Technology Study On Crack Arrest And Healing By High-density Pulsed Current

With the development of science and technology, the high-end machinery equipment has been more and more widely used in various fields of national economy. At the same time, the number of retired equipment also increases rapidly. As a strategic emerging industry, remanufacturing engineering can not only save resource and energy and protect environment, and has remarkable economic benefit. Damage forms of remanufacturing blanks are complex as well as various, of which the crack damage account for a very high share. In the past, all remanufacturing blanks with cracks were directly scrapped. Therefore, the crack arrest and healing is one of key technologies urgently needed to be solved. During processing of the remanufacturing blanks with crack, especially for high value-added machinery equipments (e.g. engineering machinery, ship, aircraft and large compressor), it is vital to arrest propagation of the cracks or heal cracks, so as to guarantee the effectiveness of the repair processes to follow and achieve the purpose of extending the life of the products. On account of pulsed current characteristics, it is used in material preparation and physical and mechanical properties improvement. So far, there is short of systematic theoretical analysis and experimental study for crack arrest and healing by pulsed current at home and abroad. The aim of this paper is to study the crack arrest and healing by pulsed current based on detouring and Joule heating effect. The research works are carried out from three perspectives:theoretical analysis, experimental research and numerical simulation. The main research contents and results are as follows:(1) Based on the theory of complex function, the theory model of crack arrest by electro pulsing is set up. Refer to the theory of fracture mechanics, the process of crack arrest by pulse current is simplified to an infinite thin plate with an elliptical hole applied electric current at the remote field. The solving process is divided into two states and the expressions of current density, temperature and thermal stress are obtained through adding the results of two states. When an elliptical approaches to a crack, a singular behavior appears at the crack tip of the electric current, the Joule heat, the heat flux and the thermal stress. The concepts of intensity used to describe the degree of electric, thermal and stress fields in vicinity of the crack tip are introduced modeled on those of fracture mechanics.(2) Based on pulse power techniques, high-density electropulsing device of type HCPC-I is designed and build. The device is mainly composed of main circuit, trigger circuit and discharge circuit. Components specifications are calculated, including voltage regulator, transformer, current-limiting resistance, rectifier bridge, and capacitor, etc.. High pressure gas spark switch with three electrodes and trigger circuit are designed. Through the analysis of the discharge circuit by capacitor, the technological parameters of experimental research are the capacitance and the charging voltage of the capacitance. With the increase of capacitance, the current density can be improved and the discharging time is prolonged. The current density can increase with the increase of charging voltage.(3) The influence rules of materials, specimen sizes and technological parameters on fusion zone size and microstructure at the crack tip are studied and the crack arrest mechanisms by electro pulsing are discussed through experiments on AISI316L, Inconel625and FV520B. In course of electro pulsing treatment, due to the large space between the two crack surfaces, the electric current cannot flow through the crack. As a result, the electric current detour effect and Joule heating become very obvious in front of the crack tip. The local range around the crack tip heats intensely and the temperature becomes high enough to initiate melting. A blow hole is formed at the crack tip, the curvature radius of the crack tip is increased, and the state of stress concentration is improved. As the pulse current discharging time is very short, the crack tip experiences rapidly heating and cooling. Solid state phase changes occur and make crystalline grain thin around fusion zone. The current density away from the crack tip is relatively low and Joule heat effect is not obvious in this region, those show that the pulse current has selective effect and little impact on no crack region.(4) Plastic deformation appears in front of the precrack tip by using tensile and microstructure in this region presents fibrous along tensile axis. Mechanisms of crack arrest and local deformation recovery are discussed in the course of electro pulsing treatment on AISI316L. Morphology, microstructure and dislocation around the crack tip are analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after electro pulsing treatment. The research results show that the sharp crack tip becomes into elliptic melting hole and the curvature radius of the crack tip increases so as to reduce the stress concentration ahead of the crack tip. Also recrystallization occurs around the crack tip and fine equiaxial grains are formed in local region ahead of the crack tip these result in local deformation recovery.(5) The internal micro crack is precast by drilling and compression. The feasibility of crack healing using electro pulsing treatment is initially explored. From the energy perspective, the mechanism of crack healing is discussed, and technological parameters and treatment times are analyzed. In course of electro pulsing treatment, the effective atoms move into the crack and the long crack transforms into a row of short cracks. In the process of crack healing, short cracks become into voids, the number of voids reduces and the size gradually gets small until the whole crack heals. The efficiency of crack healing can be improved by smaller technological parameters and more treatment times.(6) The mechanical properties changing trends influenced by the morphology and microstructure of the crack tip are analyzed. The measurement of nano indentation hardness indicated that the microhardness values of heat affected zone are higher than that of matrix after electro pulsing treatment. The residual stress measurements from X-ray analysis show that the strong compressive stress is generated around the crack tip in the process of electro pulsing treatment. The results of tensile tests show that, the tensile strengths for AISI316L, Inconel625and FV520B exhibit increasing trait, but the elongations of three materials possess different traits, that is closely related to material types. The actions of technological parameters on tensile property and fatigue performance are studied. The area of fusion zone of the crack tip can expand with increase of capacitance and charging voltage. But too large melting hole will reduce the mechanical properties of specimen. The technological parameter range for the best crack arrest effectiveness can be determined by the relation curve of fusion zone area and mechanical properties under present experimental condition. Tensile and fatigue properties of specimen with pre-stretched crack are improved after electro pulsing treatment. The main factors of mechanical properties improvement are that, sharp crack tip gets blunt and stress concentration reduces, especially the deformation recovery, microstructure refinement and dislocation density increase ahead of crack tip.(7) Based on the electric-thermal-structural coupling theory, phenomena of detour and concentration effect and Joule heat release at the crack tip are displayed visually using ANSYS software. According to experimental processes, numerical simulations of electro pulsing treatment are conducted on AISI316L. The influence rules of thickness, crack length and technological parameter on area of fusion zone are calculated. The error between measurements and calculations is less than12%. The stress field distribution in course of electropulsing treatment and residual stress are calculated. The calculated residual stress and measured one have same order of magnitude.