Study On Machinability And Surface Integrity Of Aluminum Alloy 7055-T6I4 Under Mixed Lubrication Multi-medium Of Wide Temperature

7055 aluminum alloy has been widely applied to space and aeronautics,mostly as large structural members,as one of the first choices for lightweight application for its low density,high strength,high rigidity and extraordinary corrosion resistance.Nevertheless,given the hardly controllable deformation,poor machining surface quality and heavy tool wear of the material during cutting process,considerable residual stresses and consequently material exposure to salt mist corrosion are almost unavoidable.In view of this,studying the cutability and corrosion resistance of 7055 aluminum alloy is of particular importance.This paper examines the cutability and corrosion resistance of the material through testing and simulation,using 7055-T6I4 aluminum alloy as an example.A wide-temperature range shear stress model is established with the result of a Hopkinson dynamic impact test under 90℃~350℃,based on the quantitative relations between dynamic yield stress and shear stress;a wide-temperature range triaxial cutting force prediction model is established using a two-dimensional unequal shear zone model and a three-dimensional orthogonal cutting model;considering the "three highs" of the material during cutting process(high strain,high strain rate and high temperature),the wide-temperature range triaxial cutting force prediction model is corrected using the reverse method and the cutting force under minimum quantity lubrication(MQL).Verification reveals an error smaller than 10% between the modified triaxial cutting force yielded by the prediction model and that yielded by test.Under extreme environmental conditions,when dry cutting is applied,the residual cutting stresses of 7055 aluminum alloy under high temperature unexceptionally exist in the form of residual tensile stresses;under low and room temperatures,the residual cutting stresses of the material unexceptionally include residual tensile stresses and residual compressive stresses;the maximum residual compressive stresses are all negatively correlated to the cutting velocity,cutting depth and feed rate,while the maximum residual tensile stresses are positively correlated to the cutting parameters.Under single-medium MQL,when the same cutting parameters are used,the workhardening behavior of the material is the optimum when cut under high temperature.Under low or room temperature,the priority of the maximum residual compressive stresses is: water lubrication >emulsion lubrication >oil lubrication while that of the maximum residual tensile stresses is just the opposite.Under multimedia mixed MQL,under low temperature,the residual stresses are all residual compressive stresses while under room and high temperatures,the residual stresses appear in the form of both residual tensile stress and residual compressive stress.Under the same cutting conditions,the maximum residual compressive stress is positively correlated to the water jet velocity,and the maximum residual stress is negatively correlated to the water jet velocity;under low temperature,the maximum residual compressive stress is approximately three times that under high temperature.Corrosion resistance studies of 7055 aluminum alloy during cutting process indicate that under low and room temperatures,the radius of the electrochemical impedance spectrum of 7055 aluminum alloy is obviously positively correlated to the water-oil jet ratio,while under high temperature,the electrochemical impedance spectrum is shaped like a C.Precipitation-free zone(PFZ)observation demonstrates that under low temperature,as the water-oil jet ratio continues to increase,the grain boundary precipitates of the materials become closer to each other and less continuous;the sizes of the PFZs and the intragranular precipitates also seem to reduce with the increase of the water-oil jet ratio,while under high temperature,at the water-oil jet ratio of 1.5,the grain boundary precipitates mainly appear in discontinuous chains;at the water-oil jet ratio of 2.5,the grain boundary precipitates appear in two parallel rows.Using the theoretical relationships between shear stress and dynamic yield stress,a Johnson-Cook constitutive equation and a damage constitutive model accommodating the "three highs" are established;the adiabatic shearing and microstructure during dynamic impact are also analyzed.Under the dynamic impact temperature of-60℃,the strain rate exceeded 2000s-1.Microcracks of varying sizes began to appear inside the structure;brittle fracture of second-phase particles also occurred within the TEM structure.Under the dynamic impact temperature of 20℃~150℃,as the strain rate continued to increase,adiabatic shear bands(ASBs)are present throughout theevolution of the metallographic structure of 7055 aluminum alloy;convergent microcracks also exist within the ASBs.Under the dynamic impact temperature of250℃~350℃,the microstructure of the material under dynamic impact is typically in the form of deformation zones;dynamic recrystallization is also observed within the TEM microstructure.Two-dimensional and three-dimensional finite element(FE)models are established using the constitutive equation and frictional model so established.The simulation result indicates that the rake face stress of the tool is obviously positively correlated to the cutting depth,so does the length of the rake face contact zone;the contact stress of the flank is obviously positively correlated to the cutting depth,and the arc radius of the nose of the tool is obviously negatively correlated to the contact stress of the flank;the cutting velocity is positively correlated to the chip crimp radius,chip width and chip temperature.