【作者】 姜峰；

【导师】 李剑峰；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2009， 博士

【摘要】 钛合金在航空航天领域有着广泛的应用,被誉为一种使人类走向空间时代的战略金属材料。然而,由于具有导热系数低、高温化学活性高和弹性模量小特点,钛合金又是一种典型的难加工材料。在切削过程中,容易产生很高的切削温度,导致刀具磨损加快、表面质量难以控制,而低的切削速度导致加工效率难以提高。同时,由于航空钛合金零部件的整体结构件设计特点,大量的材料需要从整块坯料中去除,造成较高的切削成本。因此,低下的加工能力和加工质量与不断增长的加工需求已构成矛盾,成为制约航空航天制造业发展的瓶颈之一。针对航空钛合金Ti6A14V的高速铣削加工,本课题从不同冷却润滑条件下硬质合金刀具的磨损机理出发,将温度和锯齿状切屑作为刀具磨损的重要影响因素,建立了锯齿-连续切屑转变图谱,为工艺参数的选择提供了理论基础:建立了Ti6A14V的本构、失效材料模型和摩擦传热模型,借助有限元方法预测了加工区域的温度分布,为切削温度的控制提供了理论基础。最后进行了不同冷却润滑条件下的刀具耐用度实验,以一定金属去除率下的刀具耐用度最大为优化目标,对铣削工艺参数进行了优化,为实际工程应用提供了工艺支持。钛合金切削加工容易引起刀具快速磨损,严重影响加工效率和加工成本。借助SEM观察和能谱分析等手段,对不同冷却润滑条件下钛合金铣削加工刀具的磨损机理进行了研究。钛合金铣削加工刀具磨损和失效是在多种机理下共同作用的结果,主要表现为粘结磨损、氧化磨损、磨粒磨损、扩散磨损、刀具材料剥落、刀具塑性变形等形式。其中Ti6A14V粘结物与刀具材料在高温高压下的扩散作用加剧了粘结磨损;Ti6A14V粘结物氧化得到的高硬度氧化物为磨粒磨损提供硬质点;刀具表面热裂纹和机械裂纹的扩展造成刀具表面的材料剥落:随着刀具刃口部位的磨损和刀具材料的氧化变质,刀具发生塑性变形。除此之外,钛合金加工中的锯齿状切屑导致切削力的高频波动,从而使刀具表面粘结层周期性开裂、剥落,刀具表面材料不断被粘结层带走;同时切削力的高频波动造成刀具刃口部位应力的高频波动,从而加速了刀具表面裂纹的扩展,造成刀具表面严重的贝壳状剥落。切削液的施加对刀具表面产生冲刷作用,使得粘结层周期性开裂剥落的频率增加,粘结磨损加剧:同时切削液的施加使得钛合金切屑的锯齿化程度增加,进一步加速了粘结层和刀具的疲劳。因此降低Ti6A14V加工用刀具磨损需要从三个方面着手:控制锯齿状切屑的产生;控制切削温度和选择合理的冷却润滑方法。钛合金加工锯齿状切屑的产生机理主要包括绝热剪切和周期性断裂,综合了上述两个机理,提出了基于裂纹动态变化的锯齿状切屑产生机理,认为锯齿状切屑的产生从工件表面的裂纹萌生开始,剪切区滑移加剧,造成温度升高和失效应变增大,最终裂纹不再扩展,锯齿状切屑最终形成,同时已形成锯齿状切屑下方一个新的锯齿状切屑开始形成。切屑的锯齿化程度受切削速度、进给量及冷却润滑方式等因素的影响,通过大量的实验研究发现,在实验速度范围内(20～120 m/min),50～100m/min速度范围内不容易产生锯齿状切屑,而过低和过高的切削速度都会导致切屑锯齿化程度的加剧。同时切削液条件下,切屑的锯齿化程度比干切削条件下明显增加。另外刀具前角对切屑的锯齿化程度也有较大影响,前角越小,切屑的锯齿化程度越大。建立了干湿两种切削条件下0°、5°、10°三种刀具前角Ti6A14V连续-锯齿状切屑的转变图谱,为加工工艺参数的优选提供了理论基础。选择了两种冷却润滑方法用于Ti6A14V的加工中:切削液和最小微量润滑(MQL),其中MQL利用冷风作为载体,冷风温度和出油量可调,相应的干切削实验作为对比研究。利用正交铣削实验和热交换实验,建立了不同冷却润滑条件下的摩擦模型和传热模型,并对MQL喷嘴的布置方式和出油量进行了优化。利用霍普金森压杆实验得到的Ti6A14V高应变率和高温条件下的应力-应变关系,建立了基于Power-Law方程的材料本构关系模型,分析了Ti6A14V应变强化、应变率强化和温度软化对流动应力的影响。利用扭转、拉伸、压缩等基本力学实验得到的材料失效应变,分析了Ti6A14V材料失效应变与应力三轴度和温度的关系,建立了考虑应力三轴度、温度、Rebinder效应的材料失效模型,并利用直角车削实验对模型的参数进行了求解。使用切削有限元仿真软件AdvantEdge,结合建立的材料模型、摩擦传热模型,建立正交切削有限元模型,仿真了Ti6A14V锯齿状切屑的形成过程,提取了切削温度、切削力与相应的正交车削实验进行了对比,验证了有限元模型的正确性。对铣削过程进行了合理的二维简化,并对铣刀片的刃口微观结构进行了建模。结合不同冷却润滑条件下的材料模型、摩擦模型和传热模型,对不同冷却润滑条件下的铣削过程进行了动态物理仿真。仿真切削力与实验结果进行了对比,进一步证明了有限元仿真的准确性。对不同冷却润滑条件下的切屑形貌、切削力、刀具最高温度变化、刀具热应力进行了分析,发现MQL方式下和切削液方式下刀具表面最高温度的降低程度相仿,但是切削液条件下切屑的锯齿化程度明显增加,切削力波动变大;同时刀具在空行程中被快速冷却,刀具的温度梯度大,热应力高,对刀具寿命会有负面影响,因此MQL方式是切削液方式的有效替代。选择合理的铣削加工工艺参数,对干切削和MQL切削两种不同冷却润滑方式下的刀具磨损过程进行分析,以ISO通用的磨钝标准为基础得到不同工艺参数、不同冷却润滑条件下刀具的耐用度。使用指数和二次项两种刀具耐用度模型分别分析加工工艺参数对刀具耐用度的影响,并建立刀具耐用度的预测模型。针对钛合金铣削加工存在的高成本、低效率问题,以一定金属去除率下刀具耐用度最大为优化目标,提出一种优化方法,得到了不同金属去除率下的最优铣削加工工艺参数,为企业工艺技术人员制定工艺方案提供决策支持。研究了刀具磨损对加工表面粗糙度的影响,发现在开始阶段刀具磨损对表面粗糙度的影响不大,但当刀具磨损量达到一定值时,已加工表面的粗糙度值急剧增加,加工表面质量迅速恶化。研究了刀具磨损对切屑形貌的影响,发现切屑锯齿化程度随刀具磨损行程增加而加剧,随刀具磨损行程增加,切屑变形增加,出现扭曲甚至破碎现象。研究了刀具磨损对切削力的影响,发现随着刀具磨损,刀具的实际前角变小,进给方向的切削力持续增大,而垂直于进给方向切削力的方向发生变化,轴向切削力变化不大。更多还原

【Abstract】 Titanium alloy Ti6Al4V is widely used in aircraft industry, marine and commercial applications due to its high specific strength and excellent corrosion resistance. However, Ti6Al4V is a typical difficult-to-machine material due to its poor thermal conductivity, low elastic modulus and high chemical activation. High cutting temperature during Ti6Al4V machining usually leads to fast tool wear and poor surface quality. On the other hand, most materials need to be removed from roughcast due to the design characteristics of components used in the aircraft industry. Therefore, the contradiction between increasing machining demand and poor machining performance has been one of the bottlenecks, which blocks the development of aerospace industry. In this study, the carbide tool wear mechanism during high speed milling titanium alloys Ti6Al4V was systematically researched. The cutting temperature