| High efficiency precision low damage grinding of titanium alloy has become an indispensable method in aerospace field.However,the deterioration of workpiece surface integrity due to thermal damage is the technical bottleneck of titanium alloy grinding.Minimum quantity lubrication is a green cleaning technology which has been applied to auxiliary titanium alloy grinding,but it still has the defects of insufficient heat dissipation ability and limited lubrication antifriction ability.The use of cryogenic air instead of normal temperature air to carry micro-lubricants can effectively improve the oil film heat transfer and lubrication performance in grinding zone,significantly inhibit the workpiece surface thermal damage.At present,the cryogenic air minimum quantity lubrication technology is still in the experimental stage of effectiveness verification in grinding processing.The effects of low temperature on the physical characteristics of lubricants and atomization effect,the dynamics mechanism of cryogenic droplet formation and the heat transfer mechanism of flowing liquid film in grinding zone have not been revealed.Based on this,the paper takes the lead in carrying out theoretical studies on the evolution law of lubricant physical properties under the condition of cryogenic air,the influence mechanism of low temperature on atomization effect,atomizing droplet group film formation mechanism,thermodynamic action law of titanium alloy grinding and heat transfer mechanism of flowing liquid film in grinding zone.The main research contents are as follows:(1)The influence law of cryogenic air minimum quantity lubrication parameters on lubricating oil physical characteristics and atomization effect is studied.The quantitative mapping relationship between cryogenic air temperature and lubricating oil temperature,kinematic viscosity and surface tension coefficient is established.The cryogenic air temperature is negatively correlated with the droplet size and contact angle.When the cryogenic air is at the lowest-50°C,the droplet size and contact angle are 155.566μm and 33.5°,respectively.The pressure is positively correlated with the atomizing cone angle and droplet velocity.When the pressure is up to 0.5MPa,the cone angle and droplet velocity are 34.6°and 12.24m/s,respectively.The dynamic behavior of droplet impact on wall and liquid film under different physical/kinematic parameters is simulated and analyzed,which laid a foundation for exploring the cooling lubrication mechanism of cryogenic air minimum quantity lubrication in grinding process.(2)The film formation kinetics mechanism of atomized droplets with cryogenic air minimum quantity lubrication is studied.The influence mechanism of the nozzle position on the effective region of the liquid film is analyzed.The Angle of the nozzle between 12°and 20°is helpful for the liquid film of atomized droplets to enter the grinding zone effectively.The influence mechanism of normal and tangential momentum sources generated by gas impingement perturbation flow and droplet impingement steady flow on the driving effect of liquid film flow is revealed.The thickness distribution models of liquid film in the impingement zone and non-impingement zone of gas-liquid two-phase flow under different cryogenic air temperatures are established.The thickness ranges of liquid film are[0.92mm,1.26mm]and[0.85mm,1.21mm],respectively.The surface topography model of CBN grinding wheel is established based on the stochastic distribution of abrasive geometry,and the pose relationship of any three adjacent abrasive particles is analyzed,and the calculation model of abrasive clearance volume is established.The distribution domain of dynamic grain clearance volume in the grinding zone is[70.46mm~3,78.72mm~3],and the total grain volume distribution domain is[140.84mm~3,155.67mm~3].(3)The stochastic distribution grinding force model is established based on the geometric space characteristics of grinding grains on the grinding wheel surface.The determination mechanism of different microscopic contact states between the effective abrasive particle and the workpiece in the grinding zone is studied,the influence law of the geometric space characteristics of the abrasive particle on the effective area of the abrasive particle and the workpiece is analyzed,and the synergistic effect law of the geometric parameters(top angle and cutting depth)and spatial parameters(three-dimensional rotation angle)of the abrasive particle on the grinding force is revealed.The results of numerical calculation and experimental measurement are in good agreement,and the error values are all less than 10%.(4)The surface temperature field model of discretized heat source workpiece in grinding zone is established.The heat distribution mechanism between grinding wheel and workpiece at different contact stages is analyzed,and the attenuation and superposition evolution law of temperature field in grinding zone is revealed.A method is proposed to compare the temperature between the regional numerical calculation and the experimental measurement.When the grinding depth a_p=30μm,the comparison results of 64 groups of experimental measurement and numerical calculation showed that there are 56 areas with error less than 10%,accounting for 86%.(5)The heat transfer mechanism of liquid film at the interface between grinding wheel and workpiece is studied.A theoretical model of heat transfer of liquid film with micro-clearance in grinding zone is established.Furthermore,a model of convective heat transfer coefficient of low-temperature liquid film is established.When the cold air temperature is-10°C,the error between the theoretical and experimental values is 8.5%.An analytical method for superimposed heat dissipation in grinding zone is proposed,and the effect of cryogenic air minimum quantity lubrication on heat production in grinding zone,heat transfer in liquid film and heat transfer to workpiece is revealed.The variation trend of workpiece surface temperature is consistent with the theoretical value.When the grinding depth is 30μm and cryogenic air temperature is-40℃,the error is 7.7%.(6)The quantitative mapping relationship of multi-factor parameters is studied in the cryogenic air grinding of titanium alloy.The effects of different cryogenic air parameters and grinding parameters on the characterization parameters of grinding effect were analyzed.Under the same grinding parameters,compared with dry grinding,the normal force,tangential force,specific grinding energy,friction coefficient,temperature and surface roughness Ra decreased by 47.6%,44.3%,45.1%,19.8%,43.9%and 46.3%,respectively.Based on the multi-factor orthogonal experiment,the quantitative mapping relationship between different grinding parameters and the influence factors of cryogenic air is analyzed to achieve the optimal workpiece surface quality.When the line speed,feed speed and grinding depth of the grinding wheel were30m/s,6mm/s and 30μm,the optimal grinding effect is obtained by the combination of cryogenic air temperature-50℃,air pressure 0.4MPa and nozzle angle 16°. |
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