| 摘要: | 鋁合金熱壓縮行為研究的文章多使用均質態的鑄坯，其晶粒為等軸晶；然而針對商業鋁合金鍛造所使用的坯件狀態多為擠製態，其晶粒為已具備應變能之細長條狀；上述兩種坯件在熱壓縮過程的變形行為勢必有所差異。為了提供鍛造製程工程師和模具設計師更接近實際鍛造行為的微結構變化，本論文以擠製態7005鋁合金(Al-4.8Zn-1.8Mg)為研究對象，進行等溫熱壓縮試驗(溫度300-550oC，應變速率0.001-1s-1，應變量1.2)。
比較試件在壓縮前後不同條件之巨觀結構與微觀結構演化，壓縮前的試片晶粒組織呈現細長條狀，細部放大可看到部分具等軸晶外型的再結晶晶粒。除了壓縮溫度550oC以外，其餘條件壓縮後的試片從巨觀結構可看到試件內部依受力狀況以及變形狀況不同可區分為三個區域，分別為難變形區(應變量最小)、容易變形區(應變量最大)、自由變形區(應變量居中)。而從EBSD微觀結構看出提高變形溫度以及降低應變速率較容易驅動動態回復以及動態再結晶使流變應力降低。根據晶粒方向差角判斷可發現，擠製態7005鋁合金之動態再結晶粒依形成過程之差異可分為連續動態再結晶、不連續動態再結晶以及幾何動態再結晶；後兩者形成位置為沿著原始晶界。
本研究根據動態材料模型(DMM)，求出功率耗散效率(η)與失穩值(ξ)，建構包含真應變量在內的三維度(3D)擠製態7005合金熱加工圖(processing maps)；並搭配微觀結構選出最佳製程區間。結果顯示，η值隨溫度或應變量的增加而增加，但隨應變速率的增加而遞減。失穩區的範圍隨溫度的降低或應變速率的增加而擴展。η<0.20通常位於失穩區，可發現Micro crack及Flow localization等微觀缺陷；而穩定區的微觀組織為晶粒細小之再結晶晶粒，η值皆在0.30以上。最佳製程區間為加工溫度425–500度C，應變速率0.1～0.01s-1。最後，本研究將η值與ξ值結合數值模擬並率先應用在車輪轂閉模鍛造參數最佳化分析，使商業鍛造模擬軟體具有預測微觀結構演變的功能，簡化模具設計與決定製程參數時的試驗流程。
;The billets commonly utilized in the commercial forging of aluminum alloys are typically in an extruded state, characterized by elongated grains containing pre-existing strain energy. However, numerous studies examining the hot compression behavior of aluminum alloys often employ homogeneous cast ingots with equiaxed grains. It is anticipated that the deformation behaviors of these two types of billets during the hot compression process will differ. In order to offer process engineers and mold designers with microstructural changes that closely reflect actual deformation behaviors, this study concentrates on the extruded state of 7005 alloys (Al-4.8Zn-1.8Mg), conducting isothermal compression tests within a temperature range of 300-550°C, strain rates of 0.001-1s-1, and a strain of 1.2.
In comparing the evolution of macrostructure and microstructure of specimens under various conditions before and after compression, it was observed that pre-compressed samples displayed elongated grains, with some recrystallized grains exhibiting an equiaxed shape upon closer examination. Post-compression specimens under conditions other than a compression temperature of 550°C exhibited three distinct regions in the macrostructure characterized by internal stress and deformation: a hard-to-deform zone with minimal strain, an easily deformable zone with maximal strain, and a free deformation zone with mid-range strain. Microstructure analysis using Electron Backscatter Diffraction (EBSD) indicated that higher deformation temperatures and lower strain rates promoted dynamic recovery and dynamic recrystallization, leading to a reduction in flow stress. Examination of grain misorientation revealed that dynamically recrystallized grains in as-extruded 7005 alloys could be classified into continuous dynamic recrystallization, discontinuous dynamic recrystallization, and geometric dynamic recrystallization, with the latter two processes occurring along the original grain boundaries.
In this investigation, the Dynamic Material Model (DMM) is being employed to analyze the power dissipation efficiency (η) and instability value (ξ) using data derived from hot compression tests in order to construct a three-dimensional (3D) processing map for as-extruded 7005 alloys, incorporating true strain. Microstructure analysis was utilized to determine the optimal processing range. Findings reveal that η rises with temperature or strain yet declines with increasing strain rate. The region of instability expands with decreasing temperature or increasing strain rate. η values below 0.20 are typically in the instability region, linked to micro-defects like microcracks and flow localization. Conversely, in stable regions characterized by refined recrystallized grains, η values consistently exceed 0.30. The ideal processing range is temperatures from 425-500oC and strain rates between 0.1 and 0.01 s-1. This study is pioneering in optimizing closed die forging processing parameters by integrating power dissipation efficiency (η) and instability value (ξ) with numerical simulations. It introduces a predictive model for microstructure evolution through commercial simulation software, intending to enhance efficiency in trial processes for mold designers and forging engineers. |
