| Summary: | 博士 === 義守大學 === 材料科學與工程學系 === 105 === In order to reduce cracks and to improve strength of the laser repair-welded CM247LC nickel base superalloy, this study was designed as a two stages work. At the first stage, the pre-welding heat treatment (PWHT) processes were conducted to reduce the low melting point phases (eg. γ+γ’ eutectic phase), or to increase the material ductility (coarsening of γ’). It is expected to decrease or avoid cracks forming in the weld fusion zone (WFZ) and the heat-affected zone (HAZ). If the cracks are still inevitable, the second stage work would be conducted. At this stage, the hot isostatic pressing (HIP) technique is used to heal cracks of laser repair-welded pieces.
Referred to the solid solution and precipitation temperatures of the phases measured by the differential scanning calorimeter (DSC), at first stage, three PWHT conditions were designed for CM247LC superalloy. After heat treatment, with different laser welding parameters, direct re-melting (simulated homogeneous filler wire for repair welding) process was conducted to investigate the effects of PWHT and laser welding parameters on the microstructure and cracking behavior of CM247LC. The results indicated, the solidification cracks in the WFZ and liquidation cracks in the HAZ were still observed in all specimens. Even repair welded with an IN-625 filler addition, cracks are more likely formed in the HAZ. Therefore, to entirely resolve two major problems, cracks and the under-strength of the repair-welded zone, the second stage work was conducted. At this stage, various cracks are produced, after direct surface re-melting of CM247LC. An IN-625 filler addition was employed for surface sealing before HIP (to reduce the adverse effects of surface cracks containing gas or impurities on subsequent healing). Then, HIP technique under preselected HIP parameters was employed for the high pressure and high temperature healing. The test results indicated that, through a proper HIP process, all cracks in the WFZ and the HAZ of the repair-welded pieces were healed. Some nano- and micro-sized carbides tended to form discontinuously along the healed crack trace. Volume fraction and size of the precipitations in the WFZ, HAZ and BM were also reduced. Moreover, when a micro-hardness test was conducted in the repair-welded area, the results were similar to that of the as-cast material.
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