S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Material Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Development of twin roll cast magnesium alloys with high formability and high strength
고성형성 고강도 TRC 마그네슘합금 개발
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 8. 신광선.
- World demand for magnesium alloys has rapidly grown due to interests in vehicle weight reduction and fuel efficiency. Magnesium alloy components have been mainly manufactured by the high-pressure die casting (HPDC) process because of high efficiency and low production cost. However, magnesium components manufactured by the HPDC process suffer from porosity and have the limitations in mechanical properties such as tensile strength and elongation. The demand for magnesium components with high plasticity wrought magnesium alloys has dramatically increased during the past decade in the automobile industry. Wrought magnesum components are mainly proudced by the extrusion or rolling procss which are more expensive than casting processes in production cost. Therefore, new process for wrought magnsium components should be necessary to produce Mg alloy sheets with low production cost and high production speed. Recently, twin roll casting (TRC) process which is a combination of rolling and casting process has been paid attention to produce low cost TRC Mg-Al-Zn alloy sheets. However, those alloys are not suitable for TRC process due to the appearance of centerline segregation and cracks, which could significantly alter their mechanical properties. It is mandatory to consider the mechanical properties and controlled segregation of Mg alloys developed by TRC process, because it mainly affects the surface quality and mechanical properties of the products.
The first objective of this study is to understand the solidification and deformation behavior in TRC Mg-6Al-X alloys to develop the new TRC Mg alloys. Simulations of TRC process were carried out in order to develop suitable TRC Mg alloy by considering both thermal and thermodynamic properties. The TRC simulation results showed that AX60 alloy had the lower segregation tendency, while AZ60 had the highest segregation tendency because of its different solidification behavior and thermal properties. Compared to the as-cast microstructure, segregation area in as-cast was well matched with the melt to roll nip distance predicted in simulation. Annealed Mg alloys with addition of Ca or Sr elements showed weaker texture when compared to A6 alloy rolled at 350oC. In addition, there was a significant change in (0002) pole figures from strong basal textures to random texture when rolling temperature increased from 350oC to 450oC. It may be attributed to the activity of non-basal slip system at high rolling temperature. Results of visco-plastic self-consistent (VPSC) simulation revealed that critical resolved shear stress (CRSS) of tension twin increased with increasing rolling temperature resulting in suppression of the activity of tension twin in compression, which was associated with enhancing the yield isotropy and formability of Mg-6Al-X alloys. Furthermore, the relative activities of basal slip in AX60 alloy were higher than the other Mg alloys resulting from weaker basal textures, which were responsible for enhancing the formability and yield isotropy of Mg alloys.
The second objective of this study is to understand the effects of alloying elements such as Al, Mn, Ca, Sn, Sr, and Zn on yield isotropy and deformation behavior of Mg-xAl and Mg-6Al-X alloys. Furthermore, mechanical properties, microstructure and texture evolutions of Mg-xAl and Mg-6Al-X alloys have also been investigated. All alloys showed the improved yield isotropy with increasing alloying elements. Especially, addition of Ca element played a role in significant increase of yield isotropy compared to other elements, and Mg-6Al-1Ca alloy exhibited the enhanced yield isotropy with over the 0.80. From the VPSC results, Mg alloys with weaker basal textures had more relative activity of basal slip at both tension and compression deformation, and that was considered to be responsible for CRSS value. In addition, it showed that there is a close relationship between yield isotropy and the relative activity of tension twin in compression deformation. Higher value of yield isotropy could be obtained by restricting the initiation of tension deformation.
The third objective of this study is to develop high strength and high formability TRC Mg alloys with low pre-heating temperature. Formability and VPSC simulations of Mg-4Zn-X-Ca alloys have been carried out to understand the relationship between deformation behaviors and room temperature formability. Microstructure, texture and mechanical properties of Mg-4Zn-X-Ca alloys have also been investigated. All the alloys showed sound TRC microstructure without occurrence of the inverse segregation. Annealed Z4 and ZSX400 alloys exhibited strong basal textures, however, the rest of Mg alloys showed weaker basal textures with a splitting to transverse direction resulting from their different types of static recrystallization. The formability and yield strength of ZX40 alloys significantly increased by Ca addition compared to Z4 alloy. Especially, Erichsen value of ZX40 alloy was 7.2mm, however, that of Z4 alloy was 3.8mm due to low yield isotropy (compressive yield strength/tensile yield strength) in Z4 alloy. Among the Mg-4Zn-X-Ca alloys, ZAX400 alloy exhibited high yield strength of 189.3MPa and excellent formability of 7.5mm which was comparable with those of Al alloys. Higher values of formability for Mg-4Zn-X-Ca alloys were closely related to modified deformation behaviors resulting from texture evolutions. Higher formable Mg alloys, ZX40, ZAX400, ZCX400, and ZWX400 alloys, had the higher relative activity of basal slip at compression deformation resulting in improved yield isotropy. However, normal formable Mg alloys, Z4 and ZSX400 alloys, had the relatively lower activity of basal slip at compression modes caused by their different CRSS ratio (tension twin/basal ).