Microstructure and Mechanical Properties of Narrow Gap Laser-Arc Hybrid Welded 40 mm Thick Mild Steel
Both laser-arc hybrid welding and narrow gap welding have potential for the fabrication of thick sections, but their combination has been seldom studied. In this research, 40 mm thick mild steel was welded by narrow gap laser-arc hybrid welding. A weld with smooth layer transition, free of visible defects, was obtained by nine passes at a 6 mm width narrow gap. The lower part of the weld has the lowest mechanical properties because of the lowest amount of acicular ferrite, but its ultimate tensile strength and impact absorbing energy is still 49% and 60% higher than those of base metal, respectively. The microhardness deviation of all filler layers along weld thickness direction is no more than 15 HV0.2, indicating that no temper softening appeared during multiple heat cycles. The results provide an alternative technique for improving the efficiency and quality of welding thick sections.
The penetration efficiency of thick plate laser-arc hybrid welding
Double-sided fiber laser-arc hybrid welding was used to join 45 mm thick high strength steel over a wide range of parameters in order to investigate the efficiency of the process. Air gap size, I- and Y-groove type preparation, pulsed and cold metal transfer pulsed arc modes, arc-laser setup, and travel speeds were compared, and in all cases, sufficient filler material was provided to fully fill the gap. The welds were investigated using high speed imaging and cross-sectional analysis to identify penetration depths, morphology, and imperfections. Larger joint air gaps were found to contribute most to weld penetration depth. Surprisingly, increased line energy decreased penetration efficiency in most cases. The laser-arc interdistance was also investigated, revealing an arc size and melt flow dependency for achieving higher penetration depth for a leading arc. It was found that, although penetration can be optimized, solidification cracking can be a limiting factor in the application of deep penetration hybrid welding for thick steel section joining.
Overview of materials research for fusion reactors
Materials research for fusion reactors is overviewed from Japanese, EU and US perspectives. Emphasis is placed on programs and strategies for developing blanket structural materials, and recent highlights in research and development for reduced activation ferritic martensitic steels, vanadium alloys and SiC/SiC composites, and in mechanistic experimental and modeling studies. The common critical issue for the candidate materials is the effect of irradiation with helium production. For the qualification of materials up to the full lifetime of a DEMO and Power Plant reactors, an intense neutron source with relevant fusion neutron spectra is crucial. Elaborate use of the presently available irradiation devices will facilitate efficient and sound materials development within the required time scale.