微铣削切削力特性及表面质量的实验研究

为了研究微铣削加工过程中切削用量对切削力的影响规律及加工质量,采用直径150μm的硬质合金微铣刀进行了微铣削加工实验.一方面,采用单因素实验方法,研究了铣削微槽时主轴转速、轴向切削深度和每齿进给量对切削力的影响规律,实验发现主轴转速和轴向切削深度是影响切削力的重要因素,而每齿进给量的影响最小.以每齿进给量与切削刃刃口半径的比值fz/Re为变量,研究了切削比能的尺寸效应现象,发现随着fz/Re的减小,切削比能增大,当fz/Re1.0以后,切削比能急剧增大.另一方面,从毛刺宽度和表面粗糙度两个方面研究了fz/Re对微铣削加工质量的影响规律,其结果表现出明显的尺寸效应现象,随着每齿进给量的的减小,毛刺宽度和表面粗糙度均先减小后增大.当fz/Re=0.5时毛刺最小;当fz/Re=1.0时,表面粗糙度最小.本研究可以为实际加工中合理选择切削用量以及分析微铣削尺寸效应现象提供理论依据.     

双顶角钻头钻削CFRP复合材料的刀具磨损机制

为了研究碳纤维增强树脂基(CFRP)复合材料切削中刀具在不同部位的磨损机制和规律,以典型的硬质合金双顶角钻头作为研究对象,主要研究对出口分层影响较大的横刃和对最终制孔成型影响较大的第二主切削刃的磨损机制及规律。通过减小磨损测量间隔,并引入切削刃钝圆半径以及后刀面磨损带宽度,表征了横刃和第二主切削刃在加工中的衰变过程。基于显微刃口观测和钝圆半径变化,揭示横刃易崩刃和第二主切削刃磨损后又受到重新刃磨的磨损机制,获得了此类型钻头不同部位的磨损规律。同时,基于上述的磨损表征,研究不同切削部位磨损量对钻削轴向力和力矩的影响,横刃轴向力与横刃钝圆半径变化相关性较小,而钻削最大力矩与第二主切削刃后刀面磨损变化规律相一致。     

高体积分数SiCp/Al复合材料铣削残余应力有限元模拟

高体积分数碳化硅颗粒增强铝基(SiCp/Al)复合材料具有优良的综合物理力学性能,其难加工性制约着该材料的应用与发展,切削加工产生的表面残余应力严重影响了工件的结构和尺寸精度稳定性。为了研究高体积分数SiCp/Al复合材料的铣削加工表面残余应力,采用有限元分析软件ABAQUS建立了基于正交切削的平面应变模型,并对建立的宏观等效模型进行模拟分析。在模拟结果中,分析并得出了不同切削速度和每齿进给量对工件加工表面残余应力的影响规律以及沿工件层深方向的残余应力分布情况。     

圆弧刃精密切削试验及其三维有限元分析

在商业化软件Marc的基础上建立了圆弧刃精密切削三维有限元模型,研究不同半径的圆弧刃精密切削条件下主切削力、切屑形状和切削温度场分布．该模型运用更新拉格朗日法的有限元方程、网格重划分准则,其刀一屑摩擦采用修正的库仑摩擦模型．模拟结果表明,圆弧刃的圆弧半径对主切削力、切屑形状和切屑的最高温度都有较大的影响．通过试验研究精密切削马氏体3J33在不同半径的圆弧刃条件下对主切削力的影响,结果表明模拟的主切削力与试验值在一定程度下是吻合的.     

