Showing papers by "Ming Gao published in 2020"

Effects of circular oscillating beam on heat transfer and melt flow of laser melting pool

Abstract: Circular oscillating laser welding can improve welding quality, but its thermodynamics of the melt pool is not clear. In this study, the basic mechanism during circular oscillating laser welding process was studied by computational fluid dynamics method. The calculation model was verified by experimental results. The results show that the circular oscillating laser reduced the peak temperature and temperature gradient and formed a new vortex in the melt pool. The ascending oscillation frequency and amplitude reduced the temperature gradient and increased the velocity of melt flow significantly. Related mechanisms were revealed at the end. This study proves that circular oscillating laser provides a new way to optimize the laser melting process and have great application values for welding and additive manufactering.

Laser-arc hybrid additive manufacturing of stainless steel with beam oscillation

Abstract: A novel additive manufacturing approach integrating an oscillating laser beam and a cold metal transfer arc was developed to balance the surface accuracy, deposition efficiency, and mechanical properties of the deposited parts. The new method was termed as oscillating laser-arc hybrid additive manufacturing (O-LHAM). The sample properties of the wire-arc additive manufacturing (WAAM), laser-arc hybrid additive manufacturing (LHAM), and O-LHAM processes were compared. It was found that some new phenomena were induced by beam oscillation. First, both the surface roughness and minimum processing margin of the O-LHAM sample were reduced to 20 % of the WAAM sample, because the droplet transfer was stabilized by the laser-arc synergic effects. Second, the grains were refined, and the {001} -cube texture content was decreased to 1.6 %, as the oscillation induced a strong stirring effect on the molten pool. The nondestructive X-ray test suggested that the visible porosity within the O-LHAM sample was suppressed by beam oscillation when the periodically oscillated laser keyhole could “capture” the bubbles, while the porosity within the LHAM sample reached 24 %. Due to the microstructure changes and the porosity suppression, the O-LHAM almost eliminated the anisotropy of tensile strength and improved the elongation by up to 34 %.

Influence of welding angle on the weld morphology and porosity in laser-arc hybrid welding of AA2219 aluminum alloy

Abstract: The influence of welding angle on weld morphologies and porosity characteristics was studied in laser-arc hybrid welding of AA2219 aluminum (Al) alloy. The upward sloping and vertical weld (USVW) was performed better than the downward sloping and vertical weld (DSVW). The high-speed camera was used to observe the welding process. It is found that the welding process becomes stable when the transitional position of the droplet was 2 mm away from the keyhole. By analyzing the force act on keyhole and characteristics of the pool, it is found that the weld morphologies and porosity characteristics were associated with gravity-laser angle (β), which affects the keyhole stability and characteristics of the pool. The results showed that the flow velocity was accelerated when the β increased, which was a benefit for decreasing porosity. This is because the gravity in the direction of the welding accelerates the pool flow. Due to the increasing of the β, the static pressure and the vapor pressure in the keyhole were increased, which improved the stability of the keyhole and decreased the porosity.

Magnetic Interconnected Macroporous Imprinted Foams for Selective Recognition and Adsorptive Removal of Phenolic Pollution from Water

Abstract: Here, novel magnetic imprinted porous foams (MIFs) were prepared by Pickering oil-in-water high internal phase emulsion polymerization, using renewable cellulose nanocrystals (CNCs) derived from cotton source as the stabilizer, and surfactant Tween 85 and iron oxide magnetic nanoparticles as additive in aqueous phase, for selective adsorption of 4-nitrophenol (4-NP). Interconnected porous foam structure was formed and optimized by controlling the addition amount of solid stabilizer CNCs, Tween 85, and oil phase volume, the optimum value of which was 7.5 wt%, 5.0 wt%, and 85 %, respectively. The as-synthesized imprinted material had the good thermal stability and magnetic responsivity. The adsorption equilibrium data was fitted well by the Langmuir isothermal model with a maximum adsorption capacity of 287.5 µmol/g. Pseudo-second-order kinetics model could better describe the kinetics data. MIFs showed excellent selective ability for 4-NP as compared with these structural analogues. Besides, Thomas model explained the dynamic breakthrough curves better. The regeneration ability of MIFs was also satisfying after several reuse.

Cold rolling performance for austenitic stainless steel with equilibrium and non-equilibrium microstructures

Abstract: Aiming to provide effective theoretical guidance for continuous rolling, the cold rolling with reduction of 45% was carried out for the equilibrium and non-equilibrium microstructures of austenitic stainless steel (ASS). The microstructural evolution, texture components and tensile properties were investigated. The results showed that the fraction of martensite transformed from austenite is 21.1% for the equilibrium base metal (BM), while that for the non-equilibrium weld metal (WM) achieves 32.0%. Whether employing rolling or not, the fraction of misorientation larger than 15° within the BM is much higher than that of the WM because of the large amount of twins. Moreover, the texture of the rolled BM is mainly composed of Goss component with small amount of S and Brass, while that of the WM is composed of mainly S and small amount of Brass, with formation of Copper component. The tensile strength of the rolled BM and WM reaches approximately 1300 MPa, the elongation rate of the BM decreases from 53% to 5.7%, while that of the WM decreases from 32% to 3.5%. The microstructure evolutions of the BM and WM during rolling were compared, and the microstructure-mechanical properties relation was established and discussed based on martensitic nucleation.