| Welding assisted by magnetic field is a new technology which has been improved in recent years. Liquid metal in molten pool can be stirred by magnetic field, which can reduce cracks and the inhomogeneity of chemical element in welds, meanwhile the microstructure of welds can be controlled. Arc welding and CO2 laser welding under magnetic field was researched by some foreigner and domestic scholars, while studies about magnetic field assisting fiber laser welding were rarely reported. In this paper, the influence of axial or transverse magnetic field on laser welding and laser hot wire welding technology was researched using aluminum alloy and stainless steel by putting a magnet next to specimen.The seam appearance morphology, joint profile and microstructure were studied when adding different magnetic field. It was found that there was no obvious influence on stainless steel welding seam when the magnetic flux density was less than 100 mT. When magnetic flux density increased to 380 mT, the weld profile of partial penetration mode varied from wide and shallow shape to narrow and deep shape, and the weld width increased by 19.7%. In addition, the weld profile of full penetration model changed from funnel to X type, meanwhile, the bottom weld width increased by 40%. It was also found the polarity and direction of magnetic field had no significant impacts on the process or weld joint of laser welding. Partial melted zone would exist in weld of stainless steel with magnetic field, which was composed of block austenite. The number of “crystal whisker” in stainless steel joint rose when applied with magnetic field, at the same time the fusion line of joint would be blurry.The impacts of magnetic field on laser hot wire welding was researched, and it revealed the mechanism of seam appearance and weld profile changing with different magnetic field. The results displayed that the performance of weld seam were improved using a magnetic field. In addition, the shape of cross section altered with the changing of direction and polarity, and the width inclined to different side when magnetic field transformed. Under axial magnetic field, the fusion line would also be ambiguous while the amount of cellular crystal was larger in stainless steel joint. The flow of liquid metal was affected by electromagnetic driving force and ampere force, the former drove liquid metal flowing to bottom of molten pool. Ampere force controlled the shape of cross section, the density and direction of ampere force was affected by magnetic field and direct current. The influence of ampere force was more significant on molten pool of aluminum alloy than on stainless steel when applied same magnetic field and current.The molten pool was analyzed by means of high-speed photography, which showed the coupling theory of magnetic field and fiber laser. The results reflected that magnetic flux density has significant impact while the polarity and direction of magnetic have no effect on molten pool in laser welding. Liquid aluminum alloy were stirred violently, with the area of molten pool increasing under 280 mT magnetic field. The area of stainless steel pool was smaller, what's more, the process were more stable when add a magnet. The influence of magnetic field on molten pool was more apparent with large heat input. The flow of liquid stainless steel was altered by Lorenz force caused by magnetic field. In stainless steel molten pool, the backwards flow of liquid metal near to keyhole was hindered, which leading to heat focusing in front of molten pool. Besides, the Lorenz force in molten pool of aluminum was strong enough to making liquid metal flowing violently. |
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