Large Strain Extrusion Machining AZ31Magnesium Alloy At Room Temperature And The Microstructure Evolution

As the lightest structural metal material, magnesium alloys possess great commercial value.The normal severe plastic deformation approaches can significantly refine the microstructureand improve the material properties, but due to low efficiency and high processing cost, theseapproaches are limited in further applications. In comparison, large strain extrusion machining(LSEM) has the advantages of high speed continuously processing, no preheating, simpletechnics and low cost. As a result, LSEM has great potential values in research and application.This paper focuses on the workability of AZ31magnesium alloy by LSEM at room temperature.With the help of DEFORM2D finite element modeling software, it analyses the deformationof the Mg AZ31chips during the LSEM process under different chip compressing ratios or rakeangles of the combined tool. It also analyses the variations of equivalent strain, equivalent strainrate, temperature and the maximum principal stress inside the chips, analyses the change of theshear angle and its influence, predicts the initiation of crack and investigates the fracture bySEM, checks the grain refinement and analyses the mechanism, and tests the mechanicalproperties of the microstructure. The results indicate that the chip compressing ratio is the mostcritical parameter of LSEM, because of its direct and significant influences to chip formability,equivalent strain, temperature, the maximum principal stress and the shear angle. A proper chipcompressing ratio is essential for producing well-formed and grain-refined Mg AZ31strips.The rake angle has important effect to the factors mentioned above, and the variations of themexhibit some regularities. But the rake angle has significant impact to the chip’s extrusionproperties. LSEM has a remarkable effect of grain refinement to Mg AZ31strips, which isdirectly influenced by the chip compressing ratio. The grains can be refined in to the fined scaleor ultra-fined scale by adopting different parameters. The mechanical shearing and dynamicrecrystallization are considered as the main mechanisms of grain refinement, which affect thegrain refinement differently under different conditions. The shear banding phenomenon canlead to a mass scale dynamic recrystallization inside the areas and eventually causes materialflow localization, which performs unignorable adjustment during the deformation process. Thecracks initially generate inside the shear bands and extend along the interior of shear bands, which may be related to softening effect inside the shear bands caused by dynamicrecrystallization. According to the analysis of SEM images of fracture, the strips produced withsmall chip compressing ratio exhibit better plastic properties. The test result also shows that thestrips produced by LSEM has a significant improvement in mechanical properties of themicrostructure.