and sawtooth chip were taken as the main influence factors of tool wear. The transition maps from sawtooth chip to continuum chip were built to guide the selections of cutting parameters. The material constitutive and failure models, friction model and heat transfer model were built to predict the maximal cutting temperature based on finite element method (FEM). The tool life in the different cutting conditions was experimentally obtained. The prediction models of tool life were built based on the exponential and quadratic equations, respectively. Based on the quadratic model of tool life, the milling parameters were optimized, taking the maximal tool life in certain material removal rate as the optimization target.The fast tool wear during Ti6Al4V cutting influences the machining efficiency and machining cost greatly. The wear mechanism of carbide tool in milling titanium alloy Ti6Al4V under the different cooling/lubrication conditions was experimentally studied by means of scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The wear mechanisms mainly include: adhesive wear, oxidation wear, abrasive wear, diffusion wear, tool fracture, tool plastic deformation,etc. The wear mechanisms interact each other. The diffusion between adhesive layer and cutting tool aggravates the adhesive wear. Ti6Al4V adhesive layer is oxidated and the oxide with high hardness supplies the abrasive particle in the abrasive wear. The thermal and mechanical cracks propagation results in the fracture of cutting tool surface. The plastic deformation of cutting tool occurs due to the oxidation of cutting tool materials. The saw-tooth chip generated in the milling process causes the cutting forces fluctuation with high frequency, which resultes in the fatigue of Ti6Al4V adhesive layer and tool materials. Cutting fluid erodes the surface of cutting tool and leaded to the thermal impact, which aggravates the fatigue of Ti6Al4V adhesive layer and tool materials. So three measurements are taken to control the tool wear: the control of sawtooth chip formation; the control of cutting temperature and the selection of reasonable cooling/lubrication methods.The formation machenisms of sawtooth chip mainly include cyclic crack theory and adiabatic shear theory. A new theory based on the dynamic cracks propagation was proposed to explain the formation of saw tooth chip. In this theory, the cracks initialize in the unmachined surface and expand towads the cutting tool point. The shearing slip aggravates and subsequently the temperature in the shearing zone increases. The failure strain increases with the increase of cutting temperature. The cracks propagation is retrained due to the increase of failure strain and finally stops. A new sawtooth chip begins to form after the formed sawtooth chip. The formation of sawtooth chip depends on the cutting speed, feed rate and cooling/lubrication methods. It found the chip was easy to be contimuun at the cutting speed 50-100m/min. The sawtooth chip was easy to form at higher or lower cutting speed. In the same cutting parameters, the sawtooth level was more obvious in the wet cutting than it in the dry cutting. The rake angle of cutting tool influences the formation of sawtooth chip greatly. Smaller the rake angle is, easilier the sawtooth chip forms. Based on the investigation of chip morphology from orthogonal turning experiments, the transition maps from sawtooth chip to continuum chip in the dry and wet cutting with rake angle 0°, 5°, 10°were built to guide the selections of cutting parameters.Cutting fluid and minimal quantity lubrication were