以晶體塑性模型探討 6000 系鋁合金析出物與溫成形之影響

Abstract

近年來因應汽車輕量化的趨勢，輕金屬逐漸被應用於汽車構件中，其中又以6000系鋁合金為具有發展潛力的輕金屬之一，然而，鋁合金之顯微結構以及溫成形技術，將是應用鋁合金於汽車產業上最重要兩個議題。欲同時連結兩項因素，晶體塑性有限元素模型(Crystal Plasticity Finite Element Method, CPFEM)將是一套不可或缺的工具，然而，針對析出物於CPFEM之模擬分析，以物理組成律模型(又稱隱式模模型)缺少考慮析出物之幾何，而過往考慮幾合之模擬分析則缺少微觀機制之探討。 因此本研究目的在於延伸隱式模型，加入析出物之幾何效應，稱為顯式模型，並針對析出物的效應提出較合理之組成律假設，同時考慮差排增生與溫度效應，可以與隱式模型互相比較。 組成律模型在本研究中被推導並與實作於Abaqus UMAT上，析出物之形狀、尺寸、分佈、方向、體積分率等因素可以利用Dream.3D建置。相較於隱式模型，顯式模型可以捕捉局部差排密度、應力應變以及晶體方位之改變。除此之外，不同晶體方位、溫度與析出物排向、形狀，將影響啟動的滑移系統，造成晶體方位與析出物方向的改變。在多晶系統分析中，發現析出物造成局部變形的不匹配效應，將抑制整體晶粒旋轉，但也同時產生較多差排密度與應力貢獻，而強烈的金屬組織會產生橘皮組織，並發現溫度與析出物效應相反，會加劇橘皮效應；AA6016-T4P之拉伸變形則成功模擬出差排密度集中於晶界與析出物周圍之效應，此外，在拉伸過程中，整體晶粒方向將逐漸往<100>//RD集中。

Lightweighting is an important technology trend in the automotive industry. To this end, aluminum alloys have now emerged as a strong candidate of lightweight materials for the industry. For 6000-series aluminum alloys, both microstructural controls and warm forming techniques are the key issues to be resolved in the automotive application. Crystal plasticity finite element method (CPFEM) that links slip activities with mechanical properties is a natural choice to study these issues. The objective of this study is to incorporate proper precipitate hardening and temperature effects into CPFEM. It is well-known that in the physical-based CPFEM (or implicit model), the effect of geometrically necessary dislocation (GND) is averaged in the constitutive law. This study also extends the implicit model with different precipitate geometries (aka, explicit model) and investigate their effects on texture and mechanical behavior. The constitutive model is implemented in Abaqus UMAT. Geometries of precipitate such as shape, size, distribution and volume fraction are considered by Dream.3D. In contrast with the implicit model, the local dislocation density, stress concentration and misorientation around precipitate can be predicted by the explicit model. Moreover, the activating slip systems are analyzed under various crystal orientations and types of precipitate. Finally, we apply CPFEM to study surface roughening effects and texture under plane stain tensile loading. We find that texture band causes surface roughening and higher temperature increases surface roughening effect. In addition, deformation incompatibility between precipitate and crystal limit grain rotation. A 6000 series aluminum alloy, AA6016-T4P, is analyzed under tensile loading. Dislocation density accumulated around precipitate and grain boundary was successfully predicted. Grain orientations after deformation are found to be preferable in <100>//RD.