基于强脉冲电流金属材料裂纹止裂及愈合技术研究

发布时间：2018-05-04 20:57

本文选题：再制造 + 脉冲电流　；参考：《大连理工大学》2014年博士论文

【摘要】：再制造工程作为一项战略型新兴产业不仅能够节省资源、能源,保护环境,而且具有显著的经济效益。再制造毛坯损伤形式复杂多样,其中裂纹损伤所占比重很大,在以往的再制造过程中,无论什么形式的裂纹损伤一经被检测出来直接报废,因此,金属材料裂纹止裂与损伤愈合技术是机械装备再制造领域急需解决的关键问题之一,尤其对于含有裂纹损伤的高附加值机械装备(如工程机械、船舶、飞机、大型压缩机)的核心部件实施再制造时,首先要修复裂纹损伤才能保证后续再制造工艺的有效性。脉冲电流由于其自身的一些特性被应用于材料制备及物理机械性能改善领域。关于脉冲电流裂纹止裂、愈合技术.目前国内外缺少系统的理论分析和实验研究。本文围绕脉冲电流的绕流效应和焦耳热效应在裂纹止裂、愈合中的应用,从理论分析、实验研究和数值模拟三个角度开展研究工作。本文的主要研究内容和研究成果如下： (1)基于复变函数理论,建立脉冲电流裂纹止裂过程理论模型。根据自由边界处电流密度为零的边界条件,将带有贯穿椭圆形切口的无限大薄板中通入电流问题的求解过程划分为无椭圆孔和存在椭圆孔扰动两种状态的叠加,简化了裂纹边界条件处理过程。当椭圆孔短轴趋于零时,可以将其近似看作裂纹,电流密度、焦耳热源功率和热应力在裂纹尖端具有奇异性,仿照断裂力学中应力强度因子概念,引入强度因子的概念描述裂纹尖端附近区域电、热、热应力场,为设计实验方案和选择工艺参数提供理论基础。 (2)基于脉冲功率技术,自行设计并搭建强脉冲电流放电装置HCPD-I。通过对电容器放电回路的分析,确定实验研究的工艺参数为电容量和电容充电电压；增加电容量能够提高回路电流密度和延长脉冲电流作用时间；增大电容充电电压只能够提高回路电流密度。 (3)选用工业中应用广泛的AISI316L和压缩机转子叶轮材料Inconel625和FV520B为研究对象,通过实验研究材料种类、试样尺寸和工艺参数对裂纹尖端熔化区尺寸和微观组织的影响规律,探讨利用脉冲进行裂纹止裂的机理。脉冲电流处理过程,由于两裂纹面间距较大电流无法通过,电流在裂纹前端绕流集中,在焦耳热作用下,裂纹尖端局部温度升高甚至瞬间熔化,裂纹尖端曲率半径增大,改善了裂纹前端的应力集中状态。由于脉冲电流作用时间非常短暂,裂纹前端在快速加热和冷却作用下,熔化区周围局部组织发生固态相变,组织均匀细化。而远离裂纹尖端的基体电流密度相对较低,焦耳热作用不明显,表明脉冲电流具有选择作用,对无裂纹部位影响很小。 (4)采用拉伸方式预制裂纹的尖端组织发生严重的塑性变形,沿着拉力轴线方向呈纤维状分布。以AISI316L材料为例探讨脉冲电流裂纹止裂同时,对裂纹尖端局部塑性变形区的恢复机理。利用扫描电镜和透射电镜分析脉冲电流处理前后裂纹尖端形貌、显微组织和位错形态的变化,研究结果表明,脉冲电流处理后,尖锐的裂纹尖端变为椭圆形熔孔,裂纹尖端曲率半径增大,降低了裂纹尖端的应力集中,并且裂纹前端局部发生再结晶,形成细小等轴晶粒,局部塑性变形组织发生恢复。 (5)采用钻孔压缩法预制内部微裂纹,初步探索脉冲电流用于裂纹愈合的可行性。分析工艺参数和脉冲电流处理次数对裂纹愈合效果的作用规律,并从能量的角度探讨脉冲电流作用对微裂纹愈合的机理。脉冲电流处理过程,在绕流集中和焦耳热效应的作用下,裂纹周围形成压应力,使裂纹面间距变窄,裂纹周围原子在电流作用下向缺陷部位移动填充,将长裂纹分隔成断续短裂纹,短裂纹在愈合过程产生孔洞,孔洞尺寸缩小,数量减少,直至裂纹全部愈合。小工艺参数、多次处理更有利于延长裂纹愈合区域尺寸,提高愈合效率。 (6)分析裂纹尖端形貌和微观组织对力学性能演变趋势的影响。纳米压痕硬度测量结果表明,脉冲电流处理后裂纹前端热影响区硬度高于基体。X射线残余应力分析结果表明,脉冲电流处理后裂纹前端产生了强大的压应力。脉冲电流处理后,AISI316L、 Inconel625和FV520B三种材料的抗拉强度均呈现不同程度的提高,而延伸率变化趋势各异,脉冲电流对塑性的影响与材料种类有关。研究工艺参数对拉伸性能和疲劳性能的作用规律,增大放电回路的电容量和电容充电电压均可以扩大裂纹尖端的熔化区面积,但裂纹尖端熔化区尺寸过大反而会降低其力学性能。通过建立熔化区尺寸与拉伸性能和疲劳性能关系曲线,确定本实验条件下裂纹止裂效果最佳的熔化区尺寸区间。含有拉伸裂纹的试样经过脉冲电流处理后,拉伸性能和疲劳性能均得到改善。尖锐的裂纹尖端变钝,降低了裂纹尖端的应力集中,尤其是裂纹前端局部塑性变形恢复,组织细化、位错密度增大是提高拉伸裂纹力学性能的主要原因。 (7)基于电-热-结构耦合理论,应用ANSYS有限元分析软件直观展示脉冲电流处理过程电流在裂纹前端绕流集中和焦耳热释放现象。以AISI316L试样为例,按照实验过程进行建模、施加载荷和边界条件,模拟计算试样厚度、裂纹长度和工艺参数对裂纹前端熔化区尺寸的影响规律,并对比实验测量结果,两者误差小于12%；计算了脉冲电流处理过程中试样的应力场分布和残余应力应力场分布,模拟计算的残余应力场与实验测量结果数量级相同。
[Abstract]:As a strategic emerging industry , it can not only save resources , energy , protect the environment , but also have obvious economic benefits . In the process of remanufacturing the core components of mechanical equipment , such as engineering machinery , ship , aircraft and large - scale compressor , it is necessary to repair the crack damage in order to ensure the effectiveness of the subsequent re - manufacturing process .

