A detailed analysis of metal flow into

a porthole die to produce a square aluminum alloy tube by means of three-dimensional FE simulation was performed.An insight into the metal flow rules and the

longitudinal weld seam formation were orchestrated subsequently.Corresponding to the variable-section-porthole of die,the process includes five distinctive stages of upsetting,

dividing,filling the clearance of portholes,solid welding and steady extruding.

The commonly observed defects on the extrudate surfaces are due to the entrapped air between porthole and work-piece during the stage of filing the clearance of portholes.The results also show, that solid welding first starts at the mid-height of welding room while the upper part is the last as a result of friction ,which also makes little air entrapped under the bridge in the upper part of the welding room causing poor welding quality at the beginning of an extrusion cycle.Finally, an improving project was proposed for the porthole design of “regular section” in order to avoid surface defects.