| dc.description.abstract | 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. | en_US |