| As the lowest density of all metallic constructional material, magnesium alloys have many advantages such as high specific strength and stiffness, good processing performance, abundance and recyclability, which have wide prospect in automobile, aerospace and aeroplane fields. However, most of magnesium alloys show poor ductility due to their intrinsic HCP crystal structure, which limits the formability and industrial application of magnesium alloys to some extend. Grain refinement has been demonstrated as effective method to improve the mechanical properties of magnesium alloys. Friction stir processing(FSP) is a novel severe plastic deformation technique, which has a high strain rate and large plastic deformation, and thus it has great potential in fine and ultrafine-grained metallic materials preparation. Due to higher cooling rate, submerged FSP(SFSP) can prepare much finer-grained alloys.Consequently, this work selects hot-rolled AZ31 and as-cast AZ91 magnesium alloy, studies the relationship of processing parameters and microstructures in the NFSP(processed in air) and SFSP(processed under water) alloy, obtains fine-grained magnesium alloy by optimizing processing parameters, explores the relationship between the microstructure characterization and mechanical properties, especially superplastic behavior, and provides theoretical basic and key technology of fine-grained magnesium alloy preparation by FSP.Firstly, hot-rolled AZ31 alloy is subjected to FSP in air and underwater, respectively. The aim is to investigate the effect of rotation speeds on microstructures and mechanical properties of the processed alloy, record the thermal histories during two FSP, and study the grain refinement mechanism of the FSP AZ31 alloy based on microstructure observation and temperature measurement. With the rotation speeds increasing, the area, the bottom size and the average grain size in the stir zone of the two FSP alloy all increase. Compared with NFSP, SFSP can prepare much finer-grained AZ31 magnesium alloy. The grain refinement mechanism during NFSP and SFSP is attributed to continuous dynamic recrystallization. Owing to much finer microstructures, the microhardness of the FSPed alloy significantly increases. Moreover, the tensile strength of the FSPed alloy slightly decreases while the elongation remarkably increases after FSP. With rotation speed increasing, both the tensile strength and elongation of the FSPed AZ31 alloy decrease. The SFSP alloy shows superior tensile properties compared with NFSP.Secondly, NFSP and SFSP are conducted on as-cast AZ91 alloy. The aim is to analyze the relationship of processing parameter(rotation speed and processing speed) with microstructure, microhardness and tensile properties, compare the microstructure, thermal histories and tensile properties of the NFSP and SFSP AZ31 alloy, and investigate the grain refinement mechanism during FSP. Due to the existence of the second phase, FSP can prepare much finer-grained AZ91 magnesium alloy than AZ31 alloy. The grain size of the NFSP and SFSP AZ91 alloy shows different change trend with rotation speed and processing speed, which is mainly attributed to the synergetic effects of material deformation degree and heat input. Because of lower peak temperature, shorter duration time at high temperature and a higher cooling rate, the microstructures of SFSP are much finer with better mechanical properties. SFSP can prepare the finest microstructures with the average grain size of 1.2 μm at a rotation speed of 800 rpm and a processing speed of 60 mm/min.Thirdly, based on previous results, the AZ31 alloy processed at rotation speed of 900 rpm and processing speed of 60 mm/min, and the AZ91 alloy processed at rotation speed of 800 rpm and processing speed of 60 mm/min are selected for high temperature test, respectively. The research is to investigate the microstructure evolution, the deformation mechanism and the fracture mechanism during superplastic deformation. Compared with AZ31 alloy, AZ91 alloy shows excellent superplasticity, especially having high strain rate superplasticity with an elongation of 990% at 623 K and 2×10-2 s-1. During superplastic deformation, grain boundary sliding is the main deformation mechanism, and the growth and connection of cavities is the main fracture mechanism. Cavities easily exist at the triple grain boundaries and around the second particles during the whole superplastic deformation.Finally, AZ91 alloy at different conditions(as-cast and casting after solution treatment) is subjected to FSP, and the microstructures and mechanical properties of the experimental materials are investigated. Subsequent aging treatment is conducted on the FSP AZ91 alloy, which aims to explore the aging treatment on the precipitation of the second phase and mechanical properties and investigate the effect of heat treatment on superplasticity of the SFSP alloy. Pre-solution treatment can result in the dissolution of the β-Mg17Al12 phase. Aging treatment leads to two kinds of precipitations, i.e. discontinuous precipitation within the grains and continuous precipitation at the grain boundaries. Moreover, pre-solution treatment can significantly improve the ductility, and subsequent aging treatment can improve the tensile strength. The SFSP AZ91 alloy also shows excellent superplasticity after heat treatment. |
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