냉간성형용 마그네슘 합금의 성형성 개선에 관한 연구

Abstract

In order to clarify the effect of various factors on the formability of wrought Mg alloys, the influences of thermomechanical treatment conditions and ECATD(Equal Channel Angular Tensile Deformation) on the microstructural change, recrystallization behavior, development of texture, phase change and tensile properties in Mg-Al-Zn and Mg-Li-Zn alloys have been studied. The results obtained are as follows; (1) The sheet hot-rolled at lower hot-rolling temperature and higher reduction rate revels many cracks at the edge of the hot-rolled sheet in Mg-Al-Zn alloy while hot-rolling process without cracks is possible even at lower hot-rolling temperature and higher reduction rate in Mg-Al-Zn alloy (2) Recrystallization temperature of grains deformed by hot-rolling depends on hot-rolling temperature; Recrystallization limit temperature of the sheet hot-rolled at lower hot-rolling temperature is fallen due to introduction of lattice defects such as deformation twin, dislocation and lattice distortion but recrystallization limit temperature of the sheet hot-rolled at higher hot-rolling temperature is raisen due to lower driving force for recrystallization, results in coarser grains. (3) The yield strength of hot-rolled sheet is increased while the elongation of that is decreased owing to the formation of deformation twin and deformed grains with falling hot-rolling temperature. (4) Comparing tensile strain of the sheets which is hot-rolled at different temperature with annealing temperature, the largest elongation is obtained in the sheet hot-rolled at 230 ℃ and then annealed at 200 ℃ and/or the sheet hot-rolled at 180 ℃ and then annealed at 300 ℃. (5) With increasing annealing temperature of the hot-rolled sheet, the size of dimples on the fracture surface is decreased and is uniformed throughout fracture surface, but fracture mode is changed to intergranular fracture by annealing above 400 ℃. (6) For optimal ECATD extrusion conditions without fracture, the temperatures of mold and both ends of the sheet to be ECATD shall be unifrom and constant. Especially, when temperatures at entrance and exit sides of the mold for ECATD are not equal, yield strength of the sheet becomes lower than that of the friction force at oblique point of ECATD mold, results in unexpected elongation and fracture of sheets. (7) At lower ECATD mold temperature and higher extrusion rate, the friction force at oblique point of ECATD mold is increased while the degree of elongation of grains and the amount of deformation twin are increased. (8) Recrystallization temperature is fallen with increasing extrusion rate after ECATD deformation; recrystallization temperature of the sheet which is ECATD deformed at a speed of 40mm/min is about 250 ℃, but recrystallization temperatures of the sheets being ECATD deformed at speeds of 10 and 20 mm/min are higher than that of the sheet being ECATD deformed at a speed of 40 mm/min. (9) By hot-rolling at 130 ℃, {00·1}<10·0> basal texture being inclined toward TD is developed, results in double poles of {00·1} basal plane on the stereographic projection. The slope of basal plane {00·1} is decreased with increasing hot-rolling temperature. (10) Comparing annealing texture with hot-rolling and annealing temperatures, the slope of basal plane {00·1} is decreased with increasing annealing temperature and the angle of inclination of {00·1} basal plane is increased in the sheet hot-rolled at lower temperature. (11) Double poles of {00·1} basal plane being developed by hot-rolling are changed to single pole by ECATD deformation, the change from double poles to single pole is dominated with increasing extrusion speed. (12) With increasing temperature of ECATD deformation, the intensities of {00·1} and {10·0} poles are increased and double poles of {00·1} basal plane are changed to single pole. (13) In Mg-7%Li-1%Zn alloy, α and β phases deformed by hot-rolling are recrystallized at different annealing temperature; α phase which is matrix and hcp crystal structure is recrystallized at 200 ~ 250 ℃ but β phases which is precipitated at grain boundary is recrystallized at 300 ℃. (14) Yield strength and strain of hot-rolled Mg-7%Li-1%Zn alloy are 15.2 kgf/㎟ and 14.2 %, respectively. However, yield strength is decreased to 1.8 kgf/㎟ and 11.15 kgf/㎟ and strain is increased to 24.3 % and 36.3 % by annealing at 300 ℃ and 400 ℃, respectively. In all the specimens, tensile strength is decreased with increasing strain after yielding due to the crystallographic characteristics of Mg alloy.