| As the lightest structural materials, magnesium alloys have many attractive advantages such as low density, high specific strength and recyclability and they are widely used in automobile and aerospace fields. However, most magnesium alloys exhibit poor ductility and formability at room temperature as a result of its hexagonal lattice structure, which limits the usage of magnesium alloys to some extent. As one of the most widely used magnesium alloys, cast AZ91 magnesium alloy generally exhibits low strength and ductility because of its coarse grain size and the network-like second phase distributed at the grain boundaries. As a novel severe plastic deformation technique, friction stir processing(FSP) which is convenient and flexible to operate has a great potential in grain refinement and modification of microstructure. On one hand, multi-pass friction stir processing(MP-FSP) can further achieve grain refinement through the accumulation of strain; on the other hand, it can also achieve more uniform structure through adding the processing passes.The present work studies the influence of processing parameters on microstructures and mechanical properties of AZ91 magnesium alloy, achieves ultrafine-grained material by rapid cooling, explores the relationship between microsturcture of ultrafine-grained AZ91 alloy and its mechanical properties, especially superplasticity. In addition, this work also studies the relationship between microsturcture of large area fine-grained plate prepared by MP-FSP and mechanical properties, explores the method of preparing large bulk material by friction stir processing.Firstly, AZ91 alloy is subjected to single-pass and two-pass FSP respectively to investigate the microstructure evolution and mechanical properties of single-pass and two-pass FSP. Meanwhile, the influence of rotation speed and processing speed on microstructure, microhardness and tensile properties is studied. Compared with the single-pass FSP, the onion ring feature disappears and the shapes of the stir zone are different in two-pass FSP AZ91 alloy. Basin-shaped, elliptical and rectangle stir zone are observed in MP-FSP. With the reducing of rotation speed and increasing of processing speed of the second pass, the average grain size of ?-Mg become smaller. The finest grain size of 3.6 ?m is achieved with the processing parameters: first pass with a rotation speed of 800 rpm, a traverse speed of 60 mm/min; second pass with a rotation speed of 400 rpm, a traverse speed of 60 mm/min.Secondly, three different types of processing methods, including two-passes normal FSP, one pass normal FSP and one pass submerged FSP, and two-passes submerged FSP are conducted on AZ91 magnesium alloy. The submerged FSP is conducted in water. The aim is to investigate the effect of cooling condition on microstructure and mechanical properties of MP-FSP AZ91.The results show that the average grain size of the two-pass submerged FSP is 0.8~1 ?m, and its microhardness, tensile strength and elongation are 94.7 HV, 355.5 MPa and 31.5%, respectively. It has the best comprehensive mechanical properties compared to the other two processing methods.Thirdly, two-pass submerged FSP specimen is adopted to investigate the microstructure evolution, the deformation mechanism and the fracture mechanism during superplastic deformation. The specimen exhibits good superplasticity at 275-350 ℃and 3×10-3- 3×10-2 s-1. The best elongation of 2300% was at the temperature of 300 ℃and the strain rate of 3×10-3 s-1. In addition, this specimen also exhibits excellent high strain rate superplasticity and the best elongation of 1900% was achieved at the temperature of 325 ℃and the strain rate of 1×10-2 s-1. Grain boundary sliding is the dominant deformation mechanism and the growth and connection of cavities is the main fracture mechanism during superplastic deformation.Finally, the effect of overlapping ratio on microstructure and mechanical properties of FSP AZ91 alloy is studied. The experiments are carried out with overlapping ratios of-1, 0, 0.5 and 1, and large fine-grained plates with no obvious macro-defects are obtained. With the increasing of overlapping ratio, the overlapping area of stir zone becomes bigger, ranging from zero overlapping to complete overlapping. The microstructure and microhardness distribution of the MP-FSP part with the overlapping ratio of 0.5 are relatively uniform. Tensile strength and elongation are higher than that of specimen with the overlapping ratio of 0, showing better mechanical properties. |
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