Ildong Choi

The effect of retained austenite stability on high speed deformation behavior of TRIP steels

The safety of passengers is important during an automobile collision. Because a collision event involves high speed deformation, it is necessary to develop property data and understand the applicable deformation mechanisms to aid in the selection of proper materials for crash-related automotive components. Therefore, dynamic mechanical properties of low carbon TRIP steels with varying retained austenite stabilities were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 102 to 6×102 s1 using standard ASTM E-8 specimens with an elastic strain gage attached to the sample grip end to measure load, and a plastic strain gage mounted onto the gage section to measure strain. Ultimate tensile strengths (UTS), strain rate sensitivities, and strain hardening behaviors are reported. TRIP steel with high stability retained austenite exhibited higher yield stress, lower UTS and lower strain hardening than TRIP steel with low stability retained austenite.

Study on Coating Melting Behavior on Weld Growth Mechanism for Al-Si coated Hot-Stamped Boron Steels in Resistance Spot Welding

The effect of coating melting behavior on nugget growth during resistance spot welding of hotstamped boron steels was evaluated. A comparative analysis of the dynamic resistance for bare hot stamped and Al-Si coated hot stamped boron steels was carried out. The dynamic resistance curve of Al-Si coated hot stamped boron steel was higher than bare hot stamped boron steel during the whole stages of weld time. Higher α-peak and β-peak for Al-Si coated hot stamped boron steel resulted in larger nugget size at higher than 4.0 kA weld current owing to non-linear layer structure of alloyed coating, which appears to restrict the current passage through the faying interface. In the case of Al-Si coated hot-stamped boron steel, most of the Al-Fe intermetallic coatings are expected to be pushed away while in the liquid state at the faying interface. The pressure build up during the early stage of weld pushes the liquid Al-Fe intermetallic coating towards the edge of the faying interface. The continuous liquid state of Al-Fe intermetallic through the faying interface ensures the large contact area for the current passage to enlarge the nugget diameter. †(Received June 1, 2014)

Effect of the Strain Rate on the Tensile Properties of the AZ31 Magnesium Alloy

정승훈 ・ 박지연 ・ 최일동* ・ 박성혁 한국해양대학교 대학원 재료공학과 한국기계연구원 부설 재료연구소 Effect of the Strain Rate on the Tensile Properties of the AZ31 Magnesium Alloy Seunghun Jeong, Jiyoun Park, Ildong Choi*, and Sung Hyuk Park Department of Materials Engineering, Korea Maritime University, Busan 606-791, Korea Korea Institute of Materials Science, Changwon 641-831, Korea Abstract: The effect of the strain rate at a range of 10‒4 ~ 3 × 10 s‒1 on the tensile characteristics of a rolled AZ31 magnesium alloy was studied. The normal tensile specimens were tested using a high rate hydraulic testing machine. Specimens were machined from four sheets with different thicknesses, 1, 1.5, 2 and 3 mm, along three directions, 0 , 45 , and 90 to the rolling direction. The results revealed that all the specimens had a positive strain rate sensitivity of strength, that is, the strength increased with increasing strain rate. This is the same tendency as other automotive steels have. Our results suggest that the AZ31 magnesium alloy has better collision characteristics at high strain rates because of improved strength with an increasing strain rate. Ductility decreased with an increasing strain rate with a strain rate under 1 s‒1, but it increased with an increasing strain rate over 1 s‒1. The mechanical properties of the AZ31 magnesium alloy depend on the different microstructures according to the thickness. Two and 3 mm thickness specimens with a coarse and non-uniform grain structure exhibited worse mechanical properties while the 1.5 mm thickness specimens with a fine and uniform grain structure had better mechanical properties. Specimens machined at 0 and 45 to the rolling direction had higher absorbed energy than that of the 90 specimen. Thus, we demonstrate it is necessary to choose materials with proper thickness and machining direction for use in automotive applications. †(Received December 28, 2012)

Microstructure and Mechanical Properties of Reaction Squeeze Cast Hybrid Al Matrix Composites

Hybrid Al matrix composites reinforced with Kaowool ceramic fiber (Al2O3 · SiO2) and reaction formed intermetallic compound particles, starting compositions of (10%Al2O3 · SiO2 + 5%Ni)/Al and (10%Al2O3 · SiO2 + 5%TiO2)/Al were fabricated employing squeeze casting technique. Microstructures of the Al composites were examined, and the reactivity of Ni and TiO2 powders added to Kaowool short fiber preform with infiltrating molten Al, resulting products of Al3Ni and Al3Ti, during squeeze casting was analyzed. Hardness, strength (at 25 and 300°C) and wear resistance of the composites were characterized. To investigate the fracture behavior of the composites, in-situ fracture test was performed within SEM. The hybrid Al composites revealed the microstructure of uniformly distributed reinforcements (ceramic short fiber and reaction formed intermetallic compounds). Ni powder added hybrid Al composite achieved the highest hardness, strength and wear resistance among the composites. It was found that the strength drop at elevated temperature (300°C) is effectively reduced by hybridization with ceramic fiber and intermetallic compounds reinforced in Al. In-situ fracture observation demonstrated the fracture behavior of Ni powder added hybrid Al composite as microcrack initiated mainly by short fiber/ matrix interfacial debonding and the crack propagated through the cluster of intermetallic compound particles as loading is raised.