300M超高强度钢车削加工性能仿真模拟分析研究

目的研究不同切削参数对300M超高强度钢切削性能的影响。方法通过单因素试验法,采用Advant Edge切削仿真软件,建立300M钢三维有限元模型,对不同切削参数下车削300M钢的切削力、刀片温度、刀片应力及切屑形状进行分析。结果在300M钢车削过程中,刀片温度随着切削速度增大而增大,但切削力和刀片应力反之;背吃刀量和进给量越小,切削力、刀片应力及刀片温度越小;切削刃半径越小,切削力越小,但小的切削刃半径使得刀片应力变大,容易导致刀片磨损。车削300M钢的切屑呈锯齿螺旋状,切屑温度为带状分布,切削速度越高,进给量、背吃刀量越大,切削刃半径越小,切屑温度越高。结论在300M钢车削加工中,应选用较高的切削速度,适中的切削刃半径,较小的进给量和背吃刀量。     

切屑数值模拟研究之中碳钢的高速加工

通过建立的正交切削有限元模型,采用Cockroft-Latham切屑断裂标准作为工件材料失效准则对45号钢高速加工过程进行了数值模拟,得到了崩碎状切屑的形成过程。并对切削过程中获得的切削力曲线及切削温度场分布进行了分析。加工过程中由于崩碎状切屑的不断产生导致切削力及切削温度产生较大的波动。数值模拟结果为进一步研究45号钢高速加工切削机理及切削参数优化、刀具几何尺寸的设计提供了基础。     

硬质合金刀具钝圆半径的仿真与实验研究

在切削过程中,合理的钝圆半径不仅可以保证加工零件的表面质量,而且也可以大大延长刀具的使用寿命。本文以前角5°、后角10°的刀具车削45#钢为例,针对硬质合金刀具刃口钝圆半径问题进行了研究,基于Abaqus仿真模拟其加工过程,分析了钝圆半径对于切削温度、切削力和工件表面加工质量的影响。同时完成了相应的车削实验,得出了钝圆半径与刀具寿命的关系。采用仿真分析与实验验证相结合的方法确定了硬质合金刀具合理的钝圆半径,对刀具的设计和生产加工具有重要的参考意义。     

CFRP/Al叠层结构制孔缺陷的有限元仿真

应用Abaqus有限元仿真软件,建立了CFRP/Al钻削有限元仿真模型,对CFRP/Al钻孔过程中碳纤维复合材料的撕裂、分层损伤及毛刺的特征和形成机理进行了仿真研究,并进一步分析了切削速度、进给量对CFRP/Al叠层结构制孔缺陷的影响规律。结果表明:麻花钻横刃对碳纤维的拉伸作用是造成撕裂的主要原因;毛刺主要产生在纤维被切削时处于拉伸状态的区域,铝合金切屑对CFRP的划伤也是毛刺形成的原因之一;在CFRP/Al叠层结构制孔缺陷中分层损伤最显著,进给量和切削速度对出入口分层损伤成正相关且对入口处分层影响更大;在这种叠层结构中,铝合金板的支撑作用有效减小了碳纤维复合材料层出口的分层损伤。     

微观状态SiCp/Al复合材料铣削残余应力模拟分析

为了获得更高精度的高体积分数SiCp/Al复合材料铣削加工表面残余应力数据,采用有限元分析软件ABAQUS建立了基于正交切削的微观平面应变模型。复合材料两相材料属性被分别定义,改善了在宏观状态下整体定义材料属性的精度缺陷,并对其微观状态切削残余应力产生过程进行模拟分析。分析并得出了不同切削参数对工件加工表面残余应力的影响规律以及沿工件层深方向的残余应力分布情况。结果表明切削速度和每齿进给量对SiCp/Al复合材料已加工表面残余应力有着相似的影响规律,但每齿进给量的影响作用较为显著。     

微磨削与超声振动复合加工技术研究现状与展望

微细切削技术是传统加工工艺向微观尺度的延伸,在微加工领域具有重要的作用,尤其适用于三维零件及微结构的加工。与其他微细切削技术相比,微细磨削技术具有加工零件棱边精度高、适于硬脆性材料加工等优势,但其存在加工效率低、磨削热量大、微砂轮易磨损等缺陷。已有研究表明,于机械加工辅加超声振动的复合加工技术可有效降低切削力、切削温度,增大脆性材料脆-塑转变临界切削深度,改善加工表面质量等。因而超声振动辅助微磨削技术被认为是一种可有效解决微磨削加工现存缺陷的技术。主要从微磨削技术研究现状、尺寸效应机理研究、脆性材料塑性域去除机理研究、超声振动切削实验研究、超声振动切削断续切削机理研究及微磨削动态有效磨刃密度建模研究六个方面,对微磨削技术及超声振动辅助切削技术相关领域研究进行综述,并探讨超声振动辅助微细磨削技术加工机理研究及未来发展需注重解决的问题。     

