically investigated.The effect of EMQL on tool life,cutting forces,cutting temperatures,surface quality,and mist concentration under different processing parameters was studied.In addition,SEM equipped with an EDS analysis was used to analyze worn tools and cutting mechanisms of EMQL.The results indicate that when EMQL was used,the fluid droplets could be electrostatically charged,which would increase their adsorption and wetting on the cutting interface,and therefore improved the lubricating and cooling effects on the machining area.This minimized the friction and temperature at the tool-workpiece/chip interfaces and improved machining properties,which therefore achieved a low tool wear and a high surface finish.