Minjung Kang

Weld strength of laser-welded hot-press-forming steel

There is an increasing demand for ultrahigh-strength steel in automotive industries to facilitate the manufacture of lightweight vehicles without compromising their security features. Although transformation-induced plasticity and dual phase steels have strengths of under 1 GPa, boron-alloyed steel obtained after the hot-press process has a strength of over 1500 MPa. Al–Si-coated steel has been developed to prevent excessive oxidation during high-temperature processes, but the Al–Si coating is known to lower weld properties. In this research, the laser weldability of hot-press-forming steels with and without Al–Si coating was examined. The specimens were laser-welded in butt and overlap joints by using disk and fiber lasers. The influence of the process parameters on the weld strength was investigated.

Analysis of Laser and Resistance Spot Weldments on Press-Hardened Steel

For the manufacture of safe, lightweight vehicles, the demand for ultra-high-strength steel in the automotive industry is increasing. Although transformation-induced plasticity (TRIP) and dual-phase (DP) steels have a strength of under 1 GPa, boron-alloyed steel produced using the hot press forming process has a strength of more than 1500 MPa. Laser and resistance spot welding processes are used to join press-hardened steel, but the characteristics of the resulting weldments are not yet fully understood. In this study, the thermal cycles for both welding processes were investigated using finite element (FE) analysis. Resistance spot welding was analyzed using a combination of thermal, electric, and mechanical models, whereas the thermal behavior of laser welding was predicted using only a thermal model. The calculated bead shapes were compared with experimentally measured ones to validate the simulation models. The mechanical and metallurgical characteristics of the weldments were explained using the thermal history of each welding process.

Laser welding for hot-stamped tailor-welded blanks with high-strength steel/high-energy absorption steel

In this research, the laser weldability for tailor-welded blanks (TWBs) made of dissimilar hot-stamping steels was examined. Usibor and Ductibor from ArcelorMittal were combined, since they are steels that possess a high-strength and high-energy absorption, respectively. Additionally, combination of Usibor and SPRC440 was investigated for comparison. After laser butt welding, the specimens were heat-treated at high temperatures, and the subsequent quenching conditions were similar to those in the hot-stamping process. The mechanical and metallurgical characteristics of the welds were then analyzed. The TWBs with Usibor and Ductibor exhibited a high degree of elongation after hot-stamping but those with Usibor and SPRC440 showed a relatively low elongation. The AlSi coating on the base metal was included in the fusion zone, and it led to ferrite formation. However, in the static and high-speed tensile tests, the AlSi coating did not affect the tensile strength of the TWBs, and all tensile specimens were fr...

Microstructure and mechanical properties of friction stir welded and laser welded high entropy alloy CrMnFeCoNi

The high entropy alloy CrMnFeCoNi has been shown to have promising structural properties. For a new alloy to be used in a structural application it should be weldable. In the present study, friction stir welding (FSW) and laser welding (LW) techniques were used to butt weld thin plates of CrMnFeCoNi. The microstructure, chemical homogeneity and mechanical behavior of the welds were characterized and compared with the base metal. The tensile stress-strain behavior of the welded specimens were reasonable when compared with that of the base metal. FSW refined the grain size in the weld region by a factor of ∼14 when compared with the base metal. High-angle annular dark field transmission electron microscopy in combination with energy dispersive X-ray spectroscopy showed chemical inhomogeneity between dendritic and interdendritic regions in the fusion zone of LW. Large fluctuations in composition (up to 15 at%) did not change the crystal structure in the fusion zone. Hardness measurements were carried out in the weld cross section and discussed in view of the grain size, low angle grain boundaries and twin boundaries in FSW specimens and the dendritic microstructure in LW specimens.

Laser Welding Characteristics of Ultra High Strength Steel for Automotive Application

자동차산업에서는 환경문제와 국제유가 상승으로 인한 차량경량화에 대한 요구가 부각되면서 알루미늄합금, 특수 강화 플라스틱과 같은 경량소재의 사용이 증가하고 있지만 아직 철강소재를 대체하기까지 가격, 강도 등 해결해야 할 문제점을 많이 가지고 있다. 또한, 운전자를 보호하기 위한 충돌 안정성 강화 등이 강력하게 요구되고 있어 강도와 성형성이 좋은 UHSS (Ultra High Strength Steel)를 사용하여 차량을 경량화하면서도 충분한 강도를 확보하려 하고 있다. UHSS는 일반적으로 600MPa를 상회하는 인장강도를 가지는 강재이며, DP, TRIP, CP, Martensitic, Boron 강 등이 개발되어 있다. 위와 같은 UHSS는 알루미늄합금이나 복합재료와 비교해 지난 십년간 가장 빠른 추세로 성장하였고 향후에도 사용이 꾸준히 증가할 것으로 예상된다

