Liu, Zhou, and Duszczyk (2008) carried out a finite element analysis for the extrusion of a magnesium square tube using porthole die and found that the average temperature and pressure on the welding plane will increase and the quality of the longitude welding could be enhanced with the growth of extrusion speed.
"Numerical Simulation of Temperature Field during Whole Extrusion Process of Aluminum Hollow Profile with Porthole Die Extrusion Forming." The Chinese Journal of Nonferrous Metals 23 (10): 2769-2778.
"FE Analysis of Metal Flow and Weld Seam Formation in a Porthole Die during the Extrusion of a Magnesium Alloy into a Square Tube and the Effect of Ram Speed on Weld Strength." Journal of Materials Processing Technology 200: 185-198.