Feed-direction ultrasonic vibration−assisted milling surface texture formation

The formation mechanism of surface texture for feed-direction ultrasonic vibration?assisted milling (UVAM) was investigated in this study by establishing a kinematic model and a pressing model of UVAM. The kinematic model showed that the cutter tip, which was supposed to be totally sharp, produced closed scratches by crossed trajectories. The variation trends of the interval for closed textures were of sine function. A comparative experiment was carried out by UVAM and conventional milling. A dividing line close to the X coordinate divides the surface feature into two parts. The pressing model showed that the tool minor cutting edge left clear traces with certain width because of the tool minor cutting edge angle. Scratches by tool minor cutting edge were intermittent and regularly varied when feed-direction vibration was introduced. All the surface feature changes are in the radial direction and the trajectory intersections shall always be the scratch grooves or ridges. The ratio between ultrasonic vibration frequency and spindle speed, tool radius, and the located cutter rotation angle affected the changing rule of scratches by tool minor cutting edge. The analytical models and the experimental results proved to each other reasonable.     

Investigation on tool wear and tool life prediction in micro-milling of Ti-6Al-4V

Short tool life and rapid tool wear in micromachining of hard-to-machine materials remain a barrier to the process being economically viable. In this study, standard procedures and conditions set by the ISO for tool life testing in milling were used to analyze the wear of tungsten carbide micro-end-milling tools through slot milling conducted on titanium alloy Ti-6 Al-4 V. Tool wear was characterized by flank wear rate,cutting-edge radius change, and tool volumetric change. The effect of machining parameters, such as cutting speed and feedrate, on tool wear was investigated with reference to surface roughness and geometric accuracy of the finished workpiece. Experimental data indicate different modes of tool wear throughout machining, where nonuniform flank wear and abrasive wear are the dominant wear modes. High cutting speed and low feedrate can reduce the tool wear rate and improve the tool life during micromachining.However, the low feedrate enhances the plowing effect on the cutting zone, resulting in reduced surface quality and leading to burr formation and premature tool failure. This study concludes with a proposal of tool rejection criteria for micro-milling of Ti-6 Al-4 V.     

半固态高碳(C＞1%)工具钢压缩变形的组织演变

采用Gleeble-1500热/力模拟机,对电磁搅拌法制备的高碳钢半固态坯料进行压缩实验,研究半固态高碳(C>1％)工具钢在不同温度、不同变形程度和不同变形速率下压缩变形的组织演变规律.结果表明,随变形程度的增加,试样心部和边部晶粒的尺寸逐渐减小;随温度的升高,固相率降低,试样心部和边部晶粒的尺寸变大;随变形速率的提高,试样心部的晶粒尺寸减小,边部的“大结构”破碎。     

Effect of welding parameters on microstructure and mechanical properties of AA7075‐T6 friction stir welded joints

The effects of advancing speed and rotational speed on the microstructure and the mechanical properties of friction stir welded 7075‐T6 aluminium alloy sheets were studied. The fatigue strength of sound joints was measured and compared to tensile testing results. Macrographs and microhardness maps were carried out to reveal the microstructure transformations. Fractographic observations were made to identify the failure mechanisms. The effects of welding parameters on the fatigue strength are discussed in terms of welding pitch k (mm/rev) and heat input (J/mm). At a high welding pitch, crack initiation at the root of the circular grooves left by the tool on the weld surface is the most detrimental failure mechanism. As the size and the depth of the grooves are related to the welding pitch, the fatigue strength increases when the welding pitch is reduced. However, when the heat input is excessive, the failure is caused by sub‐surface defects produced after abnormal stirring and/or by softening of the heat‐affected zone. Lateral lips on the weld surface edges also have an effect on the fatigue strength for intermediate welding pitch values.     

