- 學位論文
熱處理及熱均壓癒合製程對CM247LC雷射銲補件裂縫影響之研究
Effects of the heat treatment and the hot isostatic pressing process on crack healing of the laser repair-welded CM247LC superalloy
摘要
為了降低CM247LC 鎳基超合金在雷射銲補後之破裂行為及提升過去銲補強度不足的缺點，本研究設計兩階段的工作；第一階段主要是透過銲前後熱處理，減少碳化物、γ+γ’共晶等相，避免其介面組織產生液化問題，並增加材料延韌性(使γ’粗大化)，期待減少或避免雷射銲補後WFZ與HAZ形成裂縫；若第一階段測試後，裂縫仍不可避免，即進入第二階段工作；本階段主要是運用本計畫設計之HIP技術的癒合程序，對銲補件進行裂縫之癒合。 針對第一階段工作，本研究依CM247LC各微組織固溶及析出溫度，設計三組銲前熱處理條件後，以不同的Nd:YAG雷射銲補參數進行直接熔融測試(模擬添加同質銲線，藉以提升WFZ強度)，觀察銲前熱處理與銲接參數對 CM247LC顯微組織及破裂行為之影響，經試驗後作顯微觀察，所有熱處理後試片經雷射銲補後，皆仍會造成WFZ與HAZ產生各式裂縫以及液化組織，即使利用IN-625銲補依舊無法完全解決。因此，為了徹底改善CM247LC銲後裂縫及WFZ強度不足等弱化問題，本研究進入第二階段之試驗；在此階段中，雷射直接熔融銲件表面產生各式裂縫後，以IN-625銲料進行CM247LC銲道表層密封(降低表面裂縫中氣體或雜質對後續癒合不利的影響)；隨後應用HIP技術搭配預選的參數，對銲件各微區裂縫進行高溫高壓癒合。實驗結果顯示，經過HIP癒合處理後，CM247LC的WFZ、HAZ各式裂縫得以完全癒合，而裂縫癒合後，有部分接近次微米或奈米尺寸MC型碳化物顆粒沿著癒合跡線存在，同時HIP後也助於降低WFZ、HAZ及BM等區域的偏析相比率，而銲補後之區域其機械性質與原始母材差異性不大。
並列摘要
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.