Effect of Chemical Composition of Nut Material on the Fracture Behavior in Nut Projection Welding of Hot-Stamped Steel Sheet

Abstract The use of materials for modern lightweight auto-bodies is becoming more complex than hitherto assemblies.The high strength materials nowadays frequently used for more specific fields such as the front and rear sub frames, seat belts and seats are mounted to the assembled body structure using bolt joints. It is desirable to use nuts attached to the assembled sheets by projection welding to decrease the number of loose parts which improves the quality. In this study, nut projection welding was carried out between a nut of both boron steel and carbon steel and ultra-high strength hot-stamped steel sheets. Then, the joints were characterized by optical and scanning electron microscope. The mechanical properties of the joints were evaluated by microhardness measurements and pullout tests. An indigenously designed sample fixture set-up was used for the pull-out tests to induce a tensile load in the weld. The fractography analysis revealed the dominant interfacial fracture between boron steel nut weld which is related to the shrinkage cavity and small size fusion zone. A non-interfacial fracture was observed in carbon steel nut weld, the lower hardnessof HAZ caused the initiation of failure and allowed the pull-out failure which have higher in tensile strengthsand superior weldability. Hence, the fracture load and failure mode characteristics can be considered as an indication of the weldability of materials in nut projection welding.Key Words : Nut projection welding, Hot stamped steel, Carbon steel nut, Boron steel nut, Failure mode, Shrinkage cavityISSN 2466-2232Online ISSN 2466-2100

A study on tensile shear characteristics for weld-bonded 1.2GPa grade TRIP steels with changes in nugget diameter for automotive body application

High strength steels have been continually being developed to improve in fuel economy in automotive and ensure safety of passengers. New bonding and welding methods have been required for improving weldability on high strength steels. In this study, resistance spot welding and Weld-bond with nugget diameters of 4.0mm, 5.0mm, 6.0mm and 7.0mm were produced and tested, respectively. In order to confirm the effect of nugget diameters on tensile shear characteristic of the Weld-bond, tensile shear characteristics of Weld-bond were compared with those of resistance spot welding and adhesive bonding. Peak load of Weld-bond were increased as the nugget diameter increases. After appearing maximum peak load continuous fracture followed with second peak owing to load being carried by resistance spot weldment. Fracture modes of the adhesive layer in Weld-bond fractures were represented by mixed fracture mode, which are cohesive failure on adhesive part and button failure at resistance spot welds. The results showed that the tensile shear properties can be improved by applying Weld-bond on TRIP steel, and more apparent with nugget diameter higher than 5t.

Effect of Fine Copper Sulfides on the High Cycle Fatigue Properties of Bake Hardening Steels for Automotive

Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5 MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45 MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth. (Received July 14, 2010)

Effects of microstructure and alloy contents on the lüders line formation in Al-Mg alloys

There have been several efforts to replace steel with aluminum for automobile weight reduction. Main efforts have been on the development of 5000 series Al alloys (Al-Mg alloys) because of their good formability. However, Al-Mg alloys suffer from the formation of various types of Lueders line. Particularly, type A Lueders lines formed under 1% elongation make a surface completely unacceptable for outer auto body panels. There are various nomenclatures of Lueders line according to researchers. Thompson`s classification will be used in this study because it classifies Lueders lines by surface appearance and shape of stress-strain curve. The effect of grain size and microstructure on the formation of type A Lueders lines in Al-Mg alloys has been investigated in this study. Additionally, the effect of Mg content on the mechanical properties and Lueders line formation in Al-Mg alloys has also been evaluated.

A Study on Tensile Shear Characteristics of Dissimilar Joining Between Pre-coated Automotive Metal Sheets and Galvanized Steels with the Self-Piercing Rivet and Hybrid Joining

*Dept. of Advanced Materials & Industrial and Management Engineering, Dong-Eui University, Busan 614-714, Korea **Dept. of Material Science and Engineering, Pusan National University, Busan 609-735, Korea ***Dept. of Materials Engineering, Korea Maritime and Ocean University, Busan 606-791, Korea ****Transportation & Machinery System Center, KITECH, Busan 618-270, Korea *****Advanced Welding & Joining R&BD Group, KITECH, Incheon 406-840, Korea ******Dept. of Advanced Materials Engineering, Dong-Eui University, Busan 614-714, Korea

Modeling the Hall-Petch Relation of Ni-Base Polycrystalline Superalloys Using Strain-Gradient Crystal Plasticity Finite Element Method

Abstract A strain-gradient crystal plasticity constitutive model was developed in order to predict the Hall-Petch behavior ofa Ni-base polycrystalline superalloy. The constitutive model involves statistically stored dislocation and geometrically necessarydislocation densities, which were incorporated into the Bailey-Hirsch type flow stress equation with six strength interactioncoefficients. A strain-gradient term (called slip-system lattice incompatibility) developed by Acharya was used to calculate thegeometrically necessary dislocation density. The description of Kocks-Argon-Ashby type thermally activated strain rate was alsoused to represent the shear rate of an individual slip system. The constitutive model was implemented in a user materialsubroutine for crystal plasticity finite element method simulations. The grain size dependence of the flow stress (viz., the Hall-Petch behavior) was predicted for a Ni-base polycrystalline superalloy NIMONIC PE16. Simulation results showed that thepresent constitutive model fairly reasonably predicts 0.2 %-offset yield stresses in a limited range of the grain size.Key wordscrystal plasticity, strain gradient plasticity, finite element method, hall-petch relation, polycrystal.