IN-738超合金之冷噴銲塗層研究

Abstract

本研究專注於電廠氣渦輪機高溫區段IN-738葉片之冷噴銲塗層製程探討。一般而言，電廠基於成本考量，每隔固定時間即會進行氣渦輪機之檢修，將受損之動葉片進行銲補再生。先進之雷射銲補製程係採用雷射同軸輸送IN-738粉末至葉片受損區域，唯預熱溫度需達到800℃以上方可得到良好的銲補效果，製程較為複雜且耗時。冷噴銲具有銲補速度快且塗層孔隙率低的特性，若應用於IN-738葉片之銲補，可能取代部份銲補再生製程。 實驗結果顯示，IN-738粉末在氮氣載流氣體溫度780至830℃間，噴銲於IN-738基材上可獲得良好的塗層。金相觀察顯示IN-738塗層與基材有良好之接合，且孔隙率相當低(<5％)。SEM更可觀察到粉末邊緣之絕熱剪切應變區，此區為粉末間或粉末與塗層之重要鍵結機制。TEM觀察發現塗層有奈米級細晶區與粗晶區，證明粉末外圍(晶粒較細)與中心(晶粒較粗)所受到之塑性變形量有極大差異。塗層若經過熱處理，內部有γ΄析出，與IN-738基材類似。若經1205℃/10 min熱處理(模擬硬銲製程)後，奈米晶粒消失，並可觀察到不規則形狀之γ΄ (~200 nm) 析出，粉末間之γ΄則較為粗大，此係因粉末邊緣為快速擴散路徑，γ΄成長較為快速。若經1180℃/2 h + 850℃/16 h熱處理，內部呈現方型(邊長200 nm)與圓型(直徑50 nm)兩類型之γ΄，此與IN-738基材經二段式熱處理者相同。此外，粉末間為連續型之γ΄，鈮和鉈的含量較多，故亦為碳化物成核之位置。XRD分析As-sprayed塗層繞射峰並與粉末相較，發現前者γ之半高寬變寬且峰值往低角度偏移。此現象乃因IN-738粉末受到高速衝擊，使塗層晶粒細化、差排密度與氧含量上升所致。IN-738冷噴銲塗層經熱處理後，塗層內γ΄之析出使基地內Al、Ti原子濃度下降，晶格常數變小，繞射峰往高角度移動，此現象在IN-738粉末與冷噴銲塗層中皆可觀察到。試片經不同熱處理後，因γ΄析出非常快速，故峰值角度上差異不大。冷噴銲塗層厚度可達0.5mm，有可能取代IN-738 + DF4B之硬銲再生製程，並可避免因DF4B所導致之硬脆鉻硼化合物的生成，具有相當的應用價值。

IN-738 nickel-base superalloy has been widely utilized for fabricating hot-section components, e.g., turbine blades, in the industrial gas turbines. The purpose of this study was to investigate the cold spray process and the coating properties of IN-738 powder on an IN-738 substrate. During recent years, the laser cladding process has been used in power plants to restore damaged turbine blades and has proven to be cost effective. However, the drawback of such a process is that the turbine blades need to be heated to high temperatures; in addition, the process is complicated and time consuming. Satisfactory coatings can be obtained on a workpiece by using the cold spray process, which is a fast process to produce low-porosity coatings. Experimental results revealed that IN-738 powder could be sprayed on an IN-738 substrate using nitrogen carrier gas at 780~830℃. Metallographic analysis revealed that the coatings had low coating porosity (<5%) and good adhesion to the substrate. In addition, SEM examinations revealed the formation of an adiabatic shear strain zone at the outer layer of the powder particles. The adiabatic shear strain zone is the main mechanism that governs the bonding of the powder particles as well as the coatings to the substrate. TEM observations confirmed that the coatings had a nanograin zone and a coarser grain zone. The diffraction patterns revealed that the nanograin zone occurred at the outer layer of the powder particles and that the coarser grain zone occurred at the inner portion. These results implied that the difference in plastic deformation between the outer and inner portions of the powder particles was substantial. After a heat treatment of 1205℃/10 min (similar to a brazing process), the formation of irregular shaped γ΄ precipitates (~200 nm) the absence of nanograins in the coatings were resulted. Moreover, the coarsening of γ΄ at inter-particle regions was obvious due to fast surface diffusion. When the coatings subjected to a two-step treatment (1180°C/2 h + 850℃/16 h), γ΄ precipitates of either cubical (200 nm edge length) or spherical particles (50 nm diameter) were observed. Such microstructral features were similar to those of the IN-738 substrate after the same heat treatment. The inter-particle regions, which had higher contents of Ta and Nb, were likely to be the nucleation sites for carbides. Additionally, continuous γ΄ precipitates were formed along the inter-particle regions. X-ray diffraction (XRD) analysis was also performed on the raw powder as well as the as-sprayed coatings. The results indicated that the γ peaks obtained for the as-sprayed coatings shifted to lower angles and also broaden relative to the peaks obtained for the raw powder. This phenomenon could be attributed to the presence of fine grains, high dislocation density and increased oxygen content in the coatings. After heat treatment of the coatings or the raw powder, the formation of γ΄ lowered the lattice parameter of γ owing to reduced Al and Ti concentrations in the matrix, i.e., the peaks moving toward the higher angles. A comparison of the coatings after different heat treatments revealed that the diffraction peaks were nearly unchanged. This could be due to the fact that the rate of γ΄ precipitation was fast and less dependent on the heat treatment. By using the cold spray process, coatings with thicknesses of 0.5 mm can be obtained. This process also has the advantage over the brazing process (using IN-738 + DF4B powder mixtures brazed at 1205℃/10 min) to eliminate the formation of brittle chromium borides in the coatings. It is clear that the cold spray process can have many new applications and can possibly replace the brazing process for restoring turbine blades, in particular, refurbishing worn surfaces of the turbine blade.