Jaeyeong Park

Tensile property improvement of TWIP-cored three-layer steel sheets fabricated by hot-roll-bonding with low-carbon steel or interstitial-free steel

TWIP-cored three-layer steel sheets were newly fabricated by hot rolling of TWIP steel sheet surrounded by low-carbon (LC) or interstitial-free (IF) steel sheets. TWIP/LC or TWIP/IF interfaces were well bonded without pores or voids, while a few pearlites were thinly formed along the interfaces. The strengths and elongation of the TWIP-cored sheets increased as the volume fraction of TWIP-cored region increased, and were also well matched with the ones calculated by a rule of mixtures based on volume fraction or force fraction. According to digital image correlation and electron back-scatter diffraction analyses, very high strain hardening effect in the initial deformation stage and active twin formation in the interfacial region beneficially affected the overall homogeneous deformation in the TWIP-cored sheets without any yield point phenomenon occurring in the LC sheet and serrations occurring in the TWIP sheet, respectively. These TWIP-cored sheets can cover a wide range of yield strength, tensile strength, and ductility levels, e.g., 320~498 MPa, 545~878 MPa, and 48~54%, respectively, by controlling the volume fraction of TWIP-cored region, and thus present new applications to multi-functional automotive steel sheets requiring excellent properties.

Correlation between bonding strength and mechanical properties in Mg/Al two-ply clad sheet

The effect of interfacial bonding strength has been investigated in relation to mechanical properties of roll-bonded two-ply Mg/Al clad sheets in this study. Two types of thin reaction layers (Mg17Al12 and Mg2Al3 phase layers) were observed to form along the Mg/Al interface and their thickness was found to increase with annealing time. The maximum average bonding strength was achieved after annealing Mg/Al clad sheet for 10 min at 300 °C. The fracture strain of Mg alloy in this annealed clad sheet was found to increase up to 22.5% compared to 17.8% of monolithic AZ31 Mg alloy. In addition, the formability of clad Mg alloy was also found much higher compared to those of monolithic Mg alloy. This ductility enhancement of clad Mg alloy appears due to more prolonged homogeneous deformation, caused by delayed localized necking.

Adiabatic shear banding and cracking phenomena occurring during cold-forging simulation tests of plain carbon steel wire rods by using a split Hopkinson’s pressure bar

Adiabatic shear banding and cracking phenomena occurring during cold forging of plain carbon steel wire rods, whose carbon content was varied from 0.2 to 0.8 wt%, were analyzed by forging simulation test using a split Hopkinson’s pressure bar. The test results indicated that the 0.2C and 0.3C steels were dynamically compressed without surface defects after the fifth hit, whereas a deep crack was formed along the 45° direction in the 0.8C steel. In all the steels, adiabatic shear bands were formed diagonally inside forging-simulated specimens, and grains were extremely elongated within shear bands. The higher the volume fraction of pearlite was, the easier was the adiabatic shear banding. Particularly in the 0.8C steel, the shear band was white-colored and narrow, along which a long crack was formed. After the spheroidization treatment of the 0.8C steel, adiabatic shear bands or cracks were not found during the forging simulation test as the steel was relatively homogeneously deformed, which indicated that the spheroidization effectively prevented the adiabatic shear banding or cracking. The present forging simulation test plausibly evaluated the cold-forging performance by controlling the number and amount of hit, and provided an important idea on whether the spheroidization was needed or not.

Three-Ply Al/Mg/Al Clad Sheets Fabricated by Twin-Roll Casting and Post-treatments (Homogenization, Warm Rolling, and Annealing)

Abstract When thin Al alloy sheets are clad on to twin-roll-cast Mg alloy melt, inherent drawbacks of Mg alloys such as poor formability, corrosion resistance, and surface quality can be effectively complemented. In this study, three-ply Al/Mg/Al clad sheets were fabricated by twin-roll casting and post-treatments. Brittle interfacial layers composed of γ (Mg17Al12) and β (Mg2Al3) phases were inevitably formed, but their proper thickening during the post-treatments led to improvement of interfacial bonding and resultant tensile properties. In particular, warm rolling was an effective way to modify interfacial microstructures and tensile properties by minimizing deformation inhomogeneity and stress concentration.

Dynamic compressive properties obtained from a split Hopkinson pressure bar test of Boryeong shale

Dynamic compressive properties of a Boryeong shale were evaluated by using a split Hopkinson pressure bar, and were compared with those of a Hwangdeung granite which is a typical hard rock. The results indicated that the dynamic compressive loading reduced the resistance to fracture. The dynamic compressive strength was lower in the shale than in the granite, and was raised with increasing strain rate by microcracking effect as well as strain rate strengthening effect. Since the number of microcracked fragments increased with increasing strain rate in the shale having laminated weakness planes, the shale showed the better fragmentation performance than the granite at high strain rates. The effect of transversely isotropic plane on compressive strength decreased with increasing strain rate, which was desirable for increasing the fragmentation performance. Thus, the shale can be more reliably applied to industrial areas requiring good fragmentation performance as the striking speed of drilling or hydraulic fracturing machines increased. The present dynamic compressive test effectively evaluated the fragmentation performance as well as compressive strength and strain energy density by controlling the air pressure, and provided an important idea on which rock was more readily fragmented under dynamically processing conditions such as high-speed drilling and blasting.

Effect of Texture on High Temperature Deformation Behaviors of Ti-6Al-4V Alloy

Textures developed during hot rolling process may affect the high temperature deformation behaviors of Ti alloys, but their relation has not been well understood or quantitatively analyzed yet. A series of load relaxation and creep tests for hot rolled Ti-6Al-4V alloy has been conducted in this work to clarify the effect of textures on the deformation behaviors of the alloy under 700 °C and the result was analyzed by using an internal variables approach. The internal strength σ* was found to vary significantly by the textures, but not by the temperature change, while the texture effect was found to decrease at higher temperatures.

Effect of Texture on Deformation Behavior of AZ31 Mg Alloy

Hot rolled Mg alloy has a preferred orientation, which affects the deformation behavior of Mg alloy. In this work, load relaxation and tensile tests after hot rolling process were performed to clarify the effect of the texture on deformation behavior of AZ31 Mg alloy and the results were analyzed based on an internal variable theory proposed by one of the authors. The analysis of the deformation behavior is then correlated to XRD pole figure results of the texture. The analysis result of Mg alloy was found to be comparable to the results obtained from the test of Ti alloys, which has the same HCP structure.

On the Low Strain Rate Regime of Structural Superplasticity - an Internal Variable Approach

The superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS flow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.