变极性频率对铝合金变极性等离子弧穿孔立焊熔池稳定性的影响机理

Impacting Mechanism of Variable Polarity Frequency on Weld Pool Stability in Variable Polarity Plasma Arc Keyhole Vertical Welding of Aluminum Alloy

为研究变极性频率(VPF)对变极性等离子弧焊熔池稳定性的影响机理,采用变极性有限元模型分析了小孔熔池的热力耦合过程。该模型基于计算流体力学(CFD)技术,能够实现变极性电弧对熔池热-力作用的周期性变化。此外,为了更准确地表达小孔边界上热量和力分布沿小孔深度的变化,还考虑了小孔的二次压缩效应对热流和电弧压力的影响。比较了不同VPFs条件下小孔边界的热-力振荡强度和熔融桥在临界穿透时的受力平衡情况。结果表明,在小孔形成过程中,随着VPFs的增大,小孔边界的电弧压力、热流密度和流动速度的“振荡强度”减小。另外,不同的热-力振荡条件引起熔池温度场的变化,进而导致熔池表面张力的变化,改变了熔融桥在穿透时的受力平衡条件,影响穿孔熔池的稳定性。当EP相电流占空比为1/5,VPFs大于33,小于83时,匙孔边界的热-力“振荡强度”较弱,熔融桥在临界熔透状态下能保持力平衡,熔池稳定。当VPFs小于33时,“振荡强度”较强,并且熔融桥的受力平衡被打破,熔池趋于被切割。当VPFs大于83时,虽然此时熔融桥“振荡强度”较弱,但因熔融桥受力平衡被打破,熔池处于不稳定状态。最后,通过熔合线形状、小孔穿透时间和焊缝背面小孔尺寸的比较,验证了所建立模型的准确性。

To study the impacting mechanism of variable polarity frequency(VPF)on the weld pool stability in variable polarity plasma arc welding(VPPAW),the thermal-mechanical coupling process of the keyhole weld pool was analyzed by a variable-polarity finite element model.The model was developed based on the computational fluid dynamics(CFD),which can realize the periodic variation of thermal and mechanical effects of variable polarity arc on the weld pool.Moreover,in order to more accurately express the heat and force distribution on the keyhole boundary along the keyhole depth,the secondary compression effect of the keyhole on heat flux and arc pressure was taken into account.The thermal-mechanical“oscillations intensity”on the keyhole boundary and the force balance of the molten bridge were compared at different VPFs.The results show that the“oscillation intensity”of the arc pressure,heat flux and the flow velocity on the keyhole boundary decreases with increasing the VPFs during the keyhole formation.In addition,the variation of the temperature field in the weld pool caused by different thermal-mechanical oscillations results in the change of surface tension and the force balance condition of the molten bridge is changed,which affects the keyhole weld pool stability.When the VPFs are more than 33 and less than 83 and the duty radio of current in EP phase is 1/5,the thermal-mechanical“oscillations intensity”on the keyhole boundary is weak,and the molten bridge can stay force balance in the critical state of being penetrated,so the weld pool can maintain stable.When the VPFs are less than 33,the“oscillations intensity”is strong and the force balance of the molten bridge is broken,and the weld tends to be cut.If the VPFs are more than 83,the weld pool is unstable due to the broken force balance of the molten bridge although the“oscillations intensity”is weak.Finally,the accuracy of the developed model was verified by the comparison of the fusion zone,the keyhole penetration time and keyhole dimensions on the backside of the weld.