| Magnesium alloy and Aluminum Alloy is lightweight metal materials widely used in many manufacturing fields,such as Aeronautics and Astronautics,shipbuilding and rail transit industry.FSJ(Friction Stir Joining)joint has less defects and high strength,which is an ideal way to realize the connection of light alloya as a kind of solid state connection technology.In recent years,the demand of connection for Mg/Al dissimilar alloys in the field of aerospace and other areas is increased,but the lack of process research.Thus,the influence of process parameters and tools on microstructure and complex material flow of joint is necessary to be study.In this paper,the effects of process parameters and tools with different geometries and sizes on joint microstructure,mechanical properties and plastic metal flow behavior of dissimilar FSJ between 2024-T3 aluminum alloy and AZ31 B magnesium alloy for 3.2mm sheet were studied.The process parameters were optimized through the establishment of BP neural network model on this basis.To reveal the mechanism and engineering application of dissimilar alloy FSJ has important theoretical significance and application value.The main work and the achievements accomplished in this article are listed as following:1.Study on flow of plastic materials in Mg/Al dissimilar alloy FSJ process.The material flow behavior of the joint on the horizontal direction and the thickness direction was studied by step section of the joints directions in different process parameters.The visualization of the flow material was accomplished by 3D reconstruction of the joint,which shows the flow pattern on advancing side and retreating side of joint and the reason of the defect,and provides reference for the numerical simulation results.A defect free joint was formed under flow equilibrium condition and joint with defect such as hole was formed under insufficient or excess material flow condition.2.Study on simulation of plastic material flow in Mg/Al dissimilar alloy FSJ processA 3D mathematical model of material flow in FSJ process was developed based on computational fluid dynamics theory.The effects of process parameters and tools geometries and sizes on microstructure,mechanical properties and plastic metal flow behavior were studied.The results show that joining speed has limited effect on the flow pattern of joint materials when the heat input for joining is sufficient on an appropriate ratio of rotating speed and jointing speed.The material flow is more sensitive to rotating speed on a high ratio of rotating speed and jointing speed.The material flow on joint upper part can be promoted for using a tool with larger shoulder diameter.3.Study on the effects of process parameters and tools geometries and sizes on joint microstructure and mechanical properties.The process parameter range was determined through massive experiments of Mg/Al dissimilar alloys FSJ and the effects of process parameters and tools geometries and sizes on joint microstructure and mechanical properties were analyzed based on obtention of defect free joint.The results show that there were two reasonable technological parameters in the FSJ joint of Mg/Al dissimilar alloy for defect free joint,low and high ratio of rotating speed and jointing speed parameter ranges.The two forming mode of the joints is completely different.The former joint material is not mixed,the joint strength is mainly affected by the mechanical lock degree of two material and the latter two materials are fully mixed in stir zone,an obvious demarcation line can be observed between thermo-mechanically affected zone and stir zone on aluminum alloy side,which is tensile fracture area and the weak point of joint.A large number of metal compounds can be found at high ratio of rotating speed and jointing speed parameters and through EDS analysis and XRD phase analysis showed that the intermetallic compound is mainly Al12Mg17 and Al3Mg2.4.Study on the effects of tool shoulder size,pin shape and size on microstructure and mechanical properties of joint.FSJ experiments using 10 tools with different shoulder or pin shape a size were analyzed,shows that the joint was not formed when the tool shoulder diameter less than 8mm or more than 6mm for 3.2mm thick magnesium alloy and aluminum alloy sheet for FSJ.The heat input is usually insufficient using tool with 10 mm shoulder diameter at a reasonable range of process,resulting in the dynamic recrystallization of magnesium alloy grain and material flow is not sufficient.The pin with thread structure has a limited effect to promote joint material flow due to the covered pin by joint material during the FSJ process at low ratio of rotating speed and jointing speed parameters.The pin with thread structure and cylindrical pin is more conducive to the plastic material flow,especially the material at the bottom of the joint.At the same,larger shoulder can increase the mechanical lock structure and stir zone area,thus increasing the strength of the joint.5.Study on tensile property prediction of Mg/Al dissimilar alloy FSJ joints.A process optimization model of Mg/Al dissimilar alloy FSJ was established by a BP artificial neural network model based on a large number of experiments,the rotating speed,joining speed and tool shoulder diameter as the input layer and the tensile property of the joints as the output layer.The network was trained by the experimental data and this system can predict the tensile property accurately and optimize the welding parameters. |
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