通过分流组合模挤压成形是镁合金空心型材生产的主要方式。在成形过程中，实心坯料被分流孔劈开几股金属流，然后在固态下被挤压焊合，最后从模口流出。挤压中形成的纵向焊缝可能会成为空心挤压型材的薄弱之处，尤其是材料韧性和动力学性能会下降。目前，大多的挤压焊合研究关注的是铝合金型材，由于镁合金与铝合金的晶体结构和成形性能不同，镁合金的热挤压特别是挤压焊合具有自身的复杂特点。因此，本项目将深入揭示镁合金的挤压焊合机理，分析影响挤压焊合的各种因素，控制镁合金空心型材的微观组织，研究镁合金挤压焊合区的动态再结晶、晶粒度变化以及焊缝处材料扩散等。通过研究，拟在一些方面取得突破，包括镁合金挤压焊合的物理模拟方法，焊缝质量评估方法等。建立可靠的适用于镁合金的挤压焊合的准则，并将其应用到空心型材挤压的有限元模拟中，预测挤压焊合质量。

The extrusion through a porthole die is a main forming method for the hollow profile of magnesium alloys. During the process, a solid billet is split into several metal streams. Before flowing through die orifice, these metal streams are re-bonded in the welding chamber of die. Longitudinal weld-seams may negatively influence product quality, especially ductility and dynamic mechanical properties. The researches on extrusion welding of aluminum alloys were focused on, but few investigations on magnesium alloys were conducted. There are some complicated characteristics in extrusion welding of magnesium alloys, which are different from aluminum alloys. The ultimate aim of this research is to control accurately the microstructure of extruded hollow profile of magnesium alloys. We will investigate the mechanism of extrusion welding of magnesium alloys, and analyze influencing factors of weld-seam formation. The microstructure evolution in welding area will be focused on, including the dynamic recrystallization, grain size and diffusion of welded metal. It is expected to make a breakthrough in physical simulation method of extrusion welding, evaluation method of extruded weld seam and mechanical properties tests of weld seam. The criterion of welding integrity will be established and applied in numerical simulation of the profile extrusion processing of magnesium alloy.