Young Hoon Chung

Controlling the textures of the metal strips via the continuous confined strip shearing(C2S2) process

Abstract A new concept process, a so-called as C2S2 process based on equal channel angular pressing, for fabricating the A1 alloy sheets with a high formability is introduced. The C2S2 machine was designed so that it can feed the metal strip in a continuous manner at a relatively high speed in the range of ∼20 m/min. A significant amount of the shear deformation could be achieved by passing the sample through the ECAP die with Φ = 120°. The pole figures were calculated both to demonstrate the evolution of the textures of the 1050 A1 alloy sheets and to elucidate the feasibility of the proposed process as a means for enhancing the formability of the metal strips.

Texture evolution of the strip cast 1050 Al alloy processed by continuous confined strip shearing and its formability evaluation

Abstract Studies on texture evolutions were conducted on strip cast 1050 Al alloy sheets to assess the feasibility of equal channel angular pressing (ECAP) and subsequent heat treatments as a means for enhancing the formability while reducing the earing of the sheets. ODFs were calculated to investigate texture evolutions due to ECAP and subsequent heat treatments. The r -value and Δr-value were determined based on the measured pole figures to judge the formability and the earing of the samples. Experiments showed that the production of the 1050 Al alloy sheets with high formability ( r =1.05) and low earing (Δr=0.05) could be possible by ECAP and the heat treatment at appropriate temperature.

An analysis of accumulated deformation in the Equal Channel Angular Rolling (ECAR) process

Accumulated deformation during a new ECAR process is analyzed. It is found that the effect of the normal strain component is not negligible. The minimum accumulated strain is measured as 0.408 in one-pass ECAR deformed 6063 aluminum alloy sheet. Submicron grain structure is obtained in a 6063 Al alloy sheet by 3-pass ECAR deformation and recrystallization at 250°C for 30 min.

Evolution of textures and microstructures in IF-steel sheets during continuous confined strip shearing and subsequent recrystallization annealing

Interstitial free (IF) steel sheets were deformed by continuous confined strip shearing (CCSS) based on the equal channel angular pressing (ECAP). The samples were deformed by CCSS up to three passages and subsequently recrystallized at 700°C for 1 h. The strain history of IF steel sheets in the CCSS die-channel was tackled by finite element method (FEM) simulations. The deformation by CCSS led to the shear deformation and consequently the formation of shear texture components. With increasing number of CCSS passages, the intensity of the deformation texture was hardly increased. The recrystallization texture resembled the deformation texture. The orientation stability was discussed by mean of Taylor deformation model and the formation of recrystallization textures was discussed by occurrence of the discontinuous recrystallization. Observations by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) revealed the formation of ultra-fine grains in IF sheets deformed by CCSS.

Effect of Initial Textures on Texture Formation in AA 3004 Sheets during Continuous Confined Strip Shearing and Subsequent Annealing

The effect of initial textures on the texture formation during continuous confined strip shearing (CCSS) and subsequent annealing was investigated in AA 3004 sheets. The CCSS tools were designed to provide a constant shear strain of the order of 0.5 per pass while preserving the original sheet shape. During the CCSS deformation, the initial texture disappeared, and shear texture components developed. However, the intensity of the shear texture components did not further develop with an increasing number of CCSS passes. The softer initial sample depicting the cube texture displayed the evolution of {111}//ND fiber orientations. Upon recrystallization annealing, the textures developed during CCSS were randomized through the suppression of oriented nucleation and selected growth which generally dominate the evolution of recrystallization texture in cold rolled aluminum alloy sheets.

The effect of external stress on the discontinuous precipitation in an AlZn alloy at high and low temperatures

Abstract The effect of external tensile stress on continuous precipitation (DP) has been investigated in an Al21.8 at.% Zn alloy at high (215°C) and low (75 and 50°C) temperatures. The ratio of the macroscopic lattice diffusivity, D , to the DP boundary velocity, v , ( D / v ) is estimated to be larger than the interatomic spacing, λ, at the high temperature, and smaller than λ at the low temperatures. Under tensile stresses, the DP rates are enhanced at the grain boundary segments oriented transverse to the stress direction and suppressed at those oriented parallel to it at both high and low temperatures. Furthermore, Yi and Park show the DP rate changing continuously with temperature over the range where D / v increased from values much smaller than λ to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, with D / v λ , no solute diffusion is usually assumed to occur in front of the moving DP boundaries.

