Yumi Choi

Bio: Yumi Choi is an academic researcher from Seoul National University. The author has contributed to research in topics: Formability & Hardening (metallurgy). The author has an hindex of 3, co-authored 6 publications receiving 49 citations.

Mechanical Properties and Formability of Heat-Treated 7000-Series High-Strength Aluminum Alloy: Experiments and Finite Element Modeling

Abstract: The feasibility of room-temperature (cold) forming of peak-aged 7075 high-strength aluminum alloy (7075-T6) sheets after solution heat treatment followed by water quenching was investigated. The anisotropic plastic properties of the heat-treated aluminum sheet (7075-WT) were experimentally characterized by performing uniaxial tensile tests (along different loading directions) and a balanced biaxial tension test. Limit dome height (LDH) and cylindrical cup drawing experiments were conducted to evaluate the formability at room temperature. With the obtained anisotropic mechanical properties, different yield functions, including Yld2000-2d, Hill 1948, and von-Mises, were determined and implemented in the finite element simulations to numerically predict the formability at room temperature. The formability of the 7075-WT sheet at room temperature could be significantly enhanced owing to the prior heat treatment, and the finite element simulations performed using the anisotropic yield functions and Voce hardening law were in good agreement with the measured LDH. The LDH of the 7075-WT sheet in the plane strain and biaxial stress modes and the earing in the circular cup drawing tests could be better predicted non-quadratic Yld2000-2d function than those by isotropic and Hill 1948 quadratic yield functions.

Application of Combined W-Temper and Cold Forming Technology to High-Strength Aluminum Alloy Automotive Parts

Abstract: In this study, a room temperature forming process for high-strength aluminum alloys was investigated. To overcome the low formability of the peak-aged 7075 aluminum alloy (7075-T6) at room temperature, the sheet metal was subjected to solution heat treatment followed by rapid quenching. Mechanical tests, including uniaxial and balanced biaxial tension tests, were performed, and the Nakajima test was conducted to evaluate the formability of the sheet metal. On the basis of the plastic deformation behavior, finite element models including the plastic yielding and strain hardening laws were constructed. The proposed forming process incorporating the W-temper heat treatment was successfully applied to a realistic automotive part, which could not be manufactured by conventional room temperature forming without prior heat treatment. The numerical accuracies among various plasticity models were evaluated from a comparison of the thickness profiles. In addition, the paint-bake process was applied to the automotive component to evaluate the strength recovery.