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Friction Effect of Surface Treated Tools used for Warm Forming of Mg Alloy Sheets
Lee, Youngseon,Kim, Sangwoo,Park, Sung-Young,Yoo, Jaimoo,Moon, Younghoon Korean Society for Precision Engineering 2014 International Journal of Precision Engineering and Vol.15 No.12
Sheet forming of Mg alloy is usually performed at moderately high temperature since the formability of these alloys is very low at room temperature. However, it is difficult to use lubricant especially in a multi-stage sheet forming at moderately high temperatures. Therefore, tool coating is a better than lubricant application for Mg alloy sheet warm forming. In this study, some tool coating methods were investigated to reduce friction and prevent adhesion between the tool and sheet. Tools with three different non-metallic coating layers, namely, TiBCN, DLC, and TAC, were compared with a no-coated tool for friction effects by a pin-on-disc friction test. The friction coefficients for the different coating layers were measured at room temperature, 170 and <TEX>$250^{\circ}C$</TEX>. AISI H-13 and AZ31were used as pin and disc materials, respectively. Two of the tool coatings, namely, TiBCN and DLC, were not good enough to prevent sticking between the tool and AZ31 sheet. The coulomb friction coefficient in the case of these coatings was 0.4 in ambient air. On the other hand, the friction coefficient of the TAC coating layer was lower(< 0.2~0.3) than the coefficients of these two coatings before the adhesion of the Mg alloy with the tool.
Lee, Youngseon,Lim, Jeongsuk,Moon, Younghoon The Japan Institute of Metals 2012 Materials transactions Vol.53 No.5
<P>It is advantageous for industrial pipes to have small bending radii because of several advantages such as smaller space and less costs. However, the mechanical properties of such bent pipes often deteriorate, especially at the outer sections, due to the smaller thickness; in general, the properties worsen as the pipe bending radii decrease. Therefore, it is important to investigate the mechanical properties of bent pipes having smaller radii than is conventional. In this study, to verify the applicability of such bent pipes with small bending radii, the fatigue properties for a small radius of curvature from 1.5–2.0 diameter ratio (DR) have been investigated. Our pipe material was low carbon steel (SA106Gr-B) having the chemical compositions of 0.3C, 0.29Mn, 0.4Cr, and 0.4Cu (mass%). The high cycle fatigue, microstructure, hardness, and residual stress of two bent sections — extrados and intrados regions — were compared with those of the raw material section. Low carbon steel pipes have better fatigue characteristics at the bent section than at the raw material section. The improvements in the fatigue strength of specimen resulted from grain refinement and phase transformation, while compressive residual stress at surface of bent pipe remained by thermo-mechanical processing can give better fatigue properties in fields.</P>
Lee, Ho Won,Kang, Seong-Hoon,Lee, Youngseon 한국정밀공학회 2014 International Journal of Precision Engineering and Vol.15 No.6
In this study, dynamic recrystallization during nonisothermal hot compression test was numerically simulated by finite element analysis using new grain aggregate model for dynamic recrystallization. This model was developed based on mean field approach by assuming grain aggregate as representative element. For each grain aggregate, changes of state variables were calculated using three sub-models for work hardening, nucleation, and nucleus growth. A conventional single parameter dislocation density model was used to calculate change of dislocation density in grains. For modeling nucleation, constant nucleation rate and nucleation criterion developed by Roberts and Ahlblom were used. It was assumed that the nucleation occurs when the dislocation density of certain grain reaches a critical nucleation criterion. Conventional rate theory was used to model nucleus growth. The developed dynamic recrystallization model was validated by comparing with isothermal hot compression of pure copper. Then, the finite element analysis was conducted to predict the local changes of microstructure and average grain size by using the grain aggregate model. The predicted results were compared with nonisothermal hot compression results. The simulation results were in reasonably good agreement with experimentally obtained microstructures and the calculation time was much shorter than cellular automata-finite element method.
이영선(Youngseon Lee),권용남(Yongnam Kwon),권용철(Yongchul Kwon),이정환(Junghwan Lee) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6
The dimension of forged part is different from that of die. Therefore, a more precise die dimension needs to produce the precise part, considering the dimensional changes from forging die to final part.<br/> In this paper, both experimental and FEM analysis are performed to investigate the effect of several features including die dimension at each forging step and heat-treatment on final part accuracy in the closed-die upsetting. The dimension of forged part is checked at each stage as machined die, cold forged, and post-heat-treatment steps. The elastic characteristics and thermal influences on forging stage are analyzed numerically by the DEFORM-2D<SUP>TM</SUP>. The effect of residual stress after heat-treatment on forged part could be considered successfully by using DEFORM-HT<SUP>TM</SUP>.
냉간 단조 스퍼기어의 치수 정밀도 향상을 위한 실험 및 해석 연구
권용철(Yongchul Kwon),이영선(Youngseon Lee),이정환(Junghwan Lee),이춘만(Chunman Lee) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6
The high dimensional accuracy of the cold forged part could be acquired by the accurate dimensional modification for the die, which is, the dimensional changes from the die through forged part to final part after heat treatment were considered. The experimental and FEM analysis are performed to investigate the dimensional changes from the die to final part on cold forged part, comparing with the machined gear. The dimension of forged part is compared with the die dimension at each stage, such as, machined die, cold forged part, and heat-treated-part. The elastic characteristics and thermal influences on forging stage are analyzed numerically by the DEFORM-3D<SUP>TM</SUP>. The analyzed residual stress of forged part is considered into the FE-analysis for heat treatment using the DEFORM-HT<SUP>TM</SUP>. The effects of residual stress affected into the dimensional changes could be investigated by the FEA. Each residual stress of gears was measured practically by laser beam type measurement.
이혜경(Hyaekyung Yi),이건엽(Gunyeop Lee),이성문(Sungmun Lee),이영선(Youngseon Lee),문영훈(Younghoon Moon) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
In this study, the characteristics of aluminum tube hydroformed products at different extrusion type and heat treatment conditions were investigated. For the investigation, as-extruded, full annealed and T6-treated Al 6061 tubes at different extrusion type were prepared. To evaluate the hydroformability, free bulge test was performed at room temperature to 300℃. Also mechanical properties of hydroformed products at various pre- and post-heat treatments were estimated by hexagonal hydro forming test at 250℃. And the tensile test specimens were obtained from hexagonal prototype hydro formed tube. As a results, hydroformability of full annealed tube is 25% higher than that of extruded tube. The tensile strength and elongation were more than 330㎫ and 12%, respectively, when hydroformed 'part was post-To treated after hydroforming of pre- full annealed tube. However, T6 pre treated hydroformed product represents high strength, 330㎫ and low elongation, 8%. Therefore, Hydroformability of AI6061 tube showed similar value for both extrusion types. But flow stress of seam tube showed 20~50㎫ lower value than seamless tube.