高速湿式铣削内冷刀具失效机理和工件固液边界热传输机制及其对加工质量的影响研究

In high speed wet machining of difficult-to-machine materials, internal spray can make the coolant accurately reach cutting zone of the rake face and chip, and optimum cooling effect can be obtained. However, the internal cooling holes may weaken the tool strength. The tool is placed in a harsh environment with thermal shock, cavitation and nearly ultrasonic impact due to high speed. The wet workpiece and heat transfer of surface also seriously affect the in-site and online measurement accuracy. Aiming at the wear and breakage failure as well as online measurement error of the workpiece, this project will study the heat transfer mechanism of the solid-liquid boundary, and combine thermal deformation, erosion error of strong fluid with chips and online compensation. Through experimental research, the critical speed of the internal cooling end mill that generates high temperature vaporized cavitation, high-speed water jet and nearly ultrasonic effects will be determined. The failure mechanism of fragmentation, wear and breakage of internal cooling tool in high speed wet machining will be discovered. According to the analysis results of integrated discrete element model of cutting zone, the structural strengthening design of inhibiting cavitation and anti-cavitation will be studied, which can provide scientific basis for improving machining quality and ensuring tool safety. The research results of this project will have important theoretical significance and practical value to the transformation and upgrading of flexible production line as well as optimization and development of internal cooling tools in the high-end manufacturing industry of automotive, aerospace and mold, in which difficult-to-machine materials are getting more and more applications.

在高速湿式加工难加工材料时，内喷淋可使冷却液准确直达刀具前刀面与切屑之间的切削区，取得最优的冷却效果。但内冷孔削弱了刀具强度，高速则使刀具置于热冲击、空泡气蚀、近超声波冲击等恶劣环境中。同时，全湿的工件及表面热传递也严重影响在位、在线测量精度。本项目针对内冷却刀具的各类磨损、破损失效和工件在线测量误差，深入研究固液边界热传输机制，综合热变形和夹屑强流体冲刷误差及在线补偿；通过试验研究确定内冷刀具产生高温空泡气穴、高速水射流和近超声波等效应的临界转速；揭示高速湿式加工中内冷却刀的热淬碎裂、磨损、破损等失效机理；根据切削区一体化离散元模型分析结果，完成内冷却刀具抑制气穴、抗气蚀的结构强化设计，为提高工件加工质量、保证刀具安全提供科学依据。项目预期成果对于应用难加工材料越来越多的汽车、航空航天和模具等高端制造领域的柔性生产线转型升级，优化设计并研制新型内冷却刀具，都具有重要的理论意义和实用价值。

干式加工技术虽然取得了长足进展，但并不适合所有切削过程，特别是高温合金等难加工材料的高速切削。因此，湿式加工一直沿用并且呈上升趋势。近年，为了最大限度地降低切削液带来的危害，先进的生产线采用封闭防护和集中处理系统。为了获得更好的冷却效果，由外冷转为内冷。高速内冷式湿式加工则使刀具处于高频率热冲击和高速混合流体场的恶劣环境之中。同时，全湿的工件的差异性表面传热机制及不均匀热变形也影响在线测量精度。.项目组分析了高速湿式铣削加工切削区夹杂有切屑颗粒的固液混合流体场，揭示了该流体场的压强和速度矢量的分布规律。结果表明压强最高区出现在刀尖附近，压强最低负压区出现在侧后刀面附近及容屑槽内部。得到了高速铣孔时，达到空化条件的临界主轴转速。并以抑制极值负压为优化目标设计出立铣刀容屑槽截形，从而得到可抑制空化效应的高速立铣刀，为更高转速下的高速立铣刀设计提供了设计思路、方法和依据。对铣孔所得的各种形态切屑颗粒进行了球形等效建模，采用DPM离散相模型对切屑颗粒进行了模拟，得到了颗粒/粒子束自切削刃切离工件之后的运动轨迹。发现切屑颗粒从切削刃分离之后，与刀具后刀面和孔壁均有不同频率的撞击冲刷，导致刀具和工件加工表面磨损。干式和湿式铣削加工过程切削温度演变规律的对比研究表明，刀具内部切削温度的振幅湿式高于干式。高速内冷却加工中切削液虽然带走了大量的切削热，但对于刀具而言相当于水中淬火，其最大拉应力已经触及硬质合金材料的抗拉强度，导致淬裂，形成典型的井字型特征裂纹。.本项目研究揭示了高速湿式铣削加工切削区固液混合流体场压强和速度矢量的分布规律，切屑颗粒的运动轨迹及其对刀具和加工表面撞击冲刷作用机理，达到空化条件的临界主轴转速，高速刀具热淬性裂纹的生成机理，并研制出可抑制空化效应的高速立铣刀。本项目研究成果对于高速湿式加工生产系统的安全运行，新型内冷刀具研制，以及超高速切削都具有重要科学意义和实用价值。