S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
ELECTROPLASTICITY IN LIGHTWEIGHT ALLOY : 경량 금속에서의 통전 소성 변형 및 메커니즘 규명에 대한 연구
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- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Lightweight alloy ; Electroplasticity ; Annealing ; Dislocation ; Elastic modulus ; Diffusion
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 한흥남.
- Electrically-assisted forming is a promising alternative forming technique, in which the mechanical property of a metal alloy is altered by simply applying electricity to the target alloy during deformation. The reduced flow stress and increased ductility, which are often called the electroplastic effect, are generally observed in electrically-assisted deformation.
Even though a number of researches have been conducted on the effect of electric current on deformation, the amount of experimental data to clearly understand the phenomenon of electroplasticity is not sufficient yet. Also, the underlying mechanism of electroplasticity is still unclear. The objective of present study is mainly consisted of two part. Firstly, the effect of electric current on lightweight alloy is investigated based on microstructural perspective. Through the understanding of electric current-assisted phenomenon, finally, the underlying mechanism of electroplasticity is investigated.
Firstly, electroplasticity in non-heat treatable aluminum alloy was investigated with subsequent microstructural analysis. Al-Mg alloy was selected as non-heat treatable aluminum alloy. The elongation of both as-received (H32 treated) and cold-rolled specimens increases drastically with softening of flow stress by applying electric current during deformation. Recrystallization and grain growth were observed after fracture from the pulsed tensile test and it could be expected that thermal effect would be dominant after severe necking. However, increase of formability before severe necking was still observed strongly and it could not be explained by usual thermal effect. It was confirmed that the recovery occurs at a given electric pulsing condition comparing with nonpulsed tensile test. This study proves that the electric current could induce annealing as a distinct role from Joule heating.
Secondly, electroplasticity in precipitation hardened aluminum alloy was investigated with subsequent microstructural analysis. Al-Mg-Si alloy was selected as precipitation hardened aluminum alloy. Specimens with three different heat treatment conditions, solution treated, naturally aged, and artificially aged (as-received) conditions, are prepared. In solution treated specimen, the elongation and flow stress increase by applying pulsed electric current during plastic deformation compared to the result of non-pulsed tension. The Portevin-Le Chaterlier (PLC) phenomenon, which is clearly observed in non-pulsed tensile test, nearly disappears by applying electric current during deformation. For the naturally aged specimen, the flow stress decreases while the elongation significantly increases under a pulsed electric current compared to the result of non-pulsed tensile test. In case of artificially aged specimen, both elongation and flow stress decrease under a pulsed electric current. From XRD analysis, it was observed that thermal and electric current-induced annealing occur in all the specimen under the electric current. Also, the formation of early stage of precipitation from a supersaturated state might be accelerated by applying electric current with a distinct effect of Joule heating, which causes the increase of flow stress and the disappearance of PLC phenomenon in the solution treated specimen. In addition, the microstructural observation shows that electric current accelerates the formation of microvoid around the precipitates at grain boundary, which results in earlier fracture in the artificially aged specimen.
Lastly, underlying mechanism of electroplasticity was investigated based on effect of electric current on elastic modulus. From the results discussed in Al-Mg alloy and Al-Mg-Si alloy, it was suggested that electric current can enhance the atomic diffusion with a distinct effect of Joule heating. For diffusion, bonding energy of atoms with neighbors is an important factor to activate the diffusion. Bonding energy is closely related to the lattice potential energy. When material is excited by external energy, it will cause the changes in lattice potential energy. Elastic modulus is closely related to the atomic potential energy, which is derived from the second derivative of the potential energy-atomic distance curve. Therefore, the change in potential energy can be expected by measuring an elastic modulus. Laser ultrasonic method was used as a non-contact with high accuracy measuring technique to detect change in elastic modulus. For aluminum and magnesium alloy, electric current can induce additional decrease in elastic modulus more than Joule heating effect. Therefore, it can be suggested that electric current may induce the additional change in lattice potential energy with a discrete effect of Joule heating, which can cause the enhance of diffusion by weakening the atomic bonding force. Also, effect of grain boundary on electric current itself effect in electroplasticity was studied for each alloy with different grain size of specimen. It was confirmed that the decrease in elastic modulus by applying electric increases with increasing the fraction of grain boundary in both aluminum and magnesium alloy. From these results, it can be said that the grain boundary is dominantly influenced by applying electric current to decrease in elastic modulus.
This study provides an important insight to apply electric current-assisted forming. As a positive aspect, recovery occurs due to the electric current-induced annealing as well as the thermal one by applying electric current. This can enlarge the capacity for deformation. In addition, electric current-assisted aging occurs during deformation with less time and lower temperature compared to conventional aging. However, electric current can accelerate the formation of microvoid around the particles, which results in earlier fracture as a negative way in formability. Therefore, electric current should be carefully applied to material considering the microstructural features to obtain enhanced formability without degradation of mechanical property.
From this study, electroplasticity in lightweight alloy was investigated well. Also, the underlying mechanism of electroplasticity, which has not been clear up to now, is suggested. The discussion on electroplasticity and suggested underlying mechanism can provide insight to apply electrically-assisted manufacturing in real industry as well as academic interests for electroplasticity.
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