Vacuum hot pressing of Fe–Si–B–Nb-based amorphous powder cores and their high-frequency magnetic properties

FeSiBNb amorphous powder was prepared by gas atomization and subsequently consolidated into bulk amorphous cores by hot pressing after creating an oxide layer on the surface of the powder. FeSiBNb amorphous powders exhibiting a saturation magnetization of 127emu∕g and a coercivity of 0.34 Oe were successfully obtained by gas atomization. Hot pressing enabled consolidation of the FeSiBNb amorphous powder core within the supercooled liquid region. The core prepared from the FeSiBNb amorphous powder covered with an oxide layer of 1μm thickness showed excellent high-frequency characteristics with a stable permeability over 10 MHz and a core loss of 400mW∕cm3 at 50 kHz for Bm=0.1T. The oxide layer formed on the FeSiBNb amorphous powder was effective in improving the frequency dependence of magnetic properties and in reducing the core loss for the FeSiBNb amorphous powder cores.

Surface Oxidation and Magnetic Properties of Fe-Si-B-Nb Amorphous Alloy

In the present work, the forming mechanism of oxide layer during wet oxidations is presented. There can be three kinds of stress sources that develop cracks in the oxide layer during oxidation. One is the volume change by oxidation, another is thermal expansion by the heat of oxidation, and the third is the gas pressure evolved. In this investigation, the surface oxidation mechanism of Fe-Si-B-Nb amorphous powder during wet oxidation treatment is studied by the analysis of the composition and magnetic domains of the oxide layer. Fe-Si-B-Nb amorphous powder used in this research is 53-75 mum in diameter; and the morphology of oxide layers can be controlled by changing oxidation treatment time. The phase and morphology of composing insulating layer of consecutive oxide layers is characterized by SEM, XPS, EDS, TEM and Lorentz TEM.

Controlling the Thickness Uniformity in Equal Channel Angular Rolling (ECAR)

As the surface friction between feeding rolls and metal sheet generates the feeding power of ECAR, the generated feeding power is low, and the friction between the metal sheet and ECAR die should be minimized. However, for obtaining a large shear deformation by ECAR, the metal sheet should be tightly contacted with the wall of ECAR die. In this condition, the thickness of the metal sheet is continuously increased during ECAR. A new ECAR apparatus is developed for maximizing the shear deformation and obtaining sheet thickness uniformity, and succeeding continuous ECAR with such a limited feeding power. By controlling the outlet gap of the ECAR die with elastic unit, the thickness of the metal sheet is kept uniform. Detailed thickness control mechanism during the new ECAR process is analyzed. A sheet of Al 6063 alloy that is 1-pass deformed with the new ECAR apparatus shows below ±0.037 mm of thickness variation and 0.61 of shear strain.

Recrystallization behaviour of Al-2Li-3Mg-1Cu-0.12Zr-X Mn(X = 0, 0.5) alloy

Grain size control by thermo-mechanical treatment is investigated in aluminum-lithium alloys. Rolling and subsequent recrystallizing treatment have been widely employed to obtain a fine grained structure. Humphreys has reviewed the recrystallization process in particle containing alloys and proposed a new deformation zone model. He thought that subgrain size or dislocation density increased abruptly beyond the deformation and boundary. He considered that a recrystallization nucleus formed and grew fast within the deformation zone. For nucleation of new grains, the maximum misorientation within the deformation zone should be sufficient to form a high angle boundary in the matrix, and for continuous growth of the nucleus, the deformation zone should be larger than critical size R{sub cr}. In this work, the recrystallization behaviors of Mn-free and Mn-bearing Al-Li alloys at various recrystallization temperatures were investigated to confirm the deformation zone model proposed by Humphreys.