Research On The Crack Arrest In A Metal Die By Using Electromagnetic Heating

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 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...