【作者】 周军；

【导师】 李剑峰； 孙杰；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2010， 博士

【摘要】 随着航空航天、国防工业、微电子工业、现代医学以及生物工程技术的发展,对精密／超精密三维微小零件(特征尺寸在微米级到毫米级)的需求日益迫切。目前的多种微细加工技术中,针对硅基材料的刻蚀技术、薄膜技术以及LIGA技术等应用广泛。但是它们的主要加工对象被限制在硅基材料上,并且工件的几何形状基本上是简单的二维形状。而微切削技术是把常规切削技术应用到微观领域,有其自身优势。使用CNC加工中心可实现2D、2.5D简单特征到复杂的3D复杂形貌或者自由曲面的微加工,通过此方法加工微小模具可达到批量生产的目的。另外,微切削加工被加工材料几乎不受限制,可以满足微型零件对材料多样性的要求。由于工件、刀具尺寸的大幅减小导致微切削加工中表现出许多与常规切削加工不同的现象。微切削加工机理研究方面,刀具刃口半径的存在对切屑形成、尺度效应、最小切削厚度现象等有非常明显的影响。另外微切削力建模、微切削中的表面完整性与微毛刺的产生及微切削加工过程仿真等方面的研究,是目前微切削加工研究中的关键技术问题。切削力是研究切削加工过程的重要物理量之一,研究切削力对进一步弄清切削机理,对计算功率消耗,对刀具、机床、央具的设计,对制定合理的切削用量,优化刀具几何参数等,都具有非常重要的意义。通过对微铣削加工过程向二维直角切削加工的简化,建立体现微切削特点的二维切削力模型,并对切削区域进行重新区划。为了便于使用Kistler 9257B型测力仪进行加工过程切削力的测量,对工件进行预处理,使用数控铣削中心实现直角车削试验过程。考虑刀具刃口半径的存在对微切削加工过程的影响,试验方案在不同切削速度下变换切削深度,实现单因素直角切削。试验过程中测得不同切削用量下的切削力,对切削力进行处理后得到不同切削参数下的平均切削力,发现切削力和切削抗力随切削深度变化呈现不同的变化规律；通过切削抗力与切削力的比值计算,发现随着切削深度的减小,三种速度下切削力比值均呈非线性增大趋势,尤其是当切削深度小于25μm时,增大趋势愈加明显,说明微切削加工时存在摩擦尺度效应；求得单位切削力,发现切削速度对单位切削力和单位切削抗力的影响并不是十分明显,而切削深度的变化对两种单位切削力的影响却非常大。当切削深度小于25μm的情况下,随着切削深度的减小,两种力的单位切削能量都随着切削深度的减小而成非线性增加,表明使用硬质合金刀具对铝合金7050-T7451过程中存在尺度效应,且刃口半径是引起尺度效应的重要原因。微切削加工中,由于最小切削深度的存在,切屑的形成过程与传统切削加工有较大不同,切屑形貌也有自身特点。在试验基础上得到不同切削加工条件下的微切屑形貌,发现在相同切削深度不同切削速度下产生的切屑卷曲程度不同,较高速度下产生的切屑卷曲程度比较低切削速度下的切屑卷曲程度小,在切削深度较大时该现象更为明显；另外,相同切削速度下,切削深度小于最小切深时,产生挤碎的褶皱切屑,切削深度大于最小切削深度时,产生平滑的连续切屑。切削深度非常小的情况下,直角车削的切削力发生周期性振动；而切削深度较大的情况下,切削力非常平稳,该现象反映了当切削深度小于最小切削深度时,工件材料在刀具刃口前端由堆积到形成切屑的过程；同时建立描述微切削加工的等效剪切角模型,研究切屑变形系数及其影响因素,可为表面完整性、刀具寿命测以及铣削参数优化等提供支持。微切削加工中由于刃口半径造成的尺度效应及最小切深的存在,切削刃附近的应变、应力分布及温度状况极为复杂,在试验中难以直接观测,同时也很难用解析力学的方法求解。使用有限元方法进行分析,并且对于试验和经验方式来说,是非常好的补充。材料模型的构建是进行切削加工有限元仿真的重要前提,同时也是该领域研究的难点。根据霍普金森压杆试验获得的不同应变、应变率、温度下的流动应力,对铝合金7050-T7451建立了基于Power-Law的材料本构关系。将铣削加工过程进行简化,建立考虑刃口半径的二维有限元CAD模型,对不同切削条件下的加工过程进行仿真,并将仿真切削力与试验结果进行了对比,发现仿真结果与试验值有较好的一致性。基于材料流动方式,从仿真结果中发现微切削加工中由于刃口半径存在,刀刃前区存在着金属停滞区,并获得硬质合金刀具切削铝合金7050-T7451的最小切削深度大约为刀具刃口半径的30%左右。通过变切削深度及变刃口半径的两组不同仿真,分析刀具刃口半径和切削参数对尺度效应、切削区域温度分布、剪切区域内的塑性应变和应变率分布等的影响规律。表面完整性是零件在加工或处理之后所具有的表面纹理和表层状态,其评价指标有表面粗糙度、加工硬度、残余应力以及微观组织等方面。通过单因素直角切削试验,得到了不同切削参数下的已加工表面。对已加工表面的粗糙度、表层微观组织、加工硬化以及残余应力进行测量,并对其受刀具刃口半径及切削参数等的影响规律进行分析。根据测得的表面粗糙度,验证试验过程中刀具刃口形貌保持完好；试验表明直角车削条件下,已加工表面的应力状态为残余拉应力,切削深度的变化对残余应力值有明显的影响,随着切削深度的减小,拉应力的值也随之减小,且当切削深度小于刃口半径(25μm)时,刀具的刃口半径影响明显。研究已加工表面残余应力的形成机制,并尝试根据位错理论建立加工硬化理论模型。微铣削加工技术是实现微小工件制造或者进行微小特征尺寸加工的重要方式。经过切削加工获得的零件表面,总是存在一定的表面粗糙度、塑性变形层、金相组织变化和表面裂纹,对于微小工件而言,其表面面积与体积的比值比传统切削加工要大的多,表面形貌和毛刺对于其尺寸精度和使用性能的影响更为突出。基于铣削加工表面形成理论分析,提出微切削加工过程已加工表面粗糙度的预测模型,该模型在传统理论粗糙度模型基础上,考虑最小切削深度、刀具形貌以及加工参数的影响,以微铣削铝合会7050-T7451试验为依据,对微铣削表面粗糙度、毛刺形成等特征进行研究,分析了铣削用量、刀具磨损等对表面形貌及毛刺生成的影响规律,为微铣削加工表面形成特征与状态的研究、合理选择切削参数提供依据。通过正交微铣削试验发现,切削深度对槽铣底面表面粗糙度的影响明显。试验结果表明,微槽铣铝合金7050-T7451的毛刺形式为顶部毛刺,且顶部毛刺的产生受最小切削深度及尺度效应的影响,同时切削深度、进给量及刀具磨损对毛刺尺寸也有直接影响,顺铣方式比逆铣方式能够更好的控制毛刺尺寸。更多还原

【Abstract】 High-accuracy miniaturized components are increasingly in demand for various industries, such as aerospace, biomedical, electronics, environmental, communications, and automotive. Numerous researchers have investigated the feasibility of using other fabrication processes, such as LIGA, laser, ultrasonic, ion beam and micro-electro discharge machining methods, to manufacture commercially viable micro-components. However, the majority of these methods are slow, and limited to a few silicon-based materials and essentially planar geometries. Micro cutting bring many advantages to the fabrication of micro-sized features. It is capable of fabricating 3D free-form surfaces, which is essentially important for the production of