【作者】 付宇明；

【导师】 白象忠；

【作者基本信息】 燕山大学 ， 材料加工工程， 2003， 博士

【摘要】 裂纹止裂技术是当前军事、航空、航天、船舶、石油、原子能、供电器械、机械制造等许多工业生产和实际应用中急需解决的关键问题。利用电磁热效应来遏制金属构件内部和表面裂纹的扩展是新颖而又行之有效的方法之一。关于利用电磁热效应实现金属模具裂纹止裂的技术，在国内外均缺少系统研究。生产实践和理论发展迫切需要对该技术进行深入研究。本文正是以此为出发点，结合理论分析、数值模拟和实验开展研究工作，所得结论可以为工程实际问题的解决提供指导。研究内容概括为以下五部分。(1) 采用复变函数方法，建立了金属模具电磁热效应空间裂纹止裂的理论分析模型。在该模型基础上研究了带有半埋藏环形裂纹的金属模具在通入电流瞬间，裂尖附近的热源功率、温度场和热应力场，以及裂纹尖端处的温度、热应力与通电电流密度、裂纹尺寸等参数之间的关系。本文还研究了在金属模具电磁热裂纹止裂中，同时有外机械载荷作用情况下，其裂纹尖端温度场和热应力场的复变函数解。(2) 建立了在瞬间超强脉冲电流作用下，金属模具裂纹止裂的有限元耦合场分析方法。通过商用有限元软件Marc、Ansys和自编程序，采用热—电瞬态耦合过程模拟计算了带有单边裂纹和中间裂纹的金属模具材料电磁热裂纹止裂时的温度场、温度梯度场，接下来通过热—机械耦合过程计算了放电瞬间的热应力场；讨论了各种影响因素及它们在数值计算中的处理方法，并对电磁热裂纹止裂效果与多裂纹绕流屏蔽、通路尺寸和裂纹走向关系进行了研究。(3) 采用电、热和机械场三场顺次耦合的有限元分析方法模拟了金属模具中空间裂纹电磁热裂纹止裂中电流—温度—力的演变过程；具体计算了带有半埋藏环形裂纹的金属模具电磁热止裂时的温度场、温度梯度场和热应力场；同时研究了金属模具中局部裂纹实施跨越止裂时的温度场和热应力场。数值模拟分析结果与理论研究结果相吻合。(4) 采用自制的ZL-1型脉冲电流发生装置，对多种金属模具材料如塑料模具钢、冷作模具钢和热作模具钢进行了电磁热裂纹止裂实验研究；利用电子高速摄像机记录了止裂过程，验证了理论研究和数值模拟的结果；通过金相显微分析和电镜分析了裂纹尖端周围强度、韧性和硬度均高于基体的白亮层微观组织结构，阐明了其遏制裂纹扩展的微观本质；对止裂后的裂纹前缘处进行了力学性能测定，并且采<WP=5>用SPM进行了纳米尺度下力学性能的测试；自主设计了改进型ZL-2超强脉冲电流发生装置，采用它实现了金属模具试件中裂纹的电磁热止裂。(5) 研究了各种实际金属模具中裂纹电磁热止裂后的修复技术，如电火花放电熔焊搭桥、强化堆焊、刷镀等；对修复中多种工艺参数的影响进行了研究，同时对修复后的裂纹处微观组织进行了分析和测试，为将电磁热裂纹止裂技术应用到金属模具裂纹止裂中进行了基础实验研究工作。更多还原

【Abstract】 The application of crack prevention techniques is badly needed by industry. There are many practical applications such as in military affairs, space flight, aviation, shipping, oil production, nuclear engineering, electrical supply equipment and machine manufacturing. Heating by an electromagnetic field is a novel and effective method for stopping internal and surface crack propagation in metal components. However, there is a lack of systematic research on crack arrest in metal dies, both domestically and abroad. Theoretical analysis, numerical simulation and experimental surveys will be used for proposed research on crack prevention in metal dies using electromagnetic heating. The results obtained can provide the theoretical basis for solving practical problems.The main work can be outlined as follow:(1) The complex function method is used as a basis for the theoretical model of the space crack prevention in metal dies using electromagnetic heating. The heat source power, temperature and stress fields are studied around the crack tip in metal dies with half-embedding round the crack when the pulse current is switch on. The relationship between the temperature and thermal stress around the crack tip, with the parameters of current density and crack dimensions, is given from calculations. When crack arrest in a metal die takes place by electromagnetic heating, the calculations demonstrate the conditions around the crack tip under a mechanical load. (2) The finite element coupled field theory is used as a foundation for the method of space crack prevention in metal dies when a super-density pulse current is switched on. Using the commercial finite element software Marc, Ansys and self-programming, the temperature field and temperature gradient field are calculated by a thermal-electricity coupled process when the edge crack and middle crack arrest takes place in metal dies using electromagnetic heating. Continuously, the thermal stress is calculated by a thermal-mechanical coupled process when the current is switched on. The effective parameters and their methods of treatment are discussed. The dimensions of the current path and the crack direction are studied in the case of crack prevention with multiple cracks. (3) The electrical, thermal and mechanical fields are coupled in the <WP=7>process of the calculations. The evolutive process of current-temperature-stress is simulated by numerical analysis. The temperature and thermal stress are found using electromagnetic heating applied to cracks, both localized and half-embedding, with partial crossovers in metal dies. The temperature field and stress field results from numerical simulation correspond to the results of theoretical analysis.(4) The experimental has been conducted on a pulse current generator (model ZL-1) developed by the authors. Many kinds of materials for metal dies are studied such as plastic die steel, cold-applied die steel and hot applied die steel. The process of crack arrest is recorded by an electric pickup camera. The results of theoretical analysis and numerical simulation are verified by experimental. The results of structural microscopic and electrical microscopic observation show that: a white-bright layer is formed around the crack tip. The hardness, density and toughness durability is higher than the base. The microscopic principle of crack prevention by a pulse current discharge is clarified. The mechanical performance of the structure around the crack tip is tested after crack prevention. Also, Scanning Probe Microscopy is used to finish testing at the nanometer scale. The space crack arresting in metal die has been conducted on a new super-density pulse current generator model ZL-2 designed by author.(5) The techniques for repair are studied after crack arrest in metal dies using electromagnetic heating, such as electrical spark melt joining, reinforced build-up welding and brush electroplating. The technique’s parameters are studied during the repair process. The structure around the crack is analysed and tested after crac更多还原