Microstructure and Mechanical Properties of AZ91 Magnesium Alloy Tubes Processed by New Method of Improved Tube Cyclic Expansion Extrusion

In present study, an improved severe plastic deformation process named improved tube cyclic expansion extrusion process has been introduced. The idea of this process is taken from the conventional tube cyclic expansion extrusion process, and in this novel process, it is tried to solve some important problems of the conventional process. Improved tube cyclic expansion extrusion process is capable of severe plastic deforming and improving microstructure and mechanical properties of tubular components. Also, this process can be considered for producing relatively long tubes. For this purpose, the improved tube cyclic expansion extrusion process was successfully performed on AZ91 magnesium alloy tubes, up to two passes. Then, the microstructure evolution and the mechanical properties improvement were scrutinized. The results showed that the microstructure and mechanical properties were improved considerably. In this way, after two passes of this process, an ultrafine grained (UFG) microstructure was formed, and the values of ultimate strength (UTS), hardness (Hv) and ductility (EL%) became 3.6, 1.83 and 1.8 times higher, respectively. Also, the comparison of the results of the improved tube cyclic expansion extrusion process with those of the conventional tube cyclic expansion extrusion process indicated that ultimate strength and hardness of the improved process were near to those of the conventional process, but the value of elongation to failure of the improved process is considerably higher than the value of the conventional process. This can be considered as one of the important advantages of the improved process over the conventional process.