Papers by Author: Yong Yun Lee

Abstract: Recent 7xxx aluminum alloys have been designed for the finite use of thick semiproduct with contolled amount of constituent phases which mostly evolve during ingot preheat. In this study, the effects of constitutional change and preheat conditions of 7175 and 7050 type alloys on the evolution of constituent phases [M-, T-, S-phase and dispersoid] are presented. The constiuents evolve depending on the constitutional effect, primarily the change of Zn:Mg ratio, preheat condition comprising temperature and cooling rate following preheat. T- and M-phase are reprecipitated during cooling after preheat, depending on the alloy constitutions. S-phase is evolved depending on the constitution and preheat temperature, rather than preheat cooling rate. Prominent precipitation temperature interval of constituents are discussed in view of quaternary phase evolutions. In addition, evolutions of dispersoids together with M-phase are discussed. Specific alloy designs and preheat conditions could provide controlled microstructures for the thick 7xxx semiproducts.

Abstract: The high cycle fatigue properties of two kinds of wrought Al 2519 alloys without and with scandium of 0.10% were investigated. The fatigue strength was determined at R = 0.1 under constant amplitude loading conditions in air. The alloy with scandium of 0.10% showed a little lower tensile yield strength and higher fatigue strength values. The fine grained Al-0.10Sc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained Al 2519 alloy. The results can be explained mainly with the microstructural differences between both alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine subgrain structure.

Abstract: After extrusion, cold rolling and T-6 treatment, microstructure, texture development, and deep drawability of Al-5.09%Zn-1.83%Mg-0.32%Cu-0.25%Mn-alloy with 0.1% Sc are studied. After T-6 treatment, recrystallized equiaxed grains with an average grain size of 1~2㎛ are obtained. During extrusion the very strong <111>-fiber + medium sharp <100>-fiber + weak <210>-fiber texture is developed, which influences the formation of rolling texture. The texture, after 80% cold rolling, can be described by the strong {112}<111>(Cu) + {123}<634>(S) component in the cross section of the extruded rod, the strong b-fiber + weak {110}<001>(Goss) components in the longitudinal section, and the strong {110}<112>(Bs) + weak {001}<100>(Cube) components in the transverse section. The types of rolling texture are not changed after the T-6 treatment, but the maximum density of ODF is higher. The calculated mean r-values, rm, and the planar anisotropy, Dr, are relatively high, which are dependent on the texture.

Abstract: An effort has been made to present optimum alloy designs of commercial 7175 and 7050 type alloys to be used in thick forgings with proper microstructures and properties. The effects of changing alloy constitutions, primarily [Zn+Mg+Cu] and Zn:Mg ratio, on the evolutions of the coarse equilibrium phases and age hardening response are investigated. As a result, equilibrium phases (M,T,S) are evolved depending on alloy constitutions and cooling rate following solutionizing. The formation of the T- and S-phases is effectively controlled in the novel alloys (dilute and with high Zn:Mg ratio). In the slow quench, the redesign promotes homogeneous precipitation of η-phases, leading to higher mechanical properties than conventional alloys. The novel alloys provide microstructural homogeneity and extended heating range, ensuring 7xxx thick forging applications.