Effects Of Biomimetic Spontaneous Transport Surface Microstructure On The Performance Of Interrupted Cutting Tools

The tool is more prone to failure due to the harsh working condition in interrupted turning of hardened steel.The existing studies have revealed that tool surface microstructure can effectively enhance the performance of the tool and improve the surface quality of the workpiece.In addition,people are increasingly concerned about environmental issues.Being a typical green machining method,vegetable oil based minimal quantity lubrication（MQL）cutting has attracted comprehensive attentions.Taking biomimetics and spontaneous droplet transport theory into consideration,the present work proposed a biomimetic spontaneous transport surface microstructure suitable for soybean oil based MQL interrupted turning of hardened steel.Effects of the proposed microstructure on tool performance were revealed in this work.First,taking the impact resistance of the scale of Manis pentadactyla into account,surface microstructure with functions such as impact resistance and spontaneous transport was designed and fabricated on the rake face of the tool.The theory of infiltration gradient-driven droplet transport was employed in the design and fabrication process.The geometry features and the fuctions of the microstructure were studied in the work.Then,this study established a theoretical index of tool performance on the basis of continuous damage mechanics and microfracture mechanics.This index integrated the external cutting load,fracture toughness,and internal micromechanical properties of the tool.The influences of the proposed surface microstructure on the performance index were analyzed theoretically.Finally,the experimental performance of the tool was studied by means of soybean oil based MQL interrupted turning experiments.The theoretical performance index was validated by the experimental tool performance.Then,this work revealed the correlation between the experimental tool performance and the surface roughness of the workpiece.The present work found that the proposed biomimetic spontaneous transport surface microstructure resulted in stable and durable lubricant transport towards the tool-chip contact area,improving the performance of the lubricant.The established performance index can be used to pre-evaluate the performance of microstructured tools.The proposed microstructure can effectively reduce tool wear,thereby enhancing the surface quality of the workpiece.The details are as follows:（1）Theoretical and experimental researches were conducted to analyze design,preparation,morphology and performance of the biomimetic spontaneous transport surface microstructure.First,biomimetic spontaneous transport surface microstructure was designed on the rake face of the Al₂O₃/Ti C ceramic tool.The microstructure surface was divided into two zones such as oleophilic zone Z_L and oleophobic zone Z_O.The geometry of the scale of Manis pentadactyla was considered in the design of zone Z_L for acquiring relatively good impact resistance of the microstructure.There was striped microstructure parallel to the transport direction in zone Z_L,which was beneficial to the efficient transport of droplets.Square convex microstructure was distributed in zone Z_O.This kind of microstructure was expected to improve the wear resistance of zone Z_O.The combination of the oleophilic zone Z_L and the oleophobic zone Z_O formed wettability gradient,resulting in the spontaneous and directional movement of the soybean oil droplets.In addition,the impact resistance of the microstructure ensured the durability of the transport process.Then,biomimetic spontaneous transport surface microstructure was fabricated by means of laser processing and surface modification based on the design results.The wedge angle A_w of the microstructure and the inclination angleθ_l of the laser beam have great influences on the transport performance of the the microstructure.Thus,this work focused on paraters such as A_w（45°～65°at an interval of 5°）andθ_l（60°～120°at an interval of 15°）.25 different types of microstructure were prepared due to the variation of A_w andθ_l.Furthermore,analysis of the microstructure was carried out to reveal the morphology characteristics,impact resistance,and the effects of the parameters（A_w andθ_l）on the transport performance.The results indicated that the morphology characteristics of the prepared surface microstructure met the design requirements.The microstructure had relatively good impact resistance.Spontaneous directional transport of soybean oil droplet was achieved on the prepared surface microstructure.The average transport speed V_tsa became higher with the increase of A_w andθ_l.The highest V_tsa appeared at the parameter combination of A_w=65°,θ_l=120°.Moreover,the variation of V_tsa was relatively small within the parameter range near this parameter combination.（2）Modeling of the theoretical performance index was conducted for the tool used in MQL interrupted turning.First,theoretical performance index P_ib of the tool with the proposed microstructures was established based on continuum damage mechanics and microfracture mechanics.This index integrated the external cutting load,fracture toughness and internal micromechanical properties of the tool.Smaller the P_ib value corresponded to better the tool performance.Then,this work analyzed effects of the wedge angle A_w and the laser angleθ_l on the equivalent uniaxial tensile stressσ_u,fracture toughness K_IC,and theoretical performance index P_ib.Correlation analysis was performed forσ_u,K_IC and P_ib.The results showed thatσ_u gradually decreased with the increase of A_w andθ_l.K_IC increased first and then decreased as A_w andθ_l grew.P_ib decreased gradually as A_w andθ_l increased.P_ib had the smallest value at the parameter combination of A_w=65°,θ_l=120°.The value of P_ib varied little in the parameter range around A_w=65°andθ_l=120°.σ_u was positively correlated with P_ib.On the contrary,K_IC was negatively correlated with P_ib.The correlation betweenσ_u and P_ib was found to be closer.（3）Experimental researches were carried out to investigate effects of the biomimetic spontaneous transport surface microstructure on the performance of the tools utilized in MQL interrupted turning.First,interrupted turning of hardened steel was performed for conventional tools under dry turning conditions,microstructured tools under dry turning conditions,conventional tools under soybean oil based MQL condition and microstructured tools under soybean oil based MQL condition.Then,this study revealed influences of the wedge angle A_w and the laser angleθ_l on the tool wear volume V_wr.Comparison of P_ib and V_wr was performed to verify the correctness of the theoretical performance index.Furthermore,surface roughness R_aa of the machined workpiece was analyzed considering the close relationship between the tool performance and R_aa.Correlation analysis of the average transport speed V_tsa,the tool wear volume V_wr and the surface roughness R_aa was conducted with the relationship among them considered.This study found that better tool performance can be obtained due to the use of the proposed microstructure in soybean oil based MQL.The developing trend of V_wr with A_w andθ_l was similar to that of P_ib.The experimental results of tool performance verified the correctness of the theoretical index.Both V_wr and R_aa decreased with the increase of A_w andθ_l.The smallest values of V_wr and R_aa appeared simultaneously at the parameter combination of A_w=65°,θ_l=120°.The variation range of V_wr and R_aa was small in the vicinity of this parameter combination.V_tsawas negatively correlated with V_wr,V_wr was positively correlated with R_aa.V_tsa was negatively correlated with R_aa.The parameter combination of A_w=65°andθ_l=120°was found to be the optimal for the transportat speed,the theoretical performance index,the tool wear volume and the surface roughness.The biomimetic spontaneous transport surface microstructure proposed in this study can effectively improve the tool performance and thus enchance the surface quality for the cutting condition under consideration.