selected to apply in Ti6Al4V machining. MQL method uses the cutting fluid of only a small amount, typically less than 50ml/h, which is typically jetted into the cutting zone with a flow of compressed cold-air. The oil amount and cold air temperature were both adjusted. The friction and heat transfer models under the different cooling /lubrication conditions were built by means of orthogonal milling experiments and heat convention experiments. The configuration of MQL nozzles and the oil amount were optimized based on the friction and heat transfer models. Split Hopkinson pressure bar (SHPB) test was employed to obtain the flow stress in the different strain, strain rate and temperature. The material constitutive model was built based on the power-law equation. The failure strains in the different stress triaxiality and temperature were measured by the torsion, tensile and compression tests. A material failure model which considered the effect of Rebinder effect, stress triaxiality and temperature on the material failure strain was built. The orthogonal turning experiments were employed to modify the built material failure model. The formation of saw tooth chip was simulated by the aid of FEM software AdvantEdge. The simulated cutting forces and temperature were compared with the experimental results and show good agreement with the experimental results.The milling processes were reasonably simplified to two dimension cutting process. The edge structure was accurately modeled in the finite element model to simulate the milling process with small uncut chip thickness. The simulated cutting forces showed good agreement with the experimental results, which validated the accuracy of finite element model. The simulated chip morphology, cutting forces, maximal cutting temperature and thermal stress under the different cooling/ lubrication conditions were extracted. It found that the sawtooth extent in wet cutting was higher than the sawtooth extent in dry and MQL cutting. The maxiamal cutting temperature in MQL cutting was almost as same as it in wet cutting. The cutting tool was cooled rapidly in the idle stroke, so the temperature gradient and thermal stress were higher in the wet cutting than in the dry and MQL cutting. The sawtooth chip leaded to the mechanical impact and temperature gradient leaded to thermal impact, which influenced the tool life greatly.The tool life tests in dry and MQL milling were performed and the tool life in the different cutting conditions was obtained based on ISO standards for too life. The exponential and quadratic equations were employed to get the predition model of tool life and analyze the effect of cutting parameters on tool life. Based on the quadratic model of tool life, the milling parameters were optimized, taking the maximal tool life in certain material removal rate as the optimization target. The effects of tool wear on the surface roughness, sawtooth chip and milling forces were analyzed. It found the tool wear had little effect on the surface roughness in the beginning of tool wear, but the surface roughness increased rapidly when tool wear reached some extent. It found the sawtooth trend increased with the increase of tool wear. The chip formation increased. The distortion even fracture of chip occurs, which increase the formation energy and cutting temperature. It found the catual rake angle of cutting tool decreased with the tool wear. So the milling forces follow the feed direction increased; the milling force direction perpendicular to the feed direction changed; the axial milling forces had little change.更多还原