【作者】 刘洋；

【导师】 王春举；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2019， 博士

【摘要】 产品在保证性能的前提下追求便携性和微型化已成为当今生产和生活中的共同趋势,这对微型零件的加工制造水平带来了挑战。微成形技术具有高产、高效、低成本的明显优势。然而,尺寸效应导致了微成形零件质量和精度下降,是微成形技术中难以规避的瓶颈问题。超声振动辅助微成形技术能够利用体积效应和表面效应,降低材料的变形抗力,改善界面摩擦,提高材料的塑性成形性能,有望解决微成形中的尺寸效应问题。超声振动作用下材料塑性变形机理尚不明确,超声振动与尺寸效应耦合后材料的变形行为更加复杂,国内外对此鲜有研究。本文在国家自然科学基金的支持下,采用试验研究和理论分析相结合的方式,系统地研究了超声振动辅助T2紫铜塑性变形行为机理和超声振动辅助微冲裁成形工艺,分析了超声振动与微成形尺寸效应耦合作用下材料的塑性变形行为,揭示了超声振动软化效应机理,实现了高质量微型箔板零件的精密成形。首先,研究了超声振动参数对T2紫铜软化效应的影响规律。结果表明,超声振动软化效应随超声能量密度的增大而增强,残余软化效应随超声振动作用时间的增加而增强。采用EBSD方法测试了小角度晶界、KAM值等微观组织结构分布,发现超声振动作用下材料小角度晶界数量减少,KAM值减小,表明超声振动使材料塑性变形更加均匀。根据不同超声能量密度下的流动应力降低幅度,建立了基于晶体塑性理论的超声振动软化效应模型,揭示了超声振动应力叠加和声学软化共同作用下材料软化机理。其次,开展了超声振动辅助微拉伸尺寸效应研究。建立了基于超声振动能量的晶粒尺寸效应模型,从超声能量降低位错发射所需能量的角度,以Hall-Petch斜率的变化,描述了超声振动与尺寸效应耦合作用下材料塑性变形微观机理。通过对试验数据拟合,建立了e指数函数表达式,描述了流动应力降低幅度与试样几何尺寸因子之间的关系。采用DIC全场应变测量方法对微拉伸断裂行为进行研究,结果表明超声振动增大了箔板的r值,改善了箔板均匀塑性变形能力。由于超声振动加快了材料从颈缩到断裂的进程,导致微拉伸极限应变降低。在此基础上,开展了超声振动辅助微冲裁剪切变形机理研究。从凸、凹模间隙、晶粒尺寸、剪切速度等方面对比分析了超声振动对最大剪切强度的影响,并对剪切精度和断面形貌进行测量。结果表明,超声振动显著降低了最大剪切强度,能够通过延缓剪切裂纹的萌生、减小裂纹角度、缩小剪切变形区宽度来提高断面光亮带比例,以超声振动特有的表面效应改善了断面粗糙度。最后,开展了超声振动辅助精密圆环零件微冲裁成形工艺研究。对超声振动辅助微冲裁工艺得到的箔板圆环零件进行尺寸测量和断面SEM观察。结果表明,超声振动作用下,圆环同心度及内外圆直径均在误差允许范围内。光亮带比例提高,圆角减小,断面粗糙度和零件平整度得到显著改善。研究表明,超声振动辅助微冲裁成形工艺方法能够成形出高质量、高精度的箔板微型零件。更多还原

【Abstract】 Pursuing portability and miniaturization without losing the performance of the product has been a common trend in our industrial production and daily life.And this brings challenges to the manufacturing of micro parts.Microforming has advantages like high productivity,high efficiency and low cost.However,size effect decreases the quality and accuracy of micro parts and is always the bottleneck in microforming.Ultrasonic vibration assisted(UVA)forming can reduce the deformation resistance and friction by volume effect and surface effect,which is a promising way to deal with size effect.Mechanism of plastic deformation in UVA forming is still a topic on which no consensus has been reached,especially for the deformation behavior coupled with size effect.For now,few studies focus on this research.Supported by the National Natural of Science Foundation of China,the mechanism of UVA deformation behavior and UVA blanking process of T2 copper foil have been carried out experimentally and theoretically.Deformation behavior of copper foil under the coupling of ultrasonic vibration and size effect has been studied.Mechanism of softening effect under UVA forming has been specified.The forming of foil parts with high quality and high accuracy has been realized.Firstly,softening effect of T2 copper foil in UVA tension has been studied and the results show that softening effect increases with the increase of ultrasonic amplitude and the residual softening effect increases with increase of ultrasonic duration.Misorientation angle and KAM distribution are studied by Electron Backscattered Diffraction(EBSD)method.Both the misorientation angle and KAM value are reduced by ultrasonic vibration.The analysis indicates that the plastic deformation is more uniform with the assistance of ultrasonic vibration.According to the flow stress reduction under different ultrasonic energy density,theoretical model of softening effect has been established based on the crystallographic theory considering stress superposition and acoustic softening.Secondly,size effect in UVA tension has ben conducted.The grain size effect model based on ultrasonic energy has been developed.Micro mechanism of plastic deformation under ultrasonic vibration coupling with grain size effect is described by the change of Hall-Petch slope from the view point of dislocation emission.By curve fitting of exponential function,e exponential function of geometry size effect is modeled and the experimental results can be accurately predicted.Digital Image Correlation(DIC)full field strain measurement has been conducted to study the fracture behavior and results show that r value and formability of the material are improved.Limit strain is decreased because the process from necking to fracture is shortened by ultrasonic vibration.Based on the research above,mechanism of UVA shearing deformation is carried out.The influence of shearing clearance,grain size,and shearing speed on the maximum shearing strength are studied,as well as the accuracy and fracture appearance.Results show that maximum shearing strength is greatly decreased by ultrasonic vibration.Also,the smooth zone of the fracture surface is improved by delaying the initiation of shearing crack,and by reducing both the crack angle and shearing deformation area.Surface roughness is decreased by surface effect of ultrasonic vibration.Finally,UVA circular ring part blanking process has been carried out.The results of dimension measurement and fracture surface SEM observation show that concentricity and diameter of the circular ring part are satisfied by UVA blanking process.The rollover is reduced and ratio of smooth zone is increased.Both the surface roughness and flatness of the part are improved.This research indicates that the UVA micro blanking process is a promising way for the forming of micro foil parts with high quality and high accuracy.更多还原