Effect Of Ambient-temperature ECAP Processing On Microstructures And Properties Of Pure Magnesium

Mg and Mg alloys, regarded as green metallic structural material for the 21st century, have great potentials in industrial applications, especially in aviation field. However, relative low absolute strength, low ductility and poor corrosion resistance of Mg alloys greatly limit their wide applications. To solve these problems, a new-type equal channel angular pressing (ECAP) process is designed based on three-dimensional stress conditions in materials during deformation, which makes hard-working materials such as pure magnesium be able to be subjected to severe plastic deformation by ECAP at room temperature.In this work, pure magnesium sample cladded with pure Fe plate was subjected to ECAP processing for multipasses in route C. The extrusion speed is 1.5 mm/s. Microstructure, mechanical properties and corrosion resistance of Mg samples before and after ECAP processing were observed and measured by using optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffraction (XRD), universal tensile machine, electrochemical testing method and so on. The conclusions are as follows.(1) It is feasible to ECAP process pure Mg at room temperature by cladding Mg sample with pure iron plate and pure magnesium sample has no cracks and defects after equal channel angular pressing. There is close relationship between the surface quality of ECAPed pure magnesium samples with the thickness of pure iron outer wall.(2) Pure magnesium after ambient-temperature ECAP processing was significant refined by using this new ECAP process. With the increasing of ECAP passes, the grain size was reduced gradually and homogenized, and the most fine (～2um) and uniform microstructure was attained after four passes of equal channel angular pressing. Morever, grain orientation of pure magnesium after ECAP processing changed significantly. The originally random grain orientation in as-cast pure magnesium was oriented to prismatic and pyramidal planes after ambient-temperature equal channel angular pressing (ECAP).(3) Hardness and ambient-temperature tensile properties of pure magnesium after multi-pass equal channel angular pressing significantly increased. With the increasing of pressing passes, the hardness increases granuallly, and the tensile strength gets to the highest value after four pass equal channel angular pressing (ECAP), but the maximum elongation is attained in 2-pass ECAPed sample. Ambient-temperature tensile fracture analysis shows that as-cast pure magnesium exhibits brittle fracture, while after equal channel angular pressing (ECAP), the fractrure mechanism of pure magnesium was transformed into ductile fracture.(4) Self-corrosion test results show that corrosion points on the surface of as-cast magnesium sample is localized, while those on the extrused sample surface are more homogeneously distributed and the corrosion feature of pure Magnesium was converted localized corrosion into uniform corrosion with the increase of ECAP passes. Further, corrosion performance test results show that athough pure Mg was significantly refined after room-temperature ECAP processing, concomitant crystalline defects such as grain boundary were produced in large quantity which makes pure Mg liable to be corroded, and with a result the mass-loss rate and corrosion current in polarization curves in ECAPed sample are higher than thoes in as-cast sample. Moreover, internal defects in ECAPed pure Mg promote the formation of surface oxide film in atmosphere and aqueous solution and thus increase the corrosion potential of pure Mg, but the formed MgO film or Mg(OH)2 films couldn’t improve the passivation capability of Mg in NaCl solution for a long time.