- 學位論文
光纖及Nd: YAG雷射於肥粒鐵不銹鋼與高強度銅合金銲接及銲補特性之研究
Investigation into the Welding and Repair Welding Characteristics of the Fiber and Nd: YAG Laser on the Ferritc Stainless Steel and High Strength Copper Alloy
摘要
為了探討雷射銲接技術應於先進不銹鋼及模具用銅合金之銲接或銲補特性，本論文研究共區分成兩個主題：一、以光纖雷射銲接(Fiber Laser Welding)先進肥粒鐵系不銹鋼(SUS445)並探討其銲接性；為了比較其優劣勢，本研究採用傳統氬氣鎢極銲接(Argon Tungsten inert gas welding,TIG) 方式進行比較研究，二、使用了Nd：YAG雷射程序以及各種熱處理程序及銲補參數來探討熱處理和銲補參數對Cu-Ni-Si-Cr（Cu-6.7Ni-1.6Si-0.3Cr）模具合金微觀結構組織和性能之影響。 針對第一項研究主題，測試結果顯示，SUS 445不銹鋼銲件因經過不同的銲接製程，使得銲件各區域因受熱程度不同，其顯微組織呈現相當的差異性。以TIG銲件而言，若與Fiber Laser 銲接件做比較，可以發現有較寬廣的銲道(Weld Fusion Zone,WFZ)及熱影響區(Heat Affected Zone, HAZ)；就WFZ而言，兩者中皆沒有發現明顯的析出物產生；而在HAZ方面，TIG銲件中除了有寬廣的晶粒粗大化區域，在敏化HAZ中含有數量略多及尺寸較大的Laves相。在機械性質方面，TIG銲接件寬廣的晶粒粗大化區域中，呈現拉伸強度的降低，同時提升延伸率。但Fiber Laser銲接件則銲道區域顯現較高強度，而在HAZ強度的提升則較不明顯，造成拉伸斷裂位置皆位於母材，使得銲接件顯現良好的拉伸特性。就耐蝕性方面，由於較少、較小Laves相的存在，使得Fiber Laser 銲接件相較TIG銲接件的耐蝕性為佳。 針對第二項研究主題，為了消除Cu-Ni-Si-Cr模具合金在雷射銲接程序的限制，例如高熱導率和高熱膨脹係數，採用了許多改進的途徑，包括銲接前熱處理（預期降低熱導率），雷射形狀控制和預熱（降低冷卻速率，以避免過度膨脹和收縮），另外針對Nd:YAG銲補所形成微小的WFZ區域及容易表面氧化等特性，本研究銲後熱處理(Post Welding Heat Treatment, PWHT)過程是採用真空熱處理程序。結果顯示，經過銲接前熱處理和預熱後，造成了較深的滲透及無缺陷的銲道。然而，在WFZ內沿晶界的區域，觀察到連續的Ni/Si析出物，對銲道的機械性質是有負面的影響，可經由銲後熱處理（PWHT）來改善此問題。檢測PWHT後之修補銲件（包括WFZ和HAZ）可以發現，本研究的修補方案可以提供Cu-Ni-Si-Cr模具合金修補區域之硬度，拉伸強度和熱物性質等方面呈現良好組合而在銲件中的高強度，是由於奈米級Ni/Si析出相的形成所提供。
並列摘要
In order to investigate the laser welding and laser repair welding characteristics of the ferrite stainless steel and high strength copper alloy, this study focused on two topics : In the First topic, the Fiber Laser was used to weld the advanced ferritic stainless steel (SUS 445) and investigated its weldability. The traditional inert gas shielded tungsten (TIG) welding was also used for the comparison purpose. In the Second topic, the Nd：YAG laser process with various heat treatment process, and repair welding parameters was used to investigate their effects on the microstructure and properties of the Cu-Ni-Si-Cr（Cu-6.7Ni-1.6Si-0.3Cr）mold alloy welds. Regarding to the first topic, the test results showed that because of the different thermal conditions the microstructures in the weld fusion zone (WFZ) and heat affected zone (HAZ) of the TIG and FL welded samples were different. In the WFZs of the two welding samples, different morphologies of the grains were obtained. No obvious precipitation formed in these zones. In the HAZ of the TIG weldment, more volume fraction and larger grain sizes of the Laves phase and larger matrix grains were observed, which significantly affected its corrosion resistance and mechanical properties. However, in the HAZ of the FL weldment, only small amounts of volume fraction and small grain size Laves phases were observed. Furthermore, the width of the HAZ in the FL weldment is relatively narrow, and the matrix grain growth in this zone is not obvious; therefore, it offers better corrosion resistance and has better mechanical properties. Regarding to the second topic, To eliminate the disadvantages of the laser welding process for the Cu–Ni–Si–Cr mold alloy, such as high thermal conductivity and high thermal expansion coefficients, many improved approaches have been used, including pre-weld heat treatments (expected to reduce thermal conductivity), control of the laser shape, and preheating (to slow down the cooling rate and avoid excessive expansion and contraction). The results showed that after the pre-weld heat treatment and preheating, deeper penetration and defect free welds were obtained. However, continuous Ni/Si phase precipitates were observed along the grain boundaries in the welding fusion zone (WFZ), which may have harmful effects on the mechanical properties of the welds. This problem was improved by post-weld heat treatment (PWHT). The repair welds (including WFZ and HAZ) after PWHT showed a good combination of hardness, tensile strength, and thermal properties. The high hardness or tensile strength in the weld was attributed to the formation of a nano-sized Ni/Si phase, which was identified via a transmission electron microscope.