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A study on the CO2 laser welding characteristics of high strength steel up to 1500 MPa for automotive application

100%

Kim C.-H. , Choi J.-K. , Kang M.-J. , Park Y.-D.

Journal of Achievements in Materials and Manufacturing Engineering

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2010

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tom Vol. 39, nr 1

79-86

EN

Purpose: This paper presents the mechanical and metallurgical characteristics of laser weldments for automotive steels with high strength ranging from 370 MPa to 1500 MPa. Design/methodology/approach: Butt joint welding was conducted on high strength steel sheets by using a CO2 laser with 6 kW output power. For sound welds with full penetration, the proper welding conditions were chosen and the cross-sectional bead shape, tensile strength, hardness profile and micro-structure were evaluated for each case. Findings: Laser welding is known to be a low heat input process because of its high heat density and welding speed. But for laser welding of ultra high strength steel with strength over 780 MPa, micro-structural softening was observed in the HAZ (heat affected zone), resulting from dissolved martensite. Practical implications: The tensile strength reduction in laser welding of ultra high strength steed should be considered in the design of car body structures. The HAZ softening that occurs in butt joint welding can reduce the tensile shear strength for overlap joint welds, which are predominantly mostly used in the BIW (body-in-white) structure. Originality/value: This paper quantitatively demonstrates the occurrence of HAZ softening and a tensile strength reduction in laser welding of automotive steel with 780 MPa strength and more.

2

Position welding using disk laser-GMA hybrid welding

100%

Kim C.-H. , Lim H.-S. , Kim J.-K.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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tom Vol. 28, nr 1

83-86

EN

Purpose: Position welding technology was developed by using disk laser-GMA hybrid welding in this research. Design/methodology/approach: The effect of hybrid welding parameters such as the shielding gas composition and laser-arc interspacing distance were investigated for the bead-on-plate welding. The pipe girth welding was implemented and the adequate arc welding parameters were selected according to the welding position from a flat position to an overhead position. Findings: The optimized shielding gas composition and laser-arc interspacing distance for disk laser-GMA hybrid welding were 80% Ar-20% CO2 and 2mm, respectively for the bead-on-plate welding. The sound welds could be achieved even in the pipe girth welding, but the proper joint shape should be prepared. Research limitations/implications: The laser-arc hybrid welding was implemented for pipe girth welding as a kind of 3-dimensional laser welding and the process parameters could be optimized according to the various target materials and sizes. Practical implications: The optimized process parameters for the disk laser-arc hybrid welding can extend the application of the laser hybrid welding technology. Originality/value: This research showed the possibility of the disk laser-GMA hybrid welding as new pipe girth welding technique. The behaviour of molten pool and droplet transfer could enhance understanding of the hybrid welding.

3

Effect of plant growth regulators, temperature and sucrose on shoot proliferation from the stem discs of Chinese jiaotou (Allium chinese) and in vitro bulblet formation

63%

Xu Z. , Um Y.-C. , Kim C.-H. , Lu G. , Guo D.-P. , Liu H.-L. , Bah A. A. , Mao A.

Acta Physiologiae Plantarum

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2008

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tom 30

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nr 4

EN

In vitro shoot proliferation from stem disc of Allium chinense, a vegetatively propagated plant, was investigated in this experiment. In the present study, shoots were formed directly on stem discs on a medium containing 1 mg/l N⁶-benzyladenine (BA) and 0.5 mg/lα-naphthaleneacetic acid (NAA). These shoots were further cultured on MS media supplemented with various levels of BA in combination with NAA, and new shoot clusters developed easily from the explants cultured despite considerable differences in the induction of shoot clusters with different levels of BA and NAA. The most productive combination of growth regulators proved to be 1.0 mg/l BA and 1.0 mg/l NAA, in which about 17 shoots were produced per cluster in 8 weeks culture. Most of the formed shoots were rooted 15 days after being cultured on MS media supplemented with 0.1–1.0 mg/l NAA. The survival rate of the plantlets under ex vitro conditions was 95% in pots filled with a peat: sand (2:1 v/v) mixture after two weeks. In vitro bulblet formation were strongly promoted by the high temperature of 30℃ compared to that at 25℃, 20℃ and 15℃, and 12% (w/v) sucrose appeared to be optimal for bulblet development. Results from this study demonstrated that A. chinense could be in vitro propagated by using stem discs and in vitro bulblet formation could be achieved.