Study On Microstructure And Properties Of Fine-grained ZK60 Magnesium Alloy Processed By Equal Channel Angular Pressing

In recent years,magnesium and its alloys have attracted great attention as the most attractive light weight structural materials.However,magnesium alloys exhibit limited cold workability and toughness at room temperature,compared to other conventional structural materials such as aluminum or steel.At present,the purpose of improving its mechanical properties can be achieved by large plastic deformation（SPD）.At present,among many SPD methods,equal channel angular processing（ECAP）technology is considered to be one of the most promising methods for industrial applications due to its advantages of being able to deform multiple times,without changing the cross-sectional area of the material.In this experiment,A method of equal channel angular processing（ECAP）is used on the extrusion of ZK60 magnesium alloy along BC path under the temperature of 240℃.The following conclusions were obtained:（1）The average grain size of the as-cast microstructure was 97μm.The ECAP experiment was carried out using a self-made mold.After the fourth pass,the average grain size reached a minimum of 3.46×m,and the grain refining effect was reached to 96.5%.After increasing the amount of deformation,the grain size does not continue to decrease,and there is even a tendency to grow.ZK60 magnesium alloy undergoes dynamic recrystallization under the action of temperature and shear deformation.A large number of dislocations are formed inside the structure and entanglement occurs.The dislocations interact to form a subgrain structure.The proportion of small-angle grain boundaries in the structure increases rapidly.As the deformation increases,the small-angle grain boundaries gradually change to a high-angle grain boundary.Finally,the initial coarse grains inside the tissue are replaced by fine recrystallized grains.（2）There are two main phases in the extruded structure of ZK60 magnesium alloy,namely MgZn and MgZn2.These two phases can hinder the sliding of the grain boundary and the movement of dislocations during the deformation process,thereby facilitating the grain refinement of the alloy structure.It was found by SEM and TEM that the second phase structure was broken and refined during the ECAP process,and eventually spread evenly within the microstructure.This phenomenon has the eff’ect of dispersion strengthening on the whole microstructure.（3）During the test of mechanical properties of ZK60 magnesium alloy,the tensile strength of the initial as-cast ZK60 structure was 202.9MPa,and after four passes of equal channel angular extrusion,the tensile strength rose to the highest point of 256.1MPa.The strength increased by 26.2%,and the elongation rate reached the highest level of the experiment,which was 24.8%,nearly 19%increase in elongation compared to as-cast microstructure.The hardness also reached a maximum of 76.6HV in the fourth pass,which was 3.5%higher than the initial state.Continue to increase the amount of deformation,these three mechanical properties have a tendency to decline.