| Grain refinement is the only way to reinforce the strength of metals and simultaneously maintain a good plasticity.One effective way for grain refinement is conducting plastic deformation and recrystallization.Therefore,the study of grain refinement and severe plastic deformation(SPD)has attracted much attention by scholars all over the world in recent years.It is shown by the work of others that the grains can be refined to ultrafine grain size less than 1?m,and high strength and good plasticity can be obtained by appropriate heat treatment of the sever plastic deformed metals.It indicates a direction to improve the microstructures and properties of metals.Up to now,there are lots of works on the study of SPD and ultrafine grained(UFG)materials,which lead to several processing technique come into being.However,it is still hard to treat the large size of metals or long strips.A new method named equal-thickness-extrusion was proposed by our team to process much long strip,which has overcome the disadvantages of other SPD methods.The advantages are obvious such as the simple technological process and long strip can be extruded,etc.This thesis experimentally studied the effect of SPD with equal-thickness-extrusion on the micro structural changes and properties of aluminum alloy strips.It is of significance to the grain refinement of metals,the improvement of properties as well as to the development of equal-thickness-extrusion technique.In this thesis,5052 and 5083 aluminum alloy strips of 2.00 mm thick were selected as the experimental materials.Some interesting results have been obtained by the equal thickness extrusion experiment and the subsequent heat treatment of the material.It is shown from the experimental results that multi-passes of SPD deformation can be achieved for aluminum alloys with single pass strain up to 0.65 on the equal-thickness-extrusion experiment.The plasticity of 5052 aluminum alloy is so good that 6 passes of equal thickness extrusion were fluently processed.And consequently an obvious grain refinement was obtained with an average grain size of 14.91 ?m in cross section and of 18.52 ?m in longitudinal section.The Vickers-hardness increases from 78.94 HV to 104.12 HV.The electric conductivity,changing from 35.8%IACS down to 35.4%IACS,nearly has no change.The plasticity of 5083 aluminum alloy is poor,which leads to fracture of the strip after 2 passes of deformation.The deformed 5083 aluminum alloy strip has an average grain size of 10.53 ?m in cross section and of 9.87 ?m in longitudinal section after 2 passes of equal thickness extrusion.Grain refinement is not obvious.The tensile strength increases from 323.05 MPa to 330.06 MPa,while the yield strength increases from 145.62 MPa to 195.41 MPa.The Vickers-hardness increases from 89.03 HV to 126.07 HV.The electric conductivity,changing from 28.2%IACS down to 27.9%IACS,nearly has no change.It can be seen from the comparison of the results of above two alloys that the yield strength of 5052 alloy is close to that of 5083 alloy,while the electric conductivity of 5052 alloy keep higher.The results illustrate that the properties of 5052 alloy were greatly improved approaching that of higher grade 5083 aluminum alloy.This work is of great significance to the study of the grain refinement and the property improvement of aluminum alloys.It is also of practical value to the development of equal-thickness-extrusion technique. |
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