( 1 ) Based on the theory of complex function , a theoretical model of the crack arrest process of the pulse current is established . According to the boundary condition of zero current density at the free boundary , it is divided into two states : no elliptical hole and existence of elliptical hole disturbance . It can be considered as crack , current density , Joule heat source power and thermal stress . The concept of stress intensity factor in fracture mechanics can be considered . The concept of strength factor is introduced to describe the electrical , thermal and thermal stress fields near the crack tip . The theoretical basis is provided for designing experimental scheme and selecting process parameters .

( 2 ) Based on the pulse power technology , HCPD - I was designed and built . By analyzing the discharge circuit of capacitor , it is determined that the process parameters of the experimental study are capacitance and capacitor charging voltage .
increasing the electric capacity can improve the current density of the loop and prolong the action time of the pulse current ;
increasing the charge voltage of the capacitor can only improve the circuit current density .

( 3 ) In the process of pulse current treatment , the local temperature of the crack tip is increased and the stress concentration state of the front end of the crack is improved . The current density of the crack tip is relatively low due to the large current of the crack tip , and the thermal effect of the Joule heat is not obvious .

The results show that the sharp crack tip becomes elliptical hole , the radius of curvature of the crack tip is increased , the stress concentration of the crack tip is reduced , and the crack tip is locally recrystallized to form fine equiaxial crystal grains , and the local plastic deformation structure is restored .

( 5 ) The effect of pulse current on the healing of cracks is investigated by means of drilling compression method . The effect of pulse current on the healing of cracks is analyzed .

( 6 ) The influence of the morphology of the crack tip and microstructure on the evolution tendency of the mechanical properties is analyzed . The results of the nano indentation hardness measurement show that the hardness of the front end of the crack is higher than that of the matrix after the pulse current treatment . The results show that the tensile strength and the fatigue property of the crack tip are improved after the pulse current treatment . The sharp crack tip becomes dull , the stress concentration of the crack tip is increased , and the microstructure is refined . The increase of dislocation density is the main reason to improve the mechanical properties of the tensile crack .

( 7 ) Based on the theory of electro - thermal - structural coupling , ANSYS finite element analysis software is applied to show the flow concentration and Joule heat release phenomenon of the pulse current in the front end of the crack . Taking the AISI316L specimen as an example , the influence law of the thickness , crack length and process parameters on the size of the crack tip melting zone is simulated according to the experimental procedure , and the experimental results are compared with the error of less than 12 % .
The stress field distribution and residual stress field distribution of the sample during the pulse current processing are calculated . The residual stress field of the simulated calculation is of the same order of magnitude as the experimental measurement result .

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TH16;TG661

【参考文献】