7N01铝合金电子束定点焊熔池输运现象预测

Prediction of transport phenomena of 7N01 aluminum alloy electron beamspot welding pool

摘要:

为有效改善真空电子束焊焊缝成形并抑制成形缺陷产生,本文以12 mm厚7N01铝合金为研究对象,基于CFD软件 Ansys Fluent深入分析了电子束定点焊匙孔钻取过程及熔池传热和流体输运现象并进行了实验验证.为真实反映电子束流能量密度空间分布特点,建立了考虑束流活性区特征的自适应热源模型并采用VOF算法对气液界面进行实时追踪.数值分析结果表明,束流能量密度分布及与瞬态熔池/匙孔之间的耦合作用是直接决定焊缝成形良好与否的关键.当束流处于下聚焦模式时,特有的能量分布形式及其诱导的等离子体保温作用、金属蒸汽反冲压力、Marangoni流以及热浮力的耦合向上输运作用(最大流体速度为15 m/s左右),导致最终焊缝余高的形成及钉头区域扩展.由于深度方向上束流能量密度先增加后大幅降低,导致熔深熔宽的增加速度出现类似的演变规律,并且在匙孔底部可能诱发钉尖缺陷的产生.此外,研究还发现,随着匙孔深度增加,能量波动、金属蒸汽反冲压力与表面张力竞争作用逐渐加剧,促使匙孔钻取过程具有周期性.

Abstract:

To effectively improve weld forming and inhibit forming defects, taking 12 mm-thick 7N01 aluminum alloy as research object, the keyhole drilling process as well as the heat and fluid transport phenomena of electron beam spot welding (EBSW) were analyzed by Ansys Fluent and validated by experiments. In order to reflect the beam energy density distribution, a self-adaptive heat source model considering the characteristics of the active zone of beam was established, and VOF algorithm was used to track the gas-liquid interface in real time. Numerical analysis results show that the beam energy density distribution and the coupling between the beam and the transient pool/keyhole were the key factors to determine weld forming. When the beam was in lower focus mode, the unique energy distribution and the induced plasma insulation, metal vapor recoil pressure, Marangoni flow, and upward transportations of thermal buoyancy (maximum flow rate at about 15 m/s) led to the weld reinforcement and the extension of nailhead area. The beam energy density in the depth direction increased first and then decreased significantly, which resulted in similar evolutions for the welding in depth and width directions and might induce the spiking defects adjacent to the keyhole bottom. In addition, it was found that with the increase in keyhole depth, the energy fluctuation as well as the competition between recoil pressure and surface tension gradually increased, which promoted the periodicity of keyhole drilling process.