【作者】 李笑雨；

【导师】 王立君；

【作者基本信息】 天津大学 ， 材料加工工程， 2014， 硕士

【摘要】 节能、安全和环保是当今世界汽车发展的主题。高强钢和超高强钢，特别是兼顾良好碰撞吸能和成形性能的双相（DP）钢的应用日益增加。这类钢既能保证车身的安全性能，又能显著降低车身的重量，如今已经成为汽车生产商广泛使用的特殊材料。激光焊接能够保持高强钢较高的强度，防止较大的变形，因此激光焊接在高强钢中的应用日益广泛。数值模拟与仿真在焊接过程的研究中发挥着不可替代的作用，可以缩短实验周期，降低实验成本，提高效率，对焊接工艺的制定与分析有一定的理论指导意义。本文采用计算流体动力学软件FLUENT对DP1000薄板激光焊接熔池温度场与流场的分布规律和影响因素进行了数值分析，采用旋转高斯曲面体模拟激光热输入，模拟计算的熔池形貌与实际焊缝吻合良好。模拟中特别研究了所用夹具的铜基座对温度场及熔池形态的影响。结果表明，铜基座对焊件有较强的热扩散作用，会降低熔池的最高温度，减小焊缝尺寸。另外，研究发现，DP1000钢薄板焊接的熔池流动主要由重力差异引起的浮力驱动，熔池最大流动速度的数量级为mm/s。对DP1000薄板焊接接头进行维氏硬度测试，热影响区硬度最多比母材下降约70HV0.2。为了进一步分析其热影响区明显的软化现象，应用ANSYS的APDL语言对激光焊接过程进行了数值模拟，得到距焊缝中心不同距离点的温度-时间变化历程。若将热影响区划分为过热区与软化区，软化区所经历的温度循环范围在600℃到1000℃之间，软化区组织中马氏体减少，析出部分贝氏体，使得此区域硬度值降低，降低了焊接接头强度。更多还原

【Abstract】 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.更多还原