| Summary: | 碩士 === 義守大學 === 材料科學與工程學系碩士班 === 95 === To eliminate crack of the precipitation hardened nickel base superalloy IN 738LC during the repaired welding process, a low power pulsed Nd:YAG solid state laser welding technique is used in this study. To obtain the optimal weldment and find out their crack mechanism, a series of welding parameters are designed. From the laser welding factor analysis, the most important factors, laser voltages and pulse duration, crack size, are used as welding variables. By using those variables, a series of welding processes are carried out. As the welding processes are accomplished, each sample is then heat treated. The experimental result shows, the laser voltages must be decreased with hole size, otherwise it may crack between weld and substrate because of its high cooling rate. When the laser voltages is too low, it would produces a lot of holes due to the incomplete melting. However, if the welding voltages are properly used, no matter the size of the holes, it can be successfully repaired, that is, no defects and cracks are produced. After the heat treatment, there is needle like of δ phase and CrB compound can be observed in the weld and heat affect zone. The needle like of δ is Nb-rich phase, which consumes Nb substrates while precipitation, resulting in decrease the amount of γ" phase and the strength of material. But δ phase has its certain precipitation direction, it can block the slip if it precipites perpendicular in the grain boundary, which enhance the creep resistance. Therefore, if the suitable amount of δ phase is contained, it is favorable and harmless to the material. This conditis can also observed is this study. And the CrB compound is a hard and brittle phase, although it does not produce cracks in the weldment, it may become the initial nuclear sites of crack.
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