Existing FEM software cannot simulate the welding stage as the hollow profile with weld planes cannot be simplified as rigidity symmetric planes. In this paper, the metal flowing and die stress in the welding stage during porthole extrusion of a special pipe are analyzed based on Deform⁃3D commercial software and the reverse engineering method. The results show that the metal streams are welded as the extrusion stroke is 33.1 mm. At this moment, the welding temperatures are 477~496 ℃, the hydrostatic pressures in welding chamber are 169~3 463 MPa, which is enough to meet the welding requirements. The maximum stress located at the welding angle of porthole bridge is about 205 MPa, suggesting the die structure is reasonable.%针对采用有限元模拟时焊合面不能设置为刚性对称面的空心型材，其焊合过程不能模拟计算的问题，提出了基于有限元法结合逆向工程技术的焊合面网格重构技术的解决方法，并采用该方法对异形管材分流模挤压的焊合过程进行了模拟分析．研究结果表明：当挤压行程为33．1 mm时完成了焊合，焊合温度范围477～496℃，焊合室内静水压力为169～3463 MPa，满足焊合要求；分流桥的焊合角部位所受应力最大，约为205 MPa．模具结构设计合理．
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