micro-injection molds. Moreover, it can process a variety of metallic alloys, composites, polymers and ceramic materials to form functional devices. Micro cutting raises a great number of issues mainly due to size or scale. In the last few decades, there have been extensive studies of tool life, edge radius effect, surface generation, size effect, minimum chip thickness, micro structural effects as well as finite element modeling and molecular dynamics simulation of micro cutting. However, fundamental understanding and general consensus on the mechanism that dominates mechanical machining at the micro scale is lacking.Cutting force is an important parameter in metal cutting process. Research of the cutting forces has significant meaning for clarifying the cutting mechanism, calculating of power consumption, designing of cutting tools, machine tools and fixtures, selecting reasonable cutting parameters and optimizing tool geometry, etc. Simplify the cutting process from 3D micro-milling to 2D orthogonal cutting, built a model for 2D cutting forces which embody the characteristics of micro-cutting process, and make new divisions for the cutting zone. In order to facilitate the use of Kistler 9257B type dynamometer for cutting force measurement, the workpiece was pretreated, then the orthogonal turning experiments can be realized in CNC milling center. Considering the influence of tool edge radius in micro cutting process, various depths of cut under different cutting speeds were selected in the single factor experimental scheme. The cutting forces were measured during the experiment, and then the average cutting forces were calculated. The cutting forces and thrust forces present different patterns of change with various depth of cut under different cutting speeds. The ratio of thrust force to cutting force was calculated, and the ratio increases nonlinearly as the depth of cut decreases. The same trend was found under different cutting speeds, especially when the depth of cut less than 25μm. The so called frictional size-effect exists in micro cutting. The specific cutting force and specific thrust force were calculated. The cutting speed has little influence on both specific cutting forces, while the depth of cut has large effect. Both specific cutting forces increase as the depth of cut decreases, especially when the depth of cut less than 25μm. This phenomenon indicates that the size effect existing in micro cutting process and the tool edge radius does influence the specific cutting force.In micro cutting process, the chip formation mechanism is different from the conventional cutting process because of the existence of the minimum chip thickness. The chip geometry also has its own characteristics. Different chips under various cutting parameters were derived from the orthogonal cutting experiment. It is noticed that the curl of the chip increases with increased depth of cut at different cutting speeds. The chips produced under very small depth of cut are very thin and fragile, and wavy, while under large depth of cut the chips are smooth and continuously. Under very small depth of cut, the cutting forces present periodically vibrations at different cutting speed, while under large depth of cut, the cutting forces are stable. This phenomenon reflects the procedure that the workpiece material piles up before the tool edge till a specific thickness and then the chip formed. An effective shear angle model was