Failure Analysis of a Diesel Engine Connecting Rod

A failure investigation has been conducted on a diesel engine connecting rod. The fracture occurred at the small head of the connecting rod. Visual and scanning electron microscopy observations show that a lot of axial grooves appear on the internal surface close to the fracture and the fatigue cracks initiated from the axial grooves. Fractography indicates that the multiple-origin fatigue fracture is the dominant failure mechanism. The machining or assembling process was responsible for the formation of the axial grooves.     

变速箱二轴的断裂分析

汽车变速箱二轴在工作中发生多起断裂事故。采用金相检验、扫描电镜和化学成分分析等手段，对断裂变速箱二轴进行了检验分析。结果表明，轴的断口具有明显的疲劳特征，疲劳源位于二轴花键槽根部。由于渗碳淬火工艺不当，在二轴花键槽根部的组织中产生了网状碳化物和粗大的针状马氏体，在外力作用下形成沿晶显微裂纹并扩展，是导致二轴疲劳断裂的主要原因。     

MJ系列碳纤维微观结构的剖析

利用SEM、XRD和Raman光谱研究了高强高模PAN基碳纤维M40J、M55J、M60J的表面形貌和微观结构参数.结果表明:3种纤维的表面均存在沟槽,说明其原丝可能均为湿法纺丝.但沟槽的深度与均匀性又有所不同.XRD结果显示:M40J的石墨化程度最低,M55J次之,M60J最高.Raman光谱数据显示,M40J的Rs、Rc、Rsc值均相对较大,M55J次之,M60J最小.说明M60J的表面和内部的石墨化程度均较高,但皮芯结构也最严重,这可能是导致其抗拉强度较低的原因之一.     

粉末冶金上盖油槽加工裂纹分析

某批空调压缩机用粉末冶金上盖在加工油槽时出现多件开裂现象,通过对开裂上盖进行化学成分分析、金相检验以及零件加工、物理和力学性能对比试验,发现烧结网带速度快、零件密度低是造成该批上盖零件加工开裂的主要原因。最后对分析结果进行了验证试验,使故障重现,并提出了有效的改进建议。     

The fragmentation of primary dendrites during shearing in semisolid processing

Semisolid slurries with non-dendritic microstructure can be produced by solidification under forced convection. Dendrite fragmentation is a commonly accepted mechanism for shearing semisolid slurry. However, it requires from experimental observation to confirm the fragmentation of dendrite arms occurring during shearing process. Experiments are undertaken to investigate the dendrite fragmentation in Sn-15wt%Pb alloy under shearing by virtue of a twin-screw extruder together with the wetting and penetration of pure tin grain boundaries submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy. It is found that the primary dendrites can be fragmented rapidly into small particles and there are penetration grooves existing in the partially fragmented dendrites. Such grooves are similar to that found at the grain boundaries in pre-cast pure tin polycrystals, which have been submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy.     

Anisotropic wetting of microstructured surfaces as a function of surface chemistry

In order to study the influence of surface chemistry on the wetting of structured surfaces, microstructures consisting of grooves or squares were produced via hot embossing of poly(ethylene-alt-tetrafluoroethylene) ETFE substrates. The structured substrates were modified with polymer brushes, thereby changing their surface functionality and wettability. Water droplets were most strongly pinned to the structure when the surface was moderately hydrophilic, as in the case of poly(4-vinylpyridine) (P4VP) or poly(vinyl(N-methyl-2-pyridone) (PVMP) brush-modified substrates. As a result, the droplet shape was determined by the features of the microstructure. The water contact angles (CA) were considerably higher than on flat surfaces and differed, in the most extreme case, by 37° when measured on grooved substrates, parallel and perpendicular to the grooves. On hydrophobic substrates (pristine ETFE), the same effects were observed but were much less pronounced. On very hydrophilic sampes (those modified with poly(N-methyl-vinylpyridinium) (QP4VP)), the microstructure had no influence on the drop shape. These findings are explained by significant differences in apparent and real contact angles at the relatively smooth edges of the embossed structures. Finally, the highly anisotropic grooved microstructure was combined with a gradient in polymer brush composition and wettability. In the case of a parallel alignment of the gradient direction to the grooves, the directed spreading of water droplets could be observed.