Porosity Reduction during Gas Tungsten Arc-Gas Metal Arc Hybrid Welding of Zinc Coated Steel Sheets (I) - Effect of Preceding Gas Tungsten Arc

The Zn coating on automotive galvanized steel sheets can improve corrosion resistance. However, the boiling temperature of Zn is lower than the melting temperature of steel and it causes well-known spatter and porosity problem. One of most prominent solutions is a pretreatment of Zn coating by an additional welding arc prior to the main welding process. In this research, GTA and GMA are selected as heat sources for pretreatment and main welding processes, respectively. The authors suggested three possible mechanisms to reduce weld defects by the GTA pretreatment: (1) Formation of gap between the sheets; (2) Evaporation of Zn layer; (3) Oxidation of Zn layer. Among them, Zn Oxidation is the most important mechanism to reduce weld defects in the GTA-GMA hybrid process.

Cold-Metal-Transfer Arc Joining of Al 6K32 Alloy to Steel Sheets

In the arc joining of dissimilar metals, such as steel and Al alloys, a brittle intermetallic phase is formed at the interface. The thickness of this phase should be minimized because it causes solidification cracking. Cold metal transfer (CMT) arc welding technology is widely used throughout the automotive industry because of low spatter formation and heat input. A CMT arc was used to investigate arc joining between the Al 6K32 alloy and mild steel sheets. The behaviors of the interface layers were analyzed on the basis of filler wires and the coating type of the steel sheets. The thickness of the intermetallic phase was maintained under 10 μm and increased joint strength was achieved by selecting the appropriate filler wire and coating type.

Applicability Study of 2-pass Laser Welding on Galvanized Steel Sheets

During laser overlap welding of galvanized steel sheets, explosion of weld pool by the high pressure zinc vapor induces weld defects like porosity and blowhole. In this study, laser 2-pass welding was implemented to prevent the weld defects. Through the 1st pass welding, zinc layers on the faying surfaces were removed when proper heat input was applied. Excessive heat input could result in explosion even during the 1st pass welding and insufficient heat input could not remove enough region of zinc layer for the 2nd pass welding. Coating weights of and were considered and for both cases sound welds without weld defects could be achieved. In spite of 2-pass welding, softening of weld and heat affected zone was not observed and Zn coating was not diluted into the weld metal.

Effect of CaO Content on Yb: YAG Disk Laser Weldability of AZ31 Mg Alloy

Magnesium alloys have recently attracted increased attention as structural materials owing to their low specific gravity. However, they must typically be casted, formed, melted, and even heated under a protective shielding gas to prevent them from oxidizing and igniting. So-called “ECO-Mg” alloys have recently been developed by adding CaO to conventional Mg alloys. ECO-Mg alloys exhibit higher oxidation resistance during melting and superior castability, even when beryllium is not added or toxic protective gasses such as SF6 are not used. We laser-welded AZ31 Mg alloys with various CaO contents and examined the mechanical properties of the welds. Increasing the CaO content in the AZ31 increased the burning resistance of the ECO-Mg alloys, thereby suppressing plume generation during laser welding; thus the ECO-Mg alloy welds could be fully penetrated faster than conventional Mg alloy welds. The weld strength also increased with increasing CaO content in the AZ31 because the CaO refined the grains.

Corrosion Assessment of Al/Fe Dissimilar Metal Joint

The use of light-weight Al alloys in the automotive industry is increasing to meet requirements for fuel efficiency and emission reduction. Joining Al alloy to the conventional steel sheet is also very important issue with the increased use of Al alloy, and several joining processes have been introduced to enhance joining strength between dissimilar metals. This paper deals with a galvanic corrosion in the dissimilar metal joining. Salt spray tests up to 2000 hours were conducted on a self-piercing rivet, spot welded, adhesive bonded and weld-bonded joints, and cross-sections and tensile shear strength according the salt spray duration were analyzed at every 500-hour. Self-piercing rivet joint had relative low initial strength but the joint strength did not change regardless of the salt spray duration. The strength of other joints (spot welded, adhesive bonded and weld-bonded joints) decreased with the increase of salt spray duration and the corrosion behaviour of each joint was discussed.