established to describe the mechanism of micro cutting. The coefficient of chip deformation was calculated and the influence factors such as cutting parameters and tool edge radius were studied.In micro cutting process, size effect and minimum chip thickness caused by the big round tool edge have significant influence on the distribution of plastic strain, strain rate and temperature around the tool edge and in the cutting zone. It’s difficult to directly observe the complex distributions via experiments. The finite element analysis is a good supplement to experiments and analytical methods. The constitutive model of workpiece material is essential for FEM simulation. The constitutive model for A17050-T7451 was established based on Power Law from the data derived from Hopkison bar experiment. The CAE model was established for simulation and the results were compared to the experiments to verify the model. It is found from the simulation results that the stable build-up edge does exist and the minimum chip thickness for micro cutting A17050-T7451 with carbide tool at about 30%of the tool edge radius. Size effect was also found in simulation result. The effect of tool edge radius and cutting parameters on the distribution of the maximum temperature, the plastic strain and strain rate were also study.Surface integrity is the state of the surface texture after machining or processing, its evaluation indicators including surface roughness, work hardness, residual stress, microstructure, and so on. Different machined surfaced were derived and saved from the single factor orthogonal cutting experiment. The surface roughness, microstructure, work hardness, and residual stress of each machined surface under different cutting parameters were measured, and the effect of tool edge radius and cutting parameters are also investigated. It is confirmed that the tool edge was at good conditions during the experiments from the measurement result of surface morphology. It is also found that the residual stress of the machined surface is tensile stress in orthogonal cutting process. The cutting edge radius does affect the residual stress. The tensile stress decreases as the depth of cut increases, especially when the depth of cut is smaller than the cutting edge radius (25μm). The formation mechanism of residual stress was investigated, and work hardening model was also established.Micromilling process is a significant way for manufacturing small features or miniaturized components. For small parts, the ratio of surface area to volume is much larger than traditional components. The effect on the accuracy and performance of surface integrity and burr is much greater. The surface roughness prediction model for the bottom surface in slot milling was proposed based on the theoretical analysis. The minimum chip thickness, size effect and cutting parameters were considered in the model. The full factor orthogonal micromilling experiment was conducted. It if found from the result that the axial depth of cut has greatest effect on the bottom surface roughness than other parameters. The bottom surface morphology was measured. The top burr was clearly observed via SEM. The minimum chip thickness and size effect has obvious influence on burr formation, and the size of top bur was also influenced by cutting parameters and tool wear. Further more, the top burr formed in down milling process is much smaller than that in up milling process.更多还原