| Energy conservation, safety and environmental protection have now become themain themes of the automotive industry. High strength steels (HSS) and ultra highstrength steels (UHSS), especially the dual-phase (DP) steel with goodcrashworthiness and formability are increasingly applied. Cold reduced dual phasesteel sheet which can augment the automobile load capacity and reduce the total costis the first choice for automotive structural parts such as door beam, bumper and otherreinforcements. Laser welding, with its high efficiency, has great potential to be usedfor the advanced HSS. Numerical analysis, as a cost-effective way, has been widelyapplied to investigate the welding thermal process. It has guidance for developing thewelding parameters.In this paper, a three-dimensional transient finite element model is developedusing FLUENT to study the molten pool formation and temperature field in thecontinuous laser welding of DP1000steel. A Rotary-Gauss body heat source isemployed in the study to model the laser beam. A series of welding experiments havebeen performed, and a good agreement has been acquired between calculated welddimensions and experimental results. The condition of heat dissipation resulted fromthe bottom copper plate has a great effect on temperature evolution during continuouslaser welding for DP1000steel; it accelerates the cooling of molten pool and affectsthe shape and size of the weld. Besides, it is observed that the molten pool flow isdriven by buoyancy caused by gravity difference, and the order of quantity of themaximum flow velocity is mm/s.The vickers hardness values across the welded joint are measured, the lowesthardness value of the Heat Affect Zone (HAZ) is70HV0.2less than the base metal.The paper investigates HAZ softening and temperature time history of different pointsthrough using APDL language of the software ANSYS. The HAZ is divided intooverheated zone and soft zone, and the soft zone experienced the maximumtemperature ranges from600℃to1000℃. The decreae of the amout of martensiteand the increase of the amout of bainite reduces the hardness of soft zone, and thatdeteriorates the strength of welded joint. |
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