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.
Analysis of mechanisms of underfill in full penetration laser welding of thick stainless steel with a 10 kW fiber laser
Abstract With the aim to explore the formation mechanisms of surface underfill, full penetration laser welding of thick stainless steel was conducted, with the use of a 10 kW fiber laser. A modified “sandwich” specimen was used, so as to directly observe the dynamic behaviors of the keyhole, vapor plume, and melt pool with the formation of underfills. On the basis of the experimental investigations, the formation mechanisms of the underfills at the top surface and bottom surface were analyzed. The results show that the downward flow of the molten metal caused by the recoil momentum is a crucial driver for formation of the underfill on the top surface. At full penetration of the melt, a deep underfill with a periodic wide–narrow–wide serrated pattern is formed on the top surface of the weld owing to the periodic fluctuation of the rear keyhole wall. At full penetration of the keyhole, the formation of a deep underfill on the top surface of the weld and undercut on the bottom surface of the weld is presented with massive direct melt loss